The Ansys Fluent NO_x model provides the capability to model thermal, prompt, and fuel NO_x formation, as well as NO_x consumption due to reburning in combustion systems. It uses rate models developed at the Department of Fuel and Energy at The University of Leeds in England, as well as from the open literature. NO_x reduction using reagent injection, such as selective non-catalytic reduction (SNCR), can be modeled in Ansys Fluent, along with an N₂O intermediate model that has also been incorporated.

To predict NO_x emissions, Ansys Fluent solves a transport equation for nitric oxide (NO) concentration. When fuel NO_x sources are present, Ansys Fluent solves additional transport equations for intermediate species (HCN and/or NH₃). When the N₂O intermediate model is activated, an additional transport equation for N₂O will be solved. The NO_x transport equations are solved based on a given flow field and combustion solution. In other words, NO_x is postprocessed from a combustion simulation. It is therefore evident that an accurate combustion solution becomes a prerequisite of NO_x prediction. For example, thermal NO_x production doubles for every 90 K temperature increase when the flame temperature is about 2200 K. Great care must be exercised to provide accurate thermophysical data and boundary condition inputs for the combustion model. Appropriate turbulence, chemistry, radiation, and other submodels must be employed.

To be realistic, you can only expect results to be as accurate as the input data and the selected physical models. Under most circumstances, NO_x variation trends can be accurately predicted, but the NO_x quantity itself cannot be pinpointed. Accurate prediction of NO_x parametric trends can cut down on the number of laboratory tests, allow more design variations to be studied, shorten the design cycle, and reduce product development cost. That is truly the power of the Ansys Fluent NO_x model and, in fact, the power of CFD in general.

In laminar flames and at the molecular level within turbulent flames, the formation of NO_x can be attributed to four distinct chemical kinetic processes: thermal NO_x formation, prompt NO_x formation, fuel NO_x formation, and intermediate N₂O. Thermal NO_x is formed by the oxidation of atmospheric nitrogen present in the combustion air. Prompt NO_x is produced by high-speed reactions at the flame front. Fuel NO_x is produced by oxidation of nitrogen contained in the fuel. At elevated pressures and oxygen-rich conditions, NO_x may also be formed from molecular nitrogen (N₂) via N₂O. The reburning and SNCR mechanisms reduce the total NO_x formation by accounting for the reaction of NO with hydrocarbons and ammonia, respectively.