### 9.13.1. Introduction to the NO Model

The NO model calculates mass fractions of NO (with user extensions) formed in the combustion process. It solves additional transport equations for these variables but does not affect the main combustion calculation. NO is treated as a regular component, but because NO concentrations are typically very low, the effect on the global flow and combustion is negligible.

The NO is created or destroyed through four mechanisms: thermal NO, prompt NO, fuel nitrogen, and NO reburn. The fuel nitrogen mechanism only affects coal and oil combustion. In CFX, the temperature variance and NO are solved together with the other equations. NO is treated as a regular component.

In CFX, the NO model is implemented by means of generic reactions with Arrhenius Temperature PDF reaction rates. Control for these reactions is the same as for Arrhenius rates, but with the following extensions:

- Temperature Limit List: The Upper and Lower Bounds should be specified for temperature integration range (2 values).

- Control for temperature variance transport equation currently is for expert users (editing the CCL in CFX-Solver input files)

The NO formation model consists of several parts:

- Predefined reaction schemes for NO formation that are provided by the `REACTIONS` database (available in CFX-Pre) and that are user-adjustable and extensible.

- Integration of the reaction rates for NO formation over a presumed Probability Density Function (PDF) in order to account for turbulent fluctuations of temperature.

- Solving a transport equation for temperature variance.

#### 9.13.1.1. NO Model with Eddy Dissipation / Finite Rate Chemistry / Combined Model

To model NO, select the reaction scheme with the suffix `NO PDF` (for example, `Methane Air WD1 NO PDF`) when creating your reacting mixture material. This will introduce NO as an additional component and add reactions for thermal and prompt NO.

The fuel nitrogen mechanism can be added by additionally selecting the `HCN NO PDF` multi step reaction (further adding HCN and HCO to the components).

When the fuel is methane, NO reburn can be enabled by adding the `Reburn NO Methane PDF` reaction. For other fuels the corresponding reaction is not predefined in the `REACTIONS` database. However, for a given fuel the reaction may be created by copying from the `Reburn NO Methane PDF` reaction, and changing the fuel and stoichiometry as appropriate.

#### 9.13.1.2. NO Model with Flamelet Model

When using the Flamelet model, you will need to define a new multi-step reaction scheme before you create your variable composition mixture, as the reaction is a multi-step reaction.

In the Reaction details view, create a new multistep reaction scheme and select the following reactions:

- The flamelet model library (for example, `Methane Air Flamelet 298K 1 bar`)

- `Thermal NO O Radical PDF` implements NO formation by the thermal NO mechanism using O radical information provided by the Flamelet library. This is in contrast to the standard `Thermal NO PDF` reaction that approximates O radical concentration from O2 concentration and temperature. There is no need to do this when running the Flamelet model since O is one of the components.

- `Prompt NO <fuel> PDF` (where <fuel> is the name of the fuel; for example, `Prompt NO Methane PDF`). This reaction accounts for NO formation by the `prompt NO` mechanism.

  The fuel nitrogen mechanism and the reburn mechanism can be added in the same way as for the Eddy Dissipation / Finite Rate Chemistry / Combined Model as described above.

The fuel nitrogen mechanism or the NO reburn mechanism can be added in the same way as for the Eddy Dissipation / Finite Rate Chemistry / Combined Model described above. For details, see NO Model with Eddy Dissipation / Finite Rate Chemistry / Combined Model.

#### 9.13.1.3. NO Model for Coal Combustion / Hydrocarbon Fuel Model

See Hydrocarbon Fuel Model Setup for a description of how to set up the combustion of a solid hydrocarbon fuel with NO formation.

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