Steam Generator Design Research Articles

IMPLEMENTATION OF THE HIERARCHICAL APPROACHIN THE MATHEMATICAL MODELLINGOF ONCE-THROUGH STEAM GENERATORS

A comprehensive system analysis of a once-through steam generator was carried out as well as a multilevel structure of its model was developed. The application of the procedure of decomposition of a complex object at the initial stages of modeling made it possible to single out multidimensional subsystems of directed action. This makes it possible to use advanced computer simulation software. We distinguished the subsystems of the steam generator as a whole, the subsystem of the steam generator, including the screen tubes, separator, mixer, filter, circulation pump, and connecting pipelines in the resulting structural model. The zones of screen tubes determined by the state of the working medium (heating zone with a single-phase medium, evaporation zone I and evaporation zone II with a two-phase medium), finite-dimensional models of screen tube sections of heating and evaporation zones we considered separately. It was found that the models of zero-level subsystems are described by systems of differential and algebraic equations, between the internal variables of which there is no cause-effect relationship. Any subsystem of the first and higher levels can be represented by a subset of subsystems of the immediately lower levels and a set of oriented connections between them. The principle of recurrent explanation was implemented in the problem of simulating a once-through steam generator. The set-theoretic, matrix and graphical methods are used to describe the relationships between subsystems. It is shown that hierarchical models are forms of description, ready for implementation in high-level programming languages. Systematic analysis of processes, technology and design of a once-through steam generator, as well as the proposed research methodology in the time and frequency domains, the calculation methods and simulation methods used, allows you to select the types and classes of mathematical models, forms of their presentation, as well as software.

제4세대 원자로 중간열교환기-일체형 증기발생기의 고온 설계 및 건전성 평가

본 연구에서는 제4세대 원자로인 소듐 냉각고속로에 적용하고자 하는 중간열교환기 일체형 증기발생기(ICSG)의 구조 개념을 도출하고, 동 설계에 대해 고온 건전성 평가를 통해 구조적 구현 가능성을 분석하였다. ICSG는 열유체 설계를 통해 M자형(serpentine) 튜브 형상으로 설계되었고 동 전열관 형상을 갖는 새로운 개념의 증기발생기 구조에 대해 건전성 평가를 수행하였다. 동 기기는 크리프 영역의 고온에서 가동되기 때문에 3D 유한요소 해석 기반 온도 및 응력 해석을 수행하고 프랑스의 고온 설계 기술기준인 RCC-MRx를 따라 건전성 평가를 수행하였다. 평가 결과, M자형 튜브를 채택한 신개념 ICSG는 모든 설계 기술 기준의 요건을 만족하여 기계구조적으로 구현 가능한 구조인 것으로 나타났다.

Microchannel steam generators for design for integral inherently safe light water reactors

A steam generator design for the Integral Inherently Safe Light Water Reactor (I2S-LWR) based on microchannel heat exchanger (MCHX) technology is discussed. An experimentally validated heat transfer and pressure drop model is applied to the development of steam generator design for this LWR. The potential of MCHX technology to provide compact, high power density steam generators for integral inherently safe reactors is demonstrated. A Rankine cycle model for a system with a once-through MCHX steam generator is developed and the plant thermal efficiency is quantified. Heat transfer and pressure drop in the MCHX steam generator design for I2S-LWR is compared with those in a conventional tubular steam generator design. A simple exergy analysis is performed for two alternative I2S-LWR power cycles, the flash Rankine cycle and a Rankine cycle with once-through steam generation, to demonstrate the advantages of using an integral steam generator.

Design and application of a novel coal-fired drum boiler using saline water in heavy oil recovery

In this paper, the design and operation of a novel coal-fired circulating fluidized bed (CFB) drum boiler that can generate superheated steam using saline water were introduced. The natural circulation water dynamics with a drum was adopted instead of the traditional once-through steam generator (OTSG) design, so that superheated steam can be generated for the better performance of the steam assisted gravity drainage (SAGD) technology in heavy oil recovery. The optimized staged evaporation method was proposed to further decrease the salinity of water in the clean water section of the boiler. The evaporating pipes of the salted water section were rearranged in the back pass of the boiler, where the heat load is low, to further improve the heat transfer safety. A CFB combustion technology was used for coal firing to achieve a uniform heat transfer condition with low heat flux. Pollutant control technologies were adopted to reduce pollutant emissions. Based on the field test, the recommended water standard for the coal-fired CFB drum boilers was determined. With the present technology, the treated recovery wastewater can be reused in steam-injection boilers to generate superheated steam. The engineering applications show that the boiler efficiency is higher than 90%, the blowdown rate is limited within 5.5%, and the superheat of steam can reach up to 30 K. Besides, the heavy oil recovery efficiency is significantly improved. Moreover, the pollutant emissions of SO2, NOx and dust are controlled within the ranges of 20–90 mg/(N·m3), 30–90 mg/(N·m3)and 2–10 mg/(N·m3) respectively.

