Showing papers in "Jsme International Journal Series B-fluids and Thermal Engineering in 1993"

Numerical Analysis of a Single Bubble by VOF Method.

Abstract: In order to examine the feasibility of direct simulation of bubbly flow, the applicability of the VOF (volume of fluid) method to analyses of a single rising bubble was examined in this study. Calculated bubble shapes and terminal velocities under wide ranges of Eotvos number and Morton number were compared with the experimental data summarized by Grace et al. Except for the cases in which bubble shapes were spherical-cap and skirted, the VOF method could predict them well by assigning only eight cells to the bubble diameter. Hence, it was confirmed that some modification of this method will enable us to simulate bubbly flow directly under a wide range of flow conditions. The relationship between bubble shape and velocity distribution was also examined within the ranges in which the VOF method is valid. It was found that the secondary vortex appearing in wobbling bubbles induces a velocity component normal to the bubble interface, and this velocity is one of the causes of the wobbling shape of the bubble.

Transient Induced Flow Through a Vertical Annulus

Abstract: Transient laminar free convection in a vertical annulus is investigated numerically. The induced flow is due to a step change in the outer wall temperature, while the inner wall is adiabatic. The range of Grashof number considered is 4lesGγ*les50 000, while the Prandtl number is 0. 7. Results are presented showing the variations of the temperature and velocity profiles with time and location. Also, variations with time of the induced flow, the heat absorbed by the fluid, and the pressure distribution along the annulus are given.

Compressibility Factors and Virial Coefficients of Difluoromethane(HFC-32) Determined by Burnett Method.

Abstract: The compressibility factors of difluoromethane (HFC-32) in the gaseous phase have been determined by means of the Burnett method, and the vapor pressures have also been measured using the same apparatus. The cell constant of the Burnett apparatus has been calibrated by measuring the thermodynamic properties of gaseous helium. Using the cell constant, we measured 95 compressibility factors at temperatures from 290 to 370K, and pressures from 0.15 to 6.6 MPa. These experimental compressibility factors for temperatures from 290 to 370 K have been discussed along each isotherm by applying the virial-type equation having second, third and fourth virial terms. We have also measured 9 vapor pressures at temperatures from 280 to 350 K. On the basis of the present vapor-pressure measurements, the critical pressure of HFC-32 has been determined so as to correspond to the critical temperature directly measured in our laboratory. The present data were also used to develop the vapor-pressure correlation which are effective for temperatures from the normal-boiling-point temperature to the critical temperature. The present data are represented within a deviation of ± 0.035% ( ±0.6 kPa) by the derived vapor-pressure correlation. The uncertainty of temperature measurements was estimated to be better than ±10 mK and that of pressure measurements was estimated to be better than ±0.8 kPa. Experimental uncertainty for the compressibility factor was evaluated as not lower than ± 0.2%. The purity of the HFC-32 sample used was higher than 99.98 wt%.

SEM studies of particles produced by cavitation erosion

Abstract: Scanning electron microscopic investigations of particles produced by cavitation erosion and fracture surfaces for 304 stainless steel, revealed that the dominant mode of cavitation erosion is fatigue failure. In addition, the surface roughness facilitates the separation of particles, especially the longitudinal ones. The gathered particles can be classified into three types according to their shapes, namely the longitudinal, the triangular and the mixed quadrangle hexagon. The spherical particles, which were reported in a previous study on cavitation erosion, are not observed here

Initial Stage of a Three-Dimensional Vortex Structure Existing in a Two-Dimensional Boundary Layer Separation Flow : Visual Observation of Laminar Boundary Layer Separation over a Circular Cylinder from the Side of a Separated Region

Abstract: It was found in the preceding study that the boundary layer separation over a circular cylinder seems to have three-dimensional separation characteristics and its three-dimensionality appears to be related to the three-dimensional wake structure. In the present study, to investigate the structures of three-dimensional boundary layer separation, the flow of the separated region over a circular cylinder was observed by means of the flow visualization technique in the range of Reynolds numbers from 230 to 1340 in a towing water tank. As a result, it was determined that the three-dimensional features of boundary layer separation over a circular cylinder were characterized by the recirculating flow in the separated region, and the relationship between three-dimensional instability in the separated region and Reynolds number was investigated.

