Benchmark Simulation of Turbulent Flow through a Staggered Tube Bundle to Support CFD as a Reactor Design Tool. Part II: URANS CFD Simulation

In Part II, we described the unsteady flow simulation and proposed a modification of a traditional turbulence flow model. Computational fluid dynamics (CFD) simulations of an isothermal, fully periodic flow across a tube bundle using unsteady Reynolds averaged Navier-Stokes (URANS) equations, with turbulence models such as the Reynolds stress model (RSM) were investigated at a Reynolds number of 1.8 × 10⁴, based on the tube diameter and inlet velocity. As noted in Part I, CFD simulation and experimental results were compared at five positions along (x; y) coordinates. The steady RANS simulation showed that four diverse turbulence models were efficient for predicting the Reynolds stresses, and generally, SRANS results were marginal to poor, using a consistent evaluation terminology. In the URANS simulation, we modeled the turbulent flow field in a manner similar to the approach used for large eddy simulation (LES). The time-dependent URANS results showed that the simulation reproduces the dynamic stability as characterized by transverse oscillatory flow structures in the near-wake region. In particular, the inclusion of terms accounting for the time scales associated with the production range and dissipation rate of turbulence generates unsteady statistics of the mean and fluctuation flow. In spite of this, the model implemented produces better agreement with a benchmark data set and is thus recommended.     

Experimental Study of the Unsteady Actuation Effect on Induced Flow Characteristics in DBD Plasma Actuators

The main aim of this paper is to investigate unsteady actuation effects on the operation of dielectric barrier discharge(DBD) plasma actuators and to study induced flow characteristics of steady and unsteady actuators in quiescent air.The parameters affecting the operation of unsteady plasma actuators were experimentally measured and compared with the ones for steady actuators.The effects of excitation frequency and duty cycle on the induced flow pattern properties were studied by means of hot-wire anemometers,and the smoke visualization method was also used.It was observed that the current and the mean induced velocity linearly increase with increasing duty cycle while they are not sensitive to excitation frequency.Furthermore,with increasing excitation frequency,the magnitude of vortices shedding from the actuator decreases while their frequency increases.Nevertheless,when the excitation frequency grows beyond a certain level,the induced flow downstream of the actuator behaves as a steady flow.However,the results for steady actuators show that by increasing the applied voltage and carrier frequency,the velocity of the induced flow first increases and then decreases with actuator saturation and the onset of the emission of streaky glow discharge.     

Simulation of Flow Around Cylinder Actuated by DBD Plasma

The electric-static body force model is obtained by solving Maxwell's electromagnetic equations. Based on the electro-static model, numerical modeling of flow around a cylinder with a dielectric barrier discharge(DBD) plasma effect is also presented. The flow streamlines between the numerical simulation and the particle image velocimetry(PIV) experiment are consistent. According to the numerical simulation, DBD plasma can reduce the drag coefficient and change the vortex shedding frequencies of flow around the cylinder.     

Effect of DBD plasma excitation characteristics on turbulent separation over a hump model

In this paper, the effect of dielectric-barrier discharge plasma excitation characteristics on turbulent boundary layer separation over a hump is investigated using computational fluid dynamics. Four different turbulence models were used for verification. The Reynolds stress model showed the best agreement with the experimental data, in general. Based on the verification and validation, the effect of duty cycle and excitation frequency on the turbulent flow separation were investigated. The results showed that the pulsed plasma excitation could effectively suppress the flow separation by mixing augmentation. With increasing duty cycle and excitation frequency, the flow separation first increased, then decreased again. The optimal duty cycle was 0.75 and the optimal excitation frequency was 50 Hz.     

Numerical study of the flow structures in flat plate and the wall-mounted hump induced by the unsteady DBD plasma

In this work,the dielectric-barrier-discharge plasma actuator was employed to study the flow structures induced by the plasma actuator over a flat plate and a wall-mounted hump.A phenomenological dielectric-barrier-discharge plasma model which regarded the plasma effect as the body force was implemented into the Navier–Stokes equations solved by the method of large eddy simulations.The results show that a series of vortex pairs,which indicated dipole formation and periodicity distribution were generated in the boundary layer when the plasma was applied to the flow over a flat plane.They would enhance the energy exchanged between the near wall region and the free stream.Besides,their spatial trajectories are deeply affected by the actuation strength.When the actuator was engaged in the flow over a wall-mounted hump,the vortex pairs were also produced,which was able to delay flow separation as well as to promote flow reattachment and reduce the generation of a vortex,achieving the goal of reducing dissipation and decreasing flow resistance.     

