Modeling criteria for extraction regime transitions for microscale in-situ vapor extraction applications

A major detriment of two-phase microscale flow systems is a significantly large pressure drop. For flow boiling the potential for flow instabilities is also a major concern. Both disadvantages may be suppressed by extracting vapor through a hydrophobic porous wall of the channel as a means to reduce the channel vapor fraction. The vapor extraction may occur as different regimes either as evaporation, bubble extraction or a combination of both. In the design of vapor extraction systems, it is important to accurately predict extraction rates, different extraction regimes, and the effect of extraction on the heat transfer and channel flow conditions. This study focuses on two parts: the development of physic-based models for the transition criteria among (i) the extraction mechanism regimes, and (ii) the extraction flow regimes for microscale flow boiling. The identification and conditions for the various extraction regimes are discussed and criteria for transition are developed based on physical concepts. Six potential extraction mechanism regimes are identified: (a) no extraction, (b) evaporation, (c) bubble extraction, (d) bubble extraction with partial liquid blockage, (e) bubble extraction with evaporation, and (f) liquid breakthrough. Based on the criteria for the extraction mechanism regimes, the rate of vapor extraction is modeled and used to analyze the effects of vapor extraction on the dynamics of two-phase flow boiling. The results show six extraction flow regimes for two-phase flow boiling: (i) single-phase evaporation, (ii) two-phase evaporation - bubble collapse, (iii) full extraction - stable, (iv) full extraction - unstable, (v) partial extraction - stable and (iv) partial extraction - unstable.