Study of local solid volume fraction fluctuations using high speed electrical impedance tomography: Particles with low Stokes number

Abstract

The instantaneous local solid volume fraction in a two-phase solid–liquid fluidization system is measured using high-speed Electrical Impedance Tomography (EIT). The unique aspect of EIT is that it provides local volume fraction fluctuations even for opaque, concentrated mixtures. A two-meter tall and 10.16 cm diameter fluidized bed loop is used for the experiments. Mono-sized spherical Delrin particles (dp = 3.34 mm, ρs = 1400 kg/m3) with relatively low Stokes number (≈100) are tested as the solid phase. The root mean square (RMS) of the solid fraction fluctuations in the near-wall region was found to be 3–4 times higher than the RMS at the center of the bed. The maximum magnitude of the fluctuations occurred at the bulk solid fraction (C‾) of 0.31. Increasing C‾ had a notable effect on both the magnitudes and extent of the rates of decay in the power spectra. The spectra suggest that void fraction waves arising naturally in the bed act as an external driving force coupled with the turbulent spectrum, which leads to a predominantly two-dimensional turbulence within the bed. Cross-sectional-averaged fluctuations for a wide range of C‾ were found to be in good agreement with previous studies. Since most of the previous experimental studies on solid fraction fluctuations have been limited to cross-sectional-averaged values, the local measurements reported here provide a better understanding of the nature of small-scale fluctuations.