Showing papers in "Transactions of the Japan Society of Mechanical Engineers. B in 1997"

Slip of Newtonian Fluids at Solid Boundary

Abstract: A real fluid does not slip at the surface of a solid body. Most experimental results of a Newtonian fluid satisfy this condition. If a real fluid can slip freely over the surface of a solid body, how can we deduce the slip velocity? Although slip is a basic, interesting problem in fluid mechanics, fundamental data are lacking. The purpose of this study is to clarify fluid slip experimentally. Velocity profiles of tap water and 20 wt% glycerin solution flowing in a square duct with a highly water-repellent wall were measured by means of a hot-film anemometer. Consequently, slip of Newtonian fluids at the solid wall was revealed and explained by Navier's theory.

Direct Numerical Algorithm for Multiphase Flow with Free Surfaces and Interfaces.

Abstract: This paper is to describe a new direct numerical procedure for solving general multiphase flow problems with many free surfaces and interfaces. Since conventional cell volume fraction approaches, such as SOLA-VOF, use the donor-acceptor procedure for the transport of fluid-segments, they cannot transport the interface slope with sufficient accuracy. In the present study, a new accurate algorithm for interface advection and its reconstruction, called MARS (Multi-interfaces Advection and Reconstruction Solver), was developed. It consists of the continuum surface force (CSF) model for surface tension and a new line-segment algorithm for capturing and connecting the interface between neighborhood computational cells. In order to show the potential of the MARS, some direct numerical simulation results for the formation and agglomeration of bubbles in a bubble column were presented as an example.

Effect of Guide Vanes on the Performance of a Wells Turbine.

Abstract: In order to improve the performance of a Wells turbine, the effects of the gap between rotor and guide vane and solidity of the guide vane have been investigated by model testing. The results have been compared with those of the case without guide vanes. It is found that the overall characteristics of the turbine are considerably improved by the guide vanes. Furthermore, a suitable choice of design parameters such as the gap and the solidity of the guide vane has been suggested.

Design and Control of Crossflow Instability Field

Abstract: In order to find out the transition mechanism of crossflow dominant boundary layers in detail, experimental model which is composed of yawed flat plate with displacement upper body, is designed and full transition process from onset of transition to fully turbulent state is generated in the flat plate boundary layer Systematic measurement is conducted on this crossflow instability field using hot wire velocimetry with accurate traversing mechanism and effective flow visualizations. Results show that such a complicated flow condition, where several different disturbances are appearing and interacting, is occurring in the transition region. Such flow condition is quite similar to that of swept wing boundary layer flow. In the transition region, stationary crossflow vortices, crossflow instability unsteady mode, high frequency secondary instability mode are also measured. Schematic sketch of the obtained flow field where these instabilities are most amplified is drown. Directions and phase velocities of obtained disturbances are also measured, and indicated in one figure together with twisted boundary profiles. Secondary instability is successfully visualized by smoke visualization technique, and physical structure of the secondary instability is compared with the results of hot wire measurement. Good agreement is obtained especially for the travel direction of the secondary instability, discussions are also made concerning obtained interesting results.

Effect of Oxygenated Fuel on Exhaust Emission from DI Diesel Engines.

Abstract: Smoke emission from single-cylinder DI and IDI diesel engines was found to strongly depend on oxygen content in the fuel regardless of the oxygenate molecular structure. Thus, various kinds of oxygenate were assessed based on blending properties with commercial diesel fuel and potential cost. The combined use of cetane improver with glycol ether oxygenate reduced the amount of particulate, HC, and CO emission, but not that of NOx. The oxygenate seldom affected deterioration of fuel system rubber of the vehicle and storage stability.

Flow Patterns and Frictional Pressure Drop in Gas-Liquid Two-Phase Flow in Vertical Capillary Channels with Rectangular Cross Section.

Abstract: In a compact heat exchanger, a gas-liquid two phase mixture flows in a noncircular channel with a narrow cross sectional area. Therefore it is necessary to clarify the flow phenomena and the pressure drop in such a small channel. In the present experiment, we used capillary vertical rectangular channels. The shorter side length is constant at 1 mm, and the longer side length of each duct is 1.0, 2.0 or 5.0 mm. The effects of the aspect ratio of the channel on the velocity characteristics, flow patterns and two-phase frictional pressure drop are discussed. The modified correlations for the estimation of frictional pressure drop were obtained by taking the effect of aspect ratio into account. These formulae agree well with the experimental data.

