STUDY OF THERMOHYDRAULIC PERFORMANCE OF SOLAR AIR HEATERS PROVIDED WITH ARTIFICIALLY ROUGHENED DUCT

Abstract

Solar air heaters being among the cheapest and most widely used collection devices have found applications in heating of buildings, crop drying and many other low temperature applications. Solar air heaters generally have poor thermal efficiency because of their inherently low heat transfer rates from the absorber plate to the air in the ducts. Due to the low heat transfer rate the losses from the absorber plate increases. To overcome these losses the heat transfer coefficient between absorber plate and air is required to be increased. There are several methods to increase the thermal efficiency of a solar air heater. Providing artificial roughness at underside of the absorbing plate is one of the most effective method to improve the thermal performance of solar air heaters. The use of an artificial roughness on a surface on the underside of the absorber plate is an effective technique to enhance the rate of heat transfer to fluid flow in a solar air heater duct. Artificial roughness can be produced in many forms, such as sand grain roughness, rib roughness and wire roughness. Sufficient information is available in literature about fluid flow and heat transfer• in roughened circular tubes and channels in fully rough flow region. Many investigators investigated heat transfer and fluid flow characteristics in rectangular ducts with only one wall having two dimensional roughness, the flow conditions usually encountered in the case of solar air heaters have also been extensively investigated. However, these studies were carried out independently; no study has been reported in order to compare the results of these studies. Under the present work an attempt has been made to compare the results investigated by various investigators on solar air heaters provided with roughened duct.