ANSYS Fluent Combustion Modeling
#+CATEGORY: openfoam
ANSYS Fluent Combustion Modeling :: Premixed, Non-Premixed, and Partially-Premixed Combustion
ANSYS Fluent provides many combustion models. The models cover all combustion regimes. They range from simple finite-rate chemistry to complex detailed chemistry mechanisms. The choice depends on the combustion regime (premixed vs non-premixed vs partially-premixed). It also depends on the time scale separation between chemistry and turbulence. Finally, it depends on the available computational resources.
Non-Premixed Combustion Models (Diffusion Flames)
Non-premixed combustion is the main model for diffusion flames. Mixing controls the burning. Fuel and oxidizer mix before combustion occurs. The two main approaches are: 1. Probability Density Function (PDF) based methods 2. Flamelet based methods (flamelets/flamelet-progress variable)
Non-Premixed Combustion Model Family
| Model | Description | Use Case |
| ------- | ------------- | ---------- |
| Non-Premixed Combustion (PDF) | Mixture fraction + PDF integration | General non-premixed combustion |
| Flamelet Model | Precomputed laminar flamelets | Turbulent non-premixed |
| Flamelet/Progress Variable (FPV) | Flamelet + progress variable | Non-premixed with finite-rate effects |
| Steady Laminar Flamelet | Steady flamelets only | Steady-state non-premixed |
| Tabulated Chemistry | Prescribed chemical properties | Rapid setup, parameter sweeps |
Non-Premixed PDF Model
The PDF model is the most general non-premixed combustion model. It solves transport equations for the mixture fraction $Z$ and its variance $Z'$ (or scalar dissipation rate). It also integrates a probability density function over the reaction manifold. This gives species mass fractions and temperature:
The governing equations are: 1. Mixture fraction ($Z$): conserved scalar representing fuel/oxidizer ratio 2. Mixture fraction variance ($Z'$): scalar dissipation rate 3. Enthalpy: energy transport
The probability density function determines the distribution of mixture fraction values at each cell. The most common PDF form is the β-PDF:
where $B$ is Beta function and $p, q$ are parameters computed from $\tilde{Z}$ and $\tilde{Z}'$.
Flamelet Models
Flamelet models assume the turbulent flame contains laminar flamelet structures. The flamelet equations solve offline (precomputed) and get tabulated.
The flamelet equation (steady, laminar):
where $\xi$ is the mixture fraction coordinate, $\chi$ is the scalar dissipation rate, $\dot{\omega}_k$ is the chemical source term for species $k$, $Y_k$ is the mass fraction, and $D$ is the diffusion coefficient.
Flamelet/Progress Variable (FPV) extends the basic flamelet model by adding a progress variable transport equation to capture finite-rate chemistry effects:
- $\tilde{Z}$: mixture fraction
- $\tilde{Z}'$: mixture fraction variance
- $\tilde{C}$: progress variable
The lookup table stores species mass fractions and temperature as functions of $(\tilde{Z}, \tilde{Z}', \tilde{C})$.
Premixed Combustion Models
Use premixed combustion models when fuel and oxidizer mix before ignition (or mix near-fully). The primary models are:
| Model | Description | Use Case |
| ------- | ------------- | ---------- |
| Eddy-Dissipation Model (EDM) | Finite-rate chemistry with eddy dissipation | Premixed/partially-premixed with chemistry |
| Eddy-Dissipation Concept (EDC) | Finite-rate in turbulent eddies | Detailed chemistry |
| Laminar Eddy-Magnitude | (empty) | Premixed |
where $Y_j$ is the species mass fraction, $k$ is turbulent kinetic energy (eddy dissipation), $\phi$ is a dimensionless parameter, and $\nu_{jk}$ is the stoichiometric coefficient.
The Eddy-Dissipation Concept (EDC) is more physically realistic than EDM. It assumes reactions occur only in fine-scale turbulent structures. The fine-scale volume fraction comes from the ratio of chemical to turbulent time scales.
Partially-Premixed Combustion
Partially-premixed combustion combines premixed and non-premixed models: 1. Non-premixed component: handles regions where fuel and oxidizer are not mixed 2. Premixed component: handles regions where fuel and oxidizer are premixed 3. A model fraction parameter determines the local combustion regime
Ignition and Combustion in Fluent
Fluent supports:
- Chemical mechanism files: CHEMKIN-format input for detailed chemistry
- Multi-step finite-rate: tabulated Arrhenius reaction rates
- Single-step finite-rate: one Arrhenius reaction
- Arrhenius kinetics: user-specified activation energies, pre-exponential factors
Fluent supports detailed chemistry integration:
- CHEMKIN-format mechanisms
- User-defined kinetics
- Tabulated chemistry from external solvers (Cantera, CHEMKIN, ANSYS Chemkin-Pro)
Summary (ANSYS vs OpenFOAM Combustion)
| Model | ANSYS Fluent | OpenFOAM Equivalent |
| ------- | ------------- | ------------------- |
| Non-premixed (PDF) | NonPremixedCombustion | chemkinReactingFoam |
| Flamelet | FlameletModel | flameletModels in OpenFOAM |
| Flamelet/Progress Variable | FPV | flameletModels |
| Eddy Dissipation (EDM) | EddyDissipationModel | eddyDissipation |
| Eddy Dissipation Concept (EDC) | EDC | edc |
| Tabulated Chemistry | TabulatedChemistry | tabulatedChemistry |
Summary
1. Non-premixed combustion is modeled via mixture fraction + PDF or flamelet tabulation 2. The β-PDF determines species/temperature from the mixture fraction statistics 3. Flamelet/Progress Variable (FPV) adds finite-rate chemistry effects 4. Premixed combustion uses EDC or EDM 5. Partially-premixed blends premixed and non-premixed 6. Detailed chemistry (CHEMKIN format) is available for all combustion regimes
See Also