[[File:Compressor wheel Napier NA357.JPG|thumb| Centrifugal impeller with a highly polished surface likely to improve performance]]
[[File:Saturn MD-120 cutaway.jpg|thumb | [[Jet engine]] cutaway showing the centrifugal compressor and other parts]]
[[File:Impeller inlet meridional triangles.PNG|thumb|Inlet velocity triangles for centrifugal compressor impeller]]
[[File:Impeller exit meridional trianges.PNG|thumb|Exit velocity triangles for centrifugal compressor impeller]]
[[File:AeroTurboDomain.PNG|thumb|Figure 1.1 – Aero-thermo domain of turbomachinery]]
[[File:PhysTurboDomain.PNG|thumb|Figure 1.2 – Physical domain of turbomachinery]]

'''Centrifugal compressors''', sometimes termed '''radial compressors''', are a sub-class of dynamic axisymmetric work-absorbing [[turbomachinery]].<ref name="Shepard"/>

The idealized compressive dynamic turbo-machine achieves a pressure rise by adding [[kinetic energy]]/[[velocity]] to a continuous flow of [[fluid]] through the rotor or [[impeller]]. This kinetic energy is then converted to an increase in [[potential energy]]/static pressure by slowing the [[fluid dynamics|flow]] through a diffuser. The pressure rise in impeller is in most cases almost equal to the rise in the diffuser section.

== Theory of operation ==
Imagine a simple case where flow passes through a straight pipe to enter a centrifugal compressor. The simple flow is straight, uniform and has no vorticity. As illustrated below α1=0 deg. As the flow continues to pass into and through the centrifugal impeller, the impeller forces the flow to spin faster and faster. According to a form of [[Euler]]'s fluid dynamics equation, known as "[[Euler's pump and turbine equation|pump and turbine equation]]," the energy input to the fluid is proportional to the flow's local spinning velocity multiplied by the local impeller [[tangential velocity]].

In many cases the flow leaving centrifugal impeller is near the [[speed of sound]] (340 metres/second). The flow then typically flows through a stationary compressor causing it to decelerate. These stationary compressors are actually static [[guide vane]]s where energy transformation takes place. As described in [[Bernoulli's principle]], this reduction in velocity causes the pressure to rise leading to a compressed fluid.<ref name="Shepard"/>

== Historical contributions, the pioneers ==
Over the past 100 years, applied scientists including [[Aurel Stodola|Stodola]] (1903, 1927–1945),<ref name="Stodola">
{{cite book
|author=Aurel Stodola
|title=Steam and Gas Turbines
|publisher=P. Smith
|year=1945
|publication-place = New York
|ol = 18625767M
}}</ref> Pfleiderer (1952),<ref>{{cite book|last=Pfleiderer|first=C.|title=Turbomachines|year=1952|publisher=Springer-Verlag|location=New York}}</ref> Hawthorne (1964),<ref name="Hawthorne">
{{cite book
|author=W. R. Hawthorne
|title=Aerodynamics Of Turbines and Compressors
|publisher=Princeton University Press
|year=1964
|publication-place = Princeton New Jersey
|lccn=58-5029}}</ref> Shepard (1956),<ref name="Shepard">
{{cite book|author=Shepard, Dennis G.
|title=Principles of Turbomachinery
|year=1956
|publisher=McMillan
|isbn=0-471-85546-4
|lccn = 56002849}}
</ref> Lakshminarayana (1996),<ref name="Lakshminarayana">
{{cite book|author=Lakshminarayana, B.
|title=Fluid Dynamics and Heat Transfer of Turbomachinery
|year=1996
|publisher=John Wiley & Sons Inc.
|place=New York
|isbn=0-471-85546-4}}
</ref> and Japikse (many texts including citations),<ref name="Japikse"/><ref name="Japikse&Baines">
{{cite book|author=Japikse, David & Baines, Nicholas C.
|title=Introduction to Turbomachinery
|year=1997
|publisher=Oxford University press
|place=Oxford
|isbn=0-933283-10-5}}</ref><ref name="JapikseDifTech"/><ref name="JapikseExpTech"/> have educated young engineers in the fundamentals of turbomachinery. These understandings apply to all dynamic, continuous-flow, axisymmetric pumps, fans, blowers, and compressors in axial, mixed-flow and radial/centrifugal configurations.

This relationship is the reason advances in turbines and axial compressors often find their way into other turbomachinery including centrifugal compressors. Figures 1.1 and 1.2<ref name="Peng">
{{cite book|author=Peng, W. W.
|title=Fundamentals of Turbomachinery
|year=2007
|publisher=John Wiley & Sons Inc.
|place=New York
|isbn=978-0-470-12422-2}}
</ref><ref name="Wislicenus">
{{cite book|author=Wislicenus, George Friedrich
|title=Fluid Mechanics of Turbomachinery in two volumes
|year=1965
|publisher=Dover
|place=New York
|isbn=978-0-486-61345-1}}
</ref> illustrate the domain of turbomachinery with labels showing centrifugal compressors. Improvements in centrifugal compressors have not been achieved through large discoveries. Rather, improvements have been achieved through understanding and applying incremental pieces of knowledge discovered by many individuals.

Figure 1.1 represents the [[aerodynamics|aero]]-[[thermodynamics|thermo]] domain of turbomachinery. The horizontal axis represents the energy equation derivable from The [[First Law of Thermodynamics]].<ref name="Shepard"/><ref name="Wislicenus"/> The vertical axis, which can be characterized by Mach Number, represents the range of fluid compressibility (or elasticity).<ref name="Shepard"/><ref name="Wislicenus"/> The Z-axis, which can be characterized by [[Reynolds Number]], represents the range of fluid viscosities (or stickiness).<ref name="Shepard"/><ref name="Wislicenus"/> [[mathematics|Mathematicians]] and [[physics|Physicists]] who established the foundations of this aero-thermo domain include:<ref name="wood"/><ref name="streeter"/> [[Sir Isaac Newton]], [[Daniel Bernoulli]], [[Leonhard Euler]], [[Claude-Louis Navier]], [[Sir George Gabriel Stokes]], [[Ernst Mach]], [[Nikolay Yegorovich Zhukovsky]], [[Martin Wilhelm Kutta]], [[Ludwig Prandtl]], [[Theodore von Kármán]], [[Paul Richard Heinrich Blasius]], and [[Henri Coandă]].

Figure 1.2 represents the physical or mechanical domain of turbomachinery. Again, the horizontal axis represents the energy equation with turbines generating power to the left and compressors absorbing power to the right.<ref name="Shepard"/><ref name="Wislicenus"/> Within the physical domain the vertical axis differentiates between high speeds and low speeds depending upon the turbomachinery application.<ref name="Shepard"/><ref name="Wislicenus"/> The Z-axis differentiates between axial-flow geometry and radial-flow geometry within the physical domain of turbomachinery.<ref name="Shepard"/><ref name="Wislicenus"/> It is implied that mixed-flow turbomachinery lie between axial and radial.<ref name="Shepard"/><ref name="Wislicenus"/> Key contributors of technical achievements that pushed the practical application of turbomachinery forward include:<ref name="wood"/><ref name="streeter"/> [[Denis Papin]],<ref name="98-GT-22 International Gas Turbine & Aeroengine Congress & Exhibition in Stockholm, Sweden.">{{cite web|last=Engeda|first=Abraham|title=From the Crystal Palace to the pump room|url=http://www.memagazine.org/backissues/membersonly/february99/features/crystal/crystal.html|work=Mechanical Engineering|publisher=ASME|year=1999}}</ref> Kernelien Le Demour, [[Daniel Gabriel Fahrenheit]], John Smeaton, Dr. A. C. E. Rateau,<ref>{{cite web|last=Elliott Company|title=Past, Present, Future, 1910-2010|url=http://www.elliott-turbo.com/Files/Admin/100th%20Anniversary/1930s%20-%20O%20Line%20Blowers.pdf|publisher=Elliott|accessdate=1 May 2011}}</ref> [[John Barber (engineer)|John Barber]], [[Alexander Sablukov]], Sir [[Charles Algernon Parsons]], [[Ægidius Elling]], [[Sanford Alexander Moss]], [[Willis Carrier]], [[Adolf Busemann]], [[Hermann Schlichting]], [[Frank Whittle]] and [[Hans von Ohain]].

=== Partial timeline ===
{| class="wikitable"
|-
| <1689
| Early turbomachines
| Pumps, blowers, fans
|-
| 1689
| Denis Papin
| Origin of the centrifugal compressor
|-
| 1754
| Leonhard Euler
| Euler's "Pump & Turbine" equation
|-
| 1791
| John Barber
| First gas turbine patent
|-
| 1899
| Dr. A. C. E. Rateau
| First practical centrifugal compressor
|-
| 1927
| Aurel Boleslav Stodola
| Formalized "slip factor"
|-
| 1928
| Adolf Busemann
| Derived "slip factor"
|-
| 1937
| Frank Whittle and Hans von Ohain, independently
| First gas turbine using centrifugal compressor
|-
| >1970
| Modern turbomachines
| 3D-CFD, rocket turbo-pumps, heart assist pumps, turbocharged fuel cells
|}

== Turbomachinery similarities ==
Many types of dynamic continuous flow *[[turbomachinery]] are treated in Wikipedia. As stated in the main turbomachinery article, what is notable about turbomachinery is that the fundamentals apply almost universally. Certainly there are significant differences between these machines and between the types of analysis that are typically applied to specific cases. This does not negate that they are unified by the same underlying physics of fluid dynamics, gas dynamics, aerodynamics, hydrodynamics, and thermodynamics.

A few of these machines have physical characteristics related to the centrifugal compressors, such as the following;

{| class="wikitable"
|-
| Similarities to axial compressor
Centrifugal compressors are similar to [[axial compressor]]s in that they are rotating airfoil based compressors as shown in the adjacent figure. <ref name="Lakshminarayana"/><ref name="Japikse&Baines"/> It should not be surprising that the first part of the centrifugal impeller looks very similar to an axial compressor. This first part of the centrifugal impeller is also termed an ''inducer''. Centrifugal compressors differ from axials as they use a greater change in radius from inlet to exit of the rotor/impeller.
| [[File:Turboprop T-53.jpg|thumb|Cutaway showing an axi-centrifugal compressor gas turbine]]
|}

{| class="wikitable"
|-
| Similarities to centrifugal fan
Centrifugal compressors are also similar to [[centrifugal fan]]s of the style shown in neighboring figure as they both increase the flows energy through increasing radius.<ref name="Shepard"/> In contrast to centrifugal fans, compressors operate at higher speeds to generate greater pressure rises. In many cases the engineering methods used to design a centrifugal fan is the same as those to design a centrifugal compressor. As a result they can at times look very similar.

This relationship is less true in comparison to a [[squirrel-cage fan]]s as shown in figure farthest right.

For purposes of generalization and definition, it can be said that centrifugal compressors often have density increases greater than 5 percent. Also, they often experience relative fluid velocities above [[Mach number]] 0.3<ref name="Lakshminarayana"/><ref name="API673">{{cite book|last=API|title=Std 673-2002 Centrifugal Fans for Petroleum, Chemical and Gas Industry Services|year=July 2002|publisher=API|location=New York|url=http://global.ihs.com/doc_detail.cfm?item_s_key=00392744/}}</ref> when the working fluid is air or nitrogen. In contrast, fans or blowers are often considered to have density increases of less than five percent and peak relative fluid velocities below Mach 0.3.
| [[File:CentrifugalCompressor.jpg|thumb|A low speed, low pressure centrifugal compressor or [[centrifugal fan]], with upward discharging cone used to diffuse the air velocity]]
| [[File:CentrifugalFan.png|thumb|A [[squirrel-cage fan]], without a discharge diffuser]]
|}

{| class="wikitable"
|-
| Similarities to centrifugal pump
Centrifugal compressors are also similar to [[centrifugal pump]]s<ref name="Shepard"/> of the style shown in the adjacent figures. The key difference between such compressors and pumps is that the compressor working fluid is a gas (compressible) and the pump working fluid is liquid (incompressible). Again, the engineering methods used to design a centrifugal pump are the same as those to design a centrifugal compressor. Yet, there is one important difference: the need to deal with [[cavitation]] in pumps.
| [[File:Centrifugal Pump-mod.jpg|thumb|A 3D-solids model of a type of [[centrifugal pump]]]]
| [[File:Centrifugal Pump.png|thumb|Cut-away of a [[centrifugal pump]]]]
|}

{| class="wikitable"
|-
| Similarities to radial turbine
Centrifugal compressors also look very similar to their turbomachinery counterpart the [[radial-inflow turbine]] as shown in the figure. While a compressor transfers energy into a flow to raise its pressure, a turbine operates in reverse, by extracting energy from a flow, thus reducing its pressure.<ref name="Japikse&Baines"/> In other words, power is input to compressors and output from turbines.
| [[File:RadialInflowturbine.JPG|thumb|Cutaway showing a radial-inflow turbine that looks similar to the turbine shown in Figure 3.1]]
|}

===Turbomachinery using centrifugal compressors===

A partial list of turbomachinery that may use one or more centrifugal compressors within the machine are listed here.

