Experimental investigation of the conduit slope impact on mixed convection in solar collectors

An experimental investigation on the mixed convection heat transfer of air flowing in circular conduits of a solar collector is conducted. The test rig consists of three different copper pipes with a length of 70 cm. The pipes are heated at constant heat flux. The effect of the pipe inclination angle (theta = 0 degrees, 60 degrees, and 90 degrees), length-to-diameter ratio (L/D = 13.8, 18.4, and 36.7), and heat rate (Q = 5, 11, 19.6, 30, 40.5, and 57.8 W) is studied in the present study. The influences of Richardson number (Ri), inclination angle, and length-to-diameter ratio on the average Nusselt number are examined as well. The results show that the Nusselt number (Nu) is improved by 41.5% as the inclination angle is changed from 0 degrees to 90 degrees at L/D = 13.8. Further, as the length-to-diameter ratio was reduced from 36.7 to 13.8 at 90 degrees, up to 54.9% enhancement in heat transfer is found. Also, it is seen that by increasing the heat rate from 5 W to 57.8 W at 90 degrees and L/D = 13.8, the exchange in heat is improved by 44%. Three empirical correlations were proposed for each studied pipe inclination angle. A good agreement was found between the measured Nu and the calculated Nu from the proposed correlations.