Numerical Study On The Characteristics Of The Internal Flow In A Injector And Near-field Spray At The Initial Stage Of Injection

The internal flow in a diesel injector nozzle affects its primary breakup and the characteristics of spray directly,further influence performance of combustion and emission.In order to understand the development of cavitation and internal flow in the nozzle at the initial injection and their influence on near-field spray under different conditions of fuel injection and needle valve opening,a three-dimensional coupled numerical model of internal flow inside nozzle and near-field spray outside the nozzle was established based on Eula-Lagrange algorithm,furthermore,the effects of injection pressure,fuel temperature and needle valve opening speed on the internal flow and near-field spray were simulated.The results show that:With the opening of needle valve,cavitation can be divided into three regions:single flow zone,cavitation development zone and super-cavitation zone.The flow of cavitation is accompanied by the occurrence of cloud cavitation.The higher the injection pressure,the earlier the cavitation initiation time and the faster the development speed.After the super-cavitation,the breakup appears in the upstream of the near-field spray main stream because the cavitation bubbles,and then the breakup area extends to downstream of near-field spray gradually to form more droplets of small particle diameter.The cavitation increases the radial movement trend of spray,improves the near-field spray cone angle.Cavitation area shows the characteristics of small vortex structure and large rotation intensity.When the cavitation enters the chamber,the disturbance to the spray is enhanced,which makes the spray core thinner and promotes the spray break-up on the outside of the spray core angle.In the single flow zone and cavitation development zone,the velocity,mass flow rate and discharge coefficient of the exit of nozzle increase with the increase of the lift of the needle valve.When the flow reaches the super-cavitation,the fuel velocity rises sharply,while the mass flow rate and discharge coefficient drop sharply at the exit of nozzle,and then both tend to be stable.At the same temperature,comparing low carbon single-component fuels with high carbon single-component fuels,the cavitation initial time advances with the saturated vapor pressure increase,the fuel velocity increases with the viscosity decreases,and the decrease of fuel surface tension and viscosity makes the spray surface unstable wave increase and the spray breakup enhances.The initial cavitation time advances occurrence and the development rate of cavitation increases with the increase of fuel temperature at the same fuel.At the same time of injection,the fuel velocity and mass flow rate at the outlet of the injection hole increase with increase of the fuel temperature.The increase of fuel temperature makes the spray cone angle and the spray penetration distance increase,but when the fuel temperature approaches its critical temperature,the rate of droplet evaporation accelerates and the spray cone angle decreases conversely.At the same injection pressure and abiential pressure,the faster the needle valve opens,the severer the cavitation phenomenon at the sealing face the needle,and the larger the backsuction at the outlet of the the nozzle hole resulting in the longer the lag time of the initial cavitation in the nozzle hole.The faster the needle valve opens,The rate of pressure rise in the sac increases with the improving of the needle valve opening speed,which increases the development speed of cavitation in the nozzle hole and the earlier the time of reaching the super-cavitation.The breakup effect of cavitation on near-field spray inproves to cause the better automation quality with the needle valve opening speed increased.