Experimental studies and CFD calculations for buoyancy driven mixing phenomena

In nuclear reactor safety the mixing of borated and deborated water is a critical issue that needs investigation, assessment and prediction. Such mixing is buoyancy driven and numerical codes must correctly model momentum transfer between fluids of different density. To assess and develop CFD models for buoyancy driven mixing we set up a simple vertical mixing test facility (VeMix) and equipped it with a newly developed planar electrical imaging sensor. This imaging sensor acquires conductivity images of the liquid at the rear channel wall with a speed of 2,500 frames/s. By adding NaCl tracer to the denser fluid we were able to visualize the mixing process in high spatial and temporal detail. Furthermore, an image processing algorithm based on the optical flow concept was implemented and tested which allows the measurement of flow pattern velocities. Selected experiments at different Richardson numbers were run with two components of different density (pure water and glucose-water mixture) simulating borated and deborated water in a light water reactor scenario. These experiments were compared to CFD calculations using standard turbulence models. Good agreement between experimental data and CFD simulations was found.     

CFD simulations of light gas release and mixing in the Battelle Model-Containment with CFX

In this validation work two turbulence models (k– and SST model) and two grids (a finer hybrid grid and a tetrahedral coarser grid) are considered in order to model helium release and dispersion. Simulation results are compared against an experiment of jet release phenomena in the Battelle Model Containment facility (BMC), a multi-compartment facility with a total volume of about 560 m³. In the selected test, HYJET Jx7, helium was released into the containment at a speed of 42 m/s over a time of 200 s. Although the k– model is the most commonly used turbulence model in most Computational Fluid Dynamics (CFD) applications, it does not provide the most accurate predictions for this application. Alternatively the SST turbulence model has been employed giving more accurate results. This investigation provides a further confirmation that the validation of commercial CFD codes is always required in order to select the more suitable physical models and computational grids for each specific application.     

Experimental and computational thermal-hydraulic results of fluid and thermal mixing tests

Experimental and computational analyses of a mixing test of cold and hot water flows in a rectangular tee model of the cold leg downcomer geometry of pressurized water reactor were performed. Results obtained from COMMIX-1A computer code calculations showed reasonable agreement with the experimental findings. Counter-current flow and thermal stratification in the cold leg were observed in both the experimental and calculated results for certain ranges of test parameters.     

Modeling of containment atmosphere mixing and stratification experiment using a CFD approach

An experiment on containment atmosphere mixing and stratification, which was originally performed in the TOSQAN facility in Saclay (France), was simulated with the Computational Fluid Dynamics code CFX4.4. The TOSQAN facility consists of a large cylindrical vessel in which gases are injected. In the considered experiment, steam, air and helium were injected during different phases of the experiment, with steam condensing on some parts of the vessel walls. During certain phases, steady states were obtained when the steam condensation rate became equal to the steam injection rate, with all boundary conditions remaining constant. In the present work, three such intermediate steady states were simulated independently. The essential purpose was to reproduce the non-homogeneous structure of the vessel atmosphere, given that condensation is simulated in such a way to obtain the proper condensation rate. A two-dimensional axisymmetric model of the TOSQAN vessel for the CFX4.4 code was developed. The flow in the simulation domain was modeled as single-phase. Steam condensation on vessel walls was modeled as a sink of mass and energy. Calculated profiles of temperature, steam concentration, and velocity components are compared to experimental results and discussed. The comparison suggests that atmosphere mixing and stratification in an NPP containment at accident conditions could be successfully simulated using the proposed CFD approach.     

