Characterization of inertia- and shear-induced instabilities of liquid films through analysis of wave evolution, energy transfer, and force balance

Understanding the hydrodynamics of liquid films formed during microchannel flow boiling is essential for developing high-fidelity models that inform the design of high-performance heat sinks. To probe these dynamics, we performed two-phase adiabatic flow experiments spanning multiple channel sizes, fluids, and mass fluxes, enabling isolation of thin-film behavior and interrogation of instability mechanisms from a fluid-mechanics perspective. The experiments revealed two primary wave behaviors: solitary and periodic waves. Solitary waves, characterized by high velocity and amplitude, are driven by shear forces resulting from the velocity difference between the two phases. In contrast, periodic waves are slower, low-amplitude waves associated with inertial forces within the thin liquid film. Both wave types were found to be influenced by surface tension. A new long-wave evolution model, extending a previous wave growth model to incorporate inertia, successfully predicted the onset of periodic waves across various wavelengths. Additionally, an energy transfer model linked the growth of inertial and shear instabilities to the observed wave behaviors. These models demonstrated that unstable films grow due to both inertial and shear instabilities. New predictive metrics were developed based on force balance. For periodic waves, these metrics include the Weber (We) and Bond (Bo) numbers, while the Richardson (Ri) and Bo numbers are used for solitary waves. The study offers valuable insights into the interplay between inertia, shear forces, and surface tension in two-phase microchannel flow, providing improved analytical tools for predicting wave behaviors in liquid films.