Laser ignition in supersonic channel flow

Laser ignition of premixed flames is studied in supersonic channel flow. Breakdown of natural gas air kernels occurs via a focused frequency-doubled (532 nm) Nd:YAG laser. A calibrated double photodiode energy measurement system is employed to differentiate incident energy from deposited energy. Supersonic ignition kernels are similar to their subsonic counterparts: they are elliptic at early times, have a third lobe resulting from vorticity associated with plasma back-streaming, and have similar growth with respect to time. However, supersonic kernels have some interesting differences: the third lobe appears offset from the laser axis and the kernel size depends on the Mach number because Mach number can affect the static density. Kernel data are processed to expose the dependence of flame growth on the ignition event. The influence of the breakdown event decays as the time from breakdown increases, i.e. the breakdown memory fades with time. Suggested laser settings and independence times to minimize the influence of the breakdown event on flame speed measurements are given. The influence of Mach number and equivalence ratio on the ignition event are also discussed. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.