A population balance approach considering heat and mass transfer—Experiments and CFD simulations

Bubble condensation in sub-cooled water is a complex process, to which various phenomena contribute. Since the condensation rate depends on the interfacial area density, bubble size distribution changes caused by breakup and coalescence play a crucial role.Experiments on steam bubble condensation in vertical co-current steam/water flows have been carried out in an 8 m long vertical DN200 pipe. Steam is injected into the pipe and the development of the bubbly flow is measured at different distances to the injection using a pair of wire mesh sensors. By varying the steam nozzle diameter the initial bubble size can be influenced. Larger bubbles come along with a lower interfacial area density and therefore condensate slower. Steam pressures between 1 and 6.5 MPa and sub-cooling temperatures from 2 to 12 K were applied. Due to the pressure drop along the pipe, the saturation temperature falls towards the upper pipe end. This affects the sub-cooling temperature and can even cause re-evaporation in the upper part of the test section. The experimental configurations are simulated with the CFD code CFX using an extended MUSIG approach, which includes the bubble shrinking or growth due to condensation or re-evaporation. The development of the vapour phase along the pipe with respect to vapour void fractions and bubble sizes is qualitatively well reproduced in the simulations. For a better quantitative reproduction, reliable models for the heat transfer at high Reynolds number as well as for bubble breakup and coalescence are needed.     

Boron dilution transients during natural circulation flow in PWR—Experiments and CFD simulations

Coolant mixing in the cold leg, downcomer and the lower plenum of pressurized water reactors is an important phenomenon mitigating the reactivity insertion into the core. Therefore, mixing of the de-borated slugs with the ambient coolant in the reactor pressure vessel was investigated at the four loops 1:5 scaled Rossendorf coolant mixing model (ROCOM) mixing test facility. In particular thermal hydraulics analyses have shown, that weakly borated condensate can accumulate in the pump loop seal of those loops, which do not receive a safety injection. After refilling of the primary circuit, natural circulation in the stagnant loops can re-establish simultaneously and the de-borated slugs are shifted towards the reactor pressure vessel (RPV).In the ROCOM experiments, the length of the flow ramp and the initial density difference between the slugs and the ambient coolant was varied. From the test matrix experiments with 0 resp. 2% density difference between the de-borated slugs and the ambient coolant were used to validate the CFD software ANSYS CFX. To model the effects of turbulence on the mean flow a higher order Reynolds stress turbulence model was employed and a mesh consisting of 6.4 million hybrid elements was utilized. Only the experiments and CFD calculations with modeled density differences show stratification in the downcomer. Depending on the degree of density differences the less dense slugs flow around the core barrel at the top of the downcomer. At the opposite side, the lower borated coolant is entrained by the colder safety injection water and transported to the core. The validation proves that ANSYS CFX is able to simulate appropriately the flow field and mixing effects of coolant with different densities.     

CHF experiment and CFD analysis in a 2 × 3 rod bundle with mixing vane

In this study, the CHF enhancement using various mixing vanes is evaluated and the flow characteristics are investigated through the CHF experiments and CFD analysis.CHF tests were performed using 2 × 2 and 2 × 3 rod bundles and with R-134a as the working fluid. The test section geometry was identical to that of commercial PWR fuel assembly not including the heated length (1.125 m) and number of fuel rods. From the CHF tests, it was found that the CHF enhancement using mixing vanes under higher mass flux (1400 kg/m² s) and lower pressure (15 bar) conditions is larger than the CHF enhancements under other conditions. Among the mixing vanes used in this study, the swirl vane showed the best performance under relatively low pressure (15 bar) and mass flux (300-1000 kg/m² s) conditions and the hybrid vane performed best near the PWR operating conditions.The detailed flow characteristics were also investigated by CFD analysis using the same conditions as the CHF tests. To calculate the subcooled boiling flow, the wall partitioning model was applied to the wall boundary and various two-phase parameters were also considered. The reliability of the CFD analysis in the boiling analysis was confirmed by comparing the average void fractions of the analysis and the experiments: the results agreed well. From the CFD analysis, the void fraction flattening as a result of the lateral velocity induced by the mixing vane was observed. By the lateral motion of the liquid, the void fraction in the near wall was decreased and that of the core region was increased resulting in the void fraction flattening. The decrease of the void fraction in the near wall region promoted liquid supply to the wall and consequently the CHF increased. For the quantification of the void flatness, an index was developed and the applicability of the index in the CHF assessment was confirmed.     

