Akira Yamaguchi

Resistance and Fluctuating Pressures of a Large Elbow in High Reynolds Numbers

For the Japan Atomic Energy Agency sodium-cooled fast reactor, an experimental study on the fluctuating pressure of the hot legs was carried out with tests in a 1/3-scale model. The total resistance coefficient is consistent with published data, and, additionally, our research has given data up to the Reynolds number of 8.0×106. The flow visualization and velocity measurement confirmed the independence of the flow on the Reynolds number. Pressures on the pipe wall were statistically examined to predict the characteristics of fluctuating pressures of the hot legs. It reveals that generation of fluctuating pressure is dominant on the boundary of flow separation and reattachment.

Numerical Approach to the Safety Evaluation of Sodium-Water Reaction

A numerical simulation method of multi-dimensional and multi-phase reacting flow (SERAPHIM code) has been developed to evaluate the sodium-water reaction (SWR) phenomena in a steam generator of liquid metal fast reactor (LMFR). A compressible multi-fluid and one-pressure model is adopted and pressure and velocity fields are updated simultaneously by the HSMAC method. Two types of reaction models are considered; one is a surface reaction and the other is a gas-phase reaction. The surface reaction model assumes that water vapor reacts with the liquid sodium at the gas-liquid interface. If chemical reaction heating is large enough, liquid sodium is vaporized resulting in a gas-phase reaction. In the surface reaction, the reaction rate is assumed to be infinitely large. Several overall reaction equations are taken into account in the gas-phase reaction and the reaction rates are described in the form of the Arrhenius law. In the present study, adequacy of the analytical procedures for compressible multi-phase flow is validated by a benchmark calculation of the Edwards pipe blowdown problem. As a numerical example, two- and three-dimension analyses of the single-tube geometry and the two-dimension analyses of the 43-tubes geometry are carried out. It is concluded that the numerical quantification of the SWR accident by the SERAPHIM code is practicable and further use of the SERAPHIM code is useful to resolve safety issues immanent in the SWR.

Validation study of computer code sphincs for sodium fire safety evaluation of fast reactor

Abstract A computer code sphincs solves coupled phenomena of thermal hydraulics and sodium fire based on a multi-zone model. It deals with an arbitrary number of rooms, each of which is connected mutually by doorways and penetrations. With regard to the combustion phenomena, a flame sheet model and a liquid droplet combustion model are used for pool and spray fires, respectively, with the chemical equilibrium model based on the Gibbs free energy minimization method. The chemical reaction and mass and heat transfer are solved interactively. A specific feature of sphincs is detailed representation of thermalhydraulics of a sodium pool and a steel liner, which is placed on the floor to prevent sodium–concrete contact. The authors analyzed a series of pool combustion experiments, in which gas and liner temperatures are measured in detail. It has been found that good agreement is obtained and the sphincs code has been validated with regard to pool combustion phenomena. Further research needs are identified for pool spreading modeling considering thermal deformation of steel liner and measurement of pool fluidity property as a mixture of liquid sodium and reaction products. The sphincs code is to be used mainly in the safety evaluation of the consequence of a sodium fire accident in a liquid metal cooled fast reactor as well as fire safety analysis in general.

Numerical investigation of multi-dimensional characteristics in sodium combustion

Abstract Multi-dimensional sodium combustion behavior has been numerically investigated in the present paper. A new computer code AQUA-SF has been developed for this purpose. The code includes two sodium combustion models (so called ‘spray combustion’ and ‘pool combustion’), a mass and heat transfer model considering a six-flux gas radiation and a coagulation and sedimentation model of sodium oxide and hydroxide aerosols. The sodium spray combustion rate is evaluated by a summation of the combustion rate of each sodium droplet with an individual diameter. A flame sheet model is applied to situations where sodium spreads out on the floor and a pool combustion takes place. The model assumes an infinitely thin flame above the pool surface and is based on a mass and energy balance in the flame. As the results of numerical analyses of a sodium spray combustion test, a location of high-temperature core region and a maximum temperature agrees with the experiment. Good agreements of an overall transient behavior are obtained in a large-scale sodium combustion test analysis. The numerical analyses also demonstrate that the distributions of temperature and chemical species concentration vary with sodium combustion modes. If sodium scatters and the spray combustion is dominant, the distributions vary in space. When a large amount of sodium exists on a floor and the pool area is enlarged, the distributions are more uniform in space.

Numerical Methodology of Sodium-Water Reaction with Multiphase Flow Analysis

Abstract A numerical methodology of sodium-water reaction (SWR) and a coupling method of SWR and multiphase flow analysis are proposed. Two SWR models are considered. One is a surface reaction model, which assumes that water vapor reacts with liquid sodium at the gas-liquid interface. The surface reaction is likely to be dominant in the initial phase of SWR. The analogy between mass and heat transfers is assumed to evaluate the diffusion-controlled reaction rate. The other is a gas-phase reaction model. If chemical reaction heating due to the surface reaction is large enough to vaporize the liquid sodium, it turns over in the gas-phase reaction. In the gas-phase reaction, water vapor reacts with sodium gas. The reaction mechanisms in the gas-phase reaction are investigated using an ab initio molecular orbital method. The reaction rate of the gas-phase reaction described by the Arrhenius law is obtained from the transition-state theory or the capture theory. The reaction models are employed in a compressible multifluid and one-pressure model using the Highly Simplified Marker and Cell method for multiphase flow analysis. As numerical examples, surface reaction with multiphase flow analysis and simplified gas-phase reaction analyses are carried out. It is confirmed that the present method is practically applicable to the coupling phenomena of SWR and multiphase flow.

