In some aging geothermal steam turbines, the increased steam consumption is found out due to time deterioration of the turbine parts, mainly caused by erosion, corrosion damages or deposits of impurities on the steam paths. Furthermore, the heavy damage due to stress corrosion cracking or corrosion fatigue damage, etc. are observed on rotors, blades and other parts and components. On the other hand, in other units, the turbine output capacity decreases according to aging decrease of geothermal well pressure, that is, inlet steam pressure of turbine. Under these circumstances, upgrading and life extension are required for reliability and performance on geothermal steam turbines, particularly the existing ones. And as the effective utilization of geothermal energy is important from the viewpoint of decreasing carbon dioxide on environment problem, these technologies can, needless to say, be applied to new geothermal projects as well as the existing ones. This paper describes development and application of advanced steam path design such as nozzle and blade for improving reliability and performance, and of advanced rotor design and material including overlay coating technology for improving reliability and extending life. And also it describes uprating of the existing units in opposition to aged decreasing in the inlet steam pressure.

View full abstract

A large and growing portion of electricity is produced by aging thermal power plants. Although excellent, high quality materials such as CrMoV steel and 12% Cr steel, etc. are used for the steam turbines, various forms of metallurgical degradation, due to creep and fatigue, etc. affect the parts and components during long-term operation at high temperature. Extending the life of steam turbines and ensuring high reliability requires life assessment technology, scheduled repairing, conversion, modification and upgrading of components in order to provide a stable power supply. As the high temperature parts and components of aged steam turbines are mainly metallurgically damaged by creep, fatigue and the interaction of both, life assessment combined with analytical and nondestructive methods is essential for realizing strategic plant life extension. We have developed a life assessment technology that takes material degradation into consideration, and have applied the procedure to more than 650 units and 2500 components since 1983. A rotor bore replication device was developed in 1989 for the purpose of nondestructive observation of creep voids and supporting the validity of life prediction results. This paper describes the technical features and applied experience of recent life assessment technology for existing high temperature steam turbines.

View full abstract

Electricity generation utilities are increasingly looking for cost-effective solutions to maximise the value of aging steam turbine generator plant assets. To this end, retrofits of steam turbines after many years of operation have been carried out for the purpose of life extension of units, performance improvements, capacity up-rating, availability improvement, and improved environmental compliance. Major steam turbine manufacturers have continued to push forward the development of advanced technologies to satisfy demand from utilities by provided retrofit design that optimise the above criteria. This paper describes the advanced technologies adopted in the recent retrofits, including advanced steam path design and new last stage blades of improved efficiency, improved reliability, and of simplified or no maintenance. Retrofit case-studies of capacity up-rating and life extension are introduced to illustrate how these technologies have been applied and what has been the gain.

View full abstract

The 3000rpm 48inch blade for steam turbine was developed as one of the new standard series of LP end blades. The new LP end blades are characterized by the ISB (Integral Shroud Blade) structure. In the ISB structure, blades are continuously coupled by blade untwist due to centrifugal force when the blades rotate at high speed. Therefore, the number of the resonant vibration modes can be reduced by virtue of the vibration characteristics of the circumferentially continuous blades, and the resonant stress can be decreased due to the additional friction damping generated at shrouds and stubs. In order to develop the 3000rpm 48inch blade, the latest analysis methods to predict the vibration characteristics of the ISB structure were applied, after confirming their validity to the blade design. Moreover, the verification tests such as rotational vibration tests and model turbine tests were carried out in the shop to confirm the reliability of the developed blade. As the final verification test, the field test of the actual steam turbine was carried out in the site during the trial operation, and the vibration stress of the 3000rpm 48inch blade was measured by use of telemetry system. In the field test, the vibratory stress of the blade was measured under various operating conditions for more than one month. This paper first presents the up-to-date design technology applied to the design of the 3000rpm 48inch blade. In the second place, the results of the various verification tests carried out in the shop are presented as well as their procedure. Lastly, the results of the final verification tests of 3000rpm 48inch blade carried out in the site are presented.

View full abstract

This paper deals with an application of computational fluid dynamics (CFD) to partial admission stages in a steam turbine. The calculation of partial admission stages requires unsteady analysis and full circle modeling. Therefore, quasi-3-dimensional (Q-3D) analysis of the mean radius is conducted to reduce computational load. First, an experiment using the air turbine is carried out. The result is in good agreement with the result of CFD analysis under the same conditions as the experiment, and the application of the Q-3D method to partial admission stage analysis is validated. Using this method, 2-stage analysis of partial admission is conducted. The influence of the circumferential position of the admitted arc on stage efficiency is discussed. The efficiency difference is related to the windage loss caused by pressure distribution in the circumferential direction. It is found that there is an optimum circumferential position of the admitted arc from the point of view of turbine efficiency.

