超临界压力下考虑密度脉动的湍流模型开发

目前超临界水传热数值程序中使用的湍流模型都是针对亚临界压力下常物性开发的,未考虑密度脉动。超临界水的物性随温度变化显著,密度脉动对湍流模型的贡献很大,不可忽略。本文建立了考虑密度脉动的模拟方法,模拟方法中同时考虑了热膨胀系数的脉动。将建立的密度脉动模型用于AKN湍流模型中,并采用实验数据进行验证评价。评价结果表明,考虑密度脉动的湍流模型模拟得到的结果与实验值符合得更好。建议在以后的超临界水传热的湍流模拟中考虑密度脉动。     

超临界水四棒束传热数值分析

超临界水冷堆(SCWR)开发的关键是棒束内超临界水(SCW)的热工水力特性。本文针对超临界水四棒束流动传热实验进行CFD数值模拟,SSG湍流模型的计算结果与实验结果吻合良好。分析结果表明,流动方向对棒束截面内流量分布有显著影响。与下降流相比,尽管上升流时棒束间流动搅混较弱,但上升流时棒束截面流量及壁面周向温度分布更加均匀,加热棒壁面温度更低。可见,棒束横截面上的流量分布是影响加热棒壁面流动传热的主要因素。     

带格架四棒束超临界水流动传热数值分析

棒束内超临界水流动传热是超临界水堆堆芯热工水力研究的重要内容,但对其认识还十分有限。本文针对四棒束内超临界水的流动传热现象开展数值模拟,特别分析了定位格架对棒束通道内流动和传热的影响。结果表明,采用SSG湍流模型计算所得到的棒束壁面温度和实验结果吻合良好,定位格架的存在影响下游流体的速度分布,显著提高格架下游的传热特性,交混系数有大幅上升,使得加热棒周向壁面温度分布更加平均,最高温度出现位置发生改变。     

10 mm垂直单管通道内超临界水传热弱化现象的实验与数值分析

超临界压力下的流体因拟临界点附近物性的剧烈变化,形成了非常奇特的传热现象。因流体密度突变,在低流量下会引起强烈的浮升力作用,对超临界流体的流动和传热均有极大影响。本工作通过实验获得10 mm单管内传热弱化现象的实验数据,并采用改进的低雷诺数湍流模型,使用数值方法模拟该传热弱化现象。计算结果表明,不同于以往传统的模型会高估壁面温度,改进的低雷诺数湍流模型能较好预测实验结果。数值模拟结果还揭示了浮升力对湍流剪切应力和速度分布的影响,进而引起传热弱化和传热恢复。     

竖直圆管内超临界流体混合对流传热特性理论分析

理论推导了变密度引起的浮升力效应和流动加速效应对超临界流体混合对流传热特性的影响。结果表明,浮升力效应和流动加速效应通过改变壁面边界层外缘的切应力影响湍流对传热传质的贡献,进而改变超临界流体混合对流传热特性。浮升力效应通常在加热区域入口及上游区域表现明显,而流动加速效应在主流区流体温度达到拟临界温度时更显著。与实验研究结果对比发现,新建立的浮升力因子和流动加速因子可较好地预测竖直圆管内超临界流体混合对流条件下拟临界区域的局部传热特性。     

超临界压力CO2在垂直管内对流换热数值模拟

对于超临界压力CO2在垂直圆管(din=2 mm)内高进口雷诺数(Re=9 000)条件下向上流动时的对流换热进行了数值模拟.通过与实验数据进行对比来验证湍流模型的可靠性,并研究变物性和浮升力对壁面温度和湍动能的影响.结果表明:在热流密度较高的情况下,向上流动时出现了局部换热恶化和换热强化现象,这主要归因于浮升力对湍动能分布的影响;采用LB湍流模型能较好地模拟这种换热现象;在热流密度较低的情况下,未出现上述换热现象.     

二维瞬态熔融池传热特性分析程序开发与验证

针对高瑞利数熔融池自然对流传热特性实验(COPRA),建立了熔融池自然对流传热模型,并基于SIMPLE算法开发了二维瞬态熔融池传热特性分析程序。熔融池模型采用近壁修正的低雷诺数(Re)k-ε湍流模型模拟全区域的自然对流,同时考虑了熔融物硬壳凝固熔化的相变传热过程,以及硬壳与下封头壁面的导热温度场计算。采用瞬态分析程序针对COPRA实验工况进行模拟,计算得到的熔融池温度、圆弧壁面热流密度和硬壳厚度结果与COPRA实验数据符合得较好,验证了本文模型和二维瞬态熔融池传热特性分析程序的可靠性。     

三角形子通道超临界水热工水力特性数值分析

目前国际上对超临界水冷堆进行了大量的研究,但对其堆芯内超临界流体流动传热的认识还十分有限.本文采用CFX对超临界水冷堆典型三角形子通道内的流动传热特征进行了CFD研究,对比分析了包壳壁面等热流密度和燃料芯块等体积热流密度两种情况.计算结果表明,不锈钢包壳层的周向导热显著强化了燃料棒圆周上温度分布和传热系数的均匀性,但对二次流和湍流脉动的影响不大.间隙区的湍流脉动主要受几何参数P/D的影响,当P/D<1.3时,湍流交混系数在0.02～0.025之间,当P/D>1.3时,湍流交混系数较小,在温度拟临界点附近区域,存在交混系数的突变.     

