上游带弯头的T型管内混合流动数值模拟的湍流模型评价

T型管是研究冷热流体混合流动及其引起的温度振荡现象的典型几何模型,而上游带弯头的T型管又是一不可忽视的特殊情形。本文运用计算流体力学软件,采用3种湍流模型（RNG k-ε 模型、SSG雷诺应力模型、LES模型）对上游带弯头T型管内冷热流体的交混现象进行模拟,并与实验数据进行了对比。结果表明：非混合区域如上游弯头内,RNG k-ε 模型、SSG雷诺应力模型的模拟结果与实验结果较吻合,而在混合区内LES模型的模拟结果更能表征实际流动。     

平行喷口流动引起温度振荡流固热耦合数值模拟研究

本文基于FLUENT软件,采用大涡模拟(LES)方法对平行喷口冷热流体搅混产生的流固热耦合现象进行了数值模拟。首先将模拟值与实验值进行了对比,验证了模拟方法的正确性。其次,针对多种入口工况进行计算和分析,得出:某一时刻,流体温度处于波谷时,热量从固体传递给流体,下一时刻,当流体温度处于波峰时,热量则从流体传递给固体,故而形成了流固间周期性的热传递现象;随着固体厚度的增加,温度振荡的振幅呈非线性减小,但频率不变;随着入口速度的增加,流体和固体温度振荡的主频逐渐增大,当速度从0.5 m/s变化到1.5 m/s时,主频率可从2 Hz升高到11 Hz。     

竖直圆管内气泡运动的大涡模拟研究

基于两流体模型框架,使用雷诺平均N-S方程(RANS)和大涡模拟(LES)两种湍流模型对竖直圆管内的绝热离散气 液两相流动进行数值模拟研究。计算结果表明,采用恰当的相间相互作用模型,两种模型的时均模拟结果同实验均符合较好。气泡的壁面聚集现象被准确预测,速度场预测也较为准确。与基于RANS的SST湍流模型相比,采用WALE亚网格应力的大涡模拟得到的结果同实验符合得更好,且大涡模拟可给出流动的瞬态细节。     

基于大涡模拟(LES)的格架外条带区域压力和速度瞬态特性研究

分别运用雷诺平均模拟(RANS)方法和大涡模拟(LES)方法对燃料组件格架外条带区域的流场和压力场进行瞬态求解。分析发现:采用2种方法所计算出的速度分布大体一致,但是RANS方法不能捕捉到速度和压力的瞬态变化特性,而LES方法能够有效观察到流场的瞬态特性。通过2种方法的对比,可发现部分局部区域计算结果有较大差异。LES计算结果呈现出计算域内明显的压力波动,频谱分析发现格架不同区域具有不同的压力波动特性。     

等速入口三喷口流动的温度振荡数值模拟

由于来自核反应堆堆芯不同通道冷却剂的温度存在差异,它们在上腔室混合后将产生复杂的温度振荡现象,可能干扰堆芯出口的温度测量,也可能造成周围固体结构的热疲劳破坏。本文基于FLUENT软件,采用大涡模拟湍流模型(LES)对三维平行三喷口流动进行数值模拟,并与实验结果进行了比较。结果表明：随着入口速度的增加,流体混合越来越剧烈,同时温度振荡的最大振幅逐渐减小,而相应的频率则逐渐增加。     

基于大涡模拟的波形板汽水分离器数值研究

采用Lagrange-Euler方法对波形板汽水分离器内离散气-液两相流动进行了数值模拟研究。对于连续相,使用大涡模拟(LES)湍流模型替代常见的雷诺平均模型(RANS)进行数值模拟。大涡模拟方法将湍流分为大、小两种尺度,大尺度涡采用直接数值求解,只对小尺度湍流脉动建立模型。此方法不仅可给出更准确的分离效率等整体性能,同时可得到流动的瞬态细节和湍流脉动对液滴的影响,进而获得更为准确可信的液滴轨迹。计算结果表明,大涡模拟得到的结果同实验结果符合良好;与雷诺平均模型相比,大涡模拟可为汽水分离器的机理研究和优化设计提供更基础的模型。     

