Thermophysical characteristics of a wickless heat pipe in microgravity – Constrained vapor bubble experiment

Wickless heat pipes are being studied for use in cooling critical components of spacecraft. The wickless design is thought to produce a simpler and lighter heat transfer system than heat pipes containing wicks or mechanically driven systems. The constrained vapor bubble experiment (CVB) is one such system tested on the International Space Station where the Bond Number (ratio of gravitational force to surface force) is small maximizing the affects of capillarity. The CVB is essentially a square, fused silica spectrophotometer cuvette evacuated and then partially filled with pentane as the working fluid. Along with temperature and pressure measurements, the two-dimensional thickness profile of the menisci formed at the corners of the quartz cuvette was determined using an interferometry based system contained with the station's Light Microscopy Module (LMM). The CVB can be viewed as a hollow fin and its behavior analyzed using a simple, one-dimensional heat transfer model. That model, coupled with the visual observation of the vapor-liquid distribution inside the fin, provides an enhanced understanding of what the measured temperature and pressure profiles represent and the heat transfer mechanisms controlling the operation of the device. The internal heat transfer processes were found to be very complicated, multi-dimensional, and greatly dependent on internal and external radiative heat transfer. Internal radiative exchange was found to be more significant than originally anticipated as was the effect Marangoni forces on internal convective heat transfer. An analysis of the temperature profiles in conjunction with vapor-liquid interface mapping showed that the system could be separated into a number of discrete operation zones depending on the dominant mode of heat transfer. (C) 2014 Elsevier Ltd. All rights reserved.