Performance Testing of Loop Heat Pipe in a 1kW Heat Transport/Rejection System

Abstract

The use of Loop Heat Pipes (LHPs) has become a common practice in today’s spacecraft thermal control systems especially for Central Thermal Bus heat transports. For the most part, the LHP operational capability and versatility provide the engineers many design options to meet even the most challenging thermal requirements. Like conventional 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. A typical “full-size” LHP is capable of transporting more than 1kW over a distance of 5 meters in 6mm-diameter lines. Utilizing the capillary action for fluid pumping, however, has certain drawbacks regarding the LHP performance verification in ground testing for micro-gravity operation. 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 head 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. The U.S. Naval Research Laboratory was tasked to evaluate the performance of a LHP built by Thermacore Inc. of Lancaster, PA for potential space utilization. The evaluation testing focused on (i) start-up behaviors, (ii) the transport capacities of the primary and secondary wicks under adverse gravity-induced conditions, (iii) the overall thermal conductance and (iv) the transient response with respect to changes in the power input and sink temperature. The testing and evaluation process are still on-going. In this paper, description of the LHP system, test setup, plans/procedures and preliminary results will be presented.