Research On Dynamical Process Of Droplet Impacting On Curved Surface

As a typical phenomenon of the free surface flow problem, droplet of liquid impact onto solid surface is widely appeared in many industries for its strong mass transfer and heat exchange capacity. Comprehensive studies have been done for the droplet impact on flat and inclined surface, while research focused on the droplet impact on curved surface is less concerned. The study of the phenomena of droplet impact on curved surface, the scientific description and explanation of the impact process, and development of testing technology and computational model for the micro phenomenon of droplet can give great value to further research of the theory of liquid flow and heat and mass transfer under the conditions of droplet impact, and also provide a better basis for the development of advanced practical engineering technologies.The experimental studies of three different droplets impact onto flat, inclined and curved surfaces were carried out.The effects of the properties of droplet on impact process and on different test wall are systematically analyzed. The aluminum surface was used in the experiments of droplet impact onto flat and inclined surface, and by adjusting the height of droplet, the effect of velocity on impact process was studied. Besides, the critical velocity of alcohol droplet splash partly was found during the experiments. By observing the plan view of water drpoplet and alcohol droplet impact on flat surface, the influence of surface tension on recoil process was discussed. In the experiments of droplet impact on inclined surface, the front spreading factor and back spreading factor, and the time to achieve the maximum front spreading factor and maximum back spreading factor were discussed quantitatively as the variation of velocity and inclination angle. The droplet impacts on spherical and cylindrical surfaces with low velocity were investigated.and outcomes after impact, such as spreading, nucleating, the formation of secondary droplet and its split, were described scientifically and analyzed in details. The effects of diameter of curved surface and velocity of impacting on curved surface are studied during the dynamic process. Based on the results of three droplets impact on different surfaces, the force analyses in different stages were discussed.Three numerical methods which used in simulation of fluid motions are introduced in this dissertation. The main idea and characteristics of Lattice Boltzmann method, and LBGK model which used widely in LBM were introduced. Three multiphase flow models including color model, pseudo-potential model and free energy model, for the complex multi-component and multiphase system were compared and the pseudo-potential model were selected as the simulation method. In order to validate the model, simulation results of droplet impact on flat surface were compared with experimental data. By altering the wettability of surface, the droplet rebound from the wall was achieved for bad wettability of surface. With single variable control method, the intensity of viscosity and surface tension during different spreading stage were studied quantitatively. Then the simulation of droplet impact onto curved surface was carried out with this method. The flow state of the droplet on curved surface was analyzed in details, and four stages were defined for the spreading process, that are moving, spreading, nucleating and dripping. For some special circumstances, such as high speed or bad wettability of surface, the droplet splash partly on surface edge. Based on the velocity field distribution of droplet when droplet splash partly, the formation mechanism of droplet splash was revealed. The flow state of droplet impact on curved surface under different velocities and wettability are studied exhaustively, and critical Weber number of droplet drips down and splash partly on different wettability of surface were obtained.