*[[Auxiliary power unit]]
*[[Centrifugal type supercharger]]
*[[Gas turbine]]
*[[Turbocharger]]
*[[Turboprop]]
*[[Turboshaft]]

== Components of a simple centrifugal compressor ==

A simple centrifugal compressor has four components: inlet, impeller/rotor, diffuser, and collector.<ref name="Shepard"/> Figure 3.1 shows each of the components of the flow path, with the flow (working gas) entering the centrifugal impeller axially from right to left. As a result of the impeller rotating clockwise when looking downstream into the compressor, the flow will pass through the volute's discharge cone moving away from the figure's viewer.

[[File:Turbocharger.jpg|thumb|300px|Figure 3.1 – Cut-away view of a [[turbo-charger]] showing the centrifugal compressor (blue) on the right end of the rotor]]

=== Inlet ===

The inlet to a centrifugal compressor is typically a simple pipe. It may include features such as a valve, stationary vanes/airfoils (used to help swirl the flow) and both pressure and temperature instrumentation. All of these additional devices have important uses in the control of the centrifugal compressor.

=== Centrifugal impeller ===

The key component that makes a compressor centrifugal is the centrifugal impeller, Figure 01. It is the impeller's rotating set of vanes (or blades) that gradually raises the energy of the working gas. This is identical to an axial compressor with the exception that the gases can reach higher velocities and energy levels through the impeller's increasing radius. In many modern high-efficiency centrifugal compressors the gas exiting the impeller is traveling near the speed of sound.

Impellers are designed in many configurations including "open" (visible blades), "covered or shrouded", "with splitters" (every other inducer removed) and "w/o splitters" (all full blades). Both Figures 0.1 and 3.1 show open impellers with splitters. Most modern high efficiency impellers use "backsweep" in the blade shape.<ref name="Japikse">{{cite book|author=Japikse, David
|title=Centrifugal Compressor Design and Performance
|year=
|publisher=Concepts ETI .
|isbn=0-933283-03-2}}
</ref><ref name="Whitfield, Baines">{{cite book
|author=Whitfield, A. & Baines, N. C.
|title=Design of Radial Turbomachinery
|year=1990
|publisher=Longman Scientific and Technical
|isbn=0-470-21667-0}}</ref><ref name="Aungier">
{{cite book|author=Aungier, Ronald H.
|title=Centrifugal Compressors, A Strategy for Aerodynamic Design and Analysis
|publisher=ASME Press
|year=2000
|isbn=0-7918-0093-8}}</ref>

Euler’s pump and turbine equation plays an important role in understanding impeller performance.

=== Diffuser ===

The next key component to the simple centrifugal compressor is the diffuser.<ref name="Japikse&Baines"/><ref name="Aungier"/><ref name="JapikseDifTech">
{{cite book|author=Japikse, David and Baines, N.C.
|title=Diffuser Design Technology
|year=1998
|publisher=Concepts ETI .
|isbn=0-933283-01-6}}
</ref> Downstream of the impeller in the flow path, it is the diffuser's responsibility to convert the kinetic energy (high velocity) of the gas into pressure by gradually slowing (diffusing) the gas velocity. Diffusers can be vaneless, vaned or an alternating combination. High efficiency vaned diffusers are also designed over a wide range of solidities from less than 1 to over 4. Hybrid versions of vaned diffusers include: wedge, channel, and pipe diffusers. There are turbocharger applications that benefit by incorporating no diffuser.

[[Bernoulli's principle|Bernoulli's fluid dynamic principle]] plays an important role in understanding diffuser performance.

=== Collector ===

The collector of a centrifugal compressor can take many shapes and forms.<ref name="Japikse&Baines"/><ref name="Aungier"/> When the diffuser discharges into a large empty chamber, the collector may be termed a ''Plenum''. When the diffuser discharges into a device that looks somewhat like a snail shell, bull's horn or a French horn, the collector is likely to be termed a ''volute'' or ''scroll''. As the name implies, a collector’s purpose is to gather the flow from the diffuser discharge annulus and deliver this flow to a downstream pipe. Either the collector or the pipe may also contain valves and instrumentation to control the compressor.

== Applications ==

Below, is a partial list of centrifugal compressor applications each with a brief description of some of the general characteristics possessed by those compressors. To start this list two of the most well-known centrifugal compressor applications are listed; gas turbines and turbochargers.<ref name="Lakshminarayana"/>

[[File:DH Goblin annotated colour cutaway.png|thumb|300px|Figure 4.1 – [[Jet engine]] cutaway showing the centrifugal compressor and other parts.]]

[[File:Turbojet operation-centrifugal flow-en.svg|thumb|300px|Figure 4.2 – [[Jet engine]] cross section showing the centrifugal compressor and other parts.]]

* In [[gas turbine]]s and auxiliary power units.<ref>{{cite book|author=Saravanamuttoo, H.I.H., Rogers, G.F.C. and Cohen, H.
|title=Gas Turbine Theory
|year=2001
|publisher=Prentice-Hall
|isbn=0-13-015847-X}}
</ref> Ref. Figures 4.1–4.2
:In their simple form, modern gas turbines operate on the Brayton cycle. (ref Figure 5.1) Either or both axial and centrifugal compressors are used to provide compression. The types of gas turbines that most often include centrifugal compressors include turboshaft, turboprop, auxiliary power units, and micro-turbines. The industry standards applied to all of the centrifugal compressors used in aircraft applications are set by the FAA and the military to maximize both safety and durability under severe conditions. Centrifugal impellers used in gas turbines are commonly made from titanium alloy forgings. Their flow-path blades are commonly flank milled or point milled on 5-axis milling machines. When tolerances and clearances are the tightest, these designs are completed as hot operational geometry and deflected back into the cold geometry as required for manufacturing. This need arises from the impeller's deflections experienced from start-up to full speed/full temperature which can be 100 times larger than the expected hot running clearance of the impeller.

* In automotive engine and [[diesel engine]] [[turbocharger]]s and [[supercharger]]s.<ref name="Baines">
{{cite book|author=Baines, Nicholas C.
|title=Fundamentals of Turbocharging
|year=2005
|publisher= Concepts ETI .
|isbn=0-933283-14-8}}
</ref> Ref. Figure 1.1
:Centrifugal compressors used in conjunction with reciprocating internal combustion engines are known as turbochargers if driven by the engine’s exhaust gas and turbo-superchargers if mechanically driven by the engine. Standards set by the industry for turbochargers may have been established by [[SAE International|SAE]].<ref name="SAE">{{cite web|title=SAE Standards|url=http://www.sae.org/standards/|work=SAE/standards/power and propulsion/engines|publisher=SAE International|accessdate=23 April 2011}}</ref> Ideal gas properties often work well for the design, test and analysis of turbocharger centrifugal compressor performance.

* In [[pipeline transport|pipeline compressors]] of [[natural gas]] to move the gas from the production site to the consumer.<ref name="API617"/>
:Centrifugal compressors for such uses may be one- or multi-stage and driven by large gas turbines. Standards set by the industry (ANSI/API, ASME) result in large thick casings to maximize safety. The impellers are often if not always of the covered style which makes them look much like pump impellers. This type of compressor is also often termed an ''API-style''. The power needed to drive these compressors is most often in the thousands of horsepower (HP). Use of real gas properties is needed to properly design, test and analyze the performance of natural gas pipeline centrifugal compressors.

*In [[oil refineries]], [[natural gas processing]], [[petrochemical]] and [[chemical plant]]s.<ref name="API617">{{cite book|last=API|title=Std 617-2002 Axial and Centrifugal Compressors and Expander-compressors for Petroleum, Chemical and Gas Industry Services|year=July 2002|publisher=API|location=New York}}</ref>
:Centrifugal compressors for such uses are often one-shaft multi-stage and driven by large steam or gas turbines. Their casings are often termed ''horizontally split'' or ''barrel''. Standards set by the industry (ANSI/API, ASME) for these compressors result in large thick casings to maximize safety. The impellers are often if not always of the covered style which makes them look much like pump impellers. This type of compressor is also often termed ''API-style''. The power needed to drive these compressors is most often in the thousands of HP. Use of real gas properties is needed to properly design, test and analyze their performance.

* [[Air-conditioning]] and [[refrigeration]] and [[HVAC]]: Centrifugal compressors quite often supply the compression in [[vapor-compression refrigeration|water chillers]] cycles.<ref name="ASHRAE">{{cite web|last=ASHRAE|first=American Society of Heating, Refrigeration and Air-Conditioning Engineers|title=Standards & Guidelines|url=http://www.ashrae.org/technology/page/548|publisher=ASHRAE|accessdate=23 April 2011}}</ref>
:Because of the wide variety of vapor compression cycles ([[thermodynamic cycle]], [[thermodynamics]]) and the wide variety of workings gases ([[refrigerant]]s), centrifugal compressors are used in a wide range of sizes and configurations. Use of real gas properties is needed to properly design, test and analyze the performance of these machines. Standards set by the industry for these compressors include ASHRAE, ASME & API.

* In industry and manufacturing to supply compressed air for all types of [[pneumatic tool]]s.<ref name="API672"/>
:Centrifugal compressors for such uses are often multistage and driven by electric motors. Inter-cooling is often needed between stages to control air temperature. Note that the road repair crew and the local automobile repair garage find screw compressors better adapt to their needs. Standards set by the industry for these compressors include ASME and government regulations that emphasize safety. Ideal gas relationships are often used to properly design, test and analyze the performance of these machines. Carrier’s equation is often used to deal with humidity.

* In air separation plants to manufacture purified end product gases.<ref name="API672">{{cite book|last=API|title=Std 672-2007 Packaged, Integrally Geared Centrifugal Air Compressors for Petroleum, Chemical, and Gas Industry Services|year=October 2007|publisher=API|location=New York}}</ref>
:Centrifugal compressors for such uses are often multistage using inter-cooling to control air temperature. Standards set by the industry for these compressors include ASME and government regulations that emphasize safety. Ideal gas relationships are often used to properly design, test and analyze the performance of these machines when the working gas is air or nitrogen. Other gases require real gas properties.

* In [[oil field]] re-injection of high pressure natural gas to improve oil recovery.<ref name="API617"/>
:Centrifugal compressors for such uses are often one-shaft multi-stage and driven by gas turbines. With discharge pressures approaching 700 bar, casing are of the barrel style. Standards set by the industry (API, ASME) for these compressors result in large thick casings to maximize safety. The impellers are often if not always of the covered style which makes them look much like pump impellers. This type of compressor is also often termed ''API-style''. Use of real gas properties is needed to properly design, test and analyze their performance.

== Performance ==

[[File:Brayton cycle.svg|thumb|300px|Figure 5.1 – [[Brayton cycle]]. Illustration of the Brayton cycle as applied to a gas turbine.]]
[[File:Cent comp map.PNG|thumb|300px|Figure 5.2 – Example centrifugal compressor performance map.]]

While illustrating a gas turbine's Brayton cycle,<ref name="wood">{{cite book|last=Wood|first=Bernard D.|title=Applications of Thermodynamics|year=1969|publisher=Addison - Wesley Publishing Company|location=Reading, Massachusetts|lccn=75-79598}}</ref> Figure 5.1 includes example plots of pressure-specific volume and temperature-entropy. These types of plots are fundamental to understanding centrifugal compressor performance at one operating point. Studying these two plots further we see that the pressure rises between the compressor inlet (station 1) and compressor exit (station 2). At the same time, it is easy to see that the specific volume decreases or similarly the density increases. Studying the temperature-entropy plot we see the temperature increase with increasing entropy (loss). If we assume dry air, and ideal gas equation of state and an isentropic process, we have enough information to define the pressure ratio and efficiency for this one point. Unfortunately, we are missing several other key pieces of information if we wish to apply the centrifugal compressor to another application.

