Effects of Registration and Bonding on Microchannel Array Performance

Several previous investigations have found that the registration step is critical in microlamination. Wangwatcharakul (2002) found that laminae misregistration on the order of 20 |im can cause malfunction of out-of-plane microvalves on the order of 100 |im in diameter. Ashley reported that layer-to-layer bearing gaps on the order of 10 |im were necessary in order to operate the microscale turbine generator developed at MIT (Lohner et al. 1999). In each of these applications, precision alignment of laminae was necessary in order to produce a functional device. However, the effect of misregistration on microchannel array heat exchanger performance has been found to be negligible (Wattanutchariya 2002) though Paul, et al. (2000) found that poor registration in intermetallic microchannel devices resulted in an amplification in the fin warpage found within the device.

Fig. 14.16. The relation of channel deviation and the number of channel needed as well as percent increase in area to compensate the loss in heat transfer performance

As suggested, the major technological concern in bonding MECS devices is to eliminate fin warpage. Because of the difference in the cross-section of flow channels due to fin warpage, the working fluid will normally flow at different rates between channels. More fluid tends to flow in the larger cross-sectional channels resulting in flow maldistribution. This flow maldistribution results in a longer diffusion path for heat transfer to or from the wall, resulting in a less effective heat transfer coefficient for the total surface area as well as the overall heat transfer. Wattanutchariya performed an analysis of the effect of flow maldistribution on microchannel heat exchanger performance. Fig. 14.16 illustrates the results of this analysis indicating the percent change in area and change in the total number of microchannels needed to compensate for an average percent channel deviation between adjacent channels within the heat exchanger. Here, percent channel deviation indicates the percent change in channel cross-section as a result of warpage. Fig. 14.16 shows that for a 20% channel deviation, the percent increase in the total number of channels, and, consequently, the heat transfer surface, would need to be 50% to compensate for the loss in heat exchanger performance caused by this deviation. This suggests that channel deviations on the order of 20% of the channel dimension can result in an increase in size of almost 50%.

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