The objective of this research was to investigate analytically and experimentally the use of free radicals for ignition and flameholding in supersonic flows. An analytical investigation of the effects of adding small quantities of radicals to a stoichiometric mixture of hydrogen and air was performed using a finite-rate chemical kinetics code. The results of these calculations indicate that small additions of hydrogen atoms, oxygen atoms, nitrogen atoms, or hydroxyl radicals are effective in promoting ignition. These analytical results were qualitatively verified in a Mach 2 flow experiment using hydrogen atoms generated by a plasma torch. The supersonic combustion tests were conducted in a direct-connect mode at atmospheric pressure with either ambient temperature air or burner-heated vitiated air with total temperatures from 1200 to 4000 R. Both semi-freejet and ducted configurations were used. The experimental results indicate that hydrogen atoms from a low-power plasma torch provide an effective ignition and flameholding source for hydrogen-fueled Mach 2 flows at total temperatures as low as 1065 R, the lowest temperature tested. A reduction in the minimum total temperature required for ignition of several hydrocarbon fuels was also demonstrated.

A piloted fuel injector configuration designed to take maximum advantage of the hydrogen atoms from the plasma torch was conceived and fabricated. The injector design consisted of five small upstream pilot fuel injectors, a rearward-facing step and three primary fuel injectors downstream of the step. The hydrogen atoms from the plasma torch were injected in the recirculation region downstream of the step. Three other ignition sources were also tested as comparisons: an argon plasma, a pyrophoric mixture of silane and hydrogen, and a surface discharge device. Hydrogen-fueled supersonic combustion tests were conducted at conditions similar to those described earlier. Hydrogen atoms generated by the plasma torch proved to be the most effective ignition source, causing ignition for a torch input power of 780 W, the lowest power tested. The combination of the hydrogen atoms and the piloted fuel injector was shown to be a very effective igniter and flameholder for scramjet operation over a simulated flight envelope (Mach 3 to Mach 6, low to moderate altitudes).