考虑颗粒间碰撞的稠密气/固流动二阶矩两相湍流模型

将统一二阶矩两相湍流模型和颗粒动力学理论结合,推导并封闭了稠密两相流考虑颗粒间碰撞的统一二阶矩两相湍流模型.该模型用颗粒动力学理论模拟颗粒之间的碰撞,用各向异性的统一二阶矩模型考虑气相和颗粒相的湍流脉动,并用输运方程描述气固两相湍流之间的相互作用.最后用该模型对狭窄槽道内的气粒两相流动进行了模拟,模拟所得的颗粒水平方向和垂直方向的雷诺应力和实验结果吻合良好.结果表明,考虑颗粒间碰撞之后,颗粒水平方向雷诺应力的预报得到了明显改进.     

双尺度二阶矩两相湍流模型和水平槽道内两相流动的数值模拟

对有颗粒碰撞的两相流动,常常采用将颗粒湍流模型和反映颗粒碰撞作用的动力学模型叠加的方法来构造稠密两相流动的二阶矩湍流模型,在理论上不协调.基于将颗粒脉动分成湍流引起的大尺度脉动和颗粒间碰撞产生的小尺度脉动的概念,建立了两相流动的双尺度二阶矩湍流模型.用该模型对水平槽道内两相流动进行了数值模拟.预报结果和实验结果符合,和单尺度二阶矩湍流模型的结果接近,表明了本模型的可行性.模拟结果还给出了大尺度和小尺度雷诺正应力分布,发现在同一方向上前者比后者大.     

A dual-scale turbulence model for gas-liquid bubbly flows☆

A dual-scale turbulence model is applied to simulate cocurrent upward gas–liquid bubbly flows and validated with available experimental data. In the model, liquid phase turbulence is split into shear-induced and bubble-induced turbulence. Single-phase standard k-εmodel is used to compute shear-induced turbulence and another transport equation is added to model bubble-induced turbulence. In the latter transport equation, energy loss due to interface drag is the production term, and the characteristic length of bubble-induced turbulence, simply the bubble diameter in this work, is introduced to model the dissipation term. The simulated results agree well with experimental data of the test cases and it is demonstrated that the proposed dual-scale turbulence model outperforms other models. Analysis of the predicted turbulence shows that the main part of turbulent kinetic en-ergy is the bubble-induced one while the shear-induced turbulent viscosity predominates within turbulent vis-cosity, especially at the pipe center. The underlying reason is the apparently different scales for the two kinds of turbulence production mechanisms:the shear-induced turbulence is on the scale of the whole pipe while the bubble-induced turbulence is on the scale of bubble diameter. Therefore, the model reflects the multi-scale phe-nomenon involved in gas–liquid bubbly flows.     

后台阶气固流动的双流体大涡模拟和二阶矩两相湍流模型的验证

用基于气体Smagorinsky亚网格应力模型和颗粒动理学模型的双流体大涡模拟（LES）和统一二阶矩两相湍流模型的RANS模拟（USM-RANS）,对后台阶气固流动进行了数值模拟。瞬态模拟结果给出各向异性两相湍流结构和颗粒弥散的发展过程。LES经过统计平均得到的颗粒速度及颗粒均方根脉动速度和USM-RANS的模拟结果与实验结果的对照表明,两种模拟结果和实验结果在定量上吻合较好。因此USM模型基本上得到了LES的验证。但是在剪切流区域中,LES得到的颗粒-气体纵向脉动速度关联的峰值大于USM-RANS模拟的结果,这就表明LES和USM-RANS模拟还需要进一步验证和改进。     

A second-order moment three-phase turbulence model for simulating gas-liquid-solid flows

To simulate the bubble, liquid and particle turbulence properties and their interactions in three-phase flows, a second-order moment three-phase turbulence model for gas-liquid-solid flows is proposed. The bubble, liquid and particle Reynolds stress equations, bubble-liquid and liquid-solid two-phase correlation equations are derived using the mass-weighed and time averaging and the closure models of diffusion, dissipation and pressure-strain terms similar to those used in single-phase flows. The two-phase correlation equations are closed with a two-time-scale dissipation term. The proposed model is applied to simulate gas-liquid flows and gas-liquid-solid flows in a channel. The prediction results for two-phase flows are in good agreement with the PIV measurement results. The prediction results for three-phase flows give the gas, liquid and solid velocities, volume fractions and Reynolds stresses, showing that in the case studied the turbulent fluctuation of 5 mm bubbles is stronger than that of liquid, while the turbulent fluctuation of 0.5 mm particles is weaker than that of liquid. Bubbles enhance liquid turbulence, while particles reduce liquid turbulence.     

