Fluorescence imaging of NO and O2 in a spray flame combustor at elevated pressures

Abstract

Planar laser-induced fluorescence images of the distributions of nascent NO and high-temperature O2 have been obtained in the injector near field of a spray flame combustor at pressures up to 0.8 MPa. The model combustor consists of a pressure-atomizing spray nozzle injecting liquid ethanol into a high-pressure oxygen and dilute nitrogen flow at room temperature. This flow is used to assess quantitatively fluorescence strategies in preparation for application to more practical combustor geometries. Fluorescence is excited near 226 nm using a low-power, excimer-pumped dye laser and imaged with an intensified CCD array camera. The camera is spectrally filtered to reject the strong elastic laser scatter from fuel droplets and soot particles. The NO and O2 mole fractions in the flow field are known and controlled at several discrete chamber pressures between 0.1 and 0.8 MPa. Detailed models of linear fluorescence using the available spectroscopic database are shown to be sufficient to predict the pressure scaling of the fluorescence signal for the known values of the NO and O2 mole fraction. This result suggests that atmospheric pressure calibrations may be extended to higher-pressure combustors without additional detailed measurements of specific temperature and major species composition fields. As with high-pressure OH imaging measurements using 283-nm excitation, hydrocarbon interferences are found to be a ubiquitous and major contributor to the fluorescence signal for the simple flow field studied to date. This contribution must be subtracted properly for quantitative measurements.