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Selective Catalytic Reduction (SCR) Simulation Using ANSYS

Introduction

This tutorial illustrates the setup and solution of a SCR system calculation using ANSYS Fluent. The selective catalytic reduction (SCR), of nitrogen oxides by N-containing reducing agents has received increasing attention in the automotive industry for removing the harmful and EPA-regulated NOx emissions. Ammonia is usually used as an agent to react with NOx in the presence of catalysts due to the lower exhaust temperature in the diesel engines.
However to ensure safe and convenient storage and operation, the urea-water solution is often used in automobile after-treatment systems. The urea-water solution is injected into the exhaust gas upstream of the SCR catalyst. The liquid jet goes through steps including the liquid atomization, evaporation/decomposition and hydrolysis to form a mixture consisting of ammonia, isocyanic acid, H2O, oxygen and other species. The mixture reacts with NOx in the SCR catalyst to reduce the NOx in the exhaust gas stream.
SCR performance is measured on the basis of its de-NOx efficiency, ammonia and iso-cyanic acid slip rates. These parameters heavily depend on the exhaust temperature, the NOx concentration, and the mixture quality at the catalyst inlet, which is mainly determined by the urea injection and decomposition rate.
Mesh file is provided with this tutorial, however following points need to be considered while creating the geometry and meshing the volume.

Geometric Consideration

  1. Computational domain consists of three parts:
    Injector (spray development region)
    Mixer
    Catalyst
  2.  Separate fluid zone to be created for catalyst as it is modeled as porous region.
  3. In place of resolving insulation it can be modeled using thick-wall approach.

Problem Description

In this tutorial you will simulate the injection, liquid atomization, evaporation/decomposition and mixing, that takes place inside the automotive SCR systems. To check the performance of the SCR system you will use a concept of uniformity index, which gives an indirect indication of the de-NOx efficiency of the SCR system. The area-weighted uniformity index of a specified field variable Φ is defined as,


The mass-weighted uniformity index of a specified field variable ϕ is defined as,


Where, A and v indicate the facet area vector and faced velocity vector of that face A. ρ represents density.

Summary

This tutorial illustrates the setup, simulation and postprocessing of a typical SCR study. Mixture preparation upstream of the catalyst is modeled and mixture quality is measured in terms of NH3 mass fraction and velocity uniformity. Simulation gives detailed insight of the spray development, atomization, vaporization, and mixing processes. Further simulation can be done to optimize the static mixer design/location for optimum mixture quality.Workbench can be used for quick design exploration and optimization.

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