Relative flow between granular material and gas can create phenomena in which particles behave like a liquid with bubbles rising through them. In this paper, magnetic resonance imaging (MRI) is used to measure the velocities of the gas and solid phases in a bubbling fluidized bed. Comparison with theory shows that the average velocity of gas through the interstices between particles is predicted correctly by classic analytical theory. Experiments were also used to validate predictions from computer simulations of gas and solid motion. The experiments show a wide distribution of gas velocities in both bubbling and emulsion regions, providing a new direction for computational and analytical theory.
Computational fluid dynamics—discrete element method (CFD‐DEM) simulations were conducted and compared with magnetic resonance imaging (MRI) measurements (Boyce, Rice, and Ozel et al., Phys Rev Fluids. 2016;1(7):074201) of gas and particle motion in a three‐dimensional cylindrical bubbling fluidized bed. Experimental particles had a kidney‐bean‐like shape, while particles were simulated as being spherical; to account for non‐sphericity, “effective” diameters were introduced to calculate drag and void fraction, such that the void fraction at minimum fluidization (εmf) and the minimum fluidization velocity (Umf) in the simulations matched experimental values. With the use of effective diameters, similar bubbling patterns were seen in experiments and simulations, and the simulation predictions matched measurements of average gas and particle velocity in bubbling and emulsion regions low in the bed. Simulations which did not employ effective diameters were found to produce vastly different bubbling patterns when different drag laws were used. Both MRI results and CFD‐DEM simulations agreed with classic analytical theory for gas flow and bubble motion in bubbling fluidized beds. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2555–2568, 2017
Magnetic resonance (MR) was used to measure SF6 gas velocities in beds filled with particles of 1.1 mm and 0.5 mm in diameter. Four pulse sequences were tested: a traditional spin echo pulse sequence, the 9-interval and 13-interval pulse sequence of and a newly developed 11-interval pulse sequence. All pulse sequences measured gas velocity accurately in the region above the particles at the highest velocities that could be achieved (up to 0.1 m s-1). The spin echo pulse sequence was unable to measure gas velocity accurately in the bed of particles, due to effects of background gradients, diffusivity and acceleration in flow around particles. The 9-and 13-interval pulse sequence measured gas velocity accurately at low flow rates through the particles (expected velocity < 0.06 m s-1), but could not measure velocity accurately at higher flow rates. The newly developed 11-interval pulse sequence was more accurate than the 9-and 13-interval pulse sequences at higher flow rates, but for velocities in excess of 0.1 m s-1 the measured velocity was lower than the expected velocity. The increased accuracy is arose from was because of the smaller echo time that the new pulse sequence it enabled, reducing selective attenuation of signal from faster moving nuclei.October 14, 2015 Dear Editor, Please would you consider the attached paper, entitled "11-interval PFG pulse sequence for accurate measurement of gas velocity in granular materials" for publication in Journal of Magnetic Resonance? It is an original paper, which has not been published or submitted for publication in any other journal. The paper describes a new pulse sequence we have developed to accurately measure gas velocity in heterogeneous systems such as granular materials and porous media. The paper demonstrates that the new pulse sequence, which builds on the framework of the landmark work by , measures gas velocity more accurately in packed beds of particles than the pulse sequences from this previous work as well as a standard spin echo pulse sequence. The key development of this new pulse sequence comes from using a double spin echo segment on each side of a z-storage period, in order to significantly reduce the echo time necessary to allow for flow encoding gradients with finite ramping time. By providing a variety of comparisons varying pulse sequences and echo times while measuring signal intensity and velocity, the paper concludes that the increased accuracy of the measurements comes from the reduced echo time, which reduces selective signal attenuation from fast moving nuclei.This new pulse sequence enables gas phase velocity imaging in fluidized beds. We also anticipate that the pulse sequence described here will be advantageous in other systems in which background gradients caused by heterogeneity in the sample may be prominent. Thus, we expect the technique to have immediate application to studying flows in porous media, such as rock cores.Yours sincerely, Chris BoyceCover Letter *Graphical Abstract (for review) Highlights: New 11-interv...
Magnetic resonance imaging (MRI) was used to measure directly gas velocity and gas velocity distribution in the freeboard region of a fluidized bed (52 mm dia.) under bubbling fluidization and just below minimum fluidization. The bed consisted of poppy seed particles 1.1 mm in diameter and was fluidized using SF 6 gas at 7.5 barg for MRI purposes. In the system, bubbles approximately 20 mm in diameter rose through the centre of the bed. In the case of bubbling fluidization, time-averaged velocity maps at different vertical positions in the freeboard showed downward moving gas in the centre of the bed and upward moving gas near the walls for this particular bed. However, below minimum fluidization conditions, the profiles of gas velocity in the freeboard were flat, with respect to the radial dimension, with minor and random spatial variance, indicating that the profiles observed during bubbling arose from bubble breakthrough. The reasons for these observed patterns of flow are discussed.
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