Operational Flexibility of Two-Phase Flow Test Rig for Investigating the Dynamic Instabilities in Tube Boiling Systems

The safe operation of a two-phase heat exchanger can be performed by determining the instability threshold values of power plant parameters. Thus, the power plant parameters must be designed outside these thresholds to avoid undesirable instability. The fluctuations in mass flow and system pressure are undesirable processes, resulting in system failure. In diverse heat transfer distribution, that can lead to burn-out of heat exchanger tubes. Therefore, the maintaining of flow stability in a power plant is of particular relevance. The researchers and engineers can predict the threshold of flow instability with dynamic models validated with experimental records. The phenomenon of dynamic flow instabilities in two-phase flows is an important issue that has high relevance for many industries. The awareness about the triggers and the effects of fluid dynamic instabilities is of great importance in the design and operation of steam generators, refrigeration systems, thermosiphons or boiling water reactors. These instabilities manifest themselves in variations of mass flow, pressure, and fluid properties. In this work, the dynamic instabilities involved in evaporation processes were briefly discussed with simple models to understand the mechanism of its different types. Additionally, it is presented the design and construction of a two-phase flow test rig using the similarity and the scaling criteria. A heat recovery steam generator (HRSG) designed by Doosan Heavy Industries and Construction is scaled down to 4 × 4 × 2 m3. The combination of the flow diagram and the thermohydraulic design is represented in three-dimension model of the test rig using the Siemens NX 10.0 software. The flexibility of operation was taken into account in the design of this test rig. Finally, we provided preliminary results to showcase some functionalities and the capability of the test rig to characterize the existing flow pattern through the evaporator and investigate the internal characteristic curve of the evaporator.

Operational Flexibility of Two-Phase Flow Test Rig for Investigating the Dynamic Instabilities in Tube Boiling Systems

The safe operation of a two-phase heat exchanger can be performed by determining the instability threshold values of power plant parameters. Thus, the power plant parameters must be designed outside these thresholds to avoid undesirable instability. The fluctuations in mass flow and system pressure are undesirable processes, resulting in system failure. In diverse heat transfer distribution, that can lead to burn-out of heat exchanger tubes. Therefore, the maintaining of flow stability in a power plant is of particular relevance. The researchers and engineers can predict the threshold of flow instability with dynamic models validated with experimental records. The phenomenon of dynamic flow instabilities in two-phase flows is an important issue that has high relevance for many industries. The awareness about the triggers and the effects of fluid dynamic instabilities is of great importance in the design and operation of steam generators, refrigeration systems, thermosiphons or boiling water reactors. These instabilities manifest themselves in variations of mass flow, pressure, and fluid properties. In this work, the dynamic instabilities involved in evaporation processes were briefly discussed with simple models to understand the mechanism of its different types. Additionally, it is presented the design and construction of a two-phase flow test rig using the similarity and the scaling criteria. A heat recovery steam generator (HRSG) designed by Doosan Heavy Industries and Construction is scaled down to 4 × 4 × 2 m3. The combination of the flow diagram and the thermohydraulic design is represented in three-dimension model of the test rig using the Siemens NX 10.0 software. The flexibility of operation was taken into account in the design of this test rig. Finally, we provided preliminary results to showcase some functionalities and the capability of the test rig to characterize the existing flow pattern through the evaporator and investigate the internal characteristic curve of the evaporator.

