The Measurement of Particle Concentration with Optical Fiber Probes

UNPUBLISHED-1-SUMMARY Optical fibers are frequently used to measure particle concentration in dense gas-solid flows, because they are simple, they yield strong signals, and they create minimum disturbances, see for example, Hartge, Rensner, and Werther [Chem. (1989)]. Despite their apparent simplicity, their signals do not always yield an unequivocal measurement of particle concentration. Using a Monte-Carlo technique, we simulate the optical system consisting of the fiber and the suspension. We predict the response of the instrument at several volume fractions and the extent of its measurement volume. We examine in detail the performance of a single step index fiber and a design aimed at limiting the size of the measurement volume, which includes two converging emission and detection fibers. We find that probes with distinct emission and detection regions exhibit blind spots that may produce ambiguous signals, particularly for small particles. We reformulate the simple analysis of Rensner and Werther [Part. interpret our results and to explain the behavior of these probes.