## 5 Results and Discussion
### Results of Combustion Experiment
In addition, it was found that the temperature under the fuel-rich condition with an equivalence ratio of 1.00 or more decreased, as in the fuel-lean condition. When the amount of air was reduced at an equivalence ratio of 1.00 (fixed fuel rate) or more of the combustion reaction, unburned CO was generated. The temperature was expected to decrease owing to the incomplete combustion. In contrast, when the equivalence ratio was 1.00, the unreacted air cooled the heat of the flame, which lowered the flame temperature and the oxygen concentration increased [31]. Figure 11b is a graph comparing the H\({}_{2}\)O concentration measured by TDLAS with the H\({}_{2}\)O concentration derived by Chemkin in the premixed CH\({}_{4}\)/air flame. Chemkin simulations were performed using a GRI 3.0 based chemical reaction mechanism suitable for CH\({}_{4}\)/air flames, and a premixed laminar burner-stabilized flame code was considered [32]. As mentioned in the experimental setup, because the light source is transmitted to a height of 5 mm above the surface of the metal fiber burner, the simulated H\({}_{2}\)O concentration based on a height of 5 mm above the burner surface was considered. The H\({}_{2}\)O concentration derived from Chemkin according to the equivalence ratio decreased to 0.85 (16.30%) and 0.70 (13.70%) under the fuel lean condition. The H\({}_{2}\)O concentration was expected to decrease significantly, owing to the increase in unreacted air in the combustion reaction under fuel-lean conditions, and the H\({}_{2}\)O concentration under the fuel-rich condition of 1.15 (18.80%) was derived to be higher than the equivalence ratio of 1.00 (18.40%). Table 4 shows the flame temperature and concentration measured using TDLAS based on multi-peak Voigt fitting at the equivalent ratio from 0.70 to 1.30. Figure 11: (**a**) Comparison of measured temperature using TDLAS and thermocouple, and (**b**) analysis of the H\({}_{2}\)O concentration using Chemkin and TDLAS according to various equivalence ratios in a combustion environment. In addition, the measured temperature and concentration results showed standard deviation values through 10 repeated experiments. The changes in the analyzed temperature and concentration deviation were derived in the range of about 11-16 K and 0.30-0.33%, respectively. In general, the H\({}_{2}\)O concentration was maximized when it was slightly fuel-rich (\(\Phi\) = 1.15), but not when it was stoichiometric. This phenomenon occurred because the heat combustion and specific heat decreased simultaneously when the equivalence ratio was larger than 1.00. That is, when the heat combustion was larger than the equivalence ratio (\(\Phi\) = 1.05) corresponding to the maximum temperature, it decreased more rapidly. The decrease in specific heat is a phenomenon that occurs mainly because the number of product moles decreases compared to the number of reactant moles [29,33]. Based on this result, the H\({}_{2}\)O concentration tendency measured by TDLAS according to the various equivalence ratio is consistent with the H\({}_{2}\)O concentration tendency calculated by Chemkin.