A spectral scattering method for determining size distribution function and optical characteristics of droplets ensembles in fuel sprays

A new optical technique for determining size distribution functions and optical characteristics inside liquid fuel sprays is presented. The technique is based on the spectral measurements of the horizontally polarized scattering coefficients by an ensemble of droplets and of polarization ratios in the wavelength range between 300 nm and 450 nm and at scattering angles near the Brewster angle. These spectral profiles are respectively elaborated through an iterative non linear inversion procedure and a Newton formula application whose sensitivity to experimental errors has been numerically tested. The retrieved size distribution functions are compared to size distribution profiles measured through the visibility method on two light oil isothermal sprays generated either by a pressure nozzle or an air assisted atomizer and a reasonable agreement between the two techniques is reached. The technique is particularly valuable in determining size distribution function in dense sprays where single drop sizing methods cannot be used and it is only limited by the occurence of wavelength dependent attenuation of incident and scattered light in soot laden regions. The spectral dependence of the imaginary part of the refractive index of droplets inside the central region of sprays in burning condition has been obtained by combining ensemble spectral scattering measurements and LDV visibility ones. The increase in absorptivity of the droplets respects to isothermal conditions shows that the liquid phase undergoes a mild pyrolisis during which heavier aromatic compounds than those present in the original fuel are formed.