Cryogenic Nitrogen Thermosyphon Developed and Characterized

A two-phase nitrogen thermosyphon was developed at the NASA Glenn Research Center to efficiently integrate a cryocooler into an insulated liquid-nitrogen-filled tank as part of an advanced development zero-boiloff (ZBO) ground test. NASA Marshall Space Flight Center's (MSFC) Advanced Space Transportation Program supported this test to improve the performance of in-space propulsion system concepts. Recent studies (ref. 1) have shown significant mass reductions and other advantages when incorporating active cooling in a ZBO configuration, enabling consideration of high-performing cryogenic propellants for long-duration applications in space. Active cooling was integrated via a thermosyphon, made of copper, 42 in. (1070 mm) long with an inner diameter of 0.436 in. (11 mm). It was charged with nitrogen to 225 psia at 300 K, which provided a fill ratio of 15 percent. The temperatures and heat flows through the thermosyphon were monitored during the startup phase of the ZBO test, and steady-state tests were conducted over a range of increasing and decreasing heat flows. The results also were compared with the initial design calculations and with results for a similar thermosyphon. They show that the thermal resistance of the thermosyphon was one-half of that expected--0.2 K/W at a heat flow of 8.0 W. The design calculations also showed that this resistance can be made relatively constant over a wider range of heat flows by making the ratio of evaporator area to condenser area 3:1. The better-than-expected results will translate into reduced integration loss for the ZBO concept.