EXPERIMENTAL STUDY ON PERFORMANCE OF A DOUBLE PASS SOLAR AIR HEATER

Abstract

Energy is available in multiple forms and plays a significant role in world wide economic growth and industrialization. The growth of world population accompanied with rising material needs intensified the rate of energy usage. Continuous increase in energy usage characteristics of the past 50–100 years cannot continue indefinitely as demarcated energy resources of earth are exploitable. On the other hand, environment degradation with use of fossil fuels is a menace to life. In view of world’s depleting fossil fuel reserves and environmental threats, development of renewable energy sources received importance. Among many alternatives, solar energy stands out a noticeable energy source for meeting the demand. It is considered as suitable renewable energy source due to its huge potential. The freely available solar radiation provides an infinite and non polluting reservoir of fuel. The easiest way to utilize solar energy for heating applications is to convert it into thermal energy by using solar collectors. Solar air heater is one such method to convert solar energy to thermal energy. Thermal efficiency of solar air heater is less due to low heat transfer capability between absorber plate and fluid flowing in the duct. Double pass solar air heaters have large area for heat absorption hence thermal efficiency increases. Various studies have been carried out based on experimental and by mathematical modeling. It is reported in literature that heat transfer rate can be increased by many ways like use of fins, energy storage, recycle, use of artificial roughness of various shape. Artificial roughness of different shape has been studied by researcher. It is found to increase heat transfer capability of ‘double pass solar air heater’. Many experimental investigations are reported but very few studies are done based on CFD. Under the preset study a CFD based performance analysis of a double pass solar air heater has been carried out. A ‘Double Pass Solar air heater’ with artificial roughness in the form of transverse circular shape roughness element with relative roughness height of 0.0216, 0.0325, and 0.0433 are investigated. Detailed investigation is done on heat transfer and fluid flow characteristics of the heater. Attempt has been made to study Nusselt number, friction factor, thermo-hydraulic efficiency as a function of Reynolds number and roughness height. Nusselt number found to increases up to 2 times with use of artificial roughness. Enhancement in heat transfer is accompanied with increase iv in pressure drop, and thus the pumping power requirement is increased. Thus to predict performance of solar air heater thermo-hydraulic efficiency is calculated and is found to be maximum for roughness height of 2mm at Reynolds number of 6000. Further in order to validate the CFD based results, a roughened plate having similar geometry has been tested experimentally. Based on the comparison, it is found that good agreement is present between simulation results and experimental results.