Akiko Kaneko

Development of Prediction Technology of Two-Phase Flow Dynamics Under Earthquake Acceleration — (4) Measurement of Bubble Deformation Near Wall Under Flow Rate Fluctuation

In order to clear the two-phase flow behavior under earthquake, a systematic study is done experimentally and numerically. The present study is one on the series of the study on two-phase flow under earthquake, and focuses on the flow rate fluctuation. The flow rate fluctuation was added to bubbly or plug flow in a horizontal pipe, and flow behavior was measured by PIV and image processing. The bubble deformation near the pipe wall was observed and the velocity field around the bubble was shown. The bubble coalescence was also observed under the flow rate fluctuation condition.Copyright

Fluctuation of Void Fraction in the Microbubble Generator With a Venturi Tube

Microbubbles are tiny bubbles with less than 1 mm diameter. These bubbles are utilized in various engineering fields, and it is very important to understand physics of flow with microbubbles. Especially, void fraction is one of the significant parameter for two-phase flow. Thus, developments of real-time measurement systems of void fraction are required. In the nuclear power engineering, electrical void fraction measurement methods have been proposed as one of the real-time measurement techniques. In the present study, we apply this method to a microbubble generator with a venturi tube and examine the performance of the generator. Constant electrical current method is adopted as electrical measurement method of void fraction. Microbubbles are generated with a bubble collapse phenomenon through a venturi tube. We can generate microbubbles in high void fraction. However, mechanism of bubble collapse in a ventrui tube is not made clear and void fraction distribution toward flow direction is less understood. The applicability of constant electrical current method in bubbly flow and the process of the bubble breakup in a venturi tube are discussed. In this experiment, a voltage between two electrodes in the generator is measured with various gas-liquid volume flows as inlet conditions. From results we succeeded to measure the void fraction profile in the venturi tube with constant electrical current method. The void fractions achieve a peak before the bubble collapse and it decreased drastically for 10 mm after collapse.Copyright

Study on the Interfacial Behavior and Thermal Characteristics in Liquid-Vapor Flow of the Supersonic Steam Injector

Supersonic steam injector is a passive jet pump which operates without rotating power source or machinery and it has high heat-transfer performance due to the direct contact condensation between supersonic steam flow and subcooled water jet at the mixing nozzle. Since the supersonic steam injector has a quite simple and compact structure, it has been considered to apply to the safety system for the Next-generation nuclear power plant. There are various researches about the formulation and modeling of operating, flow structure and heat transfer characteristics of both vapor and liquid flow. However, there are few models which are capable of evaluating heat and momentum exchange at the boundary layer between supersonic steam flow and water jet. Since heat and momentum exchange is considered to have a major impact to operating characteristics of a supersonic steam injector, it is necessary to formulate the model which simulates such complex phenomena at the boundary layer with high accuracy. The objective of the present study is to investigate the relation between the thermal characteristics and interfacial behavior between the flows to develop a model which is able to assess the heat and momentum transfer characteristic and the flow structure of the supersonic steam injector in detail. In the present study, a visible test section of water jet-centered supersonic steam injector was adopted to conduct visualization of the water jet with high speed video camera. In addition, special measurement instrumentations of temperature and pressure were applied to obtain radial distribution of temperature and pressure in the mixing nozzle of the injector. There were large velocity and temperature gaps between the water jet and the supersonic steam flow which indicated the existence of large momentum and heat exchange at the boundary layer of the flows. It was clarified that there was pressure gradient which was considered to stabilize the water jet in the mixing nozzle from calculation of radial distribution of total pressure gradient. From the visualization measurement, it was also clarified the existence of a complex wavy behavior on the surface of the water jet. The wave velocity was estimated by the image processing technique and the cross-correlation method. It was found that there was a relation between the wave velocity and heat transfer characteristics in the supersonic steam injector. It is suggested that the enthalpy ratio between the liquid subcool enthalpy and steam condensation enthalpy as well as the Jacob number between both flows could be an indication factor for the effect of the wavy behavior on the condensation.Copyright

Oscillation of a rotating levitated droplet: Analysis with a mechanical model.

A droplet of millimeter-to-centimeter scale can exhibit electrostatic levitation, and such levitated droplets can be used for the measurement of the surface tension of the liquids by observing the characteristic frequency of oscillatory deformation. In the present study, a simple mechanical model is proposed by considering a single mode of oscillation in the ellipsoidal deformation of a levitated rotating droplet. By measuring the oscillation frequency with respect to the rotational speed and oscillation amplitude, it is expected that the accuracy of the surface tension measurement could be improved. Using the proposed model, the dependences of the characteristic frequency of oscillatory deformation and the averaged aspect ratio are calculated with respect to the rotational angular velocity of a rotating droplet. These dependences are found to be consistent with the experimental observations.

