曝气池中气液两相流粒子图像测速技术

在粒子图像测速原理的基础上研究了基于快速傅立叶变换的粒子图像测速算法,并编制算法程序进行验证。通过试验研究了气液两相流在曝气池试验装置中的流动情况,应用粒子图像测速技术对装置中3种状态下气液两相流的运动参量进行测量,获得了各种流体运动参数,并应用粒子图像测速程序进行计算分析。试验结果表明当气液两相流处于状态3时,气泡在装置中具有最长的停留时间和较大的速度紊动强度,氧转移速率和效率大大提高,这为曝气池选型提供了依据。     

基于图像二值化处理的气泡羽流不稳定结构分析

气泡羽流是在曝气过程中产生的一种复杂的两相流动。对气泡羽流中气泡速度信息的提取是粒子图像测速算法(PIV)的核心部分。速度信息的提取即数字图像的预处理过程。本研究主要是在读取灰度值后,通过大津法得到相应的最佳阈值分割点,再采用该灰度阈值对图像进行二值化,以此来减少噪声对流场的影响,降低图像后处理的难度,这是气泡羽流的运动规律研究的一个重要过程。图像的二值化处理能有效地提高PIV技术计算流场的精度,为研究气泡羽流在曝气过程中产生的不稳定结构奠定了基础。     

一种改进的粒子图像测速混合算法研究

粒子图像测速技术作为一种新的流场测速方法能够在不干扰流场的情况下获得整个流场的速度信息。粒子图像测速技术最关键的步骤在于粒子匹配。针对粒子密度分布不均匀、流场不同等实际情况,提出了混合算法,即结合互相关和松弛算法能够更准确地搜索粒子,进而对粒子进行匹配。对3种匹配算法的匹配概率进行比较分析,发现混合算法能更准确地分析粒子的运动状态,减少错误矢量的产生;另外,对松弛算法进行改进,通过优化筛选加权因子发现改进的松弛算法在运行速度上相比原始算法有了较大提高,匹配率与原始算法基本一致。     

PIV 适用于两相流稀相微粒速度场测量的算法探讨

用PIV(Particle Image Velocimetry)技术测量两相流中稀相微粒速度场时，稀相微粒起着示踪粒子的作用，其浓度低，粒子图像离散性强，需要发展跟踪单个粒子图像的算法才能确定粒子的速度矢量。本文介绍了基于BICC(Binary Image Cross-Correlation algorithm)算法的PIV粒子图像跟踪技术。用数值模拟的方法对几个典型的粒子图像场进行了粒子对的识别，并且对该算法模式中粒子对数、粒子半径等问题作了讨论。最后用所述方法对直管道中玻璃球-空气两相流50微米玻璃球粒子图像进行了实例分析，结果令人满意。     

采用乳化空气泡做示踪粒子的PIV测速技术

本文介绍一种适用于水洞力学实验的采用乳化空气泡作为示踪粒子的ＰＩＶ测速技术。选用乳化空气泡做ＰＩＶ测速示踪粒子，取代了通常采用的固体颗粒，由两台ＹＡＧ大功率脉冲式激光光源组合系统做为ＰＩＶ底片记录的双脉冲曝光光源。乳化空气泡的最大优点是对实验水洞或水槽无污染，且产生方便、经济，又可以形成高质量的杨氏条纹图像。这一技术能够使得ＰＩＶ测速技术在水洞力学应用测量中真正达到实用。     

基于PIV测量技术的变曝气量下气液两相流速度场研究

粒子图像测速技术(PIV)作为一种无扰的全流场速度测量手段,在多相流研究领域具有较高的学术意义和实用价值。本研究针对曝气过程中的复杂气液两相流动,使用高速摄影机获取气液两相流流场的高帧图像,再通过图像预处理手段及PIV测速算法得到气液两相流速度场信息,并对不同工况下气液两相流的流态流速规律进行了研究分析。本文提出的图像预处理方案,通过采用图像的运动目标分割、降噪与反相等手段进行图像预处理,可以有效地减少后续速度场计算中的信息量及误差。研究结果为工业生产过程中有效提高曝气效率提供了重要的参考依据。     

双重裂隙介质内微观粒子测速研究

微流控实验结合微观粒子图像测速技术(μPIV)是解析孔隙尺度复杂流体流动过程的重要手段。裂隙介质中存在不同大小的裂隙结构，影响溶质运移等行为。为探究复杂裂隙结构对μPIV测速结果的影响，该文设计了双重裂隙网络结构，以此对比μPIV粒子测速结果和孔隙尺度模型模拟结果。结果显示：主裂隙中μPIV测速结果与孔隙尺度模型模拟结果基本一致；微裂隙中粒子运动受边界影响较大，且优势通道的存在使进入微裂隙内的粒子数较少，导致部分区域测速结果与模拟结果存在较大偏差。该文总结了复杂介质结构中使用μPIV进行粒子测速可能导致偏差较大的区域并分析其成因，为提高μPIV粒子测速的准确性提供了新的认识。     

