| With the increasingly stringent emission regulations,Selective Catalytic Reduction(SCR)has become an important technical means for diesel engine emission control.The conversion efficiency of NOx in SCR system is mainly affected by the speed uniformity and NH3 distribution uniformity of SCR system.By directly observing the gas velocity and concentration distribution on the cross section,we can only see the changing trend,but can’t give the quantitative analysis results.Therefore,it is of great significance to evaluate the speed uniformity and NH3 distribution uniformity at the front end of SCR carrier through some indicators.In this paper,by combining simulation and experiment,and introducing deviation from average square sum formula,an index that can accurately evaluate the velocity uniformity and NH3 distribution uniformity is derived,which is used as a key technical index to guide the optimization of SCR mixer structure,thus improving the emission performance of SCR system.The main contents are as follows:(1)Based on Star-CCM+,a CFD simulation model of a diesel engine aftertreatment system was established,and the model was verified by using the pressure drop data of the typical operating conditions of the target engine,and the maximum error was 4.8%,indicating that the model had high calculation accuracy.The mixer part of the SCR system has a significant effect on the pressure drop,and the mixer accounts for up to 41%of the total SCR pressure drop in the test working conditions.(2)By introducing the off-average sum of squares,a new evaluation method that can reflect the flow uniformity and NH3 distribution uniformity of the SCR system is proposed.The experiment of NOx single-point conversion efficiency verifies the feasibility of the uniformity evaluation method proposed in this paper,and the prediction accuracy is improved by 3.4%compared with the traditional evaluation method.Based on this uniformity evaluation method,the effects of exhaust gas temperature and exhaust gas flow rate on the velocity uniformity and NH3 distribution uniformity of the SCR system were simulated and analysed.The results show that with the increase of exhaust temperature,the uniformity of NH3 distribution on the front surface of SCR carrier becomes better and better,and the uniformity of velocity distribution becomes worse and worse;with the increase of exhaust flow rate,the uniformity of NH3 distribution and velocity uniformity on the front surface of SCR carrier becomes worse and worse.(3)Based on this homogeneity evaluation method,structural improvements were made to the SCR mixer.The results show that,compared with the original mixer,the optimized mixer solution is equipped with additional baffles to enhance the airflow disturbance,and the velocity uniformity indices at the front surface of the SCR carrier at the selected operating points are improved by 0.053,0.059 and 0.083 respectively;the NH3 distribution uniformity indices are improved by 0.025,0.023 and 0.01 respectively;the pressure drops are improved by 0.52 kPa,0.6 kPa,1.18kPa respectively,but still meet the engineering constraints.(4)The SCR system of the two mixer schemes was tested with WHSC steady-state cycle and WHTC transient cycle,and the exhaust back pressure,engine fuel consumption and SCR downstream NOx emissions of the two schemes were analyzed.The results show that compared with the original mixer scheme,the average exhaust back pressure of the optimized mixer scheme is higher during the whole WHSC steady-state cycle,and the fuel consumption increases by about 1.1%.For the NOx concentration downstream of SCR,the WHSC cycle and cold WHTC cycle tests of the optimized mixer scheme were reduced by approximately 11.6%and 17.3%,respectively,compared with the original mixer scheme,and met the China VI emission regulations.It shows that the optimization of the mixer structure has good performance,and the emission performance under transient conditions is better than that under steady state conditions. |
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