liquid cooling is now recognized to be the only solution for removal of high heat fluxes.  devices based on impingement cooling (mjca from international mezzo technologies) or normal flow cold plates (npc from mikros technologies) have been demonstrated at heat fluxes in excess of 1kw/cm², and are now commercially available.  evaporative solutions such as boiling impingement and spray cooling are also available for some applications.

in liquid or two-phase cooling, removal of the thermal energy from the coolant to the environment is an important consideration.  in most applications that require liquid cooling, both space and mass are at a premium and standard liquid-air heat exchangers are often too bulky and heavy.  this limitation is a major impediment to the deployment of liquid cooling solutions.  in this article, we report on the development of ultra-compact micro-channel heat exchangers developed at international mezzo technologies.  these heat exchangers exhibit an order of magnitude improvement over the best available compact heat exchanger in terms of heat transfer per unit volume.

the primary driving force in using micro-channels to increase the gas-side heat transfer is based on the consideration that, after an entrance region effect, the nusselt number for a fully developed flow becomes constant:

nu = hd_h/k

thus, as the hydraulic diameter of the channel (d_h) is reduced the heat transfer coefficient (h) becomes larger.  for example, a reduction of channel diameter from 4 mm to 400 microns results in a ten-fold increase in the heat transfer coefficient.  such increases are significant for gas streams where the low thermal conductivity of the gases (relatives to liquids) leads to low gas-side heat transfer coefficients.   as for a given thermal load, the size of the heat exchanger is determined by the product of heat transfer coefficient and heat transfer area (ah), a ten-fold increase in the heat transfer coefficient would result in a commensurate reduction in the heat transfer area, and therefore the size of the heat exchanger.   

the building block of mezzo’s heat exchangers is a micro-channel panel, which can be used to create a variety of heat exchanger configurations.  figure 1 shows a typical panel with a thickness of 3 mm, and the frontal area of 15 cm x 10 cm.  one stream (usually the liquid in liquid-gas applications) flows through the inside of the panel, and the other stream (gas in liquid-gas applications) flows perpendicular to the panel through slots which are 250-400 microns wide and 3-20 mm long.  figure 2 is an sem view of the same panel showing the dimensions of the channels.

figure 1.  10 cm x 15 cm panel of thickness 3 mm

figure 2.  sem of cross section of micro heat exchanger panel

the micro-channel gas-liquid heat exchanger panels have been exhaustively tested and compared with a range of commercially available compact plate fin heat exchangers in figures 3 and 4.  the data for mezzo products are actual measurements, and those for the commercial heat exchangers is extracted from published data sheets. 

in figure 3 we compare heat transfer per frontal area versus air flow rate of the mezzo panels with a large number of commercial compact heat exchangers.  heat transfer per unit of frontal area of a single mezzo panel heat exchanger matches the performance of the lower end of commercially-available compact heat exchangers. a stack of two panels matches the higher end performance of these heat exchangers.   by using thicker panels or a multi-panel stack, heat transfer per unit frontal area of mezzo panels can appreciably exceed that of commercially-available heat exchangers. 

figure 3. heat transfer per frontal area of one and two mezzo panels and commercial compact heat exchangers.  the red dots are the commercial heat exchanger data.

figure 4. heat transfer per volume; one and two mezzo panels and commercial compact heat exchangers.  the red dots are the commercial heat exchanger data.

figure 4 compares heat transfer per volume of mezzo panels with the commercial heat exchangers.  heat transfer per unit volume of mezzo panels, is 5-10 times higher than the commercial systems.

the use of microchannels to increase the air-side heat transfer coefficient does not result in a substantial pressure drop.  the air-side pressure drop data for mezzo panels is compared with the pressure drop in the commercial compact heat exchangers in figure 5.  it can be readily observed that the pressure drop penalty associated with the increase in heat transfer per unit area and volume is negligible.

figure 5. air pressure drop versus air flow rate per frontal area.  the red dots are the commercial heat exchanger data.

mezzo panels can be used to construct a wide range of heat exchangers to satisfy constraints such as thermal load, frontal area, total volume, maximum pressure drop.  examples of various configurations are shown in figures 5-7.

figure 6 is a design for a thin flat heat exchanger with multiple panels. figures 7 and 8 show heat exchangers for large thermal loads, where a number of panels are stacked in “w” and “v” configurations.  these configurations provide very small frontal area while maintaining very larger heat exchange areas.

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figure 8.  intake of  v-shaped heat exchanger, using 4 large (4” x 6”) cross-flow panels

the test data on multi-panel heat exchangers show a significant reduction in weight and volume of the micro-channel reactors, when compared to state of the art compact heat exchangers.

the mezzo heat exchaner panels are manufactured using the micro-manufacturing technique liga and can be produced at low cost.

for more information on the microchannel heat exchangers or to request prototypes for testing contact dr. kevin kelly at [email protected].  a white paper on the heat exchangers is also available.  the author can be contacted at [email protected]