Analysis of dynamical flow structure in a square arrayed rod bundle

Large eddy simulation (LES) of turbulent flow in a bare rod bundle was performed, and a new concept about the flow structure that enhances heat transport between subchannels was proposed. To investigate the geometrical effect, the LES was performed for three different values of rod diameter over pitch ratio (D/P = 0.7, 0.8, 0.9). The computational domain containing 4 subchannels was large enough to capture large-scale structures wide across subchannels. Lateral flow obtained was unconfined in a subchannel, and some flows indicated a pulsation through the rod gap between subchannels. The gap flow became strong as D/P increased, as existing experimental studies had reported. Turbulence intensity profile in the rod gap suggested that the pulsation was caused by the turbulence energy transferred from the main flow to the wall-tangential direction. This implied that the flow pulsation was an unsteady mode of the secondary flow and arose from the same geometrical effect of turbulence. This implication was supported by the analysis results: two-points correlation functions of fluctuating velocities indicated two length-scales, P-D and D, respectively of cross-sectional and longitudinal motions; turbulence stress in the cross-sectional mean flow contained a non-potential component, which represented energy injection through the unsteady longitudinal fluid motion.     

CFD analysis of flow field in a triangular rod bundle

The flow field was investigated in subchannels of VVER-440 pressurized water cooled reactors’ fuel assemblies (triangular lattice, P/D = 1.35). Impacts of the mesh resolution and turbulence model were studied in order to obtain guidelines for CFD calculations of VVER-440 rod bundles. Results were compared to measurement data published by Trupp and Azad in 1975. The study pointed out that RANS method with BSL Reynolds stress model using a sufficient fine grid can provide an accurate prediction for the turbulence quantities in this lattice. Applying the experiences of the sensitivity study thermal hydraulic processes were investigated in VVER-440 rod bundle sections. Based on the examinations the spacer grids have important effects on the cross flows, axial velocity and outlet temperature distribution of subchannels therefore they have to be modeled satisfactorily in CFD calculations.     

The prediction of fully developed axial turbulent flow in rod bundles

Results are presented from the application of a finite-volume calculation method to fully-developed axial turbulent flow in various smooth rod bundle arrangements. Simplified algebraic versions of the Reynolds stress transport equations are used in the calculation of the full three dimensional velocity field, without any special adjustments for each geometry. The predictions obtained for different rod spacings compare favourably with experiment and reveal the significant role of the cross-plane turbulence-driven secondary flow in shaping the mean flow and turbulence distributions. The success of the results obtained establish the effectiveness of the method and encourage further applications and development.     

钠冷快堆燃料组件热工水力特性数值模拟与分析

利用CFD程序CFX,分别对7、19、37、61根棒组成的三角形排列螺旋绕丝定位的钠冷快堆燃料组件棒束通道进行了热工水力特性的分析研究,并将结果与子通道程序SuperEnergy进行了对比验证。重点考察了棒束通道轴向流动分布、横向流交混效应及子通道轴向温升,分析了定位绕丝的影响。结果表明,绕丝对棒束通道的横向流交混效应、轴向流动分布及子通道温升有着重要影响,且随棒束的增多,通道内的流动趋向复杂化,轴向流动不均匀性有升高趋势。     

The structure of turbulent flow through a wall subchannel of a rod bundle

An experimental investigation was performed to establish reliable information on the transport properties of turbulent flow through subchannels of rod bundles. Detailed data were measured of the distributions of the time-mean velocity, the turbulence intensities in all directions and hence, the kinetic energy of turbulence, of the shear stresses in the directions normal and parallel to the walls and of the wall shear stresses for a wall subchannel of a rod bundle of four parallel rods. The pitch to diameter ratio of the rods equal to the wall to diameter ratio was 1.07, the Reynolds number of this investigation was Re = 8.7 × 10⁴.On the basis of the data measured the eddy viscosities in the directions normal and parallel to the walls were calculated. Thus, detailed data of the eddy viscosities in direction parallel to the walls in rod bundles were obtained for the first time. The experimental results were compared with predictions by the VELASCO code. There are considerable differences between calculated and measured data of the time-mean velocity and the wall shear stresses. Attempts to adjust the VELASCO code against the measurements were not successful. The reasons of the discrepancies are discussed.     

