Numerical Simulation Of Turbulence-Induced Droplet Dispersion Process For Ultra-High Pressure Fuel Sprays And PANS Method

The combustion efficiencies of diesel engines are largely influenced by fuel spray quality.In order to improve the fuel economy of diesel engines and achieve ultralow soot emission,fuel injection pressure of diesel engines tends to be higher and higher.At present,the high-pressure common rail diesel engine is being developed.Its fuel injection pressure is much higher than that of the traditional plunger pump fuel injection,and even has reached the level of 300 MPa ultra-high pressure.The study on ultra-high pressure fuel sprays has gradually become a hot topic in diesel engine combustion research.The numerical method based on computational fluid dynamics is widely used in studying fuel spray and engine combustion,as its advantages in overcoming the limitations in simultaneous measurements of the gas-liquid two-phase velocity field,reducing the experiment cost and providing a large amount of flow details and so on.Reynolds averaged Navier-Stokes(RANS)and large eddy simulation(LES)are the two most classical turbulence simulation methods.Their combinations with Lagrangian particle tracking(LPT)technology are the common methods for the droplet dispersion modeling for high-pressure fuel sprays.Compared to the conventional high-pressure spray,the size of fuel droplets in ultra-high spray is smaller and the movement speed would be faster.The dispersion process of fuel droplets of different sizes in the gas-phase flow field induced by free and impinging sprays is different.Droplet dispersion and dynamics caused by large-scale turbulent structures is an important phenomenon in high-speed fuel injection.The further studies on the dispersion evolution and modeling methods of fine droplets in turbulent flow under ultra-high fuel injection pressure are still needed.In this paper,the turbulence dispersion process and modeling methods of ultra-high pressure fuel spray droplet are studied in detail,over the view of engineering application,using Open FOAM platform.The main work and achievements of the thesis are given as following:Firstly,the high-precision LES method was used to simulate the different dispersion processes of free and impinging sprays under ultra-high fuel injection pressure.The transient dispersion law of the whole spray and the quasi-steady state dispersion law of the local spray were studied under different injection pressures.The results show that with the passage of time,the transient interaction between the droplet and the shear vortex leads to the droplets with lowest velocity in the downstream spray edge,and the larger droplets(d > 12 m)closer to the spray edge.The transient interaction between the droplet and the wall vortex leads to the the droplets with lowest velocity in the top of wall spray,and non-obvious trend of droplet size as a function of axial distance.With the increase of injection pressure,both in the downstream and wall sprays,the intensity of interaction between the 2～12 μm droplets and the large eddies at the quasi-steady state is basically unchanged.However,the increase of the interaction time leads to a significant increase in the maximum dispersion extent of these droplets with injection pressure,while the variation of the larger droplet dispersion range with injection pressure is obviously different.Secondly,the traditional RANS method was applied to the simulation of the turbulent dispersion processes of free and impinging sprays under ultra-high fuel injection pressure respectively.The applicability of the most used Discrete Random Walk(DRW)model and the prediction accuracy of traditional method were studied in engineering.The results show that the turbulent dispersion process of ultra-high pressure fuel droplets predicted by the traditional method combined with the DRW model completely violates the previous law of motion revealed by LES.Compared with the LES method,there is a significant difference in turbulent dispersion process of free and impinging sprays predicted by the traditional method without the DRW model due to the overprediction of the accumulation of local droplets.Then,the non-zonal hybrid RANS/LES method,the partially averaged Navier-Stokes(PANS),was applied to simulate the turbulent dispersion processes of free and impinging sprays under ultra-high fuel injection pressure respectively,and compared with LES and RANS.The results show that using PANS method instead of traditional method can not only improve the prediction accuracy of droplet dynamics and dispersion,but also significantly reduce the dependence of scale resolving simulation on the grid size.Finally,based on the PANS-LPT model,the simulation of the turbulent dispersion process of ultra-high pressure multi-stage spray was carried out.The influence of double-stage injection strategy on the shape and dispersion characteristics of free spray was studied.The results show that the multi-stage injection technology can improve the dispersion of the ultra-high pressure free spray,reduce the fuel mass in the upstream spray center and improve the mixture gas organization,which provides a theoretical basis for the actually ultra-high pressure engine to reduce the raw emission by combining multi-stage injection technology.