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Title:
气泡初始直径及气泡群曳力系数的实验研究
Author: 肖航
Issued Date: 2017-05
Keyword: 化学工程
Alternative Title: 化学工程
Abstract: Gas-liquid reactors are widely applied in many industrial processes. The performance of these reactors is strongly dependent on the bubble size and its distribution. In addition, the bubble size distribution also affects directly the flow pattern, heat transfer, and mass transfer in the reactor. However, the initial diameter of the bubbles from the gas distributor is usually obtained from experience, which leads to that the accuracy of the numerical simulation related closely to the experience of the researchers. As we know, the drag force is the resistance exerted by the fluid on the bubbles, and it is a basic parameter required in the numerical simulation of multiphase flow. Up to date, the drag coefficient adopted in the computational fluid dynamics does not take into account the interaction between bubbles, i.e., the mesoscale effect of the bubble group on the drag coefficient is usually ignored. Therefore, the precise models for predicting the initial bubble diameter and mesoscale drag coefficient are urgently demanded to provide guidance for the simulation of gas-liquid multiphase flow. The models for calculating the initial bubble diameter and drag coefficient of bubble swarm can be derived on the basis of a large amount of accurate experimental observations and measurements. There are many experimental methods to study the gas-liquid multiphase flow, in which the digital image analysis (DIA) has been widely used because it is a non-intrusive technique and does not disturb the flow field in the reactor. Additionally, the DIA technique can intuitively and accurately get the information of gas holdup, bubble shape and size distribution. However, there will be more than 40% of the bubbles overlapping each other in the image when the gas holdup exceeds 1%, and the existing DIA technique cannot recognize the overlapping bubbles precisely. This is the reason why limited experimental study on the drag coefficient of bubble swarm is available. In view of the problems aforementioned, we first develop a method for efficiently recognizing elliptical overlapping bubble swarms, which was then used in the experimental studies on the initial bubble diameter and the drag coefficient of bubble swarm. The contents and achievements of this paper are as follows: (1) In response to the problem that the overlapping bubbles cannot be effectively identified by the existing DIA technique, an efficient, reliable, non-parametric and automated analytical method was established in this work. Firstly, the critical points of the ellipses are obtained by deleting the redundant quasi-collinear points; then the connecting points of overlapping bubbles are obtained by using a novel vector rotation method; finally, the various segments of overlapping bubbles are classified and merged according to the combined principle of the average distance derivation and the globle minimum deviation. In this work, the method is successfully applied to analyze the computer-generated synthetic images and a real industrial bubble image. The results show that the method developed in this study is reliable, robust, and universal. (2) An experimental system was set up to investigate the influence of variable parameters on the initial bubble diameter. The initial bubbles, which were generated through several needles, were captured by a high-speed camera. The initial bubble diameter could be obtained by the image processing method established in chapter 2. The effect of the gas hole diameter on the initial bubble diameter was studied by using four types of needles with different inner diameters. In addition, the effect of the liquid viscosity on the initial bubble diameter was investigated by using four glycerol aqueous solutions with different concentrations. Moreover, the effect of the liquid surface tension on the initial bubble diameter was also studied by using four SDBS aqueous solutions with different concentrations. Finally, a model for the prediction of the initial bubble diameter was derived by fitting all the experimental data. Comparison between the predictions and experimental results indicates that the model has a wide application range and high accuracy. (3) For the mesoscale drag coefficient of bubble swarm, the rising behavior of bubble swarm in turbulent water is captured by a high-speed camera. The Sauter diameter, local gas holdup and the velocity of bubble swarm are obtained by the image processing method established in Chapter 2. Subsequently, the terminal slip velocity of bubble swarm is obtained and compared with that of the single bubble.
English Abstract: 中文
Language: 中文
Content Type: 会议论文
URI: http://ir.qibebt.ac.cn/handle/337004/9983
Appears in Collections:反应工程团队_学位论文

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肖航. 气泡初始直径及气泡群曳力系数的实验研究[C]. 见:.
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