Instability of liquid flows investigated for a proposed space station cooling system

The equipment on long-mission spacecraft generates heat, but with no heat conduction or convection in open space, only radiation can remove the heat. Droplet cooler-radiators are one prospective heat radiation cooling system; they atomize coolant into a flow of monodisperse droplets that efficiently remove heat and are then collected using an absorbing liquid sheet. Mathematicians assessed the theoretical linear instabilities of liquid sheets within vacuum and report their findings in Physics of Fluids.

Previous assessments of liquid sheet instabilities took place in ambient gas, with gaseous interactions causing the dominant liquid sheet instability mechanisms for Earth conditions. But these mechanisms vanish in vacuum and the role of other, minor instability mechanisms come into effect. The mathematicians applied a standard hydrodynamic approach to analyze the linear stability of the flow within vacuum. The main difference in the analysis was added complexity in finding steady flow, since the continuity and momentum equations are coupled with the temperature equation.

Calculations revealed that there were two main mechanisms of instability in linear liquid sheets in vacuum: viscosity stratification and thermocapillarity. These were caused by the temperature profile gradients across the sheet and along its surfaces that developed away from the flow origin. The change in temperature altered viscosity and surface tension, and lead to the appearance of unstable short- and long-wave perturbations, respectively.

“Our next step is to study nonlinear development of the liquid curtain,” said co-author Grigori Sisoev. This will help to determine whether or not the instability amplifications will allow for the use of liquid sheets in a droplet cooler-radiator within spacecraft parameters.

Source: “On the instability of a liquid sheet moving in vacuum,” by G. M. Sisoev, A. N. Osiptsov, and A. A. Koroteev, Physics of Fluids (2018). The article can be accessed at https://doi.org/10.1063/1.5021456 .