Experimental investigation on heat transfer and friction factor characteristics for novel hybrid roughness used in solar air heater

The influence of heat transfer and frictional characteristics of air that is turbulent and transitting rectangular channel textured by novel hybrid roughness (multi-V shaped ribs combined with hemispherical protrusion in V-fashion) has been studied experimentally. A uniform solar intensity of 1000 W/m2 is incident over the opposite side of the roughened plate. The relative roughness pitch, angle of attack, Reynolds number, hydraulic diameter, relative protrusion pitch, and relative gap width majorly influence the thermal and frictional characteristics of solar air heater. In this research paper, the effect of Reynolds number from 4281 to 18,250, relative protrusion pitch from 0.375 to 0.625 and relative gap width from 2 to 4 on heat transfer and friction factor on hybrid roughness geometry have been studied. The highest value of Nusselt number is 4.15 obtained at a relative protrusion pitch of 0.5, relative gap width of 2 compared to smooth duct and Reynolds number value of 18,250. By increasing the Reynolds number, the value of friction factor keeps on decreasing and highest value of friction factor of about 3.8 is obtained at a relative protrusion pitch of 0.375, Reynolds number value of 4,281 and relative gap width of 4 as compared to smooth duct. The results of the experiment shows that hybrid roughness performs thermally better than their solitary roughness geometries. Further, Nusselt number and friction factor correlations were also developed and show a very good match with the experimental results.