Flow patterns in vertical two-phase flow

This paper is concerned with the flow patterns which occur in upwards gas-liquid two-phase flow in vertical tubes. The basic flow patterns are described and the use of flow pattern maps is discussed. The transition between plug flow and churn flow is modelled under the assumption that flooding of the falling liquid film limits the stability of plug flow. The resulting equation is combined with other flow pattern transition equations to produce theoretical flow pattern maps, which are then tested against experimental flow pattern data. Encouraging agreement is obtained.     

颗粒物质在竖直管中下行蠕动流动的实验测量

颗粒流蠕动行为是颗粒物质在竖直管中流动时常见的一种流动现象,其产生机理较复杂。为此,本文在在内径为150mm、高为5000mm的竖直管实验装置上,以FCC催化剂为固体颗粒物料,采用PV6型颗粒速度测量仪,测量不同颗粒流率下竖直管中的颗粒下行蠕动流动速度以及颗粒固含率,系统地考察了颗粒物质在竖直管中下行流动时的蠕动流动特性及产生机理。实验结果表明,颗粒物质在竖直管中下行流动时的流动行为可划分为两种形式。在颗粒流率较小时,颗粒物质下行速度呈现脉冲式变化,有速度停滞,可称之为蠕动I型流动。随着颗粒流率的增加,颗粒下行速度停顿消失,但仍是起伏变化,为蠕动II型流动。当颗粒流率增大到一定值后,颗粒物质下行蠕动行为消失,转变为流化流动。颗粒物质下行的蠕动行为是出口区颗粒成拱与崩塌、颗粒与器壁滑动摩擦和颗粒力链作用的综合反映。     

Experimental investigation on flow characteristics in a narrow annulus

Experiments are carried out to investigate the flow characteristics with/without heat exchange in a narrow annulus. In the experiments, directions of flow include horizontal, upstream and downstream flow. Experimental results show that the flow characteristics of water through the narrow annulus are different from those in normal tubes. Flow directions have little influences on the flow friction for the fluid flow in the narrow annulus with/without heat exchange. The flow characteristics in the narrow annulus have relations to the liquid temperature difference at the inlet and outlet of the annulus. Their influences on the flow characteristics are relatively obvious in the laminar flow area. When the Reynolds number is larger than 10⁴, there are little differences between the flow friction factors with/without heat exchange. It is also found that the asymmetrical flow can make the friction factor increase, whereas the symmetrical flow can reduce the flow friction. In the experiments, the transition from laminar to turbulent flow is carefully observed. In the narrow annulus, the flow transition is initiated earlier than that in normal pipes at a Reynolds number range from 1,100 to 1,500, which is different from the heat transfer transition. The results are gained to provide bases for the further investigations on the two-phase flow in narrow annuli.     

Review of two-phase flow and flow boiling of mixtures in small and mini channels

Two-phase flow and flow boiling phenomena of fluidic mixtures in small and mini channels are becoming important in the miniaturization of thermal systems. This paper aims to present a state-of-the-art review in this important area and to identify what have been done so far and what still need to be done in the future. Firstly, various definitions of small and mini channels are described and the criteria based on these definitions are compared with each other. Comments on different viewpoints of the channel size classifications are acknowledged. Secondly, the background of two-phase flow and flow boiling of mixtures is described. Then, the current research status of two-phase flow and flow boiling of mixtures in normal size channels is presented as it is a basis for the study of two-phase flow and flow boiling of mixtures in small and mini channels. Finally, an overall review of two-phase flow and flow boiling of mixtures in small and mini channels is presented. It is concluded that the available study of two-phase flow and flow boiling of mixtures in small and mini channels is rather scarce and a systematic knowledge of two-phase flow and flow boiling of mixtures in small and mini channels is required. Based on this review, the future research directions including both fundamental and applied research in this area have been indicated.     

