On accelerative propagation of premixed hydrogen/air laminar and turbulent expanding flames

The accelerative propagation of laminar and turbulent premixed hydrogen/air flames are investigated using a constant volume combustion chamber. The relative flame accelerative characteristics are analyzed under different φ, P, and u^’. The results show that three distinct stages, namely quasi steady, transition acceleration and saturation acceleration stages are distinguished in laminar flame propagation process, and the onsets of saturation acceleration and transition acceleration nearly satisfy the relationship of Pe_cr2/Pe_cr1≈2. The temporal acceleration exponents (α_R) of laminar flames increase quickly in transition acceleration stage and nearly remain as a constant (1.1–1.25) in saturation acceleration stage. The acceleration exponents (α) of laminar flames are weakly dependent on flame conditions once the cellular structures are settled. The turbulent flame propagation follows a continuous acceleration law as S_F/S_L,b∼Pe^d_t and no evident transition point is observed. The temporal acceleration exponents of turbulent flames continuously increase as flame propagates outwardly and are susceptible to turbulence intensity. The acceleration parameters (d_t) of turbulent flames continuously increase with u^’/S_L in different flame regimes and the growth trend becomes slow in corrugated flamelets regime. Besides, the d_t of turbulent flames are promoted in the fuel-lean conditions. All of these indicate that the accelerative propagations of turbulent H₂/air flames are dominated by turbulent stretch and the diffusive-thermal instability still plays a promotion role.