Xukun Luo

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.

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.

Gas disengagement technique in a slurry bubble column operated in the coalesced bubble regime

Abstract The gas disengagement technique is discussed in detail and is applied in a two-dimensional bubble column and slurry bubble column to obtain the bubble size distribution. Flow characteristics including velocities of each phase and dispersed phase size distribution during the gas disengagement process are obtained by using an advanced image analysis tool including a particle image velocimetry technique. It is shown that in the coalesced bubble regime, the bubble and flow characteristics during the gas disengagement process are complex, i.e., there are strong bubble–bubble interactions as well as interactions between the liquid flow and bubbles. Several different analogies of the disengagement processes assuming a bimodal bubble size distribution are discussed in terms of the behavior of large and small bubbles and their influence over one another. The small bubbles, which disengage within the fast bubble flow region, must be taken into account to evaluate the bimodal bubble size distribution. The applicability of the gas disengagement technique to obtain a multimodal bubble size distribution is also elaborated upon.

Single bubble formation in high pressure liquid—solid suspensions

Abstract Bubble formation from a single nozzle is investigated analytically and experimentally in nonaqueous liquid and liquid—solid suspensions at pressures up to 17.3 MPa. A mechanistic model is proposed to predict the initial bubble size in liquid—solid suspensions, by taking into account the various forces affecting the bubble growth including those induced by the presence of the particles, such as the suspension inertial force and the particle-bubble collision force. It is found that the initial bubble size in the suspensions is generally larger than that in the liquid mainly due to the inertia effect of the suspension. The initial bubble size increases with the solids holdup. The pressure has an insignificant effect on the initial bubble size in both the liquid and liquid—solid suspensions under the conditions of this study. The model can reasonably predict the initial bubble sizes obtained in this study and those reported in the literature.

High temperature and high pressure three-phase fluidization-Bed expansion phenomena

Abstract Bed contraction and expansion are investigated in a high pressure and high temperature gas–liquid–solid fluidized bed. The bed is operated at pressures from 0.1 to 17.4 MPa and temperatures from 20 to 94°C. The bubble dynamic behavior in the bed is visualized through transparent windows. The study indicates that the pressure and temperature affect the bed expansion and contraction, mainly through the variations in bubble behavior and the changes of liquid properties. The extent of bed contraction decreases with an increase in pressure and/or temperature for 2.1 mm glass beads. This decrease is attributed to the reduction in size and number density of large bubbles. However, for 1 mm glass beads, bed contraction is observed to be more pronounced under higher pressures. The generalized wake model is modified to account for bed expansion or contraction phenomena by taking into account the bubble size distribution effects.