Testing of A Loop Heat Pipe Subjected to Variable Accelerating Forces, Part 2: Temperature Stability

Loop Heat Pipes (LHPs) are being considered for cooling of military combat vehicles and spinning spacecraft. In these applications, it is important to understand the effect of an accelerating force on the performance of LHPs. In order to investigate such an effect, a miniature LHP was installed on a spin table and subjected to variable accelerating forces by spinning the table at different angular speeds. Several patterns of accelerating forces were applied, i.e. continuous spin at different speeds and periodic spin at different speeds and frequencies. The resulting centrifugal accelerations ranged from 1.2 g's to 4.8 g's. This paper presents the second part of the experimental study, i.e. the effect of an accelerating force on the LHP operating temperature. It has been known that the LHP operating temperature under a stationary condition is a function of the evaporator power and the condenser sink temperature when the compensation temperature is not actively controlled. Effects due to accelerating force are expected to be superimposed upon those due to the heat load and sink temperature. Results of this test program indicate that any change in the accelerating force will change fluid distribution in the evaporator, condenser and compensation chamber, leading to a change in the LHP operating temperature. However, the effect is not universal, and is a function of other operating conditions. Given a sufficient time, a constant acceleration may result in an increase or decrease of the operating temperature, while a periodic spin will lead to a quasi-steady operating temperature. In addition, an accelerating force may lead to a temperature hysteresis and/or a change in the temperature oscillation. In spite of all these effects, the LHP continued to operate without any problems in all tests.