Hot shear flows promote instabilities of liquid wall films

Abstract

Liquid films are common in thermal and multiphase flow systems, while the mechanisms of their interfacial instability driven by shear, heat, and evaporation remain elusive. By combining experimental flow visualization with theoretical modeling, this study examines annular gas–liquid two-phase flows under non-isothermal conditions, in which a hot turbulent airflow ($T_g=290\sim 440K$, $Nu\sim O\left(10^2\right)$) heats a room-temperature water film ($T_l=290K$). Under a fully developed hot airflow and a uniform water film, we observe an enhancement in interfacial instability of three-dimensional ripple wave structures on the wall film: the axial wavelength of ripple waves reduces to approximately half compared to the isothermal case. This remarkable trend is attributed to the cooling effect of the water film on the gas stream, which leads to thinning of the gas-side velocity boundary layer and amplification of shear-induced instabilities visible as the shortened film wavelength. Despite the heating of the liquid film — which decreases surface tension and promotes evaporation — the cooling of the airflow dominates the mechanism for promoted instability. Incorporating the thinning gas-side boundary layer thickness, we formulate the ripple wavelengths under non-isothermal conditions through Kelvin–Helmholtz and Rayleigh–Taylor instability frameworks, well validated by experimental results. Moreover, large-scale disturbance waves are likely to emerge due to increased shear stress at elevated airflow temperatures, with their onset corresponding to a film Weber number of unity. The elucidated mechanism and derived formulation advance the understanding of gas–liquid dynamics with interfacial heat transfer.