Hideomi Fujita

Measurement of turbulent flow in a square duct with roughened walls on two opposite sides

Abstract A fully developed turbulent flow along a square duct, two opposite walls of which had been roughened by square cross-sectioned ribs, was measured with a hot-wire anemometer. This paper presents the resulting velocities and stresses and compares them with measurements taken in a square duct with four smooth walls. Symmetrical results, with respect to the axes of symmetry of the duct cross section, were obtained in every measured quantity. Terms on both sides of the vorticity transport equation were calculated, and the balance of terms was discussed. As is well known, smooth-walled square ducts yield two secondary flow cells in any given quadrant of a cross section. But in ducts whose opposite walls have been roughened, we found a hitherto unobserved phenomenon: only one relatively large cell appeared in each quadrant of a ducts cross section.

Turbulent heat transfer in a square duct

Abstract The characteristics of the flow and temperature fields in turbulent flow through a straight duct with a square cross section are considerably complex owing to the momentum and heat transport by the secondary flow of the second kind. For a basic understanding of the turbulent heat transport process in such a complex turbulent flow, the detailed characteristics of the turbulent temperature field must be made clear. This paper presents experimental results concerning the fluctuating temperature intensity, cross-correlation coefficients between the fluctuating velocity and temperature, and the turbulent heat fluxes obtained for forced-convection heat transfer in a square duct. The contours of the turbulent heat flux in the streamwise direction are distorted greatly toward the duct corner, similar to those of the fluctuating temperature intensity. Similarity has been also found between the distributions of the turbulent heat flux in the transverse direction and the turbulent shear stress. Based on the results of the measurements, the eddy-viscosity, eddy thermal diffusivity, and turbulent Prandtl number have been also obtained. The distributions of the eddy-diffusivities and turbulent Prandtl numbers on the symmetric axis of a square duct are similar to those in a circular pipe. As the duct corner is approached, however, the eddy-diffusivities become smaller, and the turbulent Prandtl number becomes larger, and thus the assumption of the constant turbulent Prandtl number is invalid for turbulent heat transfer in a square duct.

Secondary flow of the second kind in rectangular ducts with one rough wall

Abstract A study was made to clarify experimentally the influences of a rough wall in a rectangular duct on secondary flow of the second kind and to obtain fundamental data on flow characteristics. Measurements were conducted on turbulent air flows through a square duct and a rectangular (aspect ratio 2:1) duct having one rough wall using hot-wire anemometers. The secondary flow patterrn in the roughened ducts was remarkably different from that in the smooth ducts. In the roughened ducts, only one large longitudinal vortex appeared near the smooth wall on each side, and the secondary currents in the core region proceeded downward from the top smooth wall to the bottom rough wall along the midplate of the duct. The secondary flow was considerably intensified by the existence of the rough wall. The vorticity balance was examined by evaluating the production and convection terms in the vorticity transport equation using the measured turbulent stresses. The two terms almost balanced near the corner bisector of the smooth ducts. However, in the rough ducts, they did not necessarily balance near the corner region formed by the smooth and rough walls.

PLASMA PRODUCTION WITH ROTATING ION CYCLOTRON WAVES EXCITED BY NAGOYA TYPE-III ANTENNAS IN RFC-XX

A method of plasma production by ion cyclotron wave heating has been developed in RFC-XX. Nagoya Type-III antennas were used for wave excitation, and gas was supplied through a gas box. The effect of the rotating field excitation on plasma production and heating was investigated. In the m = − 1 rotational mode (rotation in the direction of ion cyclotron motion), the plasma density profile is flat within the gas box bore with a line integrated density nl = 3 × 1013cm−2, and the ion temperature is Ti ≈ 150 eV. For the m = +1 mode, a high density plasma was obtained with a different profile having a peak at the centre. In this mode, the line integrated density is nl = 3 × 1014cm−2, with the peak density n(0) = 7 × 1013cm−3.

