Study on the Ignition Method for n-Octane Pyrolysis Fuel in a Scramjet Engine

In scramjet engines, ignition must take place within a residence time on the order of milliseconds. In this study, secure ignition conditions for specified n-octane pyrolysis fuel components used in autoignition or forced-ignition by plasma jet torch in a high-speed flow were numerically investigated. First, the ignition delay time within the combustor and cavity flame-holder was estimated using chemical reaction analysis. Three fuel components (n-C8H18, all pyrolysis fuel (15 components, decomposition rate under 11%)), and pyrolysis gas fuel (eight gas fuel components, decomposition rate under 11%) could not self-ignite within the combustor and cavity residence time. Secondly, ignition using a plasma jet torch in the cavity was numerically investigated. In the case of forced-ignition by plasma jet torch, all pyrolysis fuel (No. 3) and n-C8H18 could ignite within the cavity residence time with less input energy than pyrolysis gas fuel (No. 3) under three kinds of Mach number flight conditions (M-0 = 4, 6, and 8). Moreover, the effect of shortening the ignition delay time by raising the plasma jet torch gas temperature and O radical rate within the cavity was investigated. Ignition of the three kinds of mixture fuel was more greatly affected by the torch injection temperature than the O radical rate in the cavity under all Mach number flight conditions.