An integral criterion for turbulence suppression in swirling flows

A numerical study of flow in rotating pipes was conducted to elucidate the relative importance of convection and turbulence. CFD (Computational Fluid Dynamics) simulations of flow inside a rotating pipe (D = 2 cm and L/D = 20) were carried out, using the Reynolds Stress Model, for four different Reynolds numbers and a range of rotation numbers. The objective was to gain a deeper understanding of the interaction between fluid forces in swirling flows. This widely‐studied model problem was used to ascertain the conditions under which computationally cheaper turbulence models such as the k‐? model should be accurate. We identified a dimensionless rotation parameter that delineates the condition at which decreasing turbulence force equals increasing convective force as rotational speed increases. This dimensionless number establishes a criterion for knowing which forces are dominant, and thereby a rational basis for choosing turbulence models that are both cost‐effective and accurate. We found a universal, critical threshold that determines when convective forces dominate over turbulence forces. This threshold determination is based on an ‘integral measure criterion’ of local forces in the radial direction. The threshold itself is defined by a dimensionless rotation number, N, based on the ratio of the circumferential and axial flow velocities. The critical value was found to be N_cr = 0.45. Above this, convection dominates; below it, turbulence dominates. This finding will facilitate selection of CFD models to optimize cost and accuracy for modelling swirling flows. For example, k‐? models suffice when N_cr < 0.45, but more complex models are required for higher values.

     

Testing of a spherical dual-tipped optical fiber probe for local measurements of void fraction and gas velocity in two-phase flows

An alternate and easy method of constructing an optical liber probe tip is presented for the study of different two-phase flow variables such as local time-averaged void fraction, gas velocity and interfacial bubble passage frequency. The proposed probe tip has similar response characteristics to the “U-bend” single fiber probe and is easier to construct than the 90° wedge tipped probe. The signal from the spherical tip seems to be insensitive to changes in bubble velocity as opposed to the 90° wedge and is more advantageous at higher velocities. The signal from the spherical tip has the same time duration as the signal changes from liquid to gas and vice versa for the same bubble velocity. A simplified model is presented to describe the balance of forces around the spherical tip when a bubble is penetrated. The model offers a qualitative explanation of why the non-dimensional response intensity decreases as the bubble velocity increases.     

Particle sizing using particle imaging velocimetry for two-phase flows

The major factors influencing the successful measurement of particle size from Particle Imaging Velocimetry (PIV) image data are described. Components of a standard PIV system, a high resolution CCD camera and argon ion laser, are used to capture images of stationary particles. The image data are used to ascertain the limitations of estimating particle size. The effects of the Gaussian distributed intensity variation across the depth of the light sheet and the optical collection system's depth of field are investigated. These effects provide insight into designing a balanced illumination and collection optical system necessary to obtain constant particle size estimates, independent of their position within the light sheet. Using a ‘balanced’ optical set-up, monodisperse particle images are shown to be reproducible and predictable over a range of particle sizes and fields of view. Accuracy in the particle size estimates on the order of 9% are obtained consistently. It is also shown that size distributions in a mixture of polydisperse particles can be obtained with a maximum deviation of 10–20% from the true size distribution.     

A turbulence model for rapid flows of granular materials part I. basic theory

A one-equation closure model for rapid flows of granular material is developed. A complete system of equations governing the transport of mass, momentum and fluctuation energy is derived. The functional dependences of the material parameters on the solid volume fraction are obtained and several simple examples are presented. A stress transport model for rapid flows of granular materials is also outlined.     