View full abstract

Power plants with high thermal efficiency are essential and indispensable in order to decrease the impact on the environments. In order to achieve this goal, enhancement of the steam conditions is the most fundamental and effective measure. Recent steam conditions in Japan range from 593 to 610 degree C owing to the technological development. There are many areas of technology for the realization of such steam conditions, for instance, material development, cooling design, steam path development, casing design, and so on. Not only the research and development but also accumulation of the operational results is of importance to achieve a breakthrough in turbine design. In this paper, recent development of steam turbines with high temperatures will be presented focusing on their design features including material selections. This paper also deals with further efforts targeting even higher steam conditions, which are promising for future development of steam turbine technology.

View full abstract

We developed a dual-fuel single can combustor for the Niigata Gas Turbine (NGT2BC), which was developed as a continuous-duty gas turbine capable of burning both kerosene and digester gas. The output of the NGT2BC is 920kW for continuous use with digester gas and 1375kW for emergency use with liquid fuel. Digester gas, obtained from sludge processing at sewage treatment plants, is a biomass energy resource whose use reduces CO₂ emissions and take advantage of an otherwise wasted energy source. Design features for good combustion with digester gas include optimized the good matching of gas injection and swirl air and reduced reference velocity. The optimal combination of these parameters was determined through CFD analysis and atmospheric rig testing.

View full abstract

A new optical torque measurement method was applied to diagnosis of thermal efficiency of advanced combined cycle, i.e. ACC, plants. Since the ACC power plant comprises a steam turbine and a gas turbine and both of them are connected to the same generator, it is difficult to identify which turbine in the plant deteriorates the performance when the plant efficiency is reduced. The sensor measures axial distortion caused by power transmission by use of He-Ne laser beams, small stainless steel reflectors having bar-code patterns, and a technique of signal processing featuring high frequency. The sensor was applied to the ACC plants of TOKYO ELECTRIC POWER COMPANY, TEPCO, following the success in the application to the early combined cycle plants of TEPCO. The sensor performance was inspected over a year. After an improvement related to the signal process, it is considered that the sensor performance has reached a practical use level.

View full abstract

As energy consumption is rapidly increasing in the commercial sector in Japan, the market potential for a micro-gas turbine is expected to grow significantly if thermal efficiency is improved further. One way of improving thermal efficiency is to introduce a steam injection system that uses steam from the heat recovery steam generator. We have recently carried out several tests using a micro-gas turbine (Capstone C60). Test results show that this new device utilizing steam injection can improve some key performance parameters for output, thermal efficiency and emissions. The stable operation of the micro-gas turbine with steam injection was confirmed under various operating conditions. On the basis of the above findings, we hereby propose the use of a micro-gas-turbine-based cogeneration package with steam injection driven by a heat recovery steam generator (HRSG) with supplementary firing.

View full abstract

This study focuses on the fundamental characteristics of DME (Dimethyl Ether) combustion, aiming at the development of low-NOx multi-port burner suitable for the tube-nested combustor. In the tube-nested combustion, the water tubes are moved into the furnace closely to the burner to cool the burning flame directly in the field of burning reaction leading to NOx reduction. To prevent the unburned combustibles emission, the diffusion burner used in the tube-nested combustor needs a high performance of the fuel-air mixing. Multi-port burner consists of a fuel-port and surrounding multi-air-ports, which induce a strong re-circulation flow. Thus the fuel-air mixing is enhanced so that the thermal NOx and CO emissions are significantly reduced. The NOx emission of the DME from the co-axial diffusion burner was over 130ppm at 0% O₂. On the other hand, NOx emission of DME from the multi-port burner was reduced to 60ppm at 0% O₂. With the help of the tube-nested combustion, NOx emission of DME was further reduced to 20ppm at 0% O₂.

View full abstract

Increasing attention has been given to the development of low-NOx combustion technology for DME (Dimethyl Ether). The present paper describes the R&D study for water-tube boiler carried out in Kansai University and Hirakawa Guidam Co., Ltd. under the support of DME project from METI. The major problem in DME use is the difficulty in the application of premixed flame due to its low ignition temperature and rather high burning velocity. However, the previously developed tube-nested combustor, i.e. water-tubes installed in the empty furnace, becomes effective means together with the flue-gas recirculation to overcome such difficulty in achieving low-NOx combustion. This paper begins with a brief review of the R&D study of the tube-nested combustor specifically designed for city gas. Then the further development for DME-fired water-tube boiler is described.

View full abstract

Flame properties in the ambience of steam and carbon dioxide are complicated because of the third-body effect and radiation reabsorption. Thus, we performed detailed chemical kinetic calculations that include the effect of radiation reabsorption to clarify the premixed laminar flame speed of such a mixture, being one of the most important properties for controlling combustion. Pressure was varied up to 5.0MPa to simulate the 1700°C class combined gas turbine system. The results show a marked increase in laminar burning velocity by considering radiation reabsorption. Laminar burning velocity was increased up to 150% in cases of methane-oxygen and steam or carbon dioxide mixtures. It was found that the preheating of the upstream, unburned mixture caused this increase. The influence of radiation reabsorption was much larger in the case of lower pressures.

