Measurement of Particle and Bubble Velocities by Laser Transmission Method

Abstract

Velocity distributions of particles and bubbles were measured by the laser transmission method, which has hitherto been used to measure the specific interfacial area of dispersed phases such as solid particles, liquid droplets, and bubbles. Measurements were carried out using a laser beam oscillator and two optical-fiber probes. Apparent axial velocities were estimated by dividing the distance between the two probes by the time difference between signals from the two probes. As the laser beam oscillator and probes were set outside the column, the velocities of particles and/or bubbles in the column were measured without disturbing the system.
To obtain a relationship between the apparent axial velocity measured by the present method and true axial velocity component, the effects of the direction of movement of particles and the distance between the two optical fibers were discussed based on a simple model that takes this direction of movement into account. Comparison of the apparent axial velocities measured by the present method with the velocities measured by use of a video camera showed that the present method gave reasonable results when appropriate correction was made for the effects of the direction of movement of particles.
Using the present method, we evaluated the velocity distribution of particles circulating in an external-loop airlift column. The effect of number of particles charged in the airlift column on the velocity distribution was found for low gas velocities. Also, the rising velocity distributions of bubbles in a standard bubble column were measured. Change of the rising velocity distribution with gas velocity was found to correspond with the change of flow pattern from bubbly to churn turbulent flow.