A computational study of flow mal-distribution on the thermal hydraulic performance of an intermediate heat exchanger in LMFBR

The flow and thermal non-uniformities occurring in the intermediate heat exchanger (IHX) of a liquid metal-cooled fast breeder reactor have been characterized through numerical simulations. For modeling the primary and secondary sodium flow through the IHX, an equivalent anisotropic porous medium approach has been used. The pressure drop in the equivalent porous medium is accounted through the inclusion of additional pressure drop terms in the Navier–Stokes equations, with the help of standard correlations for cross flow or parallel flow over tubes. For secondary sodium flow, the effects of a flow distributor device with orifices and baffles at the inlet have also been included, in addition to axial flow through the tubes. The heat exchange between primary and secondary streams is incorporated in the form of a volumetric heat source or sink term, which is corrected iteratively. The resulting flow distributions are in reasonable agreement with available experimental results. The study shows that the temperature of the secondary sodium flow at the exit can be made more uniform by exchanging less heat near the inner wall of IHX, as compared to the region close to the outer wall, using suitable flow distribution devices.     

Dynamic simulation of accidental closure of intermediate heat exchanger isolation valve in a pool type LMFBR

In a pool type liquid metal cooled fast breeder reactor (LMFBR), core and other internals such as pumps, heat exchangers are immersed in a pool of sodium. Heat exchange from primary sodium circuit (pool) to secondary sodium circuit (loop) is through four intermediate heat exchangers (IHX) immersed in primary sodium pool. Each IHX is provided with a sleeve valve at its primary sodium inlet window for the purpose of isolating the shell side of IHX from the sodium pool. With such a provision, an inadvertent partial or complete closure of a sleeve valve of one of the IHX during normal operation of the reactor has been considered as a design basis event for the reactor. One dimensional transient thermal hydraulic models of the primary and secondary sodium circuits have been developed to study the thermal hydraulic consequences of such an event. The main areas of concern in the plant and the availability of safety parameters for the detection of the event have been evaluated.     

Assessment of flow induced vibration in a sodium-sodium heat exchanger

The 500 MWe Prototype Fast Breeder Reactor (PFBR) is under construction at Kalpakkam. It is a liquid metal sodium cooled pool type fast reactor with all primary components located inside a sodium pool. The heat produced due to fission in the core is transported by primary sodium to the secondary sodium in a sodium to sodium Intermediate Heat Exchanger (IHX), which in turn is transferred to water in the steam generator. PFBR IHX is a shell and tube type heat exchanger with primary sodium on shell side and secondary sodium in the tube side. Since IHX is one of the critical components placed inside the radioactive primary sodium, trouble-free operation of the IHX is very much essential for power plant availability. To validate the design and the adequacy of the support system provided for the IHX, flow induced vibration (FIV) experiments were carried out in a water test loop on a 60° sector model. This paper discusses the flow induced vibration measurements carried out in 60° sector model of IHX, the modeling criteria, the results and conclusion.     

A thermal hydraulic calculation method of an intermediate heat exchanger of a loop type FBR

This paper describes a thermal-hydraulic calculation of an intermediate heat exchanger (IHX) with the computational fluid dynamics (CFD) code CFX. The motivation of this paper is to clarify a heat transfer degradation phenomenon in the IHX through three-dimensional calculation. The whole IHX of the “Monju” reactor is modeled with three parts, i.e., the primary side, the secondary side and the heat transfer region. Through a partial calculation using these models, the flows on the primary side and the secondary side are shown to be axisymmetric. Therefore, a sector model is adopted for the calculation model in the heat transfer region. The calculated temperatures in the IHX are compared with the measured results using the IHX in the “Monju” reactor. Good agreement is obtained for the predicted outlet temperatures and temperatures on the shell surface. As a result of the CFD calculation, it is evaluated that a heat transfer in the lower plenum on the secondary side is dominant under the low flow rate conditions. This fact contributes to lower the heat transfer coefficient in the IHX when the standard heat exchanger theory is applied to calculate the overall heat transfer coefficient between the primary and the secondary sides.     

