UV Raman Scattering Measurements of a Mach 2 Reacting Flow Over a Piloted Cavity

Abstract

UV Raman scattering measurements were made in a Mach 2 supersonic air flow over a cavity piloted with ethylene fuel (C2H4). The wall cavity simulated the pilot region of a scramjet combustor. In the UV Raman system, a 248 nm KrF excimer laser beam (400 mJ/pulse, 20 ns pulse length) was used to excite the Raman scattering in the combustion zone. Raman scattered light in the 254-278 nm spectral region allows measurement of the following molecular species: CO2 (257 nm), O2 (258 nm), N2 (263 nm), C2H4 (268 nm), H2O (273 nm) and H2 (277nm). To avoid damaging the fused-silica windows on the combustion test section: 1) the laser pulse was stretched from 20 ns to 150 ns using two optical delay cavities, 2) a long focal length lens (5 meters) focused the KrF beam to a relatively large diameter (1 mm diameter) and 3) the laser energy was decreased to 100 mJ/pulse. Under these conditions, the high power pulsed laser beam passed through the side fused-silica windows without inflicting damage. Raman scattered light was collected from the top fused-silica window and was focused into a 0.32 meter spectrometer that dispersed the Raman spectrum onto an intensified CCD camera. Multiple Raman spectra are obtained along an 8 mm line to yield spatially resolved measurements of chemical composition. By translating the optical table, Raman scattering spectra were obtained in a number of locations in the Mach 2 reacting flow over a piloted cavity. Raman measurements in ethylene-fuel rich zones were contaminated by laser-induced fluorescence interference from fuel derived species (e.g., polycyclic aromatic hydrocarbons). Additional problems included laser-induced fluorescence from the fused-silica windows and UV flame emission. The Raman scattering images are being analyzed to obtain information about fuel/air mixing and reaction under supersonic conditions. Efforts will be made to improve the UV Raman system for application to scramjet flows based on this first set of measurements.