Modeling Convective Boiling in Single Diverging Channel With In-Situ Vapor Extraction

The present work shows the potential of using a diverging channel with in-situ vapor extraction as a means to reduce flow instability in microscale flow boiling. It has been shown that diverging channel helps stabilize convective boiling flow. In-situ vapor extraction is proposed as an alternative method that helps further stabilize flow boiling. The main concept of in-situ vapor extraction is to reduce the vapor available inside the channel where it forms by locally extracting vapor through a hydrophobic porous membrane that forms a wall of the channel to stabilize the flow and reduce pressure drop along the channel. In-situ vapor extraction of boiling flow in a microchannel also has the potential to reduce the required pressure to drive the flow through the channel without losing the benefit of convective boiling heat transfer. In this study, four microchannels, each with a range extraction pressures, are evaluated using a one-dimensional predictive model. Each channel is 50 mm and has a mid-channel width of 500 microns and height of 500 microns. The half angle of divergence of the channels varies: 0, 0.11, 0.23, and 0.34 degrees. Wall heat flux values range from 13.3 to 133 W/cm². Extracted mass flow rates, global pressure drop across the channels, and quality at the channel outlet are presented as a function of heat flux and extraction pressure. Local variations of pressure, quality and bulk fluid temperature are also presented. Stability is predicted by a newly proposed criterion applicable for a diverging channel with in-situ vapor extraction. The results show that in-situ vapor extraction significantly reduces pressure drop for all channel configurations. Although the drop in pressure across the diverging channels is less influenced by vapor extraction than the non-diverging channel, the coupling of diverging channels and vapor extraction considerably improves the predicted stability of the flow.