| The phenomenon of spray droplet impacting onto the surface exists in many areas, and it is different in the requirements on the liquid shape and movement after impacting in different areas. In the working process of small high-speed diesel engines, the droplets become more likely impacting onto the cylinder wall with the continuous improvement of the fuel injection pressure, and it has become a hot issue to improve diesel engine performance by taking full advantage of droplets impacting onto the surface for the researchers in the internal combustion engine field. Single droplet impacting onto the surface was set as the research object in this thesis, and the fundamental of interface tracking methods for the gas-liquid two-phase flow and the liquid movements of droplet impacting onto the surface were researched. The fundamental research could be contributed to understanding the mechanism and discipline, and providing theoretical basis for the control technique of droplet impacting onto the surface.Interface tracking is the base of the research of the gas-liquid two-phase flow and droplet impinging onto a surface. According to the requirement of the research of droplet impinging onto a surface, the combined Level Set-VOF method used in interface tracking for the gas-liquid two-phase flow and the coupled solution method for the coactions of interface tracking with gas-liquid two-phase flow were promoted. The basic idea and solution chart of combined Level Set-VOF method which is used in interface tracking were presented by carrying forward the advantage of VOF method and Level Set method. The phase function initializing method, the solve method for the convection equations of phase function, the interface structure method and the re-distance method of Φ were provided. The solving processes were worked out, For example, coupled process of phase function with variables of governing equations, the process of the transferring and invoking for variables of governing equations, etc. The characteristics and influencing factors of spurious currents during the solving process of the gas-liquid two-phase flow were studied based on the zero gravity static droplet problem, the cause for the spurious currents was given, and it was proposed that the combined Level Set-VOF method could be an effective way to weaken the spurious currents. The solving performance of combined Level Set-VOF method was analyzed through the computation of some classical two-phase flows, it showed that the combined Level Set-VOF method could compute the problem of incompressible two-phase flows much more precisely than the VOF and Level Set method.Based on the methods research of interface tracking and the coupled solution for the coactions of interface tracking with gas-liquid two-phase flow, the researches of wall wetting model which reflects the interaction of liquid droplet with wall and coupled solution method for the coactions of gas-liquid two-phase flow with wall were put forward. According to the phenomena of the contact angle hysteresis, a model of hysteresis tension was established with the method of the phenomenological analysis. The cause of stress singularity was worked out by the flow analysis near the area of solid-liquid-gas three-phase contact line, and a method that could avoid the stress singularity was put forward according to the theory of precursor and entrained film. On the basis of above work, the wall wetting model that could reflect the influences of the contact angle hysteresis and the properties of the wall surface was established. The calculation methods of the contact line speed and the boundary conditions in the process of coupled solution for the coactions of gas-liquid two-phase flows with wall were presented. Based on the combined Level Set-VOF method and the wall wetting model proposed in this thesis, the coupled solution method for the coactions of gas-liquid two-phase flow with wall was achieved. The wall wetting model and the coupled solution method for the coactions of gas-liquid two-phase flow with wall were validated with experimental results.On the basis of above work, movements of droplets impacting onto the dry surface and wet surface were studied. While droplets were spreading on the dry surface after impacting, droplet shape variations, characteristic parameter variations and the effect of impacting velocity were analyzed, relation between height and wetting length of droplet was studied, the features of pressure distribution and transformation were analyzed, mechanism of bubble generation and dynamic under an impacting droplet on the dry surface were researched, the theoretical model for predicting the maximum spreading ratio was developed by introducing advancing contact angle and a pre-exponential factor, the theoretical model and mechanism for splashing after droplet impacting onto the dry surface was studied, and the criteria for splashing was presented. While the droplets impacting onto the wet surface, liquid shape variations, laws of characteristic parameter variations and influence factors were studied for the liquid movement of wave, crown and splashing, etc. The formation mechanisms of wave, crown and splashing were researched and the splashing condition while droplet impacting onto a wet surface was analyzed.The visual experiment system for diesel droplet impacting onto a surface was built. On the basis of the system, the movement process, variations of dimensionless wetting length, the critical velocity for splashing and the effect of surface and liquid layer height on the droplet movement after impacting were studied by experiments. The law of the droplet shape changing and the effects of influence factors on the droplet shape were achieved. The method for computing droplet impacting onto a surface suggested in this thesis was validated through the comparison of the calculation results with the experimental results on the condition of droplet spreading/splashing on a dry surface after impacting and the liquid movement after droplet impacting onto a wet surface. |
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