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# Micro heat exchanger

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 microchannels, channels with a hydraulic diameter below 1 mm.

## Background

Investigation of microscale thermal devices is motivated by the single phase internal flow correlation for convective heat transfer:

$h=Nu_c \frac{k}{d}$

Where h is the heat transfer coefficient, Nuc is the Nusselt number, k is the thermal conductivity of the fluid and d is the hydraulic diameter of the channel or duct. In internal laminar flows, the Nusselt number becomes a constant. This is a result which can be arrived at analytically: For the case of a constant wall temperature, Nuc = 3.657 and for the case of constant heat flux Nuc = 4.364[1]. As Reynolds number is proportional to hydraulic diameter, fluid flow in channels of small hydraulic diameter will predominantly be laminar in character. This corellation therefore indicates that the heat transfer coeficient 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.

This hypothesis was initially investigated by Tuckerman and Pease [2]. Their positive results have since fueled a hot field of research ranging from classical investigations of single channel heat transfer [3] to more applied investigations in parallel micro-channel and micro scale plate fin heat exchangers. Recent work in the field has focused on the potential of two-phase flows at the micro-scale. [4][5][6]

## Classification of micro heat exchangers

Just like "conventional" or "macro scale" heat exchangers, micro heat exchangers have either one or two fluidic passages. In the case of one passage, heat is transferred to the fluid [7] from electrically powered heater cartridges, or removed from the fluid by electrically powered elements like Peltier chillers. In the case of two fluidic passages[7], 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.

## Notes and references

1. ^ Incropera & Dewitt text - more complete reference later
2. ^ Tuckerman D.B., and Pease, R.F., High Performance Heat Sinking for VLSI, IEEE Electron Device Letters, vol. EDL-2, pp126-129, 1981
3. ^ Santiago, Kenny, Goodson, Zhang - more complete reference later
4. ^ Yen, Tzu-Hsiang; Kasagi, Nobuhide; Suzuki, Yuji; Forced convective boiling heat transfer in microtubes at low mass and heat fluxes; Int'l J Multiphase Flow; 29; 2003; pp.1771-1792
5. ^ Steinke, M. E., and Kandlikar, S. G., “An Experimental Investigation of Flow Boiling Characteristics of Water in Parallel Microchannels,” Journal of Heat Transfer, 126, no. 4, pp. 518-526, 2004.
6. ^ Mudawar - more complete reference later
7. ^ a b each of the fluids can be a gas, a liquid, or a multiphase flow