Research On The Fuel Flow Inside The Diesel Nozzle And Spray Characteristics

The fuel spray and atomization plays an extremely important role in the engine combustion and emission reduction. And it is agreed that much more research on the specific impacts of the fuel injection process on the spray formation is needed. As an upstream boundary condition, the characteristics of fuel flow inside the diesel nozzle have an important influence on the fuel jet breakup and atomization. It has been proved that the cavitation effect is one of the basic factors which leading to the fuel jet breakup. Finally the shape of spay is different because the flow condition at the nozzle exit is not the same.In this dissertation, the characteristics of fuel flow inside diesel nozzle in the real high-pressure fuel injection process and the impact on the fuel jet breakup and atomization have been further investigated. For the fundamental aim, the comprehensive experimental research using the optical imaging and diagnostic equipment and the simulation research with the three-dimension numerical simulation method are executed for insight to the jet atomization characteristics and the characteristic of fuel flow inside nozzles with different structures under various injection conditions.At first, the visualization experimental apparatus for the flow pattern inside injector nozzle has been designed and set up, and then a lot of transparent test nozzles with the real size of nozzle have been made. With the commissioning and validation for the experimental apparatus, it is found that the transparent nozzle and the duration of flash light source are the key factors which influence the test result. In this dissertation, a nanosecond light has been used. The real transient images of fuel flow in injector nozzle can be obtained under the actual injection condition. And then two new parameters are put forward to analyze quantitatively the generated density of cavitation inside nozzle. The experiment researches have been carried out under different test nozzles, different fuels and different injection conditions. The experimental results show that the cavitation inside the nozzle is inevitable under the condition of high injection pressure. The generation and development of cavitation and its intensity have been affected by many factors. The structural parameters such as the nozzle length to diameter ratio, the cone angle of nozzle hole, the nozzle hole taper, and the fuel properties are the major influence factors.In simulation research, the general Two-fluid Model and the cavitation model based on Rayleigh-Plesset equation are used to describe the gas-liquid two-phase flow inside the nozzle. The further analysis and discussion about the flow parameters such as the cavitation volume fraction, liquid phase turbulent kinetic energy, the liquid velocity and the discharge coefficient inside injector nozzle are carried out. The impacts of injection conditions, nozzle structure on the fuel flow characteristics are revealed. In addition, a cavitation model is used to make an exploration research in the erosion phenomenon which occurs as the result of the cavitation. The effects of cavitation and the injection pressure on the erosion are analyzed. The simulation results manifest that the rate of cavitation generation and the rate of cavitaion collapse both rise with the increase of injection pressure. So does the liquid phase turbulent kinetic energy. But the volume fractions of cavitation in the nozzle and the flow coefficient have changed very little with the increase of injection pressure. Inside the nozzle with small axis angle, the rate of cavitation collapse is smaller, and the cavitation volume fraction, the liquid flow velocity and turbulent kinetic energy have changed very little with the variation of injection condition. It is also found that the surface microstructure of nozzle wall has an effect on the cavitation phenomenon within the nozzle. The rough surface leads to the cavitation collapse. And it also leads to the decrease of cavitation volume fraction and the increase of turbulence kinetic energy. It has been found that the erosion always occurs in the middle of nozzle, the erosion latent time decreases with the increase of injection pressure and the erosion depth is increased.In this dissertation, the spray pattern near the nozzle outlet has been observed using the experimental apparatus of visualization. The macroscopic property of spray such as the spray shape and the spray cone angle are investigated with the different fuels and the different injection parameters. In addition, the fuel steady spray experimental equipments including a laser particle size analyzer have been set up to investigate the spray microscopic property. The distribution of spray particle size is measured with different injectors and different injection pressures using many kinds of fuels. The research results show that the spray shape is an unsymmetrical shape because of the cavitation occurring inside the nozzle. The development and change of spray shape are extremely unstable at the starting stage of main injection as the two-stage fuel injections are employed. At a certain injection time, the spray shape has been much more varied at the different injection cycles. It is also found that the spray cone angle becomes smaller when the fuel with lower surface tension and lower viscosity is injected. Accordingly the degree of atomization increases and the droplet size distribution is more homogeneous. The influences of the injection pressure and the nozzle size on the spray and atomization are finally clarified by using this kind of fuel.