Proceedings of the 26thNational and 4th International ISHMT-ASTFE Heat and Mass Transfer Conference December 17-20, 2021, IIT Madras, Chennai-600036, Tamil Nadu, India
A numerical and experimental based thermo-hydraulic performance analysis of a solar air heater having a forward NACA ribs roughness on the absorber plate
A solar air heater is a type of heat exchanger that converts solar energy into thermal energy through the use of an absorber plate. It is widely used to dry agriculture products, space heating, curing industrial products and dry construction components, wood seasoning, etc. However, one of the major disadvantages is the poor heat transfer coefficient, which is caused due to the creation of a laminar sub-layer between the absorber plate and the air, which reduces its efficiency. Artificial roughness (turbulators or ribs) on the underside of the absorber plate can increase the performance of the solar air heater by disrupting the laminar sub-layer. In the present investigation, NACA 0045, NACA 0050, and NACA 0070 profile ribs are used as artificial roughness elements on the absorber plate with varying values of roughness parameters such as relative roughness pitch (P/e = 2.86, 4, 4.44 and 10) and relative roughness height (e/D = 0.073, 0.081, 0.097, 0.108, 0.114 and 0.151) in the Reynolds number (Re) range of 6000-18000. The
solar air heater experimental setup was conceived and
developed in accordance with ASHRAE Standard 93-77 using the aspect ratio of 12. The result shows that the highest Nusselt number (Nu) was obtained with P/e = 4.44 and e/D = 0.073 among the different roughness variables. It led to a 68-129% rise in the Nusselt Number (Nu) and a 58-65% rise in friction factor (f) at various Reynolds numbers compared to the smooth duct. The numerical results were compared in terms of the thermo-hydraulic performance parameter (THPP = (Nur/Nus)/(fr/fs)1/3) to find the optimum roughness parameters. The highest value of THPP was determined to be 1.94, with the rib having a P/e of 4.44 and e/D of 0.073 at Re of 6000.