Showing papers in "Jsme International Journal Series B-fluids and Thermal Engineering in 2004"
TL;DR: In this article, the performance of a single PEFC was investigated at temperatures from -3 to -25°C and pressures from 1 to 2 atm, and the results showed that it is necessary to heat the cell with an additional heat source in order to start the fuel cell below -5°C.
Abstract: At temperatures below freezing, air humidity becomes lower and produced water at the cathode freezes on the surface of catalyst, and it is difficult to start a PEFC (Polymer Electrolyte Fuel Cell) at a cold district. The object of the work is to study the performance of the fuel cell below the freezing point by experiments and simulation. To investigate the characteristics of the starting of a temperature below freezing the performance of a single cell was measured at temperatures from -3 to -25°C and pressures from 1 to 2 atm. The results of the experiments and simulation indicate that the performance of a PEFC decreases at higher current densities and pressures, and lower cell temperatures because of ice more produced on the reactive area of the cathode. To maintain the cell performance below freezing point, it is effective to adjust the current densities and gas flow rate to balance the produced and removed water. However at -5°C, heat generated in the fuel cell is effective to warm the cell and make self-starting possible. These results shows that it is necessary to heat the cell with an additional heat source in order to start the fuel cell below -5°C.
84 citations
TL;DR: In this paper, physical phenomena associated with the dynamic spreading and dried shape of droplets on solid surfaces were demonstrated reviewing several models and discussed with regard to designing the most suitable thin-film formation processes using ink-jet printing.
Abstract: Physical phenomena associated with the dynamic spreading and dried shape of droplets on solid surfaces were demonstrated reviewing several models and discussed with regard to designing the most suitable thin-film formation processes using ink-jet printing. After droplets strike a substrate surface and expand for several microseconds, they spread semi-statically for tens of seconds and asymptotically approach the final equilibrium shape determined by droplet volume and contact angle. The contact angle and the volume, number, and impact velocity of droplets for various ink-jet-deposition applications can be designed by using semi-empirical formulas. If the contact angle at the edge of droplet on the substrate is small, a large amount of solute might accumulate there during the drying process because the evaporation rate there is high. The evaporation rate distribution on droplet surfaces should therefore be controlled to be uniform in radial direction during drying.
56 citations
TL;DR: In this paper, a new flow analysis system, namely the hybrid wind tunnel, which integrates the experimental measurement with a wind tunnel and a corresponding numerical simulation with a computer, is presented.
Abstract: This paper deals with a new flow analysis system, namely the hybrid wind tunnel ,w hich integrates the experimental measurement with a wind tunnel and a corresponding numerical simulation with a computer. Analysis here is performed for the fundamental flow with the Karman vortex street in a wake of a square cylinder. A specific feature of the hybrid wind tunnel is existence of the feedback signal to compensate the error in the pressure on the side walls of the cylinder and the feed-forward signal to adjust the upstream velocity boundary condition. Investigation is focused on evaluating the hybrid wind tunnel as a flow analysis methodology with respect to the ordinary simulation and the experiment. As compared with the ordinary simulation, the hybrid wind tunnel substantially improves the accuracy and the efficiency in the analysis of the flow. Especially, the oscillation of the flow due to the Karman vortex street reproduced with the hybrid wind tunnel exactly synchronizes with that of the experiment, while that with the ordinary simulation never behave like that. In comparison with the experiment, the hybrid wind tunnel provides more detailed information of the flow than the experiment does.
41 citations
TL;DR: In this article, a silicon-based catalytic combustor is designed and its prototype is fabricated using MEMS technologies, and the Pt/alumina catalyst layer is successfully integrated onto a silicon microchannel, and a Pyrex lid is anodically bonded onto the Si substrate.
Abstract: Micro catalytic combustion of butane in microtube is investigated. Porous alumina fabricated on the inner surface of microtube through anodic oxidation of Al is employed for the support of Pt catalyst. Exhaust gas is sampled to measure its composition and the combustion efficiency. Combustion starts at 250 ˚C, and a heat release rate up to 250MW/m 3 is achieved in a 0.6mm ID tube. A silicon-based catalytic combustor is designed and its prototype is fabricated using MEMS technologies. The Pt/alumina catalyst layer is successfully integrated onto a silicon microchannel, and a Pyrex lid is anodically bonded onto the Si substrate. It is found in a preliminary experiment that the MEMS combustor also works well, but gives somewhat smaller reaction rate due to the thinner catalytic layer. ISMME2003-119
38 citations
TL;DR: In this paper, a micromachine gas turbine with centrifugal impellers fabricated by 5-axis micro-milling is under development at Tohoku University, in conjunction with Ishikawajima-Harima Heavy Industries Co., Ltd.
