| Turbulence disturbance,aerodynamics and cavitation effect in holes are three main factors affecting fuel breakup and atomization.Cavitation effect has an essential effect on fuel split atomization.In the actual working process of injector,injection pressure,needle movement and nozzle structure(nozzle length-diameter ratio,nozzle inclination angle,nozzle entrance cone angle)will have different degrees of impact on cavitation.Firstly,Based on the three laws of physical conservation of fluid motion(mass conservation law,energy conservation law and momentum conservation law),the simulation model of gas-liquid two-phase flow is introduced.Then,based on the dynamic similarity criterion of dimensionless parameters such as Reynolds number,cavitation number and Euler number,the scale effect between the experimental scaleup nozzle model and the actual small-scale prototype nozzle was established,and the visualization experiment of cavitation characteristics was carried out.According to the results of cavitation in large-scale nozzles under different injection pressures,accurate multiphase flow model,turbulence model and cavitation model are established.Ignoring the heat transfer effect on the inner wall of GDI injector,a dynamic model of the hydraulic unit and the mechanical unit of GDI injector in a injection cycle is established.Hydraulic unit modeling includes the following parts: injector inflow volume chamber,stabilized volume chamber,eddy volume chamber,inlet and outlet throttle orifice,SAC nozzle.Mechanical unit modeling includes armature,connecting spring and needle.By synthesizing the sub-models of each part of the injector,changes of needle's velocity,displacement and injection pressure in one injection cycle are obtained,which provide the physical boundary for the numerical study of instantaneous cavitation.In the process of simulation of instantaneous characteristics,an idea of starting and running the three-dimensional simulation software Fluent through MATLAB is put forward.This idea is to embed Fluent software into Matlab as a toolbox.Fluent is started by a script file of Matlab,which controls the reading of grid file,the establishment of model(multi-phase flow model,turbulence model,cavitation model),the transfer of inlet boundary pressure and the establishment of dynamic grid model of injector according to needle movement rule.With different length-diameter ratios,nozzle inclination angles and nozzle tapers,transient flow characteristics in the nozzle are as follows:(1)The static pressure in the nozzle tends to decrease with the increase of the diameter of spray hole,and the flow velocity of oil in the nozzle decreases with the increase of the diameter of nozzle.The area,where cavitation occurs gradually increases,extending from the wall of the nozzle to the center of the nozzle.When the needle lift is maximum,the average outlet cavitation volume fraction of hole 3(L/D=2.4)is about 0.20;hole 4(L/D=3.6)is the smallest,which is about 0.13.The mass flow rate at the outlet is almost not affected by the movement of needle.The smaller the length-diameter ratio of the nozzle,the larger the average mass flow rate at the outlet.The average mass flow rate at outlet of hole 3 is about 1.30 g/s,that at hole 1(L/D=3.0)is about 0.85 g/s,and at hole 4(L/D=3.6)is only 0.58 g/s.The smaller the nozzle length-diameter ratio is,the larger the average outlet velocity of fuel is.When the needle valve moves to the maximum lift,the average outlet velocity of hole 3(L/D=2.4)is about 126 m/s,and that of hole 4(L/D=3.6)is the smallest,which is about 118 m/s.With the increase of the needle valve,the turbulent kinetic energy entrance decreases.The outlet turbulent kinetic energy is not significantly affected by the nozzle lengthdiameter ratio and needle valve motion.(2)With the increase of the inclination angle of the nozzle,the fuel extends along the flow channel to the far wall in nozzle,the cavitation at the far wall is suppressed.The low pressure zone is concentrated near the wall of the nozzle,so cavitation near the wall is strengthened.The greater the inclination angle of the nozzle is,the shorter the extension length of the super-cavitation zone and the thicker the thickness.When needle valve rises to the maximum needle valve lift position,cavitation volume fraction at outlet of hole 6(?=50°)is about 0.21,that at outlet of hole 2(?=40°)is about 0.18,and that at outlet of hole 1(?=30°)is about 0.14.During the upward and downward movement of the needle,the average mass flow rate at outlet fluctuates rapidly.When the needle stays at the maximum displacement,the average mass flow rate of the nozzle outlet is inversely proportional to the inclination angle of the nozzle.That is,the average mass flow rate of the nozzle outlet in hole 1(?=30°)is the largest,which is 0.86 g/s,followed by hole 2(?=40°)which is 0.81 g/s./ S and hole 6(?=50°)--0.77 g/s.The inclination angle of nozzle does not affect the average outlet flow rate of fuel.When the needle valve lift is maximum,the average flow rate is 122 m/s.(3)The influence of the conicity of nozzle on the transient characteristics was analysed.Compared with the straight hole,the static pressure on the far wall of expanding nozzle is smaller,and the flow velocity of fuel near the outlet of the nozzle is lower,so the degree of cavitation in nozzle is stronger than that in the straight hole.At injection time,the average cavitation volume fraction at the exit of gradual expanding nozzle(k-=)is about 0.55,while that at the exit of straight hole(k=)is only one fifth of that in expanding nozzle(k-=).The average mass flow rate at outlet of straight hole and average outlet velocity are larger than that of expanding nozzle.During the injection process,the average mass flow rate at outlet of hole 4(k=)is 0.58 g/s,the average outlet velocity is 117 m/s,and the average mass flow rate at outlet of hole 5(k-=)is only 0.41 g/s,and the average outlet velocity is 112 m/s.The turbulent disturbance intensity at the inlet of the progressive nozzle is greater than that at the straight nozzle,but for the average turbulent kinetic energy at the outlet,there is no significant difference. |
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