基于附加源项法的钠冷快堆冷热池三维分析

选择计算流体动力学(CFD)为模拟手段,建立快堆一回路钠池的三维闭式一体化CFD模型,对一回路中主要部件进行模拟,其中中间热交换器、独立热交换器、堆芯、主泵采用附加源项法进行模拟,得到中国实验快堆(CEFR)额定功率稳态运行时整个流场的三维速度场与温度场。计算值同CEFR设计值进行比较,结果符合预期,证明了模型的合理性。计算结果表明,钠池较明显地分为温度较低的冷钠池和温度较高的热钠池2个部分,热钠池温差较大,冷热流体搅混现象明显;同时冷钠池、热钠池不同高度的平均温度都很接近,说明分隔冷热钠池的热屏蔽效果较好。     

稳压器波动管内热纹振荡现象数值模拟研究

对波动管内热纹振荡现象展开研究,获得波动管内热纹振荡现象的产生原因以及启堆升功率工况下热纹振荡现象造成波动管内壁面温度振荡的幅值和频率。采用计算流体力学(CFD)方法中的动态Smagorinsky涡粘模型对波动管内热纹振荡现象进行了数值模拟。结果表明:波动管内产生热振荡现象的原因是冷热流体相对流动所产生的涡状流使得冷热交界面产生上下振荡;波动管内的热纹振荡现象使得波动管内壁面温度的振荡幅值在4~20℃范围内,频率低于5 Hz。     

稠密栅元不同子通道内湍流流动的RANS和URANS模拟

本工作采用RANS和非稳态雷诺平均纳维斯托克斯模拟（URANS）方法对稠密栅元内典型子通道——中心通道和壁面通道内的湍流流动进行CFD模拟。研究分析了稠密栅元子通道内的不同周向角度的主流速度、壁面剪应力、湍动能等参数。将模拟计算结果和实验测量结果进行对比,结果表明：RANS模拟在采用各向异性的湍流模型的情况下能较好地模拟P/D较大的稠密栅元通道,但对于P/D较小（P/D<1.1）的稠密栅元通道,CFD结果和实验数据存在较大差距。相比之下,URANS方法可模拟紧密栅元子通道间隙区的大尺度、准周期的流动振动,从而和实验数据拟合良好。推荐采用雷诺应力湍流模型(SSG,ORS)进行RANS模拟,而采用SAS湍流模型进行URANS模拟。     

带格架棒束通道交混湍流数值模拟研究

采用雷诺时均模拟(RANS)和大涡模拟(LES)对MATi S-H实验进行模拟计算,得到格架交混后棒束通道内冷态单相湍流流场,通过比较格架下游特定位置处速度分量分布,发现采用精细网格的LES能够较为准确地计算湍流流场平均速度以及脉动速度的分布,与实验结果符合较好。LES结果表明,棒束通道内格架交混湍流流场具有明显的波动,脉动峰值离散分布;子通道内瞬时时刻的涡旋因子SM沿轴向也并非单调衰减,而是具有相对持续的脉动特征,最大脉动值大约是SM最大值的5%;LES的瞬时速度场计算结果可以为进一步的力学分析提供参考。     

Large-Eddy Simulation study of turbulent mixing in a T-junction

A potential cause of thermal fatigue failures in energy cooling systems is identified with cyclic stresses imposed on a piping system. These are generated due to temperature changes in regions where cold and hot flows are intensively mixed together. A typical situation for such mixing appears in turbulent flow through a T-junction, which is investigated here using Large-Eddy Simulations (LES). In general, LES is well capable in capturing the mixing phenomena and accompanied turbulent flow fluctuations in a T-junction. An assessment of the accuracy of LES predictions is made for the applied Vreman subgrid-scale model through a direct comparison with the available experimental results. In particular, an estimation of the minimal mesh-resolution requirements for LES is examined on the basis of the complementary RANS simulations. This estimation is based on the characteristics turbulent scales (e.g., Taylor micro-scale) that can be computed from LES or RANS simulations.     

Large eddy simulation of thermal striping in the upper plenum of the PGSFR

A computational study of thermal striping in the upper plenum of the prototype generation-IV sodium-cooled fast reactor (PGSFR) being developed at Korea Atomic Energy Research Institute is presented. First, previous experimental and numerical studies on the thermal striping are briefly discussed. Both Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) approaches are employed for the simulation of thermal striping in the upper plenum of the PGSFR. For the RANS approach, the conventional k ? ? turbulence model is employed and the LES is performed using the wall-adapting local eddy viscosity model. From the RANS results, the time-averaged velocity components and temperature field in the complicated upper plenum of the PGSFR are calculated. In the LES results, the time history of temperature fluctuation at several locations of upper internal structure (UIS) and intermediate heat exchanger (IHX) are additionally stored. Comparisons of the predicted time-averaged velocity and temperature between the two methods show that the prediction by the LES shows faster thermal mixing than that by the k ? ? turbulence model. From the computed results of the temporal variation of temperature, it was possible to find the amplitude and frequency of the temperature fluctuation at the several locations of the UIS and IHXs. It was found that the location where the thermal stress is largest in the upper plenum of the PGSFR is the ?-shape region of the first grid plate.     

