A Selective Catalytic Reduction CFD simulation is one of the most important tools for designing modern, clean diesel engines. SCR is a clever technology that removes harmful nitrogen oxides (NOx) from exhaust gas. It works by injecting a liquid called urea solution (often sold as AdBlue) into the hot exhaust pipe. The heat turns the urea into ammonia gas (NH₃). This ammonia then travels with the exhaust into a special filter called a catalyst. A Catalyst CFD simulation is needed because, inside this catalyst, the ammonia reacts with the dangerous NOx and converts it into harmless nitrogen gas (N₂) and water (H₂O). This process must be very efficient, often removing over 95% of NOx to meet strict environmental laws.

Simulating this in ANSYS Fluent is very complex because many things happen at once. First, the urea solution is sprayed as tiny droplets. We use the Discrete Phase Model (DPM) to track where each droplet goes. Because the droplets contain both urea and water, we must use a special multi-component DPM model to simulate how the water and urea evaporate at different rates. Sometimes, these droplets hit the walls of the exhaust pipe and form a liquid layer. A Wall-film fluent model is essential to predict how this film spreads, evaporates, or worse, forms solid deposits that can block the system. The ammonia gas must then mix perfectly with the exhaust before it enters the catalyst, which is modeled as a Porous Fluent media to correctly simulate the pressure drop. A Selective Catalytic Reduction ansys simulation allows engineers to test and perfect this entire process on a computer. They can change the injector position, the spray angle, and the exhaust temperature to find the best design that gives the most NOx reduction without wasting ammonia or creating harmful wall deposits.

Figure 1: A schematic diagram of the Selective Catalytic Reduction (SCR) system, showing the urea injection point, the mixing duct, and the catalyst monolith where NOx reduction occurs.

Simulation process: Modeling Multiphysics in an SCR System considering wall-film & multi-component dpm particle

The SCR CFD simulation in Ansys Fluent is configured with a hybrid solver approach where the continuous gas phase (exhaust flow) uses steady-state solution, while the DPM (Discrete Phase Model) uses unsteady tracking to capture the time-dependent behavior of urea-water droplets as they inject, evaporate, decompose, and interact with turbulent eddies in the exhaust stream. The catalyst monolith is modeled as porous media with appropriate permeability, inertial resistance, and porosity values to represent the honeycomb structure’s pressure drop and flow distribution characteristics. Multi-component DPM particles are configured to represent urea-water solution droplets containing both urea (CO(NH2)2) and water (H2O) components that evaporate at different rates based on their vapor pressures and molecular weights – this multi-component approach is critical because urea and water have different boiling points (133°C for urea vs 100°C for water) and the droplet composition changes during evaporation, affecting droplet size, temperature, and chemical reaction rates. The species transport model is activated to track all chemical species in the gas phase. Three conical injection sources are positioned upstream of the catalyst to represent typical SCR injector configurations, with each injector specified using DPM surface injection boundary conditions that define droplet size distribution (typically Rosin-Rammler).

The SCR chemistry is modeled using finite rate reactions with no TCI (Turbulence-Chemistry Interaction) to ensure that reaction rates are controlled purely by chemical kinetics and temperature. Two governing reactions are implemented in the Fluent reaction mechanism: Reaction-1 represents urea thermal decomposition following

CO(NH2)2 → NH3 + HNCO

using Arrhenius rate parameters. Reaction-2 represents HNCO hydrolysis following

HNCO + H2O → NH3 + CO2

to reflect the slower hydrolysis kinetics that require sufficient water vapor and residence time for complete conversion to ammonia. Convective thermal boundary conditions are applied to catalyst walls, while steel walls use shell conduction to account for heat transfer through the metal SCR housing and potential heat loss to ambient air. Wall-film boundary conditions are activated on all solid surfaces to capture droplet impingement, film spreading, evaporation, and potential deposit formation.

Post-processing: CFD analysis of the SCR System’s Health

The simulation results allow us to perform a complete engineering audit of the SCR system, as if we were inspecting a real unit. We will check its vital signs—temperature, droplet behavior, mixture quality, and wall health. The static temperature contour in Figure 2 acts as the system’s thermometer. The verdict is Healthy. For the chemical reactions to work, the urea needs to be heated above 160°C and the catalyst needs to be in an optimal range of 280-350°C. The contour confirms this is happening perfectly. Hot exhaust enters at ~400°C, and the urea is injected directly into this hot zone, ensuring it heats up and decomposes quickly. The majority of the reactor, especially near the catalyst, operates in the ideal range (280-350°C). This shows the system has a good thermal profile for efficient NOx reduction.

Figure 2: The static temperature contour from the SCR CFD simulation in Ansys Fluent. It shows the hot exhaust gas (380-400°C, red) providing the necessary heat for the urea decomposition reactions to occur efficiently.

The DPM particle contour in Figure 3 shows us the life story of the urea droplets. The verdict is Good Atomization and Evaporation. We see large droplets (red-orange, ~0.06 mm) right at the injection point, as expected. As they travel downstream, they turn yellow, green, and finally blue, shrinking to less than 0.02 mm. This is the visual proof that the multi-component DPM is correctly capturing the evaporation process. The water and urea are turning into gas, which is the essential first step to creating ammonia. The good spread of droplets across the pipe also shows that the three-injector setup is providing good initial coverage.

The ammonia mass fraction contour in Figure 4 is the ultimate performance test. The verdict is Acceptable, but Needs Improvement. The contour shows how ammonia (the active ingredient) is distributed right before it enters the catalyst. We see some areas with high concentration and others with lower concentration. The simulation calculates a uniformity index of 68.1% (from Table 2). While this is acceptable, high-performance SCR systems aim for over 90%. An uneven mixture like this means some parts of the catalyst will get too much ammonia (risking “ammonia slip,” where unused ammonia exits the tailpipe) and other parts will not get enough (risking “NOx breakthrough,” where NOx is not converted). This is a critical insight for designers.

Figure 3: The multi-component DPM particle diameter colored by particle mass  contour, visualizing the life cycle of urea-water droplets as they evaporate and shrink from an initial size of ~0.06 mm down to less than 0.02 mm.

Figure 4: The mass fraction of NH₃ (ammonia) contour at the catalyst inlet. The contour is used to calculate the uniformity index of 68.1%, a key measure of mixing performance.

Figure 5: The static pressure field from the Fluent simulation, showing an acceptable system pressure drop of approximately 60-80 Pa, which minimizes negative impact on engine efficiency.

Table 1: Wall-Film Thickness Distribution in SCR System

Table 2: Ammonia Distribution Quality Assessment

The wall-film data in Table 1 is like a check for internal blockages. The verdict is a Significant Warning. The table reveals that the upstream section has a wall-film thickness of 0.0466 mm, which is labeled as a High risk level. This is a serious engineering concern. This thick liquid film means many droplets are hitting the wall right after injection. If this liquid film evaporates too quickly without the urea properly decomposing, it will leave behind solid urea crystals. Over time, these deposits can build up, block the exhaust pipe, increase backpressure, and cause the entire system to fail. In contrast, the catalyst walls have a very low film thickness (0.0018 mm), which is excellent. This means the mixer is doing its job of keeping the liquid away from the catalyst face.

The static pressure contour in Figure 5 measures how much the SCR system restricts the engine’s exhaust flow. The verdict is Excellent. The contour shows the pressure drops from ~120 Pa at the inlet to ~40 Pa at the outlet. This means the total pressure loss is about 60-80 Pa. This is a very low value and is well within the acceptable limits for an automotive application. It means the SCR system is not creating excessive backpressure that would harm the engine’s performance or fuel economy.