Three-Dimensional Analysis of Mass and Heat Transfer in a Loop Heat Pipe Capillary Pump/Reservoir

Abstract

Loop Heat Pipes (LHPs) have replaced the conventional heat pipes as the primary heat transports for the spacecraft thermal control systems (TCS). For the most part, the LHP operational capability and versatility provide the engineers many design options to meet even the most challenging thermal requirements of today’s spacecraft. Like heat pipes, LHPs are also passive capillary devices having no mechanical moving part to wear out or break down. Hence they are reliable, durable, and more importantly maintenance-free for space applications. Utilizing the capillary action for fluid pumping, however, has certain drawbacks regarding the LHP performance verification for micro-gravity operation with ground tests. Specifically, the liquid transport capacity of the LHP secondary wick can be greatly affected by the liquid level in the reservoir. If the liquid level is above the secondary wick, the liquid hydrostatic pressure creates favorable conditions for the fluid flow from the reservoir to the primary wick. In other words, a “successful” 1-g LHP test program may not fully verify the functionality of the secondary wick, perhaps, masking potential problems in space. To counter the positive effects induced by the reservoir liquid level in 1-g tests, the capillary pump/reservoir assembly is usually tilted with the reservoir-end down. In the tilt configurations, the buoyancy force acting on the vapor bubbles may impede the delivery of liquid to the primary wick that would not occur otherwise in space. The main objective of this research is to determine whether there exists a gravity-neutral tilt level for a particular LHP design at which both favorable and adverse effects of gravity cancel themselves out. If it does exist, the LHP ground tests should be carried out in this tilt configuration to assess more accurately the liquid transport capability of the secondary wick.