Showing papers in "Jsme International Journal Series B-fluids and Thermal Engineering in 1995"

High-Speed Observations of the Cavitation Cloud around a High-Speed Submerged Water Jet

Abstract: In this paper, we attempt to clarify the jet structure and the behavior of severely erosive cavitation clouds around a high-speed submerged water jet, using a high-speed movie camera with a framing rate of ten thousand frames. The effects of the injection pressure and the nozzle geometry on cavitation are also investigated. The experiments are performed with both a free jet and an impinging jet. It is clearly found that the cavitation clouds are periodically discharged. The cavitation clouds are also closely related to downstream instability and to the impinging erosion.

Suitable Region of High-Speed Submerged Water Jets for Cutting and Peening.

Abstract: In order to establish useful techniques of cutting, drilling, peening and flushing by high-speed submerged water jets, we systematically observe the eroded surface on aluminum-alloy specimens in relation to the characteristics of cavitating jets, especially for the impinging jet through lucite specimens, for several types of nozzles. The impulsive pressures induced by the cavitating jets are also measured by means of pressure-sensitive film. It is concluded that the erosion characteristics are quite different in two typical standoff distances, i. e., the 1st and the 2nd peak. Cylindrical, small and deep pits predominantly occur at the 1st peak, while small lots of plastic deformations predominantly occur in the 2nd peak. Therefore, the 1st peak is suitable for cutting, while the 2nd peak is suitable for peening.

Instantaneous Velocity Profile Measurement by Ultrasonic Doppler Method

Abstract: The Ultrasonic Velocity Profile (UVP) measuring method has been developed at PSI It utilizes pulsed ultrasonic echography together with the detection of the instantaneous Doppler shift frequency This method mainly has the following advantages over the conventional techniques : 1) an efficient flow mapping process, 2) applicability to opaque liquids and 3) a record of the spatiotemporal velocity field After a brief introduction of its principle, the characteristics and specifications of the present system are given Then examples such as for oscillating pipe flow, T-branching flow of mercury and circular free jet flow in fluid engineering are described, which confirm the above advantages In a further in-depth study of fluid dynamics, results of an investigation of modulated wavy flows in a rotating Couette system are presented The position-dependent power spectrum and time-dependent energy spectral density were used in order to study the dynamic characteristics of the flow, and subsequently the velocity field was decomposed into intrinsic wave structure based on the two-dimensional Fourier analysis

Bubble Behavior in Magnetic Fluid under a Nonuniform Magnetic Field.

Abstract: Experimental study is made to clarify the effects of nonuniform magnetic field on bubble behavior in a magnetic fluid. The behavior of vapor bubbles is visualized with ultrasonic wave echo under the nonuniform magnetic field. It is found that the void fraction in two-phase flow decreases with increase in the magnetic field due to the effect of magnetic body force. Furthermore, the effects of nonuniform magnetic field on translational motion and deformation of a single gas bubble in a magnetic fluid are experimentally examined using a transparent thin duct, called a Hele-Shaw cell. The experimental results indicate that a single bubble is decelerated in the region of positive magnetic field gradient and accelerated in the region of negative magnetic field gradient, and that it is elongated in the direction of the magnetic field.

Recent Developments of the GRP Method

Abstract: A review of about a decade of development of the generalized Riemann problem (GRP) scheme is presented. The method is briefly outlined, followed by various numerical and physical extensions. The range of versatile applications of the GRP method is illustrated through numerous examples.

