الفهرس 
Energy problems
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Abstract
Liquid to gas heat exchangers suffer from poor thermal characteristics. One of the passive solutions is to attach extended surfaces on the gas side. Po- rous media is a promising candidate to improve the thermal performance of such applications. This study presents a 3-D numerical model of porous media that are attached to the surface of the heat exchanger towards the gas side to enhance its thermal performance. The heat exchanger consists of eight ver- tical tubes connected in series with U bends within which hot water flows internally with temperature of 323 k and velocity of 1 m/s at inlet. The Copper porous media takes the shape of horizontal U shape is attached to the heat exchanger surface. The porous inserts have a porosity of 0.75 and permeability of 1.6 X 10-9 m2. The heat exchanger is subjected to external cross air flow inside an air duct of 50 cm x 25 cm cross sectional area with an average air velocity of 0.01 m/s up to 0.35 m/s the effect of changing both thickness and spacing of porous media is studied. Air flow is laminar while water flow is turbulent. Results show that an enhancement in Nusselt number of up to 35 times greater than the bare module could be achieved. Hydraulic performance is as significant as thermal performance because it is function of the required pumping power to achieve the desired thermal performance. Thermal performance and hydraulic performance are assessed simultaneously by comparing Nusselt number and the pressure coefficient which revealed that at the same pumping power Nusselt number improved up to 35 times compared to that of the bare tube model. An empirical equation has been derived to correlate Nusselt number as a function of Reynolds number, porous thickness to width ratio and layer-to-layer spacing to width ratio.