Effects of cooling temperature on heat pipe evaporator performance using an ideal fluid mixture in microgravity

The effect of cooling temperature on heat pipe performance has generally received little consideration. In this paper, we studied the performance of a Constrained Vapor Bubble (CVB) heat pipe using a liquid mixture of 94 vol%-pentane and 6 vol%-isohexane at different cooling temperatures in the microgravity environment of the International Space Station (ISS). Using a one-dimensional (1-D) heat transfer model developed in our laboratory, the heat transfer coefficient of the evaporator section was calculated and shown to decrease with increasing cooler temperature. Interestingly, the decreasing trend was not the same across the cooler settings studied in the paper. This trend corresponded with the change in the temperature profile along the cuvette. When the cooling temperature went from 0 to 20 degrees C, the temperature of the cuvette decreased monotonically from the heater end to the cooler end and the heat transfer coefficient decreased slowly from 456 to 401 (W m(-2) K-1) (at a rate of 2.75 W m(-2) K-2). However, when the cooling temperature increased from 25 to 35 degrees C, a minimum point formed in the temperature profile, and the heat transfer coefficient dramatically decreased from 355 to 236 (W m(-2)) (at a rate of 11.9 W m(-2) K-2). A similar change in decreasing trend was observed in the pressure gradient and liquid velocity profile. The reduced heat pipe performance at high cooling temperatures was consistent with the reduced evaporation which was indicated by the decreasing internal heat transfer and the increasing liquid film thickness along the cuvette as seen in the surveillance images. The result obtained is important for future heat pipe design because we now have a better understanding of the working temperature ranges of these devices. (C) 2016 Elsevier Inc. All rights reserved.