Two-phase redistribution in horizontal subchannel flow—turbulent mixing and gravity separation

The redistribution of two-phase flow in two horizontal interconnected subchannels caused by gravity separation and turbulent mixing has been investigated experimentally using an air—water loop. The measured redistribution data included the axial distribution of void fraction and liquid and gas flow rates in the two subchannels. The redistribution data exhibited an asymptotic behaviour, approaching certain flow distributions independent of the inlet distribution. The observed equilibrium distributions were explained as a balance between gravity forces (which tend to cause flow stratification) and turbulent diffusion (which tends to homogenize the two-phase mixture). A constitutive equation for transverse vapour drift, to account for both gravity separation and turbulent diffusion, was presented and a turbulent mixing coefficient was identified. The experimental data were used to obtain the best estimate of the empirical constants.     

RANS simulations for transition and turbulent flow regimes in wire-wrapped rod bundles

Internal flows through rod assemblies are commonly found in heat exchangers, steam generators, and nuclear reactors. One of the fuel assembly designs considered for liquid metal-cooled reactors utilizes wires helically wrapped around each fuel rod as spacers. The wires keep the fuel pins separated, enhancing the turbulent mixing, and heat transfer, but also affecting the pressure drop. It is of interest the understanding of the fluid flow phenomena in the sodium-cooled fast reactor as it is one of the Generation IV advanced reactor designs and it has been a motivating topic of research for the last decade. A wire-wrapped fuel assembly replica with 61-pins has been in operation at the Thermal-Hydraulic Research Laboratory of Texas A&M University. This facility produced high-fidelity velocity and pressure drop data for validation of computational fluid dynamics codes. This study investigates the effects of geometrical features and operating conditions on the flow behavior of the 61-pin wire wrapped bundle using Reynolds-Averaged Navier-Stokes (RANS) models to predict the axial and transverse pressure drops for a range of Reynolds numbers from 1,270 to 100,000. The friction factor predictions were in satisfactory agreement with the experimental data and the Upgraded Chen and Todreas correlation. The internal subchannel velocity results were compared with experimental data and Large Eddy Simulations (LES) and found in reasonable agreement. This study demonstrates that RANS is a suitable approach in predicting velocity and pressure fields in wire-wrapped rod bundles, with a relatively low computational effort.     

Experimental study of flow structures in a large range downstream the spacer grid in a 5 × 5 rod bundle using TR-PIV

In this paper, the time-resolved particle image velocimetry (TR-PIV) and match index refractive (MIR) techniques were used to study the flow field in a large range (0 – 22 D_h) downstream a spacer grid (SG) in a 5 × 5 rod bundle channel at different Reynolds number. The sodium chloride solution (1%) is used as the working fluid to reduce the refractive index error of fluorinated ethylene propylene (FEP) and water. The proper orthogonal decomposition (POD) background removal technique was used to minimize the FEP reflection. These methods greatly reduced the interference of background noise and improved the accuracy of cross-correlation calculation. For TR-PIV velocity fields downstream of the mixing vanes, time-averaged, statistical, spectral, and cross-correlation analysis were performed for the instantaneous full-field experimental data. The transport characteristics of coherent structures in different subchannels of rod bundles are calculated and discussed. The results show that the SG caused a relatively large transverse velocity and reduces the axial velocity. With the increase of the Reynolds number, the SG promotes the generation of transverse flow and has a great resistance to the axial flow. There is relatively large turbulence intensity downstream of the SG due to the mixing effect. The attenuation of transverse turbulence intensity component is slower than the axial component. Moreover, spectrum analysis shows that cross-arranged mixing vanes will generate periodic vortices but single mixing vane will not. These periodic vortices gradually propagate downstream along the inner subchannel and dissipate in the gap subchannel due to the effect of viscosity. The cross-correlation analysis shows that the mixing effect of the SG will reduce the scale of the coherent structure, and increase the convection velocity. The results of current research are helpful for understanding the strong anisotropic turbulence in the rod bundle channel with SG. Finally, the experimental results can be utilized to benchmark the applicability of turbulence models under different Reynolds number and the performance of partially averaged Naiver–Stokes or multiple RANS algorithms downstream of the SG.     

An analysis on effect of transverse convex curvature on turbulent flow and heat transfer

An analysis based on a model of modified mixing length by Hornby, Mistry und Barrow [1] was made on the effect of transverse convex curvature in turbulent boundary layer for incompressible axial flows along circular cylinders. The deviation of various turbulent flow and heat transfer properties from those of flat plates is presented. The agreement between the analyses and the experimental results for skin friction and heat transfer rate is good. The study demonstrated that, for a given condition, both the friction coefficient and Stanton number increase with decreasing value of the cyclinder radius and that their values are always greater than those for the flow over a flat plate.     

