OH planar laser-induced fluorescence and emission imaging in high-pressure LOx/methane flames

The application of planar laser-induced fluorescence of OH to high-pressure liquid oxygen/gaseous methane flames is investigated in this article. As pressure is increased, the maximum level of OH fluorescence decreases while an interfering light intensity increases. It is shown that suitable data can only be obtained by properly tuning the detection scheme. Narrowband filtering of OH fluorescence is required to reduce the level of interfering signals. An analysis of the interfering light indicates that it is associated with polycyclic aromatic hydrocarbon fluorescence originating from a region surrounding the flame. OH and polycyclic aromatic hydrocarbon fluorescence signal amplitudes become comparable at a pressure of 2.5 MPa which constitutes an upper bound for standard imaging. Below that limit the flame is well characterized and features thin, wrinkled OH layers developing in the vicinity of the liquid oxygen jet. The initial flame sheet is continuous but it becomes highly corrugated further downstream when the liquid oxygen jet breaks down. The flame edge standoff distance is greater than a few LOx post lip sizes indicating that stabilization is less well achieved than in the case of liquid oxygen/hydrogen flames where combustion typically begins at less than one lip size from the injector.