Solid fraction fluctuations in solid-liquid flows

The fluctuating component of the solid fraction is studied for liquid-solid mixtures using an impedance volume-fraction meter (IVFM). The experiments involve a vertical gravity driven flow using glass particles in water, and a liquid fluidized bed with either glass, steel or nylon particles. The sig nals of the IVFMs for different flow conditions are analyzed in time and fr equency domains. Results show that the magnitude of the root-mean-square (R MS) solid fraction fluctuation increases from low concentrations (<10% soli d fraction), reaches a local maximum at approximately 30% solid fraction, d ecreases again and reaches a maximum at concentrations around 45%. For high er concentrations, the RMS fluctuations decreases until the packed state. F or the same solid fraction the magnitude of the fluctuation increases for l arger diameter particles. Two main types of fluctuations were identified: l arge-amplitude low-frequency fluctuations and small-amplitude high-frequenc y fluctuations. The low-frequency fluctuations were dominant at high concen trations (>30%), while the high-frequency fluctuations became more prominen t for concentrations below 30%. When the low frequency fluctuations were fi ltered out and the RMS fluctuation was re-calculated the scaling appear to change: the filtered RMS fluctuations were larger for experiments with high er particle inertia. A comparison between the filtered RMS solid fluctuatio ns and an exiting model showed limited agreement. (C) 2000 Elsevier Science Ltd. All rights reserved.