To investigate the mechanism of drag reduction by addition of small elements in turbulent flows of Newtonian fluid, a direct numerical simulation (DNS) of turbulent channel flow has been carried out. Three types of element model (single sphere, rigid dumbbell (two beads are connected by a rigid rod), viscoelastic dumbbell (two heads are connected by a spring and a dashpot)) are used to know the essential function of the small element to reduce the drag in turbulent channel flow. The DNS results with dumbbell models show the drag reduction, while the drag increases with the single sphere model. The effects of parameters including spring constant, damping coefficient, length of the element on the flow features are investigated. In addition the mechanism of the stress deficit by the elements is discussed. Finally, it is shown the primary mechanism of drag reduction by a dumbbell element is to suppress the vortex stretching in the streamwise direction.

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In an attempt to simulate both large scale and high-intensity turbulence within a limited entry length, an innovative wind tunnel of “multi-fan type” is developed. The airflow is driven by an array of fans (9 columns×11 rows), each of which is independently controlled by a computer. Details of the wind tunnel were introduced, and two typical methods (“uniformly-active” and “quasi-grid” methods) were applied to drive the wind tunnel. Comparison with the quasi-grid method revealed the spatial structure of the turbulent flow generated by the uniformly-active method.

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Instability of near wall low-speed streaks is studied experimentally by artificially generating spanwise periodic low speed streaks in a boundary layer subjected to high-intensity turbulent fluctuations. Boundary layer suction is applied to a turbulent boundary layer through a short strip of perforated wall to laminarize the near-wall flow. Then, turbulent eddies convecting from upstream trigger the instability of artificially-generated low-speed streaks downstream of the suction area. NV and hot wire data demonstrate that the development of sinuous instability modes is dominant in the near wall turbulence though the linear instability process is little observed owing to high intensity disturbances.

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Flow structures around hypersonic TSTO launch vehicle were investigated by utilizing the electrical discharge method. First, boundary layers near the orbiter surface were observed by a visualization technique of boundary layers. Subsequently, lateral and cross-sectional shock shapes around TSTO launch vehicle were observed by a visualization technique of shock waves. From these results, it was confirmed that a separation shock wave was generated near the orbiter surface. In this study, the visualizations were carried out at Mach 10.

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The flow characteristics of an air-water two-phase jet and an evaporating steam water two-phase jet were investigated experimentally. The air-water two-phase jet has larger radius and denser droplets region near its center with the water-air flow ratio, and has similarity in velocity distributions. The evaporating steam-water two-phase jet has four types of flow patterns depending on the quality at the nozzle exit. The jet radiuses are larger and the axial decreasing rates of dynamic pressure in the center of jet are smaller in order of air single-phase jet, air-water two-phase jet, and steam-water two-phase jet.

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A mesh generation method for an adaptive Cartesian mesh is developed based on the bottom up approach in which the smallest meshes near a solid surface are generated first, then the surrounding coarser meshes are generated. The Bresenham's method, one of the rendering techniques in computer graphics, is used to detect intersection between a mesh and a solid surface. With this method, the number of the meshes to be detected can be minimized. The generated meshes are organized with the tree data structure. Utilizing the hierarchical property of the tree data structure, the surrounding meshes can be generated with a simple algorithm. Efficiency of the mesh generation method is presented.

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Air bubbles were generated from twin orifices submerged in water. Bubbling features were observed using high-speed video and the micro convection induced by the generating bubble was detected at each orifice by a hot wire anemometer. It is found that bubbling pattern differs depending on the chamber volume as well as the air volumetric flow rate. When the chamber volume is small and the flow rate is relatively low, different patterns of “reciprocal” and “counterpart” bubbling appear repeatedly with the change of flow rate. Between the two patterns, “aperiodic” bubbling takes place. To reveal the mechanism of such interactive bubbling, a simplified dynamic model is proposed, which describes the time evolution of bubbles' volumes basing on the equations of motion for gas and liquid. The distribution of air flow rate to twin orifices is governed by the effects of surface tention and inertia of water flow around the growing twin bubbles. The model explains well the appearance of three bubbling patterns and their repetition.

