Forced convection heat transfer for air side flow across flat tubes heat exchanger is investigated numerically and experimentally. The effect of inclination of flat tubes on the amount of heat transfer is investigated. Fluent 6.1 software program is used to solve this problem. Three proposed cases for inclination of tubes are studied. This first case is to make convergent and divergent channels for air flow (Case 1). The other two cases are tilting of flat tubes in forward (Case 2) and back ward (Case 3). Velocity and temperature distribution around the flat tubes for the studied cases are obtained. In turn, convection heat transfer coefficient and pressure drop for air side around flat lubes are calculated.
An experimental apparatus is designed and constructed. Air is drawn through the wind tunnel from the surroundings to flow across one column of the tested tubes with different velocities. Two separated heating fluid units are used in the experimental work to perform different wall temperatures. The first unit is a refrigeration circuit, using R406A, as a working fluid. The air cooled condenser for this unit is considered the test section for this work and fixed inside the wind tunnel. The second unit is an open circuit, using steam as working fluid where steam is condensed inside the tested tubes. Three proposed cases for inclination of tubes in the theoretical work are manufactured tested and in the experimental work. The apparatus is equipped with the required sensors.
The experimental and numerical results showed that the optimum angle of inclination for the proposed first and second cases is 4°. For convergent divergent construction of one row coil without fins (case 1), the obtained enhancement of convection heat transfer coefficient is about 46.9 % meanwhile in pressure drop increased by about 112%. The second proposed construction of tilting forward alt cubes in parallel with respect to horizontal, the enhancement in convection heat transfer coefficient is about 46 % against an increase in pressure drop by about 95%. There is an acceptable agreement between the numerical and experimental results. Empirical correlations to calculate average Nusselt number from the experimental work as a function of Reynolds number at optimum inclination angle or 4° are obtained. It is found that in case of convergent-divergent channels (inclination one tube toward clockwise direction and the next in counter-clockwise direction), and tilting all tubes in the forward direction considerable improvements are obtained but in case of tilting all tubes in the backward direction there is no improvement.