Effects of Fuel Properties on Exhaust Emission from DI Diesel Engine

Abstract: Exhaust emissions (HC, CO, NOx, PM, and polycyclic aromatic hydrocarbons in PM) from a commercial DI diesel engine were measured for petroleum-derived fuels with a wide range of fuel properties. Fuel properties such as cetane number, aromatic content and type, and distillation temperature were independent of each other. The test results showed that PM and NOx depended predominantly on aromatic content, especially polycyclic aromatic content, in fuel. PAH emission was also strongly affected by polycyclic aromatic content. The amount of sulfate in PM was proportional to sulfur content in fuel. The cetane number seldom affected the amount of PM and NOx emissions.

Characterization of Diesel Fuel Spray by Ultrahigh-Pressure Injection

Abstract: Electronically controlled and hydraulically actuated fuel injection equipment was developed to investigate the characteristics of a diesel fuel spray at injection pressurs of up to 300 MPa. The injection equipment made it possible to keep the injection pressure almost constant during the injection period. The spray angle and the Sauter mean diameter decreased with an increase in the injection pressure in the range up to 250 MPa, whereas above this injection pressure the spray did not show such variation. Mach waves were observed around the spray when the initial penetrating velocity of the spray tip exceeded the sound velocity of the ambient gas. The distribution of the penetrating velocity of the fuel along the spray axis was measured from the Mach angles at the edge of the spray.

コロケート格子による非圧縮流れの高次差分解析〔流体工学, 流体機械〕

Abstract: To apply the direct numerical simulation (DNS) and the large-eddy simulation (LES) of turbulence to flow fields of complicated geometry, a higher-order finite-difference method (FDM) has been developed for the body-fitted coordinate system. The consistency and the conservation property of FDMs are discussed for the collocated grid. As numerical examples, DNS results of decaying isotropic turbulence and DNS/LES results for plane channel flow are shown and the influence of variable arrangement is examined. The results by the consistent 'interpolation' method for gradient form on the collocated grid agree well with those by other proper FDMs and the spectral method.

二次元モデルから発生する空力騒音の実験的研究 : 第2報, 円柱の傾斜角度と角柱の迎え角の発生音への影響

Abstract: Control of the aerodynamic noise generated from two-dimensional circular and square cylinders are studied experimentally in a low noise wind tunnel. The circular cylinder of 1cm diameter is inclined against the mean flow, whereas the square cylinder of 1cm2 cross section is placed normal to the mean flow, with various angles of attack. Mean velocity of the flow is between 20.6m/s and 41.6m/s. Powere spectra of the radiated noise in various angles of inclination or angles of attack are measured in order to examine the changes in peak levels and the Strouhal numbers of the so-called Aeolian tones. For the circular cylinder, it is found that the Strouhal number based on the frequency of the Aeolian tone and the normal component of the mean velocity to the axis of the circular cylinder stays nearly constant for the angle between 0° and 45°. The peak level of the radiated noise decreases rapidly from the inclined angle from 0° to 15°. but it stays nearly constant for the angle over 15°. For the square cylinder, the Strouhal number is smallest and the peak level is highest at the angle of attack of 0°. The peak level decreases and the Strouhal number increases rapidly with increase of angle of attack up to 13°. For the angle above 15°, both the Strouhal number and the peak level stay almost constant. It is concluded that a circular cylinder should be inclined more than 15°. and a square cylinder should have an angle of attack well over 15° in order to reduce aerodynamic noise generation.

後向きステップを過ぎる低温の超音速流れ中に垂直に噴射される水素の触媒燃焼〔流体工学, 流体機械〕

Abstract: Supersonic combustion using a catalytic combustion in a cold supersonic flow field was investigated in a square duct with a backward-facing step. The free stream Mech number was Mm=1.81. Hydrogen was injected transversely behind a backward-facing step into a cold air free stream. Using a catalyst in a cold supersonic turbulent mixing layer, it was found that hydrogen reacted stably to oxygen in the air flow. The relationship between the heat release due to catalytic combustion and supersonic flow properties, which influence the supersonic combustion, was clarified experimentally. The spatial distribution of heat release generated by catalytic combustion in the supersonic turbulent mixing layer is discussed. It was found that the heat release due to the catalytic combustion had a maximum at the outer edge of the mixing layer.

