## Chapter 9: Combustion Modeling

CFX includes combustion models to enable the simulation of flows in which combustion reactions occur. The following models are currently available:

- The eddy dissipation model (EDM)

- A finite rate chemistry model (FRC)

- A combined FRC/EDM model

- The laminar Flamelet model for diffusion flames

- A model for premixed or partially combustion using the Flamelet model for the burned mixture.

The eddy dissipation model was developed for use in a wide range of turbulent reacting flows covering premixed and diffusion flames. Because of its simplicity and robust performance in predicting turbulent reacting flows, this model has been widely applied in the prediction of industrial flames.

The finite rate chemistry model enables the computation of reaction rates described by the molecular interaction between the components in the fluid. It can be combined with the eddy dissipation model for flames where chemical reaction rates might be slow compared with the reactant mixing rates. It also enables the use of user-customized reaction rate expressions via CFX Expression Language (CEL).

These models determine the rates at which a component is consumed or created in a single reaction step during the combustion process. For multiple step reactions, the reactions are added to obtain the total reaction rate.

The flamelet model can provide information on minor species and radicals such as CO and OH, and accounts for turbulent fluctuations in temperature and local extinction at high scalar dissipation rates, for the cost of solving only two transport equations.

The burning velocity model (BVM) and the extended coherent flame model (ECFM) model the propagation of a premixed or partially premixed flame by solving a scalar transport equation for the reaction progress. The BVM uses an algebraic correlation for modeling the turbulent burning velocity (propagation speed of the flame in turbulent flow). When using the ECFM, the turbulent burning velocity is closed by solving an additional transport equation for the flame surface density.

Both the BVM and ECFM are combined with the flamelet model in order to describe the composition and properties of the burnt mixture. For partially premixed cases the flamelet model also models diffusion flame ‘tails’, which may occur in the burnt mixture behind the flame front.

In the CFX-Solver, a limit has not been set for the maximum number of reaction steps or components in a fluid. However, the larger the number of components, the larger the memory requirements will be.

Note: You should be familiar with the multicomponent flow chapter before reading this chapter; for details, see Multicomponent Flow. Multiphase reactions are also available in CFX; for details, see Multiphase Reactions and Combustion.

This chapter describes:

- Reaction Models
- Using Combustion Models
- Eddy Dissipation Model
- Finite Rate Chemistry Model
- Combined EDM/Finite Rate Chemistry Model
- Reaction-Step Specific Combustion Model Control
- Laminar Flamelet with PDF Model
- Burning Velocity Model (Premixed or Partially Premixed)
- Extended Coherent Flame Model (ECFM)
- Residual Material Model
- Flamelet Libraries
- Autoignition Model
- NO Model
- Chemistry Postprocessing
- Soot Model
- Phasic Combustion
- General Advice for Modeling Combusting Flows in CFX

Release 18.2 - © ANSYS, Inc. All rights reserved.