The flow of a solid-liquid mixture is inherently unsteady. Experiments have shown that the velocities of both the fluid and solid phases and their respective volume fractions experience important fluctuations around their mean values. Multiple factors might determine the nature of these fluctuations, such as the upstream and downstream conditions, the density ratio of the phases and the mean slip velocity. The interaction between the phases is quite complex and significant momentum is transferred between the phases and with the boundaries.
The experiments involve a vertical gravity driven flow using glass particles in water (with density ratio = 2.5). By changing downstream conditions the solid fractions can be varied from 0.02 to 0.60. A non-intrusive technique to measure the instantaneous volume fraction has been developed. An Impedance Volume- Fraction Meter (IVFM) measures the instantaneous impedance of the flowing mixture, averaged over the cross section, which is a direct measurement of the volume fraction of each phase. A pair of IVFMs is mounted close to each other in the test section. The signals of the IVFMs for different parameters are analyzed in time and frequency domains. By cross-correlating the signals of the two IVFMs the velocity of the traveling disturbances is calculated. Mean mass flow rates for both phases are calculated. Results are also obtained for two different diameter test sections (d/D=33.8 and 16.9).
In addition, a technique to measure the pressure generated by collisions of individual particles is developed using a high-frequency-response dynamic pressure transducer that is mounted in the wall of the test section. When a particle collides against the active face of the transducer a computer system records the signal and, in real time, calculates the impulse exerted. A time average particle pressure is calculated as a product of the average impulse times the event rate. Results for different flowing conditions are presented as a function of the average solid fraction.