Study On Atomization Mechanism And Numerical Simulation Of Drying Process Of Air-Blast Spray

As one of the most important spray drying methods used for producing powder products, air-blast spray dryer is widely used in food, pharmacy and chemical industries, etc. Air-blast spray drying process is complex, the inner of the spray dryer can not be observed easily. In order to make scientific production plan and optimize design parameters, atomization mechanism of material, heat and mass transmission between droplet and drying air, tracks of droplet, as well as flow pattern of drying gas must be fully understood. Thorough studies on the atomization mechanism of air-blast spray and droplet drying process have been made, based on the theories of hydrodynamics and numerical heat transfer.The dispersion equation of sinuous disturbances on a liquid sheet is derived considering the influence of changes in liquid-film thickness. Then the dispersion equation is used to analyze the effect of liquid viscosity, surface tension, the density ratio of air to liquid and liquid velocity on the instability of liquid surface, and the simulation results are compared with the results that do not take account of the influence. It is indicated that the maximum growth rate considering the thinning of film thickness is less than that without considering the influence. The results show that the thinning of film thickness hinders atomization process.Taking account of both effects of surface unstable disturb wave and gas drag force on the primary atomization process, the primary atomization mode of the liquid sheet is proposed, in which the breakup of liquid sheet is divided into three stages: the formation of annular liquid sheet, the radial expanding of annular liquid sheet and the breakup of annular liquid sheet. Firstly, according to the radial effect of gas drag force on the liquid sheet, the equilibrium equation of the force is constructed by analyzing the force impact on the liquid, then the thickness of the annular liquid sheet tore off from the liquid sheet is calculated by the equation; secondly, according to the tangent effect of gas drag force on the liquid sheet, the formula of the thickness of annular liquid sheet with time is constructed; finally, the diameter of the droplet is got, based on the analysis of surface instability of annular liquid sheet; then the new primary atomization model is built. The new and the general primary model are separately used to analyze the influence of air velocity and liquid viscosity on the droplet diameter. When the air velocity or the liquid viscosity is low, the simulation results with the new model is close to the results simulated by the general model; with the rising of air velocity or liquid viscosity, the new model is more accurate. It is proven that the new model is more suitable for the simulation of air-blast atomization.Analyzing the coupling relationship of solvent percentage, droplet specific heat, saturated vapor pressure and solvent diffusion coefficient, a new evaporation model is present, taking account of the changing of temperature gradient changing and concentration gradient. The numerical method of droplet evaporation is constructed by the mesh-reconstruction technology, and the visualization of the simulation results is realized by Matlab. Then the evaporating process of the droplet is simulated by the new model, taking account of the influence of air velocity, air temperature, original diameter and original concentration of droplet, the simulation results agree well with the experiment results. The new model can embody the formation of droplet crust.Four collision outcomes are discussed, such as bouncing, coalescence, reflexive separation and stretching separation. Then the occurring condition of different collision outcomes is analyzed. Aiming at the collision-induced breakup phenomena, the expression for number of satellite droplets after collision is derived from the conservations of droplet mass, moment, and energy during collision process. Then the expression is used to predict the satellite droplet number after stretching separation and reflexive separation. The calculation results show that in the stretching separation regime, the number of satellite droplets is of the highest value in range from 0.5 to 0.7 in impact parameter; and in reflexive separation regime, the satellite droplets are formed most actively in the case of the head-on collision.Taking account of more outcomes after droplet collision, the CFD model of air-blast spray drying process is proposed. The solution domain with irregular boundary is discretized under the body-fitted coordinate system, then the problem that existing in the orthogonal curvilinear coordinate system is tackled. The gas governing equation is discretized by finite volume method, and the modified pressure equation including the source term is deduced. Then the model is used to simulate air-blast spray drying process, and the temperature and humidity distribution of the air in the dryer, the velocity and diameter distribution of the droplet at different positions in the dryer are predicted. According to the simulation results, the main factors that influence heat and mass transmission are analyzed. As more collision outcomes are considered, good consistency has been achieved between the simulation results and experimental results.