Numerical Investigation On Air Side Flow And Heat Transfer Characteristics Of A Flat Tube With The Dimpled Wave Fin

Finned tube heat exchanger has been widely used in refrigeration, air conditioning, chemical industry, electricity power plants and other fields due to the advantages of compact structure and convenient operating. The direct air cooling system is the main component of most electricity power plant,while direct air cooling condenser is the core component of direct air cooling system, its performance directly affects the cooling effect of the air cooling system. This system uses mostly the heat exchanger with brazing silicon aluminum alloy fins on both sides of single flat tube as the condenser. Because of the single tube bundle has many characteristics, such as compact construct, high thermal efficiency, easy maintenance, easy cleaning, long working life and so on, it has broad application prospect. In the most of tube bank fin heat exchangers, air is outside of the fin and tube. The small heat transfer coefficient and large thermal resistant of air effect the efficiency of the heat exchanger. It becomes an important issue to enhance the heat transfer coefficient of the fin surfaces. Punching or mounting some different shaped dimples on fin surface of tube bank fin heat exchangers is an efficient method to enhance heat transfer. When fluid flows through the channel formed by the flat tube and the dimpled fin, the dimples on the fin surface strength the disturbance on fluid and make the fluid streamlines changed. As heat transfer is enhanced, while the pressure drop is not large. This paper puts forward a new kind of finned heat exchanger with dimple vortex generator punching on the surface of fin. A numerical method is used to study the flow, heat transfer characteristics, the intensity of secondary flow and air side resistance in the channel formed by the flat tube and the dimpled fin.This paper selects the channels which formed by flat tube and the dimpled fin as the computational domain. A reasonable grid of the computational domain is obtained by FORTRAN. A careful check for the grid-independence of the numerical solutions has been made to ensure the accuracy and validity of the numerical results. The study of grid independence was carried out. In order to prove the rationality of the numerical method, it is essential to compare the data result obtained by numerical calculation with experimental data result seriously. For dimple fin and plain fin under the same geometry size, various performance mainly included the flow field and temperature field distribution of transverse section and vertical section, secondary flow characteristics, changings of Nusselt number Nu and resistance coefficient with the variation of Re in the channel are compared. Through changing geometry parameters of dimple vortex generator such as the dimple radius R, dimple pitch S, fin net spacing Tp, the effects of the various parameters on the heat transfer performance in the channel is analyzed.To investigate the fluid flow and heat transfer characteristics in the channel formed by the flat tube and the dimpled fin, and to find out the deep reasons to determine the fluid flow and heat transfer characteristics, a numerical analysis on this type of fluid flow and heat transfer is performed in the this paper. The results show that the dimple radius, the dimple pitch and the fin net spacing affect the average Nusselt number and friction factor, while the effect of the dimple radius and the fin net spacing are larger than of the dimple pitch. The average Nusselt number, the friction factor and secondary flow intensity of the dimpled fin case are larger than that of the plain fin case at the same Re, respectively, The correlations of Nusselt number, the friction factor with Re and other parameters are reported. The significant result is that whenever the same intensity of secondary flow is produced by increasing the radius of dimples or other structural parameter such as increasing the fin spacing and the dimple pitch, the same heat transfer intensity is obtained. The average Nusselt number is only strongly correlated with intensity of secondary flow, and the intensity of the secondary flow determines the heat transfer characteristics.