三相磨粒流抛光及其气泡溃灭分布特性

针对软性磨粒流在加工硬脆性材料时效率低下的问题,本文提出一种气-液-固三相磨粒流加工方法。该方法通过在加工流场内注入微尺度气泡群,利用气泡溃灭释放的能量提升磨粒流加工能力。基于计算流体力学和群体平衡模型耦合计算方法,建立气-液-固三相磨粒流流体力学模型,数值模拟结果揭示了工件表面三相磨粒流形成高速湍流涡旋流场加工特性,得到了工件表面气泡溃灭的分布规律,并探明流体黏度与气泡溃灭之间的关系。图像粒子测速实验表明,通入微尺度气泡群后,平均速度从12.50~13.50m/s提升至15.00~17.00m/s,最高平均速度可达20.00m/s以上。对比加工实验显示,经8h加工后,粗糙度从0.50μm降低到0.05μm。理论和实验研究结果说明借助微尺度气泡群的溃灭效应可有效提升软性磨粒流的加工效率和加工精度。     

Estimation of bubble size distributions and shapes in two-phase bubble column using image analysis and optical probes

A precise estimation of bubble size distributions and shapes is required to characterize the bubble column fluid dynamics at the “bubble-scale”, and to evaluate the heat and mass transfer rate in bubble column reactors. Image analysis methods can be used to measure the bubble size distributions and shapes; unfortunately, these experimental techniques are limited to resolve bubble clusters and large void fractions, and can not be applied under relevant operating conditions (e.g., high temperature and pressure). On the other hand, needle probes (i.e, optical and conductive probes) can be used to measure bubble sizes in dense bubbly flows and under relevant operating conditions; however, needle probes measure chord length distributions, which should be converted into bubble size distributions by using statistical algorithms. These algorithms rely on correlations—generally obtained for single droplets/bubbles—that predicts the bubble shapes, by relating the bubble equivalent diameter to the bubble aspect ratio. In this paper, we contribute to the existing discussion through an experimental study regarding the bubble sizes and aspect ratio in a large air-water bubble column. The experimental investigation has consisted in gas holdup, image analysis and optical probe measurements. First, the gas holdup measurements have been used to identify the flow regime transition between the homogeneous flow regime and the transition flow regime. Secondly, the homogeneous flow regime has been described at the “bubble-scale”: chord length distributions and bubble size distributions have been obtained by using an optical probe and image analysis, respectively. Based on the experimental data from the image analysis, a correlation between the bubble equivalent diameter and the bubble aspect ratio has been proposed and has been compared with existing correlations. Finally, the chord length distributions have been converted into bubble size distributions using a statistical method, supported by the aspect ratio obtained through image analysis. The proposed approach has been able to estimate correctly the bubble size distributions at the center of the column then near the wall. We have also demonstrated that the correlations used to predicts the bubble shapes are the main point of improvement in the method.     

Three-dimensional visualisation of gas-water two-phase flow based on bubble mapping method and size projection algorithm

Electrical impedance tomography (EIT) has been successfully applied on gas-water flow applications, but it is incapable to identify small bubbles or the sharp gas-water interface of a large bubble due to its relatively low spatial resolution. A new visualisation approach, bubble mapping method (BM3D), offers a good 3D visualisation of bubble size and distribution. However, the empirical thresholding value method used in BM3D might meet a challenging from various flow setups and conditions in practice. Recently, the size projection algorithm (SPA) was proposed to determine the closest thresholding value for each frame of tomogram by minimising projection error. In this paper, the performances of BM3D and SPA methods are individually analysed and evaluated. Then a new method based on the combination of BM3D and SPA methods is reported to achieve better visualisation of gas-water flow, where the SPA is employed to determine the optimised thresholding values for BM3D method. Experiments are conducted to evaluate the proposed combination method for typical gas-water pipeline flow regimes, including horizontal stratified, bubble, plug, slug, annular flow regimes and vertical bubble, slug, annular flow regimes. The results are compared with the BM3D method, colour mapping method, and high-speed camera video recorded from a transparent chamber. A brief discussion on the effects of reconstruction algorithms and thresholding value for horizontal and vertical flows visualisation is also given.     