Study on Resistive Force Theory for Flagella with Boundary Element Method

Abstract: The resistive force theory (RFT) is a conventional method for analyzing the motion of a microorganism swimming with a flagellum. This theory has been used in many analyses ; however, it has some flaws such as that hydrodynamic interaction between the cell body and the flagellum is ignored and the interaction between a portion of the flagellum and the remains is also ignored. In order to estimate these effects, the boundary element method (BEM) is applied to investigate the Stokes flow around a microorganism swimming with a rotating helical flagellum. First, force and torque of prolate spheroids and of circular cylinders are calculated to check the accuracy of the BEM. Fairly good agreement is obtained between the present results and the theoretical ones. Then, the mean propulsive velocity of a microorganism is calculated. The velocity agrees with that obtained by using the slender body theory (SBT) . The interaction between the cell body and the flagellum is small ; therefore, the RFT can be used to evaluate the force and the torque exerted on the flagellum. On the other hand, the interaction among the portions of the flagellum is quite large, and this causes differences in results between the BEM and the RFT. The results of the RFT are improved by adjusting resistance coefficients to include this interaction effect.

Generation of the Karman Vortex Street at Low Reynolds Number due to Cooling of a Cylinder. Cause and Fluid Type Effect.

Abstract: For clarification of the cause and effect of fluid type on the generation of the Karman vortex street due to cooling of a cylinder at a low Reynolds number where the Karman vortex street does not occur in an isothermal wake, the two-dimensional, laminar, time-dependent continuity equation, Navier-Stokes equations with the buoyant term of Boussinesq's approximation, and energy equation are solved numerically for a circular cylinder wake with an upward freestream of mercury, air, and water. The cause is the generation of the wake vorticity which does not occur in an isothermal wake, and a stable arrangement of vorticity. With a decrease in the Prandtl number of the fluid, the Karman vortex street is generated easily by cooling of a cylinder, and has lower frequency, lower velocity of the vortex movement, and larger scale of vortex spiral than the isothermal Karman vortex.

Pressure Drop and Heat Transfer for Flow-Boiling of Water in Small-Diameter Tubes.

Abstract: Pressure drop and heat transfer for flow-boiling of water in small-diameter tubes under atmospheric pressure were discussed. The results of experiments were compared with those of several existing correlations and models. The experiments were carried out using atmospheric-pressure water in tubes with inner diameter D ranging from 2.0 to 6.0mm, heated length L from 4.0 to 680.0mm, inlet water subcooling ΔTin from 70 to 90K, and mass velocity G from 100 to 10170kg/ (m2·s). The highest heat flux q attained was 33MW/m2. Interesting phenomena were observed in small-diameter tubes with a very long or very short heated length. The most suitable correlations for predicting the pressure drop and heat transfer in small-diameter tubes were discussed.

Propagation of Shock Waves in Dilute Bubbly Liquids

Abstract: Transient shock wave phenomena in a liquid containing noncondensable gas bubbles are investigated experimentally and numerically. In the experiment, using a vertical shock tube, the time evolution of the propagation velocity and the waveform are successfully measured. In the numerical analysis, the thermal condition of the bubble interior is directly simulated by using full equations for mass, momentum and energy conservations, and the results are combined to simulate shock waves in bubbly liquids. Relative translational motion between the bubbles and the liquid is also taken into account. Comparison between the numerical results and the experimental ones shows that there is a discrepancy of the period of pressure oscillation behind the shock front, even though the rough feature of both pressure profiles agrees well.

Measurement of Interfacial Configuration of Bubbly Flow under Normal and Microgravity Conditions using Stereo Image-Processing Method.

Abstract: One interesting phenomenon in a vertical bubbly flow is that bubbles sliding near a wall can exit under certain flow conditions. It has been reported that the lift force pushing the bubbles into the wall is generated by a bubble moving with slip velocity. We present an experimental study of bubble configuration in a gas-water bubbly flow under microgravity conditions carried out using on underground drop tower at the Japan Microgravity Center. The purpose of the experiment is to clarify the effect of reduced slip velocity between the gas and liquid phases on bubble configuration when the conditions vary from normal to microgravity. The bubble configuration, that is the diameter and position of the bubbles in the tube cross section, has been measured in detail using a stereo imageprocessing method (SIM). The results showed that the void fraction profile changes from a saddle shape that has a local void peak near the wall in sliding bubbly flow under normal gravity conditions before the drop, to a parabolic shape or a power-law shape that is approximated by a power-law function in core bubbly flow under microgravity conditions. The void fraction profile can be explained by simulation analysis.

Fluid Mixing and Local Mass Transfer Characteristics in a Grooved Channel for Self-Sustained Oscillatory Flow.

