Thin liquid films formation and evaporation mechanisms around elongated bubbles in rectangular cross-section microchannels

Thin film evaporation is the dominant mechanism of heat transfer in the microchannel flow boiling process. Hence, its understanding is paramount to development of predictive models and more efficient microchannel heat exchangers. Although microchannels mostly have a rectangular cross-section, the existing models are developed for circular cross-section microchannels. The non-uniformity of the liquid film thickness in microchannels with a rectangular cross-section has made prediction of the onset of dryout and heat transfer coefficient greatly challenging. Here, a test device capable of measuring the wall heat flux is utilized to fully characterize the thin film formation and evaporation process. The effects of flow capillary number (Ca) and channel aspect ratio on the liquid film thickness are determined in microchannels with 300 mu m width and 300, 150, and 75 mu m heights, representing aspect ratios of 1, 2, and 4, respectively. It is shown that the only existing mechanistic model for the elongated bubble regime in microchannels fails to predict the experimental heat transfer coefficient (HTC) both quantitively, by a 50-120% margin, and qualitatively, due to a fundamental difference in evaporation and dryout of a variable versus a uniform film assumed in the model. The results presented here pave the way for development of a more accurate mechanistic model for the thin film evaporation heat transfer mechanism in microchannels with a rectangular cross-section. (C) 2020 Elsevier Ltd. All rights reserved.