Numerical simulation of bubble breakup and coalescence in centrifugal impeller based on PBM

doi:10.11949/0438-1157.20231244

Abstract

Abstract:

In addressing the issue of larger and unevenly distributed bubble sizes within the impeller during the preparation of microbubbles using a centrifugal pump, this study investigates the impact of different inlet gas volume fraction (IGVF) and rotational speeds on the diameter and distribution of bubbles within the impeller. The Euler-Euler non-uniform two-fluid model is coupled with the population balance model (PBM) to solve the rotating two-phase flow field within the impeller of the centrifugal pump. Additionally, vortex identification methods and the Luo fragmentation and coalescence model are employed to analyze the distribution patterns of bubbles within the impeller. The results indicate the following: ① Vortices near the leading edge and the suction side of the blades cause gas accumulation, causing the local gas content in the flow channel to increase, where the bubble merger effect dominates. ② When the flow rate and rotational speed are constant, as the IGVF increases, the turbulence intensity in the flow channel increases, and the vortex moves backward, causing the gas phase accumulation area to also extend backward. The region of high local gas content on the suction surface significantly surpasses that on the pressure surface. Consequently, the bubble merging behavior on the suction surface becomes more pronounced, resulting in larger bubble diameters. ③ When the IGVF and flow rate are constant, increasing the rotation speed in a small range can enhance the bubble crushing effect and obtain smaller diameter bubbles.

Key words: gas-liquid flow, computational fluid dynamics, population balance model, micro bubbles, size distribution

摘要：

针对利用离心泵制备微气泡时叶轮内气泡尺寸较大且分布不均问题探究不同入口含气率（IGVF）和转速对离心泵叶轮内气泡直径和分布的影响，采用欧拉-欧拉非均匀双流体模型与群体平衡模型进行耦合，求解离心泵叶轮内气液两相旋转流场，并且结合涡识别方法、Luo破碎合并模型对离心泵叶轮内气泡分布规律进行分析。结果表明：①叶片前缘以及吸力面附近存在的涡旋导致气体聚集，引起流道内局部含气率增大，此处气泡合并效应占主导；②流量和转速一定时，随IGVF的增加，流道内湍流强度增加，旋涡后移导致气相聚集区域同样向后延伸，吸力面的高局部含气率区域增大面积显著高于压力面，因此吸力面气泡合并行为更为显著，气泡直径更大；③IGVF和流量一定时，小范围内提升转速可以使气泡破碎效应增强，获得更小直径的气泡。

关键词: 气液两相流, 计算流体力学, 群体平衡模型, 微气泡, 粒度分布

CLC Number:

TQ 021.1

Yuhui SHI, Jiyuan XING, Xuehan JIANG, Shuang YE, Weiguang HUANG. Numerical simulation of bubble breakup and coalescence in centrifugal impeller based on PBM[J]. CIESC Journal, 2024, 75(5): 1816-1829.

师毓辉, 邢继远, 姜雪晗, 叶爽, 黄伟光. 基于PBM的离心式叶轮内气泡破碎合并数值模拟[J]. 化工学报, 2024, 75(5): 1816-1829.

Figures/Tables 16

Table 1 Main structural parameters of geometry of centrifugal pump

设计流量

Q_design/(m³/h)

设计扬程

H/m

额定转速

n/(r/min)

叶轮进口直径

D₁/mm

叶轮出口直径

D₂/mm

叶片出口宽度

b₂/mm

泵出口直径

D₂/mm

叶片数

3000

115

9.2

Fig.1 Impeller y+ distribution

Fig.2 Mesh independence verification

Table 2 Discrete bubble sizes in PBM

气泡离散组	直径/mm
bin0	10.00
bin1	5.99
bin2	3.59
bin3	2.15
bin4	1.29
bin5	0.77
bin6	0.46
bin7	0.28
bin8	0.17
bin9	0.10

Table 3 Numerical simulation scheme

研究变量	设置条件
入口含气率	转速：1500 r/min
	入口含气率：0.32%、1.04%、2.21%、3.25%、4.86%
	入口气泡直径：bin4
	运行流量：42%Q_design
转速	转速：1200 r/min、1500 r/min、1800 r/min
	入口含气率：0.32%
	入口气泡直径：bin4
	运行流量：42%Q_design

Fig.3 Comparison between numerical simulation and experimental result

Fig.4 Comparison of numerical simulation and visualization values

Fig.5 Vortices identify iso-surfaces of different IGVF

Fig.6 Liquid phase velocities streamlines of different IGVF

Fig.7 Effective breakup frequency and effective coalescence frequency varied with local gas content and turbulent dissipation rate

Fig.8 Bubble distributions of different IGVF

Fig.9 Bubble distributions at different locations with different IGVF

Fig.10 Schematic diagram of effective breakup and effective coalescence frequency at different locations

Fig.11 Vortex identify iso-surfaces of different speeds

Fig.12 Bubble distributions at different positions with different speeds

Fig.13 Correction of effective breaking and effective coalescence frequency

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