View full abstract

This paper presents a method for determining coal ash emissivity in high temperature atmospheres. We applied a tube-dropping mechanism to suppress reflection from the sample, and tried other new approaches for measurements at high temperatures. One approach was a shielding time reduction using tube motorization, which reduces measurement errors to a negligible level. Another approach was determining emissivity by fitting a calculation curve to transient experimental data. In these calculations, adjustable parameters were emissivity and thermal conductivity. An ash sample was heated in an electric furnace in the range from 500 to 1300°C and the radiation intensity from the sample was measured with a digital pyrometer. Each measurement was carried out within 0.3 seconds, including the time required for shielding the sample (0.1 seconds). Once the tube had been dropped into the furnace, radiation intensities from the sample began decreasing. Emissivity characteristics were compared between Powder River Basin (PRB) coal ash and bituminous coal ash. It was found that coal-ash emissivity depends on coal types and changes significantly as a function of ash surface temperature.

View full abstract

Deposition rate of droplets in annular flow was measured to elucidate the effect of the shape of flow obstacles that were placed in a flow channel. In the present experiments, the test section was a vertical round tube of 5mm in inside diameter, air and water were used as test fluids, and double film extraction technique was adopted for the measurement. Seven flow obstacles were tested; three obstacles were tubular in shape while other four obstacles were also cylindrical but the cross-sectional areas were varied smoothly in the axial direction. It was revealed that the deposition rate of droplets markedly increased if the present small flow obstacle was placed in the test section tube. The increasing rate was dependent primarily upon the cross-sectional shape of flow obstacle at the maximum cross-sectional area.

View full abstract

In order to increase void fraction and pressure drop data in a multi-subchannel system like an actual fuel rod bundle, air-water experiments have been conducted using a vertical 2 × 3 rod bundle channel made up of two central and four side subchannels as the test channel. Void fraction and pressure drop in each subchannel were measured and the frictional pressure drop was determined mainly for slug and churn flows. The results show that both the void fraction and the frictional pressure drop are higher in the central subchannel than the side one. In order to analyze the data, the data on gas and liquid flow rates in each subchannel under the same flow condition have been used. In the analysis, the calculations by various correlations reported in literatures have been compared with the present data for validation. The recommended correlations respectively for the void fraction and the frictional pressure drop have been clarified. Results of such experiments and analyses are presented and discussed in this paper.

View full abstract

In order to obtain the data on turbulent mixing rate between triangle tight lattice subchannels, which will be adopted as the next generation BWR fuel rod bundle, adiabatic experiments were conducted for single- and two-phase flows under hydrodynamic equilibrium flow conditions. The gas and liquid mixing rates measured for two-phase flows were found to be affected by the void fraction and/or flow regime, as reported in our previous study on a simulated square lattice rod bundle channel having hydraulic diameters of about four times larger than the present tight lattice channel. Comparing the present mixing rate data with those for the square lattice channel and a triangle one in other institution, we found that the mixing rate was considerably smaller in the present channel than the other ones, i.e., a channel size effect.

View full abstract

The flow-induced vibration of a circular cylinder is simulated numerically. The finite element method is used to solve the three-dimensional incompressible Navier-Stokes equations, and the flow field is coupled with the cylinder motion. The oscillation of the cylinder in the in-line direction is shown when the reduced velocity, U_r, is smaller than 4.0. Symmetric vortices are seen for U_r≤2.5 and alternative vortices appear for U_r≥2.9, while the cylinder motion is damped at around U_r=2.6. It is found that the cross-flow oscillation of the cylinder is dominant under the alternative vortex condition. The drag and lift coefficients increase and the vortex field become finer both in space and time by the cross-flow oscillation.

View full abstract

A new thermo-chemical and electrolytic hybrid hydrogen production system in lower temperature range is newly proposed by the Japan Nuclear Cycle Development Institute (JAEA) to realize the hydrogen production from water by using the heat generation of sodium cooled Fast Breeder Reactor (FBR). The system is based on sulfuric acid (H₂SO₄) synthesis and decomposition process developed earlier (Westinghouse process), and sulfur trioxide (SO₃) decomposition process is facilitated by electrolysis with ionic oxygen conductive solid electrolyte to reduce the operation temperature 200-300°C lower than Westinghouse process. SO₃ decomposition with the voltage lower than 0.5V was confirmed in the temperature range of 500 to 600°C and theoretical thermal efficiency of the system evaluated based on chemical reactions was within the range of 35% to 55% under the influence of H₂SO₄ concentration and heat recovery. Furthermore, hydrogen production experiments to substantiate the whole process were performed. Stable hydrogen and oxygen production were observed in the experiments, and maximum duration of the experiments was about 5 hours.