Abstract: Micromachine gas turbine with centrifugal impellers of 10mm diameter fabricated by 5-axis micro-milling is under development at Tohoku University, in conjunction with Ishikawajima-Harima Heavy Industries Co., Ltd. (IHI), Tohoku-Gakuin University, and Sankyo Seiki Mfg. Co., Ltd. The development is currently at the stage of proving the feasibility of the gas turbine cycle by component tests. Micro-combustors have been developed for both hydrogen and methane fuel. Over 99.9% of the combustion efficiency has been realized in both combustors and the baseline configuration of the combustor for the gas turbine is set. A compressor of 10mm diameter has been developed as a micromachined turbocharger. The performance test of the micromachined turbocharger has been started, and ran up to 566000rpm, which is approximately 65% of the design speed. Compressor performance has been successfully measured along a constant speed line at 55% of the design speed.
31 citations
TL;DR: In this paper, the authors investigated a fin model for predicting the heat transfer capacity of the lotus-type porous copper, which is a form of copper that includes many straight pores, which are produced by the precipitation of supersaturated gas dissolved in the molten metal during solidification.
Abstract: Lotus-type porous copper is a form of copper that includes many straight pores, which are produced by the precipitation of supersaturated gas dissolved in the molten metal during solidification. The lotus-type porous copper is attractive as a heat sink because a higher heat transfer capacity is obtained as the pore diameter decreases. We investigate a fin model for predicting the heat transfer capacity of the lotus-type porous copper. Its heat transfer capacity is verified to be predictable via the straight fin model, in which heat conduction in the porous metal and the heat transfer to the fluid in the pores are taken into consideration by comparison with a numerical analysis. We both experimentally and analytically determine the heat transfer capacities of three types of heat sink: with conventional groove fins, with groove fins that have a smaller fin gap (micro-channels) and with lotus-type porous copper fins. The conventional groove fins have a fin gap of 3mm and a fin thickness of 1mm, the micro-channels have a fin gap of 0.5mm and a fin thickness of 0.5mm, and the lotus-type porous copper fins have pores with a diameter of 0.3mm and a porosity of 0.39. The lotus-type porous copper fins were found to have a heat transfer capacity 4 times greater than the conventional groove fins and 1.3 times greater than the micro-channel heat sink under the same pumping power.
30 citations
TL;DR: In this paper, a diffraction-free beam was generated using an axicon lens, and the generation was confirmed using an optical microscopy image of a laser-fabricated spot on a silicon wafer.
Abstract: Laser microfabrication using a diffraction-free beam (Bessel beam) was performed. Microfabrication with a deep focal depth is possible because a diffraction-free beam has a main lobe with a small diameter, which does not depend on propagation length. The diffraction-free beam was generated using an axicon lens, and the generation was confirmed using an optical microscopy image of a laser-fabricated spot on a silicon wafer. Nevertheless using a nanosecond pulsed Nd:YAG laser, microfabrication with a spot diameter < 1 μm was realized. As a result of the deep focal depth of the diffraction-free beam, even if we change the work distance within a range of several millimeters, we are still able to maintain the diameter of the laser- fabricated spots to approximately 1 μm. Since an almost straight penetration hole with a diameter of 3 μm was drilled into SUS304 foil (20 μm thick), it was found that the diffraction-free beam has a high feasibility for the high-aspect-ratio laser drilling of an opaque material due to its deep focal depth.
30 citations
TL;DR: In this article, the critical heat fluxes (CHFs) of subcooled water flow boiling for the inner diameter of 3 and 6 mm at the outlet pressure of around 800 kPa were systematically measured for the test tube inner diameters and the heated lengths (L = 67, 120 and 150 mm).