超临界水流动传热特性影响因素数值模拟研究

以计算流体力学(CFD)商业软件FLUENT为计算平台,通过进行网格敏感性分析和湍流模型比较,选取最优化的网格和最佳湍流模型,对圆管和圆环通道内超临界水流动传热特性进行数值模拟,研究通道的几何结构、特征距离lT以及水物性对超临界水流动传热特性的影响。结果表明,热力学当量直径对流动传热特性影响不大,可以忽略;水力学当量直径、特征距离lT以及水的物性都对超临界水流动传热特性有很大的影响。     

液态金属钠在环管中湍流传热特性研究

采用高雷诺数(Re)k-ε模型与壁面函数法对液态金属钠在环管中湍流流动传热特性进行计算,并与实验结果进行比较,结果表明计算值与实验结果符合较好。应用该方法研究湍流程度、加热条件、几何条件等因素对液态金属钠在环管中湍流传热特性的影响。研究表明,湍流程度对传热的影响主要集中在流道前半段,后半段分子扩散对传热的影响逐渐凸现出来,使不同湍流程度下传热特性的区别逐渐缩小;初始温度与加热热流密度对传热特性无明显影响;环管间隙增大,湍流传热效果增强,同等间隙时管径变化对传热特性无明显影响。     

Numerical analysis of turbulent mixed convection air flow in inclined plane channel with k-ω type turbulence model

Numerical study on turbulent mixed convection in inclined plane channels, from 15° to 90° (vertical), was carried out to examine the effect of inclination on fluid flow and heat transfer distributions. The turbulent air flows upward or downward into the duct with one wall heated from bottom. Calculation results with several kinds of k-εtype turbulence models were used to compare the experimental data with those in literatures to determine suitable model. The dependents of Nusselt number on the inclination angle of both the buoyancy-aided and buoyancy-opposed flow are discussed.     

Numerical analysis of turbulent mixed convection air flow in inclined plane channel with k-ε type turbulence model

Numerical study on turbulent mixed convection in inclined plane channels, from 15° to 90° (vertical), was carried out to examine the effect of inclination on fluid flow and heat transfer distributions. The turbulent air flows upward or downward into the duct with one wall heated from bottom. Calculation results with several kinds of k-εtype turbulence models were used to compare the experimental data with those in literatures to determine suitable model. The dependents of Nusselt number on the inclination angle of both the buoyancy-aided and buoyancy-opposed flow are discussed.     

Computations of post-trip reactor core thermal hydraulics using a strain parameter turbulence model

Coolant flows in the cores of nuclear reactors consist of ascending vertical flows in a large number of parallel passages. Under post-trip conditions such heated turbulent flows may be modified strongly from the forced convection condition by the action of buoyancy, in particular exhibiting impaired levels of heat transfer with respect to corresponding forced convection cases. The heat transfer performance of these ‘mixed convection’ flows is investigated here using two physically distinct eddy viscosity turbulence models: the recent ‘strain parameter’ (or k––S) model of Cotton and Ismael [A strain parameter turbulence model and its application to homogeneous and thin shear flows. Int. J. Heat Fluid Flow 19 (1998) 326] is examined against the benchmark low-Reynolds-number k– model of Launder and Sharma [Application of the energy-dissipation model of turbulence to the calculation of flow near a spinning disc. Lett. Heat Mass Transfer 1 (1974) 131]. Comparison is made with three sets of heat transfer data for ascending mixed convection flows, and it is demonstrated that both turbulence models are generally successful in resolving the Nusselt number distributions occurring along the lengths of mixed convection flow passages. The mechanisms by which the strain parameter model generates reduced turbulence levels, and hence impaired heat transfer rates, is explored in comparison with a fourth set of experimental data for mixed convection flow profiles.     

湍流模型对瞬态流固耦合换热数值模拟影响分析

采用ANSYS Fluent程序建立了固体加热功率指数增长的气体冷却模型,分析了瞬态过程中的流动传热特性,通过数值模拟结果与3组实验数据进行对比,研究了标准k-ε模型、标准k-ω模型、过渡SST模型和RSM模型4种典型湍流模型对瞬态过程中流固耦合换热数值模拟结果的影响。通过分析发现：热负载指数提升过程中,热功率一部分用于对流换热,而另一部分仍留在固体内,且热功率提升速率越高,对流换热的占比越低;采用不同湍流模型模拟出的等效表面热流密度均低于实验值,且采用ε类湍流模型的模拟结果与实验数据更接近。     

A numerical investigation of natural convection heat transfer in horizontal spent-fuel storage cask

