Standard heat pipes will transfer heat equally in both directions. If the nominal condenser is hotter than the evaporator, then heat will flow in reverse, from the “condenser” to the “evaporator”. A diode heat pipe is used when it is necessary to prevent heat flow in the reverse direction. There are two basic types of diode heat pipes, Liquid Trap Diodes, and Vapor Trap Diodes. Note that a thermosyphon will also act as a diode heat pipe (the thermosyphon condenser is typically wickless, so liquid is not supplied to the nominal condenser).
Liquid Trap Diode
A Liquid Trap Diode has a wicked reservoir located at the evaporator end of the diode heat pipe. The wicks in the heat pipe and reservoir are designed so that they can’t communicate with each other. During normal operation, the heat pipe behaves like a standard heat pipe. Heat applied to the evaporator and reservoir causes liquid to evaporate. The vapor travels to the condenser, and capillary action in the heat pipe wick returns the condensate to the evaporator. Since the reservoir wick is not connected to the main wick, the reservoir quickly dries out, and becomes inactive.
Figure 1. Liquid Trap Diode Heat Pipe.
When the condenser becomes hotter than the evaporator/reservoir, the role of the evaporator and condenser are switched. Vapor evaporates from the hotter nominal condenser, and travels to the nominal evaporator and the reservoir, where it condenses. Since the reservoir wick doesn’t communicate with the heat pipe wick, any liquid that condenses in the reservoir can’t return to the nominal condenser. In a short time, all of the liquid is trapped in the reservoir. The main part of the pipe contains only vapor, so the only heat transfer from the condenser to the evaporator is by conduction through the heat pipe wall and wick, which has a much, much higher thermal resistance than the resistance during normal operation.
As soon as the evaporator and reservoir become hotter than the condenser, the liquid evaporates from the reservoir, and the heat pipe resumes normal operation.
Vapor Trap Diode
A Vapor Trap Diode is fabricated in a similar fashion to a Variable Conductance Heat Pipe (VCHP). In contrast to a Liquid Trap Diode with a reservoir wick at the end of the evaporator, the vapor trap diode has a gas reservoir at the end of the condenser. In addition, wick in the vapor trap diode is connected to the wick in the liquid trap diode.
During fabrication, the heat pipe is charged with the working fluid and a controlled amount of a Non-Condensable Gas (NCG). During normal operation, the flow of the working fluid vapor from the evaporator to the condenser sweeps the NCG into the reservoir, where it doesn’t interfere with the normal heat pipe operation.
When the condenser becomes hotter than the evaporator, the vapor flow is from the nominal condenser to the nominal evaporator. The NCG is dragged along with the flowing vapor. In a few minutes, it completely blocks the evaporator, greatly increasing the thermal resistivity of the heat pipe. In general, there is some heat transfer to the nominal adiabatic section. Heat is then conducted through the heat pipe walls to the evaporator.
Figure 2. Vapor Trap Diode Heat Pipe.
Recently, ACT fabricated an alkali metal diode heat pipe for a Venus Lander application, see Figure 3. Diode mode and heat pipe (normal) mode are shown in Figure 4. The system starts in heat pipe mode, with heat transferred from the evaporator to the condenser. The evaporator, adiabatic section, and the active part of the condenser are all at a nearly constant temperature of 525°C. When the condenser is heated, the entire condenser reaches a uniform temperature. The NCG travels to the evaporator and blocks it, dropping the evaporator temperature to ~ 200°C. Finally, when heat is again applied to the evaporator, the entire system recovers and starts transferring heat in the forward direction.
Figure 3. Top: Alkali Metal Diode Heat Pipe for Venus Lander Thermal Control. Bottom: Thermocouple Locations for the Diode Heat Pipe.
Figure 4. Diode Heat Pipe Thermal Profile is nearly isothermal in heat pipe mode. In diode mode, the evaporator is colder, since it is blocked with non-condensable gas.