An improved conventional-to-micro/minichannel threshold was proposed as Bo = 4 and BoRel0.5 = 200, where Bo is the Bond number and Rel is the liquid Reynolds number. The region Bo < 4 and BoRel0.5 < 200 is termed as micro/minichannel because bubbles tend to be confined and elongated in the channel and the conventional two-phase flow theory loses its applicability. Flow-pattern based heat transfer correlations for elongated bubbly flow and annular flow in flow boiling micro/minichannels were developed separately based on a collected micro/minichannel heat-transfer database. As significant disagreement in experimental trends and heat transfer mechanisms was reported for flow boiling in micro/minichannels in the literature, it is not possible to explain the discrepancy and predict all data points by a single correlation without considering the different flow patterns. The newly developed flow-pattern based predictive tool can not only present a decent overall accuracy, but also estimate the parametric trends correctly. Over 95% of the data points can be predicted by the proposed correlations within a ± 50% error band for both elongated bubbly flow and annular flow. Therefore, the simple flow-pattern based correlations can be applied for heat exchanger design, improve understanding of the underlying heat transfer mechanisms, and guide development of further enhancement techniques for flow boiling in micro/minichannels.

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