Modelling And Numerical Calculation For Heat And Mass Transfer Of Single-row Finned Tube

The condenser of the direct air-cooling thermal power unit is composed of over 10,000 single-row finned tubes with the same appearance and function characteristics. The exhausted vapour is condensed into water after entering the finned tubes, and the Carnot cycle of the unit is closed in these tubes. Thanking for the cross flow of working fluid inside and outside the tube, the heat transfer from vapour zone to air zone conjugating on several interfaces, the two-phase flow in tube involving phase transition, and the tube covered with thousands of air-cooling wave fins, the thermo-dynamic calculation to the finned tube is very complicated. Furthermore, some transient phenomena including the flooding, film climbing and droplet entrainment, occur in the reflux finned tube. For modelling and calculation to the thermo-dynamics characteristics mentioned above, the following works have been carried out in this thesis.Considering the geometry characteristics and the installation inclination angle of the finned tube, with the Nusselt’s condensation hypothesis, the governing equations are set up for the condensate film in a 3-D coordinate system. Based on the heat and mass balance at the vapor-liquid interface, the partial differential equations and their boundary conditions are proposed to calculate the film thickness. Through the works above, a mathematical model is presented to simulate the vapour condensation in the finned tube. To calculate the mathematical model, an iterative algorithm is developed to estimate the tube wall temperature. The model and algorithm are applied to an actual physical finned tube, and the predicted result to condensation rate is compared with its nominal value for the verification purpose.An original algorithm is proposed to calculate the performance indicators for the full sized fin tube, which is named as the Cross-Criss algorithm. On the Cross’transverse direction, with the uncoupling thermal conditions at three conjugating interfaces from the vapour zone to air zone, the coupled heat transfer across the finned tube is calculated asynchronously in various zones, in which, the heat released from condensation is modelled and calculated together with the interfacial shear stress between vapour and condensate film. In the longitudinal direction, based on the continuous condition for the vapour and film flow in the tube, the full sized tube is decomposed into 282 calculation modules, which share same boundaries each other and can be called repeatedly in the computation. The Cross-Criss is applied to an actual physical finned, and the numerical heat transfer performance indicators are compared with their experimental results for the verification purpose. The flow field inside and outside the finned tube are obtained from the numerical calculation, it is found that the null speed region nearby the flat tube tail deteriorates the heat tranfer performance in air zone, meanwhile, the interfacial shear stress has reasonable influence on both the vapour condensation and the liquid film flow in vapour-liquid zone.VOF approach in multiphase flow is embeded into solving the governing equations of the two-phase flow in the downflow and reflux flow tube, the diffusion layer is used to model the species transport in the reflux flow tube, and QUICK and CICSAM algorithm are implemented to track the location of the interphase interface. With the model and algorithm above, the numerical calculation is applied to an actual physical downflow finned tube and a simulative reflux-flow finned tube respectively. In the downflow tube, the computed results are shown to effectively capture the accumulation of condensate film at the semi-circle of the tube, in addition, the two-phase velocity boundary layer can be observed obviously between two phases. From the numerical computation on the reflux-flow tube, the specie concentration scalar field is attained with regard to air and vapour, with which, the mechanism of NCG affecting the condensation is explained satisfactorily. The computation on the reflux-flow tube successfully captures some interesting phenomenon, including flooding, climbing and droplet entrainment, all of which are characteristic as to only the reflux-flow tube, moreover, the simultaneous velocity profile of two-phase is extracted and analyzed when these phenomenon occur. Finally, the numerical calculation concludes that, the flow pattern of the two-phase in downflow finned tube can be categorized to the falling film flow, and the condensate film flow on the wall of the reflux-flow finned tube, is in good agreement with the rivulet flow.