双螺杆挤出机流场数值模拟中流道进出口边界条件的探讨

在对双螺杆挤出机流场的数值模拟中,流道进出口边界条件的设置一直是一个颇具争议的问题。由于事先无法获得计算域进出口平面上的真实边界条件,研究人员在进行双螺杆挤出机的流场分析时,大都采用放松边界条件。为了考察放松边界条件对双螺杆挤出机流场数值模拟结果的影响,本文采用聚合物流动分析软件POLYFLOW,在流量恒定的前提下对双螺杆挤出机流道进出口给定三种不同分布形式的速度边界条件,对其流场进行了数值模拟。数值计算结果表明,在体积流量恒定的条件下,流道进出口不同分布形式的速度边界条件对流场的影响主要集中在进出口附近区域,但对离进出口边界较远的流场影响很小。一般而言,当计算域所对应的螺杆较长时,可以忽略流道进出口的放松边界条件所引起的误差;当计算域较短时,不宜直接采用放松边界条件,而应根据螺杆的实际构型．在计算域的进出口增加适当长度的发展段。     

Investigation of the influence of swirl on a confined coannular swirl jet

Large Eddy Simulations are used to model a turbulent confined coannular combustor and examine the effects of swirl on the flow field and mixing. Three separate simulations with relatively high mesh resolutions and different swirl numbers have been carried out using a finite volume method on a Cartesian non-uniform structured grid. A localised dynamic Smagorinsky model is used to parameterize the sub-grid scale turbulence. The snapshots of the axial and swirl velocities and velocity vector fields show the complex flow patterns developing with increased swirl number and the rapid decay of axial momentum. Precessing vortex cores (PVC) were identified for all three cases and the mean axial velocity plots indicate that the upstream extremity of the vortex breakdown bubble shifts towards the inlet as the swirl number increases. The calculated power spectra indicate the distinct precession frequency for high swirl number. Probability density functions of axial velocity showed the changes of their distributions from approximately Gaussian to non-Gaussian with increased swirl number. The swirl has a large effect on the rate of decay of the axial velocity throughout the domain, whereas only has a significant effect on the decay of swirl velocity in the near field close to the jet inlet. The relation between swirl number and the axial extent of the recirculation zone is approximately linear. Radial plots of mean passive scalar and its variance also demonstrate an increase in the rate of mixing with increasing swirl number.     

Inlet conditions for LES using mapping and feedback control

Generating effective and efficient inlet boundary conditions for large eddy simulation (LES) is a challenging problem. The most accurate way of achieving this is to run a precursor calculation to generate a library of turbulence, either prior to the simulation or concurrently with it, and to transfer the data from the library simulation to the main domain inlet. In this paper, we investigate a variant of this, in which the precursor calculation is subsumed into the main domain, its function being adopted by a mapping of data from a specified plane downstream of the inlet back to the inlet. Within this inlet section of the main domain, the flow can be affected by a number of computational manipulations, including the introduction of artificial body forces, modification of the mapped data, and direct correction of the velocity data. These modifications can be linked to feedback control algorithms to drive the solution towards specified characteristics, including mean and turbulent flow profiles, and bulk properties of the flow such as swirl. Various variants of the basic technique incorporating different levels of complexity in the control are implemented and tested on simulation of flow in a rectangular channel and in a circular pipe.     

Numerical prediction of confined swirling jets

The axisymmetric flow of a swirling viscous, incompressible fluid jet inside confining cylindrical boundaries has been numerically investigated using the well-known implicit finite-difference scheme. For a swirling jet confined by cylindrical tube, the vortex-breakdown or the formation of an axisymmetric isolated eddy occurs at high values of swirl ratios at a moderate flow rate or Reynolds number. With the introduction of artificial adverse pressure gradients, such as one studied in the case of a step-up cylindrical tube, the vortex-breakdown occurs at a relatively lower swirl ratio at a given flow rate. For a swirling jet discharging in a coaxial non-rotating surrounding stream enclosed by a cylindrical tube, the vortex-breakdown and its structure depend on various parameters such as the flow rate of the jet, surrounding stream velocity, the swirl of the jet and on the radius of the enclosing cylindrical tube. In general increasing Reynolds numbers, swirl ratio, decreasing surrounding stream velocity and increasing size of the cylindrical tube enhance the occurrence and size of the vortex-breakdown.     

