Masafumi Hirota

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.

Experimental and theoretical study of an on-wall in-tube flexible thermal sensor

We propose a novel type of on-wall in-tube flexible thermal sensor, which is able to measure the flow rate under both developing and fully developed flow conditions. We fabricated the thermal flow sensor on a flexible polyimide film by using polymer MEMS technologies and formed a ring-shaped on-wall in-tube sensor configuration by inserting the sensor into a tube. The resistance of the sensor linearly changed with the change in temperature. Its temperature coefficient of resistance is 0.0026 K?1. We obtained a constant and stable output signal of the sensor even though the sensor position was near the tube entrance region where the flow is developing a hydraulic flow condition. We concluded that the proposed sensor is able to measure the flow rate under both the developing and the fully developed hydraulic flow conditions.

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.

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.

Interfacial deformation between an impacting water drop and a silicone-oil surface.

Detailed observations have been conducted on the interfacial deformation of a silicone oil surface and a water drop falling on it. Eleven kinds of silicone oils with wide variations of kinematic viscosity, nu(T)=1-10(5) mm(2)/s, have been tested. The oil surface is disturbed by a water drop with a diameter d(L)=3.1 mm, which falls freely on it from a height of 100-1000 mm. Special attention has been directed to the maximum depth of the cavity formed on the oil surface D(M) and to the maximum diameter of the water drop spreading on the oil surface d(M). We have categorized the configurations of the oil cavity into seven patterns, and those of the water drop at the oil-water interface into five patterns. The maximum cavity depth D(M)/d(L) can be well correlated by a dimensionless group Re(TL)We(TL), where Re(TL) is Reynolds number based on d(L) and nu(T) and We(TL) is Weber number with the water density and surface tension of oil. The maximum diameter of the impacting water drop d(M)/d(L) can be correlated by the Reynolds number with a viscosity of water (Re(L)) and the Ohnesorge number (Oh). Moreover, the condition under which the impacting water drop is smashed into pieces has been also examined based on Re(L) and Oh.

Miniaturization of on-wall in-tube flexible thermal flow sensor using heat shrinkable tube

We previously proposed a novel type of on-wall in-tube thermal flow sensor, and experimentally evaluated its performance. The sensor has an advantage that it can measure the flow-rate both at near the tube entrance (hydraulically developing region) [1] and at bent tube (axially asymmetric flow) [2]. With the requirements of further miniaturization of the sensor in the fields of the portable fuel cells and chemical analyzers, we newly developed the fabrication process to miniaturize it less than 2.0 mm in external diameter by using a heat shrinkable tube. The film sensor fabricated by photolithography was inserted inside tube manually. By applying heat shrinking process, the film was automatically mounted on the inner wall surface, and the outer size of the tube was miniaturized to almost of the half size from its original. The final inner and outer diameters of the tube were 1.2 mm and 1.7 mm, respectively. We evaluated the fundamental performance of this flow sensor. The electrical resistance at the sensor linearly increased with temperature. The obtained temperature coefficient of resistance of the sensing element was 0.0023 K-1. We measured the relationship between the input power consumption and the gas flow rate, and finally evaluated the response time. We obtained a value of 100 msec by forming a cavity structure under the heat element.