Research On Optimization Of Urea Water Solution Atomization And Mixing Performance For Diesel Engine SCR System At Low Exhaust Temperature

With the gradual transition of emission regulations to the CN VI emission standards,emission limit of diesel engines'nitrogen oxide?NO_x?is further reduced.The proportion of low-speed and low-load operating conditions increases and the exhaust temperature decreases as the emission test method turns to World Harmonized Transient Cycle?WHTC?,which brings challenges to the Urea Selective Catalytic?Urea-SCR?system.Therefore,in order to solve the problems that deterioration of atomizing mixing quality,imperfect evaporative pyrolysis process,and increased risk of urea deposition caused by the low exhaust temperature,the study of spray atomization mixing process was developed to provide a theoretical basis for the ultra low emission control of CN VI.The spray atomization and wall impingement,evaporation and pyrolysis,wall film and other models of Urea Water Solution?UWS?were established by AVL Fire.Based on the model verification,the effect of injection pressure and the angle between the nozzle axis and the exhaust pipe axis?i.e.the injection angle?on the spray development,NH₃ concentration distribution,wall film thickness and other parameters were studied,and the parameters optimization was also carried out.The results show that compared with high exhaust temperature?430??,the UWS spray/wall impingement is severe and the process of evaporation and pyrolysis is hindered at low exhaust temperature?180??,resulting in uneven NH₃concentration distribution and wall film formation.For the injection system with nozzle tilted in straight pipe under low exhaust temperature condition,increasing injection pressure is beneficial to the formation of droplets with small size,which can promote the evaporation and pyrolysis of the droplets.However,it also may lead to the problems that the area of spray impingement is more concentrated and the wall film thickness increases.The decrease of the injection angle enables the droplets to be more susceptible to the exhaust gas flow and thus to move in the direction of the air flow,improving the droplet distribution within the tube and reducing the spray impingement.Therefore,increasing the injection pressure while appropriately reducing the injection angle is conducive to improvement of the uniformity of NH₃ concentration distribution and reduction of the wall film.Then,based on the optimization of UWS spray,the influence of mixer on the UWS spray mixing process under the low exhaust temperature?180??was studied.The parameters optimization and influence degree of mixer structure and position were investigated by orthogonal test.The results show that the functions of mixer is violent disturbance to the flow and better break up process of droplets,the enhancement of atomization and mixing process can thus be realized;the mixing process is significantly affected by mixer fin angle,followed by the shrinkage angle and the blade angle,but slightly by the distance from the nozzle and the width of the tapered tube,and therefore the optimal mixer structure parameters and position were determined;the uniform index of NH₃ is 0.91 of the optimal scheme with the pressure drop 120Pa.Shortening mixing distance can reduce the atomization and mixing performance.While the mixing distance is reduced to 4 times the length of the diameter for SCR equipped with mixer,good NH₃ concentration distribution can still be achieved.Finally,based on the Mie scattering theorem,a laser visualization platform of UWS droplets distribution within exhaust pipe was constructed under actual exhaust conditions.The distribution of droplets on the different sections at 180�C,200�C,220�C and 240�C was measured.The results indicate that the lower exhaust temperature is,the stronger Mie scattering light intensity becomes,and the size and distribution of droplets are larger and more uneven respectively,which restrains the evaporation and pyrolysis of UWS and demonstrates the necessity of improving spray atomization and mixing process under the low exhaust temperature.Besides,the test bench provides a platform for experimental research on the atomization and mixing process of UWS.However,due to the limited time,the experimental verification for the optimization scheme will be carried out subsequently.