Usage of the spatial filter method for measurements of local particle velocities in multiphase flows

The optical spatial filter method is based upon the evaluation of light distributions by grating-like structures. The velocity of objects moving through the measuring volume is obtained by determining. the center frequency of a signal delivered by a light receiver observing that volume. The most common realization of this method is laser Doppler anemometry. The measurement volume is located within an intersection of two focused laser beams; a grating results from interference. A single light receiver observes an integral over the intersection area. In our present work, the grating function is performed within the optical receiver. The active gratings simplify the optical arrangement. In order to apply the spatial filter measuring system to multiphase flows with high solid concentrations or optically dense fluids a special probe has been designed. This probe guides light to the measurement volume at its tip and ensures the visibility of particles in this volume for the light receiver. Thus, it is possible to measure the particle velocity at any position inside a gas/solid or fluid/solid flow. The measurement range can be adjusted to the process easily by changing the focal length of the used lens.