Fuel Injection And Spray Atomization Mechanism Analysis In High-pressure Gasoline Nozzles Using Synchrotron X-ray Techniques

The characteristics of fuel atomization directly determine the pattern and quality of the fuel-air mixture formation,which is the main factor affecting the combustion process.The improvement of atomization quality plays an important role in reducing engine fuel consumption and pollutant emissions.Improving atomization quality only by increasing injection pressure will consume more useful work of the engine,and the pressure bearing capacity of the nozzle and fuel seal of the injection system could also be challenging issues.The internal structure of the nozzle is the key factor affecting the internal flow and cavitation characteristics of the nozzle,which has a significant influence on the fuel liquid core breakup process and spray macroscopic characteristics.The optimization of fuel injector nozzle design by deepening the understanding of fuel breakup and spray atomization mechanism is a promising development direction for improving the fuel spray atomization quality.Synchrotron radiation high-energy X-ray imaging technology,which can penetrate metal nozzles,visualize the flow condition and cavitation phenomenon inside the nozzle,and accurately measure the internal structure of the nozzle,plays an essential role in revealing the high-pressure fuel injection atomization mechanism.Both experimental and simulation methods have been adopted to analyze the key boundary conditions that influence the nozzle internal flow regime.The influence mechanism of the nozzle geometry features on the nozzle internal flow and cavitation characteristics,and its correlation with the fuel breakup and spray atomization characteristics have been investigated to reveal the spray atomization mechanism of high-pressure fuel injection.Firstly,several nozzles with different internal structure features were designed and manufactured,which are made of aluminum alloy with high X-ray transmittance.Based on X-ray phase contrast imaging technology,a test system has been established to measure the needle motion and an experimental system has been built to visualize the nozzle internal flow and cavitation phenomenon.The needle lift profile has been measured and the fuel cavitation and flow separation phenomena inside the nozzle have been successfully observed.Besides,the nozzle structure measurement system was built based on X-ray micro-CT imaging technology,which enables the accurate measurement of nozzle geometry parameters and 3D reconstruction of the nozzle internal flow channel.Secondly,the imaging results of nozzle internal flow with different structures were compared to analyze the correlation between the cavitation distribution characteristics and nozzle geometry features.The analysis results have shown that the orifice conicity of the nozzle and the relative position between the nozzle sac and the orifice significantly affect the nozzle internal flow conditions and cavitation characteristics.Therefore,some nozzle geometry features with general applicability such as inlet normal inclination angle,orifice inlet included angle and orifice conicity angle are defined to better describe the structural differences between different nozzles so that the research results of nozzles with different geometry features can have better comparability.The experimental results indicate that the cylindrical and divergent orifice structures can help to facilitate the development of the cavitation phenomenon,while the convergent orifice structure can inhibit the cavitation phenomenon.The inlet normal inclination angle is not equal to zero when the normal direction of the nozzle orifice inlet section is inconsistent with the nozzle axis direction,which will cause the cavitation distribution inside the nozzle to be asymmetric.The cavitation is inhibited on the side to which the normal direction of the orifice inlet section points,while the cavitation on the other side is enhanced.The included angle between the sac wall and the orifice wall,namely the orifice inlet included angle also has a significant effect on the nozzle internal flow conditions and cavitation distribution characteristics.Both the cavitation strength and cavitation area increase when the angle is small,and the cavitation distribution inside the orifice will be asymmetrical when the included angles on both sides of the nozzle orifice are different.A sophisticated nozzle internal flow model with good prediction accuracy is constructed based on the VOF method,and the simulation results indicated that the asymmetric structure of the upstream flow channel of the orifice is the reason for the asymmetric fuel flow pattern and cavitation distribution inside the orifice.When the hydraulic flip occurs inside the nozzle orifice,the spray plume is deflected from the orifice axis causing the change of the spray envelope angle of the multi-hole nozzles,which could also affect the spatial distribution of the fuel droplets and the formation quality of the mixture inside the combustion chamber.The deflection of the spray plume may also cause nozzle tip wetting and fuel accumulation at the nozzle exit,resulting in an increase in engine particulate matter emissions.When the inner and outer orifice inlet included angles are identical,the cavitation distribution is highly symmetric.The flow coefficient of the nozzle orifice reduces linearly with the decrease of the orifice inlet included angle,the cross section of the liquid core gradually transforms from round-shaped to dumbbell-shaped,and the breakup length of the liquid core decreases accordingly.When the inner and outer orifice inlet included angles are different,the degree of cavitation asymmetry increases with the reduction of the outer orifice inlet included angle,and the length of liquid core breakup length also decreases.When the inlet normal inclination angle is not equal to zero,the cavitation distribution inside the nozzle gets more asymmetric as the increase ofφ.The cavitation phenomenon is inhibited at the side where the inlet normal inclination angle is negative but enhanced at the side where the inlet normal inclination angle is positive,and the width of