Enhancing the heat transfer performance of compact heat exchangers by minimizing the contact resistance between fins and tubes

摘要

Thermal contact resistance or, its reciprocal, thermal contact conductance is an important parameter in a wide range of thermal phenomena. It plays a significant role in heat transfer applications such as electronic packaging and nuclear reactors. This parameter also appears in fin-tube heat exchangers; however, it is often neglected in the performance calculations of heat exchangers.This thesis project explores the means by which the heat transfer performance of a finned tube heat exchanger may be enhanced.It includes experimental studies and finite element analysis investigating the effects of expansion bullets and coatings on thethermal contact conductance.An apparatus has been designed and fabricated for the experimental part of the work. A finite element model established the fintubeconfiguration to be used in the design and manufacture of the apparatus. The apparatus was specially made for measuringthermal contact conductance directly in a finned-tube heat exchanger both in vacuum and in gaseous environment. Theexperiments were done on hexagon shaped specimens with a single fin connecting seven tubes. Sixteen type-T thermocoupleshave been used to measure temperatures at several locations on the specimen.A full-scale quarter-fin model was chosen for a second finite element analysis. The model simulates the actual specimen andpredicts the temperatures. The finite element analyses have been used to validate the experimental results.The experimental results from the bare contact specimens, assembled with different sizes of expansion bullet, show that whilehigher expansions enhance the thermal contact conductance, the effect of interstitial gas such as nitrogen is beneficial for allspecimens expanded with the 9.42 mm size bullet.Applying a coating material with high thermal conductivity is also an effective way to enhance the thermal contact conductance.The results show that the highly conductive plating materials, such as zinc, tin, silver and gold, enhance the thermal contactconductance. The presence of interstitial gas such as nitrogen also results in higher heat transfer rates and higher thermal contactconductance compared to those obtained in vacuum.