Thermal performance and stability experiments of a 1.75-meter-long helium pulsating heat pipe

Abstract

Experimental studies of the performance and thermal stability of extended-length helium pulsating heat pipes (PHPs) were performed at the University of Wisconsin – Madison. Multiple distinct testing procedures were carried out on helium PHPs with an adiabatic length of 1.75 meters, such as progressively increasing evaporator heat load, randomized heat load, and extended period tests. The results of these tests show that despite the stochastic nature of the fluid flow and persisting non-equilibrium conditions, long-distance helium PHPs can maintain stable and steady operation with excellent thermal performance. The progressively increasing heat load tests serve as a baseline for the 1.75-meter PHP’s performance, where the maximum effective conductivity observed was 443.4 kW/m-K at 570 mW heat load and a fill ratio of 56.42%. Furthermore, the randomized heat load test show that, with an optimized fill ratio, PHP performance is not strongly dependent on previous operating conditions and that normal operation can be swiftly recovered from a dry-out condition. The extended period test shows a stable, pseudo-steady operation for over 50 hours with no performance degradation or temperature deviations observed. The impact of PHP build quality is also discussed.