基于固有频率分析的中间热交换器管束支撑设计

在进行示范快堆电站的中间热交换器(IHX)设计时,需考虑流致振动对管束的影响。在计算IHX换热管的固有频率时,空间弯管结构形式是一个技术难点。通过有限元方法,对中国实验快堆(CEFR)的IHX换热管固有频率进行了计算。通过对比分析,得到了合理的计算模型及其适用范围。通过敏感性分析,提出管束支撑方案的设计原则。     

R&D on high temperature components

Experimental studies were carried out to confirm the structural integrity of the key high temperature components, which are necessary to commercialize the HTGR, that is, an intermediate heat exchanger (IHX) and a coaxial hot gas duct. As for the IHX, the following tests were conducted: (1) creep collapse of tube against external pressure, (2) creep fatigue of tube against thermal stress, (3) seismic behavior of tube bundles, (4) thermal hydraulic behavior of tube bundles and (5) in-service inspection technology of tube with full-scale models. As for the coaxial hot gas duct, the thermal insulation performance as well as the structural integrity was investigated with the HENDEL-loop.     

高温气冷堆中间换热器层流边界层的传热特性分析

为提高高温气冷堆中间换热器的换热效率,对中间换热器单元流道的层流流动边界层及热边界层进行了理论计算和模拟分析。利用幂次多项式速度分布的方法,对流道内的层流流动边界层和热边界层进行计算,结果表明,三次式的速度分布更为接近Blasius精确解,也更为符合数值模拟的结果。同时,结合不同位置的温度分布,可发现上、下边界层的相遇会使流动进入泊肃叶的抛物线形式,边界层理论继而不再适用。通过模拟还发现,氦气的进口温度和出口压力对于层流边界层的换热基本无影响;恒定热流密度下,壁面温度的变化分为明显的边界层区和泊肃叶区,在这两个区域壁面温度呈线性变化。     

Thermal Performance Tests on a Sodium-to-Sodium Heat Exchanger

Thermal performance of a 3MW sodium-to-sodium intermediate heat exchanger (IHX) was evaluated under temperature conditions typical of a Fast Breeder Reactor IHX. A regenerative figure of eight loop was used with the heat exchanger at the cross over point, and a 500 kW heat source and an air cooled sink to maintain the desired test conditions. The overall heat transfer coefficient was found to vary from 4.02 to 4.87 kW/m²·K for Peclet numbers varying from 37 to 112.5 on the shell side and 44.4 to 133.5 on the tube side respectively. The Peclet numbers were representative of low turbulent regime in this case. While the overall heat transfer coefficient was found close to predictions using Lubarsky's correlation, it was somewhat lower than that predicted by later correlations of Spukunsky & Borishansky. The reasons for the lower overall heat transfer coefficient have been explained in terms of possible maldistribution of shell side flow in low turbulent regime reducing the effective heat transfer area and increased thermal contact resistance. Based on their findings the authors feel that heat transfer in a sodium-to-sodium heat exchanger at low Peclet numbers is expected to differ from that obtained with large Peclet numbers.     

Feasibility study on the applicability of a diffusion-welded compact intermediate heat exchanger to next-generation high temperature gas-cooled reactor

The development of an intermediate heat exchanger (IHX) transferring high temperature heat to a process heat application is of prime importance for a next-generation high temperature gas-cooled reactor (HTGR). The IHX needs high structural integrity and reliability over 900°C for a long duration. A plate fin type compact heat exchanger (PFCHX) has a large heat transfer area per heat exchanger volume and is expected to be used as the IHX. However, the brazing for connecting fins and plate is not reliable when existing PFCHXs are used in a high temperature condition for a long time. We have proposed a concavo-convex plate type compact heat exchanger (CPCHX) which consists of concavo-convex plates (CPs) welded by solid state diffusion and made of nickel-based superalloy Hastelloy XR. In our study, first, an optimized condition for the solid state diffusion welding between the CPs of the CPCHX was found by experiments using test pieces made of Hastelloy XR. Second, small-scale diffusion-welded CPCHXs were designed, manufactured and installed in a test loop to investigate the reliability of the diffusion welding. As a result of leakage tests, it was confirmed that the reliability of the solid state diffusion welding between the CPs of the small-scale CPCHX is sufficient. A thermal performance test revealed that the thermal conductance of the small-scale CPCHX was better than calculated. In addition, a design study for the CPCHX was performed to investigate the feasibility of the diffusion-welded CPCHX to the IHX in a next-generation HTGR.     

