Development of Nusselt number and friction factor correlations for double-pass solar air heater duct

Abstract

A solar air heater exhibits a low heat transfer rate due to the formation of a laminar thermal boundary layer across the absorber plate. Furthermore, the artificial roughness contributes to the intensification of turbulence along the absorber plate, which interrupts and breaks the thermal boundary layer. Keeping in this view, a three-dimensional simulation is performed to investigate the heat transfer and frictional characteristics of a double-pass solar air heater with semicircular tubes and perforated baffles as artificial roughness. This numerical investigation is performed for the parameters considered in the study: tube amplitude ratios (a/H) from 0.2 to 0.5, baffle height ratios (e/H) from 0.4 to 0.8, open area ratios (β) from 13 % to 30 % and Reynolds number (Re) from 3000 to 19000. The experimental testing rig is designed to validate numerical results corresponding to e/H of 0.4, an open area ratio of 30 %, a/H of 0.3, and Reynolds number varying from 3000 to 15000. Numerical results are found to be in good agreement with the experimental results, with a maximum absolute percentage error of 1.82 % and 5.28 % for the Nusselt number (Nur) and friction factor (fr), respectively. The maximum Nusselt number is observed as 274.29 at Reynolds number of 19,000 and e/H of 0.8; however, the maximum friction factor is observed as 0.6290 at Reynolds number of 3000 and e/H of 0.8 at a constant value of open area ratio of 30 % and a/H of 0.3. Within the range parameters studied, the maximum THPP value is reported to be 2.51 at Reynolds number of 3000, β of 30 %, e/H of 0.4, and a/H of 0.3. Further, correlations for the Nusselt number and friction factor in the function of a/H, e/H, open area ratio, and Reynolds number have been derived based on simulation results.