对DPIV相关分析算法的优化

在数字粒子图像测试(DPIV)技术中，目前常用数字图像相关方法获取粒子图像中的运动信息，其核心环节是提取粒子的运动信息。相关分析算法的优劣将直接影响到分析结果的准确性和可靠性。本文在前人工作的基础上，从相关窗口、相关分析区域、相关搜索路径等三个方面对相关分析算法进行了改进优化，不但大大减少了计算量，同时有效地提高了分析结果的准确性和可靠性。通过对絮凝池水流流场的量测验证表明，采用优化后的分析算法不但有效地提高了分析效率，而且具有分析结果精度高、可靠性强的特点。     

气液两相流中微气泡测量问题的预处理技术

气泡运动是气液两相流动中的基本问题，水利、海洋、环境等工程均涉及微气泡测量问题，且广泛应用于水的净化和污水处理。为确保微气泡测量工作取得良好效果，对单CCD摄影机获得的微气泡视频图像进行背景处理十分关键。对于图像背景处理，首先基于数学形态学的基本理论，选用两种不同算法：① 基于选取不同结构元素的Matlab算法；② 采用滚球法。二者通过实例进行算法比较，滚球法效果较为显著，但仍需改进，于是提出基于聚类分析的图像分割算法，并对该算法提出优化设计，采用黄金分割法进行最优搜索，加快运行速度，节省时间，并更好地实现背景估计，为微气泡测量后期工作奠定了良好基础。     

恒定流中泥沙运动速度实验

泥沙颗粒运动速度与水流运动速度是不一致的,进而会对水流输沙和河床演变产生影响。通过采用粒子追踪测速方法(Particle Tracking Velocimetry-PTV)和粒子图像测速技术(Particle Image Velocimetry-PIV)测量了水槽中2种粒径的泥沙颗粒运动速度及相应的水流运动速度。实验结果表明,泥沙颗粒运动速度沿垂线分布与水流速度不一致,在对数区,泥沙颗粒的运动速度略小于水流的速度,泥沙运动滞后于水流运动;在近壁区,泥沙颗粒的运动速度大于水流速度,且在本实验的粒径范围内,随着泥沙颗粒粒径增大,泥沙颗粒的运动速度也随之增大。     

AN IMPROVED PTV SYSTEM FOR LARGE-SCALE PHYSICAL RIVER MODEL

To measure the surface flow in a physical river model, an improved system of Large-Scale Particle Tracking Velocimetry （LSPTV） was proposed and the elements of the PTV system were described. Usually the tracer particles of a PTV system seeded on water surface tend to form conglomerates due to surface tension of water. In addition, they can not float on water surface when water flow is shallow. Ellipsoid particles were used to avoid the above problems. Another important issue is particle recognition. In order to eliminate the influence of noise, particles were recognized by the processing of multi-frame images. The kernel of the improved PTV system is the algorithm for particle tracking. A new 3-frame PTV algorithm was developed. The performance of this algorithm was compared with the conventional 4-frame PTV algorithm and 2-frame PTV algorithm by means of computer simulation using synthetically generated images. The results show that the new 3-frame PTV algorithm can recover more velocity vectors and have lower relative error. In addition, in order to attain the whole flow field from individual flow fields, the method of stitching individual flow fields by obvious marks was worked out. Then the improved PTV system was applied to the measurement of surface flow field in Model Yellow River and shows good performance.     

液固悬浮液中气泡生成的数值模拟

将Level Set方法与硬球离散颗粒方法相结合,建立了一个三相流混合模型。应用该模型对液固悬浮液中气泡从单孔及多孔中的形成过程进行了三维模拟,并与文献中已有的结果进行了定性比较。数值试验主要比较了在悬浮液与纯液体中的气泡生成与上升行为。模拟结果表明,颗粒的存在对气泡的形成与上升有重要影响;气泡诱导的流体流动对气泡与颗粒的运动均有显著地影响。     

基于回归分析的表面流场粒子匹配算法

针对表面流速图像测试中高浓度粒子条件下测量误差大的问题,改进了最邻近匹配算法,提出基于回归分析的表面流场粒子匹配算法,即多帧回归粒子匹配算法。该算法综合粒子距离、运动轨迹、运动趋势等因素,应用一元线性回归分析法拟合粒子中心坐标,通过相关系数完成粒子匹配,实现最佳同源粒子追踪。运用多帧回归粒子匹配算法开发了改进型大尺度模型表面流场图像测试系统(ILSPIV),并将ILSPIV应用到八卦洲河工模型,ILSPIV和旋桨流速仪测量的断面垂线平均流速分布形态及数值基本一致,表明多帧回归粒子匹配算法能提升高浓度粒子条件下流速测量精度,更好地满足工程实践研究需求。     

Examining the effect of length/width ratio on the hydro-dynamic behaviour in a DAF system using CFD and ADV techniques.