基于核主泵性能预测的数值模拟精度研究

为提高核主泵在全工况点的数值模拟精度,研究了数值模拟过程中近壁面网格尺度、湍流模型、流动状态3种因素对计算精度的影响。结果表明,在定常状态下,重整化群（RNG） k-ε湍流模型和标准壁面函数法在近壁面网格尺度（y+）为50左右时具有较高的计算精度,并且其计算精度高于RNG k-ε增强壁面函数法、低雷诺数k-ε和剪切应力传输（SST）k-ω这3种湍流模型的计算精度,但上述不同网格尺度和湍流模型的计算结果均存在较大的计算误差;采用非定常计算时的计算精度明显高于定常计算,能够反映出扬程曲线在关死点附近的驼峰现象,效率的计算精度也有一定改善,更适合于对核主泵进行性能预测。      

Numerical Investigation of Plasma Active Flow Control

Based on the theory of EHD (electronhydrodynamic) ,a simplified volume force model is applied to simulation to analyze the traits of plasma flow control in flow field, in which the cold plasma is generated by a DBD (dielectric-barrier-discharge) actuator. With the paraelectric action of volume force in electric field, acceleration characteristics of the plasma flow are investigated for different excitation intensities of RF(radio frequency) power for the actuator. Furthermore, the plasma acceleration leads to an asymmetric distribution of flow field, and hence induces the deflection of jet plume , then results in a significant deflection angle of 6.26o thrust-vectoring effect. It appears that the plasma flow control technology is a new tentative method for the thrust-vectoring control of a space vehicle.     

Benchmark Simulation of Turbulent Flow through a Staggered Tube Bundle to Support CFD as a Reactor Design Tool. Part I: SRANS CFD Simulation

Time-invariant and time-variant numerical simulations of flow through a staggered tube bundle array, idealizing the lower plenum (LP) subsystem configuration of a very high temperature reactor (VHTR), were performed. In Part I, the CFD prediction of fully periodic isothermal tube-bundle flow using steady Reynolds-averaged Navier-Stokes (SRANS) equations with common turbulence models was investigated at a Reynolds number (Re) of 1.8 × 10⁴, based on the tube diameter and inlet velocity. Three first-order turbulence models, standard k-ε turbulence, renormalized group (RNG) k-ε, and shear stress transport (SST) k-ω models, and a second-order turbulence model, Reynolds stress model (RSM), were considered. A comparison of CFD simulations and experiment results was made at five locations along (x, y) coordinates. The SRANS simulation showed that no universal model predicted the turbulent Reynolds stresses, and generally, the results were marginal to poor. This is because these models cannot accurately model the periodic, spatiotemporal nature of the complex wake flow structure.     

三角形排列的紧密栅元棒束内流动行为的数值模拟

对三角形排列紧密栅元通道内的空气湍流流动进行了数值研究,系统考察了涡粘性和雷诺应力两类湍流模型模拟紧密栅元通道内流动特征的适用性.结果表明:SSG雷诺应力模型对流动有较好的模拟,这说明湍流各项异性的模拟在紧密栅元中十分重要;不同雷诺数和几何结构下的模拟显示,二次流的大小和雷诺数的相关性不大.但随着棒间距和棒径比(P/D)的增大,二次流减小.     

Numerical analysis of flowing characteristics of turbulent flow in rectangular channels in ocean environment

Because of the periodic effects of ocean waves, there are great discrepancies between the operational characteristics of nuclear power systems in ocean environment and that of land-based nuclear power systems. In some special operational status, like natural circulation, the additional forces due to ocean environment may impose so great disturbance on the coolant flow that theatres the safety operation of the systems. In the present paper, the turbulent flow in rectangular channels in ocean environments is investigated theoretically with CFD code FLUENT. The effects of several parameters on turbulent flow are analyzed. The effects of rolling motion includes two parts, the first part is the additional force parallel to flowing direction, which can affect on the pressure drop of the flow and change the flowing velocity, and the other part is the additional force perpendicular to flowing direction. In ocean environments, the flowing characteristics of turbulent flow are dominated by the additional force parallel to flowing direction. The effect of additional force perpendicular to flowing direction is very limited. In rolling and heaving motions, if the flowing velocity is the same, the flowing characteristics of turbulent flow are nearly the same, too. The bigger the Reynolds number is, the more serious the oscillation of turbulent kinetic energy and frictional resistance coefficient is, and the more the oscillation of turbulent flow is. The relationship between average frictional resistance coefficient and velocity oscillating amplitude is quadratic. And the oscillating amplitude of frictional resistance coefficient is in direct ratio with velocity oscillating amplitude.     

