Simulation study of microbubbles' break-up and coalescence in centrifugal pump based on TFM-PBM coupling model

doi:10.11949/0438-1157.20210355

Abstract

Abstract:

Understanding the generation characteristics of microbubbles in centrifugal pumps is essential for optimizing the performance of existing microbubble generation devices based on rotating equipment and improving the pollutant removal rate of industrial wastewater and waste gas. Recognizing bubble break-up and coalescence, this paper examines how microbubbles evolve under different inlet gas volume fraction (IGVF) and different inlet bubble size in two-phase rotating flow field by coupling two fluid model (TFM) and population balance model (PBM), which was verified to be effective through comparing with experimental head and average bubble size. The results show that gas accumulation in impeller to be the main factor affecting the performance of bubbles with IGVF increasing, which causes bubble size increase as break-up dominated turn to coalescence dominated in impeller. Furthermore, the influence of inlet bubble size on outlet bubble size is sensitive to IGVF. Outlet bubble size increases first and then decreases with inlet bubble size increasing at low IGVF, while this influence is not obvious at high IGVF by gas accumulation in impeller.

Key words: centrifugal pump, microbubble, gas-liquid flow, TFM-PBM coupling model, gas holdup, size distribution

摘要：

了解离心泵内微气泡的发生特性，对于优化现有基于旋转流场的微气泡发生装置的性能、提高工业废水废气的污染物去除率至关重要。在考虑气泡破碎合并的前提下，通过将双流体模型（TFM）与群体平衡模型（PBM）进行耦合，求解离心泵内气液两相旋转流场，研究了入口体积气含率（IGVF）、入口气泡尺寸对泵内气泡沿程尺寸变化、出口气泡尺寸分布的影响，并结合Luo等的破碎合并模型分析成因。结果表明，随IGVF增加，叶轮内气体聚集引起局部气含率陡升，气泡由破碎主导转变为合并主导，而后在蜗壳内气含率恢复正常，气泡又变为破碎主导，总体上出口气泡尺寸逐渐增大。另外，入口气泡尺寸对出口气泡尺寸的影响对IGVF敏感，当IGVF较低时，随入口气泡尺寸增大，出口气泡尺寸先增大后减小；而当IGVF较高时，由于泵内气体聚集，入口气泡尺寸的影响并不明显。

关键词: 离心泵, 微气泡, 气液两相流, TFM-PBM耦合模型, 气含率, 粒度分布

CLC Number:

TQ 021.1

Song GAO,Yanyan XU,Jixiang LI,Shuang YE,Weiguang HUANG. Simulation study of microbubbles' break-up and coalescence in centrifugal pump based on TFM-PBM coupling model[J]. CIESC Journal, 2021, 72(10): 5082-5093.

高颂,徐燕燕,李继香,叶爽,黄伟光. 基于TFM-PBM耦合模型的离心泵内微气泡破碎合并的模拟研究[J]. 化工学报, 2021, 72(10): 5082-5093.

Figures/Tables 18

Table 1 Parameters of centrifugal pump

流量	扬程	转速	叶轮入口直径	叶轮出口直径	叶片出口宽度	出口直径D₃/mm	叶片数Z
Q/(m³/h)	H/mm	n/(r/min)	D₁/mm	D₂/mm	b₂/mm	出口直径D₃/mm	叶片数Z
15	17	3000	50	115	9.2	40	7

Fig.1 Geometric modeling of centrifugal pump fluid domain

Fig.2 Mesh independence verification at 1500 r/min

Table 2 Discrete bubble sizes in PBM

气泡组	直径/mm
bin0	10
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	0.1

Table 3 TFM-PBM simulation scheme

固定参数	对照组
1500，bin4，7.7 m3/h (42%Q_max)	$α$ (IGVF)/%：0.32，0.60，1.04，2.21，2.61，3.25，4.86
1800，bin4，9.16 m3/h (42%Q_max)	$α$ (IGVF)/%：0.32，1.04，3.28，4.86
1500，0.32%，7.7 m3/h (42%Q_max)	bin2,bin3,bin4,bin5,bin6,bin7
1500，3.25%，7.7 m3/h (42%Q_max)	bin2,bin3,bin4,bin5,bin6,bin7
1500，4.86%，7.7 m3/h (42%Q_max)	bin3,bin4,bin5,bin6

Table 3 TFM-PBM simulation scheme

固定参数	对照组
1500，bin4，7.7 m3/h (42%Q_max)	$α$ (IGVF)/%：0.32，0.60，1.04，2.21，2.61，3.25，4.86
1800，bin4，9.16 m3/h (42%Q_max)	$α$ (IGVF)/%：0.32，1.04，3.28，4.86
1500，0.32%，7.7 m3/h (42%Q_max)	bin2,bin3,bin4,bin5,bin6,bin7
1500，3.25%，7.7 m3/h (42%Q_max)	bin2,bin3,bin4,bin5,bin6,bin7
1500，4.86%，7.7 m3/h (42%Q_max)	bin3,bin4,bin5,bin6

Fig.3 Head comparison of TFM-PBM data and experimental values[30]

Fig.4 Phase distribution comparison of TFM-PBM data and visualization values[30]

Fig.5 Average bubble size of each flow domain under different inlet gas volume fraction (din=bin4)

Fig.6 Effective break-up frequency ω(B) and effective coalescence frequency ωC under different gas volume fraction α

Table 4 Turbulent dissipation rate of impeller and volute under different inlet gas volume fraction

入口体积气含率IGVF/%	蜗壳内平均湍流耗散率ε/(m²/s³)
0.32	17
0.60	17
1.04	19
2.21	26
3.25	38
4.86	42

Fig.7 Effective break-up frequency ω(B) and effective coalescence frequency ωC under different gas volume fraction α

Fig.8 Bubble number density distribution at outlet of pump under different IGVF(din=bin4)

Fig.9 Bubble size distribution at outlet of pump under different inlet gas volume fraction (by air, din=bin4)

Fig.10 Bubble size distribution at outlet of pump under different inlet gas volume fraction (by two-phase, din=bin4)

Fig.11 Average bubble size of each flow domain under different inlet bubble size(IGVF=0.32%)

Fig.12 Break-up efficiency P′(B) and coalescence efficiency P′(C) in impeller under different inlet bubble size (IGVF=0.32%)

Fig.13 Bubble size distribution at outlet of pump under different inlet bubble size (by two-phase, IGVF=0.32%)

Fig.14 Bubble size distribution at outlet of pump under different inlet bubble size (by two-phase, IGVF=3.25%)

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