Modeling in situ vapor extraction during convective boiling in fractal-like branching microchannel networks

The pressure drop penalty of convective boiling flow in microchannels may be exceedingly large. A proposed method of reducing this penalty is to extract vapor locally along the channel. A potential consequence of this extraction is that the local void fraction reduction positively influences the local heat transfer coefficient. In this study, a one dimensional model was developed to simulate convective boiling flow through a fractal-like branching microchannel network with vapor extraction through a channel wall formed using a hydrophobic porous membrane. The goal of the model is to provide a design tool that can assess the effects of vapor extraction on flow boiling heat transfer performance. Heat was applied through all walls of the channel. Vapor extraction was obtained by applying a pressure difference across the membrane. Membrane transport models of the extraction process based on local channel pressure and local saturation pressure are discussed. Predicted local conditions and global results are presented for two ranges of conditions: (i) relatively low inlet flow rate with low heat flux and (ii) relatively high inlet flow rate with high heat flux. Results shows that as the vapor extraction rate increases, there is a significant reduction in pressure drop through the channel, a reduction of the bulk fluid temperature, and a reduction in exit vapor quality.