## 2 Features of Hydrogen Combustion Processes
### Unsteady Combustion  
Pulsations in the combustion chamber are often undesirable since they can cause its complete destruction. At present, the mechanisms of pulsation generation during natural gas combustion are well described. It has been established that there are no fundamental changes in these mechanisms during hydrogen combustion; however, since the flame characteristics change, they can transform the mechanisms for generating pulsating combustion. The purpose of this section is to analyze the works devoted to the effect of hydrogen additives on unsteady combustion. In this case, several areas of analysis of the effects of hydrogen addition can be distinguished: time-lag, the shape and position of the flame, changes in frequency and amplitude, and changes in the phase delay. With an increase in the hydrogen content, the shape, size and location of the flame change significantly. As shown in [21; 23; 25; 27; 34; 54; 55; 56; 57; 58; 59], the flame becomes more compact (shorter) and pressed against the base of the burner. During combustion of a premix mixture in the swirling flow, the shape of the flame changes from M to V. According to [PERSON] [28], this change in size can occur when the percentage volume of hydrogen is over 20%. [PERSON] [23] and [PERSON] [29] demonstrate that a decrease in the size of flames in an annular combustion chamber leads to a decrease in the probability of their interaction with each other and, consequently, to the elimination of the mechanism for pulsation generation. This impact on the size and location of the flame is explained by an increase in the local heat release and the temperature in the combustion zone along with an increase in the hydrogen content in the fuel-air mixture. It is also worth noting that the addition of hydrogen leads to a change in the flame stabilization region [25], modifying the nature of the flame from the suspended to a more stable flame with a fixed stabilization point. In that instance, the most stable option would be a fixed point of stabilization of the flame through the use of a pilot circuit or a bluff body with a fixed separation point. Figure 26: Change in the critical strain rate in the flame and the strain rate in the flow with a change in the hydrogen volume fraction in the fuel [34]. [PERSON] [55] has shown that the transition to harmonic oscillations is preceded by the appearance of a flame in the outer recirculation zone, which, for the combustion of methane-hydrogen mixtures, occurs at leaner fuel-air mixture compositions than during methane combustion. This transition can be estimated using the function of the dimensionless strain rate (normalized strain rate): \(\frac{k_{M}T^{D}}{D_{\infty}}\). In the article by [PERSON] [24], a \"center of heat release\" was proposed as an indicator of stability, because a flame having the same \"heat center\" location but different operating conditions and fuel composition has almost the same shape. [PERSON] [23] studies the effect of adding hydrogen to a lean mixture of natural gas with air during combustion of a premixed mixture in a swirling flow. The hydrogen percentage in the fuel-air mixture varied up to 50% by volume. As a result, the influence of hydrogen addition on the shape of the flame, the characteristics of the structure of the precessing vortex core (PVC), and the frequency and amplitude of pulsations had to be investigated. Regarding the thermoacoustic characteristics, it was found that with an increase in the percentage of hydrogen in the fuel (as well as an increased air temperature), the speed of sound between the fuel supply point and the flame front (in the premixer) increased due to a decrease in the average molecular weight of the mixture with the addition of hydrogen (Figure 27). This characteristic reflects the transfer time of the acoustic energy from the flame front, as a source, to the place of fuel supply, as a factor that further leads to an increase in pulsations.