3-D Direct Numerical Simulation Of Near Nozzle Characteristics In Fuel Jet

Fragmentation of the fuel injection directly affects the combustion and emission properties,which further has a fundamental influence on engine performance.However,the influence factors for the breakup of the spray process,especially near nozzle fields,are still unclear.Generally,fuel injection takes place under a high temperature and pressure condition with very high Reynolds number and Weber number.Therefore,the primary and secondary breakup processes of the fuel spray suffer from turbulence effect.The evolutionary processes of the Newtonian and the power-law fuels atomization affected by turbulence are investigated in present models.A 3-D single-phase turbulent pipe flow is performed first to present the in-nozzle flow.When the flow is fully developed,after temporal and spatial average,the result is validated with the literature.Then,the outlet velocity at each time step is stored to a time-varying velocity database.By keeping the time step consistent with the singlephase flow,the database is mapped as the following injection inlet velocities boundary.In the same way,a single-phase fully developed channel flow is generated for the secondary breakup process.The jet process is a typical two-phase problem.A modified VOF(Volume of Fluid)coupled with DNS(direct numerical simulation)method is applied to track the twophase interface.Due to the development of the VOF method,it is reliable to study the deformation and breakup of the fuel spray.It is found that wavy surface,ligaments,and droplets with various scales and shapes turn up gradually under the turbulence effect and liquid-gas interaction in jet evolution process.Meanwhile,after being sheared,distorted and stretched,different ligaments separation patterns are captured.Larger Reynolds number and higher gas densities accelerate the jet break-up process.Higher injection velocities and lower power-law indexes(n<1)result in better atomization effects while it's too viscous to fragment for shear-thickening(n>1)fuel jet.What's more,similar breakup patterns are detected in shear-thinning fluid(n<1)jets compared to Newtonian fuel jet(n=1).However,the disintegration is considerable easier and more violent,especially in the later stage.After the droplet separating from the liquid column,secondary breakup occurs.Apart from critical Weber number,critical Reynolds number should also be taken into consideration.Different from the uniform flow in studying the droplet deformation,the characteristics of the droplet in a weak turbulence flow is adopted in present study.The deformation of the droplet in the weak turbulent field is not towards a certain direction and unsymmetrical.The magnitude of the deformation is also smaller than the uniform flow under the same average inlet velocities.