CEFR中间热交换器一次侧数值模拟

应用CFX程序对中国实验快堆（CEFR）一回路中间热交换器（IHX）一次侧的1/6进行三维稳态模拟。计算获得IHX一次侧速度场、换热管束的温度分布及变化规律。结果表明,中间热交换器满足设计要求,能保证主热传输系统的正常工作。计算结果可为快堆调试及运行提供技术支持和理论依据。     

Simplified optimum sizing and cost analysis for compact heat exchanger in VHTR

In this study, the optimum size of the compact heat exchanger has been developed based on its weight and pumping power for the reference design of 600 MWt very high temperature gas-cooled reactor (VHTR) system. Alloy 617 was selected as a construction material. The optimum size and a number of configurations for the reference design of the VHTR with an intermediate heat exchanger (IHX) were investigated and our initial calculations indicated that it has an unrealistically too large aspect ratio of the length and height due to its small-sized channels, which might cause manifolding problems and a large number of parallel modules with high thermal stress. The flow area and channel diameter were then adjusted to achieve a smaller aspect ratio in this paper. Achievement of this aspect ratio resulted in higher cost, but the cost increase was less than would have occurred by simply reducing the flow area by itself. The appropriate channel diameter is estimated to be less than 5.00 mm for the reference system. The effect of channel waviness enhanced the compactness and heat transfer performance, but unfavorably increased the aspect ratio. Therefore, the waviness should be carefully determined based on performance and economics. In this study, the waviness of the IHX is recommended to be selected between 1.0 and 2.5. Calculations show that reducing the duty dramatically decreases the aspect ratio, indicating that the compact heat exchanger is easy to be optimally designed for low duty, but many modules are required for high duty operation proportional to the operating power. Finally, the effect of working fluids was investigated, and it reveals that using carbon dioxide instead of helium in the secondary side reduces the size and cost by about 20% due to the lower pumping power in spite of its lower heat transfer capability by a factor of 4.     

高温气冷堆螺旋管式直流蒸汽发生器热工水力学

高温气冷堆蒸汽发生器具有一次侧氦气工质、二次侧直流、螺旋管结构、工作温度高等特点，其热工水力特性与传统压水堆自然循环蒸汽发生器存在很大区别。针对高温气冷堆蒸汽发生器的特点，对其基础热工水力及特有热工水力学问题进行了阐述，主要包括螺旋管内单相及两相流阻及换热计算、横掠螺旋管束流阻及换热计算、温度均匀性及两相流不稳定性等。同时介绍了清华大学核能与新能源技术研究院针对高温气冷堆蒸汽发生器热工设计、温度均匀性及两相流不稳定性等热工水力学问题所开发的一维稳态程序、一维瞬态程序、二维分析程序和方法，并对分析结果和结论进行了讨论。相关研究方法、程序和结论对其他相似参数螺旋管和直管式直流蒸汽发生器具有参考和借鉴意义。     

Distribution of liquid sodium in the inlet plenum of steam generator in a Fast Breeder Reactor

Experimental and Computational Fluid Dynamics (CFD) investigations have been carried out on a 1/5th scale model of the inlet plenum of steam generator (SG) used in the Fast Breeder Reactor (FBR) technology. The distribution of liquid sodium in the inlet plenum of the steam generator strongly affects the thermal as well as mechanical performance of the steam generator. In the present work, flow distribution in a scaled down model has been investigated. Various strategies adopted for obtaining uniform flow distribution have been evaluated. Experiments have been conducted to measure the axial and radial velocity distributions using Ultrasonic Velocity Profiler (UVP) under a variety of geometries. Computational Fluid Dynamics (CFD) studies have been carried out for various geometries. On the basis of these experiments and CFD simulations, various flow distribution devices have been compared.     

