It is recognized that constant conductance heat pipes (CCHPs) offer the simplest two-phase capillary-driven heat transport solution. The CCHP needs a wick structure throughout to return liquid from the condenser to the evaporator. Grooved wicks are typically used in spacecraft CCHPs, and Variable Conductance Heat Pipes (VCHPs). These grooves have a very high permeability, allowing very long heat pipes (up to ~ 3.5 m) for operation in micro-gravity. One of their weaknesses is that they are suitable only for space, or for gravity aided sections of a heat pipe. The reason is that the same large pore size responsible for the high permeability results in low pumping capability. Grooved aluminum/ammonia heat pipes are designed to work with a 0.10 inch (0.25cm) adverse elevation (evaporator elevated above the condenser). This allows them to be tested on earth prior to insertion in a spacecraft. For heat pipes operating on the Moon or Mars, grooves can only be used in gravity-aided portions of the heat pipe. Another wick must be found for sections with adverse elevations, e.g. sintered powder, screen mesh, or metal foam wicks.
The hybrid CCHP concept as shown in Figure 1 is to develop heat pipes with 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, where the axial grooves in the adiabatic and condenser sections can transfer large amounts of power over long distances due to their high wick permeability and associated low liquid pressure drop.
Figure 1. Hybrid CCHPs for planetary surface and high heat flux applications: axial grooved adiabatic and condenser sections – screen mesh, metal foam or sintered evaporator wick.
The hybrid heat pipes are also used as a thermal link between Warm Electronics Box (WEB) electronics or avionics and the radiator for landers and rovers. These CCHPs have a sintered wick in the evaporator and a grooved wick in adiabatic and condenser regions, as shown in Figure 2. Planetary hybrid-wick CCHPs are shown in Figure 3.
Figure 2. Cut-away of a hybrid-wick VCHP for a Lunar Planetary Rover
Figure 3. Planetary hybrid-wick CCHPs.
Figure 4 shows a hybrid-wick copper-monel-water variable conductance heat pipe (VCHP) design that consists of a copper evaporator (with sintered wick inside), a monel adiabatic section and a condenser both with grooved wick inside and a NCG reservoir thermally and physically attached to the evaporator. The efficiency of the hybrid wick heat pipe for planetary surface in ground and in micro-gravity environment is demonstrated successfully. ACT has been working with NASA Marshall Space Flight Center and NASA Johnson Space Center to prepare a flight test of this VCHP.
Figure 4. Hybrid wick VCHP for a future micro-gravity experiment.
Figure 5 shows the thermal control testing results for the hybrid VCHP in ground. The “standard” condition is to reject 50 W into a 50°C sink with vapor at ~ 70°C. Figures 5 and 6 show that the vapor temperature varies from 69°C to 67°C over widely varying sink temperatures between 50 and – 4°C. Consequently, the VCHP can protect the payload against extremely low sink temperatures during survival.
Figure 5. Thermal control testing for the hybrid-wick VCHP.
Figure 6. Instantaneous temperature profile within the hybrid-wick VCHP during operation: (a) blue column – steady state during hot sink exposure (i.e. sink temperature = 50 °C) (b) red column – steady state at cold sink exposure (i.e. sink temperature = – 4 °C).
For more information, see Advanced Passive Thermal Experiment for Hybrid Variable Conductance Heat Pipes and HiK™ Plates on the International Space Station, Mohammed T. Ababneh, et al. 47th International Conference on Environmental Systems (ICES 2017), July 16-20, 2017, Charleston, South Carolina