Abstract: Flow patterns and mass transfer rates in a periodically grooved channel were studied in the transitional flow regime. Self-sustained flow oscillation occurs at a low Reynolds number. Primary flow instability arises from Tollmien-Schlichting waves triggered by a shear layer above the groove, and thus there is fluid exchange between channel and groove parts through the shear layer. It is found that a further increase of the Reynolds number produces secondary instability causing a three dimensional flow at the bottom of the groove. Mass transfer was performed by the electrochemical method. The transport rate at the rib increases significantly after the primary instability, but the increment of mass transfer at the bottom of the groove is small. The secondary instability leads to marked transport enhancement at the bottom of the groove.

Quantitative Analysis of Convective Flow of Nitrogen Gas and Air under Magnetic Field Gradient.

Abstract: Recently, magnetic field gradients have been found to induce gas flow and promote combustion in diffusion flames. For example, when N2 gas stream was injected into air toward a weaker magnetic field, it was accelerated and behaved like a jet stream. This phenomenon was explained qualitatively by a magnetic attractive force acting on paramagnetic oxygen gas. In this study, such a magnetically induced jet stream of N2 gas was quantitatively analyzed. First, the magnetic body force acting on the gas flow and the governing equation of magnetic gas dynamics are presented. Using them, a numerical simulation was carried out for N2-Air convective flow under a magnetic field gradient. Calculated results show that the jet flow is accelerated to 80 cm/sec when the initial N2 gas velocity is 3.9 cm/sec and maximum magnetic induction is 1.5 T. Furthermore, calculation shows the existence of a convective air flow around the N2 jet flow. The results of the present study show good agreement with the experimental results.

Numerical Prediction of Performance of Annular-Type Jet Pump.

Abstract: Jet pumps are used in a great number of engineering applications. However, no numerical method for determining the performance has been established yet. In the present study, focusing on turbulence models, a numerical method for predicting the performance of an annular-type jet pump is investigated. The flow fields are simulated by solving Reynolds and continuity equations with standard k-e and Reynolds stress turbulence models at different Reynolds numbers. Performance parameters (i. e. flow ratio, head ratio, and efficiency) are computed using the calculated flow fields, and compared with the corresponding experimental data. It is shown that a recirculation in the mixing chamber is formed at relatively low Reynolds number, and that a standard k-e model always underestimates the head ratio and the efficiency because of the overestimation of turbulent mixing, whereas a Reynolds stress model can satisfactorily predict them.

ノズル噴孔内のキャビテーションが液体噴流の微粒化に及ぼす影響〔熱工学, 内燃機関, 動力など〕

Abstract: The purpose of this investigation is to clarify the atomization mechanism of a high speed liquid jet which issued from a single hole nozzle. The breakup process of the liquid jet is thought to be greatly affected by the behavior of the internal flow in the nozzle hole. In order to investigate this, visualization of the flow in the nozzle hole, observation of the atomization of the liquid jet and measurement of the static pressure in the nozzle hole were performed under atmospheric conditions using transparent nozzles with various length-to-diameter ratios of holes, L/D. A non-dimensional pressure coefficient in the nozzle hole was defined by Cp=(pc-pv)/(pa-pc), where pc is the static pressure at 1mm from the inlet of the nozzle hole, pv is vapor pressure of water at 293 K and pa is ambient pressure. The behavior of the internal flow in the nozzle hole where cavitation occurs was arranged by using cp. It was clarified that the primary factor in the atomization of the liquid jet is the disturbance of the liquid flow resulting from cavitation phenomena.

Simultaneous Reduction of NOx and Soot in Diesel Engines Using a New Mixture Preparation Method.

Abstract: We describe an experimental study on the simultaneous reduction of NO x and soot in diesel engines achieved using a new mixture preparation method. To produce a lean uniform mixture, a new type of injection equipment was used with various injection timings. Between the late and very early injection timings, there was an injection timing range at which the amount of NO x produced was extremely low and soot was not emitted. NO x production was suppressed to a level one hundredth that of conventional DI diesel engines. However, with increasing load, NO x production increased sharply and strong diesel knocking occurred. This indicated that NO x production was strongly influenced by the process of mixture formation.