View full abstract

The subcooled flow boiling critical heat flux (CHF) for the flow velocities (u=4.0 to 13.3m/s), the inlet subcoolings (ΔT_{sub, in}=130 to 161K), the inlet pressure (P_in=812 to 1315kPa), the dissolved oxygen concentration (O₂=5.88 and 7.34ppm) and the increasing heat input (Q₀exp(t/τ), τ=38.1ms to 8.3s) are systematically measured by the experimental water loop installed the pressurizer. The SUS304 tube of test tube inner diameter (d=6mm), heated length (L=60mm), L/d=10 and wall thickness (δ=0.5mm) with the rough finished inner surface (Surface roughness, Ra=3.18µm) is used in this work. The CHF data for high heating rate were compared with the quasi steady state ones previously obtained and the values calculated by the steady state CHF correlations against outlet and inlet subcoolings. Transient CHF correlation against inlet subcooling has been given based on the experimental data for wide exponentially increasing heat input (Q₀exp(t/τ), τ=38.1ms to 8.3s). The influence of heating rate on CHF was investigated into details and the dominant mechanism of subcooled flow boiling critical heat flux for high heating rate was discussed.

View full abstract

Boiling CHF was assumed to happen based on hydrodynamic instability, and the value was thought to be unaffected by the surface condition of heater so far. However, it has been understood that the CHF value changes systematically depending on experimental conditions according to a series of experiment of the authors. As the reason, it is assumed that CHF occurs based on heterogeneous spontaneous nucleation (HSN). Especially, it appears remarkably in transient boil CHF. Moreover, it is thought that HSN physically influences the surface condition. In this study, the transient pool boiling CHF for exponential heat generation rates with various periods for saturated and subcooled liquids at various pressures were investigated to clarify the effects of surface conditions such as commercially-available and roughly-finished surface cylinders in water, ethanol and FC-72. It was clarified that the trend of CHF for the shorter periods was significantly affected by the surface conditions.

View full abstract

Forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input (Q₀exp(t/τ)) to a horizontal cylinder (heater) was theoretically and experimentally studied. In the theoretical study, transient heat transfer was numerically solved based on a turbulent flow model. It was clarified that the surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. The values of numerical solution for surface temperature and heat flux agree well with the experimental data for the cylinder diameter of 1mm. In the experimental studies, the authors measured heat flux, surface temperature, and transient heat transfer coefficients for forced convection flow of helium gas over horizontal cylinders under wide experimental conditions. The platinum cylinders with diameters of 1.0mm, 0.7mm, and 2.0mm were used as test heaters. The gas flow velocities ranged from 2 to 10m/s, and the periods ranged from 50ms to 15s. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period τ longer than about 1s, and it becomes higher for the period shorter than around 1s. The heat transfer shifts to the quasi-steady-state heat transfer for longer periods and shifts to the transient heat transfer for shorter periods. The transient heat transfer coefficients show significant dependence on cylinder diameters, there are higher for smaller cylinder diameters. The empirical correlations for quasi-steady-state heat transfer and transient heat transfer were obtained based on the experimental data.

View full abstract

Recently, the reduced moderation spectrum BWR has been studied. The fast neutron spectrum is obtained through triangular tight lattice fuel. However, there are few thermal hydraulic test data and thermal hydraulic correlation applicable to critical power prediction in such a tight lattice bundle. This study aims to enhance the database of the thermal hydraulic performance of the tight lattice bundle whose rod gap is about 1mm. Therefore, thermal hydraulic performance measurement tests of tight lattice bundles for the critical power, the pressure drop and the counter current flow limiting were performed. Moreover, the correlations to evaluate the thermal-hydraulic performance of the tight lattice bundle were developed.

View full abstract

The correlation of liquid-solid contact fraction in transition boiling was derived by focusing on the dimensionless wall temperature θ=(T_w-T_CHF)/(T_MHF-T_CHF). The following correlation was obtained from experimental results for water, R-113 and LN₂: Γ=1.000-0.9120θ-0.1343θ², where q_tb=q_CHFΓ+q_MHF(1-Γ). In the present model, considering a pseudo-liquid solid contact right after the detachment of a bubble from a liquid-vapor interface, transient heat conduction was analyzed in it three-dimensionally. Liquid-solid contact time and area were determined by the present correlation of the fractions of liquid-solid contact: τ_contact=0.3Γ_t, A_wet=15 × 15 × Γ_a, Γ_t=Γ_a=Γ^0.5={1.000-0.9120θ-0.1343θ²}^0.5. The heat transfer during the wet period was estimated using q_wet=f(ΔT_sat^0.8). The prediction by the present model was in agreement with the present experimental data for water. Furthermore, a simulation of the rewetting was performed; the experimental results of cooling curves during the rewetting were well reproduced by the transient heat conduction model using the present correlation of transition boiling heat transfer and the present liquid-solid contact model.

