# Construction of a Chemical Kinetic Mechanism of Five-Component Surrogate Fuel for RP-3 Kerosene
## 2 Development of Chemical Kinetic Mechanisms  
Based on the actual fuel properties and the research status of each candidate component, this study selected n-tetradecane, n-dodecane, isododecane, decalin, and 1,3,5-trimethylbenzene as the main components of the RP-3 aviation kerosene simulation surrogate fuel. Among them, n-tetradecane and isododecane, which are more compatible with the actual RP-3 fuel, make the physical and chemical properties of the alternative fuel model closer to those of the actual fuel. After determining the main components of the alternative fuel mixture, the significance of each selection index parameter was comprehensively considered, and combined with the main characteristics of RP-3 aviation kerosene and the research purpose of this paper, the average molecular weight, cetane number, low heat value, hydrogen-carbon ratio, smoke point, and density were finally determined as the selection index parameters for the RP-3 aviation kerosene simulation surrogate fuel mixture to further calculate the proportion of the five components [4; 6; 24]. The calculation formulas [25], Formulas (1)-(7), for each selection index parameter of the mixed fuel are presented as follows:  
Average molecular weight of the surrogate fuel mixture:  
\[MW_{\text{mix}}=\sum_{i}x_{i}MW_{\text{i}} \tag{1}\]  
Octane number of the surrogate fuel mixture:  
\[CN_{\text{mix}}=\sum_{i}v_{i}CN_{\text{i}} \tag{2}\]  
Lower heating value of the surrogate fuel mixture:  
\[LHV_{\text{mix}}=\sum_{i}y_{i}LHV_{i} \tag{3}\]  
Hydrogen-to-carbon ratio of the surrogate fuel mixture:  
\[(H/C)_{\text{mix}}=\sum_{i}x_{i}(H/C)_{i} \tag{4}\]  
Density of the surrogate fuel mixture:  
\[\rho_{\text{mix}}=\sum_{i}v_{i}\rho_{i} \tag{5}\]  
Smoke point of the surrogate fuel mixture:  
\[TSI_{\text{mix}}=\sum_{i}x_{i}TSI_{i} \tag{6}\]  
Finally, the calculation results are controlled through an objective optimization function to minimize the difference between simulated values and actual values. The objective optimization function used in this study is as follows [26]:  
\[E=\left[\frac{\frac{N}{i}\left(\frac{E_{i,sur}-E_{i,rp3}}{E_{i,rp3}}\right)^{ 2}}{N}\right]^{\frac{1}{2}} \tag{7}\]  
where \(x_{i}\) represents the mole fraction, \(v_{i}\) is the volume fraction, \(y_{i}\) is the mass fraction, \(\rho\) is the density of the substance, \(E_{i,sur}\) is a certain selection indicator value of the surrogate fuel, \(E_{i,rp3}\) is a certain selection indicator value of the real fuel RP-3, and \(N\) is the number of selection indicators determined in the previous text. For the proportion calculation, the Matlab R2022b genetic algorithm toolbox was used for optimization calculation. The optimization evaluation parameter is the optimization function value \(E\), and the target attribute is the six selection indicator values of the selected real RP-3 fuel. In the constraint settings, according to the types of components in actual fuel, the boundary conditions for the species components were set as follows: n-dodecane and n-tetradecane were both 0 to 0.3; isododecane was 0 to 0.4; decalin was 0 to 0.4; decalin was 0 to 0.4; 1,3,5-trimethylbenzene was 0 to 0.2; and the sum of the five components was 1. The remaining conditions used default values: population size was 50; the number of elites was 0.05 times the population size; crossover rate was 0.8; and mutation probability was 0.2. After multiple calculations, the ratios of n-dodecane, n-tetradecane, isododecane, decahydronaphthalene, and 1,3,5-trimethylbenzene were obtained as 0.121, 0.135, 0.318, 0.326, and 0.1, and at this point, the value of the optimized function was approximately 0.022. At this ratio, the molar proportion of alkanes was 57.4% (with straight-chain alkanes accounting for 25.6% and branched alkanes, 31.8%), cycloalkanes accounted for 32.6%, and aromatic hydrocarbons accounted for 10%, which is close to the composition of actual fuel. Table 1 compares the six selection indicator values of the simulated surrogate fuel and real RP-3 fuel at this ratio.