A Molecular Dynamics of the Formation Process of Fullerene

Abstract: The formation mechanism of fullerene, a new type of carbon molecule with hollow caged structure, was studied using the molecular dynamics method. The classical potential developed by Brenner was employed with the simplification. The clustering process starting from isolated carbon atoms was simulated under the constant internal temperature control. Considering that the time scale of the simulation was compressed, the equilibrium of internal and translational temperature was artificially enforced. Typical result of the simulation is shown in Fig. 1. Here, 550 carbon atoms were located at random positions with the random velocity as the initial condition. Then, the total translational energy and total internal energy of each cluster were kept at 3000 K. Until 25 ps, there were only small chains up to about C10. At 60 ps there appeared some multi-cyclic rings of about C20. Then, at 125 ps, three dimensional pieces were formed. Finally at 200 ps there was a large fullerene like caged structure and some flat graphitic structures. The effect of the temperature on the finally obtained structure is complied in Fig. 2. The structures of obtained clusters depended on the control temperature, yielding to graphitic sheet for T 3500. The clustering process leading to the caged structure in contrast to the flat graphitic structure was studied in detail. In the clustering process yielding to the caged structure, the moderate sized precursors with large vibrational energy transformed to the random 3-dimensional structure. On the other hand, in the clustering process yielding to the flat structure, the precursors with the lower vibrational energy always kept the graphitic structure. The possibility of obtaining considerable amount of C60 will be discussed.

選択拡散効果を考慮した乱流燃焼速度の整理〔熱工学, 内燃機関, 動力など〕

Abstract: In our previous work, we found that the preferential diffusion in a turbulent flame played an important role in its turbulent combustion characteristics, and estimated the local burning velocity in premixed turbulent combustion experimentally, taking account of the preferential diffusion effect. In this study, a model which takes the preferential diffusion effect into consideration, is proposed to predict the premixed turbulent burning velocity, using the local burning velocity as a reference instead of the original laminar burning velocity. The model can be explained as follows. The turbulence affects the turbulent burning velocity by increasing the flame surface area and stretching the flame. Consequently, the turbulent burning velocity and quenching limit are determined by the balancing of both effects. The predicted velocities are compared with the measured turbulent burning velocities such that the fuel, equivalence ratio and laminar burning velocity were varied extensively. As a result, quantitative accuracy of this simple model is confirmed.

氷層の温度・濃度複合融解 : 第2報, 対流の可視化観察による数値予測の進展〔熱工学, 内燃機関, 動力など〕

Abstract: This paper is concerned with the melting of a vertical ice plate under the condition of an initially no temperature difference between the ice and a calcium chloride aqueous solution, in which the melting rate can not be analyzed using the classical Stefan problem where in the temperature gradient at the melting front is controlled The numerical results with the concept of a diffusion controlled melting revealed a stable concentration stratification above a layer strongly convecting, uncontaminated liquid clearly observed in flow visualization, and also successfully predicted the melting rate quantitatively having not yet considered previously

Study on NOx formation Characteristics of Medium Btu Coal Gasified Fuel.

Abstract: Medium-Btu coal gasified fuel, the higher heating value of which is about 4.2 to 12.7 MJ/m3, is composed of CO and H2, as its main combustible components. Moreover, it contains small amounts of NH3 and CH4. NH3 will be converted to NOx in the combustion process. In this study, the basic combustion characteristics of the coal gasified fuel were examined by combustion experiments using a laboratory scale combustor. The main results are as folows : (1) The amount of thermal-NOx production increases rapidly as the amount of fuel calorific value of coal gas increases. (2) When the two stage combustion method is used, the optimum equivalence ratio of the primary combustion zone by which the NOx conversion rate is made minimum exists. This optimum equivalence ratio become higher when the fuel calorific value is high. (3) The NOx conversion rate rises as CH4 concentration increases. When the two stage combustion method is used, the optimum primary equivalence ratio which makes the NOx conversion rate minimum rises as CH4 concentration decreases.

Characteristics of Low-Speed Streaks in the Flow of Drag-Reducing Surfactant Solution

Abstract: The characteristics of low-speed streaks in the turbulent channel flow of a drag-reducing surfactant solution are studied by employing the hydrogen-bubble flow visualization method. The mean velocity profiles and the distributions of streamwise turbulence intensity are also obtained. It is shown that, at a large drag reduction rate DR, the profile of the streamwise turbulence intensity has a plateau region around its peak. In the drag-reducing flow, the mean spanwise streak spacing reaches minimum at some distance from the wall. In the drag-reducing flow of high-density(1000 ppm)surfactant solution, the low-speed streaks appear intermittently at the Reynolds number smaller than those for the 'hump' of the f-Re curve(f:friction factor). The mean spanwise streak spacing very near the wall increases as DR becomes large. It is speculated that the nondimensional wall-normal distance y+ of the center of streamwise vortices is larger for the flow of large DR compared with that for the Newtonian fluid flow.