Transition to annular flow regime in microchannels is arguably one of the most complex phenomena in the flow boiling process. The instability of the vapor-liquid interface in this interstitial regime presents an intricate situation in which the interface pattern rapidly changes with the mass flow rate and surface heat flux. Although a few past studies have reported observing this regime, thermohydraulics of the process and flow and boundary conditions under which this transition occurs have remained largely unknown. The main obstacle in deciphering the physics of this process is lack of measurement tools to characterize hydrodynamics and thermal characteristics of this flow regime at microscales. The present study benefits from implementation of a novel test device that enables measuring the liquid film thickness and its rapid variations with micrometer and microseconds spatial and temporal resolutions. It is determined that each flow regime has a unique surface temperature signature that enables its clear distinction without need for high-speed visualization. Based on the dynamics of the flow, we identified that the transitional region is comprised of two regimes coalescing bubbles (CB) and semi-annular flow conditions. The difference between these two flow regimes emanates from motion of liquid film beneath the bubble.
This paper presents an experimental study on bubbles extraction from a two-phase flow in microchannels. The bubbles were extracted through a hydrophobic porous membrane covering the microchannels. The two-phase flow was generated through mixing water and air at a T-junction positioned before a microchannel on a microfabricated device. To study the effect of different parameters on the extraction rate, an extensive parametric study was conducted. The parametric space included variations of the channel depth, pressure difference across the membrane, and water and air flow rates. Differential pressure across the membrane was found to be the most critical factor impacting the bubbles extraction rate. Also, the effect of flow quality on the extraction rate was determined to be insignificant. Furthermore, it was found that at a critical velocity a liquid layer forms between the bubbles and the membrane and consequently the bubbles cease to extract.
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