Testing of A Loop Heat Pipe Subjected to Variable Accelerating Forces, Part 1: Start-up

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 first part of the experimental study, i.e. the effects of an accelerating force on the LHP start-up. Tests were conducted by varying the heat load to the evaporator, condenser sink temperature, and LHP orientation relative to the direction of the accelerating force. Test results indicate that the wall superheat at the onset of nucleate boiling was random and ranged from 0.3 °C to 2.2 °C in all tests. The superheat appeared to be independent of the heat load and/or the accelerating force. On the other hand, the temperature overshoot seemed to be dependent more on the heat load than the accelerating force. For heat load of 50W or more, there is virtually no temperature overshoot. For the heat load of 5W, a temperature overshoot of a few degrees was always observed. At 25W, the temperature overshoot ranged from 0 °C to 45 °C. In spite of all these differences, the LHP started successfully in all tests.