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Shapes and rising velocities of single bubbles rising through vertical square ducts were measured under stagnant, laminar and turbulent flow conditions to examine the applicability of available empirical correlations for singe bubbles in vertical pipes to those in square ducts. Experiments were carried out by using three different ducts, the widths W of which were 13.0, 16.2 and 27.2mm. The ratio λ of sphere-volume equivalent bubble diameter d to W was varied from 0.2 to 2.0. As a result, it was confirmed that (1) empirical correlations for evaluating terminal velocities of a bubble in a pipe are applicable to a bubble in a square duct, provided that the characteristics length of duct is evaluated as (4A/π) ^0.5 where A is the flow area, and (2) bubble shapes in square ducts are well expressed as a function of the liquid Reynolds number and the Eötvös number.

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Air water two phase flow experiment was conducted to examine the basic flow characteristics of a centrifugal gas liquid separator. Vertical transparent test section, which is 4 m in height, was used to imitate the scale of a BWR separator. Flow rate conditions of gas and liquid were fixed at 0.1 m³/s and 0.033 m³/s, respectively. Radial distributions of two-phase flow characteristics, such as void fraction, gas velocity and bubble chord length, were measured by traversing dual optical void probes in the test section, horizontally. The flow in the standpipe reached to quasi-developed state within the height to diameter aspect ratio H/D=10, which in turn can mean the maximum value for an ideal height design of a standpipe. The liquid film in the barrel showed a maximum thickness at 0.5 to 1 m in height from the swirler exit, which was a common result for three different standpipe length conditions, qualitatively and quantitatively. The empirical database obtained in this study would contribute practically to the validation of numerical analyses for an actual separator in a plant, and would also be academically useful for further investigations of two-phase flow in large-diameter pipes.

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The flow field in a steam control valve is investigated by using the 2-D experiment and CFD to clarify the causes of serious pressure fluctuations. The experiments are carried out by changing the pressure ratio and valve opening ratio. Five types of flow patterns are observed in the experiment. Two types of them are stable and one of them is asymmetric attached flow and other is symmetric free jet flow. Three types are unsteady with symmetric, asymmetric oscillations and their mixture. These types of flows could be simulated by a 2-CFD fairly well. From these results, the characteristics of each flow fields are clarified.

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A model for predicting the noise level of cross flow fans has been developed. This model uses the relative air-flow velocities on the outside of the fan's impeller. And it is based on two assumptions : (i) noise levels are in proportion to the relative velocities at the edge of the blade raised to the sixth power and (ii) the velocities on the outside of the fan's impeller have an especially close relation to the noise level. Computational fluid dynamics is used to obtain the necessary velocity field data for this model. It was found that the noise levels calculated by this model agree well with the measured noise levels of a test impeller. This agreement confirms that noise level of a conventional cross-flow fan can be predicted by this simple model, since relative velocity data is readily available from frequently executed computational analyses. Accordingly, the predicted noise level was then applied in the design of a new cross-flow fan. This new cross flow fan has a velocity field that produces a quiet flow. As a result, it has a lower noise level and higher performance.

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The fluid column separation occurring in the pressure control line of Anti lock Braking System (ABS) of vehicles was analyzed experimentally. As the experimental setup, the ABS for passenger cars installed on the test stand was used. The pressure fluctuation was measured with a pressure sensor installed on the pressure control line. The tube of the connection part of the ABS hydraulic unit and the pressure control line was changed to a transparent pipe, and the vapor column was observed. The fluid column separation occurred when the pressure control valve in the hydraulic unit was operated to increase braking pressure. The mechanism of occurrence of fluid column separation in ABS system and the influence that the time of opening the pressure control valve exerted to pressure fluctuation were analyzed in this paper.

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The performance of turbine blades under a medium vacuum was investigated using the Stokes equation in a previous paper, where we could predict the performance of the turbo molecular pump in the pressure range of under 10 Pa. For further improvement of the pumping performance of turbine blades under a medium vacuum, experiments were carried out with a pump which has a narrow clearance between the rotor and the stator blade compared with the conventional pump. As a result, we found that the narrow clearance pump had a high performance under a medium vacuum compared with the conventional pump. Therefore, we analyzed the flow field in the turbo molecular pump by the two dimensional Finite Element Method (FEM). In order to examine the influence of the inter- stage clearance, the model was changed from the single stage of turbine blade to the combination of stator blade rotor blade stator blade. The comparison between the measurements and the predicted values shows a good agreement under 10 Pa.

