THERMOHYDRAULIC OPTIMIZATION OF SOLAR AIR HEATER USING ARTIFICIAL ROUGHNESS

Abstract

Solar air heaters have been observed to have generally low heat transfer coefficient from the 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 low thermal efficiency. Thermal efficiency can be increased by several methods including by increasing heat transfer coefficient with the help of roughness elements attached to the underside of the absorber plate, but it brings about large increase in pumping power consumption also. Therefore, there is a need to select suitable artificial roughness geometry that can yield maximum heat transfer with minimum possible pumping power. Different roughness geometries have been analyzed in this work. The correlations developed for these roughnesses by many investigators used to predict the performance of solar air heater with these roughness geometries. The artificial roughness used for analysis are rib-groove, V-shaped, wedge shaped and transverse ribs. A procedure has been proposed for the prediction of thermal performance, thermohydraulic performance and thermohydraulic optimization to determine values of optimum roughness parameters. Computer programs have been developed in FORTRAN for this purpose. A net exergy flow criterion has been used for the optimization of geometrical parameters for specified operating conditions of a solar air heater. Design plots have been prepared to depict the values of individual roughness parameters that represent the optimum condition as a function of temperature rise parameter and intensity of radiation. The designer can obtain the optimum values of the individual roughness parameters for the given set of operating conditio