Heat transfer and friction factor correlations of solar air heater ducts artificially roughened with discrete V-down ribs

The heat and fluid flow characteristics of rectangular duct having its one broad wall heated and roughened with periodic 'discrete V-down rib' are experimentally investigated. Reynolds number (Re) has been varied from 3000-15000 with relative gap width (g/e) and relative gap position (d/w) range of 0.5-2.0 and 0.20-0.80 respectively. The respective variation in relative roughness pitch (P/e), angle of attack (alpha) and relative roughness height (e/D-h) have been 4-12, 30 degrees-75 degrees and 0.015-0.043. The effect of roughness parameters on Nusselt number (Nu) and friction factor (f) has been determined and the results obtained were compared with those of smooth duct. The maximum increase in Nu and f over that of smooth duct was 3.04 and 3.11 folds respectively. The rib parameters corresponding to maximum increase in Nu and f were d/w = 0.65, g/e = 1.0, P/e = 8.0, alpha = 60 degrees and e/D-h = 0.043. Correlations for the Nu and f in terms of Re and rib parameters have been developed. (C) 2011 Elsevier Ltd. All rights reserved.