Numerical and Experimental Investigations on Artificially Roughened Solar Air Heater

Abstract

Solar air heaters (SAH) implement the usage of irradiance from the sun and converts it into the useful thermal energy. The convective heat transfer coefficient of SAH is less due to formation of laminar sub layer in the vicinity of the absorber plate which leads to increase in temperature of the absorber plate and causes the thermal losses. It is recommended to break this laminar sub layer in order to boost the heat transfer at the surface. In the first phase of investigation, computational fluid dynamics (CFD) analysis of Solar air heater duct (aspect ratio W : H = 12) was carried out without and with roughness in the form of NACA 0030 (semi-airfoil) geometry. The roughness parameters considered were relative roughness pitch (p/e) and relative roughness height (e/Dh) taken in the wide range of Reynolds number varying from 6,000-18,000. The results obtained with smooth duct were compared with the analytical results and the deviation was found within 10-15%. Different airfoil roughness parameters led to 1.43 to 1.48 times increase in Nusselt number and 1.1 to 1.9 times increase in friction factor. Among various roughness parameters, the results obtained with NACA 0030 profile having relative roughness pitch (p/e) of 16.67 and relative roughness height (e/Dh) of 0.065 were found to be most favourable yielding 54.67% increase in Nusselt number and 14.43% increase in friction factor over a smooth duct. In the second phase, experimental setup was developed for testing of solar air heater as per ASHRAE Standard 93-77. The experimental investigations were carried out with smooth duct followed by artificially roughened duct. The artificial roughness was taken as the numerically optimized profile i.e. NACA 0030 profile with relative roughness pitch (p/e) of 16.67, relative roughness height (e/Dh) of 0.065 and chord length of 20 mm. The experiments were performed in the wide range of Reynolds number ranging from 6000 to 18,000. The maximum Nusselt number was found as 70.71 with roughened surface at Reynold number of 18,000. This shows 46.35 % improvement in Nusselt number and 38.91 % rise in friction factor in comparison with the smooth duct. The percentage deviation between numerical and experimental results in terms of Nusselt number and friction factor were found as 2-10% and 12-14% for the smooth and roughened ducts, which shows very good agreement of the numerical results.