A New Parameter for Predicting Crossflow Instability.

Abstract: Instability of boundary layers over a concave wall and a rotating disk which were thought to be essentially different in instability sources, are compared in order to investigate whether or not a single crossflow parameter can be defined. Using a newly defined crossflow parameter, prediction was attempted on a yawed cylinder boundary layer transition. By comparing the calculation with the experiment, it was found that this parameter can document fairly well the onset of crossflow instability.

Oscillatory Characteristics of a Pseudo-Shock Wave in a Rectangular Straight Duct.

Abstract: The oscillatory characteristics of a pseudo-shock wave with a Mach number of 1. 75 in a rectangular straight duct are experimentally investigated by static pressure measurements both on the duct axis and on the wall surface. The aspects of the pressure oscillation in the pseudo-shock region are analyzed from the treatment based on the intermittent characteristics, root-mean square, probability density function and cross correlation of the pressure signals. The correlation between the oscillatory characteristics and the flow mechanism of the pseudo-shock wave is also discussed in detail in the last section of the present paper.

Absorption of Water Vapor into a Film of Aqueous Solution of LiBr Falling along a Vertical Pipe.

Abstract: The heat transfer surface used in the experiments is a chrome-plated smooth brass pipe of 28mm diameter and 1400mm length. The film Reynolds numbers are varied in the range of 40-400, which includes the range of real machines. The average absorptions mass flux is compared with the numerical results derived from the laminar flow model proposed in the previous paper. The agreement is good up to 100 in Ref, but the experimental values are far higher above 200. This may be attributed to the wave motions on the film surface. Pictures show that the film is completely smooth only at Ref below 20. Beyond 20, ring-shaped waves appear, but the surfaces between the waves are relatively smooth at Ref below 200. The measurements of the film thickness show that its minimum is constant at about 0. 38mm, but its maximum thickness increases for Ref larger than 60.

Gas hydrate cold storage using direct-contact heat transfer of liquid-vapor phase change and natural circulation of refrigerant in closed vessel

Abstract: The proposed cold storage method using heat transfers of direct-contact liquid-vapor phase change between the heat transfer medium and water in a closed vessel, whose effectiveness for a small-size vessel was confirmed in the previous report, was adopted and expanded for the actual size of a storage vessel HCFC142b and water or water solution of a surface active agent were used as the heat transfer medium and cold storage material, respectively The chemical reaction (hydration) heat of the HCFC142b and water was utilized As a result, it was confirmed by the experiment using large-scale apparatus that efficient cold storage and release was achieved In particular, for the water solution of a surface active agent, a high solid packing factor and high rate of cold storage were maintained even for the thickest storage material layer

The Aerodynamic Characteristics of a Circular Cylinder with Tangential Blowing in Uniform Shear Flows

Abstract: The experimental results of lift and drag acting on a circular cylinder with tangential blowing immersed in a uniform shear flow are presented for seven shear parameters : K=+0. 15, +0. 045, +0. 030, 0, -0. 030, -0. 045, -0. 15 at the Reynolds number 6×104. The effect of the velocity gradient of the uniform shear flow on the characteristic values is discussed for the coefficients of momentum of the blowing ranging from 0 to 0. 4. In this study, we also discuss in depth the contribution of the characteristic values to the lift. These values include the angles and the pressure coefficients of the stagnation point and the separation points as well as the minimum pressure and base pressure. It was found in this study that the shear parameters strongly affected the coefficients of lift and drag, and different starting points for the forced reattachment phenomenon. All the characteristic variables increasing the lift coefficient can be clearly classified into three categories : the first category was influenced only by the shear parameter, the second, only by the location of the tangential jet, and the third, by both parameters. A comparison of various features such as the stagnation point clarifies the contributions of each feature to the lift and drag on the cylinder.

Two-Phase Cross-Flow-Induced Vibration of Tube Arrays. (Brief Review of Previous Studies and Summary of Design Methods).

