Computational and Experimental Study of Solar Air Heater With Various Duct Cross-Sections and Artificial Roughness

Abstract

Thermo-hydraulic performance and exergetic efficiency of solar air heater (SAH) with various duct cross-sections and artificial roughness have been investigated using numerical and experimental methodology. The RNG k- model with enhanced wall treatment is employed to study the turbulent flow behavior. Validation of the CFD results for smooth and artificially roughened SAH (triangular duct and duct with semi- cylindrical sidewalls) with theoretical correlations and experimental data indicates reasonable accuracy. In triangular duct SAH, the performance of inclined ribs and V-ribs have been studied e/D) and pitch (P/e). It is observed that V-ribs in triangular duct provides a maximum thermo- hydraulic performance parameter (THPP) of 2.01 with a 23% enhancement in exergetic efficiency compared to smooth SAH. Further, the performance of triangular duct SAH with inclined ribs in an indirect type solar dryer is studied. Dryer with ribbed triangular duct SAH exhibits a 60.4% and 55% reduction in moisture ratio for food samples robusta and nendran, respectively, for the same drying time compared to a dryer with a ribbed rectangular duct SAH. In addition, the design enhances the drying characteristics with 93.3% increase in average diffusivity coefficient for banana food samples. CFD analysis of SAH design with semi-cylindrical sidewalls and continuous W-baffles provides THPP in the range of 1.70 to 2.27. Maximum enhancement in thermal and exergetic efficiency is obtained as 40.7% and 95.4%, respectively, relative to conventional SAH at Re = 5000. Based on the optimum results obtained from CFD, an experimental setup for SAH with semi-cylindrical sidewalls and multiple discrete inclined baffles is fabricated. The experimental results indicate that THPP is further enhanced for discrete inclined baffles with the gap at the trailing apex, with a peak value of 2.69. This design has higher collector efficiency (55 to 70%) compared to ribbed rectangular SAH design exhibiting 30 to 55%. Further, the design exhibits higher exergetic efficiency owing to lower exergy losses and higher collector efficiency. Maximum exergetic efficiency of 2.2% is obtained at lower Re, higher than that obtained for rectangular duct SAH with a similar kind of artificial roughness. In addition, at low Re, this SAH design has a higher coefficient of performance (COP) than conventional SAH designs. Hence, a SAH design having lower number of sharp corners and artificial roughness capable of generating multiple secondary flow can enhance the heat transfer rate with higher thermo-hydraulic performance.