Natural convection heat transfer in a horizontally placed dry spent-fuel storage cask is numerical investigated. The commercial computational fluid dynamics (CFD) code, -3.2 is used and the laminar and turbulent model are employed. The numerical predictions obtained are compared with the experimental data reported by Nishimura et al. [J. Nucl. Sci. Technol. 33 (1996) 821]. The computational results corresponding to laminar model agree well with the experimental data, but the calculated results of turbulent model are higher. The velocity pattern and the isotherms are drawn. With the increasing of Rayleigh number, the heat transfer in the cask changes from conduction dominant mode to convection dominant mode. In the condition of Ra_m=1.3×10⁹, turbulent model prevails. The convective heat transfer is so strong that almost all temperature changes take place in the region near the wall of the cask. The Rayleigh number Ra_m and the Nusselt number Nu_m characterized by maximum temperature difference are defined to depict the heat transfer characteristics. It is found laminar and turbulent models predict the same trend but different value. The flow patterns in the cask can be divided to three regimes. In these three regimes, modified Nusselt numbers are proportional to the 0.7, 0.25 and 0 power of the modified Rayleigh number, respectively.     

Thermal hydraulic behavior in the deteriorated turbulent heat transfer regime for a gas-cooled reactor

One of the key issues in a gas-cooled system with a passive heat removal measure is the correct prediction of thermal hydraulic behavior in the deteriorated turbulent heat transfer (DTHT) regime. Here, a simple one-dimensional analysis of a gas-cooled reactor system was performed to show its strong potential for operation in the DTHT regime. A numerical calculation using computational fluid dynamics (CFD) was done to verify the capability of the Launder-Sharma (LS) turbulence model to predict wall and bulk temperatures while a gas system is operating in the DTHT regime. The results by the LS model are compared to those by the high Reynolds turbulent model, a newly proposed correlation, and recently obtained experimental data. The comparison shows that the LS turbulence model can provide a reasonable prediction in the buoyancy-induced DTHT regime. Additionally, it was found that the LS model predicted strong laminarization near the entrance which deteriorated turbulent heat transfer further compared to the measurements, but re-turbulization took place much earlier than in the experiment which recovered the heat transfer to a normal turbulent heat transfer.     

大空间内底部弧形加热段自然对流传热特性研究

为准确预测安全壳上封头的自然对流换热特性以保证堆芯余热安全排出,设计了采用底部弧形加热段的矩形封腔自然对流装置,研究导热率对底部弧形加热段和封腔内流体温度分布的影响,并基于开源软件Open FOAM,采用数值模拟方法对比分析2种湍流模型和3种湍流热通量模型的适用性。结果表明,流体沿弧形面的流动受边界层和绕流脱体强化现象的影响,局部自然对流换热强度从顶部向两端先减小后增大;材料热导率对弧形面的温度分布影响比较大,但对于加热段外的流体温度分布影响极小;经过对AFM模型进行修正,得到了更适用于实验条件的模型参数值,修正后的模型对流体速度场的模拟更为准确且在更高功率工况下也得到验证。本研究可为后续方案设计的有效性评价提供参考。     

Evaluation of system codes for analyzing naturally circulating gas loop

Steady-state natural circulation data obtained in a 7 m-tall experimental loop with carbon dioxide and nitrogen are presented in this paper. The loop was originally designed to encompass operating range of a prototype gas-cooled fast reactor passive decay heat removal system, but the results and conclusions are applicable to any natural circulation loop operating in regimes having buoyancy and acceleration parameters within the ranges validated in this loop. Natural circulation steady-state data are compared to numerical predictions by two system analysis codes: GAMMA and RELAP5-3D. GAMMA is a computational tool for predicting various transients which can potentially occur in a gas-cooled reactor. The code has a capability of analyzing multi-dimensional multi-component mixtures and includes models for friction, heat transfer, chemical reaction, and multi-component molecular diffusion. Natural circulation data with two gases show that the loop operates in the deteriorated turbulent heat transfer (DTHT) regime which exhibits substantially reduced heat transfer coefficients compared to the forced turbulent flow. The GAMMA code with an original heat transfer package predicted conservative results in terms of peak wall temperature. However, the estimated peak location did not successfully match the data. Even though GAMMA's original heat transfer package included mixed-convection regime, which is a part of the DTHT regime, the results showed that the original heat transfer package could not reproduce the data with sufficient accuracy. After implementing a recently developed correlation and corresponding heat transfer regime map into GAMMA to cover the whole range of the DTHT regime, we obtained better agreement with the data. RELAP5-3D results are discussed in parallel.     

液态铅铋合金湍流普朗特数及RANS模型优选

工程上常采用RANS湍流模型进行热工水力相关的数值模拟,然而液态铅铋合金（LBE）具有独特的热物性,常规湍流普朗特数模型和RANS湍流模型对其流动与传热模拟的适用性有待研究。为更准确地描述绕丝燃料组件内LBE的流动与换热过程,本文基于大涡模拟对湍流普朗特数模型和RANS湍流模型进行优选。首先,采用四种湍流普朗特数模型对绕丝燃料组件内LBE的流动与传热过程进行大涡模拟,对比分析实验数据和模拟结果并进行模型优选。基于优选的湍流普朗特数模型,评价RANS湍流模型对LBE数值模拟的适用性和准确性。结果表明,Cheng湍流普朗特数模型和SST k-ω模型对LBE流动与传热模拟的准确性和适用性最高。