Abstract: The critical heat fluxes (CHFs) of subcooled water flow boiling for the test tube inner diameters (d = 3 and 6 mm) and the heated lengths (L = 67, 120 and 150 mm) are systematically measured for the flow velocities (u = 40 to 133 m/s), the inlet subcoolings (ΔT sub,in =48 to 148 K), the outlet subcoolings (ΔT sub,out = 105 to 951 K), the inlet pressure (P in = 753 to 995 kPa) and the outlet pressure (P out = 720 to 887 kPa) The SUS304 tubes of L = 67, 120 and 150 mm for d = 3 mm and L = 150 mm for d = 6 mm are used The values of L/d are 22, 40 and 50 for d = 3 mm, and 25 for d = 6 mm, respectively The CHFs, q cr,sub , for a fixed ΔT sub,out become gradually lower with an increase in the L/d in the whole experimental range The CHF correlation against outlet subcooling, which has been previously derived for L/d lower than 16, was modified to new one containing the L/d effect based on these experimental data Furthermore, the relation between q cr,sub and L/d for a fixed ΔT sub,in was checked The values of q cr,sub for a fixed ΔT sub,in became exponentially lower with the increase in L/d CHF correlation against inlet subcooling has been given based on the experimental data for L/d ranging from 408 to 50 The correlations against outlet and inlet subcoolings can describe not only the CHFs obtained in this work for the inner diameter of 3 and 6 mm at the outlet pressure of around 800 kPa but also the authors' published CHFs data (1611 points) for the wide ranges of Pin = 159 kPa to 1 MPa, d = 3 to 12 mm, L = 33 to 150 mm and u = 40 to 133 m/s within 15% difference for 30 K ≤ ΔT sub,out ≤ 140 K and 40 K ≤ ΔT sub,in ≤ 151 K
28 citations
TL;DR: In this paper, a molecular-dynamics technique for simulating interface diffusion, which is one of the dominant factors in mechanical failures of thin-film devices, has been developed.
Abstract: A molecular-dynamics technique for simulating interface diffusion, which is one of the dominant factors in mechanical failures of thin-film devices, has been developed. This technique was used to find effective methods for suppressing the interface diffusion and for stabilizing interfaces. Barrier-underlayer materials effective for improving the adhesion strength with interconnect films were identified by using this technique. Ruthenium was found to be an effective underlay material for improving the adhesion with Cu interconnects. The crystal orientation of Si substrates effective for reducing atomic diffusion at interfaces between the Si substrates and high-k dielectrics (ZrO2 and HfO2) was determined. The use of Si(111) substrates was found to be effective for suppressing the formation of interfacial layers.
21 citations
TL;DR: In this paper, the authors investigated the influence of cylinder diameter on turbulent transition and local heat transfer characteristics of the cylinders and found that the diameter of a cylinder can influence the turbulent transition.
Abstract: Natural convective flows of water induced around heated vertical cylinders have been investigated experimentally. Special interests were paid to the influences of cylinder diameter on the turbulent transition and also on the local heat transfer characteristics of the cylinders. The diameters of the cylinders were varied systematically from 10 to 165 mm. Visualizations of the flows around the cylinder and of the surface temperatures of the heated cylinders have been carried out to determine the onset of turbulent transition
21 citations
TL;DR: In this article, a lattice Boltzmann method for two-phase nonideal fluid flows is proposed based on a particle velocity-dependent forcing scheme, and the resulting macroscopic dynamics via the Chapman-Enskog expansion recover the full set of thermohydrodynamic equations for non-ideal fluids.
Abstract: A lattice Boltzmann method (LBM) for two-phase nonideal fluid flows is proposed based on a particle velocity-dependent forcing scheme. The resulting macroscopic dynamics via the Chapman-Enskog expansion recover the full set of thermohydrodynamic equations for nonideal fluids. Numerical verification of fundamental properties of thermal fluids, including viscosity, thermal conductivity, and surface tension, agrees well with theoretical predictions. Direct numerical simulations of two-phase phenomena, including phase-transition, bubble deformation and droplet falling and bubble rising under gravity are carried out, demonstrating the applicability of the model
TL;DR: In this paper, a preliminary investigation was conducted for the prediction of sloshing in the propellant tank of a rocket vehicle, and the flow field in the tank during the ballistic flight of the vehicle was experimentally reproduced with the sub-scale model.
