ENHANCEMENT OF THERMAL EFFICIENCY BY USING HEMISPHERICAL PROTRUSIONS ON SOLAR AIR HEATERS

Abstract

Artificial roughness elements in the form of hemispherical protruded impressions were designed and developed on a 14-gauge commercially available aluminum sheet with the help of appropriate dies and punches. This aluminum sheet was used as an absorber for a solar air heater. Artificial roughness augments turbulence and heat transfer to the air flowing over. This aluminum solar energy absorber with protrusions had been tested for flow Reylonds number of 3500−15,000 under actual outdoor conditions in OPJU campus. The thermal efficiency of a roughened solar air heater was found to be a strong function of air mass flow rate. Maximum thermal efficiencies were observed as 80%, 70%, and 60% at mass flow rates of 1 = 0.0417, 2 = 0.0313, and 3 = 0.0208 kg/s, respectively. The collector heat removal factor (FR) varied from 0.42 to 0.50, the collector efficiency factor (F') varied from 0.45 to 0.75, and the heat removal factor based on outlet air (F0) varied from 0.55 to 0.80 at corresponding air mass flow rate. For a given value of p/e (12), the Reynolds number and increased value of e/D (0.045−0.055), there was an augmentation in the Nusselt number 2.67 and 3.62 with respect to a smooth collector. The Nusselt number was maximum for a p/e value of 12 and it decreased in either sides of 12. The values of F0Ul for roughened and smooth collectors were computed to be 10 W/m2·K and 3 W/m2·K and F0(τα) to be 0.837 and 0.377, respectively. For e/D equal to 0.055, 0.045, and 0.035 and constant p/e = 12, the values of F0, Ul, and F0(ατ) are 10, 09, and 08 W/m2·K and 0.884, 0.746, and 0.664, respectively for this solar absorber. There was a ±5% variation of experimental and predicted Nusselt number based on the modified correlation developed from experimental data analysis.