The measurement of the velocities of particles in an air-solid flow

Theoretical investigations of two-phase flows have not so far produced a useful model since the interdependence of the many variables has been difficult to predict. Progress towards such a model is dependent on accurate experimental work on two-phase flows. Particle velocity is an especially important property, but most available techniques either disturb the flow or are slow or inaccurate. The laser-Doppler velocity meter, LDV, was developed for measurements in single-phase flows, but it has been demonstrated by a few authors to be practical for particle velocity measurements in air-solid flows. The aim of the investigation was to find the range for which the LDV was suitable, and also to make useful measurements in a pipe conveying a dilute suspension of solids pneumatically. Air and solid velocity distributions across the diameter of a vertically upward flowing air-solid suspension in a 50 mm diameter pipe were made using an LDV. The solids conveyed were spherical glass balls, mean diameter 455 um, and sand, mean diameters 176 um and 366 um. The maximum ratio of solids to air mass flow rate was 2.5 and the maximum mean air velocity was 50 ms. Significant slip between the phases was found. Some of the correlations postulated between the particle velocity and other flow properties, such as the pressure drop, were investigated. Velocity measurements were also attempted with an LDV on plastic pellets, with effective diameters of 2 to 3 mm and varying degrees of success were achieved. The optical properties of the particles appears to be important when applying the laser-Doppler particle measuring technique to flows conveying particles of this size