Correlation for Nusselt Number and Friction Factor for Solar Air Heater Having Absorber Roughened by Chamfered-square Elements

摘要

The idea of introducing artificial roughness on absorber plate to improve the thermal performance of a solar air heater is very common now days. The technique uses the concept of providing artificial roughness by imbedded element in the absorber plate of the heater. Diagonally chamfered cuboids have been used as roughness element in the current study. A numerical study is performed to investigate the enhancement of the thermo-hydraulic performance of the heater for the various affecting parameters such as Relative Roughness Pitch (Transverse and Longitudinal) of 6 to 14, cross section of cuboids from 8 mm to 14 mm and relative roughness height of 0.44 to 0.088. The range of Reynolds number used in this study was 5000 to 22500. During the study a constant heat flux of 1000 W/m~2 on the absorber plate was considered. The standard k-ε turbulence model with enhanced wall treatment of the ANSYS FLUENT software has been used for numerical computation and to handle the flow turbulence. The Nusselt number and the average friction factor are determined for different values of relative roughness pitch and cross sectional areas of the roughness element. Using calculated computational data correlations for Nusselt number as well as friction factor have been developed as a function of flow and roughness parameters for solar air heaters. The predicted and computational values of Nusselt number and friction factor show a good agreement.