Energy and exergy analysis of solar air heater with trapezoidal ribs based absorber: A comparative analysis

The performance of a solar air heater (SAH) is computationally investigated in this study using COMSOL multiphysics software. The present study aims to enhance the performance of smooth SAH with the help of artificial roughness. To achieve the goal, trapezoidal ribs above (Model 1) and below (Model 2) the absorber were fixed. The thermo-hydraulic characteristics of smooth SAH were compared with the thermo-hydraulic performances of Model 1 and Model 2. The computations were made by varying the heat flux between 600 and 1000 W/m(2), Reynolds number from 4000 to 7000, relative rib height (h/H) from 0.2 to 0.7, and a constant pitch of 15 mm. The maximum enhancement in Nusselt number (Nu) compared to smooth duct was 5.54 times for Model 1 and 5.42 times for Model 2 at a h/H = 0.7 at Re = 7000. The maximum increase in friction factor was 36.97 times more than the smooth duct. The thermal performance factor (TPF) at 1000 W/m(2) ranged from 1.29 to 1.72 for Model 1 and 1.27 to 1.68 for Model 2. It is found that trapezoidal ribs of Model 1 with h/H of 0.6 has delivered a maximum TPF of 1.72 at Re = 7000. The optimized relative rib height was found to be 0.6. The maximum exergy efficiency of 10.40% was obtained for Model 1 with h/H of 0.6 and at 1000 W/m(2).