Circumferential mixing in one-phase and two-phase Taylor vortex flows

This work deals with the experimental study of the circumferential mixing in the Taylor vortex flow between coaxial cylinders. Each pair of vortices is considered as a closed chemical reactor and the residence time distribution in this reactor is determined by an electrochemical method. The tanks-in-series model with recirculation is applied to describe the flow in the vortex reactor; the circumferential mixing is characterized by the number of tanks in series and by the mixing time. Experiments were carried out with one-phase flow and with two-phase liquid—liquid flow. It is shown that stirring due to the liquid droplets increases the circumferential mixing.     

Automated setup for impedance measurements of electrochemical cells with two electrodes

An automated setup which allows impedance measurements on cells with two electrodes over a wide frequency range from 1 mHz to 10 MHz combines two impedance analysers and a temperature regulator under the control of a personal computer. Impedance spectra are measured at a programmed set of constant temperatures between room temperature and 1100 K. Alternatively measurements can be made at constant temperature and at specified time intervals. Data are recorded on disk for subsequent analysis by least-squares fitting on a separate computer. Examples for polycrystalline beta-alumina and sol-gel materials are given.This paper is dedicated to Professor Brian E. Conway on the occasion of his 65th birthday and in recognition of his outstanding contribution to electrochemistry.     

非闪蒸型气液二相流压降计算的计算机程序化

确定气液二相流的流动形式对于两相流的压力降计算非常重要,而在流型判断中,使用Baker图和GriffithWallis图进行手算尚可,却不利于计算机编程。基于此,应用Origin软件对Baker图和Griffith-Wallis图进行数据回归,得到划分区域线条的函数,再使用C++语言进行编程,实现整个计算过程的计算机程序化,可快捷、准确地判断垂直管道和水平管道的二相流流型,并使用均相法和杜克勒法计算出二相流管道压力降。计算机编程能大幅度提高计算效率,在处理杜克勒法压力降计算中的迭代运算时尤为实用。经过规范中例题的验证和工程实例运行结果,证明该方法是可行和准确的。     

Vortex shedding and turbulence scales in staggered tube bundle flows

Time-resolved LDA measurements of the axial and transverse velocity components were performed in the first 6 rows of a staggered tube bundle with relative transverse and longitudinal pitch ratios of 3.6 and 1.6, respectively. Measurements were carried out in a flow of water at a Re_d of 12,858. A flow periodicity with a Strouhal number equal to 0.26 was detected, which is in agreement with published data. This periodicity was found to broaden the turbulence levels by up to 50% in some locations. Agreement between the experimental results and previously reported CFD predictions using the k-? model improved when this periodic component was removed from the spectra using appropriate filtering techniques. In an attempt to describe the turbulence structure more extensively, time- and length-scales and dissipation rates were estimated from the filtered time-resolved data. Integral time scales were on average equal to 4 ms and Taylor microscales 0.7 ms. Comparisons of the data with numerical predictions show promise for more extensive evaluation of CFD predictions in hture than has been possible hereto.     

Directional specific shear-rate measurements in gas-liquid two-phase flow

Wall shear rate vectors in a bubble column and an airlift-loop reactor were measured using the limiting-current electrodiffusion technique. In connection with a new circular three-segment probe, the magnitudes and directions of shear rates were determined. The results for the bubble column are in good agreement with hot film-anemometry measurements in the same column. Moreover, the spiral flow structure, postulated in the shear-zone model of Franz, has been confirmed. In addition, the measuring technique was used to determine the flow structure in an airlift-loop reactor employed for the cultivation of animal cells. The region above the gas distributor of this fermenter has been identified as critical for mechanical damage to the cells.     

Prediction of pressure drop and void-fraction in annular two-phase flows

The polynomial vvelocity profile due to Pai (1953), applicable to transition and turbulent conditions in single phase flow, has been used to solve the equations of two-phase annular flow with appropriate matching conditions at the interface and at the boundaries. The results obtained enable one to predict the pressure drop and the void-fraction, when the interfacial friction factor is specified. Entrainment of the liquid in the core of the flowing gas has been taken into account by using the empirical relation of Hutchinson and Whalley (1973). It has been shown that the theory provides results in agreement with horizontal tube and vertical tube adiabatic flow data, in addition to predicting the liquid film thickness accurately.     

