Experimental and theoretical analysis of the angular pattern distribution and polarization state of the light scattered by isothermal sprays and oil flames

Abstract

The angular patterns of the most relevant elements of the scattering matrix have been measured on isothermal and burning sprays of #2 fuel oil for different heights above the nozzle. The experimental results have been compared with ray optics theory and with the Mie-Lorenz sphere model, averaged on a size distribution. The vertically polarized scattered radiation evidenced a well-resolved peak corresponding to the primary rainbow at θ = 157° for the isothermal condition. The presence of a secondary rainbow near θ = 90°, which is expected by the ray optics theory, can be inferred only by the angular discontinuity of the scattered radiation. The refracted and reflected components in the forward lobe (θ < 90°) follow quite closely the geometrical optics laws, whereas a departure from this is noticed on the dark side of the secondary rainbow. More significant departure from the geometrical optics appears in the angular pattern of the horizontal polarized components in the θ = 90–150° region. The polarization ratio has values between 0.5 and 1.4 in all the field, apart from a minimum of 0.2 corresponding to the primary rainbow. Neither Mie theory nor geometrical optics predicts any depolarization effect, whereas the experimental results show that vertical and horizontal depolarization light is nearly isotropically scattered, so that this effect can be related to multiple scattering. Measurements on burning sprays corresponding to those of the isothermal regime show that the angular patterns of intensity and polarization are intermediate between the two limit conditions in which only soot particles or fuel droplets are present. Measurements which allow discrimination between the scattering contributions of the two classes are critically discussed.