Figure 5.2, a centrifugal compressor performance map (either test or estimated), shows flow, pressure ratio for each of 4 speed-lines (total of 23 data points). Also included are constant efficiency contours. Centrifugal compressor performance presented in this form provides enough information to match the hardware represented by the map to a simple set of end-user requirements.

Compared to estimating performance which is very cost effective (thus useful in design), testing, while costly, is still the most precise method.<ref name="JapikseExpTech">
{{cite book|author=Japikse, David
|title=Advanced Experimental Techniques in Turbomachinery
|year=
|publisher= Concepts ETI
|isbn=0-933283-01-6}}
</ref> Further, testing centrifugal compressor performance is very complex. Professional societies such as [[ASME]] (i.e. PTC–10, Fluid Meters Handbook, PTC-19.x),<ref name="PTC10">{{cite book|title=ASME PTC 10-1997 Test Code on Compressors and Exhausters
|publisher=ASME
|year=1997
|location=New York
|ISBN=0-7918-2450-0
|url=http://www.asme.org/kb/standards#des=PTC}}
</ref> [[ASHRAE]] ([[ASHRAE Handbook]]) and [[API]] (ANSI/API 617-2002, 672-2007)<ref name="API617"/><ref name="API672"/> have established standards for detailed experimental methods and analysis of test results. Despite this complexity, a few basic concepts in performance can be presented by examining an example test performance map.

=== Performance maps ===

Pressure ratio and flow are the main parameters<ref name="wood"/><ref name="API617"/><ref name="API672"/><ref name="PTC10"/> needed to match the Figure 5.2 performance map to a simple compressor application. In this case, it can be assumed that the inlet temperature is sea-level standard. Making this assumption in a real case would be a significant error. A detailed inspection of Figure 5.2 shows:

* ''Flow'' — kg/s (range: 0.04 - 0.34 kg/s)
* ''Pressure'' — pressure ratio (t-t) (range 1.0 - 2.6 PR_t-t)
:::"t-t" implies the discharge total pressure is divided by the inlet total pressure (Pt_discharge/Pt_inlet).

As is standard practice, Figure 5.2 has a horizontal axis labeled with a flow parameter. While flow measurements use a wide variety unit specifications, all fit one of 2 categories:
* ''Mass flow per unit time''
* ''Volume flow per unit time''

::Mass flows, such as kg/s, are the easiest to use in practice as there is little room for confusion. Questions remaining would involve inlet or outlet (which might involve leakage from the compressor or moisture condensation). For atmospheric air, the mass flow may be wet or dry (including or excluding humidity). Often, the mass flow specification will be presented on an equivalent Mach number basis. It is standard in these cases that the equivalent temperature, equivalent pressure and gas is specified explicitly or implied at a standard condition.

::In contrast, all volume flow specifications require the additional specification of density. Bernoulli's fluid dynamic principal is of great value in understanding this problem. Confusion arises through either inaccuracies or misuse of pressure, temperature and gas constants.

Also as is standard practice, Figure 5.2 has a vertical axis labeled with a pressure parameter. The variety of pressure measurement units is also vast. In this case, they all fit one of three categories:

* ''The delta increase or rise from inlet to exit'' (Manometer style)
* ''The measured discharge pressure'' (force)
* ''The force ratio'' (ratio, exit/inlet)

Other features common to performance maps are:

* ''Constant speed lines''
::The two most common methods used for testing centrifugal compressors is to test along lines of constant shaft speed or along lines of constant throttle. If the shaft speed is held constant, test points are taken along a constant speed line by changing throttle positions. In contrast, if a throttle valve is held constant, test points are established by changing speed (common gas turbine practice). The map shown in Figure 5.2 illustrate the most common method; lines of constant speed. In this case we see data points connected via straight lines at speeds of 50%, 71%, 87%, and 100% RPM. The first three speed lines have 6 points each while the highest speed line as five.

* ''Constant efficiency islands''
::The next feature to be discussed is the oval shaped curves representing islands of constant efficiency. In this figure we see 11 contours ranging from 56% efficiency (decimal 0.56) to 76% efficiency (decimal 0.76). General standard practice is to interpret these efficiencies as isentropic rather than polytropic. The inclusion of efficiency islands effectively generates a 3-dimensional topology to this 2-dimensional map. With inlet density specified, it provides a further ability to calculate aerodynamic power. Lines of constant power could just as easily be substituted.

* ''Design point(s)'' or ''guarantee point(s)''
::Regarding gas turbine operation and performance, there may be a series of guaranteed points established for the gas turbine’s centrifugal compressor. These requirements are of secondary importance to the overall gas turbine performance as a whole. For this reason it is only necessary to summarize that in the ideal case, the lowest specific fuel consumption would occur when the centrifugal compressors peak efficiency curve coincides with the gas turbine's required operation line.

::In contrast to gas turbines, most other applications (including industrial) need to meet a less stringent set of performance requirements. Historically, centrifugal compressors applied to industrial applications were needed to achieve performance at a specific flow and pressure. Modern industrial compressors are often needed to achieve specific performance goals across a range of flows and pressures; thus taking a significant step toward the sophistication seen in gas turbine applications.

::If the compressor represented by Figure 5.2 is used in a simple application, any point (pressure and flow) within the 76% efficiency would provide very acceptable performance. An "End User" would be very happy with the performance requirements of 2.0 pressure ratio at 0.21 kg/s.

=== Surge ===

* ''Surge - is the point at which the compressor cannot add enough energy to overcome the system resistance or backpressure.<ref>{{cite book
|author=Pampreen, Ronald C.
|title=Compressor Surge and Stall
|edition=
|publisher=Concepts ETI
|year=1993
|isbn=0-933283-05-9
}}</ref>

::This causes a rapid flow reversal (i.e. surge). As a result, high vibration, temperature increases, and rapid changes in axial thrust can occur. These occurrences can damage the rotor seals, rotor bearings, the compressor driver and cycle operation. Most turbomachines are designed to easily withstand occasional surging. However, if the machine is forced to surge repeatedly for a long period of time, or if it is poorly designed, repeated surges can result in a catastrophic failure. Of particular interest, is that while turbomachines may be very durable, the cycles/processes that they are used within can be far less robust.

* ''Surge Line''
::The Surge-line shown in Figure 5.2 is the curve that passes through the lowest flow points of each of the four speed lines. As a test map, these points would be the lowest flow points possible to record a stable reading within the test facility/rig. In many industrial applications it may be necessary to increase the stall line due to the system backpressure. For example, at 100% RPM stalling flow might increase from approximately 0.170 kg/s to 0.215 kg/s because of the positive slope of the pressure ratio curve.

::As stated earlier, the reason for this is that the high-speed line in Figure 5.2 exhibits a stalling characteristic or positive slope within that range of flows. When placed in a different system those lower flows might not be achievable because of interaction with that system. System resistance or adverse pressure is proven mathematically to be the critical contributor to compressor surge.

=== Maximum flow line versus choke ===

Choke - occurs under one of 2 conditions. Typically for high speed equipment, as flow increases the velocity of the flow can approach sonic speed somewhere within the compressor stage. This location may occur at the impeller inlet "throat" or at the vaned diffuser inlet "throat". In contrast, for lower speed equipment, as flows increase, losses increase such that the pressure ratio eventually drops to 1:1. In this case, the occurrence of choke is unlikely.

* ''Choke''
::The speed lines of gas turbine centrifugal compressors typically exhibit choke. This is a situation where the pressure ratio of a speed line drops rapidly (vertically) with little or no change in flow. In most cases the reason for this is that close to Mach 1 velocities have been reached somewhere within the impeller and/or diffuser generating a rapid increase in losses. Higher pressure ratio turbocharger centrifugal compressors exhibit this same phenomenon. Real choke phenomena is a function of compressibility as measured by the local Mach number within an area restriction within the centrifugal pressure stage.

* ''Maximum flow line''
::The maximum flow line, shown in Figure 5.2, is the curve that passes through the highest flow points of each speed line. Upon inspection it may be noticed that each of these points has been taken near 56% efficiency. Selecting a low efficiency (<60%) is the most common practice used to terminate compressor performance maps at high flows. Another factor that is used to establish the maximum flow line is a pressure ratio near or equal to 1. The 50% speed line may be considered an example of this.

::The shape of Figure 5.2's speed lines provides a good example of why it is inappropriate to use the term choke in association with a maximum flow of all centrifugal compressor speed lines. In summary; most industrial and commercial centrifugal compressors are selected or designed to operate at or near their highest efficiencies and to avoid operation at low efficiencies. For this reason there is seldom a reason to illustrate centrifugal compressor performance below 60% efficiency.

* ''Stonewall''
::Many industrial and commercial multistage compressor performance maps exhibits this same vertical characteristic for a different reason related to what is known as stage stacking.

=== Other operating limits ===

::Minimum Operating Speed - the minimum speed for acceptable operation, below this value the compressor may be controlled to stop or go into an "Idle" condition.

::Maximum Allowable Speed - the maximum operating speed for the compressor. Beyond this value stresses may rise above prescribed limits and rotor vibrations may increase rapidly. At speeds above this level the equipment will likely become very dangerous and be controlled to lower speeds.

==Dimensional analysis==

{{more|Buckingham π theorem}}

To weigh the advantages between centrifugal compressors it is important to compare 8 parameters classic to turbomachinery. Specifically, pressure rise (p), flow (Q), angular speed (N), power (P), density (ρ), diameter (D), viscosity (mu) and elasticity (e). This creates a practical problem when trying to experimentally determine the effect of any one parameter. This is because it is nearly impossible to change one of these parameters independently.

The method of procedure known as the Buckingham π theorem can help solve this problem by generating 5 dimensionless forms of these parameters.<ref name="Shepard"/><ref name="Japikse&Baines"/><ref name="streeter">{{cite book|last=Streeter|first=Victor L.|title=Fluid Mechanics fifth edition|year=1971|publisher=McGraw Hill Book Company|location=New York|isbn=0-07-062191-8}}</ref> These Pi parameters provide the foundation for "similitude" and the "affinity-laws" in turbomachinery. They provide for the creation of additional relationships (being dimensionless) found valuable in the characterization of performance.

For the examples below Head will be substituted for pressure and sonic velocity will be substituted for elasticity.

=== Π theorem ===

The three independent dimensions used in this procedure for turbomachinery are:

:*'''<math>M</math>''' mass (force is an alternative)

:*'''<math>L</math>''' length

:*'''<math>T</math>''' time

According to the theorem each of the eight main parameters are equated to its independent dimensions as follows:

{| class="wikitable"
|-
| Flow:
| <math> Q = </math>
| <math> \left(\frac{L^3}{T}\right)</math>
| ex. = m^3/s
|-
| Head:
| <math> H = </math>
| <math> \left(\frac{M L}{T^2}\right)</math>
| ex. = kg*m/s^2
|-
| Speed:
| <math> U = </math>
| <math> \left(\frac{L}{T}\right)</math>
| ex. = m/s
|-
| Power:
| <math> P = </math>
| <math> \left(\frac{M L^2}{T^3}\right)</math>
| ex. = kg*m^2/s^3
|-
| Density:
| <math> \rho = </math>
| <math> \left(\frac{M}{L^3}\right)</math>
| ex. = kg/m^3
|-
| Viscosity:
| <math> \mu = </math>
| <math> \left(\frac{M}{L T}\right)</math>
| ex. = kg/(m*s)
|-
| Diameter:
| <math> D = </math>
| <math> \left(\frac{L}{1}\right)</math>
| ex. = m
|-
| Speed of sound:
| <math> a = </math>
| <math> \left(\frac{L}{T}\right)</math>
| ex. = m/s
|}

=== Classic turbomachinery similitude ===

Completing the task of following the formal procedure results in generating this classic set of five dimensionless parameters for turbomachinery. Full similitude is achieved when each of the 5 Pi-parameters are equivalent. This of course would mean the two turbomachines being compared are geometrically similar and running at the same operating point.

{| class="wikitable"
|-
| Flow coefficient:
| <math> \Pi_1 = </math>
| <math> \left(\frac{Q}{N D^3}\right)</math>
|-
| Head coefficient:
| <math> \Pi_2 = </math>
| <math> \left(\frac{g H}{N^2 D^2}\right)</math>
|-
| Speed coefficient:
| <math> \Pi_4 = </math>
| <math> \left(\frac{N D}{a}\right)</math>
|-
| Power coefficient:
| <math> \Pi_3 = </math>
| <math> \left(\frac{P}{\rho N^3 D^5}\right)</math>
|-
| Reynolds coefficient:
| <math> \Pi_5 = </math>
| <math> \left(\frac{\rho N D^2}{\mu}\right)</math>
|}

Turbomachinery analysts gain tremendous insight into performance by comparisons of these 5 parameters with efficiencies and loss coefficients which are also dimensionless. In general application, the flow coefficient and head coefficient are considered of primary importance. Generally, for centrifugal compressors, the velocity coefficient is of secondary importance while the Reynolds coefficient is of tertiary importance. In contrast, as expected for pumps, the Reynolds number becomes of secondary importance and the velocity coefficient almost irrelevant. It may be found interesting that the speed coefficient may be chosen to define the y-axis of Figure 1.1, while at the same time the Reynolds coefficient may be chosen to define the z-axis.

