Comparison of External Surface Heat Transfer Coefficients for Circular and Oval Tubes

Experimental measurements were carried out to compare the air-side heat transfer coefficients of three oval tubes (axis ratio 2, 3, and 4) with those of an equivalent circular tube (o. d. 18 mm). The tubes were tested as single tubes in a cross-flow of air. The range of the investigated Reynolds numbers Re _D was between 1000 and 11000. The effects of the area blockage and the free stream turbulence were taken into consideration in the evaluation of the thermal performance. The measurement results indicate that Nusselt numbers Nu _D for the tested circular and oval tubes are close at the lower range of the tested Reynolds numbers (Re _D < 4000) corresponding to an air velocity < 4 ms ^?1 in this work, which is the air velocity for most air conditioning applications. For Re _D > 4000, the Nu _D for the circular tube are higher than those for the oval tubes, and the Nu _D for the oval tubes decrease with the increase of the axis ratio.