INVESTIGATION OF PERFORMANCE OF A LOW POROSITY METAL MATRIX ABSORBER SOLAR AIR HEATER

Abstract

A solar air heater is a simple device to heat air by utilizing solar energy. The efficiency of a conventional solar air heater has been found to be low because of low convective heat transfer coefficient between the absorber plate and air. To enhance the efficiency of solar air heater, a number of methods have been proposed. Porous packing of the air duct is one of them. From literature review it has been found that the volumetric heat transfer coefficient is generally higher for beds with lower porosity. Presence of wire screen matrices in the solar air heater duct results in an entirely different flow arrangement because air flow is parallel to the plane of screens. Previous work in low porosity shows substantial improvement in performance, which attracts further investigation in low porosity. In view of the above facts, the present investigation has been carried out with the following objectives: 1. Collection of experimental data of heat transfer and friction in a packed bed solar, air heater using low porosity wire screen matrices system under actual outdoor conditions. 2. Development of correlations for heat transfer coefficient and friction factor on the basis of experimental data in terms of geometrical parameters of the bed. An experimental setup was designed, fabricated and used for the collection of data concerning heat transfer and pressure drop across the packed bed solar air heater. Two different set of matrices have been prepared. The first set, i.e. set A consists of A,, AZ, A3 A4 and AS where the pitch to wire diameter ratio (P, I d,,,) has been kept constant and porosity varies from 0.677 to 0.827. The second set, i.e. set B consists of B,, B2, B3 B4 and B5 where (1 / nP) remains constant and pitch to wire diameter ratio (P, / d,,) varies and the porosity varies from 0.641 to 0.895. Experimental data has been used to determine the values of Colburn factor, Jh, and friction factor, fp as function of system (bed) and operating parameters. The analysis of experimental data on heat transfer and friction characteristics revealed that the heat transfer coefficient and friction factor are strong functions of these parameters. It has been observed that the heat transfer coefficient increases with a decrease in porosity. This appears to be due to higher level of turbulence created in the flow as the i porosity decreases and the flow passage becomes more tortuous and narrower with higher solidity of such a system. Based on experimental data, the following correlations have been developed. Co/burn Jfactor Friction factor 0.58 )0.33 = J~, 0. [(J 39 1 Pr RQ-0.62 nP )0.29 )0.17] fP = 2.87 ( 1 Pr L RQ-0.as nP dW A comparison of the experimental and the values predicted by these correlations have been made. It is observed that the predicted values of J factor have an average absolute deviation of 9.5% where as the friction factor has an average absolute deviation of 8% as compared to experimental values. It has also been observed that the thermal efficiency of packed bed increases by about 15% when the porosity of matrices decreases from 0.827 to 0.677. This indicates that a substantial change in efficiency can be brought about by a proper choice of the matrix. The thermal efficiency of a packed bed solar air heater has been found to improve appreciably as porosity of the bed decreases. For a particular matrix of the bed the thermal efficiency increases as the mass flow rate increases. This happens because for a higher mass flow rate the heat transfer coefficient is higher and thermal losses decease.