View full abstract

Eight hundred MWe class PWR turbine-generators operated smoothly and continuously at sudden load reduction from full load to house load, maintaining NPSH of the feedwater booster pumps through transition even though the available NPSH never exceeded zero. NPSH of the pump at 151.3°C, at which the available NPSH was minimal during transition, was evaluated as a slightly negative value, in spite of a positive value at room temperature, applying both the Ruggeri-Moore method extended to the negative area on condition of gas venting out of the system, using data of both room temperature and 95°C of a model pump facility, and another restriction by gas presence at the impeller inlet. The above evaluated high temperature NPSH demonstrated successful operation of the units equipped with a low static suction head yielding zero available NPSH during transition, resulting in alteration of the design criteria of the feedwater system and thereby contributing to possible cost reduction.

View full abstract

Since it is generally difficult to predict the occurrence of natural disasters such as earthquakes, a performance management system that constantly maintains the safety and functionality of structures is required, particularly for critical structures like nuclear power plants. In order to realize such a system, it is becoming important to carry out detailed modeling procedures and analyses to better understand actual phenomena. The aim of our research is to determine the dynamic behavior─especially the wave propagation phenomena─of piping systems in nuclear power plants, which are complicated assemblages of parts. The spectral element method is adopted in this study, and the formulation considering a shear deformation independently for a frame element is described. The Timoshenko beam theory is introduced for the purpose of this formulation. The validity of the presented element will be shown through comparisons with the conventional beam element.

View full abstract

A steam injector (SI) is a simple, compact and passive pump and also acts as a high-performance direct-contact compact heater to heat up feedwater by using extracted steam from the turbine. To develop high performance compact feedwater heater, it is necessary to quantify the characteristics between physical properties of the flow field. Its performance depends on the phenomena of steam condensation onto the water jet surface and heat transfer in the water jet due to turbulence on to the phase-interface. The analysis was conducted by using CFD code embedded separate two-phase flow models that were confirmed by the experimental data. As the four-stage SI is compact heater, the system is expected to bring about great simplification and materials-saving effects, and high reliability of its operation. Therefore, it is confirmed that the simplification of the power plant by replacing all low-pressure feedwater heaters with the four-stage SI system, having steam extraction pressures equal to those for the existing ABWR system.

View full abstract

Post-dryout heat transfer analysis was carried out considering droplet behavior by using the Lagrangian simulation method. Post-dryout heat transfer is an important heat transfer mechanism in many industrial appliances. Especially in recent Japanese BWR licensing, the standard for assessing the integrity of fuel that has experienced boiling transition is being examined. Although post-dryout heat transfer analysis is important when predicting wall temperature, it is difficult to accurately predict the heat transfer coefficient in the post-dryout regime because of the many heat transfer paths and non-equilibrium status between droplet and vapor. Recently, an analysis model that deals with many heat transfer paths including droplet direct contact heat transfer was developed and its results showed good agreement with experimental results. The model also showed that heat transfer by droplet could not be neglected in the low mass flux condition. However, the model deals with droplet deposition behavior by experimental droplet deposition correlation, so it cannot estimate the effect of droplet flow on turbulent flow field and heat transfer. Therefore, in this study we deal with many droplets separately by using the Lagrangian simulation method and hence estimate the effect of droplet flow on the turbulent flow field. We analyzed post-dryout experimental results and found that they correlated well with the analysis results.

View full abstract

Flow structures around a hypersonic re-entry Mars Environmental Survey (MESUR) capsule traveling at supersonic/hypersonic speeds are investigated utilizing a new computational method based on the finite element method (FEM). In order to confirm the validity of the computational method, the results are compared with experimental ones obtained by the electrical discharge method for a hypersonic flow at Mach 10. From these results, it can be concluded that they were in good agreement. As an example of the numerical simulation, flowfields around the capsule traveling at speeds of Mach 5, 3, and 2 are investigated. By these simulations, differences between the flow structures for these Mach numbers are clarified. The result shows that the location of the separation behind the capsule is governed by the strength of the adverse pressure gradient. The reverse flow behind the capsule that determines the base pressure distribution is correlated to the strength of the re-circulation and the stability of the capsule.

View full abstract

The present paper describes the characteristics of a viscous micropump with a cylindrical rotor in a rectangular duct. In the model experiment, the low Reynolds number flow is realized by using glycerin as a working fluid for a centimeter-scale (non-micrometer-scale) pump model. In the numerical simulation, the commercial code, STAR-CD, is used. Qualitative and quantitative agreements between the experimental and numerical results are obtained with respect to the flow fields and the pump characteristics, i.e. pressure performance and pump efficiency. The experimental and numerical results show that there exist three recirculations near the rotor, rotating in the opposite direction of the rotor. They also show that pressure performance represents a straight line with a negative slope. In the theoretical analysis, the Buckingham’s pi theorem is applied with the low Reynolds number approximation. As a result, two new dimensionless parameters are obtained for flow rate and pressure rise. By using the two new dimensionless parameters, the unified treatment of pressure performance is possible, independent of the Reynolds number. The low Reynolds number approximation is valid when the Reynolds number is smaller than 100.

