INFLUENCE OF 2-PHASE THERMOCAPILLARY FLOW ON LIQUID RETENTION IN MICROSCOPIC PORES

An important feature of screened propellant acquisition devices is the retention capability or maximum maintainable pressure difference acro ss the porous barrier separating the liquid and gas. Previous experime nts with liquid hydrogen showed a marked reduction in retention when t he tank containing the device was pressurized with hydrogen vapor. The se tests, however, did not indicate any appreciable degradation in ret ention with helium pressurization or direct heating through the screen , The objective of this article is to determine if the thermocapillary convection arising from phase change in the microscopic pores of such screens could cause these disparities in performance. A numerical mod el of flow in a single pore suggests that the thermocapillary-induced gradient in liquid pressure along the surface can strongly affect surf ace morphology, In an evaporative environment, this gradient exerts a stabilizing influence on surface curvature, and preserves the momentum balance between the liquid and gas. With condensation, it causes a fo rce imbalance and a destabilizing suction in the middle of the pore th at reduces retention, Results also indicate that introducing an inert gas, such as helium, suppresses this retention loss mechanism by lower ing thermocapillary circulation and its associated interfacial pressur e gradient.