Mesoscale enhancement mechanism of gas-liquid mass transfer in ultrasonic microreactor

doi:10.11949/0438-1157.20220087

Abstract

Abstract:

The mass transfer processes of the gas-liquid two-phase Taylor flow in the ultrasonic microreactor were simulated by CFD method. The spatial distribution and time evolution of the mesoscale structures including surface waves, acoustic streaming and local concentration were analyzed. The simulation results effectively captured the liquid film region and correlated the liquid film thickness with the surface wave vibration. The formation of acoustic streaming near the gas-liquid interface was also discussed. According to the mass transfer characteristics of ultrasonic Taylor flow, the effects of different flow and ultrasonic conditions on the mass transfer process at the liquid slug and film were investigated separately, and the contribution of each local part on the overall mass transfer efficiency was compared. The relationship between the overall and local Sh-Pe was analyzed to discuss the characterization method of the gas-liquid mass transfer rate under ultrasonic conditions. The analysis results verify the calculation of the mass transfer coefficient of the ultrasonic microreactor from the perspective of mesoscale, and improve the theory of strengthening the gas-liquid mass transfer process in the ultrasonic microreactor.

Key words: microchannel, microreactor, ultrasonics, acoustic cavitation, bubble, mass transfer

摘要：

采用CFD方法对超声微反应器内的Taylor气液两相流的传质过程进行了模拟。针对传质过程中主要的介尺度结构，包括气泡表面波、空化声流、液相内的局部浓度，分析了其空间分布和时间演化规律。模拟结果有效捕捉了实验难以观测的液膜区域，并将液膜厚度与气泡表面波振动进行了关联，阐释了气液界面附近的空化声流对传质过程的强化作用。根据超声微反应器内Taylor流的传质特点，分别研究了不同流动和超声条件对液弹内和液膜处传质过程的影响，比较了各局部传质对整体传质效率的贡献。通过分析整体/局部Sherwood数与Peclet数间的关系，研究了超声效应对气液传质速率的影响。分析结果从介尺度角度验证了文献关于超声微反应器传质系数的计算，完善了超声微反应器内气液传质过程的强化理论。

关键词: 微通道, 微反应器, 超声, 声空化, 气泡, 传质

CLC Number:

TK 124

Feishi XU, Lixia YANG, Guangwen CHEN. Mesoscale enhancement mechanism of gas-liquid mass transfer in ultrasonic microreactor[J]. CIESC Journal, 2022, 73(6): 2552-2562.

许非石, 杨丽霞, 陈光文. 超声微反应器内气液传质过程的介尺度强化机制[J]. 化工学报, 2022, 73(6): 2552-2562.

Figures/Tables 13

Fig.1 Schematic description of the ultrasonic microreactor and the modeling unit

Table 1 Physical parameters used in this study

参数	液相	气相
参数	水	二氧化碳
密度ρ/(kg/m³)	998.2	1.7878
黏度 μ/(mPa·s)	1.003	0.0137
摩尔质量 /(kg/mol)	18.0152	44.00995
表面张力系数σ/(N/m)	0.072
饱和浓度 /(kg/m³)	—	1.688
液相扩散系数 /(m²/s)	—	2×10^-9

Table 2 Size and distribution of mesh cells for mesh independency test

项目	轴向3000 μm		径向						总数
	轴向3000 μm		中心区域200 μm		过渡区域45 μm		壁面区域5 μm
	尺寸/μm	数量	尺寸/μm	数量	尺寸/μm	数量	尺寸/μm	数量
Mesh 1	20	150	20	10	10~1.34	9	1	5	7200
Mesh 2	10	300	10	20	10~1.34	9	1	5	20400
Mesh 3	5	600	5	40	4.8~1.3	16	1	5	73200
Mesh 4	2.5	1200	2.5	80	2.5~1.2	25	1	5	264000

Fig.2 Mesh independence test and verification of the mass transfer algorithm

Fig.3 Schematic description of the liquid film and simulation results

Fig.4 Thickness of the steady and dynamic liquid film versusCa under different ultrasonic oscillation amplitudes

Fig.5 Modeling results of the acoustic microstreaming near the interface

Fig.6 Flow pattern and temporal evolution of the concentration field in the liquid slug under different oscillation amplitudes

Fig.7 Distribution of the solute concentration at the liquid film with and without ultrasonic effects

Fig.8 Temporal evolution of the ratio of the flux at the liquid film to liquid slug (bubble cap) under different oscillation amplitudes

Fig.9 Temporal evolution of Sh under different flow and ultrasonic condition

Fig.10 ShversusPe at the liquid film

Fig.11 ShversusPe at the liquid slug (bubble cap)

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