Development of flow-induced vibration evaluation methodology for large-diameter piping with elbow in Japan sodium-cooled fast reactor

This paper describes the current status of flow-induced vibration evaluation methodology development for the primary piping in Japan sodium-cooled fast reactor, with particularly emphasis on the development approach and research activities that investigate unsteady hydraulic characteristics in a short-elbow piping. The approach to the methodology development was defined: experiment-based methodology and simulation-based one as well as extrapolation logic to the reactor condition based on no dependency on Reynolds number in the high Reynolds number range from the experimental results. Experimental efforts have been made using 1/3-scale single-elbow test sections for the hot-leg piping as the main activity. Recent experiments using the 1/3-scale test section revealed that a swirl flow at the inlet of the hot-leg piping hardly influenced pressure fluctuations onto the pipe though a slight deformation of flow separation was observed. Numerical results under different Reynolds number conditions appear in this paper using the unsteady Reynolds Averaged Navier Stokes equation approach, indicating its applicability to the hot-leg piping experiments.     

The Hall effect on an unsteady flow due to a rotating infinite disc

An analysis of the effects of Hall currents on the unsteady flow of an electrically conducting, viscous, incompressible fluid due to a rotating disk has been carried out. It is assumed that the flow is under the action of a transverse magnetic field and the magnetic Reynolds number is small. Approximate solutions to the velocity and the torque are derived for small values of the time t. They are shown graphically. It is observed that the torque-coefficient decreases with an increase in the Hall parameter.     

Wave propagation in two-phase flow of magnetic fluid under a nonuniform magnetic field

The effect of nonuniform magnetic field on the linear and nonlinear wave propagation phenomena in two-phase pipe flow of magnetic fluid is investigated theoretically to realize the effective energy conversion system using boiling two-phase flow of magnetic fluid. Firstly, the governing equations of two-phase flow based on the unsteady thermal nonequilibrium two-fluid model are presented and the linear void wave propagation phenomena in boiling two-phase flow are numerically analyzed by using the finite volume method. Next, the nonlinear pressure wave propagation in gas-liquid two-phase flow is numerically analyzed by using the finite different method. According to these theoretical studies on the wave propagation phenomena in two-phase flow of magnetic fluid, it seems to be a reasonable proposal that the precise control of the wave propagation in two-phase flow is possible by effective use of the magnetic force.     

应用CFD方法研究雷诺数对烟囱气体混合均匀性影响北大核心CSCD

为研究各种流态下核设施烟囱内气体混合均匀性情况,应用计算流体力学(CFD)方法,建立了仿真模型,主烟囱内雷诺数范围800~70000。仿真结果表明:雷诺数变化对于主烟道风速分布有重要影响;8倍水力直径以下,随监测截面升高,风速分布将更为均匀,8倍水力直径以上,一定程度内增强湍流,可提高风速分布均匀性,流态处于完全湍流后,继续提高雷诺数对风速的分布均匀性无益;对于示踪气体,各监测截面均达到了较充分的混合,管道内雷诺数低于29000时,其在各截面上的混合均匀性伴随雷诺数升高有细微的降低,雷诺数超过29000后混合均匀性变化不再显著。对比仿真结果与试验结果,风速及示踪气体浓度仿真结果与试验测量值具有较好的一致性。     

Simulation of turbulent flow inside different subchannels in tight lattice bundle

In this paper, both steady and unsteady Reynolds Averaged Navier Stokes (RANS and URANS) methodology are applied to the prediction of turbulent flow inside different subchannels in tight lattice bundles.Two typical configurations of subchannels (i.e., wall subchannel and center subchannel) are chosen to be investigated. In this work the application of different turbulence models implemented in the commercial code CFX v12 is shown. The validity of the methodology is assessed by comparing computational results of axial velocity, wall shear stress and turbulent intensity distributions with the experimental data (Krauss, 1996; Krauss and Meyer, 1998). This study shows that RANS simulation with anisotropic turbulent model produces excellent agreement with experiment, whereas it failed to predict the flow behavior accurately in the case of tightly packed geometries (P/D < 1.1). On the other hand, the URANS simulation is in good agreement with the results in tightly packed geometries with flow oscillation in the gap region. The effects of the Reynolds number and the bundle geometry on the flow oscillation are investigated in details.     

雷诺数关系式对膜状凝结湍流区传热计算的影响

膜状凝结现象广泛存在于核电站安全壳和稳压器中。关于膜状凝结液膜湍流区的传热模型,目前未明确辨析基于质量和能量关系的两种雷诺数关系式的差别。本文针对管外纯蒸汽自然对流膜状冷凝传热,定量地分析雷诺数关系式对膜状凝结液膜湍流区传热计算的影响。基于液膜湍流区修正项的一般性假设,推导了膜状凝结湍流区传热系数的表达式。同时,分别与雷诺数关系式Remass和Reenergy联立,求解得到不同雷诺数关系式之间以及对应的膜状凝结传热系数之间的关系。分析表明:受普朗特数Pr的影响,在膜状凝结液膜湍流区,雷诺数关系式Remass和Reenergy差别明显,并存在关于Pr的分界点。基于Remass和Reenergy得到的膜状凝结平均传热系数及其相对偏差是Re和Pr的非线性函数。当0.1Pr4.0且Re1 600时,基于Reenergy和Remass得到的膜状凝结平均传热系数相对偏差在-60%和+60%之间。通过实验和理论验证,在膜状凝结液膜湍流区基于Reenergy得到的膜状凝结传热系数更加准确。     