Wavelet analysis for the plane turbulent jet (analysis of large eddy structure)

Abstract: In this study, wavelet analysis is applied to velocity signals of a plane turbulent jet, in order to investigate the eddy structure in the dimensions of time and scale. First, a review of the definitions and the basic properties of wavelet analysis are introduced, and a revised form of the Mexican hat and the wavelet power spectrum are proposed. To illustrate some typical behaviors of the wavelet coefficient phase, a numerically generated signal is analyzed. Then from the velocity signals of a jet on the centerline and in the mixing layer, the structural features of the eddy are analyzed in terms of instantaneous frequency and onset time/position. The results reveal that eddies of very different scale and the breakdown of a large eddy are displayed in the wavelet coefficient phase, as well as the successive branchings of a large eddy structure. Furthermore, it is found that the scale of the eddy and intermittency in the mixing layer can be obtained by wavelet analysis. The wavelet power spectra agree fairly well with the Fourier power spectra and just correspond to the actual kinetic energy per unit time at each wave number.

Rigorous Modeling of Dissipation-Rate Equation Using Direct Simulations.

Abstract: With the aid of direct numerical simulation (DNS) data, we have constructed near-wall modeling of the dissipation-rate equation for a k-e turbulence model. All the budget terms in the exact e-equation are incorporated in the modeled e-equation, and both term-to-term and overall comparisons of model predictions with the DNS data are performed. It is found that the wall region should be partitioned into at least two regions so as to reflect different mechanisms of the generation processes in the dissipation of turbulent kinetic energy. Pressure diffusions in the k-and e-equations are found to be equally important and thus should be modeled. Comparisons of the resultant k-e model with the DNS data of channel and boundary layer flows with and without pressure gradients show close agreement of various turbulent quantities. In particular, near-wall behavior of e is reproduced with the present model very well.

Nonlinear Oscillations of a Cluster of Bubbles in a Sound Field. Bifurcation Structure.

Abstract: This paper deals numerically with nonlinear oscillations of spherical bubbles in a sound field. A Governing equation of a cluster of interacting spherical bubbles is obtained by taking account of the compressibility of a liquid. This equation includes previous results for both single bubble and two bubbles as special cases. Bifurcation diagrams are computed for typical configurations of bubbles. It is shown that the bifurcation structure of multiple bubbles with the same radii which are arrayed symmetrically in a sound field is similar to that of an isolated bubble driven by sound of higher frequency. When bubbles with different initial radii oscillate in a sound field, the independent oscillation of each bubble is suppressed by bubble interactions and the bubbles take on a collective behavior.

Scale Effects in Cross Flow Fans : Effects of Fan Dimensions on Flow Details and the Universal Representation of Performances

Abstract: Laser doppler velocimeter (LDV) measurements of velocity fields and the determination of static pressure distributions in the rotors of cross flow fans with good geometrical similarity but different dimensions have been performed, and the peculiar scale effects appearing in the flow details have been shown. The internal flow fields tend to be more active with increasing rotor size, and normalized velocity, vorticity, velocity fluctuations and pressure drops in the eccentric vortex increase as the fan dimensions increase. The mechanisms of generation and dissipation of vorticity in the eccentric vortex in rotor have been discussed, and it is suggested that the scale effects may be attributed to differences in the rate of diffusion of the circulation vortices in the recirculatory region. By analyzing the influences of Reynolds number and fan dimensions on performance curves, a new procedure for representing the performance of cross flow fans in a universal form has been proposed.

A Study on Ignition Process of Diesel Sprays

Abstract: The main theme of this study is to reveal the characteristics of the fuel-air mixture that is responsible for ignition of diesel sprays. For this purpose, the ignition process of a spray was studied under diesel-like conditions realized by a rapid compression machine. The fuel-air mixture before ignition was chemically analyzed by a total gas sampling method. The condition to which fuel was subjected was estimated by combining the information on the mixture composition with pressure-time curves and photographs of sprays. The results show that the mixture before ignition has considerably higher temperature than the surrounding air. Oxidation begins very early in the ignition delay period, which supplies heat to the spray and causes cracking and gasification of fuel. It is assumed that the accumulated gas begins to release a large amount of heat triggered by the oxidation of a relatively small-scale mixture.

