Geometrical Constraints in Microchannel Arrays

In the case of heat transfer, the function of a typical microchannel heat exchanger is to transmit the heat from one fluid stream into a second fluid stream. Several different microchannel configurations can be used to accomplish this task. The most efficient heat transfer methods tend to interleave the two fluids in an alternating succession of cross-flow or counter-flow channels. In particular, counter-flow channels are known to provide better temperature distributions along the length of heat exchangers when compared to co-flow heat exchangers (Kakac and Liu 1997). Most MECS applications make use of counterflow microchannel arrays such as in Fig. 14.17. Whereas in the case of co-flow arrays relatively high aspect ratios have been successfully fabricated even in hard-to-produce materials, the aspect ratio is generally constrained in two-fluid microscale heat exchangers because of the more complex geometry.

Header an

Header an

Fig. 14.17. An exploded view illustrating a microlamination procedure
Fig. 14.18. An exploded view of the two-fluid counterflow microchannel array investigated in this study (Model courtesy of P. Kwon at Michigan State University)

The prototypical counter-flow array is produced by alternating layers of microchannels, which allow the two fluids to flow in opposite directions separated by fins. A concept of the through-cut lamina design for a microlaminated counter-flow microchannel array is shown in Fig. 14.18. It is assumed that once the laminae are stacked they are diffusion bonded according to Fig. 14.18. The plan view of the resulting monolith comprised of microchannels and fins is shown in Fig. 14.19. In order to diffusion bond the laminae, a uniform bonding pressure is applied on the stack at an elevated temperature. The pressure is transmitted uniformly through the device except at the necked of the microchannels where each individual microchannel interfaces with fluid headers (the gray regions where the cross-section AA is in Fig. 14.19. The lack of transmitted bonding pressure in these regions can cause the laminae to remain unbonded. In many cases, unbonded regions are observed in the cross-section of counterflow microchannel arrays at points A & B shown in Fig. 14.20, resulting in leakage within the device and mixing of the two fluids.

Fig. 14.19. Top view of a counterflow microchannel array comprised of microchannel and fin laminae. The-regions of the device are highlighted in gray

Fig. 14.20. Cross-section of a microchannel neck at cross-section AA in Fig. 14.19. As the width of the channels increase, the stress at points A and B eventually become zero and can even tensile yielding unbounded regions which leak

Fig. 14.20. Cross-section of a microchannel neck at cross-section AA in Fig. 14.19. As the width of the channels increase, the stress at points A and B eventually become zero and can even tensile yielding unbounded regions which leak

Fig. 14.21a shows a region within a counterflow microchannel device where pressure was directly transmitted through all of the laminae. Joints between laminae exhibit excellent bonding. Fig. 14.21b shows a region where pressure was not transmitted directly between adjacent laminae because of the counter-flow design. Fig. 14.21b exhibits unbonded regions. From the experience, the width of the necked has to be constrained to a dimension where bonding will occur. The limit on the width of the channel at which bonding takes place depends on the thickness of adjacent fin lamina. In other words, as the thickness of the fin laminae above and below the neck of the microchannels increases, the neck can be made wider. The ratio of the channel span in the necked region to the fin lamina thickness is defined as the fin aspect ratio. The fins in two-fluid microchannel heat exchangers have a certain fin aspect ratio limit beyond which poor bonding will occur within the device resulting in leakage and mixing of the two fluids in the device. Using finite element analysis, it has been found that for a given set of bonding conditions, the key bonding characteristic in all regions of the device must be that only compressive out of plane stresses exist. Maximum fin aspect ratios in the necked regions of counter-flow devices have been implemented up to 10:1 with the allowable fin aspect ratio generally decreasing with increasing fin thickness. This finding is counter-intuitive but has been verified by FEA.

Fig. 14.21. Cross section of a diffusion bounded counter-flow microchannel heat exchanger showing section through which the bonding pressure (a) was transmitted; and (b) was not transmitted. Cross section b resulted in leakage between the two fluids (Micrographs courtesy of Albany Research Center)

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