Thermocapillary flow with evaporation and condensation and its effect on liquid retention in low-g fluid acquisition devices

The steady motion, thermal and free surface behavior of a volatile, wetting liquid in microgravity are studied using scaling and numerical techniques. The objective is to determine whether the thermocapillary and two-phase convection arising from thermodynamic non-equilibrium along the porous surfaces of spacecraft liquid acquisition devices could cause the retention failures observed with liquid hydrogen and heated vapor pressurant. The study also examines why these devices seem immune to retention loss when pressurized with heated helium or heated directly through the porous structure. The problem domain consists of a rectangular liquid cavity bounded by a saturated vapor on its upper free surface, and superheated or subcooled via isothermal boundaries on its other three sides. The mathematical model is based on dimensionless parameters representing surface tension, viscosity, thermocapillary stress, vapor recoil and surface non-equilibrium. Flowfield and temperature are calculated using Galerkin-based finite elements with mixed order interpolation functions, a semi-implicit Crank-Nicholson time marching scheme and a SIMPLER pressure/velocity algorithm. The meniscus is solved via a transformation of the normal stress balance.; Several heating and convection modes are examined. The basic state, which characterizes the nonlinear influence of contact angle and non-equilibrium on interfacial temperature and stress, indicates an increase in net thermocapillary force with reduced contact angle. This result is confirmed in analyses of single and combined-mode convection which show that highly wetting fluids exhibit large negative and positive dynamic pressure gradients towards the meniscus interline when superheated and subcooled, respectively. First-order and coupled assessments of meniscus deformation indicate that for highly wetting fluids in small pores, pressure and vapor recoil dictate surface morphology. With superheating, these two terms exert the same influence on curvature and promote surface retention. With subcooling, however, the pressure distribution produces a suction that degrades mechanical equilibrium of the surface. This result indicates that thermocapillary-induced deformation arising from subcooling and condensation is the likely cause for retention loss. In addition, increasing the level of non-equilibrium by reducing accommodation coefficient suppresses deformation and explains why this failure mode does not occur in instances of direct screen heating or pressurization with a heated inert gas.