Experimental investigations of heat transfer enhancement from rectangular duct roughened by hybrid ribs

Heat transfer and flow friction measurements were conducted for a fully developed turbulent flow in a rectangular duct with its bottom wall ribbed with three different rib geometries (semi-circular, rectangular and hybrid ribs of the two). The test duct was designed with an aspect ratio of 1:4 and its bottom wall was made of a rectangular removable brass plate of 320 mm by 120 mm. Brass ribs of 3 mm height, 6 mm width and 120 mm length were used. Experiments were carried out based on a Reynolds number in the range between 12,500 up to 86,500 and a range of rib pitch to height ratio from 6.6 to 53.3 for the three different rib geometries. It was found that the enhancement in heat transfer (Nu/Nus) was achieved in whole investigated cases, varied between 1.3 and 2.14 corresponding to a friction factor ratio (f/fs) of 1.8 and 4.2. Also, the hybrid ribs provide mostly higher values for the efficiency indices compared with those of the rectangular and semi-circular ribs cases. New correlations for (Nu/Nus), (f/fs) and (Nu/Nus)/(f/fs)1/3, were obtained as functions of Re and p/e ratio for all investigated geometries and configurations.