Two-phase flow frictional pressure drop prediction in helical coiled tubes

Based on the construction of different experimental databases, the frictional pressure drop in helical tubes for single-phase and water-steam two-phase flow was analyzed empirically. The aim of the present study is to provide the scientific community with a reliable and simple to use predictive tool for estimating frictional pressure drop in helical tubes covering operating conditions of interest for the design and operation of coiled tube once-through steam generators, as those used in small modular advanced nuclear reactors.The single-phase database contains around 2000 data points from 14 different sources, including data measured with air and water as working fluids, covering Reynolds numbers ranging from 40 to 145,000 and curvatures between 0.0027 and 0.3. The prediction capability of some existing correlations for simple phase flow was analyzed, concluding that Ito correlations for laminar and turbulent regime (1959) are the best ones fitting the experimental data. The average difference between experimental data and Ito correlations was 3.75% for laminar regime and 2.11% for turbulent regime.The two-phase flow database was built with data available from open literature works. Due to the fact no existing correlation was found to successfully predict the available data, a new prediction tool based on the homogeneous equilibrium model was developed for water-steam flows at conditions of interest.The proposed tool, so-called FEMA correlation, converges to the Ito correlations for single phase flow, i.e. for thermodynamic qualities equal to 0 and 1. In addition, the correlation successfully fits the experimental data with an average error of 7.4% which is smaller than any other existing correlation. In addition, the new correlation is recommended for water-steam flow in vertical helical tubes with liquid only Reynolds numbers ranging from 20000 and 124000, pressures between 1 and 8 MPa, curvatures between 0.01 and 0.08, and thermodynamic qualities ranging from 0 to 1.

Investigation into Factors Causing Damage to Low-Pressure Loop Evaporating Tubes of Large-Capacity Heat-Recovery Steam Generators

The article addresses factors causing damage—in particular, flow accelerated corrosion wear (FAC) of tube walls—to the evaporator operating in the low-pressure loop of the Ep-258/310/35-15.0/3.14/0.44-540/535/263 (P-132) heat-recovery steam generator used as part of a 795-MW combined-cycle plant. Factors influencing the FAC, namely, improper water chemistry, availability of gas shunts in the steam generator, and high mixture motion velocity in the tubes in combination with the flow path configuration, are described. The damages inflicted to the tubes and their locations in the heat-recovery steam generator low-pressure loop are described. It is shown that gas shunts in the heat-recovery steam generator result in an increased heat absorption and higher velocity of medium in the boundary tubes of low-pressure evaporator tube banks, which intensifies the tube’s wear process. It has been found that the water chemistry used for the Kirishi District Power Plant (DPP) heat-recovery steam generator was not the factor that caused damage to the low-pressure loop tubes. The Kirishi DPP steam generator was compared with other similar large-capacity heat-recovery steam generators. For analyzing the low-pressure evaporator performance indicators, the loop and the processes occurring in it were modeled using the Boiler Designer software. An analysis of the results from comparison of the steam generator design and performance characteristics has shown that it is necessary to change the two-phase mixture motion velocity in the Kirishi DPP steam generator low-pressure loop by modifying the design or operating parameters. The wear of the low-pressure evaporator tube walls was mathematically modeled, the results of which confirm that erosion wear is one of the main factors causing damage to the tubes. The erosion is caused by the intense dynamic effect of two-phase, high-velocity flow jets on the tube walls. General recommendations for decreasing the wear of heat-recovery steam generator tubes are given. It has been determined that the increased wear of tubes in the low-pressure loop of the P-132 steam generator at the Kirishi DPP is caused by a combination of a few factors, such as high velocity of steam–water mixture, availability of bends, and unsatisfactory quality of aligning the tubes at their welding places.

Heat transfer performance of U-tube molten salt steam generator

Heat transfer and thermal performance of U-tube molten salt steam generator was experimentally reported with inlet water flow rate of 22.4-89.6 L/h, water temperature of 20-100°C, salt flow rate of 1-2.2 m3/h and salt temperature of 306.5-395.3°C. The heat transfer mechanism of U-tube molten salt steam generator was experimentally revealed, and overall heat transfer coefficient was influenced by salt convection and water boiling, while thermal efficiency was determined by heat absorption and heat loss. As inlet water flow rate rose, overall heat transfer coefficient and thermal efficiency rose initially and decreased afterwards, while they dropped with inlet water temperature rising as subcooling boiling changed to saturation boiling. As salt temperature increased, salt convection was enhanced, while boiling heat transfer coefficient rose initially and dropped afterwards with superheat rising, so overall heat transfer coefficient and thermal efficiency rose initially and dropped afterwards. As salt flow rate rose, overall heat transfer coefficient under low salt temperature rose for boiling heat transfer coefficient increment, and that under high salt temperature first rose and then dropped as boiling curve had maximum heat flux, while thermal efficiency dropped as heat loss and wall temperature increasing. Molten salt heat transfer outside U-tube bundle was remarkably higher than that inside tube, while it was lower than that outside straight tube bundle. Heat transfer correlation of salt outside U-tube bundle was first proposed, and that had good agreement with experiment data with maximum deviation 7.8%. The heat transfer mechanism and correlation in present work can provide reliable basis for design and application of U-tube molten salt steam generator.