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The effect of blade tip geometry on unsteady cavitation was investigated in a 3-bladed inducer. Experiments in inducers with and without the blade tip rounding on the pressure side were carried out and the results were compared. Through the measurement of inlet pressure fluctuation, it is found that unsteady cavitation is suppressed in the inducer with rounded tip compared with that with flat tip at relatively high flow rates. Backflow in each inducer was also investigated for various cavitation numbers because backflow may be also affected by tip geometry. It is clarified that backflow in the inducer with rounded tip is a little greater than that with flat tip and the backflow becomes weaker with the decrease of cavitation number.

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This paper deals with power up method of a cross flow type wind turbine having excellent safety. The wind turbine is set vertically within a pair of guide vanes whose horizontal cross sections are 1/4 cylindrical. On wind tunnel tests, some setting condition of guide vanes led to higher power coefficient of the wind turbine than without guide vanes. When this system was set to optimum direction for wind direction, the output of the wind turbine was about 2 times of the one without guide vanes. Mechanism of performance improvement due to two guide vanes was to be apparent by two kind of experiments ; investigating torque of one blade within 12-blades of the rotating turbine and flow around the turbine with/without guide vanes. Furthermore, effect of the guide vanes was confirmed by the field test.

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We are under obligation to coexist with natural ecosystem, for the next leap in the hydroelectric power development. In such circumstances, this paper proposes a new type hydraulic turbine, named “Gryo-Type Hydraulic Turbine”, accompanying with the principle of the rotor works and the model test results. The rotor is composed of some blades with high aspect ratio and is submerged nearly parallel to the stream. The turbine is suitable for shallow streams with high velocity in mountain torrents and/or rivers. The rotor has the self-motive power and is driven by the drag force of the blade facing to the downstream at the low rotational speed. With the increase of the rotational speed, the rotational torque comes to be caused successfully by the lift forces of the blade not only facing to the upstream but also facing to the downstream. The maximum output is at higher rotational speed, which is desirable as the turbine performance for the mini/micro hydroelectric power generation.

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The present report investigates the heat transfer in the high temperature region of impinging dilute mist flow, where no liquid films are formed by coalescence of droplets on the surface. First, the upper limit of the dilute mist flow is analyzed. In addition, heat transfer characteristics in the high temperature region are investigated experimentally, focusing on effects of sensible heat of droplets. In these experiments, the subcooling of droplets was varied from 58 K to 79 K. The experimental results were compared with a heat transfer model that took into account the liquid sensible heat. The heat transfer model based on liquid sensible heat could predict the distribution of heat transfer characteristics in the high temperature region of mist cooling when a liquid film is not formed.

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A two-phase flow thermal control system, which utilizes the latent heat of fluid, is paid attention to for the heat removal in the space station and the Space Solar Power System (SSPS). The characteristics are that the system weight is lower than that in single-phase flow and that the temperature can be controlled by changing the pressure of the accumulator. An active thermal control system which can give a maximum 1 kW heat load to an evaporator was built and an experiment on the dynamic characteristics under 1 G were carried out in the extensive quality range with a partial heat load, using alternative freon HCFC-123 as the working fluid. As a result, the effects of the operating quantities such as the heat input and the pressure in the accumulator on the dynamic response are clarified. It is shown that the effect of the operating condition on the time constant of the evaporator is correlated with the quality.

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An experiment on pool boiling in methanol was performed for a case in which the boiling space was controlled by an interference plate with many holes. The narrow space, 0.12 mm in thickness, between the heat transfer surface and the interference plate was hermetically sealed at the perimeter. Therefore, the vapor and liquid were only exchanged through the holes in the interference plate. Degree of super heat at onset of boiling was 0.7 K without overshoot at 10 mm in plate thickness, 1 mm in diameter of holes and 3.85 mm in pitch of holes. In this case of interference parameter, the critical heat flux was obtained the same value without interference plate. Interference plate was disturbed free convection and superheat layer was provided under small heat flux on the heat transfer surface. And the critical bubble diameter for the onset of boiling was decreased as the temperature of the superheat layer was increased. For this reason, the degree of superheat of onset of boiling was decreased.

