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气泡初始直径及气泡群曳力系数的实验研究
其他题名化学工程
肖航
学位类型硕士 ; 学位论文 ; 硕士 ; 学位论文
导师图像处理 ; 重叠气泡识别 ; 气泡初始直径 ; 气泡群曳力系数
2017-05
学位授予单位中国科学院大学
学位授予地点北京
学位专业气液反应器在工业过程中的应用非常普遍,气泡尺寸及其分布是影响该类反应器性能的关键参数之一,也直接影响反应器的操作流型、气液两相间的传热、传质等传递性质。气泡从气体分布器分布的初始直径大多是根据经验得到的,导致数值模拟的准确性与使用者的经验密切相关。曳力是流体施加给气泡的阻力,曳力系数是多相流数值模拟中所需要的最基本的参数。然而,目前在计算流体力学中曳力系数未考虑气泡间的相互作用,也就是气泡群的介尺度影响被忽略。因此,亟需准确的气泡初始直径模型和介尺度曳力系数模型为多相流的仿真提供指导。 气泡初始直径预测模型、气泡群曳力系数模型等都可通过大量准确的实验观察和测量而建立。迄今为止,用于两相流研究的实验方法有很多,其中图像法作为一种外置、非侵入式的测量技术,在两相流研究中得到广泛应用。该法不仅对流场内的气泡运动没有干扰,而且能够直观、准确地获得气含率、气泡形状、大小及其分布等关键参数。然而,当气含率超过1%时,高达40%的气泡就会产生重叠、聚集,现阶段的图像处理技术无法准确识别重叠气泡。这也是气泡群的介尺度曳力系数研究较少的主要原因。 针对以上难题,本文首先发展了高效识别椭圆形重叠气泡的方法,然后将之用于气泡初始直径和气泡群曳力系数的实验研究。本文的研究内容及成果如下: (1)针对现阶段重叠气泡无法进行有效识别的难题,建立了一种高效、可靠、无参数且自动化的解析方法。该方法首先通过删除近似共线的冗余点来获取椭圆的关键点,然后通过矢量旋转法提取重叠气泡的连接点,最后采用平均距离偏差和最小偏差相结合的方法对重叠气泡各线段进行有效分组和合并。本文还将这种方法成功应用到计算机合成复杂图像和实际气液流动工业图像的解析。结果表明,本研究建立的方法非常可靠,具有较强的健壮性和通用性。 (2)搭建了观测气泡初始直径的实验系统,对气泡初始直径的规律进行了研究。通过针头产生初始气泡,利用高速摄像机拍摄初始气泡的图像,通过图像处理技术解析图像可以得到气泡初始直径。通过更换4种不同内径的针头,研究了气孔内径对气泡初始直径的影响;通过调节6个不同的出孔气速,研究了出孔气速对气泡初始直径的影响;通过4种不同浓度的甘油-水体系,研究了液相粘度对气泡初始直径的影响;通过4种不同浓度的SDBS水溶液,研究了液相表面张力对气泡初始直径的影响。最后对实验数据进行拟合,建立了一个气泡初始直径预测模型。计算结果和实验测量结果的比较表明,该模型具有很广的适用范围和很高的准确性。 (3)针对气泡群的曳力系数,用高速摄像机拍摄了气泡群在湍流状态水中的上升过程。使用本研究建立的图像处理技术得到了气泡群的Sauter直径、局部气含率和运动速度等详细信息,继而得出气泡群的终端滑移速度,并比较其与单气泡的不同。
关键词化学工程
摘要中文
其他摘要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.
作者部门反应工程团队
公开日期2022-06-30
学位类型硕士 ; 学位论文 ; 硕士 ; 学位论文
语种中文
文献类型学位论文
条目标识符http://ir.qibebt.ac.cn/handle/337004/9983
专题反应工程团队
作者单位中国科学院大学
推荐引用方式
GB/T 7714
肖航. 气泡初始直径及气泡群曳力系数的实验研究[D]. 北京. 中国科学院大学,2017.
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