=== Other dimensionless combinations ===

Demonstrated in the table below is another value of dimensional analysis. Any number of new dimensionless parameters can be calculated through exponents and multiplication. For example, a variation of the first parameter shown below is popularly used in aircraft engine system analysis. The third parameter is a simplified dimensional variation of the first and second. This third definition is applicable with strict limitations. The fourth parameter, specific speed, is very well known and useful in that it removes diameter. The fifth parameter, specific diameter, is a less often discussed dimensionless parameter found useful by Balje.<ref>{{cite book|last=Balje|first=O. E.|title=Turbo Machines; a Guide to Design, Selection, and Theory|year=1961|publisher=John Wiley & Sons|location=New York|isbn=0-471-06036-4}}</ref>

{| class="wikitable"
|-
| <math> 1 </math>
| Corrected mass flow coefficient:
| <math> (\Pi_1 * \Pi_4) = </math>
| <math> \left(\frac{m (Rt / k)^{0.5}}{p D^2}\right)</math>
|-
| <math> 2 </math>
| Alternate#1 equivalent Mach form:
| <math> \left(\frac{m_1}{\rho_1 a_1 D_1^2 Cos(\alpha_1)}\right) = </math>
| <math> \left(\frac{m_2}{\rho_2 a_2 D_2^2 Cos(\alpha_2)}\right)</math>
|-
| <math> 3 </math>
| Alternate#2 simplified dimensional form:
| <math> \left(\frac{m_1 (t_1)^{0.5}}{p_1}\right) = </math>
| <math> \left(\frac{m_2 (t_2)^{0.5}}{p_2}\right)</math>
|-
| <math> 4 </math>
| Specific speed coefficient:
| <math> \frac{(\Pi_1)^{ 0.5}}{(\Pi_2)^{ 0.75}} = </math>
| <math> \left(\frac{N Q^{0.5}}{(g H)^{0.75}}\right)</math>
|-
| <math> 5 </math>
| Specific diameter coefficient:
| <math> \frac{(\Pi_2)^{ 0.25}}{(\Pi_1)^{ 0.5}} = </math>
| <math> \left(\frac{N D^{0.5}}{(g H)^{0.75}}\right)</math>
|}

It may be found interesting that the Specific speed coefficient may be used in place of Speed to define the y-axis of Figure 1.2, while at the same time the Specific diameter coefficient may be in place of Diameter to define the z-axis.

=== Affinity laws ===

The following "affinity laws" are derived from the five Pi-parameters shown above. They provide a simple basis for scaling turbomachinery from one application to the next.

{| class="wikitable"
|-
| From flow coefficient:
| <math> \left(\frac{Q_1}{Q_2}\right) = </math>
| <math> \left(\frac{N_1}{N_2}\right)</math>
|-
| From head coefficient:
| <math> \left(\frac{H_1}{H_2}\right) = </math>
| <math> \left(\frac{N_1}{N_2}\right)^2</math>
|-
| From power coefficient:
| <math> \left(\frac{P_1}{P_2}\right) = </math>
| <math> \left(\frac{N_1}{N_2}\right)^3</math>
|-
| From flow coefficient:
| <math> \left(\frac{Q_1}{Q_2}\right) = </math>
| <math> \left(\frac{D_1}{D_2}\right)^3</math>
|-
| From head coefficient:
| <math> \left(\frac{H_1}{H_2}\right) = </math>
| <math> \left(\frac{D_1}{D_2}\right)^2</math>
|-
| From power coefficient:
| <math> \left(\frac{P_1}{P_2}\right) = </math>
| <math> \left(\frac{D_1}{D_2}\right)^5</math>
|}

== Aero-thermodynamic fundamentals ==

The following equations outline a fully three-dimensional mathematical problem that is very difficult to solve even with simplifying assumptions.<ref name="Lakshminarayana"/><ref>
{{cite book
|author=Cumpsty, N. A.
|title=Compressor Aerodynamics
|year=2004
|publisher=Krieger Publishing
|isbn=1-57524-247-8}}
</ref> Until recently, limitations in computational power, forced these equations to be simplified to an Inviscid two-dimensional problem with pseudo losses. Before the advent of computers, these equations were almost always simplified to a one-dimensional problem.

Solving this one-dimensional problem is still valuable today and is often termed ''mean-line analysis''. Even with all of this simplification it still requires large textbooks to outline and large computer programs to solve practically.

=== Conservation of mass ===

:Also termed ''continuity'', this fundamental equation written in general form is as follows:

:::<math>\frac{\partial \rho}{\partial t} + \nabla \cdot (\rho \mathbf{v}) = 0</math>

=== Conservation of momentum ===

:Also termed the ''[[Navier–Stokes equation]]s'', this fundamental is derivable from [[Newton's second law]] when applied to [[Fluid dynamics|fluid motion]]. Written in compressible form for a Newtonian fluid, this equation may be written as follows:

:::<math>\rho \left(\frac{\partial \mathbf{v}}{\partial t} + \mathbf{v} \cdot \nabla \mathbf{v}\right) = -\nabla p + \mu \nabla^2 \mathbf{v} + \left( \tfrac13 \mu + \mu^v) \nabla (\nabla \cdot \mathbf{v} \right) + \mathbf{f} </math>

=== Conservation of energy ===

:The [[First Law of Thermodynamics]] is the statement of the conservation of energy. Under specific conditions, the operation of a Centrifugal compressor is considered a reversible process. For a reversible process, the total amount of heat added to a system can be expressed as <math>\delta Q=TdS</math> where <math>T</math> is [[temperature]] and <math>S</math> is [[entropy]]. Therefore, for a reversible process:

:::<math>dU=TdS-pdV.\,</math>

:Since U, S and V are thermodynamic functions of state, the above relation holds also for non-reversible changes. The above equation is known as the [[fundamental thermodynamic relation]].

=== Equation of state ===

:The classical [[ideal gas law]] may be written:

:::<math>{\ pV = nRT}.</math>

:The ideal gas law may also be expressed as follows

:::<math>{\ p = \rho (\gamma-1)U}</math>

:where <math>\rho</math> is the density, <math>\gamma = C_p/C_v</math> is the adiabatic index ([[Heat capacity ratio|ratio of specific heats]]), <math>U=C_vT</math> is the internal energy per unit mass (the "specific internal energy"), <math>C_v</math> is the specific heat at constant volume, and <math>C_p</math> is the specific heat at constant pressure.

:With regard to the equation of state, it is important to remember that while air and nitrogen properties (near standard atmospheric conditions) are easily and accurately estimated by this simple relationship, there are many centrifugal compressor applications where the ideal relationship is inadequate. For example, centrifugal compressors used for large air conditioning systems (water chillers) use a refrigerant as a working gas that cannot be modeled as an ideal gas. Another example are centrifugal compressors design and built for the petroleum industry. Most of the hydrocarbon gases such as methane and ethylene are best modeled as a [[real gas]] [[equation of state]] rather than ideal gases. The Wikipedia entry for equations of state is very thorough.

== Pros and cons ==

* ''Pros''
::Centrifugal compressors are used throughout industry because they have fewer rubbing parts, are relatively energy efficient, and give higher airflow than a similarly sized [[reciprocating compressor]] or positive-displacement compressor.

* ''Cons''
::Their main drawback is that they cannot achieve the high [[compression ratio]] of reciprocating compressors without multiple stages. There are few one-stage centrifugal compressors capable of pressure ratios over 10:1, due to stress considerations which severely limit the compressor's safety, durability and life expectancy.

* ''Pros''
::Centrifugal compressors are often used in small [[gas turbine]] engines like APUs (auxiliary power units) and smaller aircraft gas turbines. A significant reason for this is that with current technology, the equivalent flow [[axial compressor]] will be less efficient due primarily to a combination of rotor and variable stator tip-clearance losses. Further, they offer the advantages of simplicity of manufacture and relatively low cost. This is due to requiring fewer stages to achieve the same pressure rise.

* ''Cons''
::Centrifugal compressors are impractical, compared to axial compressors, for use in large [[gas turbine]] engines propelling large aircraft, due to the resulting weight and stress, and to the frontal area presented by the diffuser.

== Aerodynamic design methodology ==

Modern turbomachinery design is a compromise between fluid- and thermo-dynamics, structural mechanics and manufacturability.<ref name="Japikse"/> Making the correct compromises requires appropriate experimental data and design experience. As precise as the fundamental equations above are, resultant design methods are many and varied.

Focusing on the thermo-fluid element, two logically opposed approaches are used to solve the fundamental equations. The first is to analyze the existing performance of existing hardware: this analysis is a post-experiment activity used to explain what has happened. The second is to design new hardware: here design is a predictive activity similar to forecasting the weather.

Currently, popular turbo-machinery design methods alternately solve two problems: first the design problem, to define the air foil, and then the analysis problem, to detail the aerodynamic performance of the design. For "end users/customers" requiring the highest performance, designers incorporate numerical optimization to direct these iterations, while sometimes pursuing a parallel experimental program.

In practice, design and analysis uses simplified 1D and 2D equation sets before performing the final 3D solutions. After this expansive aerodynamic problem is solved, an equally expansive set of design problems remain involving the structural design and the manufacture of the centrifugal compressor.

== Structural mechanics, manufacture and design compromise ==

Ideally, centrifugal compressor impellers have infinitely thin airfoil blades that are infinitely strong, each mounted on an infinitely light rotor. This material would be infinitely easy to machine or cast and infinitely inexpensive. Additionally, it would generate no operating noise, have an infinite life while operating in any environment.

From the very start of the aero-thermodynamic design process, the centrifugal impeller’s material and manufacturing method must be accounted for within the design. Whether it be plastic for a vacuum cleaner blower to aluminum alloy for a turbocharger, steel alloy for an air compressor or titanium alloy for a gas turbine. It is a combination of the centrifugal compressor impeller shape, its operating environment, its material and its manufacturing method that determines the impeller’s structural integrity.

==See also==

{{multicol}}
*[[Angular momentum]]
*[[Centrifugal force]]
*[[Centripetal force]]
*[[Coandă effect]]
*[[Computational Fluid Dynamics]]
*[[Compressibility]]
*[[Coriolis effect]]
*[[Corrected speed]]
*[[Darcy–Weisbach equation]]
*[[Gustaf de Laval]]
*[[Enthalpy]]
*[[Entropy]]
*[[Euler equations (fluid dynamics)]]
*[[Finite element analysis]]
*[[Fluid dynamics]]
{{multicol-break}}
*[[Gas laws]]
*[[Ideal gas law]]
*[[Kinematics]]
*[[Real gas]]
*[[Rossby number]]
*[[Mach number]]
*[[Multiphase flow]]
*[[Navier-Stokes equations]]
*[[Reynolds-averaged Navier–Stokes equations]]
*[[Reynolds transport theorem]]
*[[Reynolds number]]
*[[Turbulence]]
*[[Viscosity]]
*[[Von Karman Institute for Fluid Dynamics (VKI)]]
{{multicol-end}}
* [[Three dimensional losses and correlation in turbomachinery]]
*[[Effects of mach number and shock losses in turbomachines]]

== References ==

{{Reflist|colwidth=30em}}

== External links ==
* [http://web.mit.edu/aeroastro/www/labs/GTL/gtl_about.html MIT Gas Turbine Laboratory]
* [http://onlinelibrary.wiley.com/doi/10.1111/j.1559-3584.1948.tb02754.x/abstract (1948), First Marine Gas Turbine in Service. Journal of the American Society for Naval Engineers, 60: 66–86. ] {{doi|10.1111/j.1559-3584.1948.tb02754.x}}
* [http://www.aardvark.co.nz/pjet/chrysler.shtml A history of Chrysler turbine cars]
* [http://www.api.org/Publications/addenda/add-ref.cfm To find API codes, standards & publications]
* [http://www.asme.org/ To find ASME codes, standards & publications]
* [http://www.ashrae.org/ To find ASHRAE codes, standards & publications]
* [http://www.grc.nasa.gov/WWW/K-12/airplane/centrf.html Glenn Research Center at NASA]
* [http://pontyak.com/index.html Pontyak Resource Centre]
* [http://authors.library.caltech.edu/25019/2/HydroPmp.pdf Hydrodynamics of Pumps, by Christopher Earls Brennen]

{{Aircraft gas turbine engine components}}

{{DEFAULTSORT:Centrifugal compressor}}
[[Category:Compressors]]
[[Category:Turbomachinery]]
[[Category:Turbines]]
[[Category:Thermodynamics]]
[[Category:Fluid dynamics]]
[[Category:Aerodynamics]]

[[pt:Compressor#Compressores Dinâmicos]]
[[ru:Лопастной компрессор#Центробежный компрессор]]

Sunflower

2014-01-27T22:32:09Z

76.120.175.135: fix link

[[File:Heavy mist.jpg|thumb|upright=1.4|Mist and clouds are aerosols.]]
[[File:DustWorkerStreet3.jpg|thumb|upright=1.4|Because dust particles mostly settle to the ground, this visible dust is a suspension, not an aerosol. Very fine dust, common in the [[Sahara Desert]], however, can constitute an aerosol as it travels on the winds for weeks.]]