基于PIV测量的超声波流量计内流场特性研究

采用PIV(Particle Image Velocimetry)测量手段,考察了小口径超声波流量计的流动特性。首先针对前端安装直管段时,不同流量条件下的流场特性建立基本认识,实验结果表明,在低流量条件下,流量计内流场存在明显的不稳定演变和非定常流动特征。进一步以上游前端安装球阀为典型案例,考察了安装条件对超声波流量计响应特性和测量偏差的影响。结合直管段的实验观测结果,发现此种结构超声波流量计的适应性与其流场非定常性的关系具有很好的一致性,即流场结构稳定则适应性强。此外,综合多参数的实验结果表明,雷诺数是判断小口径超声波流量计测量准确性的重要无量纲参数。     

孔眼流入对水平井中流动影响的实验研究

设计并建立了水平井筒变质量流动模拟实验装置。对具有射孔完井的水平井筒变质量流动进行了实验研究,利用先进的数据采集系统获取了大量的实验数据。对水平井井筒单孔眼段流动阻力损失分析可知,流动阻力由三部分组成：管壁摩擦阻力、加速损失和混合损失。对实验数据进行处理得出了混合损失和主流流速与孔眼流速之间的关系。对于一定的主流流速,混合损失随孔眼流速的增大而增大,同时,对于一定的孔眼流速,混合损失随主流流速的增大而增大,此时孔眼的流入对水平井筒中的流动造成的影响比较显著。孔眼的流入增加了井筒中流动的复杂性。对实验数据进行的回归结果表明,在本实验条件和范围内,主流流速和孔眼流速是造成混合损失的主要因素。     

Multi-scale analysis on the pulverized coal flow behaviors under high pressure dense-phase pneumatic conveying

To deeply knowledge of the flow behaviors of pulverized coal particles in dense gas–solid two-phase flow, a multi-scale analysis method based on electrostatic sensor array is applied for the multi-scale characterization of flow behaviors of dense gas–solid flow. The experimental results indicate that: for steady flow, with the increment of conveying pressure difference, the individual particles increase and the particle clusters decrease, the individual particle distribution is always inhomogeneous but the particle cluster distribution tends to be more homogeneous over the cross-section of pipe, while the average flow behavior of pulverized coal particles is always in the relatively static state. For unsteady flow, the average flow behavior of pulverized coal particles is dynamic, and the flow behaviors of the multi-scale flow structures over the cross-section of pipe are all significantly inhomogeneous. Moreover, the effect of particle size on flow behavior of pulverized coal is also investigated and validated.     

关于气液两相流流型及其判别的若干问题

气液两相流体系是一个复杂的多变量随机过程体系,流型的定义、流型过渡准则和判别方法等方面的研究是多相流学科目前研究的重点内容.本文就与气液两相流流型及其判别有关的研究状况进行了回顾和评述,力图反映近年来气液两相流流型及其判别问题研究的状态和趋势.      

Two-phase flow patterns of a top heat mode closed loop oscillating heat pipe with check valves (THMCLOHP/CV)

This research is aimed at studying the two-phase flow pattern of a top heat mode closed loop oscillating heat pipe with check valves. The working fluids used are ethanol and R141b and R11 coolants with a filling ratio of 50% of the total volume. It is found that the maximum heat flux occurs for the R11 coolant used as the working fluid in the case with the inner diameter of 1.8 mm, inclination angle of ?90?, evaporator temperature of 125?C, and evaporator length of 50 mm. The internal flow patterns are found to be slug flow/disperse bubble flow/annular flow, slug flow/disperse bubble flow/churn flow, slug flow/bubble flow/annular flow, slug flow/disperse bubble flow, bubble flow/annular flow, and slug flow/annular flow.     

Considerations for the design of swirl chambers for the cyclone cooling of turbine blades and for other applications with high swirl intensity

The focus of this study lies on turbulent incompressible swirling flows with high swirl intensity. A systematic parameter study is conducted to examine the sensitivity of the mean velocity field in a swirl chamber to changes in the Reynolds number, swirl intensity and channel outlet geometry. The investigated parameter range reflects the typical kinematic flow conditions found in heat transfer applications, such as the cooling of the turbine blade known as cyclone cooling. These applications require a swirl intensity, which is typically much higher than necessary for vortex breakdown. The resulting flows are known as flow regime II and III. In comparison to flow regime I, which denotes a swirling flow without vortex breakdown, these flow regimes are characterized by a subcritical behavior. In this context, subcritical means that the flow is affected by the downstream channel section. Based on mean velocity field measurements in various swirl chamber configurations, it is shown that flow regime III is particularly sensitive to these effects. The channel outlet geometry becomes a determining parameter and, therefore, small changes at the outlet can produce entirely different flow patterns in the swirl chamber. In contrast, flow regime II, as well as flow regime I and axial channel flows, are much less sensitive to changes at the channel outlet. The knowledge about the sensitivity of the flow in different flow regimes is highly relevant for the design of a cyclone cooling system. Cooling systems employing flow regime III can result in a weakly robust flow system that may change completely over the operating range. As a remedy, the swirl intensity needs to be decreased so that flow regime III cannot be reached, which, however, reduces the maximum achievable heat transfer in the cooling system. Alternatively, the flow has to transition back from flow regime III to flow regime II or I before the flow leaves the swirl chamber. Two practical methods are presented. These findings can be directly applied in the design processes of future cyclone cooling systems, and other applications of swirling flow.     

