Jeff Diebold, R&D Engineer
An increasing number of applications such as satellites and planetary rovers have unprecedented thermal requirements due to the need to dissipate high power while operating in extreme thermal environments. Thermal management systems need to reject large heat loads into hot environments while also having high heat rejection turn-down ratios in order to minimize power needs during periods of extreme colds, such as the 14-day-long lunar night. Variable Conductance Heat Pipes (VCHPs) are capable of passively transporting large quantities of heat and provide high thermal turn-down ratios ideal for surviving extreme cold environments. For lunar and planetary surface applications, warm-reservoir VCHPs offer superior passive thermal control.
ACT recently developed a non-integrated warm-reservoir VCHP with a hybrid wick for lunar surface applications.
The non-integrated reservoir and the hybrid wick each solve a problem common to this application
- The non-integrated reservoir improves control over the distribution of working fluid and non-condensable gas (NCG) during periods of non-operation.
- The warm reservoir provides excellent thermal control.
- The hybrid wick allows the VCHP to operate effectively in both microgravity (transit) and on the surface in a gravity-aided orientation.
Warm-reservoir Variable Conductance Heat Pipes (VCHPs) offer superior passive thermal control compared to cold-reservoir VCHPs, but over time their performance can be degraded due to the migration of working fluid into the reservoir. This process displaces non-condensable gas (NCG) from the reservoir resulting in a higher nominal operation temperature for the heat pipe. If the reservoir is not integrated with the evaporator (separated from the evaporator), then during periods of non-operation an independent heater can be applied to the reservoir in order to purge the working fluid and restore normal operation.
HEAT PIPES MAXIMIZE POWER CAPABILITY
When a heat pipe must operate in microgravity (during transit) a grooved wick structure is generally required due to the high permeability of the grooves. On the lunar or planetary surface, it is generally desirable for the heat pipe to operate in a gravity-aided orientation to maximize its power carrying capability. In a gravity-aided orientation, grooved wicks have a tendency to exhibit large temperature spikes during startup due to the working fluid pooling at the bottom of the evaporator. The hybrid heat pipe, shown in Figure 1, utilizes a hybrid wick that contains screen mesh, metal foam or sintered evaporator wicks for the evaporator region, which can operate on planetary surfaces and sustain high heat fluxes and axial grooves in the adiabatic and condenser sections that can transfer large amounts of power over long distances due to their high wick permeability and associated low liquid pressure drop.
DESIGN AND TESTING FLIGHT HARDWARE
ACT designed and fabricated the non-integrated warm-reservoir VCHP with hybrid wick shown in Figure 2, as flight hardware a future Lunar Landers. The envelope material was aluminum and the working fluid was ammonia. The envelope and porous wick in the evaporator was 3D printed and were designed to interface with the grooved extrusion of the adiabatic and condenser sections. The hybrid wick will allow the VCHP to operate in microgravity and on the lunar surface in a gravity-aided orientation. The non-integrated reservoir of NCG contained an independent heater for purging the reservoir of working fluid.
“The hybrid wick will allow the VCHP to operate in microgravity and on the lunar surface in a gravity aided orientation.”
The VCHP for Astrobotic was designed to transfer 40W at a temperature of 55°C into a sink temperature of 40°C. The maximum operating power of 40W was well below the theoretical maximum for this pipe based on flooding and capillary limits. Figure 3 shows temperature measurements on the VCHP as a function of time. Temperatures corresponding to the evaporator, adiabatic, condenser, and reservoir are indicated. At approximately 4,000 seconds a steady-state operating temperature of 58.9°C was reached at the evaporator. This high temperature was due to ammonia migrating into the reservoir and displacing NCG increasing the pipe’s thermal resistance. Two purge tests were then carried out. During the purging process, the heat source at the evaporator was shut down, the sink temperature was decreased to 0°C and a small amount of power was applied to the reservoir heater. After the first purge, the steady-state operating temperature decreased to 56.5°C and after a subsequent purge operation, the design operating temperature of 55°C was achieved.
THERMAL CONTROL TEST RESULTS
This test successfully demonstrated the ability of the non-integrated warm-reservoir VCHP to purge the reservoir of working fluid in order to maintain the nominal operating conditions. Working fluid migrates into the reservoir during long periods during which the pipe is not operating, for example prior to launch. A purging process can be applied shortly before the pipe is required to operate in order to ensure the proper operation.
Figure 4 shows the results of a thermal control test of the non-integrated warm-reservoir VCHP. The system began at the nominal operating point of 40W, an evaporator temperature of approximately 55°C, and a chiller block set point of 40°C. While maintaining the power constant at 40W, the chiller block set point (the heat sink) was decreased to -100°C (a decrease of 140°C). During this time the evaporator temperature decreased only 16°C due to the increased thermal resistance provided by the expanding NCG. This highlights the excellent thermal control capability of the warm-reservoir VCHP during normal operation. The power supplied to the evaporator was then decreased to 1W and the evaporator achieved a new steady state of approximately -40°C, a reasonable survival temperature for many electronics. This represents a turndown ratio of approximately 185:1.