View full abstract

A study of a new high-speed zero-emission transportation “Aerotrain” is being carried out in Tohoku University and the University of Miyazaki. Because the aerotrain utilizes the ground effect, research on the aerofoil section, which can harness the ground effect effectively, is important. The aerotrain moves along a U-shaped guideway, which has a ground and sidewalls, so it has many viscous interference elements. In an analysis of the ground effects on the aerodynamic characteristics of aerofoils, the boundary layers on the aerofoil surface must be considered. At first, velocity distributions on the surfaces of aerofoils in potential flows are computed using the vortex method, then the momentum integration equations of the boundary layer are solved with experimental formulas. This procedure has the following advantages: modifications of the aerofoil section are easy because it is not necessary to make complicated computational grids, boundary layer transition and separation can be predicted using empirical procedures. The aerodynamic characteristics of four types of aerofoil sections are investigated to clarify the relationship between aerofoil sections and ground effects. Computational results are compared with experimental results obtained using a towing wind tunnel to verify computational precisions. In addition, aerofoil characteristics at an actual cruise speed are analyzed.

View full abstract

We study the motion of a pair of bubbles rising in the vertical line, at intermediate Reynolds number (5<Re<150), both numerically and experimentally. It has been predicted, by analytical and numerical studies, that there exists a stable equilibrium distance between a pair of bubbles due to the balance between the potential repulsive and viscous atractive forces. However, no experimental results have ever confirmed the existence of an equilibrium distance between bubbles rising in a vertical line. Most experimental results indicated that a pair of bubbles collided. We solve this paradox and answer the question: “Is there an equilibrium distance?” by presenting both experimental evidence of an existence of the equilibrium distance and the detailed numerical results of a deforming-spatial-domain/space-time finite element method, taking bubble surface deformation effects into account.

View full abstract

This study was conducted to examine how ethylene decomposes with a non thermal plasma discharge (NTPD)-photocatalyst system. The experimental study was carried out with variables of space velocity (SV), humidity and an input power to investigate the ethylene oxidation behavior in the system. The luminescence in the catalyst coated substrate, C₂H₄ decomposition, and the power factor behavior of the system were examined. As a result, the luminescence phenomenon was able to decompose C₂H₄ by activation of photocatalyst instead of UV-radiation, and there was a proportional relation between the humidity and decomposition as well as between the power factor (PF) and UV intensity of the plasma-photocatalyst. In addition there was an optimum SV for the decomposition of C₂H₄.

View full abstract

An experimental study of the structure of the roll and trailing vortex formed around the blades of a flat paddle impeller in a stirred vessel has been carried out. Angle-resolved measurements of all three velocity components were performed by synchronizing laser-Doppler velocimetry with a rotary encoder coupled with the impeller shaft. Almost all data was obtained in the θ=0° plane, located halfway between two baffles. This experimental method makes it possible to capture the details of the vortical structure behind the impeller blade. The formation of a roll and trailing vortex is clearly shown in the resultant axial and circumferential mean velocity vectors and contours of mean vorticity. The size, shape and location of the trailing vortex are shown and compared with the results of Rushton turbines.

View full abstract

The pressure fluctuations and the radial fluid forces induced by rotor-stator interaction in a centrifugal pump were measured and their relationship was investigated. Experiments were done for various guide vanes, flow rates, and rotating speeds. It was demonstrated that both the blade pressure fluctuations and the volute static pressures are non-uniform circumferentially (not axisymmetric) under off-design operating conditions and that the two have a strong relationship. At high flow rates, the interaction-induced blade pressure fluctuations are large in areas where the volute static pressure is low. The propagating directions of the pressure fluctuations, the whirling directions of the radial fluid forces acting on the impeller and the dominant frequency components of both the fluctuations and the fluid forces are discussed. When measuring the fluid forces in the rotating frame, other frequency components, in addition to those related to the products of the number of guide vanes and the rotating frequency, may occur due to the circumferential unevenness of the pressure fluctuations.

View full abstract

Performance characteristics are experimentally studied with various nozzles, stators and rotors on a partially admitted small axial-type turbine, which could be applied to a driver of micro air grinders. When air tools adopt axial-type turbines as a driver, they could operate without friction and abrasion because the turbine rotor does not make contact with the casing. In order to maintain these merits on a small axial-type turbine without reducing power, performance characteristics are examined through measuring the specific output power with eight different stators and three different rotors and nozzles. The tested turbine consists of two-stages and its mean radius of flow passage is 9.2mm. The experimental results show that the output power improvement on the first stage is significant comparing with that on the second stage because partially admitted flow is fully diffused in the second stage. Meanwhile, the output power is increased to 16-22% by changing the nozzle blade angle from 60° to 70° because the first stage performance is directly affected by the flow spouted from the nozzle. These results indicate that blade angles greatly influence the performance of a micro turbine operating in partial admission. When an appropriate stator and rotor that are designed in accordance with the flow spouted from the nozzle are installed in the rotating part, the output power is increased up to 38% depending on the blade angle.

