基于大涡模拟的室内火灾数值模拟

采用数值模拟技术对室内发生火灾后的室内温度及烟气的流动做出模拟。对火焰的燃烧采用UDF编程以实现真实火灾发生时的场景。湍流模型采用LES模型,辐射采用P-1模型。通过温度云图,流场矢量图对火灾发生后,室内温度的变化以及烟气的流动情况做出详细的研究。     

列车车厢内火灾烟气运动的数值模拟研究

使用大涡模拟和Smagorinsky亚格子尺度模型，以及简单化学快速反应燃烧模型，对列车车厢内火灾现象进行了数值模拟。由于热烟气的弱可压缩性，采用了适合于低马赫数形式的三维非稳态Navier—Stokes方程的近似形式数值研究烟气的输运过程。通过对不同释热率情况下火灾过程的研究，初步探讨了车厢内火灾发展、烟气运动和温度分布规律。采用的研究方法和所得结论对于列车火灾的防护设计和安全管理方面有一定指导意义。     

不同烟气再循环率燃油超低NOx燃烧器的大涡模拟

利用Fluent软件中的大涡模拟方法,对燃油锅炉1/10尺缩模型进行不同烟气再循环率的燃烧过程研究,分析0%,20%和30%烟气再循环率下温度场的分布特点以及污染物的排放特性。模拟结果表明：采用烟气再循环能有效降低锅炉的最高温度、平均温度以及污染物排放水平。其中,20%烟气工况降温最为明显,最高温度相较无烟气工况下降400 K,且随着烟气再循环率的增加,焰型缩短,着火点逐渐向炉膛前部移动,温度梯度减小。相较于0%烟气工况,20%和30%烟气工况的NO_x排放浓度分别减少了85.5%和72.7%;20%烟气工况下CO排放水平最低,出口排放浓度为110 mg/m³,而30%烟气工况下Soot排放水平最低,出口体积分数为2.14×10^-5。     

基于大涡模拟的发动机缸内湍流流动及拟序结构

应用大涡模拟方法对发动机缸内湍流流场进行了三维瞬态数值分析.主要从湍流脉动、湍动能和缸内拟序结构演变等方面考察了发动机缸内流场特性.计算结果表明:相比雷诺平均模型,大涡模拟方法可以更真实地反映发动机循环过程中缸内气体流动的细节和规律.利用大涡模拟结合Q准则判别法可以较好地识别缸内大尺度湍流拟序结构;拟序结构对于缸内大尺度动能的产生及湍流的维持具有关键的作用.RANS类模型则不具备充分捕获大尺度拟序结构的能力.湍流脉动与活塞平均运行速度接近于成正比.     

湍流结构及大涡模拟研究

大涡模拟最初用于大气与环境科学的研究,之后利用大涡模拟研究大气越来越广泛并取得多方面的成果．通过对端流的认识,了解研究湍流应具备的条件,由此引出大涡模拟方法在湍流研究中的优势,简单介绍大涡模拟理论．讨论亚格子模型．根据大涡模拟在水力、航天、传热及生态环境等方面已有的应用,说明其应用前景和亟待解决的问题。     

大空间内火灾烟气充填研究

分析了烟气在大空间建筑内的充填情况,得到了不同火源下的烟气填充计算模型,并在大空间实验厅内进行了全尺寸油池火对比试验,试验结果表明,烟气在火空间内的温度并不高,但烟气层下降速度很快,计算结果和试验结果对比表明,在大空间内仍可利用火灾区域模拟方法计算烟气的运动。对于池火,其计算模型的选择应根据火源的大小来确定。     

二阶矩亚网格燃烧模型对射流火焰的大涡模拟

用二阶矩亚网格燃烧模型对美国Sandia国家实验室测量甲烷／空气射流火焰进行了大涡模拟(LES),与实验数据和用二阶矩输运方程湍流燃烧模型的雷诺平均(RANS)模拟结果进行了对比．LES得到时间平均的温度, 甲烷浓度以及温度脉动均方根值和实验值符合很好．LES时均值和RANS模拟结果接近,LES脉动均方根值优于 RANS模拟结果．LES瞬态结果显示了有燃烧时的拟序结构比无燃烧时的强,同时拟序结构强化了燃烧,湍流射流火焰呈皱折火焰面的状态．     

双射流等离子体反应器大涡模拟研究

采用LES方法模拟研究双炬射流形式强化混合的等离子体反应器炬内轴向偏转角与动量比对流场结构及流体混合的影响。从平均流场、瞬时流场、湍流统计分析和混合特征进行了详细的分析,研究结果表明：高温对撞射流会产生偏斜振荡现象,振荡的相互作用会产生涡旋运动,混合流体的流动模式由涡旋对决定。增大偏转角后,涡量强度提升,短距离内温度与速度脉动更加显著,扩散效果更强,热等离子体射流对冷空气的卷吸作用会增强,物料的混合效果更好。     

