Numerical Study Of Transport Process On Gas-Liquid Falling Liquid Film Flow

The thesis is composed by two parts. Theoretical simulation and experiments onthe falling film heat and mass transfer process are discussed in chapter2-4, whichincludes the evaporation, condensation and the effects of plate structure on liquid filmflow. In chapter5-7, A novel liquid distribution model is established and multi-scaletheory is applied to predict the liquid distribution in packed column.The gas-liquid two phases flow behavior and heat and mass transfercharacteristics have always been the focus of attention in falling-film evaporators anddesalting plates. Therefore, a two-dimensional two-phase flow CFD model using thevolume of fluid (VOF) method is presented to investigate the transport phenomena.Two important momentum source terms, surface tension and interface stress shear，areconsidered. Moreover, the change of water vapor concentration and heat transfercapacity around the interface have also been described by adding the heat and masstransfer source term in the model. Based on the simulated results, the influence of twophase temperatures, flow rates, water vapor concentration and wall flux on heattransfer coefficients, latent and sensible heat have been discussed. The simulationresults show that the heat transfer coefficients near the inlet decrease rapidly, and thelatent heat is the major mode of heat transfer in the interface. Moreover, the heatreleased from condensation process further restricts the increase of sensible heat.The heat and mass transfer process is significantly influenced by liquid film flowbehavior. For this, a novel heat transfer plate structure is proposed, and the heattransfer coefficients on three different plates have been investigated, the structures ofwhich are flat, coarse and oriented baffled plate, respectively. The experimentalresults indicate better liquid distribution and increased remixed and turbulence degreeon oriented baffled plate, which improve the heat transfer performance.In section2, multi-scale method which consists of microcosmic CFD simulationand macrocosmic calculation is applied to predict the liquid distribution in structuredpacking column. The minimum representative units reappearing in structured packingis proposed and liquid film flow on the units is also been simulated to obtain themodel parameters. Based on the parameters and established liquid distribution model,the fluid mechanics behavior in packed column is obtained. Furthermore, a refinedmodel is developed to take the condition of liquid film flowing on structured packing with holes into consideration, as well as the influence of counter-current flow of gasphase. This multi-scale method doesn’t need complex computation and anyexperimental data to regress model parameters. Therefore, it provides a new approachfor the design and optimization of the large-scale packing column, and has broadfuture in industrial applications.