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| '''Micro heat exchangers,''' '''Micro-scale heat exchangers,''' or '''microstructured heat exchangers''' are [[heat exchangers]] in which (at least one) [[fluid]] flows in lateral confinements with typical dimensions below 1 mm. The most typical such confinement are [[microchannel]]s, which are channels with a [[hydraulic diameter]] below 1 mm.
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| ==Background==
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| Investigation of microscale thermal devices is motivated by the single phase internal flow correlation for convective heat transfer:
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| :<math>h=Nu_c \frac{k}{d}</math>
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| Where <math>h</math> is the [[heat transfer coefficient]], <math>Nu_c</math> is the [[Nusselt number]], <math>k</math> is the [[thermal conductivity]] of the fluid and <math>d</math> is the [[hydraulic diameter]] of the channel or duct. In internal [[laminar flow]]s, the Nusselt number becomes a constant. This is a result which can be arrived at analytically: For the case of a constant wall temperature, <math>Nu_c=3.657</math> and for the case of constant [[heat flux]] <math>Nu_c=4.364</math>.<ref name="Convection">Incropera & Dewitt{{full|date=November 2012}}</ref> As [[Reynolds number]] is proportional to hydraulic diameter, fluid flow in channels of small hydraulic diameter will predominantly be laminar in character. This correlation therefore indicates that the heat transfer coefficient increases as channel diameter decreases. Should the hydraulic diameter in forced convection be on the order of tens or hundreds of micrometres, an extremely high heat transfer coefficient should result.
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| This hypothesis was initially investigated by Tuckerman and Pease.<ref name="Tuckerman">{{cite journal |doi=10.1109/EDL.1981.25367 |title=High-performance heat sinking for VLSI |year=1981 |last1=Tuckerman |first1=D.B. |last2=Pease |first2=R.F.W. |journal=IEEE Electron Device Letters |volume=2 |issue=5 |pages=126–9}}{{psc|date=May 2012}}</ref> Their positive results have since fueled a hot{{peacock inline|date=May 2012}} field of research ranging from classical investigations of single channel heat transfer<ref name="Stanford">Santiago, Kenny, Goodson, Zhang{{full|date=November 2012}}</ref> to more applied investigations in parallel micro-channel and micro scale [[plate fin heat exchanger]]s. Recent work in the field has focused on the potential of two-phase flows at the micro-scale.<ref name="Kasagi">{{cite journal |doi=10.1016/j.ijmultiphaseflow.2003.09.004 |title=Forced convective boiling heat transfer in microtubes at low mass and heat fluxes |year=2003 |last1=Yen |first1=Tzu-Hsiang |last2=Kasagi |first2=Nobuhide |last3=Suzuki |first3=Yuji |journal=International Journal of Multiphase Flow |volume=29 |issue=12 |pages=1771–92}}</ref><ref name="Kandlikar">{{cite journal |doi=10.1115/1.1778187 |title=An Experimental Investigation of Flow Boiling Characteristics of Water in Parallel Microchannels |year=2004 |last1=Steinke |first1=Mark E. |last2=Kandlikar |first2=Satish G. |journal=Journal of Heat Transfer |volume=126 |issue=4 |pages=518}}</ref><ref name"Purdue">Mudawar{{full|date=November 2012}}</ref>
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| ==Classification of micro heat exchangers==
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| {{Unreferencedsection|date=May 2012}}
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| Just like "conventional" or "macro scale" [[heat exchanger]]s, micro heat exchangers have either one or two fluidic passages. In the case of one passage, [[heat]] is transferred to the fluid
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| (each of the fluids can be a [[gas]], a [[liquid]], or a [[multiphase flow]]) from electrically powered heater cartridges, or removed from the fluid by electrically powered elements like [[Thermoelectric effect|Peltier]] chillers. In the case of two fluidic passages, micro heat exchangers are usually classified by the orientation of the fluid passages to another as "cross flow" or "counter flow" devices. If a chemical reaction is conducted inside a micro heat exchanger, the latter is also called a [[microreactor]].
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| ==See also==
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| * [[Micro process engineering]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| *<nowiki>http://www.fzk.de/fzk/idcplg?IdcService=FZK&node=0897&lang=en</nowiki>{{dead link|date=May 2012}} Short introduction to Micro Heat Exchangers (by [[Forschungszentrum Karlsruhe]])
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| [[Category:Microtechnology]]
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| [[Category:Microfluidics]]
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| [[Category:Heat exchangers]]
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| [[Category:Heat transfer]]
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