High pressure ignition kernel development and minimum ignition energy measurements in different regimes of premixed turbulent combustion

A high-pressure, double-chamber explosion facility possessing controllable ignition and turbulence properties is used to measure flame kernel formation and its minimum ignition energy (MIE) of methane-air mixtures at the equivalence ratio φ=0.7 with various turbulent intensities (u'/S_L) up to 80 covering from flamelet to distributed regimes, where S_L is the laminar burning velocity. The facility applies a high-power pulse generator to control ignition energies of a spark-electrode at the centre of a large inner cruciform burner that is lodged in a huge high-pressure outer chamber. The cruciform burner is equipped with a pair of counter-rotating fans and perforated plates capable of generating controllable near-isotropic turbulence. Two statistical methods for MIE determination are discussed. A high speed Schlieren imaging system is applied to see different modes of flame kernel development with island formation and quench. Results show that MIE decreases considerably with increasing pressure (p) at any given u', while MIE increases with increasing u' at fixed p. It is found that the increasing slopes of MIE_T/MIE_L=Γ curves with increasing u'/S_L change drastically from linear to exponential at different critical values of u'/S_L≈22 (1atm) and 60 (3atm) showing MIE transition, where the subscripts T and L represent turbulent and laminar values with MIE_L=0.73mJ (1atm) and 0.23mJ (3atm). We introduce a modified reaction zone Péclet number, Pe^*=Pe_RZ(p/p₀)^-1/4 estimated just at the instant of the formation of the flame kernel, indicating the surface diffusivity ratio between turbulence and chemical reaction around the kernel with pressure correction, where p₀=0.1MPa. The aforesaid two different sets of Γ vs. u'/S_L curves can be thus merged into a single Γ vs. Pe^* curve having one critical value of Pe^* across which MIE transition occurs. A model is proposed in attempt to explain these results.