Abstract: For the prediction of sloshing in the propellant tank of a rocket vehicle, the preliminary investigation was conducted. The flow field in the propellant tank during the ballistic flight of the vehicle was experimentally reproduced with the sub-scale model. The lateral acceleration as large as about 0.8G was provided with a mechanical exciter and the deformation of the liquid surface in the small vessel was visualized with a high-speed camera. The sloshing phenomena were also simulated with the CFD code, called CIP-LSM. The important features of surface deformation and wave breaking were successfully reproduced in the computation.
TL;DR: In this article, a MEMS-based active-head slider using a PZT thin film for flying height control in hard disk drives is described, where a piezoelectric cantilever integrated in an air bearing slider is used to adjust the flying height individually.
Abstract: This paper describes design and fabrication of a MEMS-based active-head slider using a PZT thin film for flying height control in hard disk drives. A piezoelectric cantilever integrated in an air bearing slider is used to adjust the flying height individually. A novel air bearing surface (ABS) geometry that minimizes the aerodynamic lift force generated beneath the head has been designed on the basis of the molecular gas film lubrication (MGL) theory. The slider with PZT actuator was fabricated monolithically by silicon micromachining. Performance of the actuator was tested by using an optical surface profiler. Furthermore, the fabricated slider was mounted on a suspension and flying height of the slider above a spinning disk has been measured by means of interferometry. Change in the head-disk spacing has been successfully confirmed by applying voltage to the PZT actuator.
TL;DR: In this article, the results of LBE loop operation under the flowing conditions of target design concept are reported and the stagnant corrosion tests were done to know the controlling parameters among the various steels.
Abstract: Lead bismuth eutectic (LBE) is favored by spallation neutron sources and coolant in the sub-critical reactor at the accelerator driven nuclear transmutation system (ADS). Technical issues of ADS are material technology of how to compromise with flowing lead bismuth, high-energy proton accelerator technology and a sub-critical reactor system technology. This paper describes LBE technology developed at JAERI. First a scenario in order to realize the ADS is shown. The concept of spallation target test facility is introduced with a target design of thermo-fluid dynamics. Base data of flow rate and temperature of Pb-Bi during LBE circulation are described. The results of LBE loop operation under the flowing conditions of target design concept are reported. The stagnant corrosion tests were done to know the controlling parameters among the various steels. The tube-type oxygen sensor with having the solid electrolyte was studied. Cleaning techniques were developed to remove LBE from materials.
TL;DR: In this article, the authors presented results of parametric investigation of grooves of traditional and new configurations of axial flow compressors and proposed a model on basis of Group Method of Data Handling (GMDH) may help designing groove configurations with improved performances.
Abstract: Circumferential grooves over a rotor blade tips are used for improving axial flow compressor performances. Such casing treatment facility extends a stable operation range in most cases, but decreases compressor efficiency as a rule. There are presented results of parametric investigation of grooves of traditional and new configurations. Development of new groove constructions must permit combining of stable operation range extension with efficiency increase. Model on basis of Group Method of Data Handling may help designing groove configurations with improved performances.
TL;DR: In this article, a Bingham type constitutive model is proposed based on Mohr-Coulomb's failure criterion and the equivalent Newtonian viscosity is derived from the cohesion and friction angle.
Abstract: A numerical method is developed for prediction of large deformations associated with a geomaterial flow The geomaterial is modeled as a viscous fluid, where a Bingham type constitutive model is proposed based on Mohr-Coulomb's failure criterion and the equivalent Newtonian viscosity is derived from the cohesion and friction angle These two parameters are very important factors for the behavior of geomaterials In solving Navier-Stokes's equations, a constrained interpolated profile (CIP) scheme is utilized The numerical method developed in this paper is used to simulate a 2-dimensional gravitational flow in order to check the performance of the constitutive model The method is also applied to two real slope failures One is due to heavy rain and the other due to earthquake In the analysis of the slope failure due to earthquake, simulated velocity of ground flow, displacement and elapsed time are compared with observed results In the case of the analysis of the slope failure due to heavy rain, a previous study about the slope failure using finite element method (FEM) is firstly introduced The method used in the previous study, however, could not describe the subsequent ground flow after the initiation of slope failure Therefore, the method developed in this paper is applied to simulate the ground flow
TL;DR: In this paper, the space-time conservation element and solution element (CE/SE) method is used to solve axisymmetric heat conduction problems with melting and/or freezing.