the hydraulic flip also increases with the increase of the inlet normal inclination angle.The breakup length of the liquid core first decreases with the increase of the inlet normal inclination angle.However,the liquid core changes from a cylindrical shape to a hollow semi-ring shape when the inlet normal inclination angle is larger than 10°,the cross section of the fuel jet changes from elliptic-shaped to crescent-shaped,and the breakup length of the liquid core conversely increases.Therefore,the contact area between the fuel jet and environmental gas increases significantly,and the gas-liquid interaction is enhanced.Finally,a systemic spray model which could comprehensively consider the influence of the nozzle internal structure,environmental factors,injection parameters,and needle motion was established based on the ELSA method.Co-field coupling simulation of nozzle internal flow and the spray atomization process was conducted with the ELSA spray model,and the results have demonstrated that even with the identical orifice sizes,the change of the orifice inlet included angle and the inlet normal inclination angle could significantly affect the spray macroscopic and microscopic characteristics.By adjusting the orifice inlet included angle and the inlet normal inclination angle,the spray plume cone angle and the particle size distribution characteristics could be significantly changed to improve the fuel atomization quality.The spray plume is relatively symmetric when the inner and outer orifice inlet included angles are equal,and the inlet normal inclination angle is equal to zero.However,the asymmetry of spray morphology obviously increases when the orifice inlet included angle on only one side decreases or the inlet normal inclination angle increases.The direction of the spray plume deviates from the orifice axis direction,and the particle size and distribution form of spray droplets also change accordingly.When both the inner and outer orifice inlet included angles are equal to 90°,the spray plume cone angle is 15.9°,the plume direction angle is 39.3°,the SMD of the spray droplets is 12.6μm,the maximum particle diameter is 94.6μm,and the particle size Span is 2.01.With the increase of the orifice inlet included angle,the cavitation intensity decreases,and the cavitation symmetry is improved so that the spray plume is more symmetric.Besides,the effective flow area of the nozzle orifice increases,and the initial particle size and spray width increase consequently.When the orifice inlet included angles are increased to 120°,the spray plume cone angle is increased to 18.3°,the plume direction angle reaches 39.8°,and the spray SMD is decreased to 12.0μm which would increase of evaporation rate of the spray droplets.Meanwhile,both the number and particle size of the large droplets at the spray center increase.The particle size span increased to 2.47,and the uniformity of particle size distribution gets worse.The proportion of the small droplets in the spray droplet group decreases while the proportions of the medium and large droplets increase.The maximum particle diameter is increased to 97.0μm so it takes a longer time for the spray droplets to completely evaporate.When the inner and outer orifice inlet included angles are not identical,the cavitation is stronger on the side with the smaller orifice inlet included angle,and the asymmetry of the spray plume is enhanced.When the inner orifice inlet included angle is 90°and the outer orifice inlet included angle is 60°,the plume direction angle is reduced to 38.5°,The particle size and the number of the droplets at the spray center decrease,and the plume cone angle increases slightly.The spray SMD is reduced to 12.1μm so that the spray evaporation rate is accelerated.The DV90 is reduced from 40.1μm to 36.9μm,and the maximum particle diameter is reduced from 94.6μm to 79.8μm.The droplet particle size distribution concentrated in the small-and middle-scale region,and the particle size span decreased slightly so that the time required for the spray droplets to completely evaporate is reduced.When the inlet section is not perpendicular to the axis direction of the nozzle orifice,namely the inlet normal inclination angle is not equal to zero,the fuel flow direction deflects towards the side that the inlet normal inclination angle is negative.Besides,the width of the fuel jet decreases,and the initial particle size of spray droplets at the nozzle exit is reduced.The fuel droplets are more dispersed,which is beneficial for the entrainment of the ambient gas and the diffusion of the spray plume.When the inlet normal inclination angle is increased to 20°,the spray plume deflects to the outer side of the orifice axis and the plume direction angle is increased from 39.3°to 44.5°.The spray plume cone angle is significantly enlarged from 15.9°to 25.8°（increased by 62.3%）.The particle size distribution curve of spray droplets is more dispersed.The number of small droplets decreased prominently,and the proportion of the medium and large droplets increased.The spray SMD is reduced from 12.6μm to10.6μm,which could accelerate the evaporation rate of the fuel droplets at the steady injection stage.The DV50 decreases from 17.4μm to 12.3μm（reduced by 29.3%）,and the DV90 decreases from 40.1μm to 24μm（reduced by over 40%）.The maximum particle diameter D_max=55.2μm,which is reduced by nearly 40μm,and the particle size span decreases from 2.01 to 1.53.The particle size distribution of spray droplets is more uniform and concentrated in the small size range.The fuel atomization quality is significantly improved,which is conducive to forming a more uniform combustible mixture.The results indicate that the difference in nozzle internal structure could significantly change the in-nozzle flow and cavitation characteristics and consequently affect the particle size of the large droplets in the spray droplet group.The spray maximum particle diameter D_max is negatively correlated with the nozzle void fractionα_n and the cavitation asymmetryη,and a prediction formula with a high coefficient of determination R-square=99.52%has been proposed to predict the spray maximum particle diameter of nozzles of different geometry features.