螺旋管直流蒸汽发生器一、二次侧耦合传热特性分析

为获得螺旋管直流蒸汽发生器（HCOTSG）螺旋换热管内两相流动换热特征，以国际革新安全反应堆（IRIS）HCOTSG为研究对象建立了HCOTSG一、二次侧耦合热分析模型，分析了稳态工况下，不同二次侧给水流量对HCOTSG热工水力参数产生的影响，并将所建立的HCOTSG一、二次侧耦合热分析模型与计算流体力学软件（CFX）三维流动换热计算相结合，对HCOTSG稳态工况下螺旋管内精细的热工水力参数进行计算。通过HCOTSG一、二次侧耦合热分析模型计算得到HCOTSG稳态工作时沿管程的相关热工水力参数；通过CFX三维模拟发现螺旋管横截面流体流速和温度分布不均匀现象，得到螺旋内侧流体温度高于螺旋外侧，螺旋内侧流体速度低于螺旋外侧，螺旋内侧流体比螺旋外侧流体先开始沸腾的结论。因此，本研究对于HCOTSG稳态运行和螺旋换热管事故分析具有指导作用。      

螺旋管直流蒸汽发生器一、二次侧耦合传热特性分析

To study the flow and heat transfer characteristics of the primary and secondary sides of the helical coil once-through tube steam generator (HCOTSG) under steady state conditions, taking HCOTSG of International Reactor Innovative and Secure (IRIS) as the research object, a primary and secondary sides heat balance calculation model for steady state operation of HCOTSG is established. The influence of different secondary side feed water flow rate on HCOTSG thermal and hydraulic parameters under steady-state condition is analyzed, and the detailed thermal and hydraulic parameters in the helical tube under steady-state condition are calculated by combining the coupled thermal analysis model with the three-dimensional flow and heat exchange calculation of CFX. The relevant thermal and hydraulic parameters along the tube side of HCOTSG during steady-state operation are calculated by the thermal analysis model. The CFX simulation results show that the velocity and temperature distribution of the fluid in the cross section of the helical tube are not uniform. The temperature of the fluid inside the helix is higher than that outside the helix. The velocity of the fluid inside the helix is lower than that outside the helix. The boiling of the fluid inside the helix occurs earlier than that outside the helix. Therefore, this study has a guiding role in the accident analysis for HCOTSG steady-state operation and spiral heat exchange tube.     

示范快堆一回路主管道断裂事故最佳估算分析研究

与安全裕量有关的研究一直是反应堆安全设计与安全分析的重点和难点问题。本文针对池式示范快堆CFR600的设计特点,对主热传输系统中的重要现象进行了分析,并建立了最佳估算模型,基于Wilks方法对CFR600一回路主管道断裂事故进行了不确定性量化计算。最佳估算分析结果表明,CFR600在一回路主管道断裂事故下,包壳最高温度95%/95%上限为851?6 ℃,相较于保守分析结果具有约91?8 ℃裕量,低于包壳破损验收准则。     

