Study On Spray Characteristics Of Non-circular Orifice In Air Assisted SCR System

With the national motor vehicle emission regulations continue to be strict,the reduction of diesel exhaust emissions,especially nitrogen oxides,is still the focus of domestic and foreign scholars.The effective reduction of NO_x emission needs to further optimize the existing urea selective catalytic reduction(SCR)system.To greatly reduce pollutant emissions,the atomization quality of the urea water solution(UWS)is of great significance in the SCR system.A good atomization quality can accelerate the evaporation of the urea water solution and the mixing with the exhaust gas,accordingly,improve the conversion efficiency of NO_x and reduce the formation of sediments.The research results of many scholars at home and abroad have shown that noncircular nozzles have great potential for improving the atomization effect.Therefore,exploring the impact of non-circular nozzles on the quality of spray atomization based on the SCR system is useful for theoretical research and practical applications.In this paper,circular orifice,triangular orifice and elliptical orifice are used as the research objects.The air-assisted SCR system is used to generate spray.With the help of the particle image velocimetry(PIV)system and laser particle analyzer,the macroscopic characteristics of the spray,the spray velocity field and the spray particles distribute are explored.The main work and research conclusions are as follows:(1)The spray images are acquired by the CCD camera in the PIV system and processed by MATLAB.First,the spray macroscopic characteristics(spray cone angle,spray penetration distance,spray projection area)was analyzed under different working parameter.Then change the diameter of the circular orifice,the side length of the equilateral triangle orifice,and the eccentricity of the elliptical orifice respectively to explore the influence of the diameter,side length,and eccentricity on the macroscopic characteristics of the spray.The results show that: increasing the injection rate,the spray cone angle decreases,the spray penetration distance increases,and the effect of increasing the injection pressure is opposite.The spray cone angle of the major plane of the elliptical orifice is the largest,and the spray cone angle of the triangular orifice is the smallest;the spray penetration distance of the triangular orifice is the largest,and the spray penetration distance of the elliptical orifice is the smallest.This is because the orifices have the same outlet cross-sectional area.And the circumference of the circular orifice and elliptical orifice is larger than that of the triangular orifice,so that the air resistance of the spray is greater during the development process.The spray projection area of non-circular orifice is larger than that of circular orifice,which shows that non-circular orifice can effectively improve the quality of gas-liquid mixing.For circular orifice and equilateral triangle orifice,the increase in the outlet diameter and the side length of the outlet improves the spray cone angle and spray penetration distance.For elliptical orifice,the eccentricity increases and the spray cone angle increases.The spray penetration distance is reduced.(2)Through the PIV commercial software Dynamic studio,based on the cross-correlation analysis method,the axial and radial velocity distribution characteristics of three orifices with equal outlet area under different working conditions are analyzed,and then change the diameter of the circular orifice,the side length of the equilateral triangle orifice and the eccentricity of the elliptical orifice respectively to explore the influence of the aperture,side length,and eccentricity on the atomization.The results show that the elliptical orifice can bring greater axial velocity due to its particularity;when the injection rate is changed,the maximum spray velocity of the circular orifice is about 16.6m/s,and the maximum spray velocity of the elliptical orifice is higher than 15%of the circular orifice,indicating there is a larger elliptical orifice spray momentum,and the exhaust gas can be mixed better.The maximum velocity of the triangular orifice spray is 11.4%lower than that of the circular orifice,but the triangular orifice spray reaches the maximum velocity fastest,indicating that the triangular orifice spray can mix with the exhaust gas faster.The change of working conditions on the selected section has no obvious influence on the radial velocity,and only has a greater influence near the axis.(3)The particle size distribution of three different orifices with equal outlet area under different working conditions and different cross-sectional positions was analyzed by a laser particle size analyzer,and then the diameter of the circular orifice,the length of the side of the equilateral triangle orifice,and the eccentricity of elliptical nozzle were changed respectively to explore the influence of the aperture,side length and eccentricity on the particle size of the spray.The study found that increasing the spray pressure and reducing the spray volume can reduce the size of the droplets;when the sprayed droplets leave the nozzle,through the effects of aerodynamic breaking and evaporation in the air,the droplet diameter gradually decreases.Compared with the circular orifice,the spray droplet particle size of non-circular orifice decreases slowly;the reduction diameter of the circular has a greater effect on improving the atomization effect,while changing the side length of the triangular orifice and the eccentricity of the elliptical orifice is more effective.