No abstract

The thermal contact conductance (TCC) at the mechanically bonded tube/fin interface of a heat exchanger may be controlled by varying the amount of initial expansion of the tube. However, in this case, the TCC also varies with the temperature because of the differential expansion between the tube and the fin. The objectives of the present study are to determine the improvement in TCC resulting from higher degrees of the tube expansion, to determine the variation in TCC with the maximum temperature, and to estimate the change in contact pressure with the temperature. This paper presents the results of heat transfer experiments on mechanically expanded finned-tube specimens. Experiments were conducted in an atmosphere of nitrogen. The results showed that the TCC is enhanced by increasing the degree of initial expansion. There is a practical limit, however, to the maximum expansion that can be attempted. For the direction of heat flow prevailing in the experiments, the TCC and the contact pressure of every specimen decreased with increasing temperature.

This paper presents the experimental results of thermal contact conductance, heat transfer, and interfacial temperature drop of finned tube heat exchanger test specimens. The results were based on the measured temperatures at several locations on the test specimen so that the thermal contact conductance could be directly determined. Each test specimen was assembled by mechanically expanding seven tubes into a single fin. The geometry of the specimens was based on a commonly used model of heat exchangers. The specimens included one bare tube (uncoated) specimen and three electroplated tube specimens. The plating materials were zinc, silver, and gold. The thickness of the plating in each case was 5 micrometers.Experiments have been conducted in both vacuum and nitrogen. The results show that the highly conductive plating material enhances the thermal contact conductance. The effect of interstitial gas such as nitrogen is beneficial for the heat transfer and the thermal contact conductance. The interfacial temperature drop alone does not fully reflect the efficiency of the heat exchanger.