Experimental study on the unusual turbulence intensity distributions in rod-to-wall gap regions

The phenomenon of unusual turbulence intensity distributions in closely spaced rod bundles was experimentally studied. Fully developed turbulence intensities in rod-to-wall gap regions, formed by a single rod regularly mounted in a trapezoidal duct, were measured with a hot-wire anemometer. Starting from a rod setting giving a small bottom rod-to-wall gap, a series of gap spacings were achieved by moving the rod upward, resulting in larger bottom gaps and at the same time smaller top gaps. A region of high turbulence kinetic energy occurred first in the bottom gap region corresponding to the small bottom gap spacing and then shifted to the top gap region for the cases of small top gap spacings. There was no evidence of a high kinetic energy patch when the top gap size was extremely small. To unveil the mechanism behind this phenomenon, measurements on secondary velocities and energy spectra were conducted for one rod setting, and these results are described in detail. It was found that the measured secondary flow patterns could not explain the abnormal turbulence intensity distributions. However, energy density spectra revealed certain peak frequencies that support the argument that cross-gap large-scale eddy motion is of importance as both a transport process and production mechanism for subchannels formed by closely spaced rod bundles.     

Flow characteristics in hydraulically equilibrium two-phase flows in a vertical 2 × 3 rod bundle channel

In order to increase data on two-phase flow distribution in a multi-subchannel system, being similar to a rod bundle, experiments have been carried out using water and air at ambient pressure and temperature as the working fluids and a newly constructed 2 × 3 rod bundle channel as the test channel. The channel contained six rods in rectangular array and two-kinds of six subchannels, simulating a BWR fuel rod bundle. Experimental data on flow distribution and pressure drop along each subchannel axis were obtained in various single- and two-phase flows under a hydraulic equilibrium flow condition. From the measured pressure drop in the single-phase flow, friction factor data in each subchannel were obtained. The two-phase pressure drop data were compared with calculations by a simple, one-dimensional, one-pressure two-fluid model. In addition, Taylor bubble velocity in each subchannel in slug-churn flows was measured with a double needle contact probe. Using the bubble velocity data, we obtained a subchannel void fraction in each subchannel, and discussed a relationship of the subchannel void fractions between two different subchannels. Results of such experiments and discussions are presented in this paper.     

Schlieren measurement of turbulent structure in a diffusion flame

The regular and random mixing structures in a turbulent diffusion flame were investigated using the quantitative, dynamic crossed-beam schlieren method. Evidence was found close to the nozzle relating to the vortexlike structure of eddies surrounding the central fuel jet flow. The observations also make possible resolution of turbulent intensity, scales, convection, and spectra within the diffusion flame without the use of seeding or intrusion of measuring probes. It is found that length scales and other turbulence parameters in the diffusion flame progressively revert to values similar to those expected and observed in scalar passive mixing as the combustion reaction intensity reduces with axial distance from the nozzle system.     

Critical heat flux and turbulent mixing in hexagonal tight rod bundles

Experimental and theoretical investigations have been performed on critical heat flux (CHF) and turbulent mixing in tight, hexagonal, 7-rod bundles. Freon-12 was used as working fluid due to its low latent heat, low critical pressure and well known properties. It has been found that the two-phase mixing coefficient depends mainly on mass flux. It increases with decreasing mass flux and ranges from 0.01 to 0.04 for the test conditions considered. More than 900 CHF data points have been obtained in a large range of parameters: pressure 1.0–3.0 MPa and mass flux 1.0–6.0 Mg/m²s. The effect of different parameters on CHF has been analysed. It has been found that the effect of pressure, mass flux and vapour quality on CHF is similar to that observed in circular tubes. Nevertheless, the CHF in the tight rod bundle is much lower than that in a circular tube of the same equivalent hydraulic diameters. The effect of wire wraps on CHF is mainly dependent on local vapour qualities and subsequently on flow regimes. Based on subchannel flow conditions, the effect of radial power distribution on CHF is small. Comparison of the test results with CHF prediction methods underlines the need for further work.     

