Effects of double orifice spacing on bubble behaviors and hydrodynamics in gas-liquid-solid systems through VOF-DEM method

In this paper, a three-dimensional calculation of the formation and rising of double bubbles in a gas-liquid-solid flow system with two air-intake orifices is done through computational fluid dynamics. The coupled method of volume of fluid (VOF) and discrete element model (DEM) is used to capture the interface of bubbles and track the movement of particles. The numerical method is validated by comparing our numerical results to experimental data (from other literature) of the double-bubble rising process under the same conditions. Two cases of the bubble formation and rising, when particles are laid at the container bottom (case I) and when particles are settling freely (case II), are studied carefully. At first, for case I, the detachment time of the first bubble when there is a proper time difference of the air intake start of two orifices is less than that when the start time is the same. Secondly, for case I, the possible reason of the coalescence of two parallel bubbles and reason of the shift of the bubble queues to the left or right are discussed by analyzing the velocity vector diagrams. By comparing case I and case II, it is disclosed that the free settlement of particles could prevent bubbles from coalescence and weaken the deviation of the bubble queues. Finally, for case II, the suitable distance between two orifices for better particle entrainment under different air intaking velocities is also studied, and it will increase as the air intaking velocity increases. The results of this paper is helpful to the deep understanding of the gas-liquid-solid fluidized system.

Figure 1

Geometric structure of the gas-liquid-solid flow system.

Figure 2

The instantaneous velocity $U_{\mathrm{b}}$ of the first bubble (a) and the bubble at the height of 25 mm (b) rising in the water for different grid sizes.

Figure 3

Snapshots of the particle swarm falling into water.

Figure 4

Comparison of bubble rising trajectory (a) and the distance between the bubble and corresponding orifice (b) in simulation and experiment.

Figure 5

The formation and rising of double bubbles for different distances $\left(d_{\mathrm{s}}\right)$ between orifices when particles are laying at bottom. (a) $d_{\mathrm{s}}=6$, (b) $d_{\mathrm{s}}=8$, (c) $d_{\mathrm{s}}=10$, (d) $d_{\mathrm{s}}=12$, and (e) $d_{\mathrm{s}}=18\mathrm{mm}$.

Figure 6

Details of the bubbles rising and velocity vector diagrams for different distances between orifices when particles are settled at bottom: (a), (b) $d_{\mathrm{s}}=8\mathrm{mm}$; (c), (d) $d_{\mathrm{s}}=18\mathrm{mm}$.

Figure 7

Calculation results of the formation and rising of two bubble queues when the air intakes start at different time through the two orifices. (a) $t=0.1$, (b) $t=0.2$, (c) $t=0.4$, (d) $t=0.6$, (e) $t=0.8$, and (f) $t=1.0\mathrm{s}$.

Figure 8

Velocity vector diagrams during the rising process of two bubble queues when the air intakes start at different time through the two orifices. (a) $t=0.4$, (b) $t=0.6$, (c) $t=0.8$, and (d) $t=1.0\mathrm{s}$.

Figure 9

Velocity vector diagrams of the container bottom for two cases, when the air intakes start simultaneously for the two orifices (a) and when the air intakes start with a time difference for the two orifices (b).

Figure 10

The formation and rising process of double bubbles with different orifice distance $\left(d_{\mathrm{s}}\right)$ when particles are freely settling. (a) $d_{\mathrm{s}}=6$, (b) $d_{\mathrm{s}}=8$, (c) $d_{\mathrm{s}}=10$, (d) $d_{\mathrm{s}}=12$, and (e) $d_{\mathrm{s}}=18\mathrm{mm}$.

Figure 11

Details of the bubbles rising and velocity vector diagrams for different distances between orifices when particles are freely settling. (a), (b) $d_{\mathrm{s}}=8\mathrm{mm}$; (c), (d) $d_{\mathrm{s}}=18\mathrm{mm}$.

Figure 12

Fitted curves of the numbers of the entrained particles at different heights for several orifice distances under the fixed air-intaking velocity ($v_{\mathrm{g}}=0.5\mathrm{m}/\mathrm{s}$) at time (a) $t=0.60$ and (b) $t=1.0\mathrm{s}$.

Figure 13

Time evolution of the percentages of entrained particles by the two bubble queues for different orifice distances under the fixed air-intaking velocity ($v_{\mathrm{g}}=0.5\mathrm{m}/\mathrm{s}$).

Figure 14

Percentage of the entrained particles under different air-intaking velocities for several orifice distances at different time when particles are freely settling. (a) $v_{\mathrm{g}}=0.6$, (b) $v_{\mathrm{g}}=0.8$, (c) $v_{\mathrm{g}}=1.0$, (d) $v_{\mathrm{g}}=1.2$, (e) $v_{\mathrm{g}}=1.4$, and (f) $v_{\mathrm{g}}=1.6\mathrm{m}/\mathrm{s}$.

Figure 15

Particle distributions in the $x$ direction for different orifice distances and air-intaking velocities at a fixed time. (a) $v_{\mathrm{g}}=0.6$, (b) $v_{\mathrm{g}}=0.8$, (c) $v_{\mathrm{g}}=1.0$, (d) $v_{\mathrm{g}}=1.2$, (e) $v_{\mathrm{g}}=1.4$, and (f) $v_{\mathrm{g}}=1.6\mathrm{m}/\mathrm{s}$.