Flow and Heat Transfer Characteristics of Swordfish Fin Microchannel

The supercritical carbon dioxide Brayton cycle is a new generation of thermal cycle used in the fourth generation of nuclear energy. As a high or low temperature recuperator of supercritical carbon dioxide Brayton cycle, the thermal hydraulic characteristics of the compact microchannel heat exchanger directly affect the power cycle efficiency. Reducing flow resistance of the temperature recuperator while maintaining high heat transfer efficiency is an important research for microchannel heat exchanger optimization design. The swordfish fin microchannel design considering bionics theory can significantly reduce the flow resistance. In this work, the swordfish fin heat exchanger model was established with supercritical carbon dioxide fluid as the flow medium. The effect of swordfish fin design with different arrangements on heat transfer characteristics was analyzed by three-dimensional numerical simulation. At the same time, the thermal hydraulic characteristics of swordfish fin design were compared with those of traditional commercial Z-shaped microchannel heat exchanger. The results show that under the same Reynolds number, the Nusselt number of the swordfish fin microchannel is twice as much as that of the Z-shaped microchannel, but the pressure drop is only half of that. Therefore, the thermal hydraulic performance of swordfish fin microchannel heat exchanger is obviously better than that of Z-shaped heat exchanger. It is obtained from optimization analysis that the optimal pitches for swordfish fin design is that the L_a＝8 mm along the flow direction, and the L_b＝6 mm perpendicular to the flow direction.     

Pool boiling heat transfer enhancement on porous surface tube

The passive residual heat removal exchanger (PRHR HX),which is a key equipment of the passive residual heat removal system,is installed in an elevated pool.Its heat transfer performance affects security and economics of the reactor,and boiling heat transfer in the liquid surrounding the exchanger occurs when the liquid saturation temperature exceeded.The smooth tubes,which are widely used as heat transfer tubes in PRHR HX,can be replaced by some enhanced tubes to improve the boiling heat transfer capability.In this paper,the pool boiling heat transfer characteristics of smooth tube and a machined porous surface tube are investigated by using high-pressure steam condensing inside tube as heating source.Compared with smooth tube,the porous surface tube considerably enhances the boiling heat transfer,and shortens the time significantly before reaching the liquid saturation temperature.Its boiling heat transfer coefficient increases from 68% to 75%,and the wall superheat decreases by 1.5oC.Combining effect of condensation inside tube with boiling outside tube,the axial wall temperatures of heat transfer tube are neither uniform nor linear distribution.Based on these investigations,enhance mechanism of the porous surface tube is analyzed.     

箭鱼形翅片微通道流动换热特性研究

超临界二氧化碳布雷顿循环是第4代核能采用的新一代热能循环系统。紧凑式微通道换热器作为超临界二氧化碳布雷顿循环的高低温回热器，其流动换热特性直接影响整体热电转化的效率。降低回热器的流动阻力，同时维持较高的换热效率是微通道换热器优化设计的重要研究内容。箭鱼形翅片微通道设计借鉴仿生学原理，理论上可显著降低流动阻力。本文以超临界二氧化碳为流动工质，建立箭鱼形翅片换热器的模型并进行三维数值模拟，分析不同排列下的箭鱼形翅片设计对换热器流动换热特性的影响。同时对箭鱼形翅片设计与传统商用折线形微通道换热器流动换热特性进行对比分析。研究分析表明，在相同雷诺数下，箭鱼形翅片微通道的努塞尔数为折线形流道的2倍，而压降仅为其1/2，所以箭鱼形翅片微通道换热器的流动换热特性明显优于折线形换热器。通过优化分析，发现箭鱼形翅片设计最优的排列间距为沿流动方向的翅片间距L_a＝8 mm，垂直于流动方向的翅片间距L_b＝6 mm。     

压水堆蒸汽发生器一、二次侧稳态流场耦合分析

蒸汽发生器（SG）在运行过程中主要面临流致振动所导致的传热管破裂事故,而流致振动分析需以SG内的三维两相流场作为输入条件。采用多孔介质模型,对SG二次侧流场进行求解,同时耦合一、二次侧换热,获得SG二次侧速度场、温度场、压力场及流动含气率分布,并获得传热管一维的一、二次侧流体温度和换热系数及传热管温度分布。由于一次侧向二次侧释热极不均匀,SG内流场分布及汽水分离器内的含气率分布极不均匀;汽水分离器内的最大、最小含气率分别为0.62和0.05,该参数可为汽水分离器负载设计提供依据。通过计算还获得弯管区速度分布,该分布可为传热管的流致振动磨损评估提供输入条件。