Investigation Of Effects Of Nozzle Number And Distribution On Fuel-Air Interaction Of Flash Boiling Sprays Of Gasoline Direct-injection Injectors

Mixture formation process is vital to the success of the combustion system.The mixture quality directly affects the combustion efficiency and stability,and finally determines the engine performance and fuel economy.The mixing process is governed by two aspects,namely fuel spray breakup process and fuel-air interaction.The liquid spray breakup process mainly depends on combined effects of various forces exerting on the liquid jet,including the inertia force,surface tension force,and aerodynamic force,which eventually lead to the liquid atomization.Flash-boiling spray have some advantages over the traditional liquid fuel spray,including more rapid atomization,smaller particle size,faster evaporation and independent of injection pressure.Because vapor bubbles initiate and grow inside the nozzle before and after fuel discharged into the ambient,which enhance the spray break up process significantly.Flash boiling spray also improves the fuel-air mixing process.However there is little research about the flash-boiling spray fuel-air interaction.In order to investigate the two-phase flow fields of fuel sprays of the Spark Ignition Direct Injection(SIDI)engines,an optimized two-color PIV(Particle Image Velocimetry)technique is applied to measure the flow fields of fuel spray and ambient air.This two-color PIV system consists of a special tracer-filter system which can measure the flow fields of the fuel spray and ambient air simultaneously.The 6-hole injector is used as the baseline of the experiment.Quantitative analysis of the fuel spray geometry under non-flash boiling condition,transitional flash boiling condition,and flare flash boiling condition is conducted.At last,normalized results of the spray width,spray plume to plume distance and the collapse region length are researched.Under the non-flash boiling condition,the spray flow field didn’t change much.Under the transitional flash boiling condition,the spray expands and the vortex motion appears.When the superheat degree increases,resulting in flare flash boiling condition,the spray plumes collapse together and the velocity increase significantly.The vortex motion becomes stronger,and the vortex diameter becomes smaller.The velocity of the ambient air-flow field increases in entrainment zoon,recirculation zoon,and spray head vortex zoon with increasing the superheat degree.In order to explain the effect of the spray neighbor plumes interaction,a 3-hole injector and a 2-hole injector are researched to analyze the fuel spray and fuel-air two-phase flow fields.With increasing the superheat degree,spray geometry of these 3 injectors change,the 6-hole injector spray collapses due to the neighbor plumes interaction;the 3-hole injector spray has little collapse phenomenon.The ambient air entrainment velocity of the 6hole is strongest,second one is the 3hole injector,the ambient air entrainment velocity of 2hole is weakest.Kinetic energy transferred from fuel spray to ambient air varies among different injectors under non-flash boiling and flash boiling conditions due to different interaction between adjacent fuel plumes especially under flare flash boiling conditions.Momentum transfer between fuel spray and ambient air is enhanced by stronger plume-to-plume interaction,thus the kinetic energy of ambient air increased.But when the ambient pressure decreases,the kinetic energy transferred from spray to ambient air decreases.This experimental research covers a wide range of working conditions of the direct injection engine.The following main conclusions have been drawn.First,flash-boiling spray is easier to atomize,producing smaller particle size and shorter spray penetration.Second,flash-boiling spray enhances the mixing process with the surrounding air,leading to faster mixture formation.Finally,it provides a potential solution solving fuel impingement,oil dilution,piston carbon deposit and soot emission.In addition,this study provides big quantity of experimental data about fuel-air two-phase flow field of the flash-boiling spray,the effects of the nozzle number and distribution on fuel air interaction of flash-boiling spray.