Abstract: The space-time conservation element and solution element (CE/SE) method, an accurate and efficient explicit numerical scheme for resolving moving discontinuities in fluid mechanics problems, is used to solve axisymmetric heat conduction problems with melting and/or freezing. The axisymmetric formulation is presented. Comparisons are made to existing analytical solutions. The CE/SE method is found to be accurate and robust for the numerical modeling of phase change problems.
Abstract: Dust separation characteristics of a counter-flow vortex tube were investigated with lime powders whose mean particle sizes were 5 and 14.6µm. The experiment showed that a vortex tube can be used as an efficient pre-skimmer to separate particles from the waste gas in industry. In case of fine particles, a trend of increasing separation efficiency was observed with inlet pressure up to 160kPa at which the flow rate was 9.5m3/hr, but there was a tendency of reduction in the coarse particles. For both powder sizes, the efficiency and the effective nozzle inlet velocity were 93% and 14.52m/s respectively, while the tested vortex tube had a small diameter of 16mm and the ratio of the nozzle hole area to the tube cross-sectional area was 0.15. Additionally, the geometric ratios of the vortex tube could be proposed as 0.44 for the ratio of the orifice diameter at clean gas exit to the tube diameter and 14.0 for the ratio of the tube length to the diameter.
TL;DR: In this article, the lattice Boltzmann method (LBM) was used to simulate a combustion field and a flame in simple flow geometry was simulated and the laminar burning velocity was obtained.
Abstract: Even though laser diagnostics have significantly improved and can obtain an instantaneous 2D flame image of the velocity field, it is still difficult to obtain data such as scalar flux or reaction rates experimentally. It is also essential to understand 3D flame structures in turbulent combustion. Chemically non-reacting turbulent flows are complex and chemical reactions make the problem more complicated. Due to practical limitations of computational costs, conventional numerical methods are very expensive for carrying out 3D numerical simulations at high Reynolds numbers with detailed chemical reactions. In this study, we have used the lattice Boltzmann method (LBM) to simulate a combustion field. The LBM is an efficient alternative for the numerical simulation of this type of flow. To confirm the validity of the LBM, a flame in simple flow geometry is simulated and the laminar burning velocity is obtained. Both 2D and 3D simulations have been completed. A jet flame has been also simulated to demonstrate the LBM capability of simulating unsteady flames with vortices. The scheme with detailed chemistry has been tested for simulation of a counter-flow flame.
TL;DR: In this article, an axial 12-step power distribution is employed to simulate the complicate heating condition in a reduced-moderation water Reactor (RMWR) with 7 rods arranged on a 14.3 mm equilateral triangular pitch.
Abstract: The Reduced-Moderation Water Reactor (RMWR) has recently becomes of great concern. The RMWR is expected to promote the effective utilization of uranium recourse. The RMWR is based on water-cooled reactor technology, with achieved under lower core water volume and water flow rate. In comparison with the current light water reactors whose water-to-fuel volume ratio is about 2-3, in the RMWR, this value is reduced to less than 0.5. Thereby, there is a need to research its cooling characteristics. Experimental research on critical power in tight lattice bundle that simulates the RMWR has been carried out in Japan Atomic Energy Research Institute (JAERI). The bundle consists one center rod and six peripheral rods. The 7 rods are arranged on a 14.3 mm equilateral triangular pitch. Each rod is 13 mm in outside diameter. An axial 12-step power distribution is employed to simulate the complicate heating condition in RMWR. Experiments are carried out under G = 100-1 400 kg/m 2 s, P ex = 2-8.5 MPa. Effects of mass velocity, inlet temperature, pressure, radial peaking factor and axial peaking factor on critical power and critical quality are discussed. Compared with axial uniform heating condition, the axial non-uniform heating condition causes an obvious decrease in critical quality. Arai correlation, which is the only correlation that has been optimized for tight lattice condition, is verified with the present experimental data. The correlation is found to be able to give reasonable prediction only around RMWR nominal operating condition.
TL;DR: In this article, the authors compared the results of the Martinelli-Nelson's and Hancox's correlations with experimental results in 4 x 5 rod bundles and seven-rod bundles.
