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MicroHX (Microscale Heat Exchangers)

Microscale heat exchangers or micro heat exchangers (microHX) are heat exchangers in which one or more fluids flow in a confined area (tube, channel, …) that has dimensions on the order of 100 micrometers (1 µm to 1000 µm). The confined area based on its shape is called: microchannel, microtube, microcavity, etc. Small sizes of the channels reduce the resistance to heat transfer. That and very high surface to volume ratios, cause very high rates of heat transfer in these devices. The convective heat transfer coefficients in microchannels can be an order of magnitude higher that conventional channels.

Similar to conventional heat exchangers, several flow configurations may be seen in different microHX’s like one-fluid or two-fluid heat exchangers, counter-flow or parallel or cross flow configurations, etc. A very familiar example of one-fluid microHX’s can be found in these days computers where a microHX is used for cooling down electronic parts (like CPUs).

Some other applications are: microchannel fuel vaporizer (fuel cell), Commercial and residential heating/cooling, electronics cooling, biomedicine.

One other way of reducing the weight of the system is using light weighted materials, for example using aluminum instead of stainless steel. The problem is aluminum might not be able to tolerate the temperatures; however, higher heat transfer rates and innovative designs can reduce the operating temperatures to levels tolerable with lighter materials. Even plastic is used to make microHX’s these days.

Heat transfers through surfaces and the more is the surface area, the better is heat transfer. Microchannels (or microtubes, etc.) have high surface to volume ratios and as the dimension decreases, this ratio increases. That causes less resistance for heat transfer. It can be clearly seen from the convective heat transfer equation: h = (Nu×k)/D

where k is the fluid thermal conductivity, D is the dimension and Nu is the Nusselt number. Nusselt number is a constant value for laminar flow in a channel (i.e. 3.65) and also thermal conductivity of a fluid is constant in this case. Therefore the smaller is the dimension; the larger is the heat transfer coefficient.

Besides all these advantages, microHXs have some down points too. The major problems are:

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