Abstract: Many heat-exchange tubes in a shell-and-tube-type heat exchanger are oscillated by means of the flow . of fluid through the heat exchanger. This flow is often cross flow and gas-liquid two-phase flow. Here, a brief review of studies on two-phase cross-flow-induced vibration is presented, and a summary of our work in this area provides a design methodology. Three types of vibration have been studied for tubes in two-phase cross-flow conditions: First, resonance of a tube due to vortex shedding is important primarily in single-phase flow, but also has been observed in homogeneous flow and even in two-phase flow. However, this vibration disappears in the slug flow or froth flow regions, which are important in numerous heat exchangers. Therefore, the vortex shedding phenomenon is not considered in this paper. Secondly, turbulent buffeting vibration is considered as the dominant phenomenon in the slug flow and froth flow regions. A method for estimating this type of random vibration is explained in this paper. Thirdly, an unstable form of vibration, so-called fluid elastic vibration, is also considered. The common method for estimating the instability boundary is similar to that used for single-phase flow; however, new insights on the method for two-phase flow are given in this paper.

Direct Numerical Simulation of Chemically Reacting Mixing Layers

Abstract: The objective of this paper is to gain insight into the process of chemical reaction in turbulent mixing layers. To achieve this purpose, we have performed direct numerical simulations of three-dimensional chemically reacting mixing layers using a spectral method. The results are reported for the case of velocity fields with initial periodic excitation. From these results, we have reached the following conclusions : 1) on the streamwise vortical structures (ribs) in the braid region, the peak of the chemical production term exists ; and 2) the core region contributes significantly to the total production in the mixing layer, whereas local production rate is not so high.

Development of an Active Attenuator for Pressure Pulsation in Liquid Piping Systems : Trial Construction of the System and Fundamental Experiments on Attenuation Characteristics

Abstract: A new technique of an active attenuator for pressure pulsation in liquid piping systems, which detects the progressive wave component of fluid-borne vibration and then controls the second source feedforwardly by this detected signal so as to completely eliminate the progressive wave in its downstream pipe by producing the canceling fluid vibration, has been presented, together with experimental results. As a preliminary investigation in this study, an experiment on the attenuation characteristics has been carried out for only the simple harmonic fluid vibration, though the present active attenuator using the transfer-function-based control method is also applicable to the wide-band random fluid vibration. A general-purpose digital signal processor has been used for the high-speed operation of converting the pressure difference signal produced by two pressure transducers into the progressive wave signal of fluid vibration and for the successive operation of producing the control signal to the second source (electrohydraulic servo cylinder). It was confirmed by the test in a model experimental pipeline that, when the present control system was operated, the pressure pulsation in a downstream pipe of the second source was reduced, as expected, to about 1/10 to 1/15 (that is, a transmission loss of about 20 to 25 decibels) of that in the case where the control system was not used over the frequency range of 20 to 250Hz.

Heat Transfer Enhancement of Natural Convection and Development of a High-Performance Heat Transfer Plate.

Abstract: A new enhancing technique was developed for natural convection adjacent to a vertical, heated plate. V-shaped plates of which the edges faced upstream, were attached to a vertical plate in a staggered layout. The overall heat transfer coefficients of that surface were measured and compared with a nontreated, flat surface and a conventional finned surface

An Unsteady Implicit SMAC Scheme for Two-Dimensional Incompressible Navier-Stokes Equations.

Abstract: A finite-difference method based on the SMAC (Simplified Marker and Cell) scheme for analyzing two-dimensional unsteady incompressible viscous flows is developed. The fundamental equations are the incompressible Navier-Stokes equations of contravariant velocities and the elliptic pressure equation in general curvilinear coordinates. With application of the Crank-Nicholson scheme, unsteady flow is calculated iteratively by the Newton method at each time step, and the elliptic pressure equation is solved by the Tschebyscheff SLOR method with alternating the computational directions. Therefore, the elliptic character of incompressible flow is well described. The present implicit scheme is stable under the proper boundary conditions, since spurious error and numerical instabilities can be suppressed by employing the staggered grid and upstream differences such as the modified QUICK scheme. Numerical results for two-dimensional flows through a decelerating cascade at high Reynolds numbers are shown. Some computed results of the surface pressure coefficient are in satisfactory agreement with the experimental data.

