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Particle velocities in a vertically upward gas-solid suspension flow have been measured using a laser-Doppler anemometer system. The electronic signal-processing equipment enables individual particle slip velocities to be measured, which in turn allows mixtures of different-sized particles to be successfully studied. Alumina particles in the size range 15-200 μm were used. In laminar pipe flow of fine particles (15 μm) the particle velocity profile is parabolic, but with increasing pipe Reynolds' number the profile becomes flatter. Near the pipe wall smaller particles were unexpectedly found to have greater slip than larger particles. Reasonable agreement was found between the measured centre-line slip velocities and those computed by extensive theoretical considerations. For alumina particles in excess of 30 μm diameter, Stokes' law is no longer applicable, and great care must be taken in order to obtain an accurate estimation of the slip.

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