Bubble dynamics in a 3‐D gas–solid fluidized bed using ultrafast electron beam X‐ray tomography and two‐fluid model

Abstract

Bubble characteristics in a three-dimension gas-fluidized bed (FB) have been measured using noninvasive ultrafast electron beam X-ray tomography. The measurements are compared with predictions by a two-fluid model (TFM) based on kinetic theory of granular flow. The effect of bed material (glass, alumina, and low linear density polyethylene (LLDPE), d p $1 mm), inlet gas velocity, and initial particle bed height on the bubble behavior is investigated in a cylindrical column of 0.1-m diameter. The bubble rise velocity is determined by cross correlation of images from dual horizontal planes. The bubble characteristics depend highly upon the particle collisional properties. The bubble sizes obtained from experiments and simulations show good agreement. The LLDPE particles show high gas hold-up and higher bubble rise velocity than predicted on basis of literature correlations. The bed expansion is relatively high for LLDPE particles. The X-ray tomography and TFM results provide in-depth understanding of bubble behavior in FBs containing different granular material types.

Eulerian–Eulerian simulation of dense solid–gas cylindrical fluidized beds: Impact of wall boundary condition and drag model on fluidization

“…In order to span a wide range of fluidization regimes (particle properties and inlet gas velocity), simulation predictions are compared with experimental measurements by Rüdisüli et al [20] and Verma et al [19] in Section 5.3 while only the former is considered and scaled-up for the sensitivity study in Section 5.4. In [20], cold fluidization was carried out in a glass column with internal diameter 14.5 cm using alumina particles.…”

“…Further, bubble velocity was measured using a bubble linking algorithm by measuring the response using two such probes placed 1 cm apart. Meanwhile, Verma et al [19] used a polycarbonate cylindrical tube with internal diameter 0.1 m. The measurements in this setup were done at three cross-sections of the bed using an ultrafast electron beam X-ray scanner acquiring data at 1000 Hz with a high spatial resolution of about 1 mm. The images were then processed using an in-house reconstruction software to reveal time and spatially resolved bubbles.…”

“…Digital Image Analysis (DIA) is first applied by converting a series of 2D images (vertical or cross sectional slices) to grey-scale using ImageJ (http://rsbweb.nih.gov/ij/) with the bubble void threshold set at 0.7 [19]. While using cross-sectional slices for bubble analysis, d b = 4A b /π is used as the equivalent bubble diameter.…”

“…This is especially true for small laboratory-scale (cylindrical or thin rectangular) fluidized beds which are commonly used to obtain experimental data for validating numerical models (e.g. [18][19][20]). On the other hand, implementing the wall boundary condition in numerical simulations based on the Eulerian representation of the solid phase is not trivial and the complexity is compounded by the lack of experimental data.…”

“…Bubble statistics are usually computed by a combination of Particle Image Velocimetry (PIA) and Digital Image Analysis (DIA) (e.g. [20,39]) or more recently, using Electrical Capacitance Tomography (ECT) [19]. Both these metrics are essential for characterizing fluidization in large cylindrical beds for which experimental data is often not available.…”