Computational fluid dynamic analysis of a guide tube in a PWR

Security in nuclear power plants demands severe limitations of the maximal drop time of rod cluster control assemblies. In February 1995, several assemblies of the Chinese plant in Daya Bay failed to comply with these requirements. Electricité De France undertook a research program to get a better insight of this problem since the plant has been built by French and also because the French new four-loops N4 reactor was equipped with the same guide tubes. This paper is limited to a numerical study of the influence of the pressure forces applied to control rods and due to flow circulation through the guide tubes. After a validation test case, a first calculation has been carried out on a simplified N4 guide tube. The sensitivity of the pressure forces to transverse flow and to modifications of the geometry has been determined. The program has been extended to guide tubes used in 1300-MW reactors and similar computations have been done. To make simulations more representative, a global computation of the flow in the whole upper internals plenum (UIP) will be achieved to provide accurate boundary conditions for local calculations with better resolution.     

Fluid mixing and flow distribution in a primary circuit of a nuclear pressurized water reactor—Validation of CFD codes

The EU project FLOMIX-R was aimed at describing the mixing phenomena relevant for both safety analysis, particularly in steam line break and boron dilution scenarios, and mixing phenomena of interest for economical operation and the structural integrity.This report will focus on the computational fluid dynamics (CFD) code validation. Best practice guidelines (BPG) were applied in all CFD work when choosing computational grid, time step, turbulence models, modelling of internal geometry, boundary conditions, numerical schemes and convergence criteria. The strategy of code validation based on the BPG and a matrix of CFD code validation calculations have been elaborated. CFD calculations have been accomplished for selected experiments with two different CFD codes (CFX, FLUENT). The matrix of benchmark cases contains slug mixing tests simulating the start-up of the first main circulation pump which have been performed with three 1:5 scaled facilities: the Rossendorf coolant mixing model ROCOM, the Vattenfall test facility and a metal mock-up of a VVER-1000 type reactor. Before studying mixing in transients, ROCOM test cases with steady-state flow conditions were considered. Considering buoyancy driven mixing, experimental results on mixing of fluids with density differences obtained at ROCOM and the FORTUM PTS test facility were compared with calculations. Methods for a quantitative comparison between the calculated and measured mixing scalar distributions have been elaborated and applied. Based on the “best practice CFD solutions”, conclusions on the applicability of CFD for turbulent mixing problems in PWR were drawn and recommendations on CFD modelling were given. The results of the CFD calculations are mostly in-between the uncertainty bands of the experiments. Although no fully grid-independent numerical solutions could be obtained, it can be concluded about the suitability of applying CFD methods in engineering applications for turbulent mixing in nuclear reactors.     

CFD simulations of hydrogen combustion in a simplified EPR containment with CFX and REACFLOW

The prediction of over-pressures and temperatures that are generated by hydrogen explosions in case of a severe nuclear accident is a crucial stage of the safety analysis of the containment. The investigation presented in this paper is a continuation of the numerical studies of validation and benchmarking that were carried out in the European co-sponsored project HYCOM. In the present work, numerical simulations of hydrogen deflagrations within a simplified, real-scale European Pressure Reactor (EPR) containment have been performed with two CFD codes, CFX4 and REACFLOW. The analysis has been focused not only on overpressure peaks and pressure oscillations, but also on pressure differences between the two sides of the same wall of internal compartments. Different geometrical configurations have been considered in term of presence of vents between internal compartments and in term of vents number, size and position. Single and multiple ignition points have also been taken into account. The paper describes the main results of the investigation and it is a demonstration of how CFD modelling can provide significant indications for real-scale safety applications within the limits of uncertainty of the accident scenarios.     

