文丘里式气泡发生器工作特性分析

气泡发生器是钍基熔盐堆脱气系统的关键部件,其功用为将载气碎化成尺寸均匀的小气泡。本文在脱气系统水实验回路实验研究基础上,采用数值模拟方法,应用SIMPLEC算法对标准的k-ε湍流模型和多相流混合模型进行耦合求解,分析了沿气泡发生器流动方向的气液两相流场速度变化、压力变化、湍动能分布规律。沿流向的速度分布表明,气相从喉部开始沿壁面流动,包围位于中心区域的水相,气相速度在扩张段入口处明显降低,速度梯度的变化形成剪切,使得气相破碎、分裂;压力分布表明,在气泡发生器的扩张段入口附近出现了压力梯度的峰值,与实验中测得的气泡集中碎化的位置相近,说明压力的迅速回升可能加速了气泡的碎化;湍动能分布表明,扩张段出口湍动能相对较大,说明此处气液两相能量交换强烈,产生强烈的剪切应力,使气液两相彼此剪切、破碎。以上结果说明,扩张端入口处由于较大的速度梯度及湍动能峰值,导致产生巨大的剪切应力,使气泡出现集中破裂现象。     

颗粒-壁面相互作用对石墨粉尘在高温气冷堆蒸汽发生器换热管表面沉积过程的影响

石墨粉尘在高温气冷堆蒸汽发生器换热管表面的沉积直接影响一回路放射性分布，因而备受学界和业界的关注。本文采用临界粘附速度模型研究石墨粉尘颗粒的碰壁过程，并将其耦合入蒸汽发生器一次侧的气固两相流模拟计算中，得到了石墨粉尘颗粒在换热管束间的输运和沉积规律。结果表明，考虑颗粒-壁面相互作用后，石墨颗粒沉积率显著减小，其与粒径的关系也有所不同。沉积率随粒径的增大先增大后减小，最大沉积率对应粒径约3 μm。空间分布规律上，小颗粒迎风第1排管沉积率相对最大，2 μm以上颗粒最大沉积率发生在第4～6排管。     

扩散角对文丘里管内湍流影响的试验研究

为研究扩散角对文丘里管内湍流的影响,采用立体粒子图像测速技术分别对扩散角度为10°、12.5°、15°以及20°的文丘里管扩散段区域进行了测量,得到了平均速度分布,并通过瞬时速度场的统计分析得到了扩散段湍动能分布情况。研究表明,不同扩散角度的文丘里管扩散段内平均速度在截面直径方向成轴对称的单峰分布,湍动能在截面直径上成轴对称的双峰分布,在各试验工况下扩散段内均发生流动分离现象。随扩散角度增加,湍动能峰值增加,主流区径向宽度未发生变化,分离流区径向宽度增加,但对分离流区所占比例的影响较小,高湍动能区变宽;随着雷诺数的增加,湍动能峰值增加,主要由轴向雷诺应力引起,分离流区所占比例略有降低,但主流区和分离流区分布变化较小。此研究为高雷诺数不同角度的文丘里管流场研究提供了实验基础。      

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.     

燃料组件典型栅元湍流激振数值研究

为掌握全长范围内的燃料棒振动响应特性，以用于燃料棒微动磨损寿命分析，本研究运用计算流体动力学（CFD）方法，对燃料组件典型栅元的湍流激振进行数值模拟分析，并通过棒表面的瞬态脉动压力分布开展不同夹持力下的单棒瞬态动力学分析。研究表明:格架上游的截面平均湍动能约为0.1 m2/s2，格架临近出口位置湍动能达到峰值的0.65 m2/s2，格架的存在显著增强了流场的湍流强度，这是造成燃料棒湍流激振的主要原因；通过瞬态动力学分析确定了均方根振幅最大的定位格架位置，并建立了该格架的均方根振幅和振动速度随夹持力变化的关联式。本研究将为后续微动磨损理论计算及实验验证奠定基础。      

