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Title: Mixing dynamics in bubbling fluidized beds

ORCiD logo [1];  [1];  [2]
  1. Dept. of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave. Cambridge MA 02139
  2. Dept. of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave. Cambridge MA 02139, National Energy Technology Laboratory, Morgantown WV 26507
Publication Date:
Sponsoring Org.:
OSTI Identifier:
Resource Type:
Journal Article: Publisher's Accepted Manuscript
Journal Name:
AIChE Journal
Additional Journal Information:
Journal Volume: 63; Journal Issue: 10; Related Information: CHORUS Timestamp: 2017-09-04 12:18:36; Journal ID: ISSN 0001-1541
Wiley Blackwell (John Wiley & Sons)
Country of Publication:
United States

Citation Formats

Bakshi, A., Ghoniem, A. F., and Altantzis, C.. Mixing dynamics in bubbling fluidized beds. United States: N. p., 2017. Web. doi:10.1002/aic.15801.
Bakshi, A., Ghoniem, A. F., & Altantzis, C.. Mixing dynamics in bubbling fluidized beds. United States. doi:10.1002/aic.15801.
Bakshi, A., Ghoniem, A. F., and Altantzis, C.. 2017. "Mixing dynamics in bubbling fluidized beds". United States. doi:10.1002/aic.15801.
title = {Mixing dynamics in bubbling fluidized beds},
author = {Bakshi, A. and Ghoniem, A. F. and Altantzis, C.},
abstractNote = {},
doi = {10.1002/aic.15801},
journal = {AIChE Journal},
number = 10,
volume = 63,
place = {United States},
year = 2017,
month = 5

Journal Article:
Free Publicly Available Full Text
This content will become publicly available on May 31, 2018
Publisher's Accepted Manuscript

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  • An investigation was conducted to determine the effect of system pressure on the Transport Disengaging Height (TDH) above a 12-inch-diameter (30.5 cm) fluidized bed. TDH was found to increase linearly with both system pressure and gas velocity.
  • In this paper, the flow hydrodynamics in a bubbling fluidized bed with submerged horizontal tube bundle was numerically investigated with an open-source code: Multiphase Flow with Interphase eXchange (MFIX). A newly implemented cut-cell technique was employed to deal with the curved surface of submerged tubes. A series of 2D simulations were conducted to study the effects of gas velocity and tube arrangement on the flow pattern. Hydrodynamic heterogeneities on voidage, particle velocity, bubble fraction, and frequency near the tube circumferential surface were successfully predicted by this numerical method, which agrees qualitatively with previous experimental findings and contributes to a soundermore » understanding of the non-uniform heat transfer and erosion around a horizontal tube. A 3D simulation was also conducted. Significant differences between 2D and 3D simulations were observed with respect to bed expansion, bubble distribution, voidage, and solids velocity profiles. Hence, the 3D simulation is needed for quantitative prediction of flow hydrodynamics. On the other hand, the flow characteristics and bubble behavior at the tube surface are similar under both 2D and 3D simulations as far as the bubble frequency and bubble phase fraction are concerned. Comparison with experimental data showed that qualitative agreement was obtained in both 2D and 3D simulations for the bubble characteristics at the tube surface.« less
  • The effect of bed thickness in rectangular fluidized beds is investigated through the CFD–DEM simulations of small-scale systems. Numerical results are compared for bubbling fluidized beds of various bed thicknesses with respect to particle packing, bed expansion, bubble behavior, solids velocities, and particle kinetic energy. Good two-dimensional (2D) flow behavior is observed in the bed having a thickness of up to 20 particle diameters. However, a strong three-dimensional (3D) flow behavior is observed in beds with a thickness of 40 particle diameters, indicating the transition from 2D flow to 3D flow within the range of 20–40 particle diameters. Comparison ofmore » velocity profiles near the walls and at the center of the bed shows significant impact of the front and back walls on the flow hydrodynamics of pseudo-2D fluidized beds. Hence, for quantitative comparison with experiments in pseudo-2D columns, the effect of walls has to be accounted for in numerical simulations.« less
  • Particles of char derived from a variety of fuels (e.g., biomass, sewage sludge, coal, or graphite), with diameters in excess of {approx}1.5mm, burn in fluidized bed combustors containing smaller particles of, e.g., sand, such that the rate is controlled by the diffusion both of O{sub 2} to the burning solid and of the products CO and CO{sub 2} away from it into the particulate phase. It is therefore important to characterize these mass transfer processes accurately. Measurements of the burning rate of char particles made from sewage sludge suggest that the Sherwood number, Sh, increases linearly with the diameter ofmore » the fuel particle, d{sub char} (for d{sub char}>{approx}1.5mm). This linear dependence of Sh on d{sub char} is expected from the basic equation Sh=2{epsilon}{sub mf}(1+d{sub char}/2{delta}{sub diff})/{tau}, provided the thickness of the boundary layer for mass transfer, {delta}{sub diff}, is constant in the region of interest (d{sub char}>{approx}1.5mm). Such a dependence is not seen in the empirical equations currently used and based on the Frossling expression. It is found here that for chars made from sewage sludge (for d{sub char}>{approx}1.5mm), the thickness of the boundary layer for mass transfer in a fluidized bed, {delta}{sub diff}, is less than that predicted by empirical correlations based on the Frossling expression. In fact, {delta}{sub diff} is not more than the diameter of the fluidized sand particles. Finally, the experiments in this study indicate that models based on surface renewal theory should be rejected for a fluidized bed, because they give unrealistically short contact times for packets of fluidized particles at the surface of a burning sphere. The result is the new correlation Sh = 2{epsilon}{sub mf}/{tau} + (A{sub cush}/A{sub char})(d{sub char}/ {delta}{sub diff}) for the dependence of Sh on d{sub char}, the diameter of a burning char particle. This equation is based on there being a gas-cushion of fluidizing gas underneath a burning char particle; the implication of this correlation is that a completely new picture emerges for the combustion of a char particle in a hot fluidized bed. (author)« less