An '''aerosol''' is a [[colloid]] of fine [[solid]] particles or [[liquid]] droplets, in air or another [[gas]].<ref name="Hinds, 1999, p. 3">Hinds, 1999, p. 3</ref> Examples of aerosols include [[haze]], dust, particulate [[air pollutants]] and [[smoke]].<ref name="Hinds, 1999, p. 3"/> The liquid or solid particles have diameter mostly smaller than 1 μm or so; larger particles with a significant settling speed make the mixture a [[Suspension (chemistry)|suspension]], but the distinction is not clear-cut. In general conversation, ''aerosol'' usually refers to an [[aerosol spray]] that delivers a consumer product from a can or similar container. Other technological applications of aerosols include dispersal of pesticides, medical treatment of respiratory illnesses, and combustion technology.<ref name="Hidy, 1984, p. 254">Hidy, 1984, p. 254.</ref> Aerosol science covers generation and removal of aerosols, technological application of aerosols, effects of aerosols on the environment and people, and a wide variety of other topics.<ref name="Hinds, 1999, p. 3"/>

__TOC__
{{clear}}

== Definitions ==
[[File:Fly Ash FHWA dot gov.jpg|thumb|Photomicrograph made with a Scanning Electron Microscope (SEM): [[Fly ash]] particles at 2,000x magnification. Most of the particles in this aerosol are nearly spherical.]]
An aerosol is defined as a colloidal system of solid or liquid particles in a gas. An aerosol includes both the particles and the suspending gas, which is usually air.<ref name="Hinds, 1999, p. 3"/> [[Frederick G. Donnan]] presumably first used the term ''aerosol'' during [[World War I]] to describe an aero-[[solution]], clouds of microscopic particles in air. This term developed analagously to the term [[Sol (colloid)|hydrosol]], a colloid system with water as the dispersing medium.<ref>Hidy, 1984, p. 5</ref> ''Primary aerosols'' contain particles introduced directly into the gas; ''secondary aerosols'' form through gas-to-particle conversion.<ref>Hinds, 1999, p. 8</ref>

There are several measures of aerosol concentration. Environmental science and health often uses the ''mass concentration'' (''M''), defined as the mass of particulate matter per unit volume with units such as μg/m3. Also commonly used is the ''number concentration'' (''N''), the number of particles per unit volume with units such as number/m3 or number/cm3.<ref>Hinds, 1999, pp. 10-11.</ref>

The size of particles has a major influence on their properties, and the aerosol particle radius or diameter (''dp'') is a key property used to characterise aerosols.

Aerosols vary in their [[dispersity]]. A ''monodisperse'' aerosol, producible in the laboratory, contains particles of uniform size. Most aerosols, however, as ''polydisperse'' colloidal systems, exhibit a range of particle sizes.<ref>Hinds, 1999, p. 8.</ref> Liquid droplets are almost always nearly spherical, but scientists use an ''equivalent diameter'' to characterize the properities of various shapes of solid particles, some very irregular. The equivalent diameter is the diameter of a spherical particle with the same value of some physical property as the irregular particle.<ref>Hinds, 1999, p. 10.</ref> The ''equivalent volume diameter'' (''de'') is defined as the diameter of a sphere of the same volume as that of the irregular particle.<ref>Hinds, 1999, p. 51.</ref> Also commonly used is the [[Aerosol#Aerodynamic diameter|aerodynamic diameter]].

== Size distribution ==
[[File:Synthetic aerosol distribution in number area and volume space.png|thumbnail|The same hypothetical log-normal aerosol distribution plotted, from top to bottom, as a number vs diameter distribution, a surface area vs diameter distribution, and a volume vs diameter distribution. Typical mode names are shows at the top. Each distribution is normalized so that the total area is 1000.]]
For a monodisperse aerosol, a single number—the particle diameter—suffices to describe the size of the particles. However, more complicated [[particle-size distribution]]<nowiki/>s describe the sizes of the particles in a polydisperse aerosol. This distribution defines the relative amounts of particles, sorted according to size.<ref>{{cite journal| last = Jillavenkatesa | first = A | last2 = Dapkunas | first2 = SJ |last3 = Lin-Sien |first3 = Lum| title = Particle Size Characterization | journal = NIST Special Publication | volume = 960-1 | year = 2001}}</ref> One approach to defining the particle size distribution uses a list of the sizes of every particle in a sample. However, this approach proves tedious to ascertain in aerosols with millions of particles and awkward to use. Another approach splits the complete size range into intervals and finds the number (or proportion) of particles in each interval. One then can visualize these data in a [[histogram]] with the area of each bar representing the proportion of particles in that size bin, usually normalised by dividing the number of particles in a bin by the width of the interval so that the area of each bar is proportionate to the number of particles in the size range that it represents.<ref>Hinds, 1999, pp. 75-77.</ref> If the width of the bins [[Limit (mathematics)|tends to zero]], one gets the frequency function:<ref>Hinds, 1999, p. 79</ref>

:<math> \mathrm{d}f = f(d_p) \,\mathrm{d}d_p</math>
where
:<math> d_p </math>is the diameter of the particles
:<math> \,\mathrm{d}f </math> is the fraction of particles having diameters between <math>d_p</math> and <math>d_p</math> + <math>\mathrm{d}d_p</math>
:<math>f(d_p)</math> is the frequency function

Therefore, the area under the frequency curve between two sizes a and ''b'' represents the total fraction of the particles in that size range:<ref>Hinds, 1999, p. 79.</ref>
:<math> f_{ab}=\int_a^b f(d_p) \,\mathrm{d}d_p</math>

It can also be formulated in terms of the total number density ''N'':<ref name="Hidy, 1984, p. 58">Hidy, 1984, p. 58</ref>
:<math> dN = N(d_p) \,\mathrm{d}d_p</math>

Assuming spherical aerosol particles, the aerosol surface area per unit volume (''S'') is given by the second [[Moment (mathematics)|moment]]:<ref name="Hidy, 1984, p. 58"/>
:<math> S= \pi/2 \int_0^\infty N(d_p)d_p^2 \,\mathrm{d}d_p</math>

And the third moment gives the total volume concentration (''V'') of the particles:<ref name="Hidy, 1984, p. 58"/>
:<math> V= \pi/6 \int_0^\infty N(d_p)d_p^3 \,\mathrm{d}d_p</math>

One also usefully can approximate the particle size distribution using a [[Function (mathematics)|mathematical function]]. The [[normal distribution]] usually does not suitably describe particle size distributions in aerosols because of the [[skewness]] associated a long tail of larger particles. Also for a quantity that varies over a large range, as many aerosol sizes do, the width of the distribution implies negative particles sizes, clearly not physically realistic. However, the normal distribution can be suitable for some aerosols, such as test aerosols, certain [[pollen]] grains and [[spore]]s.<ref name="Hinds, 1999, p 90">Hinds, 1999, p 90.</ref>

A more widely chosen [[log-normal distribution]] gives the number frequency as:<ref name="Hinds, 1999, p 90"/>

:<math> \mathrm{d}f = \frac{1}{\sigma\sqrt{2\pi}} e^{-\frac{(d_p - \bar{d_p})^2}{2 \sigma^2} }\mathrm{d}d_p</math>

where:

:<math> \sigma</math> is the [[standard deviation]] of the size distribution and
:<math> \bar{d_p}</math> is the [[arithmetic mean]] diameter.

The log-normal distribution has no negative values, can cover a wide range of values, and fits many observed size distributions reasonably well.<ref>Hinds, 1999, p 91.</ref>

Other distributions sometimes used to characterise particle size include: the [[Weibull distribution|Rosin-Rammler distribution]], applied to coarsely dispersed dusts and sprays; the [[Nukiyama-Tanasawa distribution]], for sprays of extremely broad size ranges; the [[Power_law#Power-law_probability_distributions|power function distribution]], occasionally applied to atmospheric aerosols; the [[exponential distribution]], applied to powdered materials; and for cloud droplets, the [[Khrgian-Mazin distribution]].<ref>Hinds, 1999, p 104-5</ref>

== Physics ==

=== Terminal velocity of a particle in a fluid ===
For low values of the [[Reynolds number]] (<1), true for most aerosol motion, [[Stokes' law]] describes the force of resistance on a solid spherical particle in a fluid. However, Stokes' law is only valid when the velocity of the gas at the surface of the particle is zero. For small particles (< 1 μm) that characterize aerosols, however, this assumption fails. To account for this failure, one can introduce the [[Cunningham correction factor]], always greater than 1. Including this factor, one finds the relation between the resisting force on a particle and its velocity:<ref>Hinds, 1999, p. 44-49</ref>
:<math>F_D = \frac {3 \pi \eta V d}{C_c}</math>
where
:<math>F_D</math> is the resisting force on a spherical particle
:<math>\eta</math> is the [[viscosity]] of the gas
:<math>V</math> is the particle velocity
:<math>C_c</math> is the Cunningham correction factor.

This allows us to calculate the [[terminal velocity]] of a particle undergoing gravitational settling in still air. Neglecting [[buoyancy]] effects, we find:<ref>Hinds, 1999, p. 49</ref>
:<math>V_{TS} = \frac{\rho_p d^2 g C_c}{18 \eta}</math>
where
:<math>V_{TS}</math> is the terminal settling velocity of the particle.

The terminal velocity can also be derived for other kinds of forces. If Stokes' law holds, then the resistance to motion is directly proportional to speed. The constant of proportionality is the mechanical mobility (''B'') of a particle:<ref>Hinds, 1999, p. 47</ref>

<math>B = \frac{V}{F_D} = \frac {C_c}{3 \pi \eta d}</math>

A particle traveling at any reasonable initial velocity approaches its terminal velocity [[Exponential decay|exponentially]] with an ''e''-folding time equal to the relaxation time:<ref>Hinds, 1991, p 115.</ref>

<math>V(t) = V_{f}-(V_{f}-V_{0})e^{-\frac{t}{\tau}}</math>

where:
:<math>V(t)</math> is the particle speed at time t
:<math>V_f</math> is the final particle speed
:<math>V_0</math> is the initial particle speed

To account for the effect of the shape of non-spherical particles, a correction factor known as the ''dynamic shape factor'' is applied to Stokes' law. It is defined as the ratio of the resistive force of the irregular particle to that of a spherical particle with the same volume and velocity:<ref>Hinds, 1991, p. 51</ref>

<math>\chi = \frac{F_D}{3 \pi \eta V d_e}</math>

where:
:<math>\chi</math> is the dynamic shape factor

=== Aerodynamic diameter ===
The aerodynamic diameter of an irregular particle is defined as the diameter of the spherical particle with a density of 1000 kg/m3 and the same settling velocity as the irregular particle.<ref name="Hinds, 1999, p. 53">Hinds, 1999, p. 53.</ref>

Neglecting the slip correction, the particle settles at the terminal velocity proportional to the square of the aerodynamic diameter, ''da'':<ref name="Hinds, 1999, p. 53"/>

:<math>V_{TS} = \frac{\rho_0 d_a^2 g}{18 \eta}</math>

where
:<math>\ \rho_0</math> = standard particle density (1000 kg/m3).

