Researches On The Internal Flow And Spray Characteristics For Intersecting-hole Nozzles

The pressure atomization devices has a wide range of practical applications in engineering practice,such as spray drying,spray printing and fuel injection of engines.It has been pointed out that the cavitation inside of the nozzles can promote the primary atomization of the injected fuel.The primary atomization largely determines the global characteristics of the spray.With the challenges presented by legislated limitations in both fuel consumption and pollutant emissions for diesel engines,optimizing the nozzle geometry and improving the injection pressure have become hot research topics in the field of spray and combustion in recent years.Advanced atomization and fuel–air mixture provided by the intersecting-hole nozzle seems like a potential technology satisfying the strict regulations on emission.However,the internal flow and spray characteristics are still not very clear for the intersecting nozzles.To investigate the cavitation flow inside and primary atomization outside the nozzle,two types of intersecting-hole nozzles were designed and studied experimentally and numerically.Internal/external intersecting-hole was defined according to the intersected position of the pair of sub-holes.The internal intersecting-hole(IIH)has two inlets and one outlet,while the external intersecting-hole(EIH)has two outlets whose intersection point located outside the nozzle.To investigate the influence of geometrical parameters on the internal cavitation flow and near-nozzle spray characteristics,several simplified transparent intersecting-hole nozzles with upscaled size were produced including one cylindrical nozzle,three IIH nozzles,three EIH nozzles and one special EIH nozzle which has a slightly longer distance between the center of sub-holes' outlets.Optical visualization for the internal flow and near-field spray were conducted with high-speed camera from two perpendicular views marked as intersecting plane(INTP)and dispersion plane(DISP)separately.INTP is normal to DISP,and the latter one is formed by the two axes of the sub-holes.In the experiments,the injection pressure ranged from 5.0 kPa to 1.0 MPa with the ambient pressure kept at 1 atm.The results show that internal/external intersecting-hole nozzles tend to generate a fan-shaped morphology of spray,which is much different from the conical spray from the cylindrical one.Besides,the intersecting angle between sub-holes has a great effect on fuel distribution in the DISP.In addition,the EIH nozzles provided a very strong radial dispersion,followed by the internal intersecting-hole nozzles and the cylindrical hole nozzle.In some cases,the dispersion of the EIH nozzle can even reach 180°.The spray angle of IIH nozzle increased with the rising of the angle between sub-holes,but insensitive to injection pressure.Compared with the fuel dispersion on the DISP,the spray spreading angle was much smaller on the INTP.Nevertheless,the IIH nozzles still own larger spreading angles at the INTP than that of the cylindrical hole nozzle.Moreover,fan-shaped jets showed a huge advantage on radial dispersion,in terms of 1 to 5 times larger normalized cross sectional areas than those of the cylindrical-hole nozzle.It has also showed that,there was no cavitation generated inside the IIH due to their divergent cross sectional areas along the flow direction,even under the operating conditions of very strong hydraulic flip for the cylindrical hole nozzle.The injection pressure of 0.25 MPa was the critical point of cavitation inception for EIH nozzle and cylindrical-hole nozzle.And the development and extension of cavitation was slightly depressed in EIH nozzle when compared cylindrical-hole nozzle.No hydraulic flip phenomenon appeared in the EIH even when the injection pressure approaching to the maximum supplied by the experimental equipment.Additionally,the IIH nozzle produced a 15%-50% increases in discharge coefficient compared with the cylindrical-hole nozzle.Besides,numerical research has been conducted to investigate the impact of angle between the intersecting sub-holes on the internal flow and near-nozzle spray characteristics.After validated with X-ray experimental data of the Spray A injector from Engine Combustion Network(ECN),the multiphase fluid model coupled with Rayleigh-Plesset equation were utilized to simulate the cavitation flow and spray phenomena in seven IIH nozzles with intersecting angle sweeping from 20° to 50°,at intervals of 5°.The author analyzed the effect of injection pressure and ambient pressure on the flow characteristics.Four different injection pressure including 60 MPa,120MPa,180 MPa and 240 MPa,as well as several ambient pressure such as 2MPa,6MPa,8MPa and 10 MPa were employed to simulate different operating conditions of a diesel engine.Simulation predicted that no cavitation generated inside the intersecting-hole nozzles and provided detail information on the structure of spray as well as the correlation between the spray angle and layout of sub-holes.In addition,the discharge coefficient of IIH nozzle is insensitive to the injection pressure,but decreased slightly with the increasing angle of two sub-holes.The flow coefficient can be improved by 20%-30% in IIH nozzle because of its high area coefficient for no cavitation appeared inside.In summary,the experimental and numerical results suggested that the sub-jets collision can enhance the primary atomization and enlarge the spray angle which could be reinforced further by adjusting the intersecting angle of sub-holes.And the initial mixing can be promoted with a slight penalty on the discharge coefficient.