Explosion behavior of methane-air mixtures and Rayleigh-Taylor instability in the explosion process near the flammability limits

Methane is the main component of natural gas and biogas, and it has been applied in various fields in daily life and industrial production. A better understanding of the explosion behavior of methane mixture can contribute to the safe usage of this fuel and developing advanced explosion protection technology and devices. The main purpose of this study is to investigate the explosion behavior and the Rayleigh-Taylor (RT) instability in the explosion process of methane-air mixtures using a 20 L bomb. Overpressure trajectories and flame images using high-speed schlieren technique are recorded for each experiment. Explosion behavior of methane-air mixtures is investigated at 280 K covering wide equivalence ratios (0.61-1.42) and initial pressures (50 kPa - 250 kPa). By analyzing the explosion image data, flame speed and laminar burning velocity are then calculated. Results indicate that as initial pressure rises, the maximum explosion pressure increases, while the flame speed and the laminar burning velocity decrease. The effect of initial pressure and equivalence ratio (CYRILLIC CAPITAL LETTER EF) on the spherical flame is also presented and analyzed through the combination of image data and pressure data. It is suggested that mass of reactants per unit volume and fuel composition have a strong relationship to ignition feasibility and ignition delay time. When CYRILLIC CAPITAL LETTER EF is 1.42 and the initial pressure is less than 150 kPa, the dynamic ignition process is recorded. Fores (pressure gradient, density gradient) are generated at the position of the flame front due to combustion. The density of spherical flame zone (burned zone) is lower than the unburned zone, therefore the spherical flame moves upward. The moving density stratification of the flame surface changes the structure of the flame front, which induces the RT instability. Near the flammability limits (CYRILLIC CAPITAL LETTER EF = 0.61 or 1.42), the explosion pressure is weak and flame speed is slow (-0.4 m/s). Due to the longer duration of forces, the effect of forces on the spherical flame becomes apparent. It is found that the RT instability leads to structural instability of the flame.