Experimental tests on the applicability of a radioactive tracer technique for measuring coolant mixing in nuclear reactor fuel subassemblies

A previously reported radioactive tracer method for measuring coolant mixing hot channel factor is developed here. Feasibility of the technique is demonstrated with a test section simulating part of a reactor sub-channel. The test section consists of a tube containing three hollow rods simulating the fuel rods, mounted in a small hydraulic loop. Tests were carried out by injecting a ³²P activated phosphate into the main flow at Reynolds number ranging from 20 000 to 70 000. The concentration of the tracer was measured by detecting the β-rays of ³²P with three windowless Geiger-Müller counters, placed inside the hollow rods, β-particles reach the counters through openings in various axial positions along the length of the rods, covered by a thin stainless steel foil. Measured values of the concentration are reported and corresponding values of coolant cross-flow are calculated.     

CFD analysis for thermal mixing in a subcooled water tank under a high steam mass flux discharge condition

A Computational Fluid Dynamics (CFD) analysis for a thermal mixing test was performed for 30 s to develop the methodology for a numerical analysis of the thermal mixing between steam and subcooled water and to apply it to Advanced Power Reactor 1400 MWe (APR1400). In the CFD analysis, the steam condensation phenomenon by a direct contact was simulated by the so-called condensation region model. Thermal mixing phenomenon in the subcooled water tank was treated as an incompressible flow, a free surface flow between the air and the water, and a turbulent flow, which are implemented in the CFX4.4. The comparison of the CFD results with the test data showed a good agreement as a whole, but a small local temperature difference was found at some locations. A sensitivity analysis was performed to find the reason of the temperature difference. The commercial CFD code of CFX4.4 together with the condensation region model can simulate the thermal mixing behavior reasonably well when a sufficient number of mesh distributions and a proper numerical method are selected.     

球床规模对孔隙流动特性影响的CFD模拟研究

对不同规模的有序堆积球床结构进行建模,并使用计算流体力学(CFD)方法对球床孔隙通道内的单相流动进行数值模拟。球床结构的孔隙区域采用混合网格划分策略,球床的计算规模达到11层,共141个颗粒,能够较真实地反映较大规模球床内部的孔隙流动特性。同时考察了球床的规模大小对其孔隙流动以及床层流动压降的影响规律,获得不同径向规模体心堆积球床的阻力关系式。针对截面积较小的方形通道分析了边壁效应对孔隙流动的影响机理,并通过孔隙流道的沿程压降评估了球床入口/出口效应对床层阻力的影响范围。     

Thermal hydraulic performance analysis of the printed circuit heat exchanger using a helium test facility and CFD simulations

The thermal-hydraulic performance of the PCHE was investigated using the KAIST helium test loop. Experiments were performed in the helium laminar region with 350 < Re < 1200. The hot/cold side inlet conditions were 25–550 °C/25–100 °C over the operating pressure of 1.5–1.9 MPa, respectively. Mass flow rates were controlled in the range of 40–100 kg/h. Pressure drop and temperature difference were measured at the inlet and outlet of the hot and cold sides. A global Fanning factor correlation and a global Nusselt number correlation were proposed using information only at the inlet and outlet of the hot and cold sides. A three-dimensional (3-D) numerical simulation was performed using FLUENT, a commercial computational fluid dynamics (CFD) code, to compare simulation results to the KAIST helium test data and to obtain the local Nusselt number in the PCHE. CFD predictions showed good agreement with experimental data. A local pitch-averaged Nusselt number correlation was proposed using local temperature, pressure, surface heat fluxes, and properties provided by CFD simulations. The system analysis code, GAMMA, was also utilized to identify which correlation was more applicable for system analysis. It turns out that the proposed local pitch-averaged Nusselt number correlation from CFD simulations is more appropriate than the global Nusselt number correlation developed from experimental data.     

