Reacting Species Transport

Complex Chemistry

The Complex Chemistry model employs a stiff solver to integrate the chemistry over a time-step, and uses techniques, such as Clustering, Dynamic Mechanism Reduction, In-Situ Adaptive Tabulation (ISAT), and Acceleration Factor Control to mitigate this expense. Since the Complex Chemistry model uses a detailed mechanism, you can model kinetic phenomena—such as flame ignition, slowly forming pollutants, flame quenching, and extinction. Complex chemistry mechanisms are usually imported in Chemkin format.

This model offers the most general approach in modeling finite-rate kinetics. Use this model if other models do not capture the finite-rate effects that drive the process.

The Complex Chemistry model has submodels for simulating turbulence-chemistry interactions—the Laminar Flame Concept (LFC), the Eddy Dissipation Concept (EDC), and the Turbulent Flame Speed Closure (TFC) model.

The LFC and EDC turbulence-chemistry interaction models are informal Thickened Flame Models (TFMs) since the local turbulent diffusivity acts similarly to the TFM, to thicken the flame and increase the flame propagation speed over the laminar value. However, when using the LFC model or EDC model, the turbulent flame speed is very sensitive to the mesh size—although it is possible to control this speed by changing the diffusivity (using the turbulent Schmidt number), it is generally impractical. Instead, the TFC model is available for modeling premixed and partially-premixed flame fronts where the propagation speed of the flame is explicitly specified.

These models account for turbulent-chemistry interaction through the enhanced diffusivity of the turbulence model. You can adjust the flame position by changing the turbulent Schmidt number.

Eddy Break-Up Model

The Eddy-Break-Up model assumes that reactions are limited by the rate that turbulence can mix the reactants and heat into the flame zone. The EBU model limits kinetic reaction rates by the large-scale turbulent mixing rate. Since kinetic rates are replaced by a single turbulent mixing rate, use only small mechanisms with one or two steps. The EBU model is a flame position model and is not intended for modeling kinetically dominated phenomena such as ignition and pollutants.

If kinetic data exist, you can choose between the combined time scale model and hybrid model. These models account for finite-rate kinetic effects—required for premixed flames where turbulence would otherwise ignite the mixture before the flame holder. However, for diffusion flames that are purely turbulence-controlled, the standard EBU model is adequate and efficient.

Eddy Contact Micromixing Model

The Eddy Contact Micromixing Model is a combustion model for liquids. In liquids, where species diffusivities are typically small and molecular mixing time-scales are long, the Eddy Contact Micromixing model accounts for diminished molecular diffusivity in the turbulent mixing rate by reducing the reaction rate. The Eddy Contact Micromixing model is a variant of the EBU model.

Thickened Flame Model

Premixed laminar flames are typically very thin relative to the mesh size. Since the flame speed is determined by diffusion and reaction inside the flame, accurate flame speed prediction requires sufficient resolution of the internal flame structure. One approach to overcome this stringent resolution requirement for Large Eddy Simulations is to use the TFM model, where the premixed flame front is artificially thickened by increasing the local diffusivity, so that the flame can be resolved on the mesh.

Polymerization Model

The Polymerization model is a dedicated model for simulating liquid polymerization processes—either in a multi-component liquid, or as a multi-component liquid phase in a Eulerian multiphase simulation. It tracks the polymer size distribution for live and dead polymers.

Polymerization is the process of creating long-chain molecules by chemically combining large numbers of small monomer molecules. This process starts by mixing monomer and initiator molecules in a solvent. The process of polymerization is comprised of several chemical reactions, such as initiation, propagation, chain transfer, chain branching, scission, and termination, that involve radicals of different chain lengths.

The polymerization model in Simcenter STAR-CCM+ simulates free-radical polymerization and uses Method of Moments to describe the size distribution of the polymers.

Reacting Species Transport Model Ignitors

When the Complex Chemistry, EBU, or TFM models are selected, a Continuum > Ignitors node appears which allows you to control ignition.

The Complex Chemistry, EBU, or TFM models allow you to create a Fixed Temperature Ignitor. When active, a Fixed Temperature Ignitor applies a constant temperature for the cells that are contained within the ignitor parts.

In addition to the Fixed Temperature Ignitor, the EBU model also provides the EBU Ignitor. When the EBU Ignitor is on, for the cells which are contained within the EBU ignitor parts, the ignitor forces reaction rates to be mixing limited without using products.

For more details, see the specific combustion model reference page.