Unsteady Fluid Forces on a Blade in a Cross-Flow Turbine

Abstract: The internal flow in a cross-flow turbine is nonuniform because the water passes through only part of the runner. Therefore, the unsteady fluid forces act on a blade through rotation. Experimental and theoretical studies for determination of fluid forces on the blade in a cross-flow turbine are conducted. In the experiment, the tangential and radial forces are measured on a test blade using strain gauges and slip rings. On the other hand, in the theoretical study, they are calculated numerically using the unsteady momentum theory. The calculated results are compared with experimental data and good agreement is demonstrated. Furthermore, the maximum forces are found to occur immediately before the blade leaves the nozzle exit in both the experimental and theoretical results.

Generation of High-Speed Liquid Jets by High-Speed Impact of a Projectile.

Abstract: In generating a pulsed high-speed liquid jet by the impact of a high-speed projectile on a container filled with a liquid, complicated shock wave phenomena subsequently appear. The shock waves affect the liquid jet not only at the earlier stage of its formation but also at the later stage of its propagation. This paper reports results of experiments performed in order to understand the entire process of the generation of a liquid jet. The liquid jet was driven either by the impact of a polyethylene projectile directly on a container which was filled with water and connected to a nozzle or by collision of the projectile with a metal piston which transfers the momentum of the projectile to compress the liquid. The resulting shock wave and liquid jet were observed by double-exposure holographic interferometry. Impact pressures were measured with commercial pressure transducers and also with laboratory-fabricated pressure transducers made from polyvinylidenedifluoride (PVDF) piezo film. Shock waves were emitted in front of the water jet through the liquid/gas interface at the nozzle exit whether the speed of the water jet was supersonic or subsonic in terms of the speed of sound in air. This transmission process of the shock wave was demonstrated in an experiment in which an underwater micro explosion was carried out just below the air/water interface. A numerical simulation was conducted to understand the nozzle flow and the pressure history in the nozzle.

Molecular Mechanical Engineering

Abstract: Owing to the progress of techniques and technology with high performance and high efficiency, fundamental understanding of their phenomena and functions from the elementary processes of molecular motion has increasingly been required. This leads to the introduction of "molecular mechanical engineering" which forms a bridge between microscopic molecular understanding (molecular science) and macroscopic continuum engineering (mechanical engineering). The former should be viewed not as an isolated science itself but as fundamental subsystems of the latter. With respect to "molecular mechanical engineering, " we discuss and review its scientific philosophy, how it will be developed and extended, and how it will contribute to further advances in mechanical engineering technology.

Mathematical Model of Vane Compressors for Computer Simulation of Automotive Air Conditioning Cycle

Abstract: Computer simulation of refrigerating cycles for automotive air conditioners is effective in predicting performance of the cycle. A proper mathematical model of a compressor is necessary to predict the performance exactly. In this study, the mathematical model of a vane compressor for the automotive air conditioning cycle is developed. The model consists of two control volumes, a cylinder and a rear case, and a compressor body. It takes account of the thermal effect of lubricating oil and heat transfer between refrigerant and the compressor body. Validity of the model is confirmed by comparison between experimental results and calculated ones. The transient behavior in the cycle simulation depends on the compressor modeling noticeably.

An Experimental Study of Evaporation Heat Transfer of Refrigerant HCFC22 Inside an Internally Grooved Horizontal Tube

Abstract: The characteristics of flow pattern, heat transfer and pressure drop of evaporation of HCFC 22 inside a horizontal tube are experimentally revealed. The experiment is conducted with a double-tube evaporator, where the refrigerant flows inside the inner tube and the heating water flows countercurrently in the surrounding annulus. The inner tube is an internally grooved copper tube having a 9.52 mm o. d. and an 8.72 mm mean i. d. The ranges tested for the refrigerant are as follows : mass velocity of 110 to 220 kg/ (m2s), vapor pressure of 0.4 to 0.65 MPa and heat flux of 5 to 35 kW/m2. The flow pattern observed through sight glasses changes from the wavy-annular type to the annular type and then to the mist flow type as the evaporation progresses along the tube. These transitions of flow patterns occur at much lower values of vapor qualities than those predicted from the Baker map modified by Scott for a smooth tube. The heat transfer coefficients in the wavy-annular and annular flow regions are about 2 to 4 times higher than those of smooth tubes, and are expressed as a simple function of the Lockhart-Martinelli parameters. The empirical equations for the heat transfer coefficient in the mist flow region and in the single phase heat transfer region are also presented. The accuracy of these equations along with the value of the transition quality is confirmed in a comparison between the design calculations and the experimental data.