Improvement in temperature measurement accuracy of Q-branch CARS thermometry (effects of spectral resolution of detection system)

Abstract: The temperature measurement accuracy of N2 Q-branch CARS (Coherent Anti-Stokes Raman Spectroscopy) below 1000K has been improved by improving the spectral resolution of the detection system. The total spectral resolution was improved from 1. 3cm-1 to 0. 6cm-1 by dispersion magnification (×3. 6), which was effected by adding a magnifier between a monochromator and an intensified photodiode array detector. The accuracy of temperature measurement in the temperature range of 300-800K for pressures of 0. 1-3. 0MPa has been greatly improved from ±100K, in the case without the magnifier, to ±40K. The method reported in this paper has a potential advantage for practical use because of its convenience.

Development of Thermal-Hydraulic Computer Code for Steam Generator

Abstract: Understanding of the thermal-hydraulic behavior in the secondary side of the PWR (Pressurized Water Reactor) steam generator was required to improve the reliability of steam generator. Therefore, the thermal-hydraulic computer code FIT-III has been developed to predict the thermal-hydraulic behavior in the secondary side of steam generator. The FIT-III has been verified by using the two-dimensional and three-dimensional thermal-hydraulic test results. The verified FIT-III is used for the modification and design of the steam generator.

Experimental Study of Pulsed High-Speed Liquid Jet.

Abstract: Pulsed liquid jets with speeds ranging from 100 m/s to 1000 m/s were produced using a gas gun. Kerosene, water and alcohol were injected into ambient air by the impact of a projectile driven by the gas gun operated with high-pressure helium. Jet speed was measured by output signals generated when the jet cut two laser beams at the exit of the gas gun. Double exposure holography interferometry visualized the shock wave generated in front of a supersonic jet as well as the moving jet shape. Interesting phenomena associated with jet motion and the impact of the jet on the solid surface were observed. Jet generation time and evolution of the jet shape was measured. The effect of the nozzle size on the jet speed was studied. Experimental findings indicate that the formation of a precursor jet and a bifurcated jet largely depends on the liquid-air surface tension.

TDC Flow Field Structure of Two-Intake-Valve Engines with Pentroof Combustion Chamber

Abstract: The Laser sheet particle track velocimetry procedure was improved by employing a two-color light source. Velocity vectors resolving the directional ambiguity could be obtained with high spatial resolution. The forward-scattering LDV adopting a high-power light source and large diameter / low density tracer particles revealed a potential for cycle-resolved turbulence analysis with high temporal resolution. Adopting these procedures, flow field in the pentroof combustion chamber of a two-intake-valve engine was analyzed. It was clarified that at the early and middle stages of the compression stroke, both tumble and swirl maintain distinctive vortex structures and at the late stage of the compression stroke, tumble and swirl are converted into many small eddies of relatively large scale. Experimental results demonstrated that because these eddies are distributed uniformly throughout the combustion chamber and the scale conversion proceeds, maintaining the equilibrium relationship, it is reasonable to treat these eddies as turbulence.

Effects of nozzle contour on the aerodynamic characteristics of underexpanded annular impinging jets

Abstract: Dependence of the flow behavior of underexpanded annular impinging jets on the nozzle contours is investigated experimentally. In the present study, five converging annular nozzles of different contours are fabricated and tested. The most important parameters of the annular nozzle contour are the ratio of the inner diameter d to the outer diameter D and the angle α of the ejection. The flow visualization using the shadowgraph method and the pressure measurements at the impinged surface show that the shock structure in the jets and the surface pressure strongly depend on the nozzle contour. It is demonstrated that the impinging jet issuing from an annular nozzle with large d/D and large α produces high surface pressure concentrated at the jet center. These results suggest that annular nozzles show great potential for assisting laser cutting.

Analyses of the radial distributions of average velocity and turbulent velocity of the liquid phase in bubbly two-phase flow

Abstract: The radial distributions of average velocity and turbulent velocity of the liquid phase are predicted based on the correlations of the bubble dispersion coefficient, turbulent diffusivity of liquid and the mixing length of two-phase flow previously derived by the authors. The radial distribution of average liquid velocity was predicted using basic equations of mass and momentum conservations for both phases. The prediction generally agrees with experimental data, which show a flattened radial distribution compared with the single-phase flow at the same liquid volume flux. The radial distribution of turbulent velocity was predicted using the basic equation of turbulent velocity for the case where turbulent velocity in two-phase flow is larger than single-phase flow turbulent velocity for the same liquid flux. The prediction successfully predicted the flattened distribution of turbulent velocity. Detailed consideration was given to the contributions of generation by velocity gradient, dissipation, diffusion and generation by a bubble to the turbulent velocity distribution, and it was shown that the turbulence generation by a bubble is predominant in the central region of a bubbly two-phase flow.