An investigation of impact-sliding behavior and fretting wear of tubes against different supports in steam generators

Purpose The purpose of this study is to reveal the interaction behavior between tubes and supports in steam generators and study the fretting wear of tubes in different load conditions. Design/methodology/approach The fretting wear tests were conducted to investigate the fretting wear behavior of the tubes against three kinds of supports: the drilled circular holes (DCH), anti-vibration bars (AVBs) and trefoil orifice holes (TOH), which are widely used supports in nuclear steam generators. In this paper, the comparison of the interaction characteristic with different impact factors was established such as clearances and loads in the three kinds of supports. The fretting wear volume and scars were analyzed by a scanning electron microscope and 3D profiler. Findings The results show that impact can play a more important role in the DCH and TOH supports than that in AVBs. The normal work rate can be underestimated in the DCH and TOH supports. Originality/value The results of this study can be reference of fretting wear calculation in the design of steam generators with different kinds of supports and can be guidance in the maintenance of steam generators. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-12-2019-0513/

Разработка модели процесса сепарации влажного пара в паровом пространстве парогенератора ПГВ-1000М

The issue of reducing steam humidity at the output of steam generator is relevant. The value of humidity directly affects the safety and efficiency of power plants. The optimization of steam generator design will enable to enhance its separation properties and reduce steam humidity. Creating a numerical model of wet steam separation process in a full-scale steam generator and its verification will allow proceeding to optimize the steam generator design and evaluate the model effectiveness. This article presents a preliminary study of the wet steam separation process in the steam space of PGV-1000M steam generator. To study the wet steam separation process in the steam space of PGV-1000M steam generator, a numerical model was developed in the ANSYS Fluent finite element analysis system. The following assumptions were made: the surface of the evaporation mirror is flat, drops have a spherical shape, they do not affect the movement of steam, they do not interact with each other, and there is no decay of the droplets. A three-dimensional model of the steam space of PGV-1000M steam generator which allows considering the processes of wet steam separation has been obtained. The analysis of the results has shown that the nature of the processes occurring in the model corresponds to theoretical calculations and operational data. The developed model has been verified and can be used to optimize the steam generator design. Further numerical studies of the developed model will enable to determine the most optimal design of the steam generator which provides the highest efficiency of steam separation. Moreover, it is possible and promising to study the effect of the evaporation mirror surface on the steam humidity in the steam generator. Decreasing the steam humidity at the steam generator output at existing and projected power plants will provide significant savings in funds spent on repairing the steam turbine blade apparatus, and will lead to an increase in the thermal efficiency of the plant.

A method to account for transient performance requirements in the design of steam generators for concentrated solar power applications

Concentrating solar power plants are strongly characterized by recurring start-up and shut-down procedures. This imposes new challenges for conventional components such as the steam generator systems, as frequent load variations might lead to high thermal stress cycles, affecting their lifetime negatively. In this context, the header and coil design is a favourable configuration to reduce stresses. This paper introduces a method to design the heat exchangers of the header and coil steam generator type accounting for the dynamic performance, thermal stress sensitivity and impact on the performance of the power plant. Optimal designs were determined by minimizing the cost and total water-side pressure drop of the steam generator and the levelized cost of electricity of the power plant. The steam generator dynamic model was successfully validated using operational data. The results suggest that a steam generator design characterized by a tube outer diameter of 30 mm, high steam generator pressure drop and low investment cost is the optimal solution for the power plant under consideration. A configuration featuring a large number of tube layers is optimal in order to reduce the pressure drop at the superheater while at the same time, guaranteeing acceptable stresses and good transient response.

