Decrease in Thermal Contact Conductance and the Contact Pressure of Finned-Tube Heat Exchangers Assembled With Different Size Bullets

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.