This equation gives the aerodynamic diameter:<ref>Hinds, 1999, p. 54.</ref>
:<math>d_a=d_e\left(\frac{\rho_p}{\rho_0 \chi}\right)^{\frac{1}{2}} </math>

One can apply the aerodynamic diameter to particulate pollutants or to inhaled drugs to predict where in the respiratory tract such particles deposit. Pharmaceutical companies typically use aerodymanic diameter, not geometric diameter, to characterize particles in inhalable drugs. {{citation needed|date=August 2012}}

=== Dynamics ===
The previous discussion focussed on single aerosol particles. In contrast, ''aerosol dynamics'' explains the evolution of complete aerosol populations. The concentrations of particles will change over time as a result of many processes. External processes that move particles outside a volume of gas under study include [[diffusion]], gravitational settling, and [[electric charge]]s and other external forces that cause particle migration. A second set of processes internal to a given volume of gas include particle formation (nucleation), evaporation, chemical reaction, and coagulation.<ref name="Hidy, 1984, p. 60">Hidy, 1984, p. 60</ref>

A [[differential equation]] called the ''Aerosol General Dynamic Equation'' (GDE) characterizes the evolution of the number density of particles in an aerosol due to these processes.<ref name="Hidy, 1984, p. 60"/>

<math>\frac{\partial{n_i}}{\partial{t}} = -\nabla \cdot n_i \mathbf{q} +\nabla \cdot D_p\nabla_i + \left(\frac{\partial{n_i}}{\partial{t}}\right)_{growth} + \left(\frac{\partial{n_i}}{\partial{t}}\right)_{coag} -\nabla \cdot \mathbf{q}_F n_i</math>

Change in time = Convective transport + [[Brownian motion|brownian diffusion]] + gas-particle interactions + coagulation + migration by external forces

Where:
:<math>n_i</math> is number density of particles of size category <math>i</math>
:<math>\mathbf{q}</math> is the particle velocity
:<math>D_p</math> is the particle [[Einstein relation (kinetic theory)|Stokes-Einstein]] [[Mass diffusivity|diffusivity]]
:<math>\mathbf{q}_F</math> is the particle velocity associated with an external force

==== Coagulation ====

As particles and droplets in an aerosol collide with one another, they may undergo coalescence or aggregation. This process leads to a change in the aerosol particle-size distribution, with the mode increasing in diameter as total number of particles decreases.<ref>Hinds, 1999, p. 260</ref> On occasion, particles may shatter apart into numerous smaller particles; however, this process usually occurs primarily in particles too large for consideration as aerosols.

====Dynamics regimes====

The [[Knudsen number]] of the particle define three different dynamical regimes that govern the behaviour of an aerosol:

:<math>K_n=\frac{2\lambda}{d}</math>

where <math>\lambda</math> is the [[mean free path]] of the suspending gas and <math>d</math> is the diameter of the particle.<ref name="Baron&Willeke">{{cite book|author=Baron, P. A., and Willeke , K.|year=2001|section=Gas and Particle Motion|journal=Aerosol Measurement: Principles. Techniques. and Applications}}</ref> For particles in the ''free molecular regime'', ''Kn'' >> 1; particles small compared to the mean free path of the suspending gas.<ref name="DeCarlo2004">{{cite journal|doi=10.1080/027868290903907|author=DeCarlo, P.F.|year=2004|title=Particle Morphology and Density Characterization by Combined Mobility and Aerodynamic Diameter Measurements. Part 1: Theory|journal=Aerosol Science & Technology|volume=38|issue=12}}</ref> In this regime, particles interact with the suspending gas through a series of "ballistic" collisions with gas molecules. As such, they behave similarly to gas molecules, tending to follow streamlines and diffusing rapidly through Brownian motion. The mass flux equation in the free molecular regime is:

:<math> I = \frac{\pi a^2}{k_b} \left( \frac{P_\infty}{T_\infty} - \frac{P_A}{T_A} \right) \cdot C_A \alpha </math>

where ''a'' is the particle radius, ''P''∞ and ''PA'' are the pressures far from the droplet and at the surface of the droplet respectively, ''kb'' is the Boltzmann constant, ''T'' is the temperature, ''CA'' is mean thermal velocity and ''α'' is mass accommodation coefficient{{citation needed|date=August 2012}}. The derivation of this equation assumes constant pressure and constant diffusion coefficient.

Particles are in the ''continuum regime'' when Kn << 1.<ref name="DeCarlo2004"/> In this regime, the particles are big compared to the mean free path of the suspending gas, meaning that the suspending gas acts as a continuous fluid flowing round the particle.<ref name="DeCarlo2004"/> The molecular flux in this regime is:

:<math> I_{cont} \sim \frac{4 \pi a M_A D_{AB}}{RT} \left( P_{A \infty} - P_{AS}\right)</math>

where ''a'' is the radius of the particle ''A'', ''MA'' is the molecular mass of the particle ''A'', ''DAB'' is the diffusion coefficient between particles ''A'' and ''B'', ''R'' is the ideal gas constant, ''T'' is the temperature (in absolute units like kelvin), and ''PA∞'' and ''PAS'' are the pressures at infinite and at the surface respectively{{citation needed|date=August 2012}}.

The ''transition regime'' contains all the particles in between the free molecular and continuum regimes or ''Kn'' ≈ 1. The forces experienced by a particle are a complex combination of interactions with individual gas molecules and macroscopic interactions. The semi-empirical equation describing mass flux is:

:<math> I = I_{cont} \cdot \frac{1 + K_n}{1 + 1.71 K_n + 1.33 {K_n}^2}</math>

where ''I''cont is the mass flux in the continuum regime{{citation needed|date=August 2012}}. This formula is called the Fuchs-Sutugin interpolation formula. These equations do not take into account the heat release effect.

==== Partitioning ====
[[Image:Cond n evap.svg|thumb|Condensation and evaporation]]
Aerosol partitioning theory governs [[condensation]] on and [[evaporation]] from an aerosol surface, respectively. Condensation of mass causes the mode of the particle-size distributions of the aerosol to increase; conversely, evaporation causes the mode to decrease. Nucleation is the process of forming aerosol mass from the condensation of a gaseous precursor, specifically a [[vapour]]. Net condensation of the vapour requires supersaturation, a [[partial pressure]] greater than its [[vapour pressure]]. This can happen for three reasons{{citation needed|date=August 2012}}:
# Lowering the temperature of the system lowers the vapour pressure.
# Chemical reactions may increase the partial pressure of a gas or lower its vapour pressure.
# The addition of additional vapour to the system may lower the equilibrium vapour pressure according to [[Raoult's law]].

There are two types of nucleation processes. Gases preferentially condense onto surfaces of pre-existing aerosol particles, known as '''heterogeneous nucleation'''. This process causes the diameter at the mode of particle-size distribution to increase with constant number concentration.<ref name="Hinds, p.288">Hinds, 1999, p.288</ref> With sufficiently high supersaturation and no suitable surfaces, particles may condense in the absence of a pre-existing surface, known as '''homogeneous nucleation'''. This results in the addition of very small, rapidly growing particles to the particle-size distribution.<ref name="Hinds, p.288"/>

====Activation====

Water coats particles in an aerosols, making them ''activated'', usually in the context of forming a cloud droplet{{citation needed|date=August 2012}}. Following the [[Kelvin equation]] (based on the curvature of liquid droplets), smaller particles need a higher ambient [[relative humidity]] to maintain equilibrium than larger particles do. The following formula gives [[relative humidity]] at equilibrium:

:<math> RH = \frac{p_s}{p_0} \times 100% = S \times 100%</math>

where <math>p_s</math> is the [[saturation vapor pressure]] above a particle at equilibrium (around a curved liquid droplet), ''p''0 is the saturation vapor pressure (flat surface of the same liquid) and ''S'' is the saturation ratio.

[[Kelvin equation]] for saturation vapor pressure above a curved surface is:

:<math> \ln{p_s \over p_0} = \frac{2 \sigma M}{RT \rho \cdot r_p} </math>

where ''rp'' droplet radius, ''σ'' surface tension of droplet, ''ρ'' density of liquid, ''M'' molar mass, ''T'' temperature, and ''R'' molar gas constant.

==== Solution to the General Dynamic Equation ====
There are no general [[Equation solving|solutions]] to the general dynamic equation (GDE);<ref>Hidy, 1984, p62</ref> common methods used to solve the general dynamic equation include:{{citation needed|date=August 2012}}

* Moment method,
* Modal/sectional method, and
* Quadrature method of moments.

== Generation and Applications ==
People generate aerosols for various purposes, including:
* as test aerosols for [[calibration|calibrating]] instruments, performing research, and testing sampling equipment and air filters;<ref>Hinds, 1999, 428</ref>
* to deliver [[deodorant]]s, [[paint]]s, and other consumer products in sprays;<ref>Hidy, 1984, p 255</ref>
* for dispersal and agricultural [[Pesticide_application#Spray_application|appliciation of pesticides]];<ref>Hidy, 1984, p 257</ref>
* for medical treatment of [[respiratory disease]];<ref>Hidy, 1984, p 274</ref> and
* in [[fuel injection]] systems and other [[combustion]] technology.<ref>Hidy, 1984, p 278</ref>

Some devices for generating aerosols are:<ref name="Hidy, 1984, p. 254" />
* [[Aerosol spray]]
* [[Atomizer nozzle]] or [[Nebulizer]]
* [[Electrospray]]
* Vibrating orifice aerosol generator (VOAG)

== Detection ==
Aerosol can either be measured [[In-situ#Earth_and_atmospheric_sciences|in-situ]] or with [[remote sensing]] techniques.

=== In situ observations ===
Some available in situ measurement techniques include:
* [[Aerosol Mass Spectrometer]] (AMS)
* [[Differential Mobility Analyzer]] (DMA)
* [[Aerodynamic Particle Sizer]] (APS)
* [[Wide Range Particle Spectrometer]] (WPS)
* [[Micro-Orifice Uniform Deposit Impactor]](MOUDI)
* [[Condensation Particle Counter]] (CPC)
* [[Epiphaniometer]]

=== Remote sensing approach ===
Remote sensing approaches include:
* [[Sun photometer]]
* [[LIDAR]]

=== Size selective sampling ===

Particles can deposit in the [[Human nose|nose]], [[Human mouth|mouth]], [[Human pharynx|pharynx]] and [[larynx]] (the head airways region), deeper within the respiratory tract (from the [[Vertebrate trachea|trachea]] to the [[terminal bronchioles]]), or in the [[Pulmonary alveolus|alveolar region]].<ref>Hinds, 1999, p.233</ref> The location of deposition of aerosol particles within the in the respiratory system strongly determines the health effects of exposure to such aerosols.<ref>Hinds, 1999, p. 233</ref> This phenomenon led people to invent aerosol samplers that select a subset of the aerosol particles that reach certain parts of the respiratory system.<ref>Hinds, 1999, p. 249</ref> Examples of these subsets of the particle-size distribution of an aerosol, important in occupational health, include the inhalable, thoracic, and respirable fractions. The fraction that can enter each part of the respiratory system depends on the deposition of particles in the upper parts of the airway.<ref>Hinds, 1999, p. 244</ref> The inhalable fraction of particles, defined as the proportion of particles originally in the air that can enter the nose or mouth, depends on external wind speed and direction and on the particle-size distribution by aerodynamic diameter.<ref>Hinds, 1999, p. 246</ref> The thoracic fraction is the proportion of the particles in ambient aerosol that can reach the thorax or chest region.<ref>Hinds, 1999, p. 254</ref> The respirable fraction is the proportion of particles in the air that can reach the alveolar region.<ref>Hinds, 1999, p. 250</ref> To measure the respirable fraction of particles in air, a pre-collector is used with a sampling filter. The pre-collector excludes particles as the airways remove particles from inhaled air. The sampling filter collects the particles for measurement. It is common to use [[cyclonic separation]] for the pre-collector, but other techniques include impactors, horizontal [[elutriator]]s, and large pore [[membrane filter]]s.<ref>Hinds, 1999, p. 252</ref>

Two alternative size selective criteria, often used in atmospheric monitoring are PM10 and PM2.5. PM10 is defined by [[International Organization for Standardization|ISO]] as ''particles which pass through a size-selective inlet with a 50 % efficiency cut-off at 10 μm aerodynamic diameter. PM10 corresponds to the “thoracic convention” as defined in ISO 7708:1995, Clause 6'' and PM2.5 as ''particles which pass through a size-selective inlet with a 50 % efficiency cut-off at 2,5 μm aerodynamic diameter. PM2,5 corresponds to the “high-risk respirable convention” as defined in ISO 7708:1995, 7.1.''<ref>{{cite web|url=http://diamondenv.wordpress.com/2010/12/10/particulate-pollution-pm10-and-pm2-5/|title=Particulate pollution – PM10 and PM2.5|work=Recognition, Evaluation, Control. News and views from Diamond Environmental Limited|accessdate=23 September 2012}}</ref> The [[United States Environmental Protection Agency]] replaced the older standards for particulate matter based on Total Suspended Particulate with another standard based on PM10 in 1987<ref>{{cite web|url=http://www.epa.gov/airtrends/aqtrnd95/pm10.html|title=Particulate Matter (PM-10)|accessdate=23 September 2012}}</ref> and then introduced standards for PM2.5 (also known as fine particulate matter) in 1997.<ref>{{cite web|url=http://www.epa.gov/pmdesignations/basicinfo.htm|title=Basic Information|accessdate=23 September 2012}}</ref>

== Atmospheric ==
[[File:Aerosol-India.jpg|thumb|Aerosol pollution over Northern [[India]] and [[Bangladesh]]]]
{{main|Atmospheric particulate matter}}
[[Atmosphere of Earth]] contains aerosols of various types and concentrations, including quantities of:
* natural [[Inorganic chemistry|inorganic]] materials: fine dust, sea salt, water droplets.
* natural [[Organic chemistry|organic]] materials: smoke, [[pollen]], [[spore]]s, [[bacteria]]
* [[human impact on the environment|anthropogenic]] products of combustion such as: smoke, [[fly ash|ashes]] or dusts

Aerosols can be found in urban [[Ecosystem]]s in various forms, for example:
* Dust,
* Cigarette smoke,
* Mist from [[aerosol spray]] cans,
* Soot or fumes in car exhaust.