核燃料颗粒涂铌沉积室内流态化与CVD反应的耦合数值模拟研究

利用Euler-Euler方法对沉积室内的气固两相流场进行计算流体力学(CFD)模拟计算,利用HSC Chemistry软件对还原反应产物进行稳态模拟计算,并将还原反应的计算结果与流场分布的计算结果相结合,拟合得到沉积室内的速度场和组分浓度场。研究结果表明,沉积室内燃料颗粒分布不均匀,在沉积室内存在局部颗粒堆积现象;床料粘结现象可能是由于还原反应副产物低价氯化铌的熔化产生的粘性力引起,沉积室内反应速率及Nb Cl5流量的不均匀性导致副产物生成量大幅提高,进而促进床料的粘结。     

基于源项法的闭式回路架构对主泵性能影响的分析研究

传统的主泵流动分析平台多为简化的开式流路,与真实闭式回路运行工况存有较大差异。为探究主泵在真实回路中的流动特性与机理,以包含密封口环间隙的主泵全通道水力模型为研究对象,采用源项法进行稳态、瞬态计算分析研究。稳态计算结果表明:闭式循环回路中形成漩涡流态,致使主泵进口处发生预旋,产生入流畸变,导致湍动能有所增加,能量分布不均匀;瞬态计算结果表明:相较于开式流路,闭式回路入流畸变带来流场压力、速度、湍动能、压力脉动等特性的变化,导致泵体扬程、效率均有所下降,所受径向力、轴向力增大。闭式循环回路架构针对主泵流动性能的分析更接近真实流动。      

圆管内超临界水传热恶化数值模拟及模型评价

采用计算流体力学(CFD)方法对圆管通道内超临界水的传热恶化特性进行数值模拟研究,将现有模型对超临界条件计算的适用性和可靠性进行了评估。计算结果表明,在低质量流速条件下,传热恶化发生时流道内将会出现M型的速度分布,最大速度处的湍动能明显减小;在高质量流速条件下,传热恶化时各物性参数中热导率对其传热特性有明显的影响。模型评估结果表明,本研究中SST模型能够用于高质量流速条件下传热恶化的计算。     

不同棒束结构稠密栅元通道内的湍流CFD研究

采用URANS（UnsteadyReynoldsAveragedNavierStokes）方法对不同棒束结构稠密栅元通道（P/D=1.001～1.2）内的湍流流动进行CFD模拟。研究分析了不同Re（Re=5000~215000）的湍流流动的主流速度、壁面剪应力、湍动能等参数。研究表明：在较稠密的棒束（P/D<1.1）通道内,P/D的变化对子通道内主流速度和剪应力分布均有较大影响。本文的模拟结果也验证了在达到临界P/D前（即使δ/D<0.011）,交混因子Y和δ/D成反比关系。对于固定的棒束结构（P/D=1.062）,当Re达到一定值（Re=9600）时,子通道内主流速度和剪应力分布对Re的变化不敏感。     

空间气冷反应堆堆芯流动换热数值仿真研究

以美国普罗米修斯计划反应堆为参考，建立了空间气冷反应堆堆芯结构的三维模型，利用蒙特卡罗方法计算得到了堆芯真实功率分布，并使用Star-CCM+软件开展了1/6堆芯的流动换热计算，分析得到了堆芯温度场、速度场和压力场的分布情况，评估了现有设计中仍待优化之处，并提出了相关的优化建议。计算结果表明，该反应堆设计可将冷却剂加热到工作温度，能满足基本的技术指标。但从优化角度考虑，需对堆芯入口段与出口段进行优化设计，通过改变入口管与压力容器间的角度等，可降低不必要的能量损失，提高堆芯出口温度与速度分布均匀性，同时降低冷却剂对诸如气轮机等设备的不利影响。     

Gas-solid Two-phase Flow Analysis and Structure Optimizationof Spent Fuel Shearing Machine

