Deposit Formation from Urea Injection: a Comprehensive Modeling Approach

Abstract

Long-term reliability is one of the major requirements for automotive exhaust aftertreatment systems with selective catalytic reduction (SCR) using urea water solution (UWS) as NH 3 carrier fluid. A high injection rate of UWS or unfavorable operating conditions may lead to formation of solid deposits, which decrease system efficiency by increasing backpressure and impairing ammonia uniformity. A reliable numerical prediction of deposit formation in urea SCR systems is desired for optimization of system design. However, comprehensive modeling of physical and chemical processes in the tailpipe as well as different time scale phenomena represents a challenging task. This study presents a comprehensive approach for modeling UWS injection, droplet impingement, liquid film and deposit formation based on CFD-simulation. An existing kinetic model for urea decomposition is integrated into the CFD code to predict solid by-product formation from wall films. Physical simulation time is extensively increased by substituting the Lagrange-particles with source terms of mass, momentum and energy reducing simulation time by a factor of 20. The comparison of measured and simulated results shows the capability of the presented modeling approach to predict position and chemical composition of solid deposits.

Spreading and rebound dynamics of sub-millimetre urea-water-solution droplets impinging on substrates of varying wettability

“…contact angle models in other approaches, where an explicit relation between dynamic contact angle and contact line speed is prescribed. being equal to the sphere volume yields (13) and (14) so that (15) which corresponds to Eq. ( 8) of Caviezel et al [1].…”

“…Computational fluid dynamics constitutes a valuable tool to support the optimization of automotive SCR-systems at various operating conditions [10][11][12][13][14]. The spray is typically modelled with a statistical Lagrange approach in combination with an Eulerian approach for the gas phase, being based either on the Reynolds-averaged Navier-Stokes equations or on large eddy simulation.…”

Experimental and Numerical Analysis of an Innovative Mixer Geometry for Urea Injection in SCR Applications

“…The UWS decomposition could be modeled inside a 3D-CFD analysis through different methodologies. For example, simplified models describe only the process of urea thermolysis and hydrolysis using lumped reactions, while more elaborated models consider detailed urea reaction schemes and by-product formations [15]. Simplified models have been broadly applied confirming their validity in an extensive range of exhaust temperatures [29]; however, their main drawback is the lack of urea deposit formation predictiveness.…”

“…Improvements in numerical simulations have been made to predict the process of urea thermo-hydrolysis [13], and detailed decomposition mechanisms of urea have been implemented successfully to properly simulate the deposit formation process [14,15]. However, the inclusion of a detailed chemical scheme to simulate UWS decomposition could properly predict the urea by-product formation and consequently the deposit build-up, but with high penalties in terms of computational cost.…”

Experimental Investigation of AdBlue Film Formation in a Generic SCR Test Bench and Numerical Analysis Using LES

“…Therefore, the proper numerical description of the AdBlue injection and the subsequent spray-wall interaction plays a key role for determining the result of the AdBlue impingement. For numerical modeling and analysis, Reynolds-averaged Navier-Stokes (RANS) based techniques have been widely adopted [4,15,23,[30][31][32] with limitations describing highly unsteady processes. As discussed above, the interaction of spray droplets with the system walls are governed by the properties of the incident droplets (e.g., size, velocity and incident angle) and the wall properties.…”