On the scaling of separation bubbles

Abstract: A simple analysis, based on the scaling of the Navier-Stokes equations, is presented for general flow situations involving separation bubbles. Three possible functional relations between the separation bubble length L and Reynolds number Re are derived : (i) L∝Re, (ii) L∝1/Re, and (iii) L=constant. These relations are shown to correspond to the experimentally observed behavior of laminar, transitional and turbulent separation bubbles, respectively. The results of the analysis are discussed in the context of several internal and external separated flows in various geometries, and the corresponding empirical constants are evaluated from available experimental and numerical data.

Numerical Simulation of Pseudoshock in Straight Channels.

Abstract: Pseudoshock in straight channel flows was numerically simulated by solving the two-dimensional time-averaged Navier-Stokes equations. The objectives of the present simulation were to investigate the influence of parameters on the structure of pseudoshocks and to validate the model of the pseudoshock mechanism proposed in the previous paper. In this simulation, Harten-Yee's second-order-accuracy TVD scheme was used for high resolution of the shock wave and numerical stability, and Baldwin-Lomax's algebraic model was used for turbulent flows. The computational results agreed well with the experimental ones and with the proposed model ofλ- and X-type pseudoshocks.

Hydraulic Losses of Flow Control Devices in Pipes

Abstract: Concerning the vortex shedder used for the Karman vortex flowmeter, the flow resistances of cylindrical bodies of various cross-sections in a circular pipe are given by the general equation in the previous works of Igarashi et al. In the present paper, it is confirmed that the equation is applicable to the vortex shedder having a low opening ratio. Then, the same idea is applied to the hydraulic losses of flow control devices asymmetrical to the cross-section of the pipe, such as a sluice valve, a butterfly valve and a cock. Consequently, these hydraulic losses are expressed by the following general equation: ζ=C[(1-β)/β 2 ] n , where β is the opening ratio.

Chemiluminescence Emission of C2, CH and OH Radicals from Opposed Jet Burner Flames

Abstract: Chemiluminescence emission from opposed jet burner flames was measured by a charge coupled device (CCD) camera with an image intensifier. Radial profiles of the emission intensity of OH, CH and C2 radicals were obtained by the Abel inversion of the side-on observations. The profiles of emission intensity of these radicals are similar, and each maximum is located at the center of the flame zone. The emission of C2 and CH radicals produced in the early stage of the combustion reaction was observed even on the burned-gas side of the flame zone where the mean temperature is significantly high, suggesting that unburned mixture exists on the burned-gas side of the flame zone. It was found that the profiles of OH emission intensity and NO concentration are similar. This fact suggests that NO originating from the prompt NO mechanism is predominant in the distributed reaction zone.

Enhancement of evaporation of a droplet using EHD effect : (onset of instability of gas-liquid interface under electric field applied in a stepwise manner)

Abstract: An experimental study is conducted to investigate the onset of instability of a liquid surface under an electric field applied in a stepwise manner. The critical voltages and the response times above which the liquid surface becomes unstable are measured under electric fields applied in two different ways. Instability criteria are derived theoretically using a modified Rayleigh-Taylor instability equation for the cases of a perfectly conducting liquid and a perfectly insulating liquid. According to the results of the experiment and theoretical analysis, consideration is made of the mechanism of the onset of EHD (electro-hydrodynamic) instability of the liquid surface having a long electrical relaxation time.