Research and Development of Self-Priming Venturi Scrubber for Filter Venting: Preliminary Analysis and Observation of Hydraulic Behavior in Venturi Scrubber

As revealed by Fukushima Daiichi nuclear disaster, countermeasures against severe accident in nuclear power plants are an urgent need. In particular, from the viewpoint of protecting containment and suppressing the diffusion of radioactive materials, it is important to develop the device which allows filtered venting of contaminated high pressure gas. In the filtered venting system that used in European reactors, so called Venturi scrubbers are used to realize filtered venting without any power supply. The Venturi scrubber operates without a power supply of high pressure gas filled into the containment. In this apparatus, scrubbing of contaminated gas is promoted by both gas releases through a submerged Venturi tube which is one of the major components of Venturi scrubber and liquid splay flow formed by liquid suction through a hole for suction provided at the throat part of the Venturi tube. This type of Venturi scrubbers is called self-priming ones. However, the mechanism of a self-priming Venturi scrubber including effects of gas flow rate and shape of Venturi tube is understood insufficiently in the previous studies.In this study, to provide knowledge about the detailed mechanism of filtering and evaluation method for operating condition of the filtered venting system with Venturi scrubber, both experimental and numerical research works have been performed. In this paper, as a previous step of designing and making a self-priming Venturi tube, hydraulic behavior in a rectangular Venturi tube is taken by high speed camera, by the air-water experiment under atmospheric condition to check the operation in a self-priming mode and the validity of the experimental method and the visualization technique. And numerical analysis of a circular Venturi tube is conducted as a preliminary analysis, by TPFIT (Two-Phase Flow simulation code with Interface Tracking) to predict its operation. As results, the Venturi scrubbers used in experiment and simulated in numerical analysis were operated in a self-priming mode. By observed results of the hydraulic behaviors in the Venturi tube, the validity of the experimental method and the visualization technique were confirmed. And it is expected that the simulated circular Venturi scrubber in this numerical simulation was operated in a self-priming mode in a real situation.Copyright

Development of Microbubble Generator and its Utilization to Enhance the Mass Transfer in the Bubble Plumes and Columns

Microbubble has characteristics of large surface area to unit volume and small buoyancy. We propose an effective technique to generate tiny bubbles less than 200 μm diameter utilizing a venturi tube at high void fraction. The mechanism of bubble breakup in the venturi tube is elucidated that the bubbles expanded after passing through the throat and then shrank rapidly. The tiny bubbles are generated due to the surface instability of shrinking bubbles. The effect of bubble diameter and plume structure on mass transfer efficiency in bubble plumes and columns are investigated numerically. In order to capture the detailed plume structure, the interaction between liquid and bubbles is treated by a two-way coupling Eulerian–Lagrangian method. The gas transfer from bubbles to liquid is computed by modeling the mass transfer rate of individual bubbles. The numerical results show that the dissolution efficiency changes rapidly when the initial bubble size reaches certain value. The effect of bubble-induced liquid velocity on the residence time of microbubbles increases with the decrease of initial bubble diameters, and also increases with the reduction of initial water depth. By comparing the concentrated and uniform bubble injections, the results suggest that the uniform injection provides much better mass transfer efficiency becasue the circulation of liquid induced by bubble is greatly suppressed.Copyright

Study on Bubble Breakup Mechanism in a Venturi Tube

Microbubbles are expected to be applied in various subjects such as engineering and medical fields. Thus, on-demand microbubble generation techniques with high efficiently are required. In the present study, the microbubble generator using a venturi tube (converging-diverging nozzle) is focused. Although this technique realizes generation of many tiny bubbles with less than several-hundred-micrometer diameter, there are several unsolved parts of flow structure in a venturi tube on bubble breakup behavior. The purpose of this study is to clarify the bubble breakup mechanism in a venturi tube for practical use. In the present study, using a high speed camera for detailed observation of bubble behavior, the following features were obtained. In low velocity conditions, bubbles are divided in several pieces with a jet penetrating from the top (downstream) to the bottom (upstream) part of the bubble. In high velocity conditions, bubbles collapse in countless microbubbles with a drastic bubble expansion and shrinkage. Also, in order to clarify the flow structure in a venturi tube, pressure profile is measured in detail. Under chocking condition, the pressure profile shows the tendency of supersonic flow in a Laval nozzle and sudden pressure gradient appears in the diverging section. There are strong correlations between bubble fission points and pressure recovery points. It is suggested that bubble collapse is strongly influenced with pressure recovery in the diverging section.Copyright

