## 2 Features of Hydrogen Combustion Processes
### Unsteady Combustion
Thus, both the addition of hydrogen and a decrease in the flame size lead to a decrease in the time of acoustic energy transfer from the flame front to the place of fuel supply, transforming and increasing the pulsation frequency [23]. In this event, the pulsation frequency with an M-type flame is lower than that with a V-type. Similar conclusions were obtained in the study by [PERSON] [60] devoted to the effect of adding hydrogen to the methane-air mixture on the self-excited thermoacoustic instability of the flame, including the case when the length of the combustion chamber was changed in the range from 300 to 1100 mm. With an increased percentage of hydrogen, the size of the flame also decreased (Figure 28). At a low hydrogen concentration (\(\approx\)15%), excitation took place mainly for low-frequency primary acoustic modes (<200 Hz). When the frequency of the primary oscillations (without addition of hydrogen) was below 200 Hz, the frequency tended to increase linearly with the percentage of hydrogen. However, the linear relationship between the frequency and the hydrogen fraction was associated with acoustic modes that are influenced by the chamber length. At high hydrogen concentrations (>40%), the primary acoustic modes tended to occur at higher frequencies (\(\approx\)400 Hz). Furthermore, increasing the hydrogen content leads to a decrease in flame length (Figure 28). Figure 27: The influence of the molar fraction of hydrogen on the speed of sound. In the papers by [PERSON] and [PERSON] [54; 55], the observed frequencies coincided with the frequencies of the longitudinal modes of the chamber, while in [54], the hydrogen content had no effect on the pulsation frequency, which remained equal to the frequency of the first longitudinal mode of the combustion chamber. On the other hand, in [23; 60], the authors noted an increase in the frequencies of the first modes with an increase in the hydrogen content due to an increase in the temperature of the combustion products and, as a consequence, an increased sound speed in the combustion chamber. While maintaining the temperature of the combustion products, the pulsation frequency can remain the same. [PERSON] [28] reports that with an increase in the fraction of the hydrogen content, the acoustic energy is redistributed between frequencies. Thus, when burning pure methane, the authors identified the three main frequencies of 275 Hz, 450 Hz, 600 Hz, while for burning 60% of hydrogen, the only frequency characteristic was 450 Hz. The absence of the 600 Hz band indicates a change in the acoustic damping/amplification characteristics (amplitudes) of the combustion chamber, providing a high intensity in the 450 Hz band. This combustion mode is more dangerous since it can lead to the fatigue failure of the elements of the combustion chamber. [PERSON] [55] reveals that, upon transition to the oscillation mode, the pulsation frequency during the combustion of a hydrogen-methane mixture was equal to the pulsation frequency during the combustion of pure methane (110 Hz). However, at an excess air ratio of 0.65, the instability in the combustion of the hydrogen-methane mixture escalated to a higher frequency of 180 Hz. All the observed frequencies (110, 180 Hz) coincided with the frequencies of the longitudinal modes of the combustion chamber. The relationship between the hydrogen content and the amplitude of pressure fluctuations depends on the design and operating parameters of the combustion chamber: its length, the average pressure, excess air coefficient, acoustic conditions at the inlet and outlet of the chamber, the swirl number, etc. [PERSON] [60] states that a low hydrogen content (\(\approx\)15%) can enhance pulsations, while a sufficiently high hydrogen level (\(\approx\)40%) can weaken the primary modes. In turn, in the study by [PERSON] [27], the largest amplitude was found at 25% hydrogen content in the fuel by volume and decreased with a further increase in the hydrogen content.