Velocity distribution in the peripheral subchannels of the CANDU-type 19 rod bundle

Measurements of the detailed flow velocity and turbulent intensity in the peripheral subchannels of a non-wrapped CANDU-type 19-rod test bundle were performed by Laser Doppler Anemometry in a water loop. The bundle consisted of rods, 1.905 cm in diameter and 62.28 cm long, positioned by two 3 mm thick end-plates. The test housing was an 11 cm ID transparent pipe.The transverse velocity was negligible (0.002 m/s) and the turbulence intensity uniformly distributed (7%) throughout all the channel areas. The axial flow patterns for the peripheral subchannels are given, and show velocity gradients and average and peak velocities higher in the subchannels facing triangular channels than in subchannels facing square channels, even though they have the same cross-sectional areas.Experimental results show excellent agreement with COBRA-IIIC computer code calculation of flow distribution in CANDU-type reactors if the bundle is well-aligned and well-centered. The average and peak velocities of a off-centered bundle are also given.     

Comparison of thermo-hydraulic performances of large scale vortex flow (LSVF) and small scale vortex flow (SSVF) mixing vanes in 17 × 17 nuclear rod bundle

Spacer grids in the nuclear fuel rod assembly maintain a constant distance between rods, secure flow passage and prevent the damage of the rod bundle from flow-induced vibration. The mixing vanes attached to the spacer grids generate vortex flows in the subchannels and enhance the heat transfer performance of the rod bundle. Various types of mixing vanes have been developed to produce cross flows between subchannels as well as vortex flows in the subchannels.The shapes of the mixing vane have been improved to generate larger turbulence and cross flow mixing. In the present study, two types of large scale vortex flow (LSVF) mixing vanes and two types of small scale vortex flow (SSVF) mixing vanes are examined. SSVF-single is conventional split type and SSVF-couple is split type with different arraying method. LSVF mixing vane has different geometry and arraying method to make large scale vortex. 17 × 17 rod bundle with eight spans of mixing vanes is simulated using the IBM 690 supercomputer. The FLUENT code and IBM supercomputer is employed to calculate the flow field and heat transfer in the subchannels.Turbulence intensities, maximum surface temperatures of the rod bundle, heat transfer coefficients and pressure drops of the four kinds of mixing vanes are compared. LSVF mixing vanes produced higher turbulence intensity and heat transfer coefficient than SSVF mixing vanes. Consequently, LSVF mixing vane increases the thermal efficiency and safety of the rod bundle.     

Laser doppler measurements of flow in a rod bundle

A two component laser doppler velocimeter with polarized beams and frequency shift was used to measure the turbulent flow field for axial flow between the rods of a nine rod, square pitch rod bundle. Parameters measured include mean axial and lateral velocities, turbulence intensities and the friction factor. The axial velocities for 10000 to 40000 Reynolds number are slightly higher than those reported by Rowe. The maximum lateral velocities measured are about 1% of the bulk velocity; somewhat larger than suggested by earlier authors. Axial and lateral turbulence intensities are larger than those in pipe flows.     

Turbulent flow in a model nuclear fuel rod bundle containing partial flow blockages

Local velocity and turbulence intensity measurements were obtained with a laser Doppler anemometer near flow blockages in an unheated 7 × 7 rod bundle. Sleeve blockages were positioned on the center nine rods to create area reductions of 70 and 90% in the center four subchannels of the bundle. Experimental results indicated that extensive flow disturbances existed downstream from the blockage clusters and showed that only minor disturbances can be expected upstream from the blockages. Recirculation zones for both 70 and 90% blockages were detected downstream from the blockage clusters and persisted for approximately three to five subchannel hydraulic diameters, depending on the degree of the blockage. The experimental velocity results obtained with blockage clusters located midway between grid spacers were successfully predicted using the COBRA subchannel computer program.     

Analysis of flow distribution a PWR fuel rod bundle model containng A 90% blockage

An experimental investigation, covering a Reynolds number range from 2 × 10³ to 3.5 × 10⁴, was conducted to study the velocity and turbulence intensity distributions due to the presence of a blockage in an unheated 7 × 7 rod bundle. The blockage configuration, consisting of a 4 × 4 rod array, created a maximum flow area reduction of 90% in the central nine subchannels. The blockage sleeve length was 38.3 × rod diameter and the 90% blockage zone length extended for 16.4 × rod diameter. The results showed that upstream of the blockage, the flow was not influenced by the blockage until it reached the location where the inlet taper section of the swelling started. At the downstream end, the flow disturbance was extensive and persisted over a distance of about 83 rod diameters. Compared to the downstream velocity profiles, the turbulence intensity measurements however showed a faster recovery from the blockage influence. At the higher Reynolds number, velocity profiles calculated using the COBRA subchannel computer code compared consistently with the experimental data. The general flow behaviour of the various subchannels was reasonably well predicted. However, at low Reynolds number, due mainly to the frictional form loss calculation scheme in COBRA and uncertainty in the flow transition, the flow diversion due to the blockage to the surrounding unblocked subchannels was overestimated. The influence of the degree of recovery from the rod swelling on the flow was also studied using COBRA.     

