## 2 Features of Hydrogen Combustion Processes
### Flame Blowout and Flashback  
The reliability of the combustion chamber is determined by characteristics such as lean flame blowout, flashback, and pulsating combustion. Adding hydrogen to methane significantly increases the blowout limits of both the suspended and the attached diffusion flames [48; 49] (Figure 21). The effect of adding hydrogen to the air flow is much more pronounced than that of adding it to methane (with a separate fuel supply). Figure 21 shows that the flow velocity stabilization limits are proportional to the square of the maximum laminar flame propagation velocity. Adding the diluents (CO\({}_{2}\) and N\({}_{2}\)) to the fuel reduces the resistance of the diffusion flame to a much greater extent for the suspended flame than for the attached flame. Figure 20: Dependence of NOx emissions on the adiabatic flame temperature for different volume proportions of hydrogen–natural gas [4; 47] (**a**) for methane–hydrogen mixtures with constant methane consumption; (**b**) for methane–hydrogen mixtures with different methane consumptions. [PERSON] [50] (Figure 22) presents a study into stabilization of the flame of a premixed methane-air mixture with hydrogen additives (up to 29% by mole fraction) behind a bladed swirler. It was shown that an increased combustion stability upon hydrogen enrichment was a direct result of higher concentrations of OH, H, and O radicals, which led to an increase in several key reaction rates. In this case, the flame extinction time was influenced not only by the value of the average OH concentration but also by its RMS [51]. The greatest effect was achieved with small additions of hydrogen (up to 12-20% [52] by mole fractions), while an increase to 29% by mole fractions was not so pronounced, although it had a further positive effect on the combustion stability. Figure 21: Flame blowout during combustion of methane and hydrogen–methane with different volume fractions of hydrogen [48,49]: (**a**) for methane–hydrogen fuel jet and air stream; (**b**) for methane fuel jet and air–hydrogen stream. Figure 22: Change of adiabatic flame temperature during flame blowout. [PERSON] [51] (Figure 23) reported that the addition of hydrogen to methane up to 20% by volume reduced the equivalence ratio during the blowout of a homogeneous flame by 9-10%, and the relationship between the \(\Phi\) blowout and the volume content of hydrogen in the fuel was linear, which was also confirmed in [PERSON] [49]. However, in [49], the effect of H\({}_{2}\) on the flame blowout was more significant and amounted to about 15-20% \(\Phi\) for every 20% addition of hydrogen. [PERSON] and [PERSON] [52; 53] present a map of the combustion regimes of a methane-hydrogen premix mixture with air. It was found that with an increase in the percentage of hydrogen, the limits for flame blowout and flashback were shifted to a leaner region, which was associated with an increase in the flame speed and a decrease in the ignition delay time. In this case, the flame can burn at lower temperatures, as shown in Figure 24. Confirmation of this can also be found in the study by [PERSON] [28], where at the 40% hydrogen content, the temperature of the lean flame blowout was significantly reduced. Figure 23: Influence of hydrogen volume fraction on lean flame blowout: (**a**) [52]; (**b**) [50]; (**c**) [50]. Figure 24: The influence of the volume concentration of hydrogen on the boundary of sustainable combustion for the lean flame blowout and flashback [28]. [PERSON] [28] also showed that the flame blowout to some extent correlated with the Reynolds number, which was determined by the influence of the flow turbulence intensity. In this case, an increase in the hydrogen percentage in the mixture led to a decrease in the Reynolds number while maintaining the constant flow rate. [PERSON] [34] reports that with an increase in the percentage of hydrogen in the fuel, the role of the external (corner) recirculation zones in flame stabilization decreased, since the flame was shifted closer to the place of fuel supply.