Onset of liquid films instability in microchannel flow boiling

Abstract

Microchannel flow boiling is becoming increasingly important in many applications, yet its modeling has remained a challenge due to a lack of mechanistic fluid flow models, particularly for thin liquid films. In a recent study, we determined the liquid film thickness and velocity in 300-µm-wide microchannels and used the results to calculate the shear stress at the liquid-vapor interface. A graph of the shear stress versus the liquid film thickness delineated transition to wavy-annular flow regime. Here, we have extended our studies to different channel sizes and fluids, from low to high surface tension, using a combination of flow boiling and adiabatic test studies. The results show that the onset of films instability is a function of the shear stress at the liquid-vapor interface, consistent with the Kelvin-Helmholtz (K-H) instability. Various criteria for the onset of K-H instability are evaluated. The Richardson number (Ri) as an indicator of the onset of film instability is shown to decrease greatly for thin films in microchannels and is demonstrated to be dependent on surface tension such that Bond number, Bo/Ri for different fluids are relatively close. However, this criterion does not accurately predict the onset of instability of relatively thinner films. Further analysis suggests that Taitel and Dukler's (1976) criterion can accurately predict instability of adiabatic films. However, liquid films in boiling become unstable at a significantly lower interfacial shear stresses relative to Taitel and Dukler's prediction. Additional forces and perturbation mechanisms such as evaporative momentum effects, acoustics of nucleate boiling, and local temperature-induced surface tension variations could account for deviations relative to adiabatic films.