Thermal performance analysis of artificially roughened solar air heater under turbulent pulsating flow with various wave shapes

This paper presents a numerical investigation of the operation of a solar air heater (SAH) with cylindrical shape ribs under turbulent pulsating flow based on thermal and frictional considerations. Turbulence and thermal natures of flow physics are modeled by taking the relevant governing equations into the account using RNG k-ϵ turbulence model and transient state Reynolds-Averaged Navier-Stokes (RANS) equations. The pulsating flow is modeled with four different wave shapes including rectangular, sinusoidal, triangular, and sawtooth functions. The influence of various factors such as wave shape, Reynolds number (Re), frequency, and amplitude of pulsating flow on the thermal and hydraulic performance of SAH has been evaluated. Results indicated that the SAH with ribbed absorber section under pulsed flow has a larger Nu number compared to the air heater with smooth duct under steady flow. Moreover, the pulsating flow with a sawtooth velocity profile leads to the greatest value of the thermo-hydraulic performance parameter (THPP). The growth of pulsation frequency up to 200 enhances the heat transfer coefficient of the air heater while for frequencies larger than 200, the thermal performance declines. In addition, the increase in pulsation amplitude has a negative effect on the THPP. It was proved that the increment of the Re number enhances the THPP. Up to 53.3% improvement of THPP can be achieved by applying pulsed flow with sawtooth wave shape and ribbed absorber section compared to air heat with smooth duct under steady flow.