Ensemble laser light scattering diagnostics for the study of fuel sprays in isothermal and burning conditions

Abstract

The diagnostic potentialities of ensemble laser light scattering methods based on the ratio of the polarized scattering coefficients γ =C HH /C VV at different angles ν are discussed. In the forward region (20°< ν <90°), the polarized scattering coefficient of the fuel droplets can be evaluated in the framework of ray optics and they are the sum of refracted and externally reflecte rays; the γ ratio decreases with the droplet diameter when the absorption index k is different from zero. In the side scattering region (95°< ν <130°), the scattering coefficients are computed by the Lorenz-Mie theory, and their intensity and polarization are due to the combined effects of surface waves and externally reflected rays; the γ ratio decreases as the droplet size increases, even when k =0. Numerical results are presented for two scattering angles at ν =60° and at ν =105° for different values of the absorption index, and the influence of the polydispersion of the spray is analyzed. The method is applied for three different experimental conditions, to show the potentialities of the technique. The radial profiles of the modal size of droplets of an isothermal heavy oil spray, produced by an air-assisted nozzle, are determined using combined γ measurements at ν =60° and 105°, taking into account their degree of polydispersion. The droplet sizes of light oil, heavy oil and heavy oil-coal slurry, produced by an air assisted nozzle in different atomization regimes, are compared, using polarization measurements in the side scattering region. The radial size distributions are determined inside light oil sprays under burning conditions for both air-assisted and pressure atomizers; the polarization measurements at ν =60° allow the determination of the high temperature value of the absorption index of the fuel droplets that survived the first heating process, thus evidencing the liquid-phase pyrolysis of the fuel. The final comments are devoted to the comparison of the present method with other sizing techniques and give indications on other possible theoretical and experimental developments.