Stabilized Loop Heat Pipe Architecture for Reliable Operation Under High-Power Transients

This paper analyzes test data obtained by the authors and corresponding results of numerical simulation of transients in a high-power loop heat pipe (LHP) with a small-diameter condenser bypass. Thermal-fluid oscillations (TFOs) and evaporator partial dryouts were observed for the same high-power LHP without a bypass operating at several intermediate power levels after heat load steps under certain conditions, although the LHP operated stably during most of the tests with the power levels ranging from 72 to 1000 W. Adding a small-diameter tubing condenser bypass to the LHP entirely eliminated all oscillations and partial dryouts, making its operation smooth for all power levels and most demanding transients. Comparison of steady-state and transient test data obtained with and without bypass showed only minor detrimental effects of the condenser bypass on the LHP performance. The small-diameter tubing with a low-rate vapor flow bypassing the condenser is a reliable proactive approach to prevent TFOs and partial dryouts, as it effectively reduces variations of the temperature and flow rate of the liquid returning to the LHP reservoir. Such stabilized LHP architecture, augmented with a condenser bypass, can be especially useful for high-power LHPs used in spacecraft thermal control systems with large temporal changes in environmental conditions.