Experimental investigation on heat transfer and friction factor for an inclined spherical ball roughened solar air heater

The purpose of present investigation is to determine the results of heat transfer and frictional losses for an inclined spherical ball roughened solar air heater. Experimentation was conducted under actual outdoor condition at the test rig designed and fabricated at the terrace of the Mechanical Engineering Department, NIT Jamshedpur, India. To show the effect of ever-changing environmental variables like solar radiation, wind velocity, ambient temperature, etc, on the heat transfer results, the readings were noted for every 15 minutes in the experimental hours 10:00 to 15:00 hours. The present paper deals with the experimental results drafted in the form of rise in Nusselt number (Nu) and friction factor (f) for spherical ball roughened solar air heater (SAH) over those of smooth ones. Flow and roughness geometrical parameters have been varied as relative roughness pitch (p/e) 9-18, relative roughness height (e/Dh) 0.024-0.040, ball’s height to diameter ratio (e/db) 0.5-2, angle of attack (α) 35-75 and Reynolds no (Re) 2500-18500. Parametric analysis has also been made and the effects of these parameters on Nu and f characteristics have been shown. This article reveals that maximum augmentation in ‘Nu’ & ‘f’ for varying ‘p/e’, ‘e/Dh’ & ‘e/db’ and ‘α’ was respectively found to be of the order of 2.1 to 3.54 times, 1.87 to 3.21 times and 2.89 to 3.27 & 1.74 to 3.56 times for ‘Nu’ and 0.84 to 1.79 times, 1.46 to 1.91 times, 1.67 to 2.34 times & 1.21 to 2.67 times for ‘f’ in compared to non-roughened duct. The optimum roughness parameters found under present investigation is p/e = 15, e/Dh = 0.036, e/db = 1 and α = 55. The findings of this research may serve as a criteria to determine thermal and thermohydraulic performance of such roughened solar air heaters and to understand the magnitude of useful heat gain choosing such roughness geometry. © 2018 Lavoisier.