旋转固体火箭发动机统一流场计算

为了进一步研究旋转对固体火箭发动机工作的影响,采用RSM湍流模型对内孔燃烧、内孔与端面同时燃烧管状装药旋转固体火箭发动机统一流场进行了仿真。采用UDF编程给出质量入口边界,获得了旋转条件下发动机内流场结构参数特点,并给予理论说明。计算结果表明,内孔燃烧装药发动机切向速度流场类似于典型的Rankine涡,端面和内孔同时燃烧装药发动机切向速度流场呈现出Rankine涡和由端面燃烧引起的强迫涡的复杂组合涡;在发动机前封头和喷管喉部涡核切向速度峰值非常大,使燃烧室前封头和喷管喉部工作环境显著恶化;旋转使发动机燃烧室压力沿径向逐渐增大,强迫涡附近的压力梯度远大于推进剂表面的压力梯度。     

Topology optimization of internal partitions in a flow-reversing chamber muffler for noise reduction

A topology-optimization-based design method for a flow-reversing chamber muffler is suggested to maximize the transmission loss value at a target frequency considering flow power dissipation. Rigid partitions for high noise reduction should be carefully placed inside the muffler to avoid extreme flow power dissipation due to a 180° change in flow direction from an inlet to an outlet. The optimal flow path for minimum flow power dissipation is well known to change depending on the Reynolds number, which is a function of the inlet flow velocity. To optimize the partition layout with an optimal flow path in an expansion chamber at a given Reynolds number, a flow-reversing chamber muffler design problem is formulated by topology optimization. The formulated topology optimization problem is implemented using the finite element method with a gradient-based optimization algorithm and is solved for various design conditions such as the target frequencies, rigid partition volumes, Reynolds numbers, non-design domain settings, and allowed amounts of flow power dissipation. The effectiveness of our suggested approach is verified by comparing the optimized partition layouts obtained by the suggested method and previous methods.     

Study of jet precession, recirculation and vortex breakdown in turbulent swirling jets using LES

Large eddy simulations (LES) are used to investigate turbulent isothermal swirling flows with a strong emphasis on vortex breakdown, recirculation and instability behaviour. The Sydney swirl burner configuration is used for all simulated test cases from low to high swirl and Reynolds numbers. The governing equations for continuity and momentum are solved on a structured Cartesian grid, and a Smagorinsky eddy viscosity model with the localised dynamic procedure is used as the sub-grid scale turbulence model. The LES successfully predicts both the upstream first recirculation zone generated by the bluff body and the downstream vortex breakdown bubble. The frequency spectrum indicates the presence of low frequency oscillations and the existence of a central jet precession as observed in experiments. The LES calculations well captured the distinct precession frequencies. The results also highlight the precession mode of instability in the center jet and the oscillations of the central jet precession, which forms a precessing vortex core. The study further highlights the predictive capabilities of LES on unsteady oscillations of turbulent swirling flow fields and provides a good framework for complex instability investigations.     

Simulations of confined turbulent vortex flow

Large-eddy simulations (LES) of the turbulent flow in a swirl tube with a tangential inlet have been performed. The geometry, and flow conditions were chosen according to an experimental study by [Escudier MP, Bornstein J, Zehnder N. Observations and LDA measurements of confined turbulent vortex flow. J Fluid Mech 1980;98:49-63]. Lattice-Boltzmann discretization was used to numerically solve the Navier-Stokes equations in the incompressible limit. Effects of spatial resolution and choices in subgrid-scale modeling were explicitly investigated with the experimental data set as the testing ground. Experimentally observed flow features, such as vortex breakdown and laminarization of the vortex core were well represented by the LES. The simulations confirmed the experimental observations that the average velocity profiles in the entire vortex tube are extremely sensitivity to the exit pipe diameter. For the narrowest exit pipe considered in the simulations, very high average velocity gradients are encountered. In this situation, the LES shows the most pronounced effects of spatial resolution and subgrid-scale modeling.     

Calculations of axisymmetric swirling flows by the use of parabolic type equations

Numerical solutions of boundary layer like equations are presented for steady, axisymmetric swirling flows in a pipe; the fluid is incompressible. Calculations were performed for combinations of two parameters; Wa and Ω are the velocity on the axis and the swirl strength at the upstream boundary, respectively. One of the solutions was in qualitative agreement, over some axial distance, with earlier Navier-Stokes calculations obtained by the authors.     