An analytical model for plug flow in microcapillaries with circular cross section

Plug flow in microcapillaries or microchannels offers significant advantages for the development of microfluidic applications and recently triggers many interests and studies. Recirculation is formed within liquid plugs due to the presence of interfaces. This paper presents an analytical model to investigate the recirculation flow and the flow resistance in microcapillaries with circular cross section. A fourth order partial differential equation is used to model the Stokes flow within the liquid plug. The results of the flow field show that the flow pattern is affected by the plug length. The flow resistance is determined through the force balance of the liquid plug. The comparison of the flow field and the flow resistance from the analytical model and the experiments shows good agreement.     

Stability of viscoelastic flow around periodic arrays of cylinders

Low Reynolds number flow of Newtonian and viscoelastic Boger fluids past periodic square arrays of cylinders with a porosity of 0.45 and 0.86 has been studied. Pressure drop measurements along the flow direction as a function of flow rate as well as flow visualization has been performed to investigate the effect of fluid elasticity on stability of this class of flows. It has been shown that below a critical Weissenberg number (We_c), the flow in both porosity cells is a two-dimensional steady flow, however, pressure fluctuations appear above We_c which is 2.95±0.25 for the 0.45 porosity cell and 0.95±0.08 for the higher porosity cell. Specifically, in the low porosity cell as the Weissenberg number is increased above We_c a transition between a steady two-dimensional to a transient three-dimensional flow occurs. However, in the high porosity cell a transition between a steady two-dimensional to a steady three-dimensional flow consisting of periodic cellular structures along the length of the cylinder in the space between the first and the second cylinder occurs while past the second cylinder another transition to a transient three-dimensional flow occurs giving rise to time- dependent cellular structures of various wavelengths along the length of the cylinder. Overall, the experiments indicate that viscoelastic flow past periodic arrays of cylinders of various porosities is susceptible to purely elastic instabilities. Moreover, the instability observed in lower porosity cells where a vortex is present between the cylinders in the base flow is amplifieds spatially, that is energy from the mean flow is continuously transferred to the disturbance flow along the flow direction. This instability gives rise to a rapid increase in flow resistance. In higher porosity cells where a vortex between the cylinders is not present in the base flow, the energy associated with the disturbance flow is not greatly changed along the flow direction past the second cylinder. In addition, it has been shown that in both flow cells the instability is a sensitive function of the relaxation time of the fluid. Hence, the instability in this class of flows is a strong function of the base flow kinematics (i.e., curvature of streamlines near solid surfaces), We and the relaxation time of the fluid.     

压气机近失速点旋涡流场时空演化分析

为了探索轴流压气机内旋涡流场的时空演化特征, 对一台压气机转子的流场进行了时间精确的数值模拟. 通过对转子不同轴向位置的流量分析发现, 在压气机近失速工况附近, 压气机进口流量波动约为最大流量的2%. 对旋涡流场的进一步分析发现, 叶片尾缘附近吸力面的旋涡流场摆动是导致压气机进口流量波动的主要原因.      

Visualisation of flow boiling heat transfer in a microtube

This paper presents the results of the flow boiling patterns and heat transfer coefficients of FC-72 in a small tube. The internal diameter of the tube is 0.48 mm, with a heated length of 73 mm. The mass flow rate varies from 50 to 3,000 kg/m²-s. The microtube is made of Pyrex in order to obtain the visualisation of the flow pattern along the heated channel. Different types of flow pattern have been observed: bubbly flow, deformed bubbly flow, bubbly/slug flow, slug flow, slug/annular flow, and annular flow. The experiments show the presence of flow instabilities in a large portion of the tests at low mass flow rates and low subcooling. Flow patterns in presence of flow instabilities are mainly characterized by bubbly/slug flow and slug/annular flow. Heat transfer rates have been studied in all flow pattern conditions. The two groups of results, with flow instabilities and without flow instabilities, show similar heat transfer behaviour. The heat transfer characteristics of the pipes have been studied in comparison with mass flux and vapour quality.     