Numerical simulation of the effects of trefoil tube support plates on the flow and heat transfer characteristics of a steam generator

A three-dimensional coupled model of heat transfer tube and trefoil tube support plate (TSP) is developed based on a full-size steam generator (SG). The CFX thermal phase change model is adopted to numerically investigate the flow and heat transfer behaviors of the secondary side fluid. And the effects of trefoil TSPs and inlet subcooling on the thermal-hydraulic characteristics of tube bundle are obtained. The calculation model reasonably reveals the distributions of the flow and heat transfer parameters in the SG, especially in trefoil TSP regions. Based on the across flow velocity and density of the secondary side fluid in U-bend region, the most severe flow-induced vibration damage is predicted at the angles of 110° on the hot leg and 60° on the cold leg, respectively. Due to the effects of trefoil TSPs, vortexes are induced, the outer wall temperature in the circumferential direction shows periodic characteristic in TSP regions, which increases the possibility of aggressive solutes deposition and U-tubes rupture. In addition, inlet subcooling of the secondary side fluid has significant effects on the SG. As inlet subcooling decreases, the heat transfer is enhanced. The numerical simulation results obtained may provide guidance to facilitate the optimal design and in-depth thermal-hydraulic analysis of TSPs and SGs.

Ag/polypyrrole co-modified poly(ionic liquid)s hydrogels as efficient solar generators for desalination

Solar steam generation is one of the most promising technologies to utilize solar energy, which is applied to extract fresh water from seawater or contaminated water. However, solar vapour generation is usually restricted to the underutilization of solar energy or depends on complicated light-harvesting adjuncts. Here, a novel hierarchically double-layered evaporation device was demonstrated based on poly(ionic liquid)s hydrogel with the upper surface co-modified by polypyrrole (PPy) and silver particles (named as Ag/PPy-PMBA-BrILs). The evaporation system exhibits a cooperative interaction of the double-layer with efficient light-absorbing and water transportation, leading to a high evaporation rate of 1.37 kg m−2 h−1 with a photothermal conversion efficiency of 88.7% under 1 sun illumination. The high evaporation rate is mainly due to the synergistic light absorption of polypyrrole and silver particles as well as the excellent water transfer capability of poly(ionic liquid)s hydrogel. Moreover, the hydrogel-based evaporation device also exhibits excellent salt-resistant performance for solar desilination in artificial sea water. The high-efficiency poly(ionic liquid)s hydrogel based solar steam generator with co-modified light-absorbing surface may open new opportunities for the future design and fabrication of high-performanced solar steam generator for practical applications.

ОБОСНОВАНИЕ ТЕХНИЧЕСКИХ РЕШЕНИЙ РЕАКТОРНОГО КОНТУРА УСТАНОВОК БРС-ГПГ МАЛОЙ И СРЕДНЕЙ МОЩНОСТИ С ТЯЖЕЛЫМ ЖИДКОМЕТАЛЛИЧЕСКИМ ТЕПЛОНОСИТЕЛЕМ

The analysis and new scientific and technical solutions corresponding to the evolutionary development of low and medium power reactor plants with heavy liquid metal coolants are presented. The analysis was carried out on the basis of experience in the creation and operation of reactor installation with lead-bismuth coolant and research, primarily experimental, performed at the Nizhny Novgorod State Technical University (NSTU) in rationale of the small and medium-sized power plants developed at NNSTU reactor installation with horizontal steam generators (BRS-GPG). The paper substantiates the choice of coolant. With the proposed arrangement of the reactor loop, the maximum possible natural circulation is achieved, which significantly increases the safety of reactor installation. The results of studies (simulations) of one of the most potentially dangerous accidents in reactor installation with HLMC - ″large steam generator leak″ can qualitatively reduce the consequences of the accident when using the design of a horizontal steam generator in which the pipe system is placed with minimal burial of pipes under the level of HLMC, which eliminates the flow of water into the reactor core, re-pressurization of the reactor circuit, etc. It is probably advisable to consider the characteristics of the core, analog ary accepted for SVBR or BREST-OD-300. A system is proposed for cooling the reactor and providing the reactor installation shutdown modes, its characteristics are studied and tested at the NSTU stands.

Numerical investigation of helical coil tube bundle in turbulent cross flow using large eddy simulation

A realistic five-layered helical coil tube bundle in turbulent cross flow is numerically investigated using large eddy simulation (LES). The geometry of the helical coil tube bundle, which is a 1/45 sector of a helical coil steam generator design for an advanced small modular reactor, has three counterclockwise helical layers and two clockwise helical layers stacked in alternating fashion in the radial direction. A Reynolds number of 21,800 is considered based on the mean gap velocity, diameter of the coil tube, and kinematic viscosity of the working fluid. The WALE sub-grid scale model is employed for LES to resolve scale smaller than the grid size. Additionally, LES for an ideal five-layered coil bundle model without a helical geometry is utilized to generate comparison data. Instantaneous flow fields are explored by analyzing velocity magnitude, vorticity, static pressure, and vortical structures. The detachments of vortical structures from the coil tube surface are observed by visualizing vortical structures and wall shear stress distributions simultaneously. Various turbulent statistics at multiple monitoring locations are presented with an emphasis on feature difference between realistic and ideal coil bundle models. The results of spectral analyses, including the power spectral density and continuous wavelet transform analyses, are addressed from the perspective of flow-induced vibration. The key feature of this paper is the discussion of three-dimensional effects based on the helical geometry of a coil bundle.

