An experimental investigation on the air-side heat transfer and flow resistance of finned short oval tubes at different tube tilt angles

We experimentally studied the heat transfer and flow characteristics of finned oval tubes at different Reynolds numbers, fin spacing and tube orientation and compared results with correlations from literature. As assessment parameters we used the efficiency index, the performance evaluation criterion and the global performance criterion. For tubes in horizontal orientation (main flow direction perpendicular to the tube axis) we found an improvement of Nusselt number and a reduction of friction factor, when fin spacing increases. The efficiency index and the performance evaluation criterion improve with rising fin spacing and the global performance criterion remains almost constant. A substantial impact of tilt tube angle on Nusselt number and friction factor was observed. As the tube tilt angle rises from 0° to 40° Nusselt number and friction factor strongly increase. The horizontal tube orientation outperforms the tilted orientations in all performance parameters and at all Reynolds number. Thus, the performance is highest at 0° and worst at 40° tilt angle, since the increase in pressure drop dominates over the heat transfer enhancement. Based on the experimental outcome correlations between Nusselt number, friction factor and Reynolds number, fin spacing and tube tilt angle are recommended, which can be used to design finned oval tubes.