View full abstract

This paper proposes the superior wind turbine generator, which is composed of the tandem wind rotors and the double rotational armature type generator without the conventional stator. The large-sized front wind rotor and the small-sized rear wind rotor drive respectively the inner and the outer armatures of the generator, in keeping the rotational torque counter-balanced. Such operating conditions enable to make the output higher than the conventional wind turbine and to keep the output constant in the rated operating mode without using the brake and/or the pitch control mechanisms. Such wonderful advantages in the generating mode are discussed and verified experimentally with the model turbine generator.

View full abstract

Assessment of profitability, based on the accurate measurement of the frequency distribution of wind speed over a certain period and the prediction of power generation under measured conditions, is normally a centrally important consideration for the installation of wind turbines. The frequency distribution of wind speed is evaluated, in general, using the Weibull distribution. In order to predict the frequency distribution from the average wind speed, a formula based on the Rayleigh distribution is often used, in which a shape parameter equal to 2 is assumed. The shape parameter is also used with the Weibull distribution; however, its effect on calculation of wind conditions and wind power has not been sufficiently clarified. This study reports on the evaluation of wind conditions and wind power generation as they are affected by the change of the shape parameter in the Weibull distribution with regard to two wind turbine generator systems that have the same nominal rated power, but different control methods. It further discusses the effect of the shape parameter of prototype wind turbines at a site with the measured wind condition data.

View full abstract

Oscillatory flow in loop channels consisting of a T-branch, two reservoirs, and main and side branches was experimentally investigated. The following were found: (1) Oscillatory flow composed of oscillatory and steady components was induced in the loop channels. (2) The ratio of the flow rate of the steady component in the main and side branches to the maximum flow rate of oscillatory flow in the primary branch approached constant values in the Reynolds number range of 5000 ≤Re_p,max≤10000. (3) Time-averaged pressure differences were generated between the two exits of the T-branch and between the inside and outside of the reservoirs. These pressure differences induced the steady component in the oscillatory flow in the loop channel. (4) The ratio of the flow rate of the oscillatory component in the main branch to that in the side branch was nearly equal to the ratio of the reciprocal lengths of the main and side branches.

View full abstract

The present paper describes about the effects of the inequality of blade spacing on the steady cavitation and its stability. The development process of the cavity in the cascade with unequally spaced blades is more complex in comparison with that in the cascade with equally spaced blades. The development process can be reasonably explained by the interaction between the local flow near cavity trailing edge and the leading edge of adjacent blade. The minimum cavitation number of the region of stable cavitation can be decreased by unequalizing the blade spacing. Various unstable cavitation appears in low cavitation number where the cavity is longer than the 65% of blade spacing.

View full abstract

The objective of this study is to develop an efficient turbine that can be used to extract micro hydropower potential of a water supply system. For the case of high head and critical low flow rate range of micro hydropower resources, it requires very low specific speed turbines which are lower than conventional impulse turbines’ specific speed. For this purpose, we develop a new Positive Displacement Turbine (PDT). In order to reveal the performance characteristics of the new turbine, one conventional impulse turbine, which is used for automatic water faucet system, was tested for comparison. The test results show that the PDT was much more efficient than a conventional turbine and it can sustain high efficiency under the wide range of operating conditions. In addition, the efficiency of the PDT is much improved when reducing its side clearance. The pressure pulsations at the inlet and outlet of the PDT can be considerably minimized by using simple dampers.

View full abstract

The progress of the applications of water spray systems in fire suppression has been substantial over the last decade. However, the extinguishing mechanisms and the role of spray characteristics in fire suppression has not been well understood and identified. Sometimes, water spray does not behave like a perfect gaseous agent in fire suppression and the capability of a water spray system in fire suppression is dependent on the distribution of droplet size, water flux density, and spray dynamics. The study has been undertaken to examine quantitatively the effectiveness of water spray fire suppression systems using water droplet size ranged from about 150µm to 800µm. An experiment was conducted in this investigation to study quantitatively the effect of water spray on fire suppression and extinction. In order to produce different water droplet sizes under same water supply rates, different characteristics of water spray nozzles were designed and fabricated for experimental program. Four different water droplet sizes and two different water supply rates were applied on both liquid and solid fires. Also, ventilation effect of water spray on fire suppression and extinction was proposed.

View full abstract

Experimental observations and numerical simulations are conducted on combustion processes of n-decane polydisperse spray entering a gaseous flat flame stabilized in a laminar 2D counterflow configuration. For the gaseous phase, Eulerian mass, momentum, energy, and species conservation equations are solved. For the disperse phase, all individual droplets are tracked without using a droplet parcel model. The experimental results show that blue flames and luminous flames are observed and there are unsteady changes in the behavior. The numerical results show that the spray flame structures vary depending on the supplied quantities of liquid fuel spray. Furthermore, the instantaneous flame structures are consistent with the typical flame structures observed with the experiment.

