A Study in the Application of Capacitive Techniques to the Measure of the Flow Velocity of a Two Phase Slurry
Capacitance methods can be applied to electrically measure mass flow of slurries in pipes. The mass flow relation is Q(kg/sec)=v(m/sec)·density(kg/m3)·pipe cross-sectional area (m2). A capacitive instrument designed with electrodes large (relative to pipe diameter) for density measurements and small for velocity measurements, was fabricated and tested. Preliminary calculations and tests showed that the capacity of the large electrode is related to the average density and to the difference in dielectric constants of simple two component slurry mixtures in the pipe. This study, which concentrated on the velocity electrodes, indicates that measurement of mean velocity u can be accomplished using two electrodes with known separation L by cross correlating the signals obtained from them. Local fluctuations (bubbles or vortices) in the density of the material flowing through the pipe which persist during the time to transit both electrodes (∆t=L/u) provide the signals which can be correlated to obtain the transit delay time At. Then the mean velocity u=L/ ∆t. Reproducibility of the ∆t determination depends on the signal-to-noise ratio. In principle, capacitive instruments which combine density and velocity measurements can be produced commercially and used in applications for which continuous measurement of mass flow of a slurry mixture in a pipe is required.