Time-resolved imaging of flame kernels: Laser spark ignition of H2/O2/Ar mixtures

The shape and structure of developing flame kernels in laser-induced spark ignited hydrogen/air mixtures is investigated as a function of gas composition and time. Using planar laser-induced fluorescence (PLIF) to measure the spatial distribution of OH radicals produced inside the reaction zone, we have recorded the evolution of the nascent flame kernel in a series of images following the laser-induced spark. This series provides the rate of flame growth, the evolution of the flame shape, and the intensity of the PLIF signal as a function of time for both igniting flames and nonignition events. The reaction zones grow quickly at early times, but slowly decrease in propagation rate as the energy density within the flame kernel decreases. A distinct anisotropy is observed in the expanding spark and flame kernel. At short times (t < 100 μs), a toroidal shape is observed similar to that seen previously for electrode-spark ignitions and for laser ignitions in methane/air. There is also a tendency for the flame to grow back toward the ignition laser. Successful ignitions appear virtually identical to failed ignitions during the first 100 μs. Significant differences, notably in intensity, appear between 100 and 500 μs following the spark. These observations imply that early flame kernel growth is dominated by gas motion induced by the short-duration spark. The ultimate fate of an ignition lies with the chemistry of the reactions which determines whether the gas undergoes a transition from hot plasma to propagating flame.