View full abstract

Energy transfer from fluid to a surface and molecular dynamics behavior in the vicinity of a surface were calculated by using the classical molecular dynamics method in order to investigate an effect of surface structural clearances in nanometer scale on surface energy transfer and molecular diffusion. In an unsteady state calculation, upper region in a calculation domain and a solid atomic layer at lower region were controlled at constant temperatures respectively so as to make a temperature gradient in a calculation system. In a steady state calculation, temperature in the whole system was kept at a constant temperature and thermal equilibrium state was realized. Fluid molecular behaviors in the vicinity of a surface were dependent on characteristic length scale of surface structural clearances in nanometer scale that affected the dynamic behaviors of fluid molecules in the vicinity of a surface in both unsteady and steady state systems.

View full abstract

Spontaneous Raman spectroscopy with KrF excimer laser was applied to obtain a fuel concentration distribution in a sooting flame. In the case of sooting flame, fluorescence from polycyclic aromatic hydrocarbons (PAH) and laser-induced incandescence (LII) from soot particles appeared with Raman scattering. These background emissions overlapped on the Raman scattering. In order to separate the Raman scattering and the background emissions, polarization property of laser-induced emissions was utilized. Since the background emissions were depolarized whereas the Raman scattering was highly polarized, it is possible to subtract the background emissions from the overlapping signal of the Raman scattering and the background emissions. Subtracting the emission signals for the electric vector of the laser light perpendicular and parallel to the direction of observation allows to extract the precise Raman signals. By using this technique, detailed fuel concentration distribution in sooting flames could be obtained based on Raman scattering.

View full abstract

Laser-induced breakdown spectroscopy (LIBS) with atomic emission excited with a focused high-energy ND: YAG laser was applied to quantify the concentration and the vapor fraction of liquid-gas mixtures. With LIBS it is possible to quantify local concentrations accurately even in liquid-gas mixtures as the ratio of the number of fuel-borne hydrogen atoms to nitrogen or oxygen atoms in the ambient gas. The ratio has a strong linear relation with the ratio of the peak emission intensities regardless of phase of the fuel. As the full width at half maximum (FWHM) of the emission peak from the fuel-borne hydrogen increases linearly with the liquid fraction due to the Doppler shift with micro-explosions, the FWHM yields the fuel vapor fraction. Simultaneous, high-resolution measurements of equivalence ratios and vapor fractions in an intermittent fuel spray in a pressurized atmosphere were obtained with this method. The results showed that the tip of the intermittent spray has a richer mixture with a lower vapor fraction.

View full abstract

This study tries to investigate the reduction of nitric oxide by thermally cracked hydrocarbons under rich condition during diesel combustion. Experiments using flow reactor system, which follows the chemical process of fuel at high temperature and atmospheric pressure, show that thermal cracking of fuel starts at about 1000K, and lower hydrocarbons mainly composed of C₂H₄ and CH₄ are formed. NO can be reduced when fuel is thermally cracked and oxidized. A larger amount of NO is reduced when thermal cracking hydrocarbons are increased in quantity under rich and high temperature condition. Among decomposed hydrocarbons, C₂H₄ is easily decomposed and affects deNO mechanism. Chemical kinetic calculation using CHEMKIN III reveals the mechanism. NO is reduced through the reaction of HCCO or CH₂ with NO. In these reaction paths, C₂H₂ is an essential species. The computation also shows that this deNO mechanism can be actualized in the practical diesel combustion.

View full abstract

An efficient and scalable Balancing Domain Decomposition (BDD) type preconditioner for large scale linear systems arising from 3-dimensional heat transfer problems is presented. The new method improves parallel scalability of BDD by employing an incomplete balancing technique to approximate a coarse space problem and a diagonal scaling to precondition the local fine space problems instead of the Neumann-Neumann preconditioner. It may increase the number of iterations but reduces the computation costs of the precondition process for each iteration. Consequently, total computation time and required memory are expected to be reduced. The convergence estimates may also be independent of the number of subdomains. We have implemented this algorithm on the parallel processors and have succeeded in solving some ill-conditioned large scale heat transfer problems.

View full abstract

Heat pumps using CO₂ as a natural refrigerant have been developed and are expected to contribute to energy saving in hot water supply. In residential applications, CO₂ heat pumps are used in combination with hot water storage tanks. The objective of this series of papers is to analyze the overall performance of a hot water supply system composed of a CO₂ heat pump and a hot water storage tank by numerical simulation. In the 1st report, a simulation model of a CO₂ heat pump is created based on thermodynamic equations and measured data for an existing CO₂ heat pump. In addition, the performance of a CO₂ heat pump is clarified in relation to the air temperature as well as the inlet and outlet water temperatures.

View full abstract