CFD simulation of gas–liquid flow in a hig(3)

The geometry of the high-pressure bubble column is shown in Fig.1(a):height 6600 mm,diameter 300 mm,wall thickness5 mm.Tw o conductivity probes at the height of 2550 mm and 3050 mm are used to measure the bubble size in the radial direction and the velocity of rise of bubble electrodes of Electrical Resistance Tomography(ERT)are evenly mounted on the inner wall of the high-pressure bubble column at the height of 2600 mm and 3000 mm to measure the local air holdup and average gas holdup.

diagram of the high-pressure bubble column and meshing scheme.

Compared with the 3D geometric model,the computational amount of the numerical simulation will be greatly reduced by the two-dimensional geometric et al.[18]succeeded in simulating gas–liquid flow behaviors in a bubble column by an axisymmetric refore,two-dimensional axisymmetric geometric model are used in this work.As shown in Fig.1(b):6.6 m high and 0.15 m wide.In the model,no gas distribution plate is arranged on the column bottom and the gas is directly fed from the central bottom of the column(r/R≤0.8).The heights of 2550,2600,3000 and 3050 mm were set as the sampling experimental and simulation results are compared and analyzed in the following sections.

The mesh of 2D simulation structure is generated by the Mapped Face Meshing of mesh at the wall:wall inflation,Maximum Layers:5,Grow th Rate:1.2.As shown in Fig.1(c),the obtained mesh is between the two sections from 2000 mm to 3200 mm above the bottom,and this section of 1200 mm length is enlarged to show the refine meshing at the wall.

In Fig.2,a grid independence verification was performed under the condition of 0.160 m·s?1and 0.5 MPa.The simulation results of radial gas holdup were compared and analyzed with four different grid figure indicates that the simulation results with other three grids are basically consistent except for the first ,to ensure a high computing accuracy and an acceptable computing time,the grid with 5940 cells was adopted.

of grid on the simulation results of gas holdup.

and Discussion

4.1.Gas holdup

gas holdup

holdup at different apparent velocities and operating pressures(0.5,1.0,1.5,2.0 MPa).

The simulation results of the average gas holdup between the heights 2600 and 3000 mm at different apparent gas velocities and pressure were compared with the experimental data[14]in Fig.3.It is indicated that although the gas holdup made by the CFD-PBM coupled model without Cedirectly increases with the increasing apparent gas velocity,the error is greater than that by the modified CFD-PBM model with is shown that the modified CFD-PBM coupled model can accurately simulate the average gas holdup under other experimental conditions of Qin[14].

gas holdup

In Fig.4,the simulated radial gas holdup profiles are compared with the experimental data under the operating conditions of 0.5,1.0,1.5,2.0 MPa and 0.160,0.215,0.253,0.317 m·s?1.The two diagrams at the same apparent velocity show the simulation results of the modified CFD-PBM coupled model and the CFD-PBM coupled model respectively compared with the experimental data.And the radial gas holdup showed a decreasing trend from the center to the column wall.It can be seen that the simulation results obtained by the CFD-PBM coupled model are generally smaller than the experimental data at elevated et al.[32]proposed a model for the bubble interaction time by energy conservation analysis,ignoring the effects of operating the Luo coalescence model is used directly to simulate high-pressure bubble column,as the simulation environment pressure is lower than the really pressure,the simulation result of gas holdup is can be seen that when the density correction coefficient Ceis introduced into the Luo coalescence model,the error between the simulation results and the experimental data of Qin[14]is reduced.

diameter

distribution of bubbles

Fig.5 describes the effect of operating pressure on bubble size radial distribution at the apparent gas velocity of 0.160,0.215,0.253,0.317 m·s?1.It can be seen from Fig.5 that although the bubble size decreases from the center to the edge of the column,the results obtained by the CFD-PBM coupled model are far larger than the experimental data at four different operating pressures(0.5–2.0 MPa)per gas the other hand,the results obtained by the modified CFD-PBM coupled model are more consistent with experimental ,the effect of operating pressure on the bubble diameter in the gas–liquid flow s can be well simulated by adjusting the density correction coefficient Cein the bubble coalescence model.

size distribution

Fig.6 shows the simulation results of the influence of pressure on the bubble sizedistribution at the apparent gas velocity of 0.160 m·s?1.The bubbles show a unimodal number of medium bubbles(3–8 mm)increased at elevated pressure,while the number of smaller bubbles(＜3 mm)almost was consistent with the simulation results of Yang et al.[33].This is fully showed that the bubble size became smaller and more uniform at elevated pressure.

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