## Abstract

This paper presents high-pressure minimum ignition energies (MIE) and their scaling from measurements on spark discharges in lean methane/air mixtures at the equivalence ratio φ = 0.6 by cylindrical electrodes with flat ends in near-isotropic turbulence over a range of turbulent intensities (u′/S_{L} = 0-50), from 1 to 5 atm, using a large dual-chamber, constant-pressure, fan-stirred explosion facility, where S_{L} is the laminar burning velocity. Voltage and current waveforms of spark discharges with nearly square profiles are carefully generated for accurate determination of MIE, commonly defined as the 50% successful ignitability. Applying high-speed schlieren imaging, we observe a drastic change of kernel development from turbulent flamelet to distributed like with island formation and local quench even at 5 atm, when u′/S_{L} is greater than some critical values depending on p. It is found that the scaling slopes of MIE_{T}/MIE_{L} versus u′/S_{L} change abruptly from a linear increase to an exponential increase when u′/S_{L} > (u′/S_{L})_{c}, showing ignition transition. The subscripts T and L represent turbulent and laminar properties, MIE_{L} ≈ 6.84 mJ (1 atm), 2.81 (3 atm), and 2.11 (5 atm), and the transition occurs at (u′/S_{L})_{c} ≈ 12 (1 atm), 24 (3 atm), and 34 (5 atm). It is also found that the above scattering MIE_{T}/MIE_{L} data at different u′/S_{L} and p can be merged together into a single curve when scaled with a pressure-corrected kernel (reaction zone, RZ) Péclet number, Pe^{∗} = Pe_{RZ}(p/p_{0})^{-1/4}, showing the first and fourth power dependence before and after MIE transition at a critical Pe^{∗} ≈ 3.6. Pe_{RZ} = u′η_{k}/α_{RZ}, η_{k} is the Kolmogorov length scale of turbulence, α_{RZ} is the thermal diffusivity estimated at the instant of kernel formation, and p_{0} = 1 atm. These results reveal a self-similar spark ignition phenomenon.

Original language | English |
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Pages (from-to) | 1785-1791 |

Number of pages | 7 |

Journal | Proceedings of the Combustion Institute |

Volume | 36 |

Issue number | 2 |

DOIs | |

State | Published - 2017 |

## Keywords

- High-pressure turbulent spark ignition
- Intense isotropic turbulence
- Minimum ignition energy transition
- Self-similar ignition
- Turbulent flamelet and distributed like kernels