A Simulation Study on Urea Maldistribution and Implications for NOx Reduction with a Multi-Channel Modelling Approach  
[PERSON]  
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[PERSON]  
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[PERSON]  
3 State Key Laboratory of Engine and Powertrain System, Weifeng 261061, China; [EMAIL_ADDRESS]  
[PERSON]  
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[PERSON]  
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[PERSON]  
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[PERSON]  
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Footnote 1: email: [EMAIL_ADDRESS]  
Footnote 2: email: [EMAIL_ADDRESS]  
Footnote 3: email: [EMAIL_ADDRESS]  
Footnote 4: email: [EMAIL_ADDRESS]  
urea-SCR system. In the aftertreatment system, the process of the urea aqueous solution generating NH3 through hydrolysis and pyrolysis reactions is affected by temperature changes, gas-flow drag forces, and diffusion. Eventually, the differences in ammonia concentration distribution on the front end surface of the SCR (selective catalytic reduction) carrier constitute the problem of ammonia distribution uniformity. Basically, the monolith reactor can be depicted as a single uniform channel. And the SCR model/design would typically demonstrate the supposition of single channel behaviour, featuring uniform concentration, flow, and temperature throughout all the channels of the monolith. However, due to the insufficient residue time, the packaging limitations, incomplete decomposition of the DEF, and poor DEF spray results, the urea/ammonia at the inlet of SCR within an aftertreatment system might not be evenly distributed in non-axial directions in practice, which may lead to a lower NOx conversion than expected, as shown in Figure 1, and ammonia slip in the outlet gas. There are several experimental and numerical studies focusing on the investigation related to the effect of maldistribution and the optimization of the design and operating conditions of injectors for achieving a more uniform ammonia profile [6; 7; 8; 9; 10; 11; 12]. It is generally recognized that the distribution of NH3 at the catalyst inlet significantly affects the conversion effect of nitrogen oxides. Therefore, designing different mixer structures to reduce this non-uniformity is a widely pursued research direction at present. [PERSON] et al. investigated the ammonia uniformity in the SCR system through the application of two types of urea injection devices [13]. Simultaneously, the ammonia uniformity along with urea injection time was examined to anticipate the NOx conversion reduction efficiency of the SCR system [14; 15]. [PERSON] et al. employed 1-dimensional (1D) model simulations to explore the impacts of the non-uniformity of temperature, flow velocity, and the maldistribution of NH3 on the performance of the Cu-SCR system. It was ascertained that the SCR inlet NH3 maldistribution led to a 3.5% reduction in NOx reduction efficiency throughout a surrogate HD-FTP cycle [16]. [PERSON] et al. put forward that the enhancement of the uniformity coefficient of the flow field and gas-phase components could enhance NOx conversion efficiency and minimize ammonia leakage [17]. [PERSON] et al. utilized a CFD (computational fluid dynamics) model comprising a mixing chamber coupled with an SCR convertor to study the system, and the effects of temperature, NOx-NO2 ratio, and inlet velocity were probed [18]. Despite numerous endeavours in modelling and experimental validation, a more comprehensive  
Figure 1: Effect of urea maldistribution on SCR DeNOx. understanding of the influence of various non-uniformity parameters on SCR performance is still needed. An efficient approach featuring shorter calculation durations and superior prediction outcomes is still highly sought after. In this work, a multi-channel approach was adopted for predicting transient SCR performance in the 1D model, taking account of the urea maldistribution quantified by the uniformity index for realistic SCR inlet parameters, while the urea decomposition kinetic has been explored in previous work [19]. Based on the experimental results measured in the WHSC cycle, the corresponding simulation conditions were established.