Experimental and numerical investigation of forced convective heat transfer in solar air heater with thin ribs

Ribs on the underside of the absorber plate of solar air heater enhance the convective heat transfer rate of the air flowing through it. Several experimental and numerical investigations, with different rib geometry and flow conditions, have been carried out. This paper is one such effort of experimental and numerical investigations of solar air heater. It presents the effect of rib arrangements on the heat transfer and frictional loss characteristics of a rib roughened solar air heater. The geometrical and flow conditions of the present work circumscribe with aspect ratio (W/H) of 10 for the duct, blockage ratio (e/H) is 0.1, relative roughness height (e/Dh) of 0.055, relative roughness pitch (P/e) of 10, angle of attack (α) of 90° and Reynolds number (Re) from 4000 to 16000. Two thin transverse continuous and two truncated ribs are used for one pitch length. Four different rib arrangements are considered for the heat transfer, friction factor and thermo hydraulic performance parameter (THPP) investigations. A three dimensional (3D) numerical simulation is carried out with the commercial CFD code ANSYS FLUENT ver. 16.2 and RNG k-ε turbulence model. The enhanced wall treatment as wall function is used, keeping the Y+ criteria < 1. The numerical results are in good agreement with the experimental results. Three important outcomes are observed in the present investigation. Rib arrangement 1, with mid ribs placed at 3.3% and 6.67% truncation from the sidewalls gives the highest heat transfer enhancement. Arrangement 3, with mid ribs placed at 5% truncation from the side walls gives the best overall THPP results and arrangement 4, with two transverse continuous ribs in between the truncated ribs shows the highest average friction factor.