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

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.

The Role of the Process and Design Variables in Improving the Performance of Heat Exchanger Tube Expansion

“…The effect of the expansion ball curvature on the expansion force and tube's inner groove deformation has also been studied [13,14]. Elliptical, hemispherical, and conical expansion ball shapes influence the expansion force during expansion and the tube thickness after expansion [15], and the size and operating interface temperatures affect the heat transfer efficiency [16].…”

Experimental and numerical characterization of a mechanical expansion process for thin-walled tubes

“…A study of the lubrication conditions is therefore needed in order to retain limited driving forces, which could results in lower process energy consumption, and a reduction of the gap between the fin and the expanded tube. This should also improve the heat exchange efficiency, as shown by Madhusudana and Cheng (2007).…”