Numerical investigation of convection in tubes with aluminium and carbon steel fins: evaluating the assumption of convective heat transfer coefficient as that for the tube without fins and relating physical processes with the optimum spacing between fins

Abstract

Finned tubes exist in diverse geometric configurations, usually the convective heat transfer coefficient is unknown and approximated as that for a geometry similar to the actual one. This paper presents a numerical investigation about the convective heat transfer in a horizontal finned tube. Ten geometric configurations were considered, which differed on the distance between fins, and two materials were analysed for the fins: aluminium and carbon steel. Eleven different values were considered for the temperature difference between the base of the tube and air. Therefore, a total of one hundred and ten different conditions were studied, for each material. The flow regime was laminar. The analysis showed that approximating the convective heat transfer of the finned tube as that for a tube without fins, for which correlations are available in the literature, can lead to significant errors. The maximum difference verified was for the spacing between fins equal to 2 mm, for which the convective heat transfer coefficient was about seven times lower than that for the tube without fins. For spacings equal to 6 mm and 8 mm, associated to the maximum heat transfer rate, the convective heat transfer coefficient for the tube without fins was about 30% to 50% higher than that for the finned tube. The common assumption of uniform fins surface temperature was also evaluated for the two considered materials the fins were made of. The results showed that for the steel fins the approximation leads to an error of about 10%, while for aluminium it is only about 3%. Results that allow a better understanding of the physical phenomena related to the occurrence of the optimum spacing between fins, which maximize the heat transfer, are also presented and analysed.