Using Thermoelectric Converters for Loop Heat Pipe Operating Temperature Control

This paper describes thermal vacuum testing of a miniature loop heat pipe (MLHP) with two evaporators and two condensers designed for future small systems applications requiring low mass, low power and compactness. Each evaporator contains a wick with an outer diameter of 6.4 mm, and each has its own integral compensation chamber (CC). Multiple evaporators provide flexibility for placement of instruments that need to be maintained at the same temperature, and facilitate heat load sharing among instruments, resulting in a reduced auxiliary heater power requirement. A flow regulator is used to regulate heat dissipations among all condensers, thus providing flexibility for placement of radiators on the spacecraft. A thermoelectric converter (TEC) is attached to each CC for operating temperature control and enhancement of start-up success. Tests performed include start-up, power cycle, sink temperature cycle, high power and low power operation, heat load sharing, and operating temperature control. The MLHP demonstrated excellent performance in the thermal vacuum environment. The loop started successfully and operated stably under various evaporator heat loads and condenser sink temperatures. The TECs were able to maintain the loop operating temperature within ±0.5K of the desired set point temperature at all power levels and all sink temperatures. The un-powered evaporator would automatically share heat from the other powered evaporator. The CC control heater power was reduced by more than 50 percent when a TEC was used instead of conventional electrical heaters. The flow regulator was able to regulate the heat dissipation among the radiators and prevent vapor from flowing into the liquid line.

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Effect of Sink Temperature on the Stability of the Pressure-Controlled Loop Heat Pipe

Journal of Heat Transfer ◽

10.1115/1.4043956 ◽

2019 ◽

Vol 141 (9) ◽

Author(s):

Wukchul Joung ◽

Joohyun Lee

Keyword(s):

Heat Pipe ◽

Temperature Control ◽

Control Technique ◽

Loop Heat Pipe ◽

Heat Leak ◽

Start Up ◽

Recent Emergence ◽

Compensation Chamber ◽

The Stability ◽

Pressure Controlled

Recently, a novel temperature control technique utilizing the unique thermohydraulic operating principles of the pressure-controlled loop heat pipes (PCLHPs) was proposed and proved its effectiveness, by which a faster and more stable temperature control was possible by means of the pressure control. However, due to its recent emergence, the proposed hydraulic temperature control technique has not been fully characterized in terms of the various operating parameters including the sink temperature. In this work, the effect of the sink temperature on the loop heat pipe (LHP)-based hydraulic temperature control was investigated to improve the stability of the proposed technique. Start-up characteristics and transient responses of the operating temperatures to different pressure steps and sink temperatures were examined. From the test results, it was found that there was a minimum sink temperature, which ensured a steady-state operation after the start-up and a stable hydraulic temperature control with the increasing pressure steps, due to the unstable balance between the heat leak and the liquid subcooling in the compensation chamber at low sink temperatures. In addition, the range of the stable hydraulic temperature control was extended with the increasing coolant temperature due to the decreased heat leak, which resulted in the increased pressure difference between the evaporator and the compensation chamber. Therefore, it was found and suggested that for a stable hydraulic temperature control in an extended range, it was necessary to operate the PCLHP at higher sink temperatures than the low limit.

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