Simulation Analysis And Experimental Research On SCR System Of High-power Diesel Engine

As a harmful emission of diesel engine,nitrogen oxides have great harm to the atmospheric environment and human health.With the upgrading of national environmental regulations,the requirements for diesel engine NO_X emission limits are becoming stricter.As a technology that can effectively deal with NO_X,SCR technology has received more and more attention from the society.In this paper,the SCR catalysts for a certain type of high-power diesel engine are modeled and simulated.Three urea mixing tubes are designed to match the high-power diesel SCR catalyst.The three schemes are:empty tube structure and post-mixing.Structure,front/rear mixer structure.Three urea spray cone angles were designed to couple with the mixing tube scheme.The three spray cone angles were 50°,60°,and 70°,respectively.Before studying the influence of mixing tube and spray cone angle on SCR system,briefly analyze various factors affecting SCR performance,and then propose indicators for evaluating SCR performance:distribution uniformity,back pressure,crystallization risk,NO_X conversion efficiency,NH₃ leakage Wait.In this paper,Star-CCM+software is selected as the simulation tool.Firstly,according to the experimental conditions and actual working conditions,a reasonable physical numerical model is selected,and the appropriate boundary parameters are set.Finally,the CFD simulation calculation is performed.The effects of three mixing tube schemes on the distribution uniformity,back pressure and crystallization risk of the SCR system were simulated.The results show that the pre-mixer in the front/rear mixer structure can generate swirling flow and promote urea droplets and exhaust.Mixing,NH₃ distribution uniformity can reach 0.95,compared with the rear mixer structure NH₃ distribution uniformity increased by about 4.0%,back pressure increased by 0.33kPa;compared with the empty tube structure NH₃ distribution uniformity increased by 6.7%,increased back pressure 0.77kPa.The uniformity of velocity distribution of the three hybrid tube schemes is not much different.The front/rear mixer structure does not affect the liquid film distribution on the inner wall of the mixing tube,but promotes the evaporation decomposition of the aqueous urea solution due to the influence of the swirling flow,and reduces the liquid film distribution on the surface of the post mixer.Coupling analysis of mixing tube and urea spray cone angle,found that 50°urea spray cone angle SCR system urea decomposition degree is lower than 60°,70°urea decomposition degree,50°,60°spray cone angle SCR system mixing tube inner wall liquid film distribution below 70°liquid film distribution.According to the emission regulations,combined with the actual test conditions,the emission test cycle test plan and the urea crystallization test plan were formulated and tested.The test results show that the NO_X ratio of the SCR system of the front/rear mixer structure is 1.14 g·?kW·h?^-1,which is about 6.2%higher than that of the post mixer.The NO_X ratio is reduced by calibration optimization.To 0.47 g·?kW·h?^-1;the 60°spray cone angle can reduce the possibility of urea crystallization in the SCR system,and the number of operating points where crystallization occurs at a low temperature operating point compared to the 70°spray cone angle can be applied more.High emission limits and no urea crystallization.