输气管道壁面涂料减阻机理的实验研究

用IFA-300热线风速仪以高于对应最小湍流时间尺度的分辨率精细测量了风洞中不同壁面涂料的管道湍流边界层不同法向位置流向速度分量的时间序列信号,利用湍流边界层近壁区域对数律平均速度剖面与壁面摩擦速度、流体黏性系数等内尺度物理量的关系和壁面摩擦速度与壁面摩擦切应力的关系,在准确测量湍流边界层近壁区域对数律平均速度剖面的基础上,间接测量湍流边界层的壁面摩擦阻力．对不同壁面涂料的壁湍流脉动速度信号用子波分析进行多尺度分解,用子波系数的瞬时强度因子和平坦因子检测管道湍流边界层中的多尺度相干结构,提取不同尺度相干结构的条件相位平均波形,对比研究输气管道壁面涂料的减阻机理．     

用平均速度剖面法测量壁湍流摩擦阻力

用IFA300恒温热线风速仪精细测量风洞中不同雷诺数流动条件下的平板湍流边界层近壁区域对数律平均速度剖面．利用平板湍流边界层近壁区域的对数律平均速度剖面与壁面摩擦速度、流体黏性系数等内尺度物理量的关系和壁面摩擦速度与壁面摩擦切应力的关系,在准确测量平板湍流边界层近壁区域对数律平均速度剖面的基础上,测量平板湍流边界层的壁面摩擦阻力．实现了平板湍流边界层壁面摩擦阻力的无干扰或微小干扰测量．该种方法操作简便,不需要在流场中安装测力天平、传感器等复杂的测量装置,不需要对湍流边界层的壁面进行破坏,不会影响湍流边界层壁面附近区域原有的流场条件,是一种切实可行的测量平板湍流边界层壁面摩擦阻力的简便方法．     

分级进风燃烧室内含颗粒的湍流反应流的实验研究

应用三维激光粒子动态分析仪(PDA),对分级进风燃烧室内含颗粒的湍流反应流的气 固两相瞬时速度场进行了实验测量,同时对单相湍流反应流的瞬时速度场也进行了实验测 量. 得到了两种情况下燃烧室内气固两相和单相气体的平均轴向与切向速度、轴向与切向脉 动速度均方根值和轴向-切向脉动速度二阶关联量的分布.     

Pressure drop and heat transfer characteristics of turbulent flow in annular tubes with internal wave-like longitudinal fins

Measured were pressure drop and heat transfer characteristics with uniform axial heat input using air as the working fluid in both the entrance and fully developed regions of annular tubes with wave-like longitudinal fins. Five series of experiments were performed for turbulent flow and heat transfer in the annular tubes with number of waves equal to 4, 8, 12, 16 and 20, respectively. The test tube has a double-pipe structure with the inner blocked tubes as an insertion. The wave-like fins are in the annulus and span its full width. The friction factor and Nusselt number in the fully developed region were obtained. The friction factor and Nusselt number can be well corrected by a power-law correction in the Reynolds number range tested. In order to evaluate the thermal performance of the longitudinal finned tubes over a plain circular tube, comparisons were made under three conditions: (1) identical pumping power; (2) identical pressure drop and (3) identical mass flow. It was found that under the three constraints all the wave-like finned tubes can enhance heat transfer with the tube with wave number 20 being superior. Finally, discussion on the enhancement mechanism is conducted and a general correlation for the fully developed heat transfer is provided, which can cover all the fifty data of the five tubes with a mean deviation of 9.3%.     

Computational modeling of turbulent mixing in a jet in crossflow

The jet in crossflow is a configuration of highest theoretical and practical importance, in which the turbulent mixing plays a major role. High-resolution measurements using Particle Image Velocimetry combined with Laser Induced Fluorescence have been conducted and used to validate simulations ranging from simple steady-state Reynolds-averaged Navier Stokes to sophisticated large-eddy simulation. The reasons for the erratic behavior of steady-state simulations in the given case, in which large-scale structures dominate the turbulent mixing, have been discussed. The analysis of intermittency proved to be an appropriate framework to account for the influence of these flow structures on the jet in crossflow, contributing to the explanation of the poor performance of the steady-state simulations.     

Measuring turbulence in a circulating fluidized bed using PIV techniques

This study utilized the particle image velocimetry (PIV) technique, non-invasively near the wall, in the developing region, for the measurements of laminar and turbulent properties during circulation of Geldart B type particles in the U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) riser. A novel method was used to measure axial and radial laminar and turbulent solids dispersion coefficients using autocorrelation technique.The instantaneous and hydrodynamic velocities for the solid phase were measured simultaneously in the axial and radial directions using a CCD camera, with the help of a colored rotating transparency. The measured properties, such as laminar and Reynolds stresses, laminar and turbulent granular temperatures, laminar and turbulent dispersion coefficients and energy spectra exhibited anisotropy. The mixing in the riser was on the level of clusters. The total granular temperatures were in reasonable agreement with the literature values. However, the axial and radial solids dispersion coefficients measured near the wall were slightly lower than the radially averaged values in the literature.     

