Heat transfer and friction factor correlations for multiple arc shape roughness elements on the absorber plate used in solar air heaters

Incorporating artificial roughness on the underside of the absorber plate is an established effective and economic method for improving the thermal performance of a solar air heater. The present experimental investigation is undertaken to analyze the effect of various geometrical parameters for multiple arc shape ribs on heat transfer and friction characteristics. The experiments carried out encompasses Reynolds number (Re) in the range of 2200-22,000, relative roughness height (e/D) values of 0.018-0.045, arc angle (α) range of 30-75° and relative roughness width (W/w) ranges from 1 to 7 and relative roughness pitch (p/e) range of 4-16. Thorough experimentation has been conducted to accumulate data on heat transfer and friction characteristics of a rectangular duct roughened solar air heater. Utilizing these experimental data, correlations for Nusselt number and friction factor are also developed.