Boiling Limit

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:

boiling limit equation

 

 

where:

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.

Figure 7. Specially designed vapor chamber wicks can remove 2000 W over 4 cm2, and 750 W over 1 cm2.

Figure 1. Specially designed vapor chamber wicks can remove 2000 W over 4 cm2, and 750 W over 1 cm2.

 

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 [1].

 

Hybrid CCHPs: axial grooved adiabatic and condenser sections - screen mesh or sintered evaporator wick.

Figure 2. Hybrid CCHPs: axial grooved adiabatic and condenser sections – screen mesh or sintered evaporator wick.

 

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.

 

The effect of the evaporator wick thickness on the CCHPs performance as a function of temperature.

Figure 3. The effect of the evaporator wick thickness on the CCHPs performance as a function of temperature.

 

Return to Heat Pipe Limits

 

 

[1] 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.

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