Optimum thermo-hydraulic performance of solar air heater provided with cubical roughness on the absorber surface

Abstract

Heat transfer augmentation in solar air heaters (SAHs) can be achieved economically and conveniently using artificial roughness elements on the absorber surface. Cubical roughness elements used in this investigation produces better heat transfer characteristics. However, this enhancement is particularly accompanied by increased pumping power due to increased friction. This study presents the results of the experiments carried out to determine the optimum design and operating conditions which gives a better thermohydraulic performance. Efficiency index (η) is often used as thermohydraulic performance parameter which is evaluated using correlation η = ((Nuav/Nus)/(fav/fs)). Experiments are conducted within the limits of Reynolds number (Re) from 3000 to 8000, relative roughness pitch (p/e) of 10 to 20 and relative roughness gap (w/e) of 4 to 8. Relative roughness height (e/Dh) of 0.06 and aspect ratio (W/H) of 5 is maintained constants. Substantial enhancement in the Nusselt number (59.1–72.7%) accompanied by a large increase in the friction factor (138.7–179.4%) over SAH with smooth absorber plate is achieved. Optimum thermohydraulic performance is obtained for lower values of the Reynolds number, relative roughness pitch (p/e) value 20 and relative roughness gap (w/e) of 8.