An analytical model of gas content in fluids considering the distribution of bubble radius

Gas bubbles in liquid can change the properties of liquid, such as density, viscosity and compressibility. Therefore, to predict the liquid properties, it is essential to evaluate the gas content, which depends on the bubble radius and its size distribution. This paper presents an analytical model of gas content with consideration of the distribution of bubble radius. A steady state formula of bubble radius is presented based on force equilibrium of bubble interface. From the experimental data, the distribution of bubble radius is found to agree with the logarithmic normal distribution, so a corresponding distribution function is proposed. Then, both the gas content and the average bubble radius are derived from the distribution function. Specific tests are performed to validate the developed model particularly as concerns the change of bubble radius with respect to the fluid pressure. Combined with the steady state model of bubble radius, the presented analytical model allows an accurate prediction of gas content at different pressures, implying its potentials to describe the fluid properties in two‐phase flows. Copyright © 2016 John Wiley & Sons, Ltd.     

轴承腔油气两相泡状流动的数值研究

航空发动机轴承腔过热或着火等现象与腔内油气两相泡状流动有关,需要通过数值计算研究两相泡状流动特性,从而为轴承腔润滑设计提供参考依据。针对上述问题,以某型航空发动机轴承腔为对象,建立轴承腔润滑油气液两相泡状流动模型,借助该模型研究润滑油介质运动状态。在建立模型方程时,考虑气泡直径尺寸大小和破裂聚合的影响,引入气泡种群方程和破裂聚合模型,并基于两流体模型和 两方程湍流模型,建立轴承腔内润滑油两相泡状流动计算模型。通过数值计算,模拟出轴承腔内润滑介质流场,最后利用国外文献的试验数据对理论分析进行了验证。研究表明,轴承腔润滑介质中气泡直径尺寸分布状态以及与转速的关系,揭示空泡率、转子转速和润滑介质进口速度等工况条件对轴承腔润滑油性能的影响。与国外试验数据的对比证明了该理论分析方法在航空发动机轴承腔润滑系统设计的可靠性和可行性。     

汽车液压制动系统气液两相流流型的识别

建立了汽车液压制动系统中气液两相流流型检测装置,根据压差波动信号,利用Hilbert-Huang变换(HHT)对制动液两相流流型进行识别,并利用高速摄像机采集不同工况下制动液的气液两相流流型图像。结果表明,制动时车轮转速越高,压差信号幅值越大,幅值主要集中在0～50Hz区域;识别制动时的制动液流型为一种泡状流。高速摄影的结果验证了液压制动管路中制动液为泡状流;制动转速越高,气泡越小。结论揭示了制动时汽车制动液的气液两相流流型,说明利用测量制动液的压差波动信号进行HHT就可以识别其流型。     

液压油路气液两相传质的光学测量

为研究液压系统工作压力下管路中自由气体的气液传质过程,解决油液中气泡质量变化的光学测量问题,推导了高压油液环境下气泡直径测量的恒温及零温度梯度条件,设计了高压气液传质光学测量系统,并对该系统的气泡识别与跟踪的关键算法进行研究。根据毕渥数及牛顿冷却定律推导了测量气泡的极限尺寸,提出以气泡圆度为判定阈值的气泡图元识别算法。最后,利用相邻帧间气泡中心最小向量的方法实现对气泡圆心的跟踪。实验结果表明:该系统实现了14MPa下气液传质的光学测量,气泡半径测量误差小于4%,提供了高压油气传质的试验手段,基本满足液压油路气液两相传质的光学测量要求。     

气油二相流含气率标定方法的研究

气油二相流是工程中常见的一种多相流，如何标定其含气率一直是气油二相流研究中的一个难点。本文通过对几种气油二相流含气流的标定方法研究，结合作者实践，分析了各种方法的特点，认为量筒法，压力法及浮法能力方便，准确地用来对气油二相流含气率进行标定。     

气液耦合激振与超声空化数值模拟运动学分析

提出了一种将外加气体与液压油结合起来的气液耦合激振方式,通过控制气脉冲与液压油形成的空化泡发生生长和破灭,从而产生空化作用对液压系统内部污染物进行分散与剥离;开发了以气液耦合波动为激振方式的气液激振试验系统,建立了波动发生器作用下的气泡运动学方程;数值模拟了波动发生器与超声波作用下空化泡的运动学过程,并进行了对比分析,发现低频的波动发生器作为激振源使气泡产生的空化作用是显著的,为气液耦合激振作用下气泡空化过程的可控性研究提供了部分理论依据。     

Optimisation of four-sensor probes for measuring bubble velocity components in bubbly air-water and oil-water flows

