Numerical Investigation On Air-side Laminar Flow And Heat Transfer Characteristics Of Fin-and-tube Heat Exchangers With Louvered Fin

Louver fins are widely used in heat exchangers such as air conditioning,heat pumps,automotive radiators and refrigerators.Its main structural feature is that many rows of flat sheets arranged at a certain angle are punched on the fin plane.When the fluid flows through the surface of tube rows and louvered fins,the louver structure can periodically destroy the boundary layer.At the same time,vortices will form around the fins and behind the tube.These vortices make the mixing of fluids in different temperatures more intense,so the heat transfer effect between the fluid and the fins is enhanced.It is noteworthy that when the louvered structures enhance the heat transfer performance,the flow loss at the rib side also increase.In this paper,a single-layer fin which is used in louvered fin-tube heat exchanger is numerically studied.On the basis of investigating the change of heat transfer performance and air flow characteristics on the rib side,the mechanism of heat transfer enhancement of louvered fins is further analyzed at the perspective of secondary flow theory.The type of the computational region studied in this paper is the fluid domain encapsulating the solid domain.The solid domain consists of three rows of circular tubes and a single-layer louvered fin.The computational area is meshed with unstructured grids and the grid independence is also assessed.In order to verify the rationality of the choice of computational domain,mesh generation and boundary condition setting which are used in this numerical calculation,a three-dimensional geometric model which was tested in an experiment study was simulated in advance.And the numerical results are basically consistent with the experimental results.In this paper,attack angle and longitudinal tube spacing are investigated as two geometric parameters.Nine different geometric models are built totally.Each model is studied under four working conditions which are designed by different Reynolds numbers.After processing the data obtained from the calculation results,the parameters including the average Nusselt number Nu on the fin surface,the heat transfer factor j,the friction factor f,the area goodness factor j/f and the secondary flow intensity Se were reached.The influence of the attack angle and the longitudinal tube spacing on the heat transfer performance of the louvered fins and the mechanism were studied by analyzing the interaction among the above parameters.The results show that the heat transfer performance between fins and air grows with the increase of attack angle at the same Reynolds number,but the flow loss also getting higher.When the attack angle remains unchanged,the heat transfer factor j,resistance factor f and area goodness factor j/f decrease with the increase of Reynolds number,while the heat transfer performance factor J drops with the rise of pumping power factor F.Under the same Reynolds number,the fins which are with larger attack angle,J and F are also larger;and the larger the attack angle,the smaller the j/f,which leading to a worse comprehensive performance;under the same pumping power factor F,the bigger attack angle,the greater J.when the attack angle keep the same,the friction factor f will decrease with the rise of longitudinal vorticity intensity Se.On the condition of an unchanged Reynolds number,making the longitudinal tube spacing larger cannot significantly improve the heat transfer performance of louvered fin,but will make the flow loss decrease slightly.And under the same situation,when the longitudinal tube spacing increases,j/f follows,and thus the overall performance of fins getting improved.