Fluid Flow and Heat Transfer in a Periodically Diverging-Converging Turbulent Duct Flow

Abstract: Experiments have been performed to investigate the effects of pressure gradient on the fluid flow and heat transfer in duct flow. Air flow in a fully developed turbulent state was decelerated through a linearly diverging section, and then accelerated through a linearly converging section. Two different ducts were tested, whose acceleration parameter K in the accelerating section was 2. 2×10-6 and 4. 5×10-6. The wall static pressure, the heat transfer coefficient and fluctuations of streamwise velocity as well as the time-averaged velocity profiles were measured. The heat transfer coefficient and turbulence intensity in the converging section were considerably lower than those in the diverging section, and the viscous sublayer expanded in the converging section. The behavior of the increase or decrease of the heat transfer coefficient was similar to that of the turbulence intensity in the near-wall region.

Effect of Rows of Two-Dimensional Square Ribs on Flow Property along Plane Wall

Abstract: This paper describes the detailed study of the flow structure over rows of two-dimensional square ribs on a ground plane for various values of S/D, and the optimum value of S/D to augment the turbulence of the free stream. The pitch between the centers of two adjoining square ribs was varied at S/D=2, 3, 4, 5, 7, 9, 13 and 17. The time-mean velocity, static pressure and the velocity vectors were measured by Pitot and static pressure tubes, and a cylindrical yawmeter of 6mm in diameter having three pressure holes. The turbulence intensities and autocorrelation were obtained using the data processing system and the F. F. T. analyzer connected to the hot-wire anemometer. As a result, it is concluded that the pitch ratio S/D=9 is optimum for augmenting the turbulence intensity.

Motion of Small Particles due to a Pulsatile Flow in a Chamber.

Abstract: The unsteady motion of small particles, which are uniformly distributed at first, in a square chamber with a pulsating inlet velocity is studied numerically. The motion of the particles is classified into 3 types: the exhaust type, the circulation type, and the adhesion type. The effects of the period and the amplitude of the inlet velocity on the ratio of the number of particles of each type to the total number of particles are examined taking the Reynolds number and the dimension ratio as parameters

Numerical analysis of flows around a rotating square cylinder

Abstract: Flows around a rotating square cylinder in a uniform flow are analyzed numerically. Body-fitted grid generation with moving boundaries is used to obtain the numerical solution of the incompressible Navier-Stokes equations. The time-dependent curvilinear coordinate system which coincides with a contour of the moving boundary is transformed into a fixed rectangular coordinate system in a calculational plane. The results of this method are compared with those of previous experiments, and their validity is confirmed. The changes in the stream lines, pressure distributions, CD and CL as the cylinder rotates are analyzed. The periodical flow can be simulated when the Karman vortex shedding and the rotation of the square cylinder are synchronized .

An experimental study on the permeability of screen wicks

Abstract: Several kinds of test screens, which have a large mesh number similar to the wick material of heat pipes conventionally employed, were prepared to examine the influence of wick characteristics on permeability. From the experimental results, an expression of fRe (the product of friction factor and Reynolds number) correlating to the ratio of the wick thickness per single layer and the wire diameter was proposed

Experimental considerations in an approximate method for estimating the blade performance of Darrieus-type cross-flow water turbines

Abstract: To estimate the blade performance of a Darrieus-type cross-flow water turbine, differences between the characteristics of a Darrieus blade which moves along a circular path and that of a blade at rest in a uniform steady flow are examined quantitatively through the use of experimental data for three types of blades with different cambers or chord lengths. It is found that a unique stall character strongly affects the drag force acting on the Darrieus blade. A model of the changing pattern of the drag coefficient due to dynamic stall is proposed from the results of measurement for estimating the blade characteristics. As a result, the blade characteristics and performance of the Darrieus-type runner are predicted well by the use of aerofoil data in a uniform steady flow, taking into account the effect of a circling motion and the dynamic effect of the unsteady flow around the Darrieus blade.