| Vehicle exhaust emissions regulation is becoming more stringent,and SCR emissions aftertreatment technology is facing increasing pressure to upgrade.In the SCR system,the ammonia storage characteristics of the catalyst and the flow mixing characteristics of the package structure are critical factors influencing the desired high NOx conversion and low ammonia leak rate.They are also important bases for formulating the reductant injection strategy.Therefore,this paper conducts a detailed study on the ammonia storage and spray mixing characteristics of the Urea-SCR system for vehicles.Based on the SCR transient reaction model verified by experimental data,the ammonia storage characteristics of copper-based molecular sieve catalyst under different working conditions,the relationship among ammonia-nitrogen ratio,conversion efficiency and ammonia slip were simulated.The results show that under non-oxidation and oxidation conditions,the saturated storage capacity of ammonia decreases with the increase of temperature.But with the increasing of temperature,the parasitic oxidation intensity of ammonia enhances,resulting in a lower ammonia storage capacity under oxidizing conditions than non-oxidizing conditions.In the SCR operating conditions,the time during which the ammonia storage dynamic process tends to stabilize gradually decreases with temperature increasing.But the time of ammonia release and consumption process decreases more significantly than the stored process.The SCR dynamic ammonia storage only accounts for a part of the total ammonia storage and its proportion gradually decreases with temperature increasing.In the middle temperature stage,the ammonia-nitrogen ratio is maintained at around 1,which is beneficial to increase conversion efficiency and reduce ammonia leakage.In the high temperature stage,the introduction of excess ammonia gas is beneficial to increase the NOx conversion efficiency and the amount of ammonia leakage can be substantially ignored.The spray characteristics of the non-gas-assisted injection system were studied by high-speed photography and laser particle size analyzer.It was found that the injection rate had no effect on the spray development morphology and droplet size when the injection pressure was constant.The spray model was then calibrated based on the experimental results.Finally,a compact spiral mixer was designed for the exhaust aftertreatment of the integrated DOC,DPF and SCR units.The influence of structural design parameters which is the number of turns and the pitch on the spray flow mixing was studied by CFD method.The number of spiral turns has little effect on the development of the flow field and spray wallfilm in the spiral mixer.When the number of turns is greater than 2 turns,the airflow can form a fully developed centrifugal swirl.The pitch has a great influence on the development of the flow field and the spray wallfilm.The smaller the pitch,the greater the swirling amplitude of the gas stream as it passes through the mixer,and the worser the flow field uniformity.The reasonable pitch design enables the SCR inlet speed and ammonia uniformity to be maintained above 90% at 200-600kg/h exhaust flow.Optimizing the design of the baffle structure in the spiral mixer can improve the uniformity of the flow field and significantly improve the ammonia uniformity of the SCR inlet. |
