Hiroshige Kikura

Wall shear stress determination from near-wall mean velocity data in turbulent pipe and channel flows

A novel method is proposed that allows accurate estimates of the local wall shear stress from near-wall mean velocity data in fully developed pipe and channel flows. DNS databases are used to demonstrate the accuracy of the method and to provide the reliability requirements on the experimental data.To demonstrate the applicability of the method, near-wall LDA measurements in turbulent pipe and channel flows were performed. The estimated wall shear stress is shown to be accurate to within 1%. Streamwise mean velocity and turbulence intensity profiles normalized with the wall friction velocity at several Reynolds numbers are presented.

Methods to Set Up and Investigate Low Reynolds Number, Fully Developed Turbulent Plane Channel Flows

The tripping of fully developed turbulent plane channel flow was studied at low Reynolds number, yielding unique flow properties independent of the initial conditions. The LDA measuring technique was used to obtain reliable mean velocities, rms values of turbulent velocity fluctuations and skewness and flatness factors over the entire cross-section with emphasis on the near-wall region. The experimental results were compared with the data obtained from direct numerical simulations available in the literature. The analysis of the data indicates the important role of the upstream conditions on the flow development. It is shown that the fully developed turbulent state at low Reynolds number can be reached only by significant tripping of the flow at the inlet of the channel. Effects related to the finite size of the LDA measuring control volume and an inaccuracy in the estimation of the wall shear stress from near-wall velocity measurements are discussed in detail since these can yield systematic discrepancies between the measured and simulated results.

Development of Pulse Ultrasonic Doppler Method for Flow Rate Measurement in Power Plant Multilines Flow Rate Measurement on Metal Pipe

Ultrasonic Doppler method for a flow metering system has been developed. The method has the capability to obtain instantaneous velocity profiles along the ultrasonic beam. Our purpose is to apply the ultrasonic Doppler method to a flow rate measurement of feed- or recirculation- water in power plants. The principle of the flow measurement method is based on the integration of an instantaneous velocity profile over a pipe diameter. Hence, it is expected to eliminate installation problems such as entry length, also to follow transient flow rate precisely by increasing ultrasonic transducers. In this paper, we report that the errors are less than 1% just below a bend and sudden expansion pipe employing three measuring lines. And then, for constructing a basic system of a flow rate measurement in power plants, a transmission of ultrasound through a metallic wall is investigated, at first. Afterward, since there is no ultrasonic reflectors in the feedwater in power plants, cavitation bubbles are induced as ultrasonic reflectors and the results are appeared that cavitation bubbles are effective when the pipe material is metallic.

Natural convection of a magnetic fluid in a cubic enclosure

Abstract Laminar natural convection heat transfer of a magnetic fluid in a cubic enclosure is examined experimentally. Wall-temperature distributions are visualized by thermosensitive liquid crystal sheets. The effect of the magnetic field on the transient temperature distributions, and the local and averaged Nusselt numbers are discussed.

Natural convection of a magnetic fluid in concentric horizontal annuli under nonuniform magnetic fields

Abstract Natural convection of a magnetic fluid in concentric annuli was investigated experimentally. Two concentric cylinders were made of copper and placed horizontally. The temperature of the outer cylinder was kept at 15°C, and the inner cylinder was rapidly heated from 15°C to 25°C and held there. A thermosensitive liquid crystal was utilized for temperature visualization instead of flow visualization; temperatures on a central cross-section were also measured by thermocouples. A magnetic field was applied to the cylinders using a permanent magnet. The test liquid was a magnetic fluid with a 33% weight concentration of fine magnetic particles in a water carrier. Several kinds of experiments were carried out to clarify the influences of direction and the intensity of magnetic fields on the natural convection. When there was no magnetic field, ordinary natural convection was observed. When a magnetic field gradient was applied in the same direction as the gravity, a wall-temperature distribution was observed, as if an apparent gravity had increased; however, the clear influence of the magnetic field was not found. When a magnetic field gradient was applied in the opposite direction of the gravity, the reverse natural convection was observed. Consequently, even if the intensity of the applied magnetic field was small, it played an important role in natural convection and heat transfer of a magnetic fluid. It was recognized that natural convection of a magnetic fluid could be controlled by the application of a magnetic field.

