Nusselt number and friction factor correlations for forced convective type counter flow solar air heater having discrete multi V shaped and staggered rib roughness on both sides of the absorber plate

In this work, the results of an experimental study on heat transfer and friction factor in a counter flow double pass solar air heater (DPSAH) duct with discrete multi V-shaped and staggered rib roughness on two broad surfaces of the heated plate have been investigated. The investigation covers a wide range of Reynolds number (Re) from 2000 to 20,000, relative staggered rib pitch (p′/p) from 0.2 to 0.8, relative staggered rib size (r/e) from 1 to 4 and relative roughness width (W/w) from 5 to 8. The optimum values of flow and geometrical parameters of roughness have been attained and explained in detail. For the Nusselt number (Nu), the maximum increase of 4.52 times to the corresponding value of smooth double pass duct has been achieved, however it has also been seen that the friction factor (f) enhanced by 3.13 folds as compared to smooth one. The rib parameters corresponding to maximum increase in Nu and f are r/e = 3.5, p′/p = 0.6 and W/w = 7. Further, correlations for Nu and f have also been developed on the basis of experimental data.