Design of self-righting steam generators for solar-driven interfacial evaporation and self-powered water wave detection

Solar-driven interfacial evaporation, desalination and self-powered water wave detection are synergistically achieved by developing a self-righting steam generator with a tumbler-shaped monolithic structure.

Numerical investigation of the thermal-hydraulic characteristics of AP1000 steam generator U-tubes

Steam generators (SGs) are vital heat exchangers that connect the primary and secondary sides in power plant systems. Based on the AP1000 SG, a numerical model of SG U-tubes is developed. The interior flow and heat transfer behaviors of the SG are simulated using CFX thermal phase change model. The numerical model reasonably predicts the distributions of the thermal-hydraulic behaviors, including the velocity, temperature, steam quality, void fraction and heat transfer coefficient. The slip-ratio of the secondary side increases and then decreases slowly along the axial direction. The simulated heat transfer coefficient agrees with the calculation of the Rohsenow formula. The most serious flow-induced vibration (FIV) damage is expected at the angles of θ = 60° in cold leg and θ = 110° in hot leg according to the fluid density and velocity of the secondary side in U-bend region, and the simulated results agree with the drift-flux model. The current simulation results may provide useful technical information for the optimal structural design of SGs.

Russian and foreign steam generators for NPP power units with wet steam turbines

The main aim of the current study is to analyze advantages and shortcomings of horizontal and vertical types of steam generator design. Design solutions and experience of operation of steam generators of horizontal type accepted in Russia and of vertical type applied by Westinghouse, Combustion Engineering, Siemens, Mitsubishi, Doosan were analyzed within the framework of the present study. It was established that steam generator equipment of horizontal type is characterized by disadvantages of design, technological and operational nature. Thus, horizontal steam generators with dimensions permissible for railroad transportation and, for VVER-1200 with reactor vessel diameter equal to 5 m, by water transport as well, have exhausted the possibilities for further significant increase of the per unit electric power. The demonstrated advantages of vertical-type steam generators are as follows: 1) absence of stagnant zones within the second cooling circuit; 2) uniformity of heat absorption efficiency of the heating surface that ensures improved conditions for moisture separation; 3) increased temperature drop with parameters of generated steam elevated by 0.3 – 0.4 MPa. Conclusion was made on the advisability of introduction of steam generators with vertical-type layout in the Russian nuclear power generation.

Experimental study on the circulating-cavity flow and an innovative central baffle design in a steam generator

Steam generator (SG) is the key equipment in pressurized water reactors (PWR), which transfers heat from primary circuit to secondary circuit and has the feed water vaporized into the steam. It is related to the safe, reliable and economical operation of the nuclear power plant. Many researches have been done on SG, including numerical simulation and experimental research. Since it is involved with complicated steam-water (two phase) flow in high temperature and high pressure, it is not easy to measure the key parameters such as pressure, temperature and void fraction, especially to carry out the visual observation. So the detailed working information of SG such as void fraction and flow pattern is still unknown, while this information is very important for the improvement of SG performance. In order to obtain the working information of SG and study the dynamic flow process in secondary side, a visualization scaled-down mock-up experimental bench was set up. Although its operation parameters (temperature and pressure) are much lower than the actual one in PWR, the internal dynamic flow process of SG in this facility is kept similar to the actual one by the scaling analysis and design. Appling the high speed camera, particle image velocimetry (PIV) and self-made optical fiber probes, two phase flow behavior including flow pattern, velocity and void fraction were collected and analyzed in the experiment. From the experiment, a circulation phenomenon is discovered; which is defined as circulating-cavity flow (CCF). At U-bend area, CCF flows from hot side to cold side, which may result in flow-induced vibration (FIV) and jeopardize the U-tubes. In order to limit CCF, a baffle installed in the middle of U-bend area is proposed to suppress CCF. The experimental results show that this baffle can effectively suppress the CCF. This paper may contribute to the design and safety of SG.