[[The]] constrained vapor bubble heat pipe experiment: understanding the role of interfacial flow

Abstract

The performance of a heat pipe is affected by various limits and capillary limit being one of them. This limit has been previously studied in detail and the transport equations describing it are well established. Interfacial forces can play a major role in the performance of a heat pipe in the absence of gravity. Interfacial forces such as Marangoni forces can drive the liquid from the heater end to the cooler end, reducing the effectiveness of the heat pipe by literally drying out the hot end, and this is known as the Dryout limitation. The transparent setup of the CVB shows that the increasing power input leads to the formation of a thick coating layer near the heater end. This is opposite to what is predicted from the literature. This phenomena is observed in both 30 mm and 40 mm CVB modules. An additional phenomenon is observed in the 40 mm module which shows that this limitation region stops growing down the axial position of the heat pipe at high power inputs because of the inability of Marangoni flow to further offset the capillary flow. Also, fringes are observed in the 40 mm CVB module near the heater end which looks like the formation of a jet on the flat surface of the cuvette near the heater end.

Extensive 3D models can be developed to validate the findings of the CVB and can be used to design a better heat pipe. The heat pipes developed validating the data from the CVB experiment can be a big step towards a long term manned mission in space.

An analysis of the temperature profiles for the 30 and 40 mm CVB modules in conjunction with vapor-liquid interface mapping showed that the internal system of the CVB could be separated into a number of discrete operation zones depending on the dominant mode of heat transfer. The analysis showed that the internal radiative exchange was found to be more significant than originally anticipated. Significant Marangoni forces exist which drives the liquid away from the heater end. The competition between the capillary and Marangoni forces lead to the formation of a coating flow or interfacial flow region near the heater end. This coating flow formation is a limitation to the CVB heat pipe and is called the Flooding limitation. As the power input is increased, the flooding limitation is first observed at 1.2 W in 30 mm modules and at 0.7 W in 40 mm modules. As the power input is increased to 3W, this limitation region grows to a maximum length of around 7 mm for 30 mm modules and 12 mm for 40 mm modules. Moreover, the growth of this region is arrested in 40 mm CVB modules as the length of this region stays constant from 2.2 W – 3 W. The capillary flow the cold end is balanced by the Marangoni flow from the hot end, leading to the arresting behavior of the limitation region. Identical signatures are observed for both dryout and flooding limitation which perhaps might have to lead to the misdiagnosis in opaque heat pipes. An additional phenomena in 40 mm CVB modules, where the instability in the meniscus and disjoining pressure leads to the condensation near the heater end, and the condensed liquid on the flat surface is driven by Marangoni forces away from the heater end in the form of a micro jet. The CVB experiment in microgravity environment of the International Space Station has provided a good understanding in the first performance limitation of a heat pipe.

Heat pipes are critical components in cooling of systems present in space, and are present in laptops, Hubble Space Telescope, Mars Rovers etc. Heat pipes work on the principle of heat change, i.e., they utilize the latent heat of the working fluid to transport the heat. The wickless heat pipe 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 (CVB) is a transparent, wickless heat pipe system tested on the International Space Station (ISS) where the Bond number (ratio of gravitational force to surface force) is small maximizing the effects of capillarity. The CVB is made up of a square, fused silica spectrophotometer cuvette which is then partially filled with pentane or a mixture of pentane and isohexane (94% pentane, 6% isohexane) as the working fluid. Along with the temperature and pressure measurements, the image data obtained from the experiment using the interferometry based system contained with the station’s Light Microscopy Module (LMM), can be used to determine the spatial details of the curvature of the liquid vapor interface. The motivation of the project is to study the role of interfacial phenomena in microgravity, and not to improve the heat pipe performance.