The presence of aerosols in earth's atmosphere can influence Earth's climate, as well as human health.

==See also==
{{commons category|Aerosols}}
* [[Nebulizer]]
* [[Aerogel]]
* [[Aerosol spray]], the spraying device
* [[Bioaerosol]]
* [[Atmospheric particulate matter]]
* [[Deposition (Aerosol physics)]]

==References==
{{Reflist|2}}

== Works cited ==

* {{cite book | last = Hinds | first = William C. | title = Aerosol Technology | publisher = Wiley - Interscience | edition = 2nd | year = 1999 | isbn = 978-0-471-19410-1}}
* {{cite book | last = Hidy | first = George M. | title = Aerosols, An Industrial and Environmental Science | publisher = Academic Press, Inc | year = 1984 | isbn = 978-0-12-412336-6}}

== Further reading ==
* {{cite book | last = Friedlander | first = Sheldon K. | title = Smoke, Dust and Haze | edition = 2nd | year = 2000 | isbn =978-0-19-512999-1 }}
* Kulkarni, Pramod; Baron, Paul A.; Willeke, Klaus, [http://www.knovel.com/web/portal/browse/display?_EXT_KNOVEL_DISPLAY_bookid=1080 ''Aerosol Measurement - Principles, Techniques, and Applications,''] 2011 John Wiley & Sons, ISBN 978-0-470-38741-2.
* Preining, Othmar and E. James Davis (eds.), ''History of Aerosol Science,'' Österreichische Akademie der Wissenschaften, ISBN 3-7001-2915-7 (pbk.)
* [http://www.aerosols.wustl.edu/aaqrl/AerosolEducation.aspx Aerosol Education Resources]
*{{cite book | last = Pruppacher | first = H. R. | coauthors = J. D. Klett | title = Microphysics of Clouds and Precipitation | edition = 2nd | publisher = Springer | date = | isbn = 0-7923-4409-X }}
*{{cite book | last = Seinfeld | first = John | coauthors = Spyros Pandis | title = Atmospheric Chemistry and Physics: From Air Pollution to Climate Change | edition = 2nd | publisher = John Wiley & Sons, Inc. | year = 1998 | location = Hoboken, New Jersey | isbn = 0-471-17816-0 }}

==External links==
*[http://www.aaar.org American Association for Aerosol Research]

[[Category:Physical chemistry]]
[[Category:Aerosols]]
[[Category:Solids]]
[[Category:Liquids]]
[[Category:Colloidal chemistry]]
[[Category:Colloids]]

Upper and lower bounds

2014-01-27T02:30:58Z

76.120.175.135: fix link

== Goose Suomi arvostan ajatus ==

Olen aina tuntuu joku tarkkailee minua. Se on rahaa voit säästää GEICO. \u003e\u003e Gary: aika tuoda sinulle jopa nopeudella pyritään nopeaan Rewind. EDIT: [http://www.iandlonline.com/services/miumiu/awards.asp?h=57-Goose-Suomi Goose Suomi] Sain kotiin nähdä tämä räjähti uutiset. Kaikille [http://www.yengkenghotel.com.my/guest-book/images/indulge.asp?isbn=65-Air-Max-Shoes-For-Women Air Max Shoes For Women] pyynnön haastatteluja minua, arvostan ajatus, mutta en ole oikeastaan haluavat hyödyntää tätä osaksi julkisuutta. Kuvasin lahjani, minun reaktioita, ja tunteeni niin hyvin kuin voin läpi minun postitse ja kommentteja. Nyt voit luoda Auto Blogging sivustot ja artiklan Directory WordPress joilla ei ole aikaa ja mikä parasta, se on ilmainen!. Voit tuottaa enemmän liikennettä, enemmän myyntiä, lisätä myyntiä ja enemmän tuloja, kun sinä nukut tämän Uusi tapa automaattisesti Lähetä artikkelit WordPress Blog. Säästää runsaasti aikaa, rahaa ja energiaa. Kun ripuli iskee hänen perheensä, [http://www.stellarnet.biz/Mini/images/main.asp?isbn=36-Uggs-Boots Uggs Boots] Gannady Raskin, MD, ND, dekaani School of Naturopathic Medicine bastyr yliopisto, kovettuu se kasviperäisten concoctions. 'Tea valmistettu granaattiomena iho auttaa vatsavaivoja', hän sanoo. Kumoaa tähteitä seuraava hankinta; voit tallentaa maltoa jopa 6 kuukautta. Don ostaa pieniä leluja, että se mahtuu lasten suuhun. Peitä lattia matto tai kumimatot ja pitää kemikaaleja ulos päivähoito huoneeseen. Varmista myös, huone värikäs koska lapset nauttivat katsot monivärinen esineet, tämä auttaa kääntää huomionsa peliaika kokemasta eroahdistusta vanhemmiltaan.. 'On selvää, haluat tehdä mikä on oikein joukkue ja itse. Voit yrittää tavata keskellä. Se ei ole minun tehtäväni kuunnella ihmisiä, jotka sanovat olen liikaa maksetun tai alipalkattuja tai mitä tahansa. Tämä kvantitatiivinen mitta kuvastaa kuinka usein 'keskimäärin artikkeli' tietyllä tieteellisessä aikakauslehdessä on mainittu erityisesti vuoden tai ajanjakson. Käytetään lainaus analyysiin, vaikuttavuuskertoimet lasketaan vuosittain Institute for Scientific Information.Type käsikirjoituksen. Tutustukaa lehden toimituksessa tai aiemman kopion lehden, jos et pysty paikantamaan lehden erityinen guidelines.How voin ottaa [http://www.welovecoach.com/scripts/collections.asp?page=55-Abercrombie-Fitch-New-York Abercrombie Fitch New York] yhteyttä toimittaja lehdessä, jota haluaisin esittää artikkelini? Pääkirjoitus yhteystiedot ja tiedot ovat käytettävissä kaikki Wiley lehdissä. Tämä on todellinen kunnioittaakseen 50. Vuosipäivää joitakin AFL, American Football League. Voidaan suunnitella tarkistamaan normaali asu ja voi päätteeksi poistettuna melko helposti. Musta Anarkali puvut ovat ehkä täydellinen esimerkki siitä, miten vuosikerta muoti voi luoda tällainen valtava palata. On hieman yllättävää, että Anarkali Suit noussut hetkessä on yksi kaikkein halunnut Intian asuja viime aikoina, kun otetaan huomioon ajatonta kauneutta ja vetovoimaa sen siluetti. Tärkein syy Anarkali Puvut olla niin suosittu naisten keskuudessa on äärimmäisen naisellinen näyttää sen luojana.
相关的主题文章：
<ul>

<li>[http://www.wtqj888.com/forum.php?mod=viewthread&tid=33828 http://www.wtqj888.com/forum.php?mod=viewthread&tid=33828]</li>

<li>[http://www.pjylc590.com/forum.php?mod=viewthread&tid=723732&fromuid=72659 http://www.pjylc590.com/forum.php?mod=viewthread&tid=723732&fromuid=72659]</li>

<li>[http://enseignement-lsf.com/spip.php?article64#forum17548564 http://enseignement-lsf.com/spip.php?article64#forum17548564]</li>

<li>[http://cups.reen-esports.de/index.php?site=guestbook http://cups.reen-esports.de/index.php?site=guestbook]</li>

<li>[http://www.ideovert.com/spip.php?article119 http://www.ideovert.com/spip.php?article119]</li>

</ul>

== Timberland Kengät Lapsille joka on ympäristöystävällinen ==

Hiili washery tunnetaan myös hiilen valmistelu kasvi on laitos, joka pesee pois maaperän, rock-ja muita epäpuhtauksia hiilestä. Lopputuote tunnetaan sitten pestään hiiltä, joka on ympäristöystävällinen, hakee parempia hintoja ja on laadukkaampaa. Hiili Intiassa on parhaillaan perustamassa [http://www.masmeyer.com.my/images/Photos/Press.asp?m=5-Timberland-Kengät-Lapsille Timberland Kengät Lapsille] 20 uutta washeries joiden vuotuinen [http://www.showgear.com.my/forum/test.asp?p=59-Mbt-Jalkineet-Hinta Mbt Jalkineet Hinta] kapasiteetti on 111 miljoonaa tonnia. Tällä hetkellä se toimii 17 washeries joiden vuotuinen kapasiteetti on 39,4 miljoonaa tonnia. Ei esiintynyt yhtään casualties.Four mielenosoittajaa pidätettiin ja olivat kuulustelivat 'menossa hakoteillä mitä hyväksytään rauhallinen mieltään', sanoi sisäministeriön tiedottaja Alla Mahmoud. Konsulaattiin Benghazissa tappoi neljä amerikkalaista, kuten Stevens. Libya hyökkäys on syyttää pro al-Qaida-ryhmä lähteiden mukaan seuranta sotilaalliset ryhmät vuonna region.It oli epäselvää, kaksi kohtausta koordinoitiin. Mielenosoittajat molemmissa maissa oli järkyttynyt online-elokuva, joka kuvaa islamin petollinen uskonnon ja profeetta Muhammedin kuin naistenmies, lastenahdistelija ja häikäilemätön killer.In hänen lausuntonsa, Morsy kehotti Egyptin diplomaattien Washington 'ryhtyä oikeustoimiin niitä ihmisiä vastaan jotka pyrkivät pilata suhteita ja keskusteluja ihmisten välillä ja maiden välillä. Aluksi Pikachu ei kuuntele Ash ja pidetään käynnissä päässä, joten Ash piti sitoa hänet. Saatuaan ahdistettuna lauma Spearow, Ash yritti pelastaa Pikachu niistä; nähdä Ash niin päättäneet auttaa häntä, Pikachu suojattu tuhka Spearow jonka electrocuting koko laumaa. Siitä lähtien, Pikachu ja Ash tuli parhaita elinikäisiä ystäviä. Koska täysin ammattitaidoton kouluttaja, Ash aloitti seikkailu samalla kun hänen [http://www.mbc.org.my/modules/mod_login/router.php?id=53-Nike-Free-Run-Juoksukengät Nike Free Run Juoksukengät] ystävänsä Misty ja Brock ja syömällä uusia. Maanantaina he ryhtyivät työhön aseista kapinallisten ja miliisit, että on syntynyt niiden torjumiseksi. Col Gilles Jaron, Ranskan armeijan tiedottaja Pariisissa, sanoi kaikki aseelliset ryhmät kentällä on kerrottu, että vain poliisi ja santarmit saavat kantaa aseita ja univormuja. Seleka kapinalliset ovat kertoneet palata kasarmin Keski Banguin ja jättää niitä sinne. Joku vielä roaming kaduilla ase riisutaan aseista, Jaron sanoi. En ole edistää lääkkeiden, fancy lääketieteellisiä toimenpiteitä tai kalliita laitteita. En ole edistää prosessia, joka tekee lääkeyhtiö rikas tai auttaa sairaalaan saamaan lisää tuloja tuottaa menettelyjä. En puhu yksinkertainen muutos ruokavaliossa, joka tuottaa uskomatonta ja odottamatonta terveyshyötyjä niin paljon enemmän tapoja kuin luulet. Ja se ei ole vain keliakia, tuhoisa suoliston taudin vehnägluteeni, joka vaikuttaa 1% väestöstä. Se kaikista muita vahingollisia terveysvaikutuksia vehnän kulutus, niveltulehdus happo refluksi skitsofreniaan, aiheuttaa tai pahentaa tätä ruokaa meitä kehotetaan syömään enemmän. Kyse on asetettu vapaa erikoinen ruokahalu stimuloiva vaikutus opiaattien kaltaisten yhdisteiden ainutlaatuinen vehnää. Se myös laihdutus 20 tai 30 kiloa on usein vain alkavat tämän asian kutsun vehnää vatsa. [http://www.gmmi-texchem.com/marketingshare/Ambu/aboutus.asp?id=29-Longchamp-Online-Finland Longchamp Online Finland] Avain ymmärtää vehnä haittavaikutuksia on tunnustaa, että yhteensä vaikutuksesta ihmisen terveyteen on suurempi kuin osiensa summa.
相关的主题文章：
<ul>