A Study on the Mechanism of Spontaneous Vapor Explosions with Single Molten Tin Drops and Water

Abstract: The spontaneous vapor explosion generated by single drops of molten tin submerged in water is investigated using high-speed photography and pressure trace measurement. The vapor film begins to collapse at the edge of the disk-shaped drops and the collapsed zone spreads over the rest of the drop at a velocity of several meters per second. Explosion experiments in a two-dimensional channel with a narrow space are also carried out to observe more clearly the explosion configuration and the fragmentation process. A model of the spontaneous vapor explosion with a single drop is presented on the basis of experimental observations. In this model, the size of the vapor bubble compared with the cloud of high-temperature debris is assumed to have a strong relationship with heat transfer, and the collapse of the vapor bubble is also assumed to be induced by overexpansion and condensation.

Calculation of turbulent flows with pressure gradients using a k-ε model

Abstract: A k-e turbulence model is developed to calculate wall turbulent shear flows under various pressure gradient conditions. In the present model, we set the dissipation rate of turbulent energy at a wall equal to zero, though the wall limiting behavior of velocity fluctuations is reproduced exactly. Thus, the model assures computational expediency and convergence. The proposed model is constructed to properly take into account the effect of pressure gradients on shear layers. It was found by Nagano, Tagawa and Tsuji (Turbulent Shear Flows 8, (1992), 7) that in adverse pressure gradient flows the Van Driest constant decreased with increasing dimensionless pressure gradient parameter P+. Therefore, the present model incorporates the modified Van Driest dumping function which is a function of P+. The validity of the proposed model was tested by application to a turbulent channel flow and boundary layers with P+ 0. The model predictions indicate that agreement with the experiment and the direct simulation data is very good over a wide range of pressure variations.

Effect of Sulfur Dioxide in Recirculated Exhaust Gas on Wear within Diesel Engines : Relationship between Wear and Amount of SO2 Absorbed by Lubricating Oil Film

Abstract: In both spark ignition engines and diesel engines, exhaust gas recirculation (EGR) is well known as an effective technique to reduce emissions of nitrogen oxides. However, it has not yet been applied practically to heavy-duty diesel engines because the wear on piston rings and cylinder liners is increased by EGR. It is widely accepted that sulfur dioxide in burnt gas is strongly related to this wear increase. This paper proposes a numerical model to clarify the mechanism of wear which estimates the amount of sulfur dioxide absorbed by lubricating oil film. The calculated results show that the concentration of sulfur dioxide in lubricating oil film is increased with the increase in the rate of EGR and the reduction in engine speed, and is decreased with the decrease in engine load. The measured amount of wear on piston rings and cylinder liners is related to the concentration of sulfur dioxide in lubricating oil film.

Evaporation Rate of Free Paraffin Hydrocarbon Droplets in a High-Temperature and High-Pressure Gas Stream

Abstract: A series of evaporation rate constants has been obtained for fine free droplets of paraffin hydrocarbons, n-heptane, iso-octane and n-undecane in the range of 623 to 1023K in temperature and 0.1 to 0.5 MPa in pressure as the ambient conditions. To obtain the time histories of droplet diameter a fine droplet stream generated by an electrically controlled piezoelectric element was synchronized with a flash light, and enlarged droplet images were taken on a still camera. The evaporation rate constant increases with ambient pressure and temperature and is expressed by empirical equations. A mixed-hydrocarbon droplet of n-heptane and n-undecane was also examined in terms of the evaporation rate constant, and the evaporation history, the so-called D2-t relation, displays a bending line with two separate straight lines of respective constituents.

Numerical Analysis of Transient Flow through a Pipe Orifice. Time Constant for Settling Flow.

Abstract: As a fundamental consideration in hydraulic valve dynamics, transient flow through a pipe orifice has been studied via a numerical analysis. Steady axisymmetric viscous fluid flow was first investigated to confirm the present SIMPLER-based finite volume methodology. Time-dependent calculation for a suddenly imposed pressure gradient has shown two distinct characteristic time constants for the transient state. The first characteristic time is commonly considered to correspond to the flow rate change, while the second one concerning the variation of flow structure has not been treated in earlier studies. The final settling of flow is completed in the second characteristic time which is almost ten times larger than the first one under the present condition.

