Optimization of optical particle sizing in multiphase flows

Simultaneous measurements of particle size and velocity with different medi a are important in the experimental study of multiphase flows. Especially, when one of the dispersed media has a relative refractive index close to un ity (m approximate to 1.0), the optical configuration requires extra consid eration in phase-Doppler anemometry (PDA). Such measurements are often enco untered when water flows are composed of air bubbles and glass beads with r elative refractive indices of m(1) = 0.75 and m(2) = 1.13, respectively. La rge uncertainty in measurements may arise if optical parameters fall into t he region of so-called "critical scattering angles." Furthermore, the scatt ering intensities from air bubbles and glass beads in water are found to ha ve two-order-of-magnitude differences at an off-axis angle in between 60 to 85 deg, which narrows the measurable dynamic range and causes bias in the size distribution of different dispersed phases. We use a scheme based on t he recently developed combined refraction-reflection model and Mie scatteri ng theory to optimize the off-axis angle to minimize the measurement uncert ainty. The variation of the critical scattering angles and scattering inten sity ratios of media are quantitatively described. The predictions are vali dated by conducting experiments to measure glass spheres and vapor bubbles in water. (C) 2001 Society of Photo-Optical Instrumentation Engineers.