# NOx Formation Mechanism and Emission Prediction in Turbulent Combustion: A Review
## 5 The Chemical Reaction Network Method
The CRNs approach to NOx emissions displayed a notable absolute accuracy for all power settings in all simulations. The main constituent of the CRNs method are two kinds of ideal models, which are the perfect stirred reactor (PSR model) and the plug flow reactor (PFR model). There are two other models, namely the MIX model and the partially stirred reactor (PaSR model), that could also be applied to assist the simulation. The PSR model is mainly used to simulate the area of combustion, and the PFR one is used to stimulate the diffusion area. An essential element of the perfect stirred reactor model is the assumption that the reactor is sufficiently mixed to be described well by spatially averaged or bulk properties, in which the Damkohler number approaches zero [58]. The partially stirred reactor model is similar to the PSR model, but the difference between them is the mixing time scale. When the mixing time scale is small enough that the properties inside the reactor are homogeneously mixed, the PaSR model becomes the PSR model. The PFR models describe the steady-state tube-flow reactor that could be used for process design, optimization, and control. Each model represents a small region that has a small variation in physical and chemical parameters [59]. Through the combination of these four types of models with different amounts, the inner space of the combustion chamber could be rebuilt. The quantities of ideal models depend on the geometry of combustion. In the simplest case, a main reactor and a diffusion area could be applied to express a combustor [42]. The more ideal models are applied in the simulation system, the more accurate the flow pattern rebuilt by these models is. The few quantities of the ideal model could describe simple geometry, which usually applies dozens of PSR models and several PFR models in the examples mentioned above. In general, the CRNs model could independently operate without CFD results, but with the help of CFD information, the simulation could be more accurate. The complex geometries should be divided by the CFD_CRNs method, which would describe the latter. Compared with the CFD method, the CRNs method has two advantages. First, the CRNs model decoupled with the chemical and flow process, meaning that the calculation would be decreased. Second, the calculation method of NOx emissions in the CRNs method is more accurate than that in the CFD approach. In the CFD approach to evaluate NOx emissions, the main method of calculation of the NOx reaction rate is through the concentration of compounds, but in the CRNs method, the detail radicals could be considered by thermal pathway, prompt pathway, N\({}_{2}\)O pathway, NNH pathway, and so on. The quantities of species and elementary steps in the CRNs-approach method could be ten times than the CFD-approach method. With the detailed mechanisms applied in the CRNs model, more information would be enhanced compared with the approach of the only CFD simulation. The quantities of species and elementary steps in the CRNs-approach method could be dozens of times compared with the CFD-approach method.