EFFECT OF STAGGERED ROUGHNESS ELEMENTS ON THE PERFORMANCE OF A SOLAR. AIR HEATER

Abstract

Solar air heaters have been observed to have poor heat transfer coefficient from absorber plate to air. This low heat transfer coefficient results in relatively higher absorber plate temperature leading to higher thermal losses to the environment and hence lower thermal efficiency. It is therefore necessary to improve heat transfer rate to increase the thermal efficiency of solar air heaters. Artificial roughness has been used successfully for the enhancement of heat transfer coefficient in solar air heaters. The effect of staggered roughness elements on the performance of solar air heater is studied in the present work. The absorber plate of solar air heater is roughened by staggered discrete square ribs in three and two numbers in alternate rows. The present investigation covers the following parameters. (i) Relative roughness pitch (p/e) = 10 (ii) Relative roughness height (e/D) = 0.0519, 0.0695 and 0.1038 (iii) Angle of attack (a) = 900 (iv) Range of Reynolds number = 3000 to 12000 During the period in which the experiments were conducted, the environmental conditions variation were (v) Range of the intensity of solar radiation = 400 to 1206 W/m2 (vi) Range of ambient temperature = 27°C to 320C. (vii) Range of wind velocity = 0.0 to 1.1 m/s. GThe effect of geometric and operating parameters such as relative roughness height (e/D) and Reynolds number (Re) on friction factor and heat transfer rate is studied and ice correlations for friction factor and Stanton number in the range of flow parameters usually encountered in solar air heaters are developed as given below St = 0.639 (e/D)0.47 (Re)-0.323 f = 2.447 (Re)-0.38 (e/D)0.314 It is found that (i) Friction factor is increased by a factor 3 to 4.3 that of smooth solar air heater. (ii) Stanton number is increased by a factor 2.4 to 3.54 that of smooth solar air heaters. IIIIIFurther, the enhancement of thermal efficiency of solar air heaters with staggered discrete square ribs on absorber plate is determined. The enhancement of roughened solar air heater is about 44.24 % to 51.5 % with respect to smooth solar air heater. It is also observed that the staggered discrete ribs in solar air heaters yields 10 to 15 %. higher heat. transfer coefficient compared to that of transverse rib roughness employed on the opposite walls of the duct. Thus the use of staggered discrete ribs would, in addition, result in saving of materials and lower cost compared to transverse ribs.