The criteria of flushing phenomena under microwave heating

In the microwave heating (MH) de-nitration method developed in Japan, a mixed solution of uranyl nitrate and plutonium nitrate recovered from the spent fuel in the reprocessing plant is converted directly to mixed oxide (MH-MOX) powder. This MH-MOX powder is utilized to fabricate MOX fuel with UO2 powder for FBR. The MH method is accompanied with transient boiling phenomena such as overflow and flushing. Toward high-speed and high-capacity conversion by MH-method in the future, it is required to avoid overflow and flushing and to understand optimal conditions for design and operation. At the first step for these objectives, basic knowledge of transient boiling phenomena by the MH-method has been acquired with using distilled water. It is observed that generation of singular bubble triggers flushing and distilled water just before flushing is superheated by a temperature 10 °C higher than boiling temperature. Based on these results, it is clarified that the occurrence criteria of flushing correlate with absorbed power in the water and released power from the water surface.

Influence of Hydrodynamic Interaction on Jet Breakup and Fragmentation Behavior

Mitigative measures against a Core Disruptive Accident (CDA) are important from the viewpoints of safety of a Fast Breeder Reactor (FBR). If a CDA occurs, Post Accident Heat Removal (PAHR) must be surely achieved. In the PAHR, molten materials are likely to be injected into the coolant like a jet and they must satisfy two requests simultaneously: fast ejection and stable cooling after quenched. In order to estimate the quench behavior of the molten jet, it is important to understand how the jet breaks up.The objective of this study is to clarify that the influence of hydrodynamic interaction between a jet and the surrounding fluid on jet breakup. Previous works have clarified that one cause of the jet breakup is provoked by fragmentation at the side of a jet. However, there are few detailed results describing the correlation between jet breakup and hydrodynamic interaction at the leading-edge region of a jet. Additionally, air entrainment with a jet is always observed in our past experiments using simulants, but its influence has not been discussed yet.In this study, jet injection experiments in liquid-liquid system were conducted for investigating the interaction a jet and an ambient fluid, and the effect of air entrainment on jet breakup behavior. Both simulant core materials and coolants were transparent liquids for visualization. The stored simulant core material was injected into a tank filled with the simulant coolant. In order to realize the condition without air entrainment, the air remaining within the nozzle was removed using a syringe. The jet breakup behavior was observed with a high speed video camera. A normal backlight system and a Laser Induced Fluorescence (LIF) system were employed for visualization. The inner velocity distribution of a jet was measured by Particle Image Velocimetry (PIV).As a result, in the experiments without air entrainment the jet breakup lengths were described by Epstein’s equation. In addition, a pair of vortices was observed at the leading-edge region. The vortices were generated at the leading edge and the leading edge rolled up by the vortices returned toward a jet core. Thus, it was very likely that the vortices at the leading edge region promoted jet breakup.Copyright

Development of Prediction Technology of Two-Phase Flow Dynamics Under Earthquake Acceleration: (13) Rising Bubble Motion Under Horizontal Vibration

An earthquake is one of the most serious phenomena for the safety of a nuclear reactor in Japan. Therefore, structural safety of nuclear reactors has been studied and nuclear reactors ware contracted with structural safety for a big earthquake. However, it is not enough for safety operation of nuclear reactors because thermal-fluid safety is not confirmed under the earthquake. For instance, behavior of gas-liquid two-phase flow is unknown under the earthquake conditions. Especially, fluctuation of void fraction is an important factor for the safety operation of the nuclear reactor. In the previous work, fluctuation of void faction in bubbly flow was studied experimentally and theoretically, to investigate the stability of the bubbly flow. In such studies, flow rate or void fraction fluctuations were given to the steady bubbly flow. In the case of the earthquake, the fluctuation is not only the flow rate, but also a body force on the two-phase flow and a shear force through a pipe wall. Interactions of gas and liquid through their interface also act on the behavior of the two-phase flow. The fluctuation of the void fraction is not clear for such complicated situation under the earthquake.Therefore, in this research project, the behavior of gas-liquid two-phase flow is investigated experimentally and numerically in the series of study. In this study, to investigate the effects of vibration on bubbly flow in the components and construct an experimental database for validation, we performed visualization experiments of vertical bubbly flow in a rectangular water tank on which a sine wave vibration was applied. In this paper, results of visualized experiment evaluated by the visualization techniques, including positions of bubbles, shapes of bubbles and liquid velocity distributions around bubbles, were shown. And liquid velocity distribution around bubbles by the PIV measurement was also shown. In the results, bubble behaviors were affected by oscillation. And the cycle of the bubble tilt angle was almost same as the cycle of oscillation table velocity.Copyright