| With the implementation of the most stringent emission regulations national VI,the engine will soon realize near-zero emissions,increasing thermal efficiency and reducing energy consumption are the main goal of the future engine development technology.The characteristics of spray and atomization directly determine the quality of the mixture in the engine cylinder.It is the key of the whole combustion process and has important influence on the improvement of thermal efficiency.Non-circular nozzle has great potential ability in improving fuel atomization quality and has a positive effect in reducing the engine emissions.Therefore,it is important to carry out the mechanism of non-circular nozzle spray and the basic theory of fuel-air mixture strengthen,In order to meet the demand of the high thermal efficiency engine for the preparation of homogeneous mixture,in the aspect of theoretical research and engineering application is of great significance.In this paper,elliptical and equilateral triangular orifices are taken as research objects to explore cavitation,turbulent flow and axis-switching phenomena,and the mechanism of spray quality enhancement,also provides the theory basis for design and manufacture of the non-circular orifice.The main research works of this paper are as follows:(1)Based on the Large Eddy Simulation turbulence method,the model of cavitation flow in non-circular orifice was established,and the influence of orifice shape and pressure factors on the characteristics of cavitation and turbulent flow in orifice and the outflow parameters were studied.It is found that the cavitation distribution in the elliptical orifice is asymmetrical,and the length of the cavitation distribution in the major axial plane is longer than that in the minor axial plane.Compared with the circular orifice,the turbulent vorticity at the elliptical orifice outlet is larger,while the cavitation intensity in the elliptical orifice is smaller than that in the circular one.When the cavitation number is 1.28,the discharge coefficient of the elliptic orifice with the aspect ratio of 1.4 is 2.6% higher than that of the circular orifice.The elliptical orifice can effectively improve the orifice flow performance.When cavitation occurs in the orifice,the velocity coefficient of the circular orifice is the largest,while the area coefficient is the smallest.(2)The long distances microscopic technology was adopted to capture the near field spray images at different injection pressures,and the inner flow calculation model for the actual elliptic orifice is established.The actual injection pressure was the computational boundary conditions.Combined with the inner flow calculation results of elliptical orifice and the near field spray characteristics,the effects of circular and elliptical orifices on the near field spray characteristics of elliptical orifice are studied.The vapor volume fraction at the exit of the elliptical orifice along the major axis direction is higher than that of the circular orifice,which indicated that the cavitation intensity at the elliptical orifice exit in the major axis direction is higher than that at the circular orifice.The near-field spray angle of the elliptical orifice is always larger than that of the circular orifice during the whole injection process.The cavitation intensity of the elliptical jet orifice on the major axis direction is more intense than that of the circular orifice,which is beneficial for increasing the spray cone Angle.The projected area of the near-field spray of the elliptical orifice is larger than that of the circular orifice,which indicated that the use of the elliptical orifice can effectively improve the fuel-air mixing quality in the near-field domain and thus increase the fuel atomization quality in the whole spray field.(3)Based on the gas-liquid interface capture method of LES/VOF,the primary spray model coupled inner flow was established.The influence of nozzle shape and pressure parameters on the performance of primary spray and axial-switching phenomenon were studied.It was found that mushroom like structure appeared in the early stage of the jet head.The liquid break up length of the circular hole is longer than that of the elliptical hole,indicating that the liquid break up length can be effectively reduced by using the elliptical hole.The turbulent vorticity of the elliptical jet is larger than that of the circular jet,and the turbulent vorticity of the jet field increases with the increase of the aspect ratio.The turbulent vortex core structure in the elliptical jet field is more than that in the circular jet field.The turbulent vortex core structure is concentrated in the region where the liquid phase jet is broken,which indicates that the turbulent vortex structure can cause the instability of jet;The spray width of the elliptical jet on the major and minor axial view plane is larger than that of the circular jet.The air entrainment rate of the elliptical jet is higher than that of the circular jet,and the air entrainment rate of the elliptical jet is the highest with an aspect ratio of 1.8.(4)The spray and evaporation characteristics of the elliptical orifice under different injection pressures,ambient backpressure and ambient temperature were studied based on the Mie-scattering test.It is found that the spray penetration of elliptic orifice is shorter than that of the circular orifice spray in the whole spraying process,mainly because the larger surface area of elliptic orifice spray causes the larger air resistance.The number of interactions between the spray cone angle of the elliptic orifice in the major and minor axial planes decreases with the increase of the ambient back pressure;The spray projection area of elliptical spray is larger than that of circular orifice spray;The air entrainment of elliptic orifice spray is higher than that of circular spray in both major and minor axial plane.When the injection pressure is 120 MPa,the ambient temperature is 600 K,the ambient pressure is 1MPa,and the steady spray liquid length of the elliptical spray is 34.5% less than that of the circular one.The elliptical orifice can effectively shorten the spray liquid penetration,and the shorter elliptical spray liquid penetration indicates better fuel and air mixing quality,which is conducive to improving the combustion and emission performance of the engine.(5)The atomization and mixing strengthening mechanism of equilateral triangle jet hole were studied by Mie-scattering method.Based on the simulation of equilateral triangular orifice outflow parameters,the mechanism of the difference of spray characteristics between circular and equilateral triangular orifice were analyzed.The vapor phase volume fraction at the equilateral triangle orifice exit is smaller than that of the circular orifice,while the discharge coefficient of nozzle and the turbulent vorticity magnitude at the orifice exit are all higher than that of the circular one.The spray penetration of circularorifice is always longer than that of the equilateral triangle,while the spray angle of circular orifice is smaller than that of the equilateral triangle.The spray cone angle of equilateral triangle spray is interacted between the major axis and the minor axis.The volume of air entrainment of equilateral triangular orifice spray is larger than that of circular orifice.The spray liquid length of equilateral triangle orifice is smaller than that of circular orifice,the results show that using equilateral triangular orifice can effectively accelerate the breaking of spray liquid length,and thus improve the mixing quality of fuel and air. |
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