Hideaki Monji

New measurement method for very low liquid flow rates using ultrasound

Abstract A very small ultrasonic flowmeter for liquids with measuring pipe diameter down to 0.5 mm was developed using disk ultrasonic transducers. The transducer has a hole in the center and the measuring pipe passes through the hole. The vibration mode in the radial direction of the transducer was used, and ultrasound was transmitted and received from outside the pipe wall. The flowmeter can measure a liquid flow rate below 1 ml/min, which corresponds to a Reynolds number of about 40 based on the measuring pipe diameter. The standard deviation of flow rate measurement was 0.01 ml/min (1%) at the flow rate of 1 ml/min.

Measurement of particle/bubble motion and turbulence around it by hybrid PIV

Abstract Characteristics of bubble flow are influenced by bubble motion, liquid flow and interactions between bubbles, and between a bubble and liquid phase. Thus because behavior of a single bubble and liquid around it is regarded as one of the basic elements characterizing bubble flow, the single bubble motion in stagnant water was investigated experimentally by using flow visualization and image processing methods. The bubble motion is influenced by several factors, that is, bubble size, density difference between gas and liquid, bubble shape and deformation in motion. In order to separate the effect of each factor, some solid particles with different size, shape and/or density were also measured and the characteristic of each factor was discussed. Two-dimensional water velocity field and the motion of a rising particle/bubble in the water were simultaneously measured by PIV (Particle Image Velocimetry) and PTV (Particle Tracking Velocimetry), respectively (Hybrid PIV). The experimental results showed that the large density difference between a particle and water caused high relative velocity and induced zigzag motion of the particle. Furthermore, the turbulence intensity of a bubble was about twice in the case of the spherical solid particle of similar diameter.

Thermal Mixing in T-Junction Piping System Related to High-Cycle Thermal Fatigue in Structure

The numerical simulation code “MUGTHES” has been developed by the authors to investigate thermal striping phenomena caused by turbulence mixing of fluids at different temperatures and to utilize with an evaluation method for high-cycle thermal fatigue. MUGTHES employs the large eddy simulation (LES) approach to predict unsteady thermal mixing phenomena and the boundary fitted coordinate (BFC) system to fit complex boundary shapes in a reactor. In this paper, thermal striping phenomena in a T-junction piping system (T-pipe) are numerically simulated as a code validation study and the simulation results are used to investigate temperature fluctuation generation mechanism in the T-pipe. The boundary conditions for the simulation are chosen from the WATLON experiment which was a water experiment conducted at JAEA using the T-pipe. In the simulation, the standard Smagorinsky model is employed as a turbulence eddy viscosity model with the model coefficient of 0.14 (= Cs). Numerical results of MUGTHES are compared with experimental velocity and temperature profiles. Applicability of MUGTHES to the thermal striping phenomena is confirmed through the numerical simulation in the T-pipe and the characteristic large-scale vortex structure which dominates thermal mixing and may cause high-cycle thermal fatigue is revealed.

Improvement of Gas Entrainment Prediction Method—Introduction of Surface Tension Effect—

A gas entrainment (GE) prediction method has been developed to establish design criteria for the largescale sodium-cooled fast reactor (JSFR) systems. The prototype of the GE prediction method was already confirmed to give reasonable gas core lengths by simple calculation procedures. However, for simplification, the surface tension effects were neglected. In this paper, the evaluation accuracy of gas core lengths is improved by introducing the surface tension effects into the prototype GE prediction method. First, the mechanical balance between gravitational, centrifugal, and surface tension forces is considered. Then, the shape of a gas core tip is approximated by a quadratic function. Finally, using the approximated gas core shape, the authors determine the gas core length satisfying the mechanical balance. This improved GE prediction method is validated by analyzing the gas core lengths observed in simple experiments. Results show that the analytical gas core lengths calculated by the improved GE prediction method become shorter in comparison to the prototype GE prediction method, and are in good agreement with the experimental data. In addition, the experimental data under different temperature and surfactant concentration conditions are reproduced by the improved GE prediction method.

Effect of Experimental Conditions on Gas Core Length and Downward Velocity of Free Surface Vortex in Cylindrical Vessel

This paper deals with characteristics of surface vortex in a cylindrical vessel. One of the characteristics is a gas core length, which is important to estimate the onset condition of the gas entrainment but influenced easily by the experimental condition. In the experiment using water, the effects of the water temperature, water level, and the surface tension on the gas core length were investigated. The onset condition of the gas entrainment is sometimes estimated by using the Burgers vortex model but the real flow in the vessel is different from the model. The velocity fields were measured by particle image velocimetry (PIV) and the velocity gradient of the downward flow was discussed. The proper flow conditions for the Burgers vortex model are a high water level and a high flow rate.

Numerical Investigation of Flow Structure in Pipe Elbow With Large Eddy Simulation Approach

At the JAEA (Japan Atomic Energy Agency), the simulation code “MUGTHES (MUlti Geometry simulation code for THErmal-hydraulic and Structure heat conduction analysis in boundary fitted coordinate)” has been developed. MUGTHES employs LES (Large Eddy Simulation) approach to calculate unsteady thermal-hydraulic phenomena and the BFC (Boundary Fitted Coordinate) system to simulate complex geometry in the system. In this study, numerical simulations for pipe elbow flows in various curvature radius ratio (Rc/D) conditions at several Reynolds number conditions. By the numerical simulation in pipe elbow at a laminar flow condition of Re = 700, the numerical schemes and the evaluation method of metrics in BFC system are verified and an appropriate mesh arrangement for elbow pipe is considered. By the numerical simulations in pipe elbow with the ratio of Rc/D = 2 under turbulent flow condition of Re = 60,000, the LES approach using standard Smagorinsky model with wall function law is examined in comparison with the experimental results. Moreover, numerical simulation for the 1/3-scaled water experiment at Re = 3.7×106 which simulates the primary cooling system of the JSFR (Japan Sodium-cooled Fast Reactor) is conducted. From comparisons of axial velocity profiles, applicability of MUGTHES to the elbow pipe flow is confirmed and the characteristic of three-dimensional flow structure relating to the structural integrity of the elbow pipe is discussed.© 2009 ASME

Visualization of bubble-fluid interaction by a moving object flow image analyzer system.

Abstract: This paper deals with interaction between a bubble and fluid around it, visualized by a moving object flow image analyzer (MOFIA) consisting of a three‐dimensional (3D) moving object image analyzer (MOIA) and two‐dimensional particle image velocimetry (PIV). The experiments were carried out for rising bubbles of various sizes and shapes in stagnant water in a vertical pipe. In the MOFIA employed, 3D‐MOIA was used to measure bubble motion and PIV to measure fluid flow. The 3D position and shape of a bubble and the velocity field were measured simultaneously. The experimental results showed that the interaction was characterized by the shape, size and density of a bubble. Concretely, they showed the characteristics of bubble motion, wake shedding, and flow field.

Transition mechanism from bubble flow to slug flow in a riser

An experiment was performed to examine the mechanism of flow pattern transition from bubble flow to slug flow in a riser. The flow was measured by a double resistivity probe system, and photographs of the flow were taken using strobe lights. The negative diffusion distance of bubbles was estimated using a voidage wave equation and compared with the interbubble distance. The flow pattern transition from bubble flow to slug flow occurred when the diffusion distance was larger than the interbubble distance. Conversely, when the diffusion distance was smaller than the interbubble distance, the bubble flow was sustained. Therefore, it is found that the negative diffusion caused by the instability of the voidage wave brings about the flow pattern transition.

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

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