Experimental analysis of laser-induced spark ignition of lean turbulent premixed flames: New insight into ignition transition

Abstract

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 for a wide range of equivalence ratios. Our study used a similar approach to present an additional and complete MIE data set for laser-induced spark ignitions in a flow configuration and under turbulent conditions (i.e., for different ranges of length and time scales) that differed from those chosen by Shy and his colleagues. To extend the previous analyses to very lean premixed methane/air flames, the study examined a characteristic chemical time (τCB), which was defined as the start of the chain-branching reactions. Physically, this chemical time corresponded to the time when the initial chemical reactions of the ignition process released enough heat to compensate for heat losses and to enable a self-sustained reaction in successful ignition. Values for τCB, which were experimentally obtained from the temporal evolution of the mean hot kernel emissions (in laminar flow), strongly decreased with the equivalence ratio Φ for very lean flames and remained nearly constant and equal to 150μs for Φ>0.7. We obtained a clear turbulent ignition transition on the MIE as a function of the rms velocity (u′) and reconfirmed the two modes of ignition. Temporal kernel emission recordings served to describe the transition through the study of the interaction between the turbulence and the hot kernel at the time t=τCB. For low turbulence, when the smallest time scales of the turbulence τK were larger than the τCB values, the hot kernel/turbulence interaction occurred after the initiation of the chemical reactions. The amount of deposited energy required to initiate the reactions and to attain a self-sustained flame kernel was consequently similar to that found under laminar conditions. After the transition, the smallest time scales of the turbulence were smaller than the τCB values. Turbulence may have affected and interacted with the hot kernel before the initiation of chain-branching reactions occurred. Larger amounts of deposited energy were therefore required to compensate for this turbulent dissipation and to attain a self-sustained flame. A Peclet number (PeCB) was introduced, which is equal to the ratio of the turbulent diffusivity at the time of the initiation of the chemical reactions and the thermal diffusivity. These very scattering MIE data (dependent on Φ and u′) are plotted as a function of PeCB and collapsed into a single curve with two drastically different increasing slopes showing the ignition transition.