Single-laser CH, OH and fuel PLIF in a pressurized swirl-stabilized spray flame

This paper explores the use of planar laser-induced fluorescence (PLIF) with excitation wavelengths in the R-branch of the CH C 2 Σ + − X 2 Π ( 0,0 ) in a liquid-fueled combustor at elevated pressures. Following the atmospheric pressure work of Hammack et al. Appl. Phys. B 124:34 (2018), longpass intra-vibrational band filtering was used to isolate CH PLIF, OH PLIF, and fuel signals from the laser scattering. CH PLIF images were obtained by targeting CH R-branch transitions at 310.69 nm and 311.18 nm. Interference from fuel fluorescence was mitigated through the use of a low-aromatic content synthetic aviation fuel, though it was still significant where liquid was present. Nevertheless, the CH layers in the reaction zone were distinguishable from the droplet signal even in the dense spray near the fuel injector. OH PLIF images could be obtained without CH interference using the same setup, but targeting the OH Q-branch transitions of the A − X ( 0,0 ) band at 310.75 nm. Furthermore, due to pressure-induced collisional broadening, simultaneous CH and OH PLIF could be imaged on a single frame when the laser wavelength was tuned in the vicinity of either CH R-branch transition. These results highlight the effects of collisional line broadening on PLIF measurements at elevated pressure, and their potential to enable unique imaging capabilities. There was no evidence of significant fluorescence saturation over the range of laser pulse energies tested, indicating a path for improved signal-to-noise. Increasing pressure did not diminish CH signal levels, showing promise for future studies at higher pressures. Novelty and significance statement: This work presents, to the authors knowledge, the first implementation of CH PLIF imaging at elevated pressures (up to 0.7 MPa) and in a swirl-stabilized spray flame. Additionally, it is the first implementation of combined CH-OH PLIF imaging using the C − X intra-vibrational band filtered detection technique, enabling such measurements in confined or particle laden combustors operated at elevated pressure. This work advances the capabilities of CH PLIF diagnostics, demonstrating and enabling measurements in confined, liquid fueled combustors operated at elevated pressure. This opens the door for CH PLIF to be used in studies of practical combustor configurations, e.g., in aviation gas turbine engines. Furthermore, the intra-vibrational band filtered detection technique allows for simultaneous application of other laser diagnostics, enabling the in-depth study of such practical combustion systems.