Transport property measurements in sheared granular flows

Experiments were performed in a shear cell device under four different solid fractions. The glass spheres with a mean diameter of 3 mm were used as granular materials. The motions of the granular materials were recorded by a high-speed camera. By using image processing technology and a particle tracking method, the average and fluctuation velocities in the streamwise and the transverse directions could be successfully measured and analyzed. Three bi-directional stress gages were used to measure the normal and shear stresses along the upper boundary. The effective viscosity of the granular material flow can be calculated. By tracking the movements of particles continually, the curves of the mean-square diffusive displacements versus time were plotted and were used to determine the self-diffusion coefficients from the slopes of the curves. The fluctuations and the self-diffusion coefficients in the streamwise direction were much higher than those in the transverse direction. The fluctuations were found to increase with the solid fraction, but the diffusion coefficients were greater in a more dilute flow system.     

Mixing characteristics for the single and air-water two-phase flows in miniaturized parallel multichannel-based devices

Investigation on the miniaturized parallel multichannel-based devices packed with glass beads to improve the mass exchange execution is the critical focal point of the current study. One of the essential parameters to specify the miniaturized devices' flow distribution is the residence time distribution (RTD). In the present context, the RTDs of a liquid tracer were investigated for the air-water multiphase flows (concurrent) across the multichannel-based miniaturized devices (comprising of 11 similar dimensional parallel channels). The devices were variable in height and packed with glass beads. The conductivity estimations generated the RTD curves and were addressed by the axial dispersion model (ADM). The fluid-flow rates differed within the range of 5–23 ml min⁻¹. The axial dispersion coefficients and the rate of the specific energy dispersion were investigated. The effects of pressure difference and geometry on the hydrodynamic attributes and mixing properties were well-illustrated, and the new correlations were suggested.     

Effects of turbulence on heat and mass transfer in stagnation flows

The influence of turbulence on local heat and mass transfer is systematically analyzed. Essential prerequisite is the availability of measurement methods, based on convective mass transfer, which allow the visualization of heat and mass transfer distributions. The results show that, even at low turbulence intensities, the flow and transport phenomena are decisively influenced by the specific design of turbulence grids and the orientation of the grid wires with respect to the stagnation line. Thereby, two different flow mechanisms occur which may interact. The anisotropy of grid turbulence leads to the “wire-gap effect” depending on the actual position of the grid wires in relation to the stagnation line. The other mechanism is based on the wavy wake velocity distribution behind screens and grid which, in conjunction with the centrifugal instability of stagnation flows, may lead to longitudinal vortices. The results indicate why turbulence has not been sufficiently considered in the available (semi-) empirical correlations. Information needed for an accurate prediction of heat and mass transfer in turbulence flows is discussed.     

Estimation of turbulence power spectra for bubbly flows from Laser Doppler Anemometry signals

The accuracy of the estimation of turbulence power spectra from Laser Doppler Anemometry (LDA) signals in bubbly flows is studied. Special attention is paid to the influence of the gaps in the signal created by the bubbles. The performance of reconstruction and slotting techniques for the estimation of the power spectrum is determined by application to synthetic bubbly flow LDA signals, and checked by application to real data.Estimation of spectra of bubbly flow signals with reconstruction techniques is found to give poor results. In general, the application of reconstruction techniques to LDA signals gives bias in the spectrum due to the addition of noise and low-pass filtering. The nature of the filtering and noise for bubbly flow signals differs from that for single phase flows. For bubbly flow signals, the spectrum is reliable up to a cut-off frequency which is lower than the cut-off frequency for single-phase flow signals with similar data rates. In addition, slopes close to may appear for signals which in reality have flat spectra. For single phase flow, it is possible to correct for these artefacts. The work shows that it is not possible to create similar correction techniques for bubbly flow signals.The application of the slotting technique for estimation of power spectra of bubbly flow signals leads to much better results. Estimation of the spectrum beyond the mean data rate is well possible. The performance of several improvements of the slotting technique is discussed.