# 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
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**CRNs/CFD_CRNs** & **Fuel** & **Object** & **Mechanism** & **Units** & **Deviation** & **Reference** \\ \hline \multirow{4}{*}{CRNs} & Dimethyl ether and its mixtures & \multirow{4}{*}{Flameless furnace} & \multirow{4}{*}{NUIG-Mech 1.1} & \multirow{4}{*}{2 PSRs} & 7.1 dry ppm & \multirow{4}{*}{[50]} \\  & with methane/ & & and CRECK & & 7.0 of & \\  & hydrogen & & & & experimental data & \\ \hline \multirow{4}{*}{CRNs} & \multirow{4}{*}{Hydrogen/ methane blends} & Moderate or & \multirow{4}{*}{GRI-Mech 3.0} & \multirow{4}{*}{1 Mixer and 1 PSR} & Similar tendency & \multirow{4}{*}{[51]} \\  & & intense & & & 1 Mixer to & \\  & methane blends & low-oxygen & & & and 1 & \\  & & dilution (MILD) & & & the experimental results & \\  & & combustion & & & & \\ \hline \multirow{4}{*}{CRNs} & \multirow{4}{*}{Syngas-fueled} & Gas turbine & \multirow{4}{*}{syngas} & 9 PSRs, 1 Mixer, & \multirow{4}{*}{-} & \multirow{4}{*}{[52]} \\  & & combustor & mechanism with & & & \\ \cline{1-1}  & & with RQL & 44 species and & & & \\ \cline{1-1}  & & & 251 reactions & & & \\ \hline \multirow{4}{*}{CRNs} & \multirow{4}{*}{RP-3 kerosene} & \multirow{4}{*}{LESS combustor} & a surrogate of & \multirow{4}{*}{10 PSRs, 2 PFRs} & \multirow{4}{*}{Maximum 10.13\%} & \multirow{4}{*}{[54]} \\  & & & & & mechanism & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Several references were listed to exhibit the CRN and CFD_CRNs methods in the field of NOx generation. The CRNs method is more focused on the concentrations of each reactant compared with the flow field of the CFD method. Through the more detailed reaction documents applied, the CRNs method could be based on the concentration to calculate the reaction rate in each time node. This avoids the introduction of the combustion model, which would easily lead to errors in the chemical condition. This is the key point of the CRNs method to improve accuracy. With the sacrifice of the flow characters, the steps of the time scale could decrease enough to avoid the problem of stiffness in ODE equations. This would enhance the accuracy of the calculation in comparison to the CFD approach, without introducing additional computational overhead. Finally, the CRNs method could be applied based on the condition of the early stage. Because of the independence of the geometry, the CRNs method could be applied without detailed flow information, only through the residence time, temperature, and pressure. Even though there are several advantages to the CRNs method, it still has limitations in application. First and foremost, the accuracy of the CRNs method remains contingent upon the geometric characteristics of the combustor chamber. The more units applied, the more accurate simulation would become; however, there is still a limitation of quantities of units. And with the quantities increased, the time of calculation would also increase rapidly. There is a balance between the time cost and the accuracy of NOx production. Moreover, the methods to divide models in some specific ways are still not universally relevant for each case, which needs to state the guidelines to satisfy each case.