Mathematical Modeling for Thermohydraulic Performance of Wire Mesh Packed Bed Solar Air Heater.

Abstract

Solar air heaters are used for low to moderate temperature applications like space heating, crop drying, timber seasoning and other industrial applications. Conventional air heaters are found to have poor thermal efficiency primarily due to low convective heat transfer coefficient between the absorber plate and flowing air, leading to higher plate temperature and greater thermal losses. Attempts have been made to improve the efficiency of solar air heaters by employing various techniques. Use of porous packing’s inside the duct is one of the important design improvements to achieve this objective. Packed bed collectors absorb solar radiation in depth which reduces the top layer temperature of the matrix and thus top losses are reduced. Such collectors have high ratio of heat transfer area to volume resulting in high heat transfer capability. Moreover, heat transfer coefficient between packing element and flowing air is also increased due to increased turbulence of flowing air. An attempt to enhance heat transfer is always accompanied with increase in pressure drop and thus pumping power requirement is increased. It is therefore desirable to optimize the system to maximize heat transfer,keeping frictional losses at minimum possible level. In the present work a methodology has been presented for thermohydraulic optimization of packed bed solar air heater having its flow passage packed with wire screen matrices having various geometrical parameters. The thermohydraulic performance of the system in terms of an effective efficiency is determined on the basis of actual thermal energy gain subtracted by the equivalent thermal energy required to generate power needed for pumping air through the packed bed. A mathematical model is developed to compute effective efficiency, based on energy transfer mechanism in the bed. Based on the results of mathematical model a design criterion has been suggested for selecting a suitable matrix for packing the air flow duct of solar air heater which results in best thermal efficiency with minimum pumping power penalty. The salient results of investigation are as follows: (i) Solar air heaters packed with wire screen matrix are more efficient than conventional type of air heaters. (ii) Thermal efficiency as well as effective efficiency of packed bed air heaters depends on the geometrical parameters of the packing element. (iii) A design criterion has been suggested for selecting the best geometry of matrix resulting in optimum thermohydraulic performance of air heater for given operating conditions.