5×5棒束通道内流动转捩特性研究

棒束通道的特殊结构导致其内部流动转捩情况较为复杂,探究其内部流动转捩规律具有重要意义。本文针对棒束通道内的流动转捩特性开展实验与CFD模拟研究,通过实验获得了棒束通道内沿程阻力系数的变化规律;采用不同湍流模型进行了数值模拟。结果表明,SST k-ω模型能较好地反映实验结果。进一步对比了不同雷诺数工况下通道内不同位置的沿程阻力系数与湍流强度,发现对于不同子通道,中心子通道湍流强度与沿程阻力系数高于边角子通道;对于同一子通道,子通道中心处湍流强度与壁面切应力高于子通道边缘处。这一结果说明,受壁面影响,棒束内湍流强度、壁面切应力、阻力特性具有不均匀性,这些空间上的不均匀性相互作用会引起总体上棒束转捩点不明显。     

Flow Transition Characteristics in 5×5 Rod Bundle Channel

The special geometric structure of the rod bundle channel can induce complicated flow transition of the coolant, and investigation on the flow transition rules is sufficiently important. In the current study, experimental and numerical study on the flow transition characteristics in the 5×5 rod bundle channel was carried out. Experiments were performed to obtain the variation characteristics of the resistance coefficient and CFD simulation was performed using different turbulence models in ANSYS Fluent. The results show that the simulation with SST k-ω turbulence model agrees well with the experimental data. The simulated turbulence intensity and resistance coefficient at different measurement locations and in different flow conditions were compared. For different subchannels, the turbulence intensity and the resistance coefficient are higher in the center subchannel than those in the edge subchannel. For the same subchannel, the turbulence intensity and the shear stress in the subchannel center are higher than those in the subchannel edge. This result indicates that the turbulence intensity, shear stress and resistance coefficient in the rod bundle are not uniform due to the influence of the wall surface. This non-uniform spatial interaction makes the transition point obscure.     

脉动流下棒束通道内流场与湍流特性的PIV实验研究

事故条件下路基核反应堆以及受到海洋条件附加惯性力影响的浮动核反应堆一回路冷却剂会处于非稳定流动状态,进而改变冷却剂的流动和传热特性,影响反应堆的安全运行。本文应用锁相粒子图像测速（PIV）以及折射率匹配技术分别对脉动流条件下有无定位格架棒束通道内瞬时速度进行了测量。实验结果表明：对于不带定位格架的棒束通道,加速使得靠近通道壁面附近流体速度变大,而靠近中心区域流体速度变小。此外湍流强度分量随流体加速而逐渐变小,随流体减速而逐渐增加。对于流向压力梯度驱动的周期性脉动流,横向脉动速度均方根分量u′滞后于流向脉动速度均方根分量v′,且二者都滞后于流速的变化;对于带定位格架的棒束通道,带有搅浑翼的定位格架强烈的交混作用极大地减小了流体加速度对棒束通道内速度分布和湍流强度带来的影响。实验结果有助于更加清晰地揭示脉动流在棒束通道中的作用机理。     

Experimental investigation of turbulent transport of momentum and energy in a heated rod bundle

Turbulent air flow in a central channel of heated 37-rod bundles with triangular array at two different pitch-to-diameter ratios (P/D=1.12 and P/D=1.06) was investigated. Measurements were performed with a hot-wire probe with x-wires and an additional temperature wire. Time mean velocities, time mean fluid temperatures, wall shear stresses and wall temperatures, turbulent quantities such as the turbulent kinetic energy, all Reynolds stresses and all turbulent heat fluxes were measured at two different pitch-to-diameter ratios in a central channel of the bundle. It is shown that with decreasing gap width the turbulence field in rod bundles deviates significantly from that in a circular tube. Also, data on the power spectral density functions of the velocity and temperature fluctuations are presented. These data show the existence of large-scale periodic fluctuations of velocity and temperature in the gap region of two adjacent rods. These fluctuations are responsible for the high intersubchannel heat and momentum exchange. Spectral measurements with two hot wire probes imply a distinct similarity of motion of vortices in adjacent subchannels of the bundle.     

Turbulent transports by secondary flow vortices in a rod bundle

Experimental data on secondary flow vortices in a rod bundle with pitch-to-diameter ratio show weak vortices with the average velocity magnitude only about 0.1% of the mean bulk velocity. The question then arises, how important these weak vortices could still be as a transport mechanism in turbulent flows. The transport of axial momentum by these vortices is analysed quantitatively. While a minor importance is observed for the transport in radial direction, it is found that about half of the total transport in circumferential direction is due to the secondary flow vortex convection. Based on the analogy between the transport of momentum and heat, it is expected in nonisothermal situations that, in radial direction, the contribution can improve the heat transfer coefficient and contribute to better economy of heat transfer installations. In circumferential direction, the contribution helps to smooth out circumferential temperature differences, improves the heat removal from heated surfaces and, through a decrease of the maximum surface temperature, it contributes to passive safety of heat transfer installations.     