Turbulent drag reduction by spanwise slot blowing pulsed plasma actuation

This work studies the turbulent drag reduction (TDR) effect of a flat plate model using a spanwise slot blowing pulsed plasma actuator (SBP-PA). Wind tunnel experiments are carried out under a Reynolds number of 1.445 × 104. Using a hot-wire anemometer and an electrical data acquisition system, the influences of millisecond pulsed plasma actuation with different burst frequencies and duty cycles on the microscale coherent structures near the wall of the turbulent boundary layer (TBL) are studied. The experimental results show that the SBP-PA can effectively reduce the frictional drag of the TBL. When the duty cycle exceeds 30%, the TDR rate is greater than 11%, and the optimal drag reduction rate of 13.69% is obtained at a duty cycle of 50%. Furthermore, optimizing the electrical parameters reveals that increasing the burst frequency significantly reduces the velocity distribution in the logarithmic region of the TBL. When the normalized burst frequency reaches f+ = 2πfpd/U∞ = 7.196, the optimal TDR effectiveness is 16.97%, indicating a resonance phenomenon between the pulsed plasma actuation and the microscale coherent structures near the wall. Therefore, reasonably selecting the electrical parameters of the plasma actuator is expected to significantly improve the TDR effect.     

Numerical Investigation of Gas Flow Distribution and Thermal Mixing in Helically Coiled Tube Bundle

In this study, we carried out the numerical simulation of gas flow across a helically coiled tube bundle, which is fitted in the annular space between a central spine and an outer casing. We focused on the effect of clearances at the inner and outer radial boundaries on the gas flow distribution and thermal mixing in the tube bundle. The conservation equations of mass, momentum and energy were numerically solved by computational fluid dynamics. It was demonstrated from the numerical simulation results that (i) the gas flow distribution in the tube bundle is very sensitive to clearances at the inner and outer radial boundaries, and even a very small variation of clearances would lead to a considerable change in the average velocity profile, which correspondingly leads to a great deviation of the gas flow distribution away from the original one; (ii) the clearance effect on thermal mixing is not as strong as that on gas flow distribution;(iii) the Reynolds number has little effect on the gas flow distribution and thermal mixing, the latter of which is in agreement with the results of Achenbach experiments.     

Vortex dynamics over an airfoil controlled by a nanosecond pulse discharge plasma actuator at low wind speed

The vortex dynamics of flow over an airfoil controlled by a nanosecond pulse dielectric-barrierdischarge(NS-DBD) actuator is studied at a Reynolds number of 1?×?10~5 through wind tunnel experiments and numerical simulation. The numerical method is validated through comparison of the simulated and measured results regarding the effect of the discharge of an NS-DBD actuator placed on a flat plate. The simulated results show that vorticity is mainly induced by the baroclinic torque after plasma discharge, i.e. the term(■) in the equation of vorticity evolution. Both experimental and simulated results demonstrate that after the discharge of the NS-DBD actuator a series of vortices are developed in the shear layer and pull the high-moment fluid down to the wall, enhancing the mixing of internal and external flows.     

Large Eddy Simulation of the Effects of Plasma Actuation Strength on Film Cooling Efficiency

In this article, numerical investigation of the effects of different plasma actuation strengths on the film cooling flow characteristics has been conducted using large eddy simulation(LES). For this numerical research, the plasma actuator is placed downstream of the trailing edge of the film cooling hole and a phenomenological model is employed to provide the electric field generated by it, resulting in the body forces. Our results show that as the plasma actuation strength grows larger, under the downward effect of the plasma actuation, the jet trajectory near the cooling hole stays closer to the wall and the recirculation region observably reduces in size. Meanwhile, the momentum injection effect of the plasma actuation also actively alters the distributions of the velocity components downstream of the cooling hole. Consequently, the influence of the plasma actuation strength on the Reynolds stress downstream of the cooling hole is remarkable. Furthermore, the plasma actuation weakens the strength of the kidney shaped vortex and prevents the jet from lifting off the wall. Therefore, with the increase of the strength of the plasma actuation, the coolant core stays closer to the wall and tends to split into two distinct regions. So the centerline film cooling efficiency is enhanced, and it is increased by 55% at most when the plasma actuation strength is 10.