## 2 Features of Hydrogen Combustion Processes
### Laminar Flame Propagation Speed  
One of the principal properties of a laminar flame is speed propagation. Figure 3 shows the relationship between the laminar flame propagation speed and the equivalence ratio. The first thing worth noting is that the maximum propagation speed of a laminar hydrogen flame is several times higher than the one for methane: 100-120 m/s compared to 30-40 m/s under normal atmospheric conditions. The second feature is the displacement of the maximum laminar flame propagation speed value to the region of rich mixtures. The maximum value of the flame speed of a hydrogen mixture is shifting to the region \(\varphi=1.0\)-1.2 (Figure 3), which approximately corresponds to the position of the maximum of the flame temperature. For combustion chambers, this may mean that rich mixtures will burn more intensively, thus increasing the probability of flashback. However, hydrogen combustion can occur at higher flow velocities without loss of combustion efficiency.  
Figure 2: Influence of hydrogen volume concentration on the normalized temperature of the methane–air mixture flame [3,8,9].  
The effect of pressure on the relative laminar flame speed of methane-hydrogen mixtures is shown in Figure 4.  
The addition of hydrogen has a profound effect on the flame speed of rich mixtures at high pressures (an increase in speed by a factor of \(\approx\)35 compared to the methane laminar flame speed at the pressure of 20 atm). For lean mixtures, the effect of hydrogen is somewhat smaller compared to rich mixtures, with the increase in the laminar flame of about 25 times. In general, it can be observed that the correlation between the addition of hydrogen and the flame speed largely depends on the pressure and grows as the pressure increases. This complicates the process of extrapolating the results obtained by the atmospheric studies to the real conditions in gas turbine engines, which leads to the need for additional experimental tests for the working process when burning hydrogen in combustion chambers at elevated pressures, thus making the feasibility of such studies much higher than when burning natural gas.  
Consequently, a certain pattern can be observed for any composition where an increase in normal flame propagation speed can be divided into three modes: (1) at a hydrogen content of up to 50-60%, an increase in the laminar flame speed with an increase in the proportion of hydrogen is approximately linear and does not exceed 25%, which means  
Figure 4: Relationship between the propagation laminar flame speed of methane–hydrogen mixture and the volume concentration of hydrogen [9; 11; 12] (**a**) \(\phi=0.6\); (**b**) \(\phi=1\); (**c**) \(\phi=1.4\).  
Figure 3: Influence of the molar fraction of hydrogen on the flame propagation speed of a methane-hydrogen mixture at atmospheric pressure and 300 K [9; 10].  
that hydrocarbons predominate in the chemical composition; (2) at the highest levels of hydrogen content (>75-90%), there is an exponential increase in flame speed due to the dominant effect of hydrogen oxidation chemistry, which means that the flame is considered to be predominantly in hydrogen; (3) the range within 60-75% can be considered the area of the parabolic increase in velocity.