Study of the vapor-gas front of a variable conductance thermosyphon using advanced optical techniques

Abstract

The influence of noncondensable gases on the operation of a thermosyphon was studied experimentally. The operating characteristics were studied while varying the power input, gas quantity, and gas type. The results were compared with existing models to determine how well the operating characteristics could be predicted. The experimental thermosyphon was constructed of copper with a planar design and was designed to operate with water as a working fluid at vapor temperatures ranging from 10OC-1000C. The power input was varied from 40 watts to 140 watts. The noncondensable gases studied were helium, nitrogen, and carbon dioxide. The results show the importance of the noncondensable gas type and working fluid combinations. At the lower operating temperatures, the heavier noncondensable gases did not remain in the upper portion of the thermosyphon as helium did. Instead the heavier noncondensable gases randomly circulated throughout the thermosyphon. The effects of increasing gas loads were clearly seen for the helium case. As gas loads increased condenser temperatures decreased and evaporator temperatures increased. Also as power increased a decrease in the noncondensable gas blocked length shut-off to condensation occurred for a similar gas load. Of greatest importance was the effect of wall conductivity on the vapor-gas interface. The effect of high wall conductivity on the vapor-gas front was to spread this front into a longer diffuse vapor-gas front. Although models do exist which incorporate the wall conductivity, these models were only used for low conductivity materials such as stainless steel and do not accurately predict the gas quantity for copper heat pipes at the low power inputs used in this study.