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| '''Critical heat flux''' describes the thermal limit of a phenomenon where a phase change occurs during heating (such as bubbles forming on a metal surface used to heat [[water]]), which suddenly decreases the efficiency of [[heat transfer]], thus causing localised overheating of the heating surface. | |
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| The '''Critical heat flux''' for [[Combustion|ignition]] is the lowest thermal load per unit area capable of initiating a [[combustion]] reaction on a given material (either [[flame]] or [[Smouldering|smoulder]] ignition).
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| ==Description==
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| When [[liquid]] [[coolant]] undergoes a change in [[Phase (matter)|phase]] due to the [[heat transfer|absorption]] of heat from a heated solid surface, a higher [[transfer rate]] occurs. The more efficient heat transfer from the heated surface (in the form of [[heat of vaporization]] plus [[sensible heat]]) and the motions of the bubbles (bubble-driven [[turbulence]] and [[convection]]) leads to rapid mixing of the [[fluid]]. Therefore, ''[[boiling]] heat transfer'' has played an important role in [[Industry|industrial]] heat transfer processes such as [[macroscopic]] heat transfer [[heat exchanger|exchangers]] in [[nuclear power plant|nuclear]] and fossil power plants, and in microscopic heat transfer devices such as heat [[pipe (material)|pipes]] and [[microchannel (microtechnology)|microchannel]]s for cooling [[electronics|electronic]] [[Integrated circuit|chips]].
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| The use of boiling is limited by a condition called '''critical heat flux (CHF)''', which is also called a [[boiling]] crisis or departure from [[nucleate]] boiling (DNB). The most serious problem is that the boiling limitation can be directly related to the physical burnout of the materials of a heated surface due to the suddenly inefficient heat transfer through a [[vapor]] film formed across the surface resulting from the replacement of liquid by vapor adjacent to the heated surface.
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| Consequently, the occurrence of CHF is accompanied by an inordinate increase in the surface temperature for a surface-heat-flux-controlled system. Otherwise, an inordinate decrease of the heat transfer rate occurs for a surface-temperature-controlled system. This can be explained with [[Newton's law of cooling]]:
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| :<math>q = h(T_w-T_f)\,</math>
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| where <math>q</math> represents the heat flux, <math>h</math> represents the [[heat transfer coefficient]], <math>T_w</math> represents the wall temperature and <math>T_f</math> represents the fluid temperature. If <math>h</math> decreases significantly due to the occurrence of the CHF condition, <math>T_w</math> will increase for fixed <math>q</math> and <math>T_f</math> while <math>q</math> will decrease for fixed <math>\Delta T</math>.
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| ==Correlations for critical heat flux==
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| The critical heat flux is an important point on the boiling curve and it may be desirable to operate a boiling process near this point. However, one could become cautious of dissipating heat in excess of this amount. Zuber, through a hydrodynamic stability analysis of the problem has developed an expression to approximate this point.
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| <math>{{\frac{q}{A_{max}}}}=C{{h}_{fg}}{{\rho }_{v}}{{\left[ \frac{\sigma g\left( {{\rho }_{L}}-{{\rho }_{v}} \right)}{{{\rho }_{v}}^{2}} \right]}^{{}^{1}\!\!\diagup\!\!{}_{4}\;}}</math>
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| It is independent of the surface material and is weakly dependent upon the heated surface geometry described by the constant C. For large horizontal cylinders, spheres and large finite heated surfaces, the value of the Zuber constant <math>C=\frac{\pi }{24}=0.131</math>. For large horizontal plates, a value of <math>C=0.149</math> is more suitable.
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| The critical heat flux depends strongly on pressure, mainly through the pressure dependence of surface tension and the heat of vaporization.<ref> {{cite journal|title=Fundamentals of Heat and Mass Transfer 6th Edition by Incropera }}</ref>
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| ==Applications in heat transfer==
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| The understanding of CHF phenomenon and an accurate prediction of the CHF condition are important for safe and economic design of many [[heat transfer]] units including [[nuclear reactors]], [[fossil fuel]] [[boiler]]s, [[fusion reactors]], electronic chips, etc. Therefore, the phenomenon has been investigated extensively over the world since [[Nukiyama]] first characterized it.<ref> {{cite journal|title=Film boiling water on thin wires|journal=Soc. Mech. Engng., Japan|date=1934|first=S. |last=Nukiyama|coauthors=|volume=37|issue=|pages=|id= |url=|format=|accessdate=2008-07-08 }}</ref> In 1950 [[Samson_Kutateladze|Kutateladze]] suggested the hydrodynamical theory of the burnout crisis. <ref> {{cite journal| title=Hydromechanical model of the crisis of boiling under conditions of free convection|journal=Journal of Technical Physics, USSR|date=1950|first=S.S.|last=Kutateladze|coauthors=|volume=20|issue=11|pages=1389–1392|id=|url=|format=|accessdate=2009-06-15}}</ref> Much of significant work has been done during the last decades with the development of water-cooled [[nuclear reactor]]s. Now many aspects of the phenomenon are well understood and several reliable [[prediction]] [[Model (abstract)|model]]s are available for conditions of common interests.
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| ===Terminology===
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| A number of different terms are used to denote the CHF condition: departure from nucleate boiling (DNB), liquid film dryout (LFD), annular film dryout (AFD), dryout (DO), burnout (BO), boiling crisis (BC), boiling transition (BT), etc. DNB, LFD and AFD represent specific mechanisms which will be introduced later.
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| DO means the disappearance of liquid on the heat transfer surface which properly describes the CHF condition; however, it is usually used to indicate the liquid film dryout from [[annular flow]]. BO, BC and BT are phenomenon-oriented names and are used as general terms. The CHF condition (or simply the CHF) is the most widely used today, though it may mislead one to think that there exists a criticality in the heat flux. The terms denoting the value of heat flux at the CHF occurrence are CHF, dryout heat flux, burnout heat flux, maximum heat flux, DNB heat flux, etc.
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| The term peak pool boiling heat flux is also used to denote the CHF in pool boiling.
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| ==See also==
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| *[[Leidenfrost effect]]
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| *[[Nucleate boiling]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| *[http://www.pmmh.espci.fr/~vnikol/boiling_crisis.html Modeling of the boiling crisis]
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| [[Category:Thermodynamics]]
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Marvella is what you can contact her but it's not the most feminine title out there. To gather badges is what her family members and her enjoy. Hiring is her working day occupation now but she's always wanted her own company. My family lives in Minnesota and my family members enjoys it.
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