Numerical Simulation of Non-Premixed Diffusion Flame and Reaction Product Aerosol Behavior in Liquid Metal Pool Combustion

In the present study, a numerical methodology has been developed to solve a non-premixed diffusion flame under natural convection. The methodology has been applied to the calculation of the liquid sodium pool combustion experiment at various pool temperatures and oxygen molar fractions. The authors have proposed expressions of aerosol dynamics, radiation heat transfer and evaporation of liquid sodium that are applicable to sodium combustion phenomena. The computations reproduce the experimental observations concerning burning rate, flame temperature and flame height, consistently. The aerosol release fractions are also in good agreement with the measurement. Dominant mechanisms of the mass and heat transfer are identified through the numerical simulation. An intrinsic feature found in the present study is that the liquid sodium pool combustion is self-limited and a negative feedback mechanism is at work. Interaction among the thermal-hydraulics, chemical reaction and aerosol dynamic behavior plays an important role in the phenomena and it has been successfully analyzed by the numerical simulation. The present method can be used to understand sodium combustion phenomena and applied to the modeling of sodium pool combustion for safety analyses of liquid metal fast reactors. The numerical simulation is a useful tool because it can easily employ various conditions by changing parameters.

Analysis of jet flows with the two-fluid particle interaction method

The particle interaction method called MPS (Moving Particle Semi-implicit) method has been developed in recent years, which is formulated by representing the differential operators in Navier-Stokes equation as the interaction between particles characterized with a kernel function and adopts a mesh-free algorithm. This method is suitable especially for treating liquid breakup. We extended the MPS method to two-fluid system, introduced a potential-type surface tension, and modified the calculation algorithm to simulate jet flows. The objective of this study is to evaluate the interfacial area (or, so called binary contact area) of immiscible two-fluid systems with a chemical reaction, where one is injected as a jet into a pool of the other fluid. As a first step, we investigated if the proposed method is capable of reproducing the hydrodynamics of jet flow by analyzing Tanasawas experiment. In this paper, we describe the formulation and the calculation algorithm of the method, and results of the verification studies.

Flow-Induced Vibration of a Large-Diameter Elbow Piping Based on Random Force Measurement Caused by Conveying Fluid: Visualization Test Results

A 1/3 scale flow-induced vibration test facility that simulates the hot-leg piping of the JNC sodium-cooled fast reactor (JSFR) is used to investigate the pressure fluctuations of the pipe, where the high velocity fluid flows inside the piping. By the measurement of the pressure drop in the elbow piping while changing the Reynolds number, the similarity law of this model is confirmed. To evaluate the flow-induced vibrations for the hot-leg and cold-leg pipes, the random force distributions along the pipe and their correlations are measured with pressure sensors in a water loop. It is found that a flow velocity-dependent periodic phenomenon in the rear region of the elbow, and the maximum flow-induced random vibration force in the pipe are observed in the region of flow separation downstream the elbow. Finally, a design method is proposed with power spectral densities of the pressure fluctuations classified into four sections, correlation lengths in the axial direction divided into three sections, and with correlation lengths in the tangential direction into four sections.Copyright

Numerical simulation of a free-falling liquid sodium droplet combustion

Abstract Droplet combustion is considered one of the combustion figurations in sodium-fire events; the detailed combustion mechanism by which it occurs is evaluated by a developed computational fluid dynamic (CFD) simulation code, called COMET, in which the extended MAC method is employed for calculating the reacting compressible flow coupled with the multi-component diffusion of chemical species. A single droplet combustion in steady-airflow was simulated using the COMET to analyze: (1) the spatial distributions of heat rate and temperature, and (2) the formation, decomposition, and transfer of combustion products. Next, a free-falling droplet combustion experiment was simulated for code validation, where the evaluated falling-velocity and burnt-mass showed good agreement with the experimental values.

Numerical Analyses of Flashing Jet Structure and Droplet Size Characteristics

In this paper, flashing jets are numerically simulated using the MPS method. The boiling mode for flashing is identified as surface boiling mode, based on the postulation of jets from a short nozzle under high depressurization. The Homogeneous Non-equilibrium Relaxation Model (HRM) is used for calculating the evaporation rate of flashing. The numerical simulation results show that flashing jets comprise an inner intact core which is surrounded by two-phase droplet flow. The effect of degree of superheat on the jet topological geometry is investigated. With increasing degree of superheat, the topological shape of flashing jets evolves from cylindrical core for low degree of superheat to cone-shaped core for high degree of superheat, and meanwhile the extinction length comes to decrease and tends asymptotically constant as the injection temperature approaches the saturation temperature corresponding to the injection pressure. The analyses of the droplet size distribution engendered from primary breakup of flashing jets show that: two peaks exist for droplet size distribution at lower degree of superheat; however, merely one peak for higher degree of superheat. From droplet size distribution, it is revealed that the primary breakup mechanism of flashing jets can be attributed to dominant mechanical breakup mode plus enhancement via surface evaporation.