Mechanism of transition from elongated bubbles to wavy-annular regime in flow boiling through microchannels

Abstract

Transition from elongated bubbles to annular flow regime is one of the most intricate phenomena in flow boiling in microchannels. To identify this transition, flow regime maps have been introduced primarily based on visualizations and characterized using average flow properties. However, understanding the mechanism responsible for this transition demands accurate determination of the hydrodynamic characteristics of liquid films around the vapor core. But, the challenges associated with measurements of the thickness and velocity of the liquid films have made it difficult to determine the flow hydrodynamics. Here, we utilize a measurement technique to determine the liquid film thickness along with velocities of the two phases. These measurements have enabled accurate calculation of the cross-sectional void fraction, vapor quality, interfacial shear stress and superficial velocities, which form the coordinates of the existing flow regime maps. The results show that upon transition to the annular flow regime, a remarkable increase in the liquid film thickness occurs due to a substantial increase in the liquid-vapor interfacial shear stress. This shear stress, which is a function of the liquid film thickness, vapor velocity, and channel aspect ratio, is introduced as a criterion for transition from elongated bubbles (EB) to wavy-annular (WA) flow regime.