The world’s fastest spark plug
(Brieschenk, O’Byrne, Kleine)
Scramjets are air-breathing rocket engines, which can propel vehicles to hypersonic speeds exceeding 10.000 km/h. At such velocities, the hydrogen fuel has to mix with the incoming air and combust in just thousands of seconds. The Gordian Knot of supersonic combustion is to find ways to efficiently ignite the air-fuel stream and force complete combustion within these time scales. Dr. Sean O’Byrne and PhD student Stefan Brieschenk have designed a laser-driven plasma torch and successfully demonstrated that this device can solve the long-standing problem of efficient supersonic combustion. By focussing a high-powered, pulsed laser inside the fuel injector, plasma is formed with peak temperatures approaching 100 000 degrees Celsius. At these high temperatures the hydrogen fuel forms ions and chemical radicals, which accelerate the fuel-air reaction by orders of magnitude. Pure hydrogen plasma is very short-lived and a small amount of plasma buffer gas is added to the fuel in order to increase plasma lifetimes. This makes sure that the hot fuel plasma will survive until mixing with the air stream is completed. With this technique, supersonic combustion can be achieved even at flow temperatures that are too low to spontaneously ignite. Because each laser pulse only lasts for a few billionths of a second, the laser-driven plasma torch requires powers in the order of only few kilo Watts – similar to a commercial hair dryer. The experimental campaign stretching over more than 3 years required the T-ADFA shock tunnel to be upgraded to a combustion facility, where safe fuel injection of the highly explosive gases can be guaranteed. Since hydrogen-air combustion is invisible, a combustion detection system (OH-PLIF) had to be designed and built. This system compromises a pulsed UV-laser, which is tuned to specific wavelengths to cause fluorescence of the hydroxide ion (OH-), a combustion species.