Katsumi Tsuchiya

Some aspects of high-pressure phenomena of bubbles in liquids and liquid–solid suspensions

Abstract Some aspects of bubble dynamics and macroscopic hydrodynamic properties in high-pressure bubble columns and three-phase fluidization systems are discussed. Experimental results along with discrete-phase simulations of a single bubble rising in liquids and liquid–solid suspensions at high pressures are presented. A mechanistic model is described, which accounts for the initial size of bubble from a single orifice in liquid–solid suspensions. The mechanism for bubble breakup at high pressures is illustrated by considering bubble instability induced by internal gas circulation inside a bubble, and an analytical expression is obtained to quantify the maximum stable bubble size. Experimental examinations on the roles of bubbles of different sizes indicate the importance of large bubbles in dictating the macroscopic hydrodynamics of slurry bubble columns. Further, extensive studies are made of the key macroscopic hydrodynamic properties, including moving packed bed phenomena, flow regime transition, overall gas holdup, mean bubble size, and bubble size distribution. An empirical correlation is introduced which predicts the gas holdup in slurry bubble columns of different scales. A similarity rule is revealed for the overall hydrodynamics of high-pressure slurry bubble columns, which takes into account the operating conditions, the maximum stable bubble size, and the physical properties of the gas, liquid, and solids. The heat transfer characteristics under high pressures are also investigated. A consecutive film and surface renewal model is used to characterize the heat transfer mechanism.

Near-wake structure of a single gas bubble in a two-dimensional liquid-solid fluidized bed: Vortex shedding and wake size variation

Abstract The fluid mechanic behavior of a single gas bubble and its wake in a two-dimensional liquid-solid fluidized bed was examined visually via a video camera moving at the same s peed as the bubble. The bubble wake was observed to consist of two regions: primary wake (near wake) and secondary wake. Wake formation-shedding mechanisms were illustrated for both symmetric and asymmetric shedding modes. The shedding frequency, expressed in terms of the Strouhal number, was shown to be a function of the bubble Reynolds number but independent of particle properties. The boundaries between the primary and secondary wakes were qualitatively identified for several wake structures. The primary-wake size periodically varies in the form of a saw-tooth wave function, while the liquid wake exhibits no appreciable cyclic variation in size.

Wake properties of a single gas bubble in three-dimensional liquid-solid fluidized bed

Abstract Experiments were performed to investigate the wake properties of a single gas bubble in a three-dimensional liquid-solid fluidized bed via a video camera moving at the same speed as the bubble. The solids holdup in the fluidized bed varied up to around 10%. The bubble size varied from 5 to 20 mm with corresponding bubble Reynolds numbers ranging from 1000 to 6500. The bubble was observed to have two types of wake configurations depending on the bubble size: the asymmetric/helical vortex wake for small bubbles and the symmetric wake for large bubbles. The bubble shape and relative rise velocity in the fluidized bed can be well-represented by correlations developed for single bubbles in liquid media, although the bubble shape in liquid-solid media is slightly more flattened compared to that in liquid media. The bubble rocking frequency was found to be independent of particle properties and to correspond in magnitude to the vortex shedding frequency in a two-dimensional liquid-solid fluidized bed. The average primary wake size in three dimensions is comparable to that in two dimensions.

On the rise velocity of bubbles in liquid-solid suspensions at elevated pressure and temperature

Experiments are conducted to measure the rise velocity of single bubbles in liquid-solid suspensions at pressures up to 17 MPa and temperatures up to 88°C over the bubble size range from 1 to 20 mm. It is found that the bubble rise velocity decreases with increasing pressure and with decreasing temperature. The decrease of bubble rise velocity is due mainly to the variations of gas density and liquid viscosity with pressure and temperature. The presence of solid particles also reduces the rise velocity; the extent of reduction can be examined in terms of an increase in the apparent suspension viscosity by applying the homogeneous, Newtonian analogy. A mechanistic model is developed which considers a balance of forces acting on a single bubble, including the impact force due to solid particles, as well as buoyancy, gravity and liquid drag forces. Comparisons between the model predictions and the experimental data on the bubble rise velocity in liquid-solid fluidized beds are shown to be satisfactory.

VISUALIZATION OF BUBBLE-WAKE INTERACTIONS FOR A STREAM OF BUBBLES IN A TWO-DIMENSIONAL LIQUID-SOLID FLUIDIZED BED

Abstract In-line bubble-bubble interactions were studied visually for a stream of bubbles injected from a single nozzle into a two-dimensional liquid-solid fluidized bed. The bubble size varied from 12 to 35 mm in breadth, with corresponding bubble Re ranging from 2000 to 13000. Coalescence may take place due to suction of the trailing bubble into the primary wake of the leading bubble driven by the pressure defect in this region. Breakup probably occurs when the trailing bubble roof is flattened due to free shear layer penetration and/or vortical flow in the near wake of the leading bubble. The structure of the “apparent” primary wake for bubble aggregates in close contact varies depending on the geometric configuration of the aggregates. The wake shedding frequency was obtained for both colliding and non-colliding bubbles. For a given bubble Re, the St was found to be within the same range as that for single-bubble conditions for non-colliding bubbles but to be lower for colliding bubbles.

Effects of particle properties on bubble rise and wake in a two-dimensional liquid—solid fluidized bed

The objective of this communication is to elucidate quantitatively the effects of particle properties on wake structure as well as rise characteristics of a single rising bubble in a liquid-solid fluidized bed. A visualization technique is utilized to measure the bubble rise properties such as shape and rise velocity, and the wake properties such as vortex-shedding frequency and the sizes of the primary wake and the liquid wake (a stable particle-depleted layer immediately beneath the bubble base)

Image processing technique for measurement of solids holdup in near wake behind a single bubble in a liquid—solid fluidized bed

We apply the imaging technique to the study of the behavior of the solids concentration distribution in the bubble wake. Experiments are conducted in a two-dimensional system. The boundaries of the primary wake and the liquid wake and the average solids concentration in the primary wake are examined

Effect of turbulent wake on bubble—buble interaction in a gas—liquid—solid fluidized bed

Visual experiments are carried out in a three-dimensional fluidized bed. The bubble-bubble interaction is interpreted in terms of the wake velocity which accounts for an increase in the bubble rise velocity due to the presence of the wake. Results obtained in the present system are also compared with those obtained in the two-dimensional system

Prediction of the wake size of a single gas bubble in liquid and/or liquid-solid media—the pendulum model

Le modele mecaniste mis en place ici considere une bulle ascendante et son sillage primaire comme un corps simple rigide en mouvement. On cherche a relier la frequence de detachement tourbillonnaire a la taille du sillage de la bulle. On considere des systemes bi et tridimensionnels. Ce probleme est frequemment rencontre en fluidisation gaz-liquide-solide