涡流桨搅拌槽内流动非稳定性的大涡模拟

The aim of this work is to investigate the flow instabilities in a baffled, stirred tank generated by a single Rushton turbine by means of large eddy simulation （LES）. The sliding mesh method was used for the coupling between the rotating and the stationary frame of references. The calculations were carried out on the ＂Shengcao-21C＂ supercomputer using a computational fluid dynamics （CFD） code CFX5. The flow fields predicted by the LES simulation and the simulation using standard κ-ε model were compared to the results from particle image velocimetry （PIV） measurements. It is shown that the CFD simulations using the LES approach and the standard κ-ε model agree well with the PIV measurements. Fluctuations of the radial and axial velocity are predicted at different frequencies by the LES simulation. Velocity fluctuations of high frequencies are seen in the impeller region, while low frequencies velocity fluctuations are observed in the bulk flow. A low frequency velocity fluctuation with a nondimensional frequency of 0.027Hz is predicted by the LES simulation, which agrees with experimental investigations in the literature. Flow circulation patterns predicted by the LES simulation are asymmetric, stochastic and complex, spanning a large portion of the tanks and varying with time, while circulation patterns calculated by the simulation using the standard κ-ε model are symmetric. The results of the present work give better understanding to the flow instabilities in the mechanically agitated tank. However, further analysis of the LES calculated velocity series by means of fast Fourier transform （FFT） and/or spectra analysis are recommended in future work in order to gain more knowledge of the complicated flow phenomena.     

PIV experiments and large eddy simulations of single-loop flow fields in Rushton turbine stirred tanks

The single-loop flow fields in Rushton turbine stirred tanks with clearance C=0.15T (T is tank diameter) were investigated by using particle image velocimetry (PIV) experiments and large eddy simulation (LES) methods. The velocity and turbulent kinetic energy (TKE) were carefully measured and resolved with high resolution camera. The regions with high TKE are affected by the movement of the trailing vortices generated behind the impeller blades. The effects of both geometrical configuration and Reynolds number were discussed. It is found that the Reynolds number has little effect on the mean flow for the configuration of impeller diameter D=T/3, C=0.15T. However, the single-loop flow pattern is changed into a double-loop one if D is increased from T/3 to T/2. The LES results were compared with the PIV experiments and the laser Doppler anemometry (LDA) data in the literature. The effect of the grid was validated, and the levels of local anisotropy of turbulence near the impeller discharge regions were investigated. Both the phase-averaged and phase-resolved LES results are in good agreement with the PIV experimental data, and are better than the predictions of the k–ε model. The agreement shows that the LES method can be used to simulate the complex flow fields in stirred tanks.     

T型反应器内非稳态吞噬流机理

运用激光诱导荧光（PLIF）技术和大涡模拟方法对T型反应器内的流动特性进行了研究。发现随着Reynolds数（Re）的增大,T型反应器中依次出现分离流（Re<120）、稳定吞噬流（120≤Re<190）、非稳态吞噬流（190≤Re≤300）、非稳态对称流（Re>300）四种流动模式。通过大涡模拟重点考察了非稳态吞噬流的振荡特性。结果表明,在非稳态吞噬流型下,T型反应器内撞击面上会周期性地出现旋涡合并现象,合并的涡向下传递,产生自持振荡。此时撞击区的压力、速度和涡量也发生周期性变化,且变化周期与旋涡合并周期相同,这种振荡是由速度和压力的周期性转换引起的。     

翼型桨叶片尾涡结构的PIV实测与LES模拟研究

在塔径383 mm、高径比为1的翼型桨搅拌反应器内,采用粒子图像测速(PIV)技术和大涡模拟(LES)研究了一种翼型桨的叶片尾涡结构. 对比二维PIV技术测定的翼型桨搅拌反应器平均速度场数据,验证了LES方法的可靠性. 并通过叶片端的速度矢量场、叶片区内的涡量大小及湍动能分布清楚地识别了翼型桨叶片尾涡结构. 发现该叶片尾涡为单涡结构,其尾涡轴心轨迹几乎就在桨叶尖划过的圆柱面上. 此外,还探讨了叶片尾涡对湍动能分布、湍流特性等的影响,表明最大无因次湍动能k/Vtip2约为0.04~0.06,其位置约在0.5相似文献   

Simulating turbulent swirling flow in a gas cyclone: A comparison of various modeling approaches

The single phase flow field of a gas cyclone has been simulated with a finite volume RANS model and two LES approaches — one with finite volume and one with lattice-Boltzmann discretization. In order to evaluate their quality, the modeling results have been compared with LDA velocity measurements taken from the literature. Since the steady-state RANS simulation did not result in a converged solution, an unsteady RANS simulation was performed and used for further evaluations. The peak-levels of the time-averaged tangential velocity are well predicted by the two LES approaches whereas the RANS-based simulation under-predicts them. For the mean axial velocity, all models perform equally well. Velocity fluctuation levels (in terms of the root-mean-square values of the tangential and axial velocity components) are much better predicted by the large eddy simulations as compared to the RANS simulation. This relates to vortex core precession, the frequency of which was analysed for the two LES approaches. It agreed within 10% with experimentally obtained data. In conclusion, unsteady RANS-based simulations on a relatively coarse grid can provide reasonable and industrially relevant results with limited computational effort, whereas simulations that accurately capture the flow physics require a large eddy approach on fine grids.     