Mechanical shearing systems are commonly used in reprocessing plants to crack spent fuel. The radioactive dust generated by shearing needs to be contained to ensure the full utilization of spent fuel pellets and the maintainability of the hot chamber. The airflow pattern and particle distribution and movement in the machine are of great significance to ensure the normal operation of the shearing machine. In order to supplement the deficiencies of experimental research and theoretical research, and improve dust distribution, reduce dust levels and optimize design of the shearing machine, the flow field in bar shearing machine with injected virtual particles was simulated based on the discrete particle model. By analyzing the distribution of velocity, pressure and turbulent kinetic energy of the fluid in shearing machine and the destructive effect of dust particles on the inner wall of shearing machine, the structure and boundary conditions of the shear were optimized. The results show that the eddy current, turbulence and high impact force can be effectively avoided by improving the structure of the flow passage and the input conditions. The velocity field, pressure field and turbulent kinetic energy distribution of gas-solid two-phase flow in the box are more uniform and the gradient change is smaller, and the damage of box wall is effectively reduced.     

加/减速流动下棒束通道内速度分布和湍流特性研究

事故工况及海洋条件下反应堆处于非稳态工况,堆芯燃料组件内热工水力行为复杂多变,对反应堆安全提出了更高挑战,因此有必要对非稳态下燃料组件内流动换热特性开展研究。基于粒子图像测速（PIV）技术,结合远心镜头和脉冲控制器,实现对燃料组件内复杂流场的高时空分辨率、长时间的连续测量,获得了流量波动下燃料组件内时空演变的流场结构,分析了棒束通道内速度分布、湍流强度、雷诺应力等瞬时流场信息的空间演变特性。以定常流动下流场分布特性为基准,对比分析了加速度对燃料组件内空间流场分布的贡献特点。实验结果表明：加速流动提高了棒束通道内流层之间的速度梯度,抑制了横向速度和湍流强度;减速流动减弱了棒束通道内流层之间的速度梯度,提高了横向速度和湍流强度。实验结果有助于揭示燃料组件在非稳态条件下的瞬态特性,并为燃料组件的设计和优化奠定基础。     

钠冷快堆乏燃料组件自然循环冷却瞬态过程的数值模拟研究

应用Fluent程序,对处于氩气中的钠冷快堆乏燃料组件自然循环冷却瞬态过程进行了三维数值模拟。计算获得了乏燃料组件内部冷却剂通道和外部区域的热工水力学现象及变化规律。结果表明:利用标记区域分割方法,将燃料棒间隙网格划分为绕丝网格和绕丝周边流体域网格,能在棒束区生成高质量结构化网格;在氩气自然循环冷却瞬态过程中,棒束区内子通道氩气流量增加速度落后于边子通道,内子通道升温更快;乏燃料组件棒束区温度在轴向呈现中心高、边缘低的分布特征;为避免包壳温度超过设计值,乏燃料组件处于氩气中的时间不宜超过6min。     

间距对快堆燃料组件迷宫密封结构性能影响的数值模拟

本文采用计算流体动力学（CFD）方法针对直角梯形齿迷宫密封结构进行了三维数值模拟，对比研究间距对密封性能、流场分布、压力场分布、流速控制的影响，并在最佳间距尺寸下分析了偏心度对密封性能的影响。结果显示：扩大迷宫密封结构间距可有效减少漏流量、提高密封性能，并实现较好的流速控制，还可明显改善近壁面处的流动性能，强化对燃料组件外壁面的冷却；当间距超过3.02 mm时，密封性能提高有限，说明间距超过一定限值时，不能继续通过扩大间距提高迷宫密封结构密封性能，需在结构设计与工程应用中选择合理的间距尺寸；偏心现象导致迷宫密封结构流域内流速、压力分布不均匀，且随偏心度的增大不均匀现象有扩大的趋势，不仅对密封性能产生不利影响，还使得燃料组件振动加剧。     