In a previous paper Lucas and Mishra (2005) [3] a local four-sensor conductance probe was introduced to measure the velocity vectors of dispersed bubbles in bubbly two-phase flow in which the continuous phase is water. There are a very limited number of alternative methods available for bubble velocity vector measurement with which results from, for example, computational fluid dynamic models can be compared and so the four-sensor probe technique is of interest to the multiphase flow community. In the previous paper [3] a mathematical model was presented to calculate the velocity vector of each gas bubble from seven time intervals which were measured using the output signals from each of four ‘needle’ conductance sensors located within the probe. In the present paper, a new technique for making the local four-sensor probe is introduced to minimise interference with the measured bubbles. A new signal processing method is presented using criteria to ensure that (i) the group of sensor signals from which the bubble velocity vector is to be determined are all produced by the same bubble and (ii) bubbles which contact the local four-sensor probe in an ambiguous manner are ignored. The accuracy with which the locations of each of the rear sensors in the probe relative to the lead sensor can be measured influences the accuracy with which the bubble velocity vector can be measured. However, the degree to which the accuracy of the measured velocity vector is affected by errors in the measured probe dimensions is dependent upon the geometrical arrangement of the four sensors within the probe. Experimental results and an error analysis are presented which show that the susceptibility of the velocity vector measurement technique to errors in the measured probe dimensions is reduced if the geometrical arrangement of the four sensors is optimised. As a result of this initial work, an optimised probe, known as the P30 probe, was designed and built and results obtained from the P30 probe in swirling oil-in-water bubbly flow are presented. A probe calibration factor is defined in this paper which can be interpreted as a measure of the interference of a probe with the motion of the bubbles with which it interacts. For the probes described in this paper the calibration factor was found to be much closer to unity than for previous four-sensor probes described in the literature (e.g. [3]) suggesting that these new probes have a much smaller effect on the bubbles’ motion than previous probes.     

Numerical simulation of multiphase flows with material interface on an unstructured grid system

Two-dimensional multiphase flows with material interface due to density difference are numerically simulated on an unstructured grid system by a Navier-Stokes solver developed by Myong and Kim (2006), since numerical computation for these flows is still known to be difficult, especially if the interface separates fluids of large different densities. This solver employs an unstructured cell-centered method based on a conservative pressure-based finite volume method, since the unstructured grid approach makes the solver very flexible in dealing with complex boundaries, and adopts a high resolution method (CICSAM) in a volume of fluid (VOF) scheme for the accurate phase interface capturing. The test cases are the Rayleigh-Taylor instability (density ratio of 2), the oil bubble rising in a partially filled container (density ratio of 2), the air bubble rising in a fully filled container with bubble shedding (density ratio of 100) and the droplet splash (density ratio of about 1000), which are typical benchmark problems among multiphase flows with material interface due to density difference. The present results are compared with other numerical solutions found in the literature. The present method (solver) efficiently and accurately simulates complex interface flows such as multiphase flows with material interface due to both density difference and instability.     

基于经验模态分解和BP神经网络的油气两相流流型辨识

基于经验模态分解（empidcal mode decomposition，EMD）BP神经网络，提出了油气两相流流型辨识的新方法。应用EMD将差压信号分解成不同频率尺度上的单组分之和，并提取组分的归一化能量作为流型辨识特征量。BP神经网络以这些能量特征量为输入对油气两相流不同流型（包括泡状流、塞状流、层状流、弹状流和环状流）进行分类。实验结果表明，本文提出的流型辨识方法是有效的，其中泡状流、塞状流、层状流、弹状流和环状流的辨识精度分别为100％、89．4％,93．3％、96.3％和96.9％。     

气-液-固三相磨粒流光整加工及其工艺参数优化

考虑用流体抛光法加工大尺度工件存在效率低下问题,本文提出了一种气-液-固三相磨粒流抛光方法。该方法在约束流场中引入微纳米气泡,利用气泡在溃灭时释放的能量加速驱动磨粒运动,从而有效提升抛光效率。实验显示:在加工过程中,离心泵的发热会导致流体黏度下降,进而影响工件近壁面的湍动能和动压力的大小及分布,而加工工件近壁面的湍动能和动压力会对表面纹理的均匀性和材料的去除效率有重要影响。针对上述实验结果,文中基于对磨粒流抛光机理的研究,提出一种通过改变入口流速来补偿温升带来的湍动能和动压力变化的方法,实验求得了抛光流体温度从20℃到60℃之间的9个均等点对应的最优入口流速值。实验表明,相对未加入气泡时,该抛光方法的加工效率得到提高,而调速补偿明显提升了工件表面加工质量。     

A novel concept of automatic soap flowmeter with bubble detection by closing an electrical circuit

A novel accurate method of automatic timing of bubble motion through the measuring section of a soap flowmeter is proposed. The method is based on the closure of an electrical circuit by a soap bubble moving in the calibrated tube. The accuracy of the automated flowmeter, estimated by comparison with other accurate flow measurement methods, is found to be 2%. Modification of the bubble making system aimed at increasing of the flowmeter tightness and bubble quality is developed, as well. The proposed design does not require precise assembly and admits the use of cheap and readily available components.     