Experimental and computational studies of multiphase hydrodynamics in a hydrocyclone separator system

The flow patterns within a hydrocyclone separator system have been studied experimentally using particle image velocimetry (PIV) and computationally via large eddy simulations (LES). Various aspects of the flow field such as the process of air core structure formation, pressure and velocity distributions within the system have been characterized. Detailed flow field information in the form of velocity vector maps of entire planes have been obtained experimentally and reproduced computationally. Good qualitative agreements between velocity vector fields, air core structure and averaged velocity distributions have been achieved. LES is deemed to be an adequate methodology for modelling the air core structure and flow patterns of the surrounding liquid in a hydrocyclone separator system.     

PIV experiment and large eddy simulation of turbulence characteristics in a confined impinging jet reactor

Confined impinging jet reactor(CIJR) is a typical process intensification device used in the chemical industry.In this study, two dimensional Particle Image Velocimetry(PIV) and Large Eddy Simulation(LES) method were used to investigate the flow field in a CIJR with jets of diameter 3 mm under highly turbulent condition.The results showed LES can predict the velocity and Turbulence Kinetic Energy(TKE) distributions in the reactor well by comparing with the PIV results.In the CIJR, the stagnation point fluctuates with the turbulence, and its instantaneous position accords with the normal distribution.Three methods, including s–t representation, Lumley–Newman triangle and A–G representation, were used to compare the turbulence anisotropy in the mixing chamber.It was found that the anisotropy in the impinging area and at the edge of impinging jet was strong and the maximum deviation was up to 40%.The results from 2 DPIV would lead to an overestimation of the turbulent kinetic energy as much as 20% to 30% than the results from the three dimensional numerical simulation.     

Assessment of large eddy and RANS stirred tank simulations by means of LDA

Large eddy simulations (LES) and Reynolds-averaged Navier-Stokes (RANS) calculations were performed on the flow in a baffled stirred tank, driven by a Rushton turbine at Re=7300. The LES methodology provides detailed flow information as velocity fluctuations are resolved down to the scale of the numerical grid. The Smagorinsky and Voke subgrid-scale models used in the LES were embedded in a numerical lattice-Boltzmann scheme for discretizing the Navier-Stokes equations, and an adaptive force-field technique was used for modeling the geometry. The uniform, cubic computational grid had a size of 240³ grid nodes. The RANS calculations were performed using the computational fluid dynamics code CFX 5.5.1. A transient sliding mesh procedure was applied in combination with the shear-stress-transport (SST) turbulence closure model. The mesh used for the RANS calculation consisted of 241464 nodes and 228096 elements (hexahedrons). Phase-averaged and phase-resolved flow field data, as well as turbulence characteristics, based on the LES and RANS results, are compared both mutually and with a single set of experimental data.     

STUDY OF THE TURBULENT FLOW IN AN UNBAFFLED STIRRED TANK BY DETACHED EDDY SIMULATION

Detached eddy simulation (DES) of the liquid-phase turbulent flow in an unbaffled stirred tank agitated by a six-blade, 45°-pitched blade turbine was performed in this study. The tank wall is cylindrical with no baffle and the fluid flow problem was solved in a single reference frame (SRF) rotating with the impeller. For the purpose of comparison, computation based on large eddy simulation (LES) was also carried out. The commercial code Fluent was used for all simulations. Predictions of the phase-averaged turbulent flow quantities and power consumption were conducted. Results obtained by DES were compared with experimental laser Doppler velocimetry (LDV) data from the literature and with the predictions obtained by LES. It was found that numerical results of mean velocity and turbulent kinetic energy profiles as well as the power consumption are in good agreement with the LDV data. When performed on the same computational grid, which is under-resolved in the sense of LES, DES allows better accuracy than LES in that it works better in the boundary layers on the surface of the impeller and the stirred tank walls. It can be concluded that DES has the potential to predict accurately the turbulent flow in stirred tanks and can be used as an effective tool to study the hydrodynamics in stirred tanks.     

