FLUID FLOW AND HEAT 'TRANSFER STUDIES ON TURBULENCE PROMOTORS IN SOLAR AIR HEATER DUCTS

Abstract

The heat transfer rate from the absorber plate to air in solar air heater duct is very low leading to poor thermal efficiency: In order to enhance the heat transfer rate from the absorber plate, several methods have been, employed namely turbulence promotors, packing the duct with porous bed and artificial roughness. In the present work attention is focused on the experimental investigation of heat transfer and fluid* flow characteristics of solar air heater having transverse triangular ridge type turbulence promotors on absorber plate for which very little information is available in the literature. The effect of various geometrical parameters (e/D, W/H) and operating parameter (Re) have been studied for triangular ridge type turbulence promotors on absorber plate. The overall range of parameters covered in this experimentation is given below Relative roughness height, e/D = 0.11 to 0.19 Angle of attack, a = 900 Aspect ratio.W/H = 12.6 to 22.05 Relative roughness pitch, P/e = 10 Reynolds number, Re = 4000 to 10400 As a result of the turbulence promotors on the absorber plate the heat transfer coefficients increases by a factor of' 2.0 to 3.5 as compared to that of 'ducts with all smooth walls. Similarly the friction factor has been found to increase 'by factors of 5-12 times as compared-to that of ducts with all smooth walls. Using experimental data, the following correlation have been developed for Nusselt number and friction factor in terns of geometrical and operating parameters. Nusselt number Nu = 0.110 Re0.:830 (e/p)0.490 Friction factor f = 6.980 Re-0.24 (e/D)1.294 It has been found that the thermohydraulic performance of solar air heater with roughened absorber plate having transverse triangular ridge type turbulence promotors is better as compared to the solar air heaters with smooth absorber plates