Two mathematical models of flow boiling and flow instability in rectangular expanding microchannel heat exchangers and structure optimization

Flow boiling instability in microchannels was widely studied to improve the reliability of microchannel heat exchangers. Two rectangular expanding microchannel heat exchangers have been experimentally investigated, one channel with cross-cutting and the other without cross-cutting. This study aims to optimize the heat transfer capabilities of heat exchangers. Two mathematical models containing heat transfer correlation and instability parameters have been developed. The experimental heat transfer coefficient (HTC) was used to validate them. Based on the models, the structure of the expanding microchannel was optimized, and the effect of expanding microchannel on flow instability was analyzed. As analyzed, in the models for two heat exchangers with and without cross-cutting, the mean deviations are 10.03% and 5.95%, respectively. The effect of flow instability suppression increases with the increase of the radiation angle. Moreover, the cross-cutting in expanding microchannels could further steady flow when adding fins in downstream channels. When adjusting the number of cross-cutting in cold plates from 1 to 4, the maximum increment of HTC is 58.17 kW/(m²·K). The influences of adjusting the number of fins in microchannels on the HTC are obvious for cold plates without cross-cutting. When the position of added fins is moved downstream in channels, the HTC of the heat exchanger could be increased by 26.58%. The results of this study provide some insights for inhibiting flow boiling instability and further optimizing the expanding microchannel structure to enhance heat transfer capability.