气体示踪法研究燃烧器横向湍流混合扩散特性

用气体示踪法对侧边风燃烧器弱旋流一次风和侧边直流二次风横向湍流混合扩散特性进行了详细研究，讨论了各截面混合物浓度分布规律及混合强度分布规律，分析对燃料着火的影响，同时利用湍流运动方程反算气体湍流扩散系数     

Pressure drop measurements for woven metal mesh screens used in electrical safety switchgears

The paper deals with an experimental investigation of the pressure drops and friction factors induced by sets of metal woven screens in the case of an incompressible fluid flow, namely water flow. These woven screens are of metallic type and are used in electrical safety devices, especially at the end of exhaust duct of low voltage circuit breakers. In a first step, the whole of the set-up is used to check some approximations dedicated to pressure drops due to sphere beds around 275 µm and 375 µm as diameters. Results are discussed in terms of Darcyan and non-Darcyan permeabilities compared with published data and are analyzed in terms of the Blake-type friction factor. In the second step, the pressure drops are measured for stacks composed of eight different woven screens (of plain dutch type) formed with millimetric wires. Various formulations of the pressure drops and friction factors published elsewhere are tested with a special care dedicated to the choice of the geometry for the flow pattern (hydraulic diameter, spherical diameter, cylindrical diameter) and to the consideration of laminar and turbulent contribution. We then give the formulation that characterizes the fluid flowing through stacks of woven screens used in electrical safety applications.     

Velocity and temperature dissimilarity in fully developed turbulent channel and plane Couette flows

The natural dissimilarity or decorrelation of stream-wise velocity and temperature fluctuations in fully developed turbulent channel and plane Couette flows was studied using direct numerical simulation (DNS). For both of the flow configurations, a Reynolds number of about 150 was used based on the friction velocity and half the distance between walls. Buoyancy effects were neglected, and only results with a molecular Prandtl number, Pr, equal to 1 are presented. The boundary conditions for the thermal field were a uniform source of energy in the domain and isothermal wall temperature for the channel and Couette flow, respectively. The importance of those events responsible for wall-normal turbulent fluxes in the generation of axial velocity and temperature dissimilarity was examined using conditional probability. It was found that the dissimilarity in the whole domain was higher in Couette than in channel flow. It was also found that for wall-normal turbulent fluxes (momentum and heat), the averaged dissimilarity in the whole domain was slightly more correlated with those events in the second or fourth quadrant, according to the quadrant analysis technique. For channel flow, the importance of both kinds of events was similar, while for Couette flow there was a predominance in the generation of dissimilarity by those events in the fourth quadrant. Also, for both flow configurations and throughout the wall-normal direction, it was found that in the buffer region there was a predominance of events in the fourth quadrant associated with dissimilarity for both wall-normal turbulent fluxes. In the frequency domain, the distribution of energy showed that there was a high-frequency shift experienced from the wall towards the centerline by the temperature spectrum with regards to the axial velocity spectrum, for which the action of the fluctuations of the wall-normal velocity was the main cause. In the central region of the flow, on the other hand, there was a global convergence of all spectra towards the pressure spectrum, with this convergence lower for Couette flow. Finally, it is shown that the dissimilarity in developed conditions is caused by the greater correlation existing for the temperature fluctuation with the instantaneous axial pressure gradient than for the velocity fluctuation with the instantaneous axial pressure gradient.     

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。     

强旋湍流气粒两相流动的PDPA研究

采用相多普勒颗粒分析仪（ＰＤＰＡ）对切向进气，轴向缩口出口的旋风筒内强旋单相和气粒两相流动进行了实验研究，给出了强旋流场中，两相湍流的运动及相互作用规律     

Turbulent friction factor for two-phase: air-viscoelastic flows through a horizontal tube

The fully developed two-phase turbulent isothermal Fanning friction factors for air-viscoelastic fluid flows through a horizontal tube were measured experimentally. The viscoelastic fluids studied were aqueous solutions of polyacrylamide (100, 200, and 500 ppm by weight). Over the range of the apparent Reynolds number (Re_e) from 10,000 to 100,000, the homogeneous model was found to be accurate enough for engineering prediction of turbulent friction factor for air-viscoelastic flows through horizontal tubes. A new correlation for the turbulent friction factor of airlriscoelastic plug flow is proposed.