Turbulence-induced Heat Transfer in PBMR Core Using LES and RANS

This paper introduces the results of numerical simulations on flow fields and relevant heat transfer in the pebble bed reactor (PBR) core. In the core, since the coolant passes a highly complicated random flow path with a high Reynolds number, an appropriate treatment of the turbulence is required. A set of simple experiments for the flow over a circular cylinder with heat transfer was conducted to finally select the large eddy simulation (LES) and k-ω model among the considering Reynolds-averaged Navier-Stokes (RANS) models for PBR application. Using these models, the PBR cores, whose geometries were simplified to the body-centered cubical (BCC) and face-centered cubical (FCC) structures, were simulated. A larger pressure drop, a more random flow field, a higher vorticity magnitude and a higher temperature at the local hot spots on the pebble surface were found in the results of the LES than in those of RANS for both geometries. In cases of the LES, the flow structures were resolved up to the grid scales. Irregular distributions of the flow and local heat transfer were found in the BCC core, while relatively regular distributions for the FCC core. The turbulent nature of the coolant flow in the pebble core evidently affected the fuel surface temperature distribution.     

冷热流体混合特性的数值研究

应用三维热工水力程序AQUA对冷热流体混合时发生的热现象进行了数值分析，并用简化基本议程归一化方法对影响该热现象流动参数分布的因素进行了讨论。结果表明，流场与温度场计算结果与归一化分析相符，温度波动计算结果与自然规律一致。这表明AQUA程序能较好地模拟冷热流体混合时发生的热现象。     

Application of large-eddy simulation to pressurized thermal shock problem: A grid resolution study

Life time extension assessment is a very important issue for the nuclear community. A serious threat to the life time extension of a Reactor Pressure Vessel (RPV) is an occurrence of the Pressurized Thermal Shock (PTS) during an Emergency Core Coolant (ECC) injection in a loss-of-coolant accident. Traditional system codes fail to predict the complex three-dimensional flow phenomena resulting from such ECC injection. Improved results have been obtained using Computational Fluid Dynamics (CFD) analysis based on the Reynolds-Averaged Navier-Stokes (RANS) equations. However, it has been also shown that transient RANS approaches are less capable to predict the complex flow features in the downcomer of the RPV. More advanced CFD methods like Large-Eddy Simulation (LES) are required for modeling of these flow phenomena. This paper addresses the feasibility of the application of LES for single-phase PTS. Furthermore, the required grid resolution for such LES analyses is identified by evaluation of the solution on different mesh sizes. A buoyancy-driven PTS experiment has been considered here. This experiment has been performed in the 1:5 linear scale Rossendorf Coolant Mixing Model (ROCOM) facility. In the applied numerical model, the incompressible Navier-Stokes equations are solved in the LES formulation, with an additional transport equation for a scalar, which is responsible for driving the embedded buoyancy term in the momentum equations. Instantaneous mixing characteristics are investigated based on evaluation of the scalar concentration. The analysis presented in this paper indicates that the application of LES is feasible nowadays for single-phase PTS. It is demonstrated that the mixing in the downcomer is quite sensitive to small turbulent disturbances at the ECC inlet, i.e., two simulations performed with slightly different fluctuations at the inlet result in substantially different flow in the downcomer. This complicates the analysis of the data from simulations and suggests that evaluation of the results should be performed in the frequency-amplitude domain instead of the classically employed temporal data analysis.     

