Ignition transition in turbulent premixed combustion

Recently, Shy and his co-workers reported a turbulent ignition transition based on measurements of minimum ignition energies (MIE) of lean premixed turbulent methane combustion in a centrally-ignited, fan-stirred cruciform burner capable of generating intense isotropic turbulence. Using the same methodology, this paper presents new complete MIE data sets for stoichiometric and rich cases at three different equivalence ratios φ{symbol} = 1.0, 1.2 and 1.3, each covering a wide range of a turbulent Karlovitz number (Ka) indicating a time ratio between chemical reaction and turbulence. Thus, ignition transition in premixed turbulent combustion depending on both Ka and φ{symbol} can be identified for the first time. It is found that there are two distinct modes on ignition in randomly stirred methane-air mixtures (ignition transition) separated by a critical Ka where values of Ka_c ≈ 8-26 depending on φ{symbol} with the minimum Ka_c occurring near φ{symbol} = 1. For Ka < Ka_c, MIE increases gradually with Ka, flame kernel formation is similar to laminar ignition remaining a torus, and 2D laser tomography images of subsequent outwardly-propagating turbulent flames show sharp fronts. For Ka > Ka_c, MIE increases abruptly with Ka, flame kernel is disrupted, and subsequent randomly-propagating turbulent flames reveal distributed-like fronts. Moreover, we introduce a reaction zone Péclet number (P_RZ) indicating the diffusivity ratio between turbulence and chemical reaction, such that the aforementioned very scattering MIE data depending on Ka and φ{symbol} can be collapsed into a single curve having two drastically different increasing slopes with P_RZ which are separated by a critical P_RZ ≈ 4.5 showing ignition transition. Finally, a physical model is proposed to explain these results.