CFD analysis of flow and heat transfer of turbulent pulsating flow in a tube in rolling motion

The investigation of flow and heat transfer of turbulent pulsating flow is of vital importance to the nuclear reactor thermal hydraulic analysis in ocean environment. In this paper, the flow and heat transfer of turbulent pulsating flow is analyzed. The calculation results are firstly verified with experimental data. The agreement between them is satisfactory. The effect of spanwise and wall-normal additional forces is significant in small Reynolds number, and decreases with Reynolds number increasing. The rolling axis and rolling radius contribute slight to the flow and heat transfer. The effect of velocity oscillation period on the heat transfer is limited than that of Reynolds number and oscillating velocity Reynolds number. The traditional empirical correlations could not predict the flow and heat transfer of turbulent pulsating flow in rolling motion.     

Analyses on fluid flow and heat transfer inside Calandria vessel of CANDU-6 using CFD

In a CANada Deuterium Uranium (CANDU) reactor, fuel channel integrity depends on the coolability of the moderator as an ultimate heat sink under transient conditions such as a loss of coolant accident (LOCA) with a coincidence of a loss of emergency core cooling (LOECC), as well as a normal operating condition. This study presents the assessments of moderator thermal–hydraulic characteristics in the normal operating condition and one transient condition for CANDU-6 reactors, using a general purpose three-dimensional computational fluid dynamics code. This study consists of two steps. First, an optimized calculation scheme is obtained by many-sided comparisons of the predicted results with the related experimental data, and by evaluating the fluid flow and temperature distributions. Then, in the second step, with the optimized scheme, the analyses for real CANDU-6 of normal operating condition and transition condition have been performed. The present model has successfully predicted the experimental results and also reasonably assessed the thermal–hydraulic characteristics of the real CANDU-6 with 380 fuel channels. Flow regime map with major parameters representing the flow pattern inside Calandria vessel has also proposed to be used as operational and/or regulatory guidelines.     

Investigation on the characteristics of an atmospheric-pressure microplasma plume confined inside a long capillary tube

An atmospheric-pressure microplasma plume of diameter 10 μm is generated inside a long tube. The length of the microplasma plume reaches as much as 2 cm. First, with the assistance of an air dielectric barrier discharge (DBD), the ignition voltage of the microplasma decreases from 40 kV to 23.6 kV. Second, although the current density reaches as high as (1.2−7.6)×10⁴ A cm ⁻² , comparable to the current density in transient spark discharge, the microplasma plume is non- thermal. Third, it is interesting to observe that the amplitude of the discharge current in a positive cycle of applied voltage is much lower than that in a negative cycle of applied voltage. Fourth, the electron density measured by the Stark broadening of Ar spectral line 696.5nm reaches as high as 3×10¹⁶ cm⁻³ , which yields a conductivity of the microplasma column of around 48 S m⁻¹ . In addition, the propagation velocity of the microplasma plume, obtained from light signals at different axial positions, ranges from 1×10⁵ m s ⁻¹ to 5×10 ⁵ m s⁻¹ . A detailed analysis reveals that the surface charges deposited on the inner wall exert significant influence on the discharge behavior of the microplasma.     

Unsteady numerical simulations of the single-phase turbulent mixing between two channels connected by a narrow gap

This paper focuses on the unsteady numerical simulation of the turbulent flow in two types of geometry containing a narrow gap with the explicit algebraic Reynolds stress model. The model was first validated through the comparison of simulation results inside a rectangular channel containing a cylinder and the corresponding experimental data. The structures of the oscillation were correctly reproduced. Simulation of turbulent mixing between circular channels connected by a narrow gap was carried out with the validated model. Because of the influence of the strong anisotropic turbulent flow in the gap region, the mixing rate was dramatically enhanced by the cyclic and almost periodic flow pulsation. The calculation results of the turbulent mixing rate showed good agreement with the experiment and the maximum error was less than 15%.