扩散型气柱界面R-M失稳中混合率的实验研究

Richtmyer-Meshkov(R-M)不稳定性普遍存在于众多工程问题中,激波管实验是研究R-M失稳问题的主要手段.高精度的平面激光诱导荧光(planar laser-induced fluorescence,PLIF)技术具有分子量级的示踪能力,可获得界面气体浓度(摩尔分数)分布,为研究界面失稳混合问题提供了有力工具.在弱激波(Ma=1.25)冲击扩散型气柱界面实验中,采用PLIF技术对R-M失稳引起的SF6-Air界面混合问题进行了研究.通过改变椭圆形初始界面的长短轴比,得到了3种扩散型初始界面失稳演化过程中气体摩尔分数,观察到了斜压机制下界面的简单拉伸、二次不稳定性、挤压射流等现象.利用浓度分布进一步得到了界面的瞬时混合率,通过瞬时混合率、界面整体平均混合率以及混合率的概率密度分布,分析了界面在不同演化阶段的界面混合特征,初步讨论了界面失稳混合的机制.演化初期,界面在斜压涡的作用下发生拉伸卷曲,通过增大浓度梯度来促进界面的混合.当演化进一步发展,二次不稳定性出现后,界面通过小尺度对流的方式达到湍流混合状态,而浓度梯度驱使的分子间混合逐渐减弱.由浓度梯度引起的扩散与由二次不稳定性引起的对流存在着"竞争"关系,二者共同主导了界面的混合.     

Eulerian-Lagranigan simulation of aerosol evolution in turbulent mixing layer

The formation and evolution of aerosol in turbulent flows are ubiquitous in both industrial processes and nature. The intricate interaction of turbulent mixing and aerosol evolution in a canonical turbulent mixing layer was investigated by a direct numerical simulation (DNS) in a recent study (Zhou, K., Attili, A., Alshaarawi, A., and Bisetti, F. Simulation of aerosol nucleation and growth in a turbulent mixing layer. Physics of Fluids, 26, 065106 (2014)). In this work, Monte Carlo (MC) simulation of aerosol evolution is carried out along Lagrangian trajectories obtained in the previous simulation, in order to quantify the error of the moment method used in the previous simulation. Moreover, the particle size distribution (PSD), not available in the previous works, is also investigated. Along a fluid parcel moving through the turbulent flow, temperature and vapor concentration exhibit complex fluctuations, triggering complicate aerosol processes and rendering complex PSD. However, the mean PSD is found to be bi-modal in most of the mixing layer except that a tri-modal distribution is found in the turbulent transition region. The simulated PSDs agree with the experiment observations available in the literature. A different explanation on the formation of such PSDs is provided.     

圆柱形充液室中4股贴壁燃气射流扩展特性的实验研究

为了探索高温高压周向均布4股贴壁燃气射流在受限空间中的扩展特性,设计了贴壁燃气射流在圆柱形充液室内扩展的实验装置,借助数字高速录像系统,观察了4股贴壁燃气射流在充液室中的扩展过程,发现由Kelvin-Helmholtz不稳定性引起的表面不规则一直存在于整个射流扩展过程;通过处理拍摄记录的射流扩展序列图,获得不同时刻射流扩展的轴向和径向位移; 对比了不同破膜喷射压力和喷孔结构参数对4股贴壁燃气射流扩展过程的影响。实验结果表明:喷孔面积越大,贴壁射流初期轴向扩展速度越大,但由于径向扩展达到交汇的时间较早,湍流掺混和干涉强烈,衰减也越快;破膜喷射压力越高,射流径向扩展到达交汇的时间越短; 破膜喷射压力从12 MPa升高到20 MPa,射流轴向扩展速度大幅增加,气液湍流掺混效应增强。     

Sleeve leakage gas impact on fuel assembly temperature distribution

ABSTRACT

The flow in a large section (80%) of an Advanced Gas-cooled Reactor fuel assembly is investigated in two configurations, the former dealing with the fuel assembly operating in normal plant conditions and the latter with a cold gas injected sideways at the top of Element 2 of the fuel assembly. Results show that injecting the cold gas decreases the temperature at the top of the assembly and that mixing is almost complete there.     

Detailed experimental study of a highly compressible supersonic turbulent plane mixing layer and comparison with most recent DNS results: “Towards an accurate description of compressibility effects in supersonic free shear flows”

A compressible supersonic mixing layer at convective Mach number (Mc) equal to 1 has been studied experimentally in a dual stream supersonic/subsonic wind-tunnel. Laser Doppler Velocimetry (L.D.V.) measurements were performed making possible a full estimation of the mean and turbulent 3D velocity fields in the mixing layer. The Reynolds stress tensor was described. In particular, some anisotropy coefficients were obtained. It appears that the structure of the Reynolds tensor is almost not affected by compressibility at least up to Mc = 1.The turbulent kinetic energy budget was also experimentally estimated. Reynolds analogies assumptions were used to obtain density/velocity correlations in order to build the turbulent kinetic energy budget from LDV measurements. Results have been compared to other experimental and numerical results. Compressibility effects on the turbulent kinetic energy budget have been detected and commented. A study about thermodynamics flow properties was also performed using most recent DNS results experimentally validated by the present data. A non-dimensional number is then introduced in order to quantify the real effect of pressure fluctuations on the thermodynamics quantities fluctuations.     

