PLIF Imaging of Fuel Fraction in Practical Devices and LII Imaging of Soot

In this paper we discuss the development and application of 2D imaging 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 fluorescence (PLIF) with pulsed UV lasers. Fuel concentrations may be estimated from the fluorescence of common fuel components or through the addition 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 combustion derived polyaromatic hydrocarbons (PAH) and laser‐induced incandescence from soot, are discussed. The application of the method is illustrated with an example of an isothermal mixing study in a gas turbine combustor sector rig and measurements in a simple flame. The results demonstrate 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 studies of non‐premixed flames is laser‐induced incandescence (LII) from soot particles. We discuss the role of LII both as an interference in PLIF imaging and as an imaging method for soot volume fraction. A detailed analysis of the physics of LII is presented on a theoretical model developed. Results from the model and supporting experimental data are presented. LII images from a highly turbulent sooting flame indicate that soot structures, probably in the form of thin sheets as small as 100 μm across, are formed through vortex mixing.