波瓣混合器的大涡模拟

基于开源软件OpenFOAM,对波瓣混合器中的三维复杂流场结构进行了大涡数值模拟(LES),研究了其主要涡结构的形成及其发展规律。研究表明大涡模拟能够较好地捕捉到波瓣混合器中的涡结构特征。由于波瓣混合器的特殊几何外形,使得流向涡产生于波瓣混合器的波瓣处,并在出口截面形成了反向旋转涡对,随着流向距离的增加,涡对破碎成更小尺度的涡结构。与之相对应的,由于射流剪切层中的K-H不稳定性产生的正交涡形成在波瓣混合器的出口截面,其发展过程与流向涡呈现相似的规律。     

内燃机压缩过程冷态流场的大涡模拟

通过修改发动机多维CFD计算程序KIVA-3V,建立了内燃机压缩过程冷态流场的大涡模拟（LES）计算模型。利用此模型对内燃机压缩过程中缸内流场的水平速度及湍流动能进行了详细分析,并与k-ε模型进行了比较。结果表明与采用k-ε模型计算时相比,采用LES计算时显示了更为复杂的湍流结构,而且LES所能捕捉到的涡团结构范围要大于k-ε模型。同时,采用LES计算时得到的湍流动能要远远低于k-ε模型。     

Large eddy simulation of swirling jet in a bluff-body burner

The large eddy simulation (LES) is applied to an unconfined swirling flow of an air surrounding a bluff-body having a central jet of air, and the complicated flowfield that involves the recirculation and vortex breakdown is investigated. The Smagorinsky model is used as the sub-grid scale model. The results of the present numerical simulation are compared with the experimental data of the mean and stochastic root mean square (RMS) variations of two velocity components. Although the inflow conditions are specified in a simple manner, the obtained numerical results are in reasonable agreement with the experiments, except for a part of RMS variation values near downstream of the bluff body. The present numerical calculations can successfully reproduce the two characteristics of the flow, i.e., an upstream recirculation zone established just downstream of the burner plane and the additional recirculation zone established at the more downstream location.     

Large eddy simulation of coherent structure of impinging jet

The flow field of a rectangular exit, semi-confined and submerged turbulent jet impinging orthogonally on a flat plate with Reynolds number 8500 was studied by large eddy simulation (LES). A dynamic sub-grid stress model has been used for the small scales of turbulence. The evolvements such as the forming, developing, moving, pairing and merging of the coherent structures of vortex in the whole regions were obtained. The results revealed that the primary vortex structures were generated periodically, which was the key factor to make the secondary vortices generate in the wall jet region. In addition, the eddy intensity of the primary vortices and the secondary vortices induced by the primary vortices along with the time were also analyzed.     

Large eddy simulation of turbulent buffet forces in flow induced vibration

The pressure pulse filter and Smagorinsky subgrid stress model of the Large Eddy Simulation (LES) are introduced. The fluid field in the annular plenum between the pressure vessel and the core barrel of the1:5 model in the second phase of Qinshan Nuclear Power Plant is simulated, and the distribution of the total pressure in the space and time domains is obtained. The results show that the Power Spectrum Density (PSD) of LES from the calculation and the test are in the same quantity order. Thus, the pressure of LES can be a load to stimulate the barrel vibration. __________ Translated from Nuclear Power Engineering, 2007, 28(5): 14–17 [译自: 核动力工程]     

Large eddy simulation of a 660 MW utility boiler under variable load conditions

Large eddy simulation (LES) has become a promising tool for pulverized coal combustion with the development of computational fluid dynamics (CFD) technologies in recent years. LES can better capture the unsteady features and turbulent structures of coal jet flame than Reynolds averaged Navier Stokes (RANS). The coal-fired power plants in China are now required to be operated in a wide load range and quickly respond to the electric grid. The boiler performance of variable loads should be evaluated in terms of flow, heat transfer, and combustion processes. In this paper, LES was applied to simulate a 660 MW ultra-supercritical boiler under BMCR (boiler maximum continue rate), 75%THA-100, and 50%THA-100 conditions. The predicted gas velocities agree well with the thermal calculation and the temperature error is less than 130 K. The simulation results show that the operation load has significant effects on the boiler performance. It is also proved that LES can provide guidance for the design and operation of advanced coal-fired boilers.     

