PLIF IMAGING OF FUEL FRACTION IN PRACTICAL DEVICES AND LII IMAGING OFSOOT

In this paper we discuss the development and application of 2 D imagin g methods for the study of fuel-air mixing and the in-situ measurement of soot in flames. Fuel imaging is based on planar laser-induced fluo rescence (PLIF) with pulsed UV lasers. Fuel concentrations may be esti mated from the fluorescence of common fuel components or through the a ddition of a fluorescent species or seed. PLIF of fuel is shown to be applicable to both premixed and non-premixed combustion. The choice of fluorescent seed and the role of interfering species, such as combust ion derived polyaromatic hydrocarbons (PAH) and laser-induced incandes cence from Soot, are discussed. The application of the method is illus trated with an examine of an isothermal mixing study in a gas turbine combustor sector rig and measurements in a simple flame. The results d emonstrate that PLIF can achieve high spatial resolution, circa 0.4 mm , in a combustor with dimensions of order of 400 mm. The dynamic range of the measured intensities exceeds 1500 with typical signal-to-noise ratios of better than 100:1. A major source of interference in PLIF s tudies of non-premixed flames is laser-induced incandescence (LII) fro m soot particles. We discuss the role of LII both as an interference i n PLIF imaging and as an imaging method for soot volume fraction. A de tailed analysis of the physics of LII is presented on a theoretical mo del developed. Results from the model and supporting experimental data are presented. LII images from a highly turbulent sooting flame indic ate that soot structures, probably in the form of thin sheets as small as 100 mum across, are formed through vortex mixing.