# The Period of Ignition Delay for Methane-Air Mixture with Hydrogen and Ethylene Additives
## 1 Introduction
In article [4], the authors found that the non-equidiffusive effects play a negligible role in this process because the characteristic time of hydrogen diffusion is one order of magnitude higher than the characteristic time of flame roll-up around the vortex. These data, as well as the concomitant effect of the increase in the flame temperature, were confirmed by calculations [5; 6] based on numerical simulation of turbulent combustion of H\({}_{2}\)/CH\({}_{4}\) mixture in the oxygen-containing flow. It was shown by calculation that the addition of -40% hydrogen (by volume) increased the methane combustion efficiency from 0.78 to 0.96. This is due to the lower activation energies of hydrogen combustion reactions compared to methane combustion reactions. Combustion of a small amount of hydrogen at the beginning of the flame locally increased the flame temperature and intensified the methane combustion. Combustion of depleted methane-hydrogen mixtures in air was studied in [7]. In this work, the volume fraction of hydrogen in the mixture was 20% and 30%. The work was based on calculated and experimental data of turbulent flame combustion at various pressures. The authors asserted that molecular diffusion and the Lewis number play an important role in the shape of the flame front in the depleted air-fuel mixture. It is supposed that the Lewis number, which characterizes molecular processes of transport, defines the shape of laminar flame to a significant extent. While if turbulent flow prevails, the molecular effects that occur are insignificant against the background of intensive commingling via vortexes. However, the authors assert that even in turbulent flow, this factor can also have a great influence. To prove this statement, the references to the experimental data and results of numerical modeling with the use of the DNS method (Direct Numerical Simulation) are presented. The present work mainly focuses on kinetic processes; however, it is worth mentioning that the addition of hydrogen, which possesses a strong diffusion ability, besides the intensification of kinetic processes, enables the spreading of the flame front even in a turbulent flow. In some combustion modes [8], using a high-speed chamber and the PLIF method, the effect of destabilization of the flame front and the occurrence of high-frequency pulsations caused by diffusion-thermal instability and flow turbulization during hydrogen combustion were detected. It should be noted that, according to [9; 10], flow turbulization increases the average speed of flame spreading in a multicomponent medium. The effects of diffusion-thermal instability occurrence, as well as the increase in the mixture reactivity when hydrogen is added to the turbulent flow, are also confirmed by numerical simulation [11]. In [12], the problems of turbulent flame stability in the model combustion chamber of a gas turbine, in which the burner with low swirl was used, were studied experimentally and numerically. In the experiment, depleted premixed mixtures of methane with hydrogen and air were used at flow velocities of 12-18 m/s. Flame fluctuations were recorded generally in the area of the outer shear layer at the burner rim. The flame with the addition of hydrogen was characterized by the prevalence of dissipating vortices. The authors paid particular attention to the flame base, where there was an intensive exchange of oxidizer and fuel. Based on these results and the results of [13; 14; 15], it can be assumed that the main mechanisms for excitation of oscillations are Kelvin-Helmholtz instability and the spreading of acoustic waves, which are common for a burner. Expansion of research on combustion of methane with hydrogen is presented in [16]. The change of the shape of a laminar flame of depleted methane-air mixture via the addition of hydrogen additive was studied. It was shown that the addition of hydrogen expands the ranges of the flame breakdown rate. In particular, adding 20% hydrogen increased the blowout rate by 7%, and 40% hydrogen increased the blowout rate by 35%.