Variable Conductance Heat Pipes (VCHPs) use a Non-Condensable Gas (NCG) to vary the active condenser length. NCG can also be used to aid in start-up from and shut-down to a frozen state. Such VCHPs are also known as gas-loaded, or Gas Charged Heat Pipes (GCHPs).
When the condenser of a conventional heat pipe is lowered to a temperature below freezing, the heat pipe temperature drops, and fluid starts to freeze in the condenser. Freezing is not a concern for normal-length copper/water heat pipes with a wick. In fact, ACT routinely builds copper/water heat pipe assemblies and HiK™ plates that can survive hundreds or thousands of freeze/thaw cycles.
Freezing is a concern for long water thermosyphons, as well as long grooved ammonia heat pipes. In these heat pipes, the majority of the working fluid can freeze in the condenser, preventing restart when the condenser remains below freezing. During long time periods, sublimation in water heat pipes can continue to remove water from the evaporator to the condenser.
In a gas-loaded heat pipe, the NCG allows the heat pipe to restart, by blocking the condenser as the temperature is lowered. As the temperature drops and approaches the freezing temperature, the vapor pressure within the VCHP decreases. The non-condensable gas expands to maintain pressure equilibrium at the vapor-gas interface, reducing the active condenser length in the process. This has the effect of both restricting the flow of vapor to the inactive portion of the condenser and maintaining the active condenser section at a higher temperature than an equivalent heat pipe at the same power level. As the working fluid temperature reaches a limit specified by design, the entire condenser section is occupied by non-condensable gas. Further decrease in temperature beyond this shutoff point will result in the gas expanding into the adiabatic section. When freezing occurs, the working fluid is restricted to the evaporator and adiabatic regions. Sublimation from the evaporator to the condenser is suppressed by the NCG blocking the condenser.
A 2-meter-long titanium/water gravity-aided heat pipe (thermosyphon) was fabricated to demonstrate the ability of a gas-charged heat pipe to withstand multiple freeze/thaw cycles; see Figure 1. 15 thermocouples were attached to the heat pipe:
- 3 at the evaporator, 2” spacing from the end (TC0, TC1, TC2)
- 11 along the condenser, 6” spacing from the end of the adiabatic section (TC3 to TC 13)
- 1 at the NCG reservoir, 6” from the end of the condenser section (TC14)
Since these heat pipes are intended for use in a Lunar application, the condenser was allowed to freeze for 15 days, which is a period longer than a lunar night. Power was first increased to fully activate the condenser; then the cooler was quickly cooled again. After 15 days, power was applied to the VCHP to restart it. As shown in Figure 2, the evaporator temperature increases quickly as the evaporator is thawed. As the temperature of the evaporator increases, the vapor pressure increases, so the blocked length in the condenser gradually reduces. As the front passes each thermocouple, the temperature at that position quickly increases. After about 400 minutes, the entire heat pipe was operating, and then the power was turned off at the conclusion of the test.