Nusselt number and friction factor correlation for solar air heater with staggered semi-frustum roughness on the absorber plate

This study numerically investigates the thermo-hydraulic performance of solar air heater equipped with staggered semi-frustum roughness over the absorber plate. Three-dimensional, steady-state CFD simulations were performed in ANSYS Fluent using the RNG k-ε turbulence model with enhanced wall treatment and the SIMPLE pressure-velocity coupling algorithm. Three-dimensional CFD simulations were performed for a wide range of geometric and operating parameters suitable for solar air heater. The semi-frustum roughness generates strong turbulence near the wall to promote the Nusselt number compared with a smooth duct. It is observed that the Nusselt number increases and the friction factor decreases with increasing Reynolds number. Nusselt number increases and friction factor decreases with rise in Reynolds number. The optimal performance achieved at Relative longitudinal roughness pitch of 6.67, Relative transverse roughness pitch of 3.33, and Relative exit radius of 0.0117. Regression analysis of the CFD data set yielded reliable correlations for predicting about 90 % Nu and f values, with deviations of ± 9 % and ± 6 %, respectively. The high thermo-hydraulic performance equal to 2.61, demonstrates the effectiveness of the staggered arrangement of the semi-frustum roughness elements as a potent tool for efficient solar air heater design.