Very-Large Eddy Simulation (V-LES) of the flow across a tube bundle

A new turbulence modelling approach (Very-Large Eddy Simulation; V-LES) is developed and compared to conventional RANS and LES for a flow across a tube bundle. The method, which belongs to the large-scale simulation category, represents a good compromise between efficiency and precision, and may thus be used for industrial problems for which LES remains computationally expensive under high to very-high Reynolds number flow conditions. It can also be used for gas-liquid two-phase flows such as pressurized thermal shocks. The method is a sort of blend between U-RANS and LES, in that it resolves very large structures - way larger than the grid size - and models all subscale of turbulence using a two-equation model, by reference to RANS. The original model is shown here to share the same characteristics as the Detached Eddy Simulation (DES) approach, in that when the filter width is smaller than the wall-distance at which viscous effects are negligible (f_μ = 1), the fixed filter width is replaced by the wall distance. First conclusions to be drawn from its extension here is that the flow must be resolved in three-dimensions, under transient conditions, with refined grids. Sensitivity to various computational parameters has been addressed: grid, filter width, domain size, and inflow conditions. This modelling strategy is proved to provide the flow unsteadiness in three-dimensions, while saving computational cost compared to LES. The method is computationally efficient (it can be applied using an implicit solver which permits a higher CFL than with LES; typically 1 versus 0.1), and numerically robust. The computational cost decreases with increasing filter width, though at the expenses of the quality of the results.     

Fluent analysis of a ROSA cold leg stratification test

One of the OECD ROSA project tests, investigating temperature stratification in the cold legs and the downcomer during ECCS water injection under single-phase natural circulation conditions was analysed with the FLUENT code. The guidance given in the “Best Practice Guidelines for the Use of CFD in Nuclear Reactor Safety Applications” of the OECD GAMA group was followed. Steady-state calculations were performed with the Standard k-?, the Realizable k-? and the Reynolds Stress Model, the last one being closest to the measured results. The calculations indicate the predominance of buoyancy effects in the cold leg caused by the density difference between cold and hot water, while in the test it seems, as if mixing between the cold plume and hot water would be the prevailing mechanism. It is shown that the temperature distribution in the downcomer is strongly influenced by correct modelling of the cold leg-downcomer connection. A model with an abrupt transition leads to the colder fluid flowing to the core barrel, while in the test it was flowing down along the vessel wall. Modelling the rounded transition of the ROSA facility shifts the cold stream towards the vessel wall.     

From discovery to recognition of periodic large scale vortices in rod bundles as source of natural mixing between subchannels—A review

The mixing of cooling fluid in rod bundles from one subchannel to another through the gaps between the rods reduces the temperature differences in the coolant as well as along the perimeter of the rods. The phenomenon of natural mixing was first intensively investigated in the 1960s and remains a topic of research up to the present time. The paper describes the main stations on the way to understand the nature of the flow in rod bundles and generally in compound channels with the focus on work performed at Research Center Karlsruhe (FZK).¹Earlier, it was noticed that the mixing rates where higher than could be accounted for by turbulent diffusion alone. For more than 20 years attempts were made to prove experimentally and by code application that secondary flows could account for the measured mixing rates, although the measured secondary flow velocities were much too low. Measurements of the turbulence structure by hot wire anemometry confirmed the existence of cyclic flow pulsations, which had been postulated earlier on the basis of thermocouple measurements. More sophisticated hot wire measurements revealed the nature of these pulsations as periodic, coupled to gap width and Reynolds number. Finally, the extension of the investigation to other compound channel types and flow visualization revealed the true nature of the mixing process as a vortex train moving along the gap between rods or in the narrow part of a compound channel. These findings have been confirmed by LES calculations. Based on these results CFD codes with improved turbulence models calculated successfully the flow in rod bundles including the macroscopic oscillations.     

Numerical analysis of thermal striping induced high cycle thermal fatigue in a mixing tee

Thermal striping, characterized by turbulent mixing of two flow streams of different temperatures that result in temperature fluctuations of coolant near the pipe wall, is one of the main causes of thermal fatigue failure. Coolant temperature oscillations due to thermal striping are on the order of several Hz. Thermal striping high-cycle thermal fatigue that occurs at tee junctions is one of the topics that should be addressed for the life management of light water reactor (LWR) piping systems. This study focuses on numerical analyses of the temperature fluctuations and structural response of coolant piping at a mixing tee. The coolant temperature fluctuations are obtained from Large Eddy Simulations that are validated by experimental data. For the thermal stress fatigue analysis, a model is developed to identify the relative importance of various parameters affecting fatigue-cracking failure. This study shows that the temperature difference between the hot and cold fluids of a tee junction and the enhanced heat transfer coefficient due to turbulent mixing are the dominant factors of thermal fatigue failure of a tee junction.