Numerical simulation of turbulent flow in complex geometries used in power plants

Performance degradations or improvements of coal-fired power stations depend on effective functioning of pulveriser equipment and combustion efficiency of furnaces in boilers. The function of a pulveriser is to grind the lumped coal and transfer the fine coal to the furnace for efficient combustion. However, the presence of several solid objects inside the mill, flow of air and particles takes turn around from inlet to outlet. The flow simulation process involves the geometrical modelling, grid generation and particle trajectories for the given flow conditions, and has been investigated to understand the flow path in grinding chamber, separator and classifier. The behaviour of turbulent air flow motion with fly ash particle paths on Lagrangian scale in computational domain are obtained through CFD/CAD software packages. The understanding developed with reference to recirculation flows in the inlet duct, non-uniform flow over the height of bowl mill and unequal flow at exit, provides valuable insights to designers for optimisation of components for better efficiency.     

Modeling fluid flow in fuel cells using the lattice-Boltzmann approach

Two flow problems relevant to fuel cell modeling are simulated with the lattice-Boltzmann (LB) approach. The first is a 3D viscous flow through a section of serpentine channel and the second is a 2D channel filled or partially filled with porous medium. In the first case, attention is given to the implementation details such as inlet–outlet boundary conditions, nonuniform grid, and forcing. It was shown that the flow pattern and pressure distribution depends sensitively on the flow Reynolds number as the flow Reynolds number is increased from 10 to 1000. There also appears to be some evidence that the transition to a turbulent flow occurs at Reynolds number on the order of 1000. In the second case, the effects of multiple time scales and interface between the porous medium and clear channel are considered. It was shown that, in order to obtain correct results at the interface or near the boundary, the physical time scales of the problem must be kept larger than the lattice time. This can be achieved by using a small particle velocity in the LB scheme.     

Simulation of the effect of geometric parameters on tangentially injected swirling pipe airflow

A realizable k–ε turbulence model is employed to study compressible tangentially injected swirling flow in the nozzle of air–jet spinning. The effects of the nozzle geometric parameters (the injection angle, the diameter, number and position of the injector, nozzle length and chamber diameter) on both the flow and yarn properties are investigated. The simulation results show that some factors, such as velocity distribution, reverse flow in the upstream of the injector and vortex breakdown in the downstream caused by the nozzle geometric variation, are significantly related to fluid flow, and consequently to yarn properties. With increase in the injection angle or injector diameter or injector number, in the downstream of the injector, velocities will increase somewhat, and the locations of vortex breakdown move downward. As injector number increase with the total injection area being kept constant, the strength of vortex breakdown in the downstream of the injector will slightly increase. A larger reverse flow will be not helpful to draw the fibers into nozzle, as the injector position is closer to the nozzle inlet. The flow is more turbulent for a larger chamber diameter.     

Parallel Simulations of Swirling Turbulent Flames

The feasibility of using massively paralleled computations as an engineering design tool is evaluated. A parallel Large-Eddy Simulation (LES) algorithm which simulates turbulent reacting flows using a space and time-accurate method, is used to model the complex flow found inside a realistic gas-turbine combustor. The parallelization philosophy and its implementation as a platform-independent solver is discussed. A performance analysis is carried out to determine the communication and storage requirements, and the associated overhead. As a case study, the LES methodology is used for a parametric investigation of swirl effects on the turbulent reacting flow in the gas-turbine.     

SPH中一种层流入口、出口边界的处理方法

提出一种替代周期边界条件的对入口、出口边界的处理方法,这种方法使用跟随虚粒子处理入口、出口边界。跟随虚粒子设置于入口、出口的外侧,这些虚粒子的速度、位置根据对应的内部粒子的速度、位置进行更新。该方法建立在对层流特性的分析的基础上,适用于具有层流入口或出口的低雷诺数流场中。利用该入、出口边界处理新方法,分别对Poiseuille流和渐扩管平面流进行数值摸拟,数值结果与理论解吻合良好。     

An investigation of turbulent open channel flow with heat transfer by large eddy simulation

Large eddy simulation of fully developed turbulent open channel flow with heat transfer is performed. The three-dimensional filtered Navier-Stokes and energy equations are numerically solved using a fractional-step method. Dynamic subgrid-scale (SGS) models for the turbulent SGS stress and heat flux are employed to close the governing equations. Two typical temperature boundary conditions, i.e., constant temperature and constant heat flux being maintained at the free surface, respectively, are used. The objective of this study is to explore the behavior of heat transfer in the turbulent open channel flow for different temperature boundary conditions and to examine the reliability of the LES technique for predicting turbulent heat transfer at the free surface, in particular, for high Prandtl number. Calculated parameters are chosen as the Prandtl number (Pr) from 1 up to 100, the Reynolds number (Re_τ) 180 based on the wall friction velocity and the channel depth. Some typical quantities, including the mean velocity, temperature and their fluctuations, heat transfer coefficients, turbulent heat fluxes, and flow structures based on the velocity, vorticity and temperature fluctuations, are analyzed.     