Dissolved air flotation (DAF) is a solid-liquid separation system that uses fine bubbles rising from the bottom to remove particles in water. In this study, we investigated the effect of L/W(L; length, W; width) on the hydrodynamic behavior in a DAF system using CFD (computational fluid dynamics) and ADV (acoustic Doppler velocimetry) technique. The factual full-scale DAF system, L/W ratio of 1:1, was selected and various UW ratio conditions (2:1, 3:1,4:1 and 5:1) were simulated with CFD. For modelling, 2-phase (gas-liquid) flow equations for the conservation of mass, momentum and turbulence quantities were solved using a Eulerian-Eulerian approach based on the assumption that a very small particle is applied in the DAF system. Also, for verification of CFD simulation results, we measured the actual velocity at some points in the full-scale DAF system with the ADV technique. Both the simulation and the measurement results were in good accordance with each other. We concluded that the L/W ratio and outlet geometry play an important role for flow pattern and fine bubble distribution in the flotation zone. In the ratio of 1:1, the dead zone is less than those in other cases. On the other hand, in the ratio of 5:1, the fine bubbles were more evenly distributed.     

简易粒子图像测速(PIV)技术开发与优化技巧

粒子图像测速(PIV)技术是一种瞬态流动平面二维速度场测试技术,在细部流场实测领域得到重视,但是成熟的PIV产品价格高昂。鉴于此,介绍了一种简易的PIV装置,主要由高速摄像机、激光发射器、柱面透镜和示踪粒子构成,以较低成本即可基本实现商业用PIV产品的功能。为了验证简易PIV性能,设计了PIV简易装置,采用Fluent软件模拟,并结合PIV技术对比分析了简易PIV装置的优缺点,同时对影响结果的粒径大小和粒子跟随性进行了优化。结果表明,简易PIV装置适宜选用玉米粉作为示踪粒子,并需要根据示踪粒子跟随性所能达到的最大进口水流速度选择高速摄像机的帧率,最终能够较好地实现流场实测,进而达到对PIV技术进行开发和优化的目的,其分析结果将为后续研究者提供参考。     

Numerical investigation of the time-resolved bubble cluster dynamics by using the interface capturing method of multiphase flow approach

The present paper proposes a multiphase flow approach for capturing the time-resolved collapse course of bubble clusters in various geometrical configurations.The simulation method is first verified by computing the dynamic behavior of an isolated vapor bubble placed in a uniform ambient pressure.The comparison between the numerical result and the theoretical solution indicates that the method can accurately capture the bubble shape,the characteristic time and the extremely high pressure induced by the collapse.Then the simulation method is applied to investigate the behavior of two kinds of bubble clusters in hexagonal and cubic geometrical configurations.The predicted collapsing sequence and the shape characteristics of the bubbles are generally in agreement with the experimental results.The bubbles transform and break from the outer layer toward the inner layers.In each layer,the bubbles on the corner first change into a pea shape and cave before collapsing,then the bubbles on the sides begin to shrink.It is also found that,in comparison with the case of an isolated single bubble,the central bubble in the cluster always contracts more slowly at the early stage and collapses more violently at the final stage.     

Effects of floc and bubble size on the efficiency of the dissolved air flotation (DAF) process.

Dissolved air flotation (DAF) is a method for removing particles from water using micro bubbles instead of settlement. The process has proved to be successful and, since the 1960s, accepted as an alternative to the conventional sedimentation process for water and wastewater treatment. However, limited research into the process, especially the fundamental characteristics of bubbles and particles, has been carried out. The single collector collision model is not capable of determining the effects of particular characteristics, such as the size and surface charge of bubbles and particles. Han has published a set of modeling results after calculating the collision efficiency between bubbles and particles by trajectory analysis. His major conclusion was that collision efficiency is maximum when the bubbles and particles are nearly the same size but have opposite charge. However, experimental verification of this conclusion has not been carried out yet. This paper describes a new method for measuring the size of particles and bubbles developed using computational image analysis. DAF efficiency is influenced by the effect of the recycle ratio on various average floc sizes. The larger the recycle ratio, the higher the DAF efficiency at the same pressure and particle size. The treatment efficiency is also affected by the saturation pressure, because the bubble size and bubble volume concentration are controlled by the pressure. The highest efficiency is obtained when the floc size is larger than the bubble size. These results, namely that the highest collision efficiency occurs when the particles and bubbles are about the same size, are more in accordance with the trajectory model than with the white water collector model, which implies that the larger the particles, the higher is the collision efficiency.     

曝气池中气液两相流速度场分布的实验研究与数值模拟

气液两相流广泛存在于水利工程、废水处理等领域,其在气液装置中的流型流态及速度场分布可直接影响装置运行效能。本文采用实验研究与数值模拟相结合的方法,对圆柱形气液装置进行PIV实验,获得了准确的气液两相流气相速度场分布;在充分考虑湍流、曳力、升力、湍流分散力等作用下,采用欧拉-欧拉双流体模型和气泡群平衡模型对气液两相流流场进行了数值模拟,并将模拟结果与实验结果进行验证,所得结果误差控制在10%左右。结果表明,本文提出的模拟方法在气液两相流模拟中能够得到较准确的两相流动规律和速度场分布情况。