## 2 Features of Hydrogen Combustion Processes
### Lewis Number  
It is worth noting the decrease in the thickness of the flame with an increase in the concentration of hydrogen in methane-air mixtures (Figure 30). This leads to a change in the Karlovitz and Damkohler numbers (Figure 31), which entails changes in the regimes along the Borghi diagram, as shown, for example, by [PERSON] [61]. Figure 30: The curve of dependence of flame thickness on the volume fraction of hydrogen in the mixture [30]. Figure 29: The scheme of the amplitude of pressure fluctuations in the fuel–air mixture flame. At Ka < 1, the thickness of the laminar flame is less than the smallest (Kolmogorov's) vortices. This means that small vortices can only distort the shape of the flame and not penetrate into the interior of the flame to change its structure. Thus, the hydrogen flame front is more prone to distortion than hydrocarbons [62]. It was shown in the work of [PERSON] [63] that the Lewis number had a greater effect on the combustion rate of hydrogen, in contrast to the combustion of methane, propane, and other hydrocarbons. The range of the Lewis number during the combustion of hydrogen is shown in Figure 32[64]. Determination of a universal method for estimating the effective Lewis number for fuel mixtures requires further study and generalization, since the existing data lead to conflicting conclusions due to the difference in combustion modes [65]. In turn, in one of the widely used turbulent combustion models FGM, it is not possible to change the Lewis number for Flamelet generation, which remains one of the main limitations in applying this approach to modeling hydrogen combustion processes. The local burning rate in a methane flame is relatively insensitive to flame curvature, while the data from the work by [PERSON] [66] show a very strong positive correlation of flame speed with positive curvature and a weaker positive correlation with negative  
Figure 31: The curves of dependence of the Karlovitz and Damköhler numbers on the volume fraction of hydrogen in the mixture [30]. Figure 32: Curves of dependence of the Lewis number on the equivalence ratio (composition of mixtures in volume fractions). flame curvature (Figure 33). For a thermodiffusion-unstable hydrogen flame, combustion intensifies in regions of positive curvature and has pockets of local extinction at negative curvature. [PERSON] [65] revealed that the hydrogen flame velocity is determined by the front part (from the side of unburnt reagents) of the curved front, while the back part of the flame plays the role of the burnout zone. Later, [PERSON] [67] developed this concept for the leading point model [67]. Thus, the effects of molecular diffusion play an important role in the combustion of fuels containing hydrogen. For a propane flame, the diffusion coefficient of the fuel is much less than the thermal conductivity, which allows the fuel to heat up faster than it diffuses into the flame zone. This causes the curve of the dependence of the molar fraction of fuel on temperature in Figure 34 to shift so that it is higher than the methane curve. The diffusion coefficient of hydrogen is higher, which leads to an increase in the diffusion of fuel into the flame and a downward shift in the dependence of the mole fraction of fuel on temperature. [PERSON] [68] demonstrates that as turbulent velocity fluctuations increase, turbulent diffusion begins to dominate molecular diffusion, and the curves of the dependence of the molar fraction of fuel on temperature become similar to the curve for methane (Figure 34), and thus we assume that the propane curve would do same. Since the burning rate increases as the fuel concentration and temperature increase as the flame becomes distributed, it can be expected that the burning rate decreases in the case of a high Lewis number and increases in the case of a low Lewis number. Figure 34: Distribution of normalized temperature and fuel mole fraction for four fuels with different Lewis numbers. Figure 33: Correlation of local burning speed with curvature for each flame.