Numerical study on the natural convection from horizontal finned tubes with small and large fin temperature variations

Abstract

This study numerically investigates the thermal performance of horizontal tube with circular fins under natural convection. The natural convection from horizontal finned-tube heat sink has been understood in an average sense since most previous studies were experiment-based. The present study reveals the flow and heat transfer characteristics in details with numerical analysis. The parameters include fin diameter (D), fin spacing (b), fin material, and tube diameter (d), under a fixed fin thickness of 0.5 mm and temperature difference of 50 K between the tube and the ambience. The distributions of fin temperature and heat transfer coefficients are calculated for stainless-steel (SS) and aluminum (Al) fins under various conditions. For Al fins with small fin temperature variations, decreasing tube diameter leads to higher heat transfer rates due to the larger fin surface areas; but for SS fins with large fin temperature variations, increasing tube diameter enlarges heat transfer rates. On large SS fins, negative local heat transfer coefficients may exist above the tube as the hot plume generated by the horizontal tube transfers heat back to the fin. A larger d leads to a slightly larger optimum fin spacing bopt. In addition, various existing empirical bopt formulae are examined for their reliability with respect to different D/d ratios and Al and SS fins, respectively with small and large fin temperature variations.