Large eddy simulation of flow and scalar transport in a round jet

Large eddy simulation (LES) was performed for a spatially developing round jet and its scalar transport at four steps of Reynolds number set between 1200 and 1,000,000. A simulated domain, which extends 30 times the nozzle diameter, includes initial, transitional, and established stage of jet. A modified version of convection outflow condition was proposed in order to diminish the effect of a downstream boundary. Tested were two kinds of subgrid scale (SOS) models: a Smagorinsky model (SM) and a dynamic Smagorinsky model (DSM). In the former model, parameters are kept at empirically deduced constants, while in the latter, they are calculated using different levels of space filtering. Data analysis based on the decay law of jet clearly presented the performance of SGS models. Simulated results by SM and DSM compared favorably with existing measurements of jet and its scalar transport. However, the quantitative accuracy of DSM was better than that of SM at a transitional stage of flow field. Computed parameters by DSM, coefficient for SGS stresses, C_R and SGS eddy diffusivity ratio, Γ_SGS, were not far from empirical constants of SM. Optimization of the model coefficient was suggested in DSM so that coefficient C_R was nearly equal in the established stage of jet but it was reduced in low turbulence close to the jet nozzle. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(3): 175–188, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20001     

Evaluation of layout and atmospheric stability effects in wind farms using large‐eddy simulation

Large‐eddy simulation (LES) has been used previously to study the effect of either configuration or atmospheric stability on the power generated by large wind farms. This is the first study to consider both stability and wind farm configuration simultaneously and methodically with LES. Two prevailing wind directions, two layouts (turbines aligned versus staggered with respect to the wind) and three stabilities (neutral and moderately unstable and stable) were evaluated. Compared with neutral conditions, unstable conditions led to reduced wake losses in one configuration, to enhanced wake losses in two and to unchanged wake losses in one configuration. Conversely, stable conditions led to increased wake losses in one, decreased wake losses in two and unchanged wake losses in one configuration. Three competing effects, namely, rates of wake recovery due to vertical mixing, horizontal spread of wakes and localized regions of acceleration caused by multiple upstream wakes, were identified as being responsible for the observed trends in wake losses. The detailed flow features responsible for these non‐linear interactions could only be resolved by the LES. Existing analytical models ignore stability and non‐linear configuration effects, which therefore need to be incorporated. Copyright © 2017 John Wiley & Sons, Ltd.     

Large eddy simulations of the effect of vertical staggering in large wind farms

In order to study the effect of vertical staggering in large wind farms, large eddy simulations (LES) of large wind farms with a regular turbine layout aligned with the given wind direction were conducted. In the simulations, we varied the hub heights of consecutive downstream rows to create vertically staggered wind farms. We analysed the effect of streamwise and spanwise turbine spacing, the wind farm layout, the turbine rotor diameter, and hub height difference between consecutive downstream turbine rows on the average power output. We find that vertical staggering significantly increases the power production in the entrance region of large wind farms and is more effective when the streamwise turbine spacing and turbine diameter are smaller. Surprisingly, vertical staggering does not significantly improve the power production in the fully developed regime of the wind farm. The reason is that the downward vertical kinetic energy flux, which brings high velocity fluid from above the wind farm towards the hub height plane, does not increase due to vertical staggering. Thus, the shorter wind turbines are effectively sheltered from the atmospheric flow above the wind farm that supplies the energy, which limits the benefit of vertical staggering. In some cases, a vertically staggered wind farm even produced less power than the corresponding non vertically staggered reference wind farm. In such cases, the production of shorter turbines is significantly negatively impacted while the production of the taller turbine is only increased marginally.     

Investigation of wake interaction using full‐scale lidar measurements and large eddy simulation

In this paper, wake interaction resulting from two stall regulated turbines aligned with the incoming wind is studied experimentally and numerically. The experimental work is based on a full‐scale remote sensing campaign involving three nacelle mounted scanning lidars. A thorough analysis and interpretation of the measurements is performed to overcome either the lack of or the poor calibration of relevant turbine operational sensors, as well as other uncertainties inherent in resolving wakes from full‐scale experiments. The numerical work is based on the in‐house EllipSys3D computational fluid dynamics flow solver, using large eddy simulation and fully turbulent inflow. The rotors are modelled using the actuator disc technique. A mutual validation of the computational fluid dynamics model with the measurements is conducted for a selected dataset, where wake interaction occurs. This validation is based on a comparison between wake deficit, wake generated turbulence, turbine power production and thrust force. An excellent agreement between measurement and simulation is seen in both the fixed and the meandering frame of reference. Copyright © 2015 John Wiley & Sons, Ltd.