# NOx Formation Mechanism and Emission Prediction in Turbulent Combustion: A Review
## 6 Turbulent Combustion Simulation Approach
### CFD with Different Combustion Models
#### 6.2.3 Combustion Models with CRNs Method
The results show that the combined modeling of the CFD and CRNs models could effectively and quickly predict the changes in velocity, temperature, and concentration in the flow field more accurately. [PERSON] et al. [103] used a two-step global reaction of methane to simulate combustion with CFD, followed by a GRI-Mech 3.0 mechanism to simulate NOx generation in the combustion chamber. The CRNs model includes five PSR models and one PFR model. Through the combination of the CRNs model and CFD, the prediction of NOx generationis effectively solved. [PERSON] [104] used 24 CRNs units to predict NOx emissions in a lean combustion chamber, and the error between the reaction group prediction and the test results was less than 1.2%. [PERSON] et al. [105] established two kinds of CRNs models based on CFD flow simulation results, among which the simple model included 7 PSR models and the detailed model included 15 PSR models. The reaction mechanism file in the CRNs model is the detailed JP10 reaction mechanism, which includes 82 substances reacting with 374 step elements. Compared with the experimental results, the results of the detailed CRNs model are closer to the results of the empirical formula than that of the simple CRNs model. For the CRNs combined with the CFD method, [PERSON] et al. [106] applied this method to simulate the NOx emission of ammonia-hydrogen-blend fuel on the premixed combustion. The results showed that, in a large range of equivalence ratio of 0.65-1.2, the approach tendency is correct compared with the experimental data. Through these examples, the CFD simulation could apply a global reaction or two-step mechanisms of the fuel to provide the flow field information, mainly about the temperature field. Even though this operation would reduce the accuracy of the flow field, considering that the fuel oxidation process is independent of the NOx generation, this sacrifice was acceptable for most cases. Without the complex mechanism, the CFD simulation could reduce the cost of time and CPU resources. Then, the CRNs model could add detailed mechanisms, which include the NOx-generation process to ensure the accuracy of NOx generation. The methods that were used for CFD to be combined with the CRNs built model could be summarized by several methods. In some cases, the combustor is divided by flow patterns from the CFD simulation results, such as streamlines and temperature [103; 107; 108; 109], or based on the local temperature gradient [99]. By partitioning the combustion volume based on temperature or flow-pattern characteristics, a preliminary model was developed. And then, through several iterations, the research goals would fit with the experiments. In some instances, the CFD data were utilized through a blend of unsupervised clustering and graph-scanning algorithms to automatically construct a CRN model for enhanced accuracy [52; 111]. Several references are listed in the Table 2. From the content of the table, it could be found that the CRNs and the CFD_CRNs method could predict the NOx emission with a small deviation, which is approximately 10%. And it could be widely applied under different conditions. The CRNs method and the CFD_CRNs method focused on the chemical reaction; and the types of fuel could be abundant, which could simulate the NOx emission of the aviation fuel. This is another advantage of this method.