Heat (mass) transfer in rectangular cross-sectioned two-pass channels with an inclined divider wall

Abstract Heat transfer characteristics in rectangular cross-sectioned two-pass channels with an inclined divider (inner) wall have been examined experimentally. Local heat (mass) transfer rates were measured by the naphthalene sublimation method; seven kinds of divider inclination angles were tested for three turn clearances under the Reynolds numbers of (2.0–5.0)×10 4 . The influence of the inclined divider wall on the local heat transfer characteristics is discussed in detail. Then, the optimum combination of the inclination angle and the turn clearance is examined based on the trade-off between the heat transfer enhancement and pressure loss penalty, and on the improvement of uniformity in the distribution of local heat transfer rates.

Heat (mass) transfer in serpentine flow passage with rectangular cross-section

Abstract Detailed local heat (mass) transfer characteristics in rectangular cross-sectioned serpentine ducts with a sharp 180° turn have been measured by the naphthalene sublimation technique, directing special attention to the influence of the flow-inlet condition on the local mass transfer. Two contrasting flow-inlet conditions, a contracted-flow inlet and a uniform-flow inlet, have been tested for three turn clearances in the Reynolds number of 3.5×104. The distributions of local Sherwood numbers on all the walls of the test ducts are shown in the form of 2-D maps, and the influence of the flow-inlet condition on the local mass transfer characteristics is examined in detail.

Influences of velocity gradient on hot-wire anemometry with an X-wire probe

The influence of velocity gradients on hot-wire anemometry with an X-wire probe were investigated. Analysis showed a strong possibility that the values obtained by the usual method of measurements with an X-wire probe involve serious experimental errors caused by the velocity gradients. In order to eliminate these errors, the authors devised an entirely new method of measurement on the basis of the analysis. In this method, two X-wire probes, one of which is the mirror image of the other, are used for measuring a quality at a location. Then an ultimate value of the quantity at the location is given as an arithmetic mean of the values obtained from the two probes. The validity of the analysis was confirmed by measurements of a turbulent flow in a square duct with the present method. The results showed that the present method is very effective for the measurement of quantities such as the secondary flow velocities and the turbulent shear stresses, quantities which are very sensitive to the influences of velocity gradients.

Gas-Liquid Flows in Flat Channels with Small Channel Clearance

Void fractions and pressure drops were measured on gas-liquid flows in five kinds of narrow flat channels with a width of 10 mm and channel clearances of 0.2 mm ∼ 2.0 mm. Four kinds of water solutions of ethanol were used to examine the influences of surface tension σ and viscosity μ of liquid on flow characteristics. The void fractions increased with a decrease in σ and μ and with a decrease in channel clearance. Under low liquid velocity conditions, the pressure drops were affected by the surface tension, and it was found that Chisholms equation for the Lockhart-Martinelli correlation could not be applied to channels with relatively large clearances.

Local heat (mass) transfer characteristics in rectangular ducts with a sharp 180-degree turn

Abstract Experiments on convection heat (mass) transfer in a rectangular channel with a sharp 180-degree turn were conducted to clarify the local heat transfer characteristics. The local mass transfer rate on a long-side wall of the channel whose cross section is 50 × 25 mm was measured using the naphthalene sublimation technique. The results show that the local mass transfer rate is not uniform over the wall and varies remarkably with the location, especially in the turn and after the turn sections and that the distribution is strongly influenced by the turn clearance of the channel as well as the Reynolds number.

Turbulence kinetic energy in turbulent flows through square ducts with rib-roughened walls

Abstract Turbulence kinetic energy production and convection were examined in detail for the turbulent flows through various square ducts. The following ducts were used: a smooth duct, a duct with one wall roughened, and a duct with two facing walls roughened. Rough walls were produced by gluing square ribs to a smooth surface repeatedly. By the examinations the turbulent shear stresses were found to play the most important role in the production of the turbulence kinetic energy. Near the rough walls the turbulent normal stresses also have strong influences on the energy production, although their contributions are either positive or negative, depending on the relative streamwise locations with respect to a rib roughness element. The amount of turbulence energy convected by secondary flow is considerably smaller than that of the energy produced. The secondary flow has little influence on the balance of turbulence kinetic energy in the tested ducts.