Experimental study and thermal modeling of the constrained vapor bubble heat pipe operation in a convection-free environment under the influence of gravity.

摘要

The absence of mechanical pumps in heat-transfer devices operating under interfacial free-energy gradients to control the fluid flow, renders them simple and light. They have often been proposed as reliable cooling systems in a microgravity environment. Currently, one such heat exchanger called the Constrained Vapor Bubble (CVB) is being viewed as a prototype of an enhanced microgravity heat transfer device, designed to operate in the vacuum of space. The testing and study of the device is being designed as a μg fluid physics experiment aboard the International Space Station, due for launch in 2005. The underlying aim of the space experiment is the study of thermal and fluid flow characteristics of the device operating as a wickless heat pipe.; The objective of the work presented in this dissertation was to study the operation of the heat pipe in a convection-free environment on ground, to mimic the radiative dominant ambience of space. Our aim was to evaluate the 3D-temperature field in the cell, investigate the heat transfer regimes within the cell and analyze the performance and evaporation enhancement of the device. Additionally, another objective was to develop and test experimental procedures to be used in microgravity.; The Constrained Vapor Bubble heat pipe is much smaller (3 mm x 3 mm x 39 mm I.D.) than a conventional heat pipe but larger than the micro heat pipes. The usual wicking structure is replaced by the sharp corners of the cuvette that act as arteries to circulate the working fluid. The operation involves evaporation of the liquid at one end, condensation of the vapors at the other with capillarity aiding in the re-circulation of the liquid back to the heater end.; A three-dimensional thermal model to study steady state solutions to heat transfer of the CVB cell under the influence of conduction, convection and radiation is presented. The study aims to calibrate the radiative properties of the cell for future use with heat pipe operations on ground and in space. Experiments were conducted with the heat pipe inside a vacuum chamber. Experimental temperature profile data and results of the model were used to obtain the external radiative heat transfer coefficients and the heat transfer coefficients of the liquid within the heat pipe. Results indicate that radiative exchanges will play a significant role in the operation of the heat pipe under microgravity condition. The resistance to heat transfer within the heat pipe arises from the vapor-solid interface. For ground-based operation of the vertical heat pipe in a radiative environment, the dry-out region initially lengthens rapidly with power input. It then reaches its “transport limit”, when any further increase in the power input does not cause any additional lengthening of the dry-out zone. The performance and the evaporation enhancement of the device are driven by the evaporation/condensation cycle when the dry-out zone is small. Under microgravity condition, the CVB heat pipe is expected to be capable of driving between 60 to 150 times more power (without dry-out) than on ground.