TAILIEUCHUNG - Heat Transfer Handbook part 39

Heat Transfer Handbook part 39. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | JOINT CONDUCTANCE ENHANCEMENT METHODS 371 the highest contact conductance and silver with the highest microhardness has the lowest contact conductance. The thermal conductivity of hie coating appeal s to play a secondary role. The unusual shape of the curves is a ttri bu t le to the fact that the assumed effective hardness curve shown in Fig. has three distinct zones. Moreover because the microhardness ofsilver is much closer to aluminum than are the microhardness of lead and tin respectively the transition from one region to the next is not abrupt in the silver-on-aluminum effective microhfrdness curve and this is reflected in the smoother contact conductance plot for the silver layer shown in Fig. . It should be noted that in the model that has been used the load is assumed to be uniformly applied over the apparent contact area. Ranking Metallic Coating Performance In his research on the effects of soft metallic foils on joint conductance Yovanovich 1972 proposed that the performance of various foil materials may be ranked according to the parameter k H using the properties of the foil maferlal. He showdd empirically that the higher the value ofthis parameter the greater was the improvement in the joint conductance over a bare joint. Following this thought Antonetti and Yovanovich 1983 proposed but did not prove experimentally that the performance of coaeed 8 can be i l d by See pander k H 0 93. Table shows the variation in this parameter as the layer thickness is increased. Table also suggests that even if see eITo ciIvc fafcsl il mdnefe o1 the layes-sufati cte combionitons being considered is not known the relative performance of coating materials can be estimated by assuming an infinitely thick coating where the effective microhfrdness is equal to the microhardness of hie . In this section we have shown how a thermomechanical model for coated substrates can be used to predict enhancement in thermal joint conductance. For the .

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