Abstract:

The X-43 missions in 2005 demonstrated the feasibility of a hydrogen fueled scramjet. Since that time, the cost, performance, and safety concerns surrounding the use of hydrogen have motivated researchers to investigate methods of enhancing the performance of easily stored hydrocarbon based fuels by improving their ignition, flammability, and flameholding characteristics.

The microwave enhanced combustion testbed has demonstrated a significant increase in the laminar flame speed of a premixed CH4 /air flame when 1.3 kW continuous wave (CW) microwave radiation is directed at the flame front in a high-Q resonant cavity. The main aspect of the research aimed to accurately quantify key combustion parameters in the microwave enhanced flame using laser diagnostics for improved spatial resolution and accuracy over invasive probe devices. Particle image velocimetry (PIV), filtered Rayleigh scattering (FRS), and planar laser induced fluorescence (PLIF) were used to measured flame speed, temperature, and OH radical concentrations, respectively. The PIV and FRS laser experiments measured increases in flame speed up to 20% and a deposition of only 30 W of microwave power into the flame. The temperature measurements show an increase of temperature within the flame front as well as in the post flame region that are not of large enough magnitude to account for the flame speed increase via simple joule heating. Alongside PLIF measurements of an enhanced concentration of OH in the flame zone, these results imply that the microwave enhanced combustion was a combined thermal joule heating and non-equilibrium interaction. By replacing the CW microwave source with a pulsed magnetron that is capable of generating 1 ?s wide, 30 kW peak power pulses at 1000 pulses per second, similar flame speed enhancements were achieved with 40 times less power. This effort provides the first realistic approach towards incorporating the microwave enhanced combustion concept into a scramjet combustor.