Experimental study of the natural convection heat transfer performance for finned oval tubes at different tube tilt angles

Abstract

The natural convection heat transfer of finned oval tubes was studied for different tube tilt angles (0° to 40°), fin spacing (6mm to 16mm) and Rayleigh numbers (11,000 to 130,000). Fin efficiency was determined by temperature measurements along the fin surface and temperature gradient calculations. Nusselt number and volumetric heat flux density were chosen as assessment parameters for the thermal performance. A comparison of the experimental data with correlations from literature was made and good agreement was found. Furthermore, the uncertainty of the measurements was evaluated. In the horizontal tube orientation (0°) the Nusselt number increases with fin spacing. However, the fin efficiency and the volumetric heat flux density reduce. The tilt angle of the longitudinal tube axis was found to have an essential impact on the thermal performance, in particular when the fin spacing is high. For the higher fin spacing values the horizontal orientation gives highest Nusselt number and volumetric heat flux density. At tube tilt angle of 40° the thermal performance becomes lowest for all fin spacing values. When the fin spacing is low, the effect of tube tilt angle is minor. From the experimental results correlations between Nusselt number, Rayleigh number, fin spacing and tube tilt angle are proposed to assist the future design of heat exchanger with tilted finned oval tubes.