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The observation and the measurement of the steam condensation phenomena on the wetted outer surface of the thin tubes arranged vertically were carried out. It was clarified that the condensate modes were affected by the surface tension of the condensate and the wavy shape of the condensate surface appeared due to the surface tension instability for thin tubes. That is, while the smooth surfaces of condensate with uniform thickness were observed for the tube of 1 mm in outer diameter, the wavy shape with non-uniform thickness appeared for 0.5 and 0.4 mm in diameter. The wavy shape became remarkable with decrease of the tube diameter. Although the wave lengths of the instability under low condensation rate almost coincided with the analysis of Rayleigh for free liquid jet, they increase with increases of the condensation rate and the axial length from the top.

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Recently, studies have begun on a functional heat transfer medium using fine spherical microcapsules encapsulated with phase-change material and dispersed in water. This enables the medium to maintain fluidity whether the phase-change material is solid or liquid. The present study has clarified the latent-heat storage characteristics of microcapsule slurry by increasing its heat transfer coefficient with the help of evolved heat condensing of vapor from the slurry in heat storage. Paraffin wax with the melting point of 62°C was encapsulated into the fine microcapsules and used as a phase-change material. The cooling coil surface temperature and concentration of paraffin in the microcapsule slurry were selected as the experimental parameters. As a result, the non-dimensional correlation equations of the heat release completion time and heat transfer coefficient were derived in terms of non-dimensional parameters.

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In this study, surfaces waves characteristics of fully-developed wind-driven turbulent flow, the maximum wind speed 2 m/s, were investigated used DNS data by means of MARS method for a coupled gas-liquid turbulent flow. As the results, large horizontal “scales” shaped wind-driven water surfaces consists of the low wave-number surface fluctuations, of which the spectra follow the -4th power law. High wave number surface fluctuations were inactive in surface fluctuations or fluid velocity turbulent intensity behaviors near air-liquid interface. Dynamic processes compared with the coherent turbulent flow structures over fixed wavy wall, were observed, and were caused from the control factors with the phase difference, wind, surface waves and water velocity, in the air-liquid interface interaction.

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This paper shows the effects of clearance length between a body and a duct wall, and duct height on the heat transfer characteristics and flow behavior at downstream region of the body when a blunt body was set in a parallel plate duct with some distance separating it from the duct wall as a turbulence promoter. At the ratio of clearance length to body height, C/D= 0.05-0.1, the heat transfer was characterized by the reattachment of shear flow separated from the body. Furthermore, the heat transfer depended on both the reattachment flow and the separation vortex at C/D= 0.15-0.2, and the side vortex induced by Karman vortex at C/D = 0.25 and 0.275. The reattachment flow gives a superior effect to enhance heat transfer at low Reynolds number, but at larger Reynolds number, the side vortex induced by Karman vortex becomes more effective to heat transfer enhancement.

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Experiments have been performed for a turbulent channel flow over a backward-facing step. The backward-facing step is controlled by equipping a slit at the bottom corner of the step. Low momentum fluids in the recirculating region are sucked or high momentum fluids are injected from the slit. The width of the slit is changed between 2, 3 and 5 mm, and the flow ratio is varied from 0.00 to 0.15. The wall static pressure and local heat transfer coefficient are measured behind the backward-facing step. The wall shear stress is measured using the micro flow sensor. In addition, the velocity profiles and turbulent intensities are measured by a split hot film probe. It is found that the heat transfer and pressure drop characteristics are controlled by the flow ratio. When the suction flow ratio is 0.06, the highest performance is obtained. Enhancement of the heat transfer is related to the increase of turbulence intensity.

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Natural convective flows around upward-facing horizontal heated plate with a vertical plate at the edge were investigated experimentally. The widths of horizontal heated plates, W, and the heights of vertical plates, H, were varied systematically as W=100-350 mm and H=0-150 mm. The flow fields and the surface temperatures of the horizontal plates were visualized with dye and liquid crystal thermometry. The result shows that the vertical plate blocks incoming flow from the edge, and that the flow entering from the open edge covers the whole horizontal surface when the height of the vertical plate exceeds H/W =0.14 for the adiabatic vertical plate and H/W =0.1 for the heated vertical plate. The local heat-transfer-coefficients of the horizontal plate were also measured. The results show that the vertical adiabatic plate depresses the heat transfer while that the heated vertical plate enhances the heat transfer from the horizontal plates.

