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 NH3 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.
Highlights
Selective catalytic reduction (SCR) is a widely applied exhaust gas aftertreatment measure to reduce nitrogen oxide (NOx) emissions of both light and heavy-duty vehicles
This study presents a comprehensive modeling approach for physical and chemical processes involved in the formation of solid deposits from urea
The source terms for the liquid film and the gas phase were calculated during a single injection event and further applied for the modeling of liquid and deposit formation
Summary
Selective catalytic reduction (SCR) is a widely applied exhaust gas aftertreatment measure to reduce nitrogen oxide (NOx) emissions of both light and heavy-duty vehicles. The model was further applied for simulations of a test rig By this approach, decomposition of urea to various solid by-products was modeled in the liquid film. This study presents a comprehensive modeling approach for physical and chemical processes involved in the formation of solid deposits from urea This includes injection, spray/wall interaction, evaporation, liquid film formation and urea reactions forming solid by-products. In order to speed up the simulation runtime and accomplish the modeling of deposit formation within typical time ranges of several minutes, an injection source approach was developed In this approach, the numerical parcels representing the spray are substituted by source terms of mass, momentum and energy that are directly applied to the film and gas phase. Model validation is performed by experimental results from an engine test bench and subsequent deposit analysis demonstrating the capability of the modeling approach to predict deposit formation in real scale systems
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