TRAJECTORY AMBIGUITIES IN PHASE DOPPLER SYSTEMS - STUDY OF A NEAR FORWARD AND A NEAR-BACKWARD GEOMETRY

Generalized Lorenz-Mie theory for the scattering of arbitrarily shaped beams by spherical particles has been applied to two standard phase D oppler layouts, employing receiving units at 30-degrees and 150-degree s off-axis locations. It is shown that the particle trajectory effects may lead to inaccurate size measurements for the near-forward receive r and may make the near-backward measurements totally misleading when a large particle size range (1 : 40) needs to be covered. Only limited improvements can be achieved by using two phase-shift signals from a single receiving unit for discrimination. The errors associated with t he trajectories are also detrimental to the concentration measurements based on the existing criteria. However, an extended optical system e mploying two identical receiving units, located symmetrically about th e plane of the laser beams, provides a robust solution to the trajecto ry ambiguity. It can be used to measure correctly the particle size an d the particle location in the measuring volume. The difficulties asso ciated with estimating the effective size of the measuring volume as a function of the particle diameter (in order to determine the true siz e distribution and the particle number density) may also be resolved b y employing an extended system. Hence, despite a higher cost, this arr angement is attractive, at least for obtaining some benchmark simultan eous measurements of sizes and velocities in two-phase particulate flo ws.