Natural convection heat transfer from horizontal annular finned tubes based on modified Rayleigh

The modification of the Rayleigh, Ra, number has been studied in the past. These studies have shown that the physical interpretation of the modified Rayleigh number which is commonly used is questionable. Moreover, although most of the available correlations work well, their behaviours in extremes (low or high Ra) are not correct where only a few correlations are available in low Ra. Therefore, a numerical simulation of heat transfer from annular circular finned tubes was conducted to present a comprehensive correlation for the modified Rayleigh number. As shown in this study, the flow forms a circular path around the tube. Based on this fact, a new modified Rayleigh number definition and correlation are proposed, which are valid for Nusselt numbers from 1 to 20 000. This range of Ra provides a complete picture of natural convection over circular annular finned tubes, especially at relatively low Ra. Finally, the end behaviours of the proposed correlation were compared with those of existing correlations.