Boiling Limit Equation
When a low heat flux is applied to a heat pipe evaporator, the heat is conducted through the wick, and liquid vaporizes on the inner surface of the wick, into the vapor chamber. As the heat flux increases, the temperature difference across the wick increase linearly. The boiling limit or heat flux limit takes place when the transverse heat flux into the evaporator is enough to create nucleate boiling in the wick of the evaporator section. This generates vapor bubbles, which can become trapped in the wick, blocking the liquid coming back, which can result in evaporator wick dry-out. The boiling limit can be calculated by applying nucleate boiling theory:
QBoil Boiling limit, W
LEvap Evaporator length, m
keff Effective thermal conductivity of the liquid-wick combination, W/(m K)
T Vapor temperature, K
λ Latent heat of vaporization, J/kg
ρVapor Vapor density, kg/m3
rIDWall Inner radius of the heat pipe wall, m
rODVapor Radius of the vapor core radius, m
σ Surface tension, N/m
PCapillary Capillary pressure of the wick structure, Pa
rnucleate Nucleation site radius, which can be [2.54 x 10-5 m to 2.54 x 10-7 m] for conventional heat pipes.
Experimental Boiling Limits
The following are rules of thumb for the boiling limit in some typical heat pipe wicks:
- Sintered Wicks with Water: ~ 75 W/cm2
- Screen Wicks with Water: ~ 75 Wcm2
- Grooved, Aluminum Wicks with Ammonia: ~ 15 W/cm2
In special cases, wicks can be designed with much higher boiling limits. Figure 7 shows a specially designed copper/water vapor chamber wick, which can remove 750 W/cm2 over a 1 cm2 area, shown in the center of the figure.
Increasing the Boiling Limit with a Hybrid Wick Heat Pipe
Grooved Constant Conductance Heat Pipes (CCHPs) transport heat from a heat source to a heat sink with a very small temperature difference. Aluminum/ammonia CCHPs are used for transferring the thermal loads on-orbit due to their high wick permeability and associated low liquid pressure drop, resulting in the ability to transfer large amounts of power over long distances in micro-g environment. The maximum heat flux into a CCHP is set by the boiling limit, which is roughly 5 to 15 W/cm2 for typical grooves. In order to increase the heat flux limit to more than 50 W/cm2, ACT developed heat pipes with a hybrid wick that contains screen mesh, metal foam, or sintered evaporator wicks for the evaporator region, which can 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 as shown in Figure 8 .
For 0.5” OD aluminum/ammonia hybrid heat pipe, boiling and capillary limits are shown in Figure 9 as a function of the evaporator’s sintered wick thickness in the CCHPs performance as shown in Figure 9. The boiling limit can be improved by minimizing the wick thickness in the evaporator, but the capillary limit will be reduced. As the boiling limit is more sensitive and important than the capillary limit in hybrid CCHPs, the 0.06 in. (1.5 mm) wick should be selected.
 Ababneh, Mohammed T., Calin Tarau, and William G. Anderson. “Hybrid Heat Pipes for Planetary Surface and High Heat Flux Applications.” 45th International Conference on Environmental Systems, 2015.