Study of Local Solid Volume Fraction Fluctuations in a Liquid Fluidized Bed

• Author / Creator

  Marefatallah, Maedeh

• This research aimed to investigate the local dynamic behaviour of solid–liquid mixtures through measurements of one of the important dynamic parameters of liquid fluidized beds: local solid volume fraction fluctuations. These experimental results will enhance the understanding of important interactions in a solid–liquid multiphase system: particle–particle, particle–fluid, and particle–wall interactions.
  Spatially resolved experimental measurements were made using a high-speed electrical impedance tomography system combined with signal processing techniques. The important advantage of this technique over other measurement methods such as optical techniques is that it can produce reliable results even at the high solid volume fractions, e.g. values close to the maximum packing concentration, even for opaque systems.
  In this study solid volume fraction fluctuations of different solid particles in a liquid fluidized bed were measured: mono-sized spherical Delrin, nylon, glass, and steel particles and water was the fluidizing medium. A wide range of particle Stokes numbers (94 ≤ St ≤ 3809) was tested by selecting specific particle sizes and densities. The results were compared with previous experimental studies. Previous works measured only cross-sectional averages of the fluctuations. Therefore, for comparison the local measurements of this study were spatially averaged. The comparison showed an acceptable agreement in the level of Root Mean Square (RMS) fluctuations. Importantly, this study found that: (1) The strongest fluctuations always occurred at a narrow strip by the bed wall. Moving from the wall region towards the bed center, the magnitude of RMS fluctuations decreased. (2) The magnitude of local fluctuations was linked to the particle Stokes number (St), such that, at any given radial position, the RMS fluctuations increased by increasing St. However, the shapes of the local RMS profiles did not change significantly with St. (3) Only limited agreement was observed among the trends of the measurements and the predictions of four existing mathematical and semi-empirical models.
  Furthermore, the solid fraction fluctuations obtained in the liquid fluidized bed were analyzed in both the time and frequency domains. Power spectra analysis showed that by moving from the center of the bed to the wall region the slopes and profiles of power spectra do not change significantly. However, the amplitude increases over the whole frequency domain. The slopes of decay in power spectra revealed a two-dimensional turbulent flow coexisting with an external force. Further analysis confirmed that this external force is associated with void fraction waves traveling in the bed. Results obtained over a wide range of bulk solid fractions showed that the dominant frequency of these waves decreases with increasing solid fraction. Local experimental data presented in this study enhance the understanding of the small- and large-scale fluctuations in a solid–liquid fluidized bed.
  
  Unlike the previous studies which were limited to cross-sectional-averaged values, this study provides experimental data with a higher spatial resolution. This local information not only can be used to examine the current mathematical models but also can validate and improve computational models of liquid fluidization. This will enhance the current simulation works of two-phase systems and thus multiphase flows in general.

• Subjects / Keywords

  • Fluidization
  • Fluidized bed
  • Solid–liquid flow
  • Electrical impedance tomography
  • Solid volume fraction fluctuations

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-dr5x-5a44

• License

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