NUMERICAL SIMULATION OF BROWNIAN COAGULATION AND MIXING OF NANOPARTICLES IN 2D RAYLEIGH-B?NARD CONVECTION

采用泰勒展开矩方法对二维瑞利-贝纳德热对流系统(1×10⁶ ≤Ra ≤1 ×10⁸) 中纳米颗粒群的混合和凝并特性进行了数值模拟. 结果显示颗粒群随时间演化经历了扩散阶段、混合阶段、充分混合阶段3 个阶段, 随着颗粒群混合和凝并的进行, 颗粒数目浓度减少, 颗粒群的平均体积增大; 得到了颗粒分布函数各特征量与温度相关系数以及各特征量的空间分布标准偏差在3 个阶段的不同特征; 得到了颗粒分布函数各阶矩以及平均体积长时间演化的渐近行为, 结果与零维渐近解析解一致. 最后, 本文进一步研究了无量纲数(包括瑞利数Ra, 斯密特数Sc_M, 达姆科勒数Da) 对颗粒群达到自保持分布时间的影响, 发现该时间随着Ra和Sc_M的增大呈对数率减小, 随着Da的增大呈线性增大     

A solids mixing rate correlation for small scale fluidized beds

A new first degree solids mixing rate is proposed to evaluate the mixing of solids in small scale fluidized beds. Particle mixing experiments were carried out in a 2D fluidized bed with a cross-section of 0.02 m × 0.2 m and a height of 1 m. White and black particles with average diameters of 850 and 450 μm were used in our experiments. Image processing was used to measure the concentration of the tracers at different times. The effects of four representative operating parameters (superficial gas velocity, ratio of tracer particles to bed particles, tracer particle position, and particle size) on mixing are discussed with reference to the mixing index. We found that the Lacey index depends on the concentration of the tracers. The position of the tracers affects the initial mixing rate but not the final degree of mixing. However, the new mixing rate equation does not depend on the initial configuration of the particles because this situation is considered to be the initial condition. Using the data obtained in this work and that found in literature, an empirical correlation is proposed to evaluate the mixing rate constant as a function of dimensionless numbers (Archimedes, Reynolds, and Froude) in small scale fluidized beds. This correlation allows for an estimation of the mixing rate under different operating conditions and for the detection of the end point and/or the time of mixing.     

一种基于AUSM思想的通量分裂方法

根据对流迎风分裂(AUSM)思想提出一种通量分裂方法,称为K-CUSP格式.它与传统H-CUSP和E-CUSP格式的最大差异在于总能量的分裂:K-CUSP格式将无粘守恒通量中所有的运动学量分裂到对流项,所有的热力学量分裂到压力项,即总能量被分裂成动能和静焓.对于压力项的数值通量,采用一种新的界面构造方法.数值测试表明:①K-CUSP格式继承了FVS格式的简单性和稳健性.在激波后不易出现压力过冲,在膨胀区域没有振荡,优于AUSM和WPS格式;②K-CUSP格式继承了FDS格式的分辨率.激波间断的分辨率和H-CUSP、E-CUSP格式基本相同,接触间断的分辨率高于FVS格式,低于Roe、AUSM和WPS格式.AUSM和WPS格式在计算运动接触间断时,速度存在很大振荡,而新格式不存在振荡.     

混合干涉型分布式光纤传感器PGC信号解调设计与实验分析

对一种基于马赫-曾德尔和萨格纳克混合干涉仪原理的分布式光纤泄漏检测系统调制解调技术进行研究,对PGC解调技术进行理论分析。针对所选泄漏分布式光纤传感特点设计了PGC解调硬件电路,并对PGC解调电路进行了测试。实验中待测信号为0~50 kHz,载波频率为300 kHz,截止频率为50 kHz, 移相后与待测信号的相关系数达到0.95,谐波失真小于3%,泄漏源位置为7.865 km,泄漏检测定位系统绝对定位误差为235 m,相对定位误差为2.35%。本解调定位系统具有稳定性和抗干扰性。     

An optimized mixed‐mode stationary phase based on silica monolith particles for the separation of peptides and proteins in high‐performance liquid chromatography

A phase with both hydrophobic and hydrophilic functionalities has been synthesized by modification of ground silica monolith particles with C18 and 1‐[3‐(trimethoxysilyl)propyl] urea ligands. A series of phases was prepared by changing the ratio of the two ligands to determine the optimal ratio in view of separation efficiency. The resultant optimized stationary phase was packed in narrow‐bore glass‐lined stainless‐steel columns (1 × 300 mm and 2.1 × 100 mm) and used for the separation of synthetic peptides and proteins. The average numbers of theoretical plates (N) of 52 100/column (174 000/m, 5.75 µm plate height) and 35 500/column (118 000/m, 8.47 µm plate height) were achieved with the 300 mm column at a flow rate of 25 µL/min (0.86 mm/s) in 60:40 v/v acetonitrile/30 mM aqueous ammonium formate for the mixture of peptides (Thr‐Tyr‐Ser, Val‐Ala‐Pro‐Gly, angiotensin I, isotocin, and bradykinin) and for the mixture of proteins (myoglobin, human serum albumin, and insulin), respectively. Fast analysis of the peptides and proteins was also carried out at a flow rate of 0.9 mL/min (6.88 mm/s) with the 100 mm column and all the analytes were eluted within 2 min with good separation efficiency.