Simultaneous 10-kHz PLIF and chemiluminescence imaging of OH radicals in a microwave plasma-enhanced flame

This paper examines the structure of microwave (MW)-enhanced flames through 10-kHz imaging. High repetition rate laser diagnostic methods are used to simultaneously record 2-D images of OH laser-induced fluorescence and chemiluminescence within an atmospheric plasma-enhanced flame. Collecting both OH planar laser-induced fluorescence and chemiluminescence allows for observation of OH radicals in the plane of the thin laser sheet as well as volume-integrated excited state emission. A tunable, MW waveguide plasma source - operating at 2.45 GHz and delivering 90-130 W to the flowfield - ignites and sustains a CH₄/air flame, whereas laser-induced fluorescence and chemiluminescence are acquired at a sustained framing rate of 10 kHz, using two intensified CMOS cameras and a synchronized laser. Multiple geometries and flames (premixed and nonpremixed) are studied by adjusting gas flow compositions and the plasma applicator nozzle components. A stoichiometric premixed flame configuration produces a divergent flame with large-scale fluctuations and vortex shedding into ambient air and is capable of feedstock flow velocities >20 m/s for combustion-to-plasma power ratios >10:1. Another arrangement produces plasma along the initial mixing layer of a nonpremixed flame, yielding a thin cylindrical reaction zone of coincident chemiluminescence and fluorescence. Replacing the fuel with rich premixed gases produces a narrow conical flame anchored by the circular plasma discharge with a little flamefront fluctuation. The high-speed diagnostics capture OH signals in cinematic sequences, providing new understanding of the plasma-assisted flame holding mechanism and allowing for the tracking of individual flow feature development.