Early Growth of Flames, Ignited by Fast Sparks

Spark-ignition of propane/air mixtures in a constant volume combustion bomb at 200 kPa pressure has been studied within an explored time window covering four decades (0.2?s-2.0ms). Shadowgraphy and Rayleigh imaged (1-D) laser light scattering have been applied simultaneously, in order to obtain 2-D qualitative and 1-D quantitative descriptions of the fluid dynamic and chemical processes, developing after the spark. It is shown that energy delivered lo the gas within the first tens of nanoseconds gives rise to i) a fast expansion of the spark channel, ii) the development of a shock wave and iii) the build-up of a peculiar flow field, spreading out over a quite large volume, within the first millisecond. An interpretation scheme is provided, which highlights quantitatively the relations among three phases. and based upon the consideration of the dynamics and the morphology of the shock wave. Analysis of the toroidal shapes of the ignited flames allowed us to identify two, temporally distinct, growth mechanisms; it has been shown that the spark-induced flow field conirols the behaviour of the excited gas pocket within the first hundreds of microseconds, and leads to a strong mixing between excited and fresh gases, whereas the combustion chemistry becomes dominant at later times, when laminar flame fronts are established.