Stereo‐PIV experiments and large eddy simulations of flow fields in stirred tanks with Rushton and curved‐Blade turbines

The flow characteristics in pilot‐scale stirred tanks with Rushton and curved‐blade turbines were investigated by using stereoscopic particle image velocimetry (SPIV) experiments and large eddy simulation (LES) methods. The velocity and turbulent kinetic energy (TKE) in the impeller discharge regions were carefully resolved with a high resolution SPIV system, and the detailed phase‐resolved velocity and TKE profiles were used to validate the LES results. The effects of Reynolds number and blade shape on the flow characteristics were discussed. The LES results of velocity, TKE, and the evolution of trailing vortices were compared with the SPIV experimental data, and good agreement was obtained at various phase angles. The effects of subgrid scale model and hybrid grid with different mesh resolutions on the LES results were investigated. LES is a computationally affordable method for the accurate predictions of the complex flow fields in pilot‐scale stirred tanks is presented. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3986–4003, 2013     

CFD simulations and particle image velocimetry measurements in an industrial scale rotating disc contactor

Fluid dynamics of the single‐phase and two‐phase flow in a segment of a rotating disc contactor (RDC) liquid–liquid extraction column with 450 mm inner diameter were studied by performing computational fluid dynamics (CFD) simulations and particle image velocimetry (PIV) measurements. The fluid dynamics were investigated to test the predictivity of CFD at industrial scale. Different turbulence models in conjunction with the Eulerian approach were applied in the single‐phase and two‐phase simulations. The turbulent flow characteristics were analyzed by PIV measurements to validate the CFD simulations. An iso‐optical system composed of CaCl₂/water–butylacetate allows for the two‐phase PIV measurements. Local turbulent energy dissipation was derived from velocity gradients in PIV data. In this connection, the influence of the PIV spatial resolution on the measured energy dissipation was also analyzed, and different fit functions were tested to scale the measured energy dissipation. Simulated velocity fields as well as the energy dissipation were compared with the experimental PIV data. The results from the simulations and experiments are in good agreement. The work shows that CFD can predict hydrodynamic characteristics even at bigger scales but is still subject to some minor restrictions. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

LDA measurements and CFD simulations of an in‐line high shear mixer with ultrafine teeth

Hydrodynamics of a pilot‐scale in‐line high shear mixer (HSM) with double rows of ultrafine rotor and stator teeth, including the velocity profiles and power consumptions, were measured using laser Doppler anemometry and a torque transducer, respectively. Computational fluid dynamics simulations were conducted using the standard k‐ε turbulence model with first‐ and second‐order accuracy and large eddy simulation (LES) with the standard Smagorinsky–Lilly subgrid scale model. Predictive capabilities of the different turbulence models and discretization schemes were assessed based on the experimental data. It is found that the current LES can predict accurately the flow patterns for the strongly rotating and locally anisotropic turbulent flows in the complex in‐line HSM. The results obtained are fundamental to explore potential applications of the in‐line teethed HSMs to intensify chemical reaction processes. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1143–1155, 2014     

Large Eddy Simulation of mixing effects on the course of parallel chemical reactions and comparison with k-? modeling

The way in which reagents are mixed can have a large influence on the product distribution of chemical reactions. To model effects of mixing on various scales on the course of chemical reactions the method of Large Eddy Simulation (LES) of non-premixed, turbulent reactive flows of incompressible fluids is considered in this work. The subgrid modeling of chemical reaction is based on a beta distribution of the mixture fraction in combination with a conditional moment closure based on linear interpolation of local instantaneous reactant concentration values. The predictions obtained with LES are compared with experimental data for fast parallel chemical reactions, the fluid velocity measured using Particle Image Velocity (PIV) technique and the passive tracer concentration measured using the Planar Laser Induced Fluorescence (PLIF) technique. Predictions of the model based on LES are compared as well with results obtained using the non-equilibrium multiple-time-scale mixing model combined with a standard k-? model and employing similar conditional moment closure as LES, applied, however, at larger scale. All comparisons show a very good performance of the model based on LES.     

LES/FDF simulation of a gas-particle backward-facing step flow

A newly developed filtered density function (FDF) transport equation was used to describe the fluid velocity seen by the particles in two-phase flows and the interaction between the two phases was considered. Numerical simulation of a gas-particle two-phase backward-facing flow was carried out with the LES/FDF model, in which the large eddy simulation (LES) was used for the gas-phase. The numerical results were compared with the experimental data. The comparison shows that the LES/FDF model can distinctly improve the precision of the simulation. Further investigation has been carried out on the turbulence modification by particles with different particle parameters.     