脉动流下定位格架下游时均流场分布特性研究

研究流量波动下棒束通道内定位格架下游瞬时流场演变特性对于揭示海洋条件下燃料组件内流动换热机理具有重要意义。本文应用粒子图像测速（PIV）技术获得了脉动流下棒束通道内定位格架下游时空演变流场结构,分析了脉动参数（脉动周期和脉动振幅）对定位格架下游速度分布和湍流特性的影响。结果表明,脉动流下定位格架下游时均速度与定常流动下时均速度差异较小,且基本不随脉动振幅和脉动周期变化而变化;脉动流下的定位格架下游横向速度和轴向速度均方根与定常流动下的速度均方根存在明显差异,且随脉动参数变化呈现出不同的变化趋势。本文研究结果有助于揭示燃料组件在非稳态条件下瞬态特性,并为燃料组件的设计和优化奠定基础。      

快堆燃料组件棒束通道内流动和传热现象分析与研究

利用三维计算流体力学软件CFX 12.0对由7根带螺旋状定位绕丝的燃料棒组成的快堆燃料组件典型棒束通道内的流动和传热现象进行了数值模拟。模拟得到不同Re下的压降系数曲线与Nu曲线,并将计算结果与经验公式的计算结果进行了比较,两者符合较好。研究了组件内3类典型子通道的横向流交混效应,分析了3类典型子通道的横向流分布特点,发现角子通道横向流交混强度沿轴向波动较大,而3类子通道的横向流交混强度均存在周期性。研究了中心燃料棒壁面上3个截面的局部换热效应,发现在燃料棒与绕丝接触处传热效果最差,在事故分析时应重点关注。     

Turbulent momentum transport in rod bundles

An experimental investigation was performed with air to obtain detailed information on the velocity and turbulence distribution for parallel turbulent flows through subchannels of rod bundles. Experimental results were obtained for wall and corner subchannels of rod bundles of four parallel rods. The pitch-to-diameter ratios were varied between 1.07 and 1.4. The Reynolds numbers ranged from 6 × 10⁴ to 2 × 10⁵, depending on the rod bundle arrangement.On the basis of the data measured, the eddy viscosities in the directions normal and parallel to the wall were calculated. The experimental results of the velocity and wall shear stress distributions are compared with the predictions by the VELASCO code. There are considerable differences between computed and experimental results especially for low pitch-to-diameter ratios. The reasons for the discrepancies are discussed together with the results of attempts to adjust the VELASCO code against the experimental data.     

PIV and CFD Study of Turbulent Flow Behavior in Rod Bundle

At the downstream of the spacer grid in a PWR fuel assembly, local disturbance damps out along the flow direction and the flow returns to stable eventually. The turbulent flow and mixing behavior of the coolant are key factors affecting the economy and safety of a nuclear reactor, and need in-depth investigations. In the present paper, the turbulent flow of water in a 3×3 rod bundle was studied using PIV (particle image velocimetry) and CFD. First-order mean velocity and second-order turbulent statistics were obtained. It is found that the velocity in the central subchannel is higher than that in the gap region, but the streamwise root-mean-square velocity behaves inversely. Large-scale flow pulsation induced by the strong streamwise velocity gradient between adjacent subchannels, is observed in the rod bundle, and the wave length increases with Reynolds numbers. In addition, the measured turbulent mixing coefficient is 10% higher than that predicted by the Castellana correlation for PWRs, but this deviation reduces with the increase of Reynolds numbers.     

棒束内湍流流动特性的PIV与CFD研究

在压水堆燃料组件的定位格架下游,局部扰动沿流动方向逐渐衰减,流场最终趋于稳定。光滑棒束区冷却剂的湍流流动和交混特性是影响反应堆经济性和安全性的重要因素,有必要进行深入研究。本文采用粒子图像测速（PIV）与数值模拟(CFD)相结合的方法,对3×3小规模棒束内水的流动特性进行研究,得到了一阶平均流速以及二阶湍流统计信息。结果表明,中心子通道的速度明显高于棒间隙区,但轴向均方根速度呈现出相反的变化趋势。在相邻子通道横向速度梯度的作用下,棒束内出现了大尺度的流量脉动现象,且脉动波长随雷诺数的增加而增大。此外,实验得到的湍流交混系数较压水堆采用的Castellana公式预测值偏高10%左右,这一偏差随雷诺数的增加有减小的趋势。