Simultaneous planar OH imaging of microwave plasma enhanced combustion at 10 kHz

This study examines the structure of various microwave enhanced flames through 10 kHz imaging. High-speed laser diagnostic methods are implemented to simultaneously record two dimensional images of OH laser-induced fluorescence and chemiluminescence within an atmospheric plasma enhanced flame. A tunable, microwave waveguide plasma source ignites and sustains CH4/air flame, while signal acquisition operates at a framing rate of 10 kHz, utilizing two intensified CMOS cameras and a synchronized laser. Resultant image sequences are temporally correlated between the two diagnostics, each with greater than 1000 continuous frames (100 ms time span). Multiple geometries and flames are studied by adjusting gas flow compositions and plasma applicating nozzle components, producing optically accessible premixed and non-premixed flames. The plasma source is powered by a continuous 2.45 GHz magnetron producing 360 W of power. Microwave power input to the nozzle is reduced to 90 to 130 W using stub tuners and a sliding short to control the reflected microwave power, isolated and absorbed by a circulator equipped with a dummy load. Collecting both OH planar laser-induced fluorescence with chemiluminescence allows for observation of OH radicals in the plane of the thin laser sheet as well as volume integrated excited state emission. One premixed flame geometry produces a divergent flame with large scale fluctuations and vortex shedding into ambient air, capable of stoichiometric feedstock flow velocities greater than 20 m/s for combustion-to-plasma power ratios greater than 10:1. Another arrangement produces plasma along the initial mixing layer of a non-premixed flame, yielding a thin cylindrical reaction zone of coincident chemiluminescence and fluorescence, periodically separating downstream. Replacing the fuel with rich premixed gases produces a narrow conical flame anchored by the circular plasma discharge with very little front fluctuation. The high-speed diagnostics successfully capture OH signals in cinematic sequences, allowing for tracking of individual flow feature development.