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Three dimensional linear stability analysis for onset of natural convection flow in a cubic cavity is performed by using the spectral element method. The cubic cavity is placed horizontally and is heated from the bottom surface and cooled from the upper one. The lateral boundaries are assumed to be rigid and in a thermally medium state between conducting and insulating which can be established by introducing Biot number. The fluid is in a motionless state if the Rayleigh number is small. On the other hand, natural convection occurs if the Rayleigh number exceed a critical value. The critical Rayleigh numbers for the onset of the natural convection are evaluated numerically for various Biot numbers by constituting eigenvalue problem using linear stability theory. In addition, the velocity and temperature field at the critical state are obtained and discussed.

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This paper describes an application example of thermal flow simulation to the design of a switch mode power supply (SMPS), that is natural convection air-cooled. As a simulation tool, a commercially available computational fluid dynamics (CFD) software was used and the modeling of configuration conditions was considered. In particular, the modeling of printed circuit board (PCB) and power semiconductor devices was examined by using the design of experiments method. As the heat generation sources, a field effect transistor (FET) and a diode were considered in the simulation, the calculation method of power loss is described. The difference between measured and calculated temperature values for power semiconductor devices was found to be within approximately 10 K.

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Measurements of Excitation-Emission Matrix (EEM) of polycyclic aromatic hydrocarbons (PAHs) in an atmospheric diffusion burner flame were conducted using a multi-wavelength excitation laser in order to demonstrate the potential of the EEM fluorometry for investigation of soot formation processes in flames. The PAHs in the flame were excited by a coherent multi-wavelength “rainbow” laser light generated by converting the 3rd harmonic (355 nm) of a pulsed Nd : YAG laser using a Raman cell frequency converter filled with hydrogen (0.6 MPa). The LIF from the PAHs in the flame was captured using a spectrometer and an ICCD camera as EEM images. The LIF due to 2-to 3-rings PAHs was observed around 300 to 400 nm when excited at 275 nm in the upstream region in the flame and the intensity of the LIF decreased in the downstream. The LIF due to 3-to 6-rings PAHs was observed at the emission wavelength of longer than 400 nm at all excitation wavelengths employed in the present study between 275 and 417 nm and the intensity of the LIF increased in the downstream region in the flame. It was shown that the scattered excitation laser light from soot particles on the EEM images, which severely interfered with the LIF, can effectively be reduced by applying a delay between the laser excitation and the ICCD gating.

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Crank angle resolved monitoring of formaldehyde using laser-induced fluorescence (LIF) has been conducted at homogeneous charge compression of dimethyl ether (DME) -air mixture in an externally motored single cylinder piston engine. In order to correct the raw LIF intensity profiles, which are significantly influenced by collisional quenching and pressure broadening, fluorescence lifetime and spectral linewidth of the observing transition were evaluated as a function of crank angle. It is clearly demonstrated that formaldehyde is promptly generated at the cool flame and disappears at the thermal flame in the two-stage autoignition, as expected by the simulation using the detailed DME oxidation mechanism of Curran et. al.

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A fiber optic system linked to the optical sensor installed in the spark plug, in which light can pass through the combustion chamber, was developed to determine the fuel concentration near the spark-plug successively using an infrared absorption method. A He-Ne laser with a wavelength of 3.392 pm that coincides with the absorption line of hydrocarbons was used as a light source. By exchanging an ordinary spark plug into this spark plug with the optical sensor, successive measurement of fuel concentration before the spark timing near the spark-plug was performed in a spark-ignition engine inducted with homogeneously mixed method methane-air. The fuel concentration was determined from law of Lambert Beer in consideration for the effects of pressure and temperature on molar absorption coefficient of methane.

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An infrared absorption method was used to determine the fuel concentration with a 3.392μm He-Ne laser. The effects of pressure and temperature on the molar absorption coefficients of hydrocarbon fuels were clarified, so that it increased with increasing temperature and decreased with increasing pressure. Molar absorption coefficient of multi component fuel like gasoline can be estimated using molar absorption coefficient of each component and considering mass balance. By exchanging an ordinary spark-plug for a spark plug with a developed sensor in which light can pass through mixture in the combustion chamber, successive measurement of fuel concentration was performed in a spark-ignition engine. The effects of liquid droplets, mechanical vibration, and other gas like H₂O on measurement accuracy were discussed.

