Heat Pipe Performance

Heat Pipe Assembly

Heat Pipe Assembly used to move heat away from the hot side of TEC to external heat sink.

The most important heat pipe design consideration is the amount of power a heat pipe is capable of transferring. Heat pipes can transfer much higher powers for a given temperature gradient than the best metallic conductors. However, if operating conditions cause the heat pipe to exceed its power capacity, the effective conductivity of the heat pipe will significantly reduce. Therefore, assuring heat pipes will meet your maximum system requirements is a critical aspect of design.

There are five primary heat pipe transport limitations that must be considered during design: viscous, sonic, capillary, entrainment/flooding and boiling. These limits are a function of many variables including operating temperature, wick selection and fluid properties. The most common limit for terrestrial applications is the capillary limit. ACT designed a heat pipe calculator to help customers design accordingly.

Table 1 below discusses each limit. Figure 1 shows a plot of several limits for a given heat pipe. For more information and calculation of these limits, contact an ACT engineer.

Figure 1. Heat pipe performance limits. (Top) A plot of the individual limits, showing that the entrainment and capillary limits are controlling over certain temperature ranges. (Bottom) The heat pipe performance limit curve, calculated by taking the lowest limit at each temperature.

Figure 1. Heat pipe performance limits. (Top) A plot of the individual limits, showing that the entrainment and capillary limits are controlling over certain temperature ranges. (Bottom) The heat pipe performance limit curve, calculated by taking the lowest limit at each temperature.

 

Table 1.  Heat Pipe and Thermosyphon Performance Limits.

Heat Pipe Limit Description Cause
Viscous (Vapor Pressure) Viscous forces prevent vapor flow within the heat pipe. Heat pipe operating near triple point with a very low vapor pressure – need to use a different working fluid.
Sonic Vapor flow reaches sonic velocity when leaving the evaporator, choking the flow. Too much power at lower operating temperature. Typically this is seen at start-up and will self-correct.
Heat Pipe Entrainment High velocity vapor flow strips liquid from the wick. Not enough vapor space for the given power requirement.  Occurs at low temperatures.
Thermosyphon Flooding High velocity vapor flow prevents liquid return in a gravity aided thermosyphon. Not enough vapor space for the given power requirement.  Occurs at low temperatures.
Capillary The capillary action of the wick structure cannot overcome gravitational, liquid, and vapor flow pressure drops. Power input too high. Wick structure not designed appropriately for power and orientation.
Boiling Boiling occurs in the wick which prevents liquid return High radial heat flux into the heat pipe evaporator.