| For a direct-injection diesel engine,increasing injection pressure is one of the most effective techniques to improve the atomization and air-fuel mixing performance.With the continuous increase of injection pressure in the high-pressure common rail injection system,Bosch and Delphi have already launched a system whoserail pressure is more than 200MPa and even 300MPa is in the process of development.Due to the rarely research of fuel atomization characteristics under ultra-high injection pressure,and whether there exist similarities and differences on the macroscopic and microscopic characteristics between ultra-high and conventional high injection pressure,it has the vital practical significance and engineering value to conduct a research on fuel spray under ultra-high injection pressure.In this paper,the exploratory study by trial method was carried out to discuss the penetration development characteristics and droplet size distributions in the spray field within the injection pressure range from 200MPa to 400MPa.The experimental study of the atomization process under ultra-high injection pressure is helpful to deepen people’s understanding of ultra-high injection fuel atomization mechanism,is of great theoretical significance to enrich and develop the basic theory of fuel spray atomization.In this paper,an ultra-high injection pressure fuel spray test system was built based on the ultra-high injection pressure generator.A diaphragm rupture atomizer was self-designed and manufactured inspired by the shock tube which was applicable for ultra-high pressure injection.Through the formula computing and some test experiments,reasonable structure parameters(diameter of nozzle flow,diameter of plug flow)of the atomizer,diaphragm material,and the correspondence between diaphragm thickness and injection pressure were determined.In order to meet the requirements of spray tests under high back pressure and frequently replacement of the diaphragm,a well-sealed and easily disassembled constant volume chamber was designed.By combining camera technique with Schlieren system and high-speed photography,the information of fuel atomization and induced shock waves can be obtained simultaneously.The macroscopic development of fuel spray atomization under ultra-high injection pressure was studied using high-speed photography.Through analysis of the spray configuration,spray penetration property,and cone angle character under different injection pressures,the impact of the ultra-high injection pressure on the macro characteristics of fuel spray was discussed.A spindle structural spray body with thickness in the end and fineness in the middle was obtained in atmospheric air condition.The influence law of increasing injection pressure on spray penetration and cone angle characteristics was clearly defined,and the penetration results were compared with those calculated by the traditional empirical formulas.It was found that the increasing injection pressure can effectively promote the axial penetration performance and the spray cone angle.However,the ultra-high injection pressure could weaken the promotion function with the angle significantly increased.The influence rules of nozzle diameter and back pressure on the spray macro characteristics under super-high injection pressure of 300 MPa were investigated to discuss the influence of nozzle geometric structure and environmental gas parameter on the penetration and cone angle characteristics under ultra-high injection pressure.The original empirical model was modified to apply to ultra-high injection pressure.The particle size distribution characteristics of the ultra-high pressure fuel spray field was studied using the method of Malvern laser particle size analysis.Through the analysis of the droplet size distribution/cumulative volume distribution,representative diameter of droplet and Sauter mean diameter(SMD)in different axial/radial positions,the distribution characteristics of droplet particle size in the area of near,middle,and far spray field were obtained.It was defined that the spray droplets distributed in space.The influence of increasing injection pressure up to ultra-high on the droplet size distribution was discussed.The results showed that the higher injection pressure could effectively promote the breakup of large droplets into small ones.However,the SMD of all droplets in the measurement point decreased with gradually insignificant trend,which indicated that the improvement of ultra-high injection pressure on jets breakup and atomization was weakened.Based on the analysis of droplet size distribution with different nozzle diameters,the promoting effect of small hole nozzle on the ultra-high injection pressure spray atomization of 300MPa was clarified.Based on the photography technique by the combination of Schlieren system and high-speed camera,the evolution process of leading edge shock wave induced by ultra-high injection pressure was studied.Through the analysis of propagation characteristics of the leading edge shock wave in the near field of spray nozzle,the induction mechanism of leading edge shock wave was explored,and the correlation between shock wave propagation and spray development was clarified.It was found that the leading edge shock wave shown different propagation patterns and presented two modes in the axial and radial direction.The initial induced position of the leading shock was discussed based on the experiment.The influence rules of ultra-high injection pressure fuel spray on the generation and propagation of the leading shock were revealed with the distribution characteristics of the two-mode shock junction point.Rules for the impact of injection pressure and nozzle diameter on the velocity of spherical shock reaching the local sound velocity,the development of junction points,and the oblique shock angle were illustrated.The separation cause of spray head and leading shock and the impact of back pressure on the separation phenomenon could be obtained by the analysis of the propagation rules of the super-high fuel spray and leading shock under different environment pressures.It was proposed that the essential reason would lie in the velocity difference between the fuel spray and the leading shock,and then undisturbed shock propagated in the local sound velocity.The coupling relationship between the macroscopic characteristics of fuel spray and the evolvement rule of induced shock waves under ultra-high injection pressure could be further discussed on the basis of previous research.The influence rules of the spray morphology on windward side on the propagation characteristics of the bow shock wave and oblique shock wave were defined.It was found that the correlation between the radian of bow shock wave(AB)and the obtuse degree of spray head(r/b),and also the correlation between the shock wave angle of oblique shock(β)and the deflection angle of spray head at different Mach number of the spray jets.The similarities and differences between the spray body under ultra-high injection pressure and the conical solid in the θ’-β-Ma correspondence were also discussed.The influence of the maximum windward of ultra-high pressure spray on the multiple shock waves was explored according to the penetration rule of first/second expansion wave and maximum windward side.On the basis of evolution of multiple shock waves expanding angle with injection pressure,the coupling relationship between ultra-high pressure spray and multiple shock waves was further defined. |
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