A thermal contact conductance correlation for mechanically expanded finned tube heat exchangers

Tests were conducted in dry vacuum environment, on eight mechanically expanded finned tube heat exchangers (with 3/8 inch outer diameter tubes) with different geometries including variation in fin thickness, fin collar length, and net interference. One of the eight heat exchangers was also tested under wet conditions. A mathematical model was used to calculate the thermal contact conductance from the experimental data. An error analysis was performed to estimate the range of error in determining the thermal contact conductance. A correlation of the dry contact conductance in terms of fin number (actual fin collar length in contact with the tubes), fin thickness, net interference and unexpanded tube diameter was obtained. The thermal resistances, the contact conductance, the error band of the contact conductance, and the correlation coefficients were calculated on a microcomputer. A scanning electron microscope was used to examine the contact region between the flat aluminum fin and the copper tube. Comparisons of the contact conductance were then made between heat exchangers having different fin numbers (20, 14, and 6 fins per inch), and also between heat exchangers having different expansion. For a given fin number, it was observed that the contact conductance increases as the expansion increases. Also for a given radial expansion (net interference), it was found that the contact conductance increases with increasing fin number --Abstract, page ii