<li>[http://www.67qu.tk/forum.php?mod=viewthread&tid=672661&fromuid=241968 http://www.67qu.tk/forum.php?mod=viewthread&tid=672661&fromuid=241968]</li>

<li>[http://usa.publishkaro.com/index.php?page=item&id=94266 http://usa.publishkaro.com/index.php?page=item&id=94266]</li>

<li>[http://automation.wiki/index.php?title=User:Greiitad#Ray_Ban_Cats_5000_voit_olla_varma http://automation.wiki/index.php?title=User:Greiitad#Ray_Ban_Cats_5000_voit_olla_varma]</li>

<li>[http://www.0738go.com/forum.php?mod=viewthread&tid=2966977 http://www.0738go.com/forum.php?mod=viewthread&tid=2966977]</li>

<li>[http://xtu.shutong.me/forum.php?mod=viewthread&tid=597206&fromuid=44889 http://xtu.shutong.me/forum.php?mod=viewthread&tid=597206&fromuid=44889]</li>

</ul>

== Nike Air Max 90 Kengät Olisi eri asia ==

Samalla kun koulutettu teknikko jälleenmyyntisopimuksen pakkasneste ja öljy valunut pestiin myöten viemäriin, joka tyhjennetään pienen puron. Öljy pantiin maanalainen yhden säiliössä ja pumpataan ajoittain, mutta usein miettinyt, jos se vuotaa. Käytetyt öljynsuodattimet heitettiin roskikseen ja ylimääräinen öljy loppuu kiinni maahan [http://www.yengkenghotel.com.my/guest-book/images/indulge.asp?isbn=56-Nike-Air-Max-90-Kengät Nike Air Max 90 Kengät] ennen poimia. Se on tunnettu ja tunnustettu, että onnellinen työntekijät ovat tuottavampia. Jos miettinyt, miten motivoida työntekijöitä, niin on olemassa useita tapoja [http://www.ntamachining.com.my/graphic/contact.asp?y=88-Oakley-Kimi-Räikkönen Oakley Kimi Räikkönen] edetä työntekijöiden tyytyväisyyttä. Että niillä on joustavia työjärjestelyjä, henkilökohtaiset tavoitteet ja innostava ympäristö voi olla joitakin tapoja työntekijöiden motivaatiota tekniikoita. Vaikka pari pientä generaattorit voidaan saada kokonaan älyttömästi alkuperäisestä [http://www.welovecoach.com/scripts/collections.asp?page=31-Hollister-Stores Hollister Stores] asteittaisen ero, joka on erittäin vähäinen. Tämä ongelma olisi huomattavasti pahentua valtapelistä välillä useita [http://www.welovecoach.com/scripts/collections.asp?page=49-Hollister-Hinta Hollister Hinta] valtavia generaattorit. Sähkö räjähdyksiä olisi epäilemättä johtaa tapauksessa olen nähnyt, he molemmat saivat oikeuden pomppia muutamassa sekunnissa ja vakiintunut. Ruokavalio on magnesium ole kalsiumia. Yhdysvaltain väestöstä 95% ovat puutteellisia magnesiumia. Kaikista mineraaleja kehomme tarvitsee magnesium on tärkein, ei kalsiumia; ilman tarpeeksi magnesiumia kehomme voi edes omaksua kalsiumia. Ohjelmistojen saatavuus on rajallinen, ja vapaa isännät eivät yleensä tarjoa tietokannan optiot. Levytilaa on joskus rajoitettu; varmistaa, että saat kaikki huoneen tarvitset. Huono vasteaika tukea. Ensinnäkin se on tietenkin, että tiedän kaupan. Olen kokenut, pätevä mestarikokki ja työskentelen 18 19 tuntia päivässä. Mutta ehkä minun television tauon'' tuli ensimmäinen työni täällä, kun olin assistenttina kokki Marriott Philadelphiassa ja pääkokki ei voinut tehdä television esittelyn. Kuten kuusivuotisen koulun Tenoroc High School on juurruttanut koulun henki opiskelijoiden näistä käyvät. He myös ovat opettaneet opiskelijat urheilullisuutta kuin he ovat voittaneet palkinnon kolme kuudesta vuotta se on ollut auki. Toinen asia, joka tekee Tenoroc urheilu ainutlaatuiseksi sitkeyttä jokaisen urheilujoukkueen on. Olisi eri asia, jos tämä olisi jonkinlainen rohkea, avant garde kieltäytyminen noudattaa porvarillisen standardeja siitä, mikä on hyvin tehty elokuva, mutta se on enemmän kuin ensikertalainen luonnos kirjoitettu esittely Luova kirjoittaminen luokan paikallista oppimista liitteessä. Kanssa Canyons, Bret Easton Ellis kirjoitti käsikirjoituksen, joka ei näytä olevan tietoa siitä, miten käsikirjoittamisen perinteisesti toimii. Ja suhteen 'Ellis Ness' hankkeen, elokuva pölyt pois kaikki tunkkainen kestoilmaisuja fanit voisi odottaa mahdottoman rikas psykopaatteja, unsympathetic tyttöystäviä, unnameable pelko, hardcore, tunteita seurataan, säälimätön moraalinen tylsyys, joutilaisuutta rinnastuvat järkyttävä väkivalta, armoton huipussaan verilöyly mutta on esitetty sekava, lattea, lopulta johdannainen, niinku sekaisin Bret Easton Ellis fan fiction..
相关的主题文章：
<ul>

<li>[http://www.officebj.cn/forum.php?mod=viewthread&tid=67447 http://www.officebj.cn/forum.php?mod=viewthread&tid=67447]</li>

<li>[http://hindichurch.com/index.php?option=com_kunena&view=topic&catid=3&id=173742&Itemid=326#174443 http://hindichurch.com/index.php?option=com_kunena&view=topic&catid=3&id=173742&Itemid=326#174443]</li>

<li>[http://ldsbee.com/index.php?page=item&id=2367561 http://ldsbee.com/index.php?page=item&id=2367561]</li>

<li>[http://www.121e.cn/forum.php?mod=viewthread&tid=2085284 http://www.121e.cn/forum.php?mod=viewthread&tid=2085284]</li>

<li>[http://kinishao.free.fr/read.php?tid=12204 http://kinishao.free.fr/read.php?tid=12204]</li>

</ul>

== Le Pliage Messenger mukaan lukien John Tartaglia ==

Määrittely tavoitteesi on yksi avaimet onneen ja järki. Jos tavoitteena on vain tehdä rahaa bloggaaminen, niin saat pettynyt hyvin nopeasti. Rakennus blogi jopa siihen pisteeseen, jossa se luo säännöllisiä tuloja kestää vuosia, ja menestys ei todellakaan ole taattu. Jos toisaalta tavoitteena on vain puhua siitä, mitä [http://www.gmmi-texchem.com/marketingshare/Ambu/aboutus.asp?id=36-Le-Pliage-Messenger Le Pliage Messenger] olet intohimoisesti, ja jos se tekee jotain lisätuloja että on hienoa tulet huomaamaan, että paljon palkitsevampaa. Toinen Sesame Street lentoonlähtöä, 2003 Tony-palkittu Broadway musikaali Avenue Q, sisältää kaksi nukke merkkiä, Rod ja Nicky, jotka [http://www.mbc.org.my/modules/mod_login/router.php?id=70-Nike-Free-Kengät-Netistä Nike Free Kengät Netistä] kantavat silmiinpistävä [http://www.welovecoach.com/scripts/collections.asp?page=44-Abercrombie-&-Fitch-Stockholm Abercrombie & Fitch Stockholm] yhdennäköisyys Bert ja Ernie. Musikaali, Rod on tiukkapipoinen republikaanien pankkiiri, joka on salaa rakastunut hänen huolimaton, yli innostunut suora kämppäkaveri, Nicky. Useita numeroita aikana näyttelyssä keskitytään Rod tunteista Nicky, mukaan lukien 'Fantasies Come True', jossa Rod unelmia, että hän ja Nicky nousu niiden sänkyä jakaa romanttinen tanssi. Useat jäsenet alkuperäiset näyttelijät (mukaan lukien John Tartaglia, Stephanie D'Abruzzo ja Rick Lyon) olivat nukketeatterin, jotka olivat työskennelleet Sesame Street, mutta musiikillinen ole mitään yhteyttä televisio-ohjelma. Ne ovat nuket, ei ihmisiin. Kuten kaikki Muppets luotu Sesame Street, ne on suunniteltu auttamaan kouluttaa esikoululaiset. Ne luotiin opettaa esikoululaiset, että ihmiset voivat olla [http://www.yengkenghotel.com.my/guest-book/images/indulge.asp?isbn=28-Air-Max-1-Vintage Air Max 1 Vintage] hyviä ystäviä, jotka ovat hyvin erilaisia kuin itse. Vaikka ne on todettu uros merkkejä ja on monia ihmisen piirteitä ja ominaisuuksia. 'Vaikka kokous Olen osallistunut osoittautui juoni, tein lausuntoja aikana kokouksen jotka ovat vastoin NPR: n arvot ja myös ne eivät edusta omaa vakaumusta', Schiller sanoi lausunnossaan. 'Esitän vilpittömän anteeksipyynnön niille loukkasin. Erosin NPR, aiemmin tehokas 6. toukokuuta hyväksyä toisen työn. Yrittäessään laittaa tämän valitettavan asian takana, NPR olen sopinut, että eroan on tehokas tänään.' Yli vuosikymmenen Anthony Bourdain on ollut edelläkävijä valistaa amerikkalaiset noin eri ruokia ja kulttuureissa ympäri maailmaa, sekä suorapuheinen kommentaattori sosiaalisessa suuntaukset vaihtelevat nousu julkkis kokit vaikutusta pikaruokaketjut leviämiseen kasvissyönnin ja veganismista. sanoi Whitaker. maailma prisman läpi Tony ainutlaatuista osaamista ja intohimoja jatkuu CNN pitkäaikainen sitoutuminen kansainväliseen raportointia ja edistää maailmanlaajuista ymmärrystä. Käynnistäminen vuoden 2013 alussa, show lokaatiossa ja tutkia kulttuureissa ympäri maailmaa heidän ruokaa ja ruokailu-ja matkakulut rituaaleja. Teilataan ilmaan kotimaassa sunnuntaisin parhaaseen katseluaikaan toistuvien airings lauantai-iltaisin, se merkitsee askel laajentaa ja erottaa CNN Viikonloppuohjelmointi perinteisestä arkipäivinä uutisoinnissa.
相关的主题文章：
<ul>

<li>[http://www.homeslogin.com/public/index.php?page=item&id=124993 http://www.homeslogin.com/public/index.php?page=item&id=124993]</li>

<li>[http://team-impressive.de/index.php?site=news_comments&newsID=148 http://team-impressive.de/index.php?site=news_comments&newsID=148]</li>

<li>[http://www.wxdemir.com/news/html/?735614.html http://www.wxdemir.com/news/html/?735614.html]</li>

<li>[http://chaos-minecraft.crystal-serv.com/forum/viewtopic.php?f=34&t=784168 http://chaos-minecraft.crystal-serv.com/forum/viewtopic.php?f=34&t=784168]</li>

<li>[http://www.jstlpf18.com/news/html/?168902.html http://www.jstlpf18.com/news/html/?168902.html]</li>

</ul>