# Construction of a Chemical Kinetic Mechanism of Five-Component Surrogate Fuel for RP-3 Kerosene
## 3 Modification of Kinetic Parameters for Mechanism of Surrogate Fuel Mixture
### Modification of Dynamic Parameters  
After the mechanism coupling, the prediction accuracy of the mechanism is an issue deserving attention. Due to factors such as cross-reactions between molecules, changes in component reaction pathways and rates, and the fusion of repeated elementary reactions, the ignition delay time and laminar flame speed will change. Whether it is the actual fuel RP-3 or single-component experimental data, there are significant differences in the predicted values of the initial mechanism. Therefore, the initially constructed coupled mechanism must be optimized. The optimization of the mechanism mainly involves the experimental data, by modifying the reaction product pathways and correcting the pre-exponential factors and activation energies that affect the reaction rates, to make the prediction results of the mechanism better match the experimental data. Sensitivity analysis is an analytical method for assessing the extent to which the uncertainties of different input variables in a model or system affect the output results. It can visually reflect the sensitivity effect of certain disturbance factors in the combustion system, such as reaction conditions, component concentrations, elementary reactions, etc., on the calculated result parameters, thereby determining the importance of each elementary reaction. The orthogonal sensitivity coefficient \(\widetilde{S}\) between the disturbance factor \(\alpha_{i}\) and the calculated result parameter can be expressed by Formula (8):  
\[\widetilde{S}=\frac{\alpha_{i}}{\varphi}\frac{\partial\varphi}{\partial\alpha _{i}}=\frac{\partial\ln\varphi}{\partial\ln\alpha_{i}} \tag{8}\]  
In the formula, when the disturbance factor \(\alpha_{i}\) changes, it can cause a change in the target parameter \(\varphi\), thereby obtaining the sensitivity coefficient of various elementary reactions. When the sensitivity coefficient is positive, it promotes the calculated result parameter; otherwise, it inhibits it. In this research, the optimization process of the mechanism mainly takes the experimental data as the objective. Corresponding modifications are made by comparing the differences between the predicted values of the mechanism and the experimental data, ultimately obtaining a multi-component surrogate fuel mixture mechanism with good prediction performance. The modification method mainly optimizes the mechanism by adjusting the chemical reaction rate constants in this research. [PERSON] [31], [PERSON] [42], [PERSON] [43], [PERSON] [44], and [PERSON] [45] all adopted this method to fit the experimental values. The principal steps of the modification involve identifying the elementary reactions that exert considerable influence on the system temperature through the temperature sensitivity analysis approach, modifying the kinetic parameters of the related reactions, and then conducting further comparisons and modification until a mechanism that aligns with the experimental data is attained. Temperature sensitivity analysis can gauge the sensitivity of a certain elementary reaction to temperature by means of the normalized sensitivity coefficient. A positive normalized coefficient indicates that the corresponding reaction promotes ignition, whereas a negative value implies an inhibitory effect. In Figure 7, at P = 1.0 MPa and \(\varphi\) = 1.0, temperature sensitivity analyses were performed for working conditions with initial temperatures of 600 K, 800 K, and 1200 K, respectively. As evident from Figure 7a, at the low temperature of 600 K, the R253 reaction possesses the largest sensitivity coefficient, signifying that this reaction has the greatest influence on temperature. Among the reactions with the highest sensitivity presented in the figure, all elementary reactions are large-molecule reactions. Specifically, R247, R236, R225, and R96 are all dehydrogenation reactions where a large-molecule fuel loses one H to an OH radical, while R235, R224, and R95 are all initial oxidation reactions of the fuel.