Microstructure of the flow field around a bubble in counter-current bubbly flow

Abstract Experimental study was made on the flow structure around a bubble in air–water bubbly flow. In order to measure velocity profiles around a bubble, an Ultrasonic Velocity Profile monitor was employed, which can obtain an instantaneous velocity profile along its measuring line across a channel. The experiments were carried out in a 100×10 mm 2 rectangular channel for the air–water counter-current bubbly flow whose void fraction smaller than 7%. The bubble Reynolds number was ranged between 700 and 1000. Most bubbles had ellipsoidal shapes and rose up with wobbling motions. Our experimental results plotted in the form of non-dimensional velocity profiles show that the velocity field around a bubble has a structure similar to the turbulent boundary layer on a solid wall. On the other hand, an earlier analytical study by Moore [J. Fluid Mech. 16 (1963) 161] used an assumption of a spherical bubble rising in liquid irrotationally, and the solution was derived that the flow around a bubble being composed of a thin boundary layer and its outer main stream in potential flow. In this paper, the relation between these two types of boundary layer structures is discussed.

Velocity profile measurements of magnetic fluid flow using ultrasonic Doppler method

Abstract In the present study, successful applications of the ultrasound velocity profile measuring technique to velocity fields of a magnetic fluid are presented. After a brief introduction of its principle, various examples applied for the magnetic fluid flow are demonstrated.

Assessment of Effects of Pipe Surface Roughness and Pipe Elbows on the Accuracy of Meter Factors Using the Ultrasonic Pulse Doppler Method

The velocity profile of the flow in a pipe and its influence on the profile factor used with conventional flow meters were investigated with ultrasonic pulse Doppler measurements. From the measured velocity profiles, the influences of surface roughness and Reynolds number were characterized qualitatively and quantitatively. As pipe surface roughness changes during plant operation, the velocity profile changes, producing a change in the profile factor. Variation in the Reynolds number also influences the change in the profile factor. Experiments were conducted at high temperature and pressure to evaluate the ultrasonic pulse Doppler method for measuring the flow of nuclear plant cooling water. Helium gas bubbles provided sufficiently persistent ultrasonic reflectors when injected into high-pressure water, permitting the velocity profile of the flow to be obtained under high-temperature and high-pressure conditions using this method.

Transport Mechanism of Thermohydraulic Instability in Natural Circulation Boiling Water Reactors during Startup

This paper presents experimental study on transport mechanism of thermohydraulic instability, which may occur in natural circulation boiling water reactor during startup. The research was carried out using a natural circulation experimental loop featuring twin parallel boiling channels with chimney assembly. The experiments were performed with the pressure range of 0.1 to 0.7MPa and maximum heat flux of 577kW/m2. The objective of the study is to formulate thermohydraulic stability maps required for determining rational startup procedure of the reactor, in which the instability could be prevented. The study clarified that the flow modes during startup consist of the following sequence: (1) single-phase flow, (2) geysering, (3) oscillation due to hydrostatic head fluctuation, (4) density wave oscillation, (5) transition oscillation, and (6) stable two-phase flow. The main findings of the experiments are as follows: First, low amplitude geysering still occurs at 0.7 MPa under lower heat flux and high inlet subcooling. Second, stable two-phase natural circulation is achieved with system pressure as low as 0.2 MPa, under medium heat flux, and subcooling lower than 5 K. Third, oscillation due to hydrostatic head fluctuation only occurs under atmospheric condition. Finally, thermohydraulic stability maps and rational startup procedure are formulated.

Propagation of surface waves of magnetic fluids in travelling magnetic fields

Abstract Propagation of waves on the free surface of magnetic fluid is experimentally investigated. It is found that the surface velocity of the magnetic fluid depends on the intensity and frequency of traveling magnetic fields, and depth of the channel contained with the magnetic fluid. Theoretical investigation is also carried out in order to clarify the phenomena.