面向真空注型技术的双组分材料混合脱泡方法

针对真空注型工艺(Vacuum casting, VC)过程中存在的双组分材料混合不均匀、脱泡不充分而影响最终产品质量的问题。依据混沌混合理论,提出一种几何非对称大叶片式搅拌桨,通过周期性改变搅拌转速的方法来强化混合效果,并利用计算流体力学技术对搅拌槽流场特性进行研究,分析槽内流场结构、压力分布、匀速及变速搅拌下槽内速度分布情况。同时,采用机器视觉和图像处理技术对搅拌过程材料气泡特征进行在线检测,并将检测结果作为控制系统的反馈信号,以实现气泡的自动消除。试验结果表明,所提出的方法不仅能够实现对高黏度双组分材料的均匀混合以及气泡的自动消除,而且还能够进一步缩短处理时间,为有效解决真空注型工艺中高黏度双组分材料难于混合脱泡的问题提供一种可行的方法。     

Numerical simulation of shock wave propagation using the finite difference lattice Boltzmann method

The shock wave process represents an abrupt change in fluid properties, in which finite variations in pressure, temperature, and density occur over the shock thickness which is comparable to the mean free path of the gas molecules involved. This shock wave fluid phenomenon is simulated by using the finite difference lattice Boltzmann method (FDLBM). In this paper, a new model is proposed using the lattice BGK compressible fluid model in FDLBM for the purpose of speeding up the calculation as well as stabilizing the numerical scheme. The numerical results of the proposed model show good agreement with the theoretical predictions.     

机械设备流固耦合动力分析的有效方法

提出流固耦合动力分析的新方法。该法通过结构动力学的有限元求解方程和Newmark法,导出了新的迭代公式。流场的动压力使弹性体的动态特性变为载荷非线性问题;同时,固体位移场的变形作用使流场成为可变域流场。这些问题都可用本法得以解决。文中给出算例和工程应用实例。数值解结果表明,该法收敛速度快,效率高,克服了耦合问题的求解困难。     

锚链提升器水平液压管内气泡流型及其对边界层速度梯度影响分析

为有效提取气泡对管路边界层的扰动参数,达到提前预测和判断空气对液压管路污染的目的,基于管路流动边界层理论和气液两相流动理论,通过Fluent仿真平台建立气液管路模型,计算得到气泡流型及其对边界层速度梯度的影响规律。通过实验验证了仿真模型中气泡流型变化的一致性,并提出了一种液压系统空气污染诊断的新方法。计算结果表明,该方法能通过提取边界层微观变化规律来监测液压系统宏观空气污染故障;但是,当气泡直径小于0.01 mm时,则面临监测不到边界层速度梯度变化的情况。     

水力空化装置文丘里管的模拟优化

基于FLUENT软件．采用标准的κ-ε模型和空化泡动力学模型对三种不同几何形状的文丘里管中的空化流场进行了数值模拟，井将计算结果与实验结果进行了比较．计算结果表明，理论计算的汽含率分布与实验拍摄的汽含率分布是相似的。文丘里管的结构对空化效应有着重要的影响．从而为文丘里管的优化设计提供了依据。     

Nonlinear model predictive control of managed pressure drilling

A new design of nonlinear model predictive controller (NMPC) is proposed for managed pressure drilling (MPD) system. The NMPC is based on output feedback control architecture and employs offset-free formulation proposed in [1]. NMPC uses active set method for computing control inputs. The controller implements an automatic switching from constant bottom hole pressure (CBHP) regulation to flow control mode in the event of a reservoir kick. In the flow control mode the controller automatically raises the bottom hole pressure setpoint, and thereby keeps the reservoir fluid flow to the surface within a tunable threshold. This is achieved by exploiting constraint handling capability of NMPC. In addition to kick mitigation the controller demonstrated good performance in containing the bottom hole pressure (BHP) during the pipe connection sequence. The controller also delivered satisfactory performance in the presence of measurement noise and uncertainty in the system.