Experimental study on void fraction distribution in liquid slug of vertical upward slug flow

doi:10.11949/0438-1157.20210135

Abstract

Abstract:

The fiber probe method and the high-speed photography method are used to test the void fraction distribution in the liquid slug of a vertical upward co-current air-water slug flow. Experiments were performed in a polymethyl methacrylate tube of 15 mm inner diameter. An improved image processing technique based on the machine learning was employed, which can effectively recognize various complex types of bubble boundary. By building a neural network system for bubble boundary extraction, the iterative training of the model was performed by using the constructed bubble boundary database. The radial void fraction distribution profiles of the liquid slugs were obtained and the results showed a typicalwall-peak distribution. The wake effect of the Taylor bubble had aprominent influence on the distribution form, and the wall peaks were observed to correspond to the vortex center of the Taylor bubble wakes. Regarding the local void fraction in the tube center and the radial position of the wall peak, two corresponding predictive correlations were proposed and the experimental data in the present study was found to agree well with the correlations.

Key words: two-phase flow, slug flow, void fraction, distributions, optical probe, image processing technique

摘要：

采用光纤探针法和高速摄影法对垂直上升气液两相弹状流的液弹区含气率分布进行了试验研究，测试管径为15 mm。一种基于机器学习的图像处理技术用来识别气液两相界面，通过搭建气泡边界提取的神经网路系统，使用构建的气泡边界数据库对模型进行多次迭代训练，该方法可以有效地识别多种复杂类型的气泡边界。试验得到了弹状流液弹区的径向含气率分布曲线，结果表明，壁峰分布是液弹区含气率分布的主要形式，其中泰勒气泡的尾迹效应对分布形式有重要影响，尾迹的旋涡中心和含气率分布的峰值相对应。针对弹状流液弹区径向含气率分布的两个主要特征——中心局部含气率和壁峰位置，分别提出了相应的预测公式，且与本文的试验数据吻合良好。

关键词: 两相流, 弹状流, 含气率, 分布, 光纤探针, 图像处理技术

CLC Number:

TQ 021.1

Teng WANG, Qincheng BI, Miao GUI, Zhaohui LIU. Experimental study on void fraction distribution in liquid slug of vertical upward slug flow[J]. CIESC Journal, 2021, 72(9): 4584-4593.

王腾, 毕勤成, 桂淼, 刘朝晖. 弹状流液弹区含气率分布的试验研究[J]. 化工学报, 2021, 72(9): 4584-4593.

Figures/Tables 14

Fig.1 Schematic diagram of the main hydrodynamic features and regions of a slug flow

Fig. 2 Schematic diagram of the air-water two-phase flow test loop

Fig.3 Several typical bubbly flow regimes in the liquid slug of a slug flow

Fig.4 Image processing steps based on machine learning

Fig.5 Optical probe functioning principles

Fig.6 Real time signal processing of a slug unit by the optical probe

Table 1 Measurement uncertainty

参数	最大相对不确定度/%
定性压力P/MPa	0.48
定性温度T_in/℃	1.82
表观液相速度U_l_s/（m/s）	1.5
表观气相速度U_gs/（m/s）	1.0
局部含气率α_l	8.4
气相速度U/（m/s）	12.6
气泡平均索特直径D_sm/mm	15.2
像素面积	0.5

Fig.7 Radial void fraction distribution profile of the liquid slugs

Fig. 8 Identification of the radial void fraction distribution of the discrete bubbles in liquid slugs, employing the criterion proposed by Mendez-Diaz

Table 2 Comparison between wall-peak distributions of several typical bubbly flows with that of liquid slugs in this study

研究方法	管径/mm	流型	壁峰位置	峰值含气率与中心含气率比值	文献
试验/探针法、图像法	15	弹状流液弹区	0.5R~0.8R	1.07~1.26	本文
试验/探针法	50	泡状流	0.8R~0.95R	1.18~1.58	[29]
试验/热膜风速探头	38	泡状流	0.85R~0.95R	1.16~8.4	[25]
试验/电导法	14.8	泡状流	0.75R	2.36	[27]
数值模拟	40	泡状流	0.85R	8.4	[26]

Fig. 9 Flow pattern diagram and streamline diagram of the Taylor bubble wake

Fig.10 Comparison between the radial void fraction distribution of the Taylor bubble wake region and that of the entire liquid slug region

Fig.11 Predictive correlation evaluation of the local void fraction in the center of the liquid slug region

Fig.12 Predictive correlation evaluation of the peak position of the radial void fraction distribution profiles

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