Thermal Performance of Solar Air Heater by Using Shot Peened Absorber Plate

Author:  A Jain, JL Bhagoria, MC Sharma
Source:  Conf Proc: ICSP-9 (pg 67-74)
Doc ID:  2005053
Year of Publication:  2005
Abstract:  
ABSTRACT Solar air heaters have been observed to have generally poor convective heat transfer coefficient from absorber plate to the 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 has been found that the main thermal resistance to the convective heat transfer is due to the formation of boundary layer on the heat-transferring surface. Efforts for enhancing heat transfer have been directed towards artificially destroying or disturbing this boundary layer. Ball peening on the absorber plate has been used to create artificial turbulence near the wall or to break the boundary layer. Thus, artificial roughness employed for the enhancement of heat transfer coefficient between the absorber plate and air has improved the thermal performance of solar air heater appreciably. A critical review of the heat transfer and fluid flow characteristics in artificially roughened ducts has been carried out. Investigations on heat transfer and friction characteristics have been reported pertinent to transitional flow of air in rectangular duct with aritificial roughness in the form of ball peening on one broad wall, which is subjected to solar radiation while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters. Use of artificial roughness to enhance heat transfer rate also results in considerably large increase in friction factor and hence in pumping power. The present study involved experimental work on an outdoor test facility has been designed and fabricated to generate heat transfer and friction data for flow in a rectangular duct with broad wall with ball peened surface at different air flow rate for a range of surface roughness. The roughness was controlled by different size of balls. Experimental data have also been collected for smooth duct for ensuring the accuracy of the experimental data and also to compare the heat transfer and friction characteristics of roughened and smooth duct. Three plates having different shot peened roughness have been tested for different flow rate. Five flow rates corresponding to a flow Reynolds number between 3000 to 12000 have been used for each test set and data were collected under steady state condition. On the basis of this experimental investigation it is found that the ball peening at one broad wall of the rectangular duct yields up to 1.6-2.0 fold increase in the heat transfer coefficient and 1.3-1.62 fold increase in the friction factor as compared to a smooth duct for the range of present investigation. The shape and size of shots have a significant effect on heat transfer coefficient and friction factor.