Heat pipes using high performance sintered powder metal wicks are known to operate satisfactorily with nucleate boiling occurring inside the evaporator wick. An analytical model has been developed which predicts the effects of the resulting two phase flow on evaporator wick temperature drop and dryout. The accuracy of the model has been confirmed by comparison of calculated and measured performance of a copper/water heat pipe operating against gravity with the evaporator elevated 3.5 to 30 cm above the condenser at power ranging from 2000 to 6300 watts. This paper describes the analytical model, the heat pipe experiment, and the comparison of calculated with experimental results.
The model uses a finite element approach and accounts for wick pore size distributions, wick thermal conductivity, evaporation from the surface of the wick exposed to vapor, nucleate boiling within the wick, normal and tangential vapor and liquid flow and associated pressure drops, and permeability; all as a function of vapor fraction within the wick. Several features of the analytical model are of particular significance since, to our knowledge, these features have not been reported previously as follows:
1. The model accounts for the variation of wick properties through the wick thickness. This permits modeling vapor void fraction effects normal to the wick in the direction of vapor flow as well as in the plane of the wick in the direction of liquid flow. The model typically predicts a gradient in the vapor void fraction through the thickness of the wick with the highest concentration of vapor at the wall/wick interface;
2. The model accounts for both evaporation at the wick to vapor interface as well as nucleate boiling within the wick;
3. The model accounts for wick thermal conductivity as a function of void fraction and predicts temperature drops through the wick between the heat pipe wall and vapor. It accounts for the portion of the thermal energy evaporating liquid from the wick surface as well as the remainder of the thermal energy dissipated by nucleate boiling within the wick.