Simultaneous Measurements of Flow Rates and Particle Concentrations in Heterogeneous Solid-Water Two-Phase Flows by Means of One Venturi

Abstract: In order to establish an accurate measurement method for simultaneous measurements of flow rates Q and particle concentrations Cv in heterogeneous solid-water two-phase flows, the pressure-drop ratios were investigated in detail at the exit of a typical Hershel-type venturi in both horizontal and vertical pipe arrangements for six different kinds of solid-water flows with various Q and Cv. The results were represented by a simple formula similarly to that in our previous report on throat measurements. These results obtained at the exit, together with those obtained at the throat, indicated the possibility of simultaneous measurements of Q and Cv in an easy and accurate manner by means of only one Venturi.

Experimental Study on the Effect of Metallic-Coated Junctions on Thermal Contact Conductance

Abstract: The effects of one-surface and two-surface coatings on thermal contact conductance of contacting aluminum 6061-T6 surfaces were experimentally investigated through the use of metallic coatings prepared by the ion vapor deposition (IVD) process. Three different coating materials, aluminum, lead, and indium, were evaluated using two different coating thicknesses and two different surface roughnesses over the pressure range of 100 to 500 kPa, which is suitable for microelectronic application purposes. Experimental results are presented to graphically illustrate the dimension. less thermal contact conductance of one-surface coatings and two-surface coatings as a function of contact pressure. The thermal contact conductance was found to vary from 0 to 500 percent of the uncoated value, depending on the contact surface characteristics.

Application of a Two Phase Flow Model Based on Local Relative Velocity to Gas-Liquid-Solid Three-Phase Flows

Abstract: A two-phase flow model based on local relative velocity, which was previously proposed by the authors, was extended to a model for gas-liquid-solid three-phase flows. The extension was carried out utilizing a hypothetical two-phase flow, which was conceived by removing one of the three phases. In order to examine the usefulness of the extended model, the measured area-averaged volumetric fractions of gas-liquid-solid three-phase bubbly or slug flow in vertical pipes were correlated based on the basic equations of the extended model. The accuracy of the obtained correlation was compared with those of the drift-flux correlation, the correlation based on a multiplier method and the correlation based on a gas-liquid-solid three-phase slug flow model. Consequently, it was confirmed that the extended model gives simpler and more accurate correlations for the area-averaged volumetric fractions of the gas-liquid-solid three-phase flows.

Numerical Analysis of the Diffusion Process of Intake Mixture in Dual-Intake Valve Engines.

Abstract: The diffusion process of the intake mixture (homogeneous mixture of air and gaseous fuel) in the cylinders of dual-intake valve S. I. engines has been analyzed numerically using the GTT and CIP methods and the k-e turbulence model. The following has been found : When the fuel is supplied into only one intake port in one engine in which a tumbling vortex is generated, the intake mixture is localized mainly in half of the combustion chamber and the fuel concentration is stratified. In another engine in which a swirl is generated by holding one intake valve closed, a relatively homogeneous mixture or an axially stratified mixture is formed in the combustion chamber, depending on whether fuel supply is early or late.

Numerical Study of Compression Waves Produced by High-Speed Trains Entering a Tunnel : Effects of Shape of Hood

Abstract: In this study, compression waves produced by trains which enter a hooded tunnel at high speed were numerically investigated. Equations of the one-dimensional, unsteady and compressible flow in which the area of a tube is dependent on the time and the distance were numerically solved by the method of characteristics. The calculations were performed for tunnels with various hoods of different shapes in cases in which the speed of the train was 200 km/h or 300 km/h, and the effects of the shape of the hood on the shape and the strength of compression waves were quantitatively clarified. The results show that the pressure gradient of compression waves is greatly affected by the shape of the hood.