Immersion tomographic study of the motion of bubbles in a flooded granular bed

Abstract

The microstructure and dynamics of two-phase gasliquid flows in packings and granular beds remain a challenging problem in several fields of advanced engineering, such as chemical technology, the power industry, and petroleum and gas production. The creation of physical and computational models and methods is limited by the unobtainability of visual observations of phase interactions in such flows. Immersion tomography of a gas-liquid medium in a granular bed, a recently developed technique, provides a new means for investigating the hydrodynamics in this type of object [1]. The essence of the immersion visualization technique, in this case, consists of choosing a liquid with a refractive index practically coinciding with the refractive index of the transparent particles of the packing: as a result, the flooded packing becomes an optically homogeneous transparent object in which bubble movement can be observed in any projection. This movement, which depends on the interaction of a bubble with packing elements, is three-dimensional and very complex. For three-dimensional reconstruction of the bubble trajectory, the familiar methods of optimal tomography were used [2]: an object is illuminated in various directions, and optical-parameter distribution in the bulk of the probed object is reconstructed.The granular bed was modeled by a packing of K8 glass beads with a refractive index of 1.51. The liquid used was a solution of α -monobromonaphthaline with methylene iodide in n -decane. The immersion liquid was supplied by the All-Russian Research Institute of Physical Technical and Radio Engineering Measurements of the State Standard Office of the Russian Federation (Gosstandart). Required for the calculations were the following properties of the mixture: density ρ = 990 kg/m 3 , dynamic viscosity µ = 3.5 × 10 -3 Pa s, and surface tension coefficient σ = 3.5 × 10 -2 N/m. Beads with a diameter of d p = 6 × 10 -3 m in the cell were packed in ten layers in the vertical direction and four and eight layers in the transverse directions. However, in most of the experiments, the packing consisted of beads with a diameter of d p = 3 × 10 -3 m with an approximately doubled number of layers in all directions. A capillary used to feed gas microvolumes in order to create bubbles could be positioned either inside the packing or below it in the free liquid layer. The scheme and description of the experimental setup are found elsewhere [1]. The basic change in the measuring scheme was the use of a Lepton digital computer system (Zelenograd, Russia) as the image recorder. This avoided the use of a stroboscopic system and improved image quality. THEORETICAL ANALYSISPreliminary experiments, which are described elsewhere [1], showed that bubbles rose in a flooded granular bed at velocities 1.5-4 times slower than in a free liquid volume. Experiments in the granular beds also indicated the existence of significant transverse velocity components in bubbles. Therefore, roughly, we may regard a flooded granular bed as a highly viscous disper...

Bubbles emerging from a submerged granular bed

“…These are exemplified in figure 2. In mode 1, small bubbles percolate up through the interstitial spaces in the granular bed presumably in the manner studied experimentally by Roosevelt & Corapcioglu (1998), Brooks et al (1999), Pokusaev et al (2004) and others. In the present case, these bubbles could emerge from a single site on the bed surface (at low flow rates or with large beads) or multiple sites (at high flow rates or with smaller beads).…”

“…Consequently, the emerging bubble size is determined by the balance of surface tension and buoyancy forces as previously described in the porous media literature (see e.g. Pokusaev et al 2004). This can be demonstrated in two simple but equivalent ways by considering a large bubble of diameter, D, in a packed bed of grains with diameter, d, filled otherwise with liquid of density, ρ, and surface tension, S.…”

“…We should note that there exists a literature on how bubbles rise through granular beds (e.g. Roosevelt & Corapcioglu 1998;Brooks, Wise & Annable 1999;Gostiaux, Gayvallet & Géminard 2002;Pokusaev, Kazenin & Karlov 2004) though the focus of those studies is on what happens within the bed rather than during emergence. We make reference below to that literature.…”

Characterising gas behaviour during gas–liquid co-current up-flow in packed beds using magnetic resonance imaging

“…gives a minimum detectable bubble radius of 0.6 mm. Bubbles closer together than the pixel resolution of the image (780 m) will not be individually identified, therefore 'swarms' or 'flocks' of small bubbles travelling together in, for example, bubble trains (Pokusaev et al, 2004b), will be identified as a single long bubble in the present work. As seen in Fig.…”

“…There are numerous examples of the use of optical techniques for tracking of gas bubbles; for example, Benkrid et al (2002), Pokusaev et al (2004 a,b), Bordas et al (2006), Mena et al (2008), Jo & Revankar (2009. In particular, Pokusaev et al (2004b) released small numbers of bubbles of 0.4-2 mm in diameter into the centre of a packed bed of 3 mm diameter glass spheres, with n-decane as the liquid phase. It was found that the rise velocity of the bubbles increases significantly with bubble size.…”