Variable Conductance Heat Pipes (VCHPs) are similar to standard heat pipes, with the addition of a Non-Condensable Gas (NCG). When heat is supplied to the evaporator, the working fluid vaporizes, and travels to the condenser, where it condenses. As with a standard heat pipe, the liquid is drawn back to the evaporator by capillary action in the wick.
When the heat pipe is not operating, NCG and vapor co-exist throughout the vapor space. During VCHP operation, the NCG is swept toward the condenser end of the heat pipe by the flow of the vaporized working fluid. At high powers, all of the NCG is driven into the reservoir, and the condenser is fully open; see the video below. As the power is lowered, the vapor temperature drops slightly and since the system is saturated, the vapor pressure drops at the same time. This lower pressure allows the NCG to increase in volume, expanding out of the reservoir to block a portion of the condenser. This prevents the VCHP from overcooling the electronics, allowing them to be held within a specific temperature range.
The NCG reservoir and gas charge are sized so that the NCG volume blocks all of the condenser area at very low powers. This change in active condenser length maintains the evaporator and associated electronics temperature over large changes in power and evaporator sink conditions.
Many of the benefits of the VCHP are the same as the Standard Heat Pipe, while the only disadvantage over a standard heat pipe is a slight increase in design and fabrication costs. Benefits of VCHPs include:
- Passively responds to changes in evaporator power and condenser sink conditions, minimizing the changes in the evaporator and electronics temperatures.
- High Thermal Conductivity at high powers (10,000 to 100,000 W/m K)
- Low Thermal Conductivity when the condenser is blanketed with NCG, so that conduction through the envelope is the main heat transfer path.
- Low Cost
- Shock/Vibration tolerant
The applications for VCHPs include: