Soot and pah distributions in oil spray flames, inferred by elastic and inelastic laser light scattering

Abstract

The space distribution of droplets, soot particles and fluorescing PAH species have been determined for vertical unconfined swirled oil spray flames, by analyzing light scattering, and laser-excited fluorescence coefficients, and radially inverted emission of OH radicals. The angular patterns of the scattering coefficient on vertical and horizontal polarization planes have been measured in the flame and compared with theoretical predictions for transparent and absorbing large spheres and soot particles and with the measured patterns on isothermal sprays. The fuel droplets were mainly concentrated in the initial part of the flame around the axisand the maximum concentration of the fluorescing species were detected on the periphery of this zone, which is externally bounded by a layer of intense OH emission. Soot was produced in a small toroidal zone near the burner outlet and more massively in a high temperature recirculation region downstream. The influence of swirl strength on the combustion process has been studied by means of the axial scattering and the radial fluorescence profiles. Vaporization was delayed along the centerline and the off-axis maximum concentration of the fluorescing PAH increased up to an intermediate swirl level. These effects were attributed to the outward displacement of the swirled air jet, which thus reduced the mixing rate between fuel and air in the central part of the flame. Further increase of the swirl level reduced the PAH concentration because the recirculation of hot gases accelerated the vaporization and combustion processes.