An Apparatus to Measure the Maximum Heat Transfer Rate in Heat Pipes

Abstract

In an earlier investigation of evaporative heat transfer in low-temperature heat pipes, it was concluded, that nucleate boiling was the dominating mechanism for heat removal from the wall. Experimental data for vaporization of water and freon in screen wicks was shown to agree well with a boiling correlation. Data used in the correlation were obtained from Abhat and Seban for a water/screen wick vaporization experiment and from Ponnappan and Mahefkey for a copper/water double-wall artery heat pipe. During verification testing of this heat pipe, heat input to the evaporator was provided by electrical resistance heating. In such cases the evaporator wall heat flux is the independent parameter and the temperature drop across the evaporator is measured as a function of applied heat flux. The temperature drop is given by T{sub w} {minus} T{sub s}, where T{sub w} is the wall temperature and the T{sub s} is saturated vapor temperature. The criterion used to signify that the maximum heat flux had been reached was an observed evaporator temperature drop larger than 10C. In low-temperature heat pipes, before the boiling limit is reached, dryout will usually occur due to the inability of the wick to pump sufficient liquid to the evaporator (capillarymore » limit). The capillary limit Q{sub max} is a limitation on total heat input rate Q for a given heat pipe. Temperatures in the heat pipe evaporator increase rapidly if the maximum heat input rate is maintained, and the rise in internal pressure may cause failure of the container material. In this paper, a unique apparatus is used to control the wall temperature of the heat pipe evaporator independently. With this method, an increase in T{sub w} above that corresponding to Q{sub max} can only cause a decrease in Q. Although it is more difficult to control the temperature than the heat flux, this method allows a more accurate determination of Q{sub max} and precludes the possibility of runaway temperature.« less