Parametric Studies on Artificially Roughened Solar Air Heater

Abstract

Solar air heater is subjected to low heat transfer coefficient on account of formation of laminar sub layer between absorber plate and air. The objective of the present study is to increase the heat transfer coefficient with the use of artificial roughness (turbulators) which may break the laminar sub layer. Initially, numerical analysis of solar air heater duct was carried out without and with roughness in the form of various reverse NACA profile (i.e. fluid flows from trailing edge to leading edge). To optimize the NACA profile parametric studies was done with various relative roughness pitch (p/e) and relative roughness height (e/D) in the wide range of Reynolds number from 6000 to 18000. NACA 0040 profile with relative roughness pitch (p/e) of 5 and relative roughness height (e/D) of 0.065 (i.e. C15 plate) were found to be most favorable among the all roughness variations yielding 194.58 % increase in Nusselt number and 62.10 % increase in friction factor over a smooth duct for Reynolds number of 6000. In the second phase, experimental investigations were carried out on solar air heater with smooth and roughened ducts. Among the various roughened plates, the optimum performance was found with C15 plate. It led to 97.56 % improvement in Nusselt number and 61.03 % rise in friction factor in comparison with the smooth duct. The maximum Nusselt number was found as 104.45 at Reynolds number of 18,000. The thermo-hydraulic performance parameter (Non dimensional ratio of ‘Nu’ and ‘f’) for this case was found as 2.53 at Reynolds number 6000 which is highest among all the roughened geometries available in the literature. From the experimental investigations, an empirical correlation was developed for ‘Nu’ and ‘f’ in terms of ‘Re’ (range: 6000-18000) and e/D (range: 0.043-0.087) and it has predicted ‘Nu’ and ‘f’ within ±3% range of the experimental results. The deviation between numerical and experimental results in terms of ‘Nu’ and ‘f’ were found as 0.39-3.60 % and 2.53-6.86 % in various cases. This shows very good agreement of the two approaches.