Prediction of Regime Transition in Two Phase Flow Microchannels Based on Ultrathin Liquid Film Interfacial Instability

摘要

Transition from elongated bubble (EB) to semi-annular (SA) or annular regime is one of the most complicated phenomena in two phase flow in microchannels. Visualizations have shown that this transition is associated with variations in the shape of liquid films surrounding a fast-moving vapor core. Hence, capturing the thin films hydrodynamic characteristics and onset of their instability is key to identifying transition criteria. Previous studies had to introduce the transition criteria based on the flow superficial velocity. However, this parameter by no means represents the actual velocity of the liquid film. Here, using a novel measurement technique, thickness and actual velocity of liquid films as thin as a few microns are measured, and flow characteristics can subsequently be calculated. Measurements show that during the transition, a drastic change in hydrodynamics of the liquid film occurs that makes the liquid-vapor interface unstable. To define a criterion for the onset of this instability, a linear stability analysis is utilized by solving the Orr-Sommerfeld equations for the liquid and vapor phases using the perturbation theory. Experimentally measured film thickness and velocity are used in the stability formulations. Comparison with the experimental results suggest that the stability analysis is able to predict transition from elongated bubbles to semi-annular regime.