CFD modeling of the in-cylinder flow in direct-injection Diesel engines

Three-dimensional flow calculations of the intake and compression stroke of a four-valve direct-injection Diesel engine have been carried out with different combustion chambers. A limited number of validation calculations of the compression stroke were first performed in order to explore the limits of CFD representation of the in-cylinder flow. The calculated flow field in three different combustion chambers was compared with laser Doppler velocimetry measurements; the comparison shows that the three-dimensional model is reasonably accurate for crank-angles around top dead center (TDC). In general, it performs better for low swirl combustion chambers while turbulence velocities are under-predicted when squish effects are important.In the main study, the flow characteristics inside the engine cylinder equipped with different piston configurations were compared. For this, complete calculations of the intake and compression strokes were performed under realistic operating conditions and the ensemble-averaged velocity and turbulence flow fields obtained in each combustion chamber analyzed in detail. The results confirmed that the piston geometry had little influence on the in-cylinder flow during the intake stroke and the first part of the compression stroke. However, the bowl shape plays a significant role near TDC and in the early stage of the expansion stroke by controlling both the ensemble-averaged mean and the turbulence velocity fields.     

Development of boundary conditions for direct numerical simulations of three-dimensional vortex breakdown phenomena in semi-infinite domains

The suitability of various open boundary conditions is evaluated for direct numerical simulations of three-dimensional, incompressible, spatially and temporally evolving, swirling laminar jets in domains that extend to infinity in the downstream and radial direction. From the point of view of specifying conditions at the open boundaries, this class of flows is particularly challenging due to its ability to support traveling waves. Towards this end, several radial boundary conditions are implemented and tested with respect to their ability to conserve local and global mass, to handle low and high entrainment flow, and to avoid the introduction of artificial waves propagating from the boundaries into the interior: a free-slip condition, two types of homogeneous Neumann conditions, and a radiation condition in spirit of the outflow boundary condition. Global mass is conserved automatically within machine accuracy in the free-slip and simple radiation case, while the Neumann conditions require some iterative modification to conserve mass. This yields a computationally less efficient scheme which additionally exhibits poorer conservation properties due to the limited number of iterations. The free-slip condition typically requires the largest radial extent of the computational domain due to its impermeable character which is particularly problematic for the high entrainment flow. Hence, the radiation condition has been found as the most suitable lateral boundary condition for both high and low entrainment jets.     

Modeling of axis-symmetric steam plasma flow in a non-transferred torch

This paper discusses the numerical modeling of steam plasma in a DC non-transferred torch equipped with well-type cathode, where the non-trivial azimuthal velocity component and turbulent effect are both taken into account. In order to investigate the complicated flow characteristics of thermal plasma due to the interaction between steam and electric arc, the flow filed inside the plasma torch is modeled by the magnetic–hydrodynamic (MHD) equations. The governing equations are then solved numerically using a non-staggered finite volume discretization based on Cartesian grid system. The anode location and maximum current density at cathode are fixed by the corresponding experimental measurement at a given flow rate. The predicted result suggests that the steam plasma can be accelerated to a mean velocity of 1500 m/s at torch outlet with the mass flow rate of 5 g/s and the system current of 180 A. A strong vortex flow structure is found inside the torch due to the introduction of azimuthal velocity at the gas inlet, which should help to stabilize the rotating arc during the torch operation.     

LES of intermittency in a turbulent round jet with different inlet conditions

Large eddy simulation (LES) is a promising technique for accurate prediction of turbulent free shear flows in a wide range of applications. Here the LES technique has been applied to study the intermittency in a high Reynolds number turbulent jet with and without a bluff body. The objective of this work is to study the turbulence intermittency of velocity and scalar fields and its variation with respect to different inlet conditions. Probability density function distributions (pdf) of instantaneous mixture fraction and velocity have been created from which the intermittency has been calculated. The time averaged statistical results for a round jet are first discussed and comparisons of velocity and passive scalar fields between LES calculations and experimental measurements are seen to be good. The calculated probability density distributions show changes from a Gaussian to a delta function with increased radial distance from the jet centreline. The effect of introducing a bluff body into the core flow at the inlet changes the structure of pdfs, but the variation from Gaussian to delta distribution is similar to the jet case. However, the radial variation of the intermittency indicates differences between the results with and without a bluff body at axial locations due the recirculation zone created by the bluff body.