Stability of Thermocapillary Flow in High-Prandtl-Number Liquid Bridges Exposed to a Coaxial Gas Stream

The stability of thermocapillary flow in a liquid bridge made from 5cSt silicone oil (Pr = 68) is computed numerically. To improve the numerical model as compared to the standard approach, we consider the flow in the liquid bridge fully coupled to the flow in the ambient gas, temperature-dependent material parameters, and a dynamically deforming liquid–gas interface. We address the full two-phase-flow problem of interest for the space experiment JEREMI and investigate the effect of a steady axisymmetric coaxial gas flow which is imposed at the inlet of the annular gap between the liquid bridge and the outer confining cylinder. Under zero-gravity the flow is primarily driven by the imposed temperature gradient with viscous stresses from the gas phase being small. However, the heat transfer between liquid and the gas, and thus the temperature fields are strongly affected by the forced flow in the gas phase. As a result the stability of the steady axisymmetric flow depends sensitively on the flow direction and the temperature of the gas. If the temperature of the gas is identical to that of the support rod of the liquid bridge a gas stream opposing the thermocapillary stresses strongly destabilizes the basic flow. In a co-flow configuration the basic state is stabilized. Curves of neutral Reynolds numbers as functions of the strength of the annular gas flow are discussed for two aspect ratios of the liquid bridge.