Nonlinear fluid flow laws for orthotropic porous media

Nonlinear fluid flow laws for orthotropic porous media are written in invariant tensor form. As usual in the theory of fluid flow through porous media [1, 2], the equations contain the flow velocity up to the second power. Expressions that determine the nonlinear resistances to fluid flow are presented and it is shown that, on going over from linear to nonlinear flow laws, the asymmetry effect may manifest itself, that is, the fluid flow characteristics may differ along the same straight line in the positive and negative directions. It is shown that, as compared with the linear fluid flow law for orthotropic media when for three symmetry groups a single flow law is sufficient, in nonlinear laws the anisotropy manifestations are much more variable and each symmetry group must be described by specific equations. A system of laboratory measurements for finding the nonlinear flow characteristics for orthotropic porous media is considered.     

Thermal imaging study on the surface wave of heated falling liquid films

By using thermal imaging technique and film thickness metering system, the surface wave and film thickness of the heated falling liquid film were experimentally investigated. Temperature variations of the heated film induce surface tension gradient and so-caused Marangoni flow that attempts to avoid the temperature variations. There are three kinds of Marangoni flow appearing in the heated falling liquid film. It is found that the lateral Marangoni flow (MF I) and the streamwise Marangoni flow (MF II) make the heated film thick, while the Marangoni flow in the surface wave (MF III) reinforces the wave and makes the heated film thin. The intensity of Marangoni flow is determined by the flow rate and the heating conditions. MF I and MF II are both enhanced with the increasing liquid flow rate. Moreover, MF III is prominent under moderate flow rates and is gradually weakened at high flow rates. The distance over which MF III starts, increases with a rise in flow rate, but is independent of the heating condition.     

Approximate solutions for power-law fluid flow past a particle at low Reynolds numbers

A simple perturbation approximation is proposed for describing flow behaviour of particles immersed in a uniform flow and an extensional flow of power-law fluids. The present solution for particles in a uniform flow field is in good agreement with the numerical solution in the literature. Theoretical predictions indicate that the effect of pseudoplasticity on flow around particles in an extensional flow field is small compared with that for particles in a uniform flow field.From the viewpoint of perturbation techniques, existing analytical solutions based on linearization of the equations of motion for particle in a power-law fluid are re-examined. Mass transfer to a power-law fluid from a particle is also discussed.     

Interfacial flows in corrugated microchannels: Flow regimes,transitions and hysteresis

We report simulations of gas–liquid two-phase flows in microchannels periodically patterned with grooves and ridges. A constant effective body force is applied on both fluids to simulate a pressure-driven creeping flow, and a diffuse-interface model is used to compute the interfacial evolution and the contact line motion. Depending on the body force, capillary force and the level of liquid saturation, a number of flow regimes may appear in the corrugated microchannel: gas flow, blockage, liquid flow, bubble–slug flow, droplet flow, annular flow and annular-droplet flow. A map of flow regimes is constructed for a set of geometric and flow parameters starting from a prescribed initial configuration. Some of the regimes are new, while others have been observed before in straight tubes and pipes. The latter are compared with previous experiments in terms of the regime map and the holdup ratio. The transition among flow regimes shows significant hysteresis, largely owing to the pinning of the interface at sharp corners in the flow conduit. Hysteresis is reduced if the sharp corners are rounded. Under the same operating conditions, different flow regimes can be realized from different initial conditions. The roles of geometry and wettability of the channel walls are also elucidated.     

Choking and shock phenomena in a single component two-phase flow including vibrational effects

An analytical and experimental investigation including vibratory effects of flashing flow in a tube with a sharp edged entrance is presented. A free streamline flow model is applied to predict choking in single-component two-phase flow. By identifying three separate regimes (i.e. jet flow, two-phase homogeneous flow, and single-phase liquid flow) in the flashing flow system, an expression is obtained for the prediction of the minimum stagnation pressure loss under choked flow conditions. A normal shock located between the flashing two-phase mixture and the single-phase liquid was experimentally observed. The location of the shock is predicted as a function of the stagnation pressure drop across the tube. The analytical predictions are verified by experimental data.     

基于时程深度学习的流场特征分析方法

流场的特征分析是流体力学的重要研究方向,传统方法主要根据流动参数量值的大小加以判断,所得结果受选取的参数形式及主观阈值影响大.本文提出了基于流场时程深度学习的流动特征分析新方法,建立了基于自动编码的流动特征提取模型;采用无监督训练方法充分挖掘流动时程信号中的隐含特征,进行流场中复杂时序特征的低维表征与特征分析.开展了R...