PERFORMANCE ENHANCEMENT OF A ROUGHENED SOLAR AIR HEATER DUCT PROVIDED WITH DISCRETE EXPANDED METAL MESH AS ARTIFICIAL ROUGHNESS

Abstract

Solar air heater is cheaper than the solar water heater and generally used for drying and heating of space. Because of low heat transfer capacity from absorber plate to the air present in the duct, solar air heaters have low efficiency. In order to increase the performance of solar air heater, the rate of heat transfer between the absorber plate and air is required to be increased. Heat transfer coefficient can be increased by several methods, but use of artificial roughness is one of the effective method for increasing coefficient of heat transfer in the system. There are several forms by which artificial roughness on the surface of absorber plate can be provided; such as sand grain roughness, rib roughness, wire roughness etc. For rib roughness several geometries were investigated by different investigators to increase heat transfer rate. The investigations carried out by investigators and the relations developed for nusselt number and friction factor have been reviewed and presented in this report. It is observed from the review that heat transfer coefficient increases along with friction factor. The objective of all studies is to find out the optimal geometries of roughness element to increase heat transfer with less mechanical pumping requirement in order to prevent friction losses. Therefore the investigators need to find such roughness element which can fulfill the requirement of our application. Under the present study an experimental investigation has been carried out to investigate the heat transfer and fluid flow characteristics in a roughened duct having discrete expanded metal mesh geometry as artificial roughness element. The Reynolds number value from 1900 to 13,000 was varied. The other parameters; long way length of mesh, short way length of mesh and relative height of mesh are considered as 46.87, 25 and 0.04 respectively. The relative gap distance value has been varied from 4.34 to 18.47. Considering the above parameters the effect of geometry is analyzed on heat transfer coefficient and friction factor of flow in the duct. Nusselt number and friction factor obtained in the roughened duct have been compared with the data obtained of continuous expanded metal mesh and smooth duct. From the experiment it is seen that solar air heater has been found to have optimum value correspond to relative gap distance is 14.13.