PHASE-DOPPLER SIZING OF OPTICALLY ABSORBING LIQUID DROPLETS - COMPARISON BETWEEN MIE THEORY AND EXPERIMENT

Phase-Doppler experiments applied to optically absorbent (homogeneous and inhomogeneous) liquids are described. Simultaneous size and velocity measurements of single droplets were executed at three off-axis angles phi. These angles were found to be suitable for the evaluation of the phase-Doppler technique by Mie theory. Both the sizes of the monodisperse droplets and their absorption properties were varied. At least with respect to homogeneous liquids, comparison of the droplet parameters obtained by phase-Doppler measurement and by photography (reference technique) showed good agreement. Concerning the optical properties of inhomogeneous liquids, additional phase-Doppler measurements were carried out using an alternative device based on a laser diode and photodiodes. The longer wavelength lambda (830 nm instead of 488 nm) reduced the influence of the inhomogeneities on the droplets' scattering behaviour and thereby improved significantly the phase-Doppler results. It is concluded that phase-Doppler anemometry is suitable for the sizing of optically absorbent droplets of real process fluids. Such droplets result from atomization processes often used in various branches, such as the chemical, pharmaceutical and food industries (production of powders and granules, "spray drying"), automotive engineering (dispersing fuels for combustion) and agriculture (crop spraying). Mie scattering theory is a powerful aid for describing the scattering behaviour of dispersed droplets of real process fluids with more complicated optical properties compared with water, i.e. the behaviour of optically absorbent homogeneous liquids, and for determining the necessary parameters of the optical set-up.