石墨粉尘通过高温气冷堆堆芯球床结构的运动行为研究

高温气冷堆在运行过程中产生带有放射性的石墨粉尘,对反应堆的运行安全和环境安全造成一定影响。本文选取二维球床流场,采用离散相模型分析了堆芯球床结构对石墨粉尘颗粒的扩散和沉积的影响。计算结果表明:球床结构能有效阻碍石墨粉尘颗粒的扩散;沉积在球床结构上的石墨粉尘颗粒数目随堆芯内氦气流速的增加而增大,而由于受到颗粒惯性及热泳力的作用其增长趋势逐渐放缓;石墨粉尘颗粒在球床结构上的沉积效率随粒径的逐渐增加呈现"几乎不变-快速增长-缓速增长"的态势。     

An analytical method of evaluating the effect of rod displacement on circumferential temperature and heat flux distributions in turbulent flow through unbaffled rod bundles

An analytical method of evaluating the circumferential variations of temperature and heat flux fields inside and around a displaced fuel rod in triangular rod bundles in turbulent flow is presented with illustrative examples. The analysis consists mainly of the derivation of the simultaneous solutions of a set of heat conduction equations for fuel, cladding and coolant under the assumption of fully developed flow and heat transfer conditions. The local coolant velocity distribution, which is necessary for deriving the temperature field in coolant, is determined by solving the Navier-Stokes equation and the turbulent mixing of coolant is taken into consideration. The results show how the circumferential variations in the temperature and heat flux fields on the outer surface of the cladding increase the lower the ratio and the larger the fuel rod displacement due to thermal conduction and peripheral coolant flow velocity distribution.     

Simulations of flow behavior of fuel particles in a conceptual helium-cooled spout fluidized bed nuclear reactor

Hydrodynamics of helium and fuel particles are simulated in a conceptual helium-cooled spout fluidized bed nuclear reactor. The conceptual reactor consists of an axis-symmetric column with a sharp cone inside which the fuel particles are fluidized by helium. An isothermal gas-solid two-fluid flow model is presented. The kinetic-frictional constitutive model for dense assemblies of solids is incorporated. The kinetic stress is modeled using the kinetic theory of granular flow, while the friction stress is from the normal frictional stress model proposed by (Johnson, P.C., Nott, P., Jackson, R., 1990. Frictional-collisional equations of motion for particulate flows and their application to chutes. Journal of Fluid Mechanics 210, 501-535). Detailed spatial/temporal concentration and velocity profiles have been obtained in a conceptual spout fluidized bed nuclear reactor. The influence of inlet spouting jet velocity and conical angles on flow behavior of fluid and fuel particles is analyzed. The numerical simulations show that the unique mixing ability of the spout fluidized bed nuclear reactor gives rise, as expected, to uniform particle distributions. This uniformity enhances the heat transfer and therefore the power produced by the reactor.     

Kinetic-theory-based investigation of electronegative plasma–wall transition with two populations of electrons

Kinetic theory has been employed to investigate the magnetized plasma-sheath structure and its characteristics in the presence of more than one species of negatively charged particles: hot electrons,cold electrons, and negative ions. The cold electrons and negative ions are considered to obey a Maxwellian distribution, whereas the hot electrons follow a truncated Maxwellian distribution. The Bohm sheath condition has been extended for the case of more than one species of negatively charged particles, in which the concentration of hot electrons has a crucial role in achieving the Bohm velocity. The thermal motion of hot electrons is much higher compared to cold electrons and negative ions, such that the variation of hot electron concentrations and the temperature ratio of hot to cold electrons play a key role in the determination of the plasma-sheath parameters: particle densities,electrostatic potential, the flow of positive ions towards the wall, and sheath thickness. We have estimated the deviation of the resultant drift velocity of positive ions on the plane perpendicular to the wall from the parallel component at the presheath–sheath interface. It is found that the deviation between the two velocity components increases with an increase in the obliqueness of the magnetic field. Furthermore, the results obtained from the kinetic trajectory simulation model are compared with the results obtained using a fluid model; the results are qualitatively similar, although the potential varies by less than 4% in terms of the magnitude at the wall.