Study On Spray Mixing Process Of Vehicle Diesel Engine Non-air Assisted SCR System

As countriesⅥemission regulations come on stage,the main pollutants in diesel exhaust NOx and PM are more stringent emission requirements are put forward.Single reprocessing technology is difficult to reduce all tail gas pollutants to below the limit.The development of efficient and compact integrated tail gas reprocessing device combined with in-machine technology is the current development trend of diesel vehicle emission reduction.And how to improve NOx conversion efficiency and anti-crystallization ability of urea-SCR system at low temperature is an urgent problem to be solved.In this paper,based on urea-SCR system,the spray mixing process of non-air-assisted injection SCR was studied in detail.The characteristics of non-air-assisted SCR spray are discussed by means of a visualized constant volume projectile test bench and a high-speed camera.Based on the experimental results,the SCR spray model was calibrated,and the spray development characteristics at different background temperatures were simulated by using the calibrated spray model.The results showed that:for the non-air-assisted urea-SCR injection device,when the injection pressure was constant,the injection rate had no significant influence on the characteristic parameters such as spray cone angle,penetration distance,particle size and initial velocity,and the overall spray shape was not significantly different.As secondary crushing occurs during spray droplet spraying,SMD of droplet gradually decreases with the increase of the distance from the nozzle outlet.Background temperature has little effect on the overall form of spray.In a limited space,the spray penetration distance at different temperatures hardly changes.However,increasing background temperature can accelerate droplet evaporation,effectively reduce droplet SMD,and promote spray atomization,at the same time,it can reduce the formation of liquid film.Taking the integrated tail gas post-treatment device as the research object,and build the physical models,the simulation calculation of the non-air-assisted urea-SCR spray mixing process was carried out,and the influence of different factors on the flow mixing and the development of spray liquid film was analyzed.The study showed that the change of injection amount of urea liquid had no significant effect on the uniformity of inlet speed of SCR,but the uniformity of NH₃concentration at the inlet of SCR gradually declined with the increase of ammonia nitrogen injection ratio.The quality of spray liquid film increased with the increase of the amount of urea liquid injection,while the area of liquid film almost remained the same under different ammonia-nitrogen ratios.The change of liquid film quality was mainly due to the change in the thickness of liquid film coverage.The uniformity of inlet cross section velocity of DOC,DPF and SCR all increased with the increase of exhaust temperature,but the higher the temperature was,the smaller the increase could be.The uniformity of NH₃concentration at SCR inlet is greatly affected by exhaust temperature.When the exhaust temperature is too low,the decomposition of urea is limited by low temperature,and the uniformity index is always in a low state.When the exhaust temperature is too high,the uniformity of NH3 concentration decreases due to the strong swirling flow in the mixing chamber.There is a trough on the curve of time variation of NH3 concentration uniformity,which is mainly caused by the local low temperature caused by the decomposition of urea,the hydrolysis of isocyanate and the evaporation of water.The quality of the liquid film decreases with the increase of the exhaust temperature.Increasing the exhaust temperature can not only effectively reduce the area of the liquid film,but also reduce the thickness of the liquid film,so as to inhibit the increase of the liquid film quality from the two aspects.Increasing the exhaust temperature can aggravate the disturbance effect of the airflow on the spray,change the position of the liquid droplets against the wall,and change the area of liquid film formation.