## 2 Features of Hydrogen Combustion Processes
### Flame Size and Shape  
The flame shape is an important characteristic that has an effect on the temperature field and pollutant emissions [21]. With a strong flow swirl, a typical flow structure is shown in Figure 9. The stream has two recirculation zones: the inner recirculation zone (IRZ) and the outer recirculation zone (ORZ). The fresh air-fuel mixture and combustion products are separated by an inner shear layer (ISL) and an outer shear layer (OSL). The flame is usually V-shaped if the combustion reaction occurs only at the boundary of the internal shear layer (ISL) of the fresh air-fuel mixture and the combustion products in the inner recirculation zone (IRZ). If the combustion process occurs at the boundary of the outer shear layer (OSL) of the fresh air-fuel mixture and the combustion products in the outer recirculation zone (ORZ), then the flame stabilizes with the M-shape. Thus, in the work by [PERSON] [22], it was demonstrated that the M-shape flame is more prone to unstable  
Figure 8: The influence of the equivalence ratio on the autoignition temperature. combustion than the V-shape. Changing the flame shape from M to V can increase its tolerance to pulsations. The experimental study by [PERSON] [23] is devoted to the effect of hydrogen enrichment of natural gas on the flame dynamics under atmospheric pressure conditions using a DLE (dry low emission) radially stepped burner. The volume contents of hydrogen were 0%, 5%, 11%, 21%, and 26%. The authors demonstrated that the flame shape changed along with the increased hydrogen percentage from M to V (Figure 10). Similar results were obtained in the paper by [PERSON] [24], where a change in the nature of flame stabilization from an aerodynamically stabilized M-type to stabilization in a V-type shear layer was obtained when the fuel was enriched with hydrogen, and in the works by [PERSON] and [PERSON] [25; 26], where it was shown that the flame became more compact. Similar results were obtained in the paper by [PERSON] [27], where it was shown that the flame with a higher hydrogen percentage was characterized by a more compact shape, which indicates a better reactivity of the fuel mixture (Figure 11). Figure 10: A change in the shape of the flame with the addition of hydrogen by volume [23]. Figure 9: Scheme of swirling flow in the combustion chamber [21]: (**a**) graphical description of the inner and outer recirculation zones and internal shear layer; (**b**) V-shape flame; (**c**) M-shape flame. In the work of [PERSON] [28], a premix fuel-air mixture was fed into a cylindrical chamber to explore the volume concentration range from 0 to 0.5 (where 0 is the absence of hydrogen in the mixture and 1 is pure hydrogen), which corresponds to the power fraction \(\mathrm{P_{H}}\) from 0 to 0.25. The flame shapes for the two stable operating conditions are presented in Figure 12, which shows that with an increase in hydrogen content, the flame became more compact. At \(\mathrm{P_{H}}\) = 0.2, the maximum heat release rate remained at the end of the flame brush. It also reduced the interaction with the neighboring flame, as evidenced by a much larger interaction area at \(\mathrm{P_{H}}\) = 0.1. For the case of \(\mathrm{P_{H}}\) = 0.1, it can be seen that the edges of the flames merged, which created a large vertically oriented area with a high heat release rate. This was not observed at \(\mathrm{P_{H}}\) = 0.2, although there was some evidence of the flame tip interaction. The study by [PERSON] [29] revealed that adding hydrogen at the volume content below 20% had little or no effect on the flame length, while when using the fuel with a hydrogen content of 40% by volume, the flame became much shorter (Figure 13). Figure 11: The flame shape in a swirled flow with various hydrogen additions by volume. Figure 12: The average flame shapes and corresponding longitudinal distributions of the integral heat release rate for two stable operating conditions [28]. The change in the shape of the flame is associated primarily with an increase in local heat release.