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The structure and propagation properties of edge flames were investigated for counterflow strain controlled CH4/air diffusion flames. Numerical computations incorporating detailed chemical kinetics and multicomponent diffusion were performed focusing on effects of strain (stretch) rate on edge flame structure and burning velocity. These computations gave the following results. (1) When the stretch rate is low, the edge flame forms a hook-like shape where the structure corresponds to a premixed flame in the radial direction and a diffusion flame in the axial direction. The local burning velocity varies from point to point in the equivalence ratio. The maximum local burning velocity observed at the tip of the flame front is lower than that of the maximum burning velocity of one-dimensional premixed flame of the same fuel. (2) The local burning velocity is influenced not only by the local equivalence ratio but also by the main flame properties of the high-temperature burnt gas, where the different equivalence ratio prevails. (3) For a high stretch rate, the burning velocity is negative. The negative burning velocity means that the receding velocity of edge flame is higher than the radial fluid velocity component because of flame-edge quenching. The flame edge of the negative burning velocity is a sharp edge with equicontours of the heat-release rate.

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Monitoring of OH chemiluminescence through an optical fiber was demonstrated to be a useful method in detecting self-excited combustion oscillations. OH chemiluminescence intensity detected by the optical fiber showed mostly excellent agreement with those obtained by high speed CCD camera measurements when combustion oscillations were strong. Symptoms of self excited combustion oscillation were also studied in order to predict the onset of combustion oscillation before it proceeded to a catastrophic failure. For the purpose, we have found and proposed unique measures to tell the onset of self-excited combustion oscillations based on the careful statistics of fluctuating properties in flames, such as pressure or OH chemiluminescence.

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This paper describes the temperature changes with time and microexplosion phenomena of single droplets under specified furnace temperatures and droplet diameters for two kinds of Asphalt/Water emulsified fuels (As/W-I and As/W-J). It was observed the temperature histories with time of both fuels resembled closely each other, which was different from the experiments of thermal analysis (TG/DTA) of Ref. (6). However, the microexplosion of As/W J occurred earlier and more intensively than those of As/W-I like Ref. (6). A theoretical approach was carried out for the single droplets by applying the analytical method for thermal analysis of Ref. (6). It can explain the experimental temperature histories before the rapid vaporization and the occurrence times of the microexplosion of the droplets. The chemiluminescence signals of OH and CH were detected for As/ W-J, Pitch/Water emulsion (Pi/W) and C heavy oil. The OH signals of As/W J with the microexplosion fluctuated intensively at the occurrence time of the microexplosion, which was apparently different from Pi/W and C heavy oil without the microexplosion. On the other hand, the CH signals were in agreement with the luminosity of the flame and did not fluctuate intensively different from OH signals for all three fuels. Thus, the occurrence of the microexplosion can be assured by the difference between OH signal and CH signal.

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Biodiesel is alternative diesel fuel, and a mixture of fatty acid methyl esters with other impurities. The combustion characteristics of biodiesel may be influenced by the compositions of fatty acid methyl esters in the biodiesel. In order to understand the effects of the fatty acid methyl ester compositions on biodiesel combustion, the diesel combustion of the blended fuels of the oleic and palmitic acid methyl esters are investigated using a small DI diesel engine. As the results, ignition delay and NO_x emissions decrease with higher fraction of palmitic acid methyl ester in the blended fuels. It is considered that the ignitionability of palmitic acid methyl ester is better than that of oleic acid methyl ester because of the lower boiling point of palmitic acid methyl ester.

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Economic, energy saving and environmental protection feasibilities of solid oxide fuel cell (SOFC) and SOFC/microturbine (MT) hybrid cogeneration systems are investigated based on a multiobjective optimization approach. Equipment capacities and maximum contract demands of utilities such as electricity and natural gas are determined so as to minimize the annual total cost, primary energy consumption and CO₂ emission subject to satisfaction of all the energy demand requirements. The s constraint method is adopted as a solution one for the above multiobjective optimization problem. Numerical studies are carried out on the SOFC and SOFC/MT cogeneration systems installed in a hotel by changing the capital unit costs of SOFC and SOFC/MT cogeneration units. Through the studies, the trade off relationships between the economic, energy-saving and environmental protection properties are clarified, and it is found that the SOFC/MT hybrid cogeneration system is excellent from viewpoints of energy-saving and environmental protection properties.

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