# 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  
Not only could the CRNs model be used alone to simulate the prediction of NOx generation, but it also can further improve the prediction accuracy of NOx generation based on the flow-fitting results of CFD. NOx formation is a chemical reaction process with a strong correlation between temperature and reactant concentration. However, the temperature gradient and concentration gradient in the combustion chamber vary greatly during the combustion process, after combustion, and after air mixing, so more detailed flow information and temperature-concentration information are needed to provide more accurate guidance for CRNs modeling. The CRNs method combined with the CFD information has become a common method in current research. In a complex flow field, the complexity of the flow field is related to the number of CRNs units required. In theory, more CRNs simulation units could describe the changes in temperature and concentration in the flow field with more detail; especially in areas with large temperature gradients and large concentration changes, it is more necessary to increase the number of CRNs simulation units. By comparing the fitting results of a different number of units, it can be found that the prediction results of 12 PSR models, 1 PFR model, and 3 MIX models using complex models are closer to the experimental results than that of 5 PSR models, 1 PFR model, and 1 MIX model using simple models [97]. A similar conclusion was also obtained by [PERSON] et al. [98]. They compared the precision of the CRNs model in predicting NOx and CO concentration with different numbers of PSR models. The results showed that as the number of PSR models increased, the accuracy of NOx generation progressively increased and approached the experimental results. It is important to note that an increase in the number of CRNs analog units will lead to a substantial rise in computational workload [99]. The combined method of CRNs and CFD could effectively predict the NOx generation. [PERSON] et al. [100] applied the combined CFD-CRNs method to research the impact of steam addition on combustion and NOx emissions in a model annular-type gas turbine combustor. Through the information provided by the CFD simulation, a 14-element CRNs model using the jet-A fuel was built. The CRNs model prediction was compared with the ICAO databank at different operation conditions. The whole EI NOx showed almost the same results in different power settings, except for one that is on the idle power point, in which the relative error is about 10%. [PERSON] et al. [101] analyzed the pollutant discharged NOx from a gas turbine ignition chamber using methane gas. Two kinds of CRNs models were built through the CFD simulation, providing flow information. The simple CRNs model contained 5 PSR reactors, and the complex CRNs model contained 12 PSR reactors. Compared with the experimental value, the NOx emission from the complex CRNs model showed the same tendency at the equivalence ratio from 0.5 to 0.8. the biggest error occurred at ER = 0.8, which is about 11.1%. [PERSON] et al. [102] established a model of NOx emission level in lean oil-premixed combustors by combining CFD and CRNs models. Among them, the CRNs model used 41 reaction units, and its reaction mechanism was GRI-Mech 3.0. The simulation results showed that when the equivalent ratio is 0.5, 0.6, and 0.7, the predicted results of the NOx model jointly established by CRNs and CFD are closer to the test results than those obtained by CFD post-processing. [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [97] simulated a lean combustion gas turbine using natural gas. By combining CFD and CRNs methods, it could be used to predict the change in temperature and reactant concentration. Among them, CFD is used to simulate the temperature field and velocity field of the flow field, providing effective data support for the material flow exchange in the CRNs model, where the chemical reaction uses the two-step global reaction of methane.