Detached eddy simulation on the turbulent flow in a stirred tank

A detached eddy simulation (DES), a large‐eddy simulation (LES), and a k‐ε‐based Reynolds averaged Navier‐Stokes (RANS) calculation on the single phase turbulent flow in a fully baffled stirred tank, agitated by a Rushton turbine is presented. The DES used here is based on the Spalart‐Allmaras turbulence model solved on a grid containing about a million control volumes. The standard k‐ε and LES were considered here for comparison purposes. Predictions of the impeller‐angle‐resolved and time‐averaged turbulent flow have been evaluated and compared with data from laser doppler anemometry measurements. The effects of the turbulence model on the predictions of the mean velocity components and the turbulent kinetic energy are most pronounced in the (highly anisotropic) trailing vortex core region, with specifically DES performing well. The LES—that was performed on the same grid as the DES—appears to lack resolution in the boundary layers on the surface of the impeller. The findings suggest that DES provides a more accurate prediction of the features of the turbulent flows in a stirred tank compared with RANS‐based models and at the same time alleviates resolution requirements of LES close to walls. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3224–3241, 2012     

板翅式换热器入口流场的模拟及实验研究

利用PIV粒子图像测速仪和CFD数值模拟的方法,对板翅式换热器入口结构改进前后的流场进行了研究,发现其内部流场的流动与分布规律。由于原始入口结构的不合理导致漩涡、回流等现象存在,使得其内部的物流分配极不均匀。而对于添加了打孔挡板的改进型结构,物流分配的均匀性有了很大改善。实验结果和数值计算结果吻合良好,不仅验证了计算模型的正确性,而且证明PIV技术适合于研究复杂的流动结构。     

Advanced PIV/LIF and shadowgraphy system to visualize flow structure in two-phase bubbly flows

Particle image velocimetry (PIV) is a promising technique to measure dispersed phase size, dispersed phase hold-up and velocity of both the phases. The current work reports measurement of the shape, size, velocity and acceleration of bubbles using shadowgraphy, and liquid velocity measurement obtained using PIV/LIF with fluorescent tracer particles. Measurements were performed in a narrow rectangular column at moderate gas hold-up (～5%) with wide variation of bubble sizes (0.1–15 mm). The liquid velocity field was subjected to 2D discrete wavelet transform (DWT) to visualize the flow structures in the bubbly flow. Further, the slip velocity of individual bubbles was obtained from the DWT filtered liquid velocity field. The results are compared with the slip velocity correlations reported in literature for single bubbles rising in quiescent water. The comparison shows the difference in slip velocity of single bubbles and bubbles rising in swarm. The scale wise decomposition obtained from DWT was also used to quantify the liquid velocity field in terms of wavenumber spectrum. The velocity and acceleration measurements are demonstrated on a single spherical cap bubble rising in quiescent water. The measurements show the potential of the 2D acceleration measurement to facilitate the estimation of unsteady drag on bubbles.     

Computational Investigation of an Industrial Cyclone Separator with Helical‐Roof Inlet

An industrial cyclone separator with helical‐roof inlet TsN‐11 has been numerically investigated as to pressure and flow field, pressure drop, fractional efficiency, and particle trajectories inside the cyclone. The turbulence was modeled with Reynolds stresses and large eddy simulations (LES) based on three different subgrid‐scales (SGS). The results with the different setups were compared to experimental data from previous studies. For a proper calculation of the flow field, LES combined with a dynamic SGS model was used for predicting cyclone performance. Individual particles were tracked through the unsteady flow field using the Lagrangian approach. The results of the numerical calculations of the tangential and axial velocity, pressure drop, and cut size are in good agreement with experimental measurements.     

液-液旋流分离器内流动特性的大涡模拟

基于柱坐标系下的瞬态N-S方程,采用标准Smagorinsky-Lilly模型,对旋流分离器内的单相流动特性进行了大涡模拟研究,获得了旋流分离器内的瞬时流场、时均速度与湍动能分布,并将结果与雷诺应力模型(RSM)及实测结果进行了对比。结果表明,大涡模拟方法可以得到旋流分离器内的瞬态流场,且能够反映更多的流场细节;对时均速度的预测与实测数据吻合良好,精度优于RSM模型;对湍动能的模拟与RSM方法差异显著,但其结论更符合以往理论分析与实测的结果。相比采用时均方法的RSM模型,大涡模拟方法对此类旋流湍流具有更好的模拟能力。     

Turbulent combustion of preheated natural gas-air mixture

The flame structure at small Damköhler number and large Karlovitz number is still unknown. In the present study, the reaction zone structure in a distributed reaction zone regime was investigated extensively. The OH-PLIF images suggested that there are no thin laminar flamelets and that the reacting eddies were distributed throughout the reactor. With the cross-correlation of ion signals, the scale of the reacting eddies was determined to be of the order of 4 mm, and the convection velocity of these eddies or zones was found to coincide with the mean flow velocity of the order of 100 m/s. In this combustion regime, high intensity combustion can be realized with extremely low NO_x emission. These features are very attractive for the practical use (for example, vehicles and aircrafts) to meet the environmental requirements.