Abstract: The reduced-moderation water reactor (RMWR) consists of several distinctive structures; a triangular tight-lattice configuration and a double-flat core. In order to design the RMWR core from the point of view of thermal-hydraulics, an evaluation method on pressure drop characteristics in the rod bundles at the tight-lattice configuration is required. In this study, calculated results by the Martinelli-Nelson's and Hancox's correlations were compared with experimental results in 4 x 5 rod bundles and seven-rod bundles. Consequently, the friction loss in two-phase flows becomes smaller at the tight-lattice configuration with the hydraulic diameter less than about 3 mm. This reason is due to the difference of the configuration between the multi-rod bundle and the circular tube and due to the effect of the small hydraulic diameter on the two-phase multiplier.
TL;DR: In this paper, a very low temperature poly-Si TFT manufacturing technology without excimer laser annealing and very low cost manufacturing which is characterized by very simplified display structures and very simplified manufacturing steps based on very drastic progress of various relating materials and components such as backlights, polarizer, color filter, and etc.
Abstract: A large size display for entertainment, internet, PC and other information instruments is key tool for coming IT revolutionary era, so that the large size display must be characterized by very low electric power consumption and human friendly performance without tiring user's eyes. Thus, liquid crystal (LC) and electroluminescence (EL) displays are candidates for this target. High quality poly-silicon TFT is essentially required even for LCD displays instead current amorphous Si TFT, because very large current drivability is necessary for TFT due to the increase of LCD cell capacitor with an increase of display size up to 50 inches and beyond. The key issue for this target is a creation of very low temperature poly-Si TFT manufacturing technology without excimer laser annealing and very low cost manufacturing which is characterized by very simplified display structures and very simplified manufacturing steps based on very drastic progress of various relating materials and components such as backlights, polarizer, color filter, and etc.
TL;DR: In this paper, an innovative fuel cycle system concept named BARS (BWR with an Advanced Recycle System) has been proposed as a future fuel cycle option aiming at enhanced utilization of uranium resources and reduction of radioactive wastes.
Abstract: An innovative fuel cycle system concept named BARS (BWR with an Advanced Recycle System) has been proposed as a future fuel cycle option aiming at enhanced utilization of uranium resources and reduction of radioactive wastes. In BARS, the spent fuel from conventional light water reactors (LWRs) is recycled as a mixed oxide (MOX) fuel for a BWR core with the fast neutron spectrum by means of oxide dry-processing and vibro-packing fuel fabrication. The fast neutron spectrum is obtained by means of triangular tight fuel lattice. Further study on BARS, especially on tight lattice MOX fuel, has been initiated as a joint study by Toshiba and Gifu University. The objective of this paper is to show the latest progress of the study on BARS, especially concerning the thermal-hydraulics measurements for tight lattice bundle.
TL;DR: In this article, the outline and operation status of Karita New Unit 1 is discussed. But the main focus of this paper is on the power generation system of the plant, which is the largest PFBC plant in the world.
Abstract: Karita Power Station New Unit 1, which started commercial operation on July 3, 2001, has adopted the new power generation system PFBC, Pressurized Fluidized Bed Combustion, a combined cycle generation system. The system boasts high thermal efficiency, excellent environmental characteristics and compact size. Output from the steam turbine generator and gas turbine generator is 290 MW and 75 MW, respectively, with total output adjusted to 360 MW, making the unit the largest PFBC plant in the world. In October 2002, combustion tests were conducted using an anthracite coal from Yangquan; and in April 2003, a world record was achieved for continuous operation hours of a PFBC plant. This paper focuses on the outline and operation status of Karita New Unit 1.
TL;DR: In this article, a 1.829m (6ft) diameter industrial large flow-rate axial fan operated at 1770rpm was investigated by measuring the pressure distributions on the blade suction and pressure surfaces and the results were discussed by comparing with analytical formulations and CFD.
Abstract: A 1.829m (6ft) diameter industrial large flow-rate axial fan operated at 1770rpm was studied experimentally in laboratory conditions. The flow characteristics on the fan blade surfaces were investigated by measuring the pressure distributions on the blade suction and pressure surfaces and the results were discussed by comparing with analytical formulations and CFD. Flow visualizations were also performed to validate the flow characteristics near the blade surface and it was demonstrated that the flow characteristics near the fan blade surface were dominated by the centrifugal force of the fan rotation which resulted in strong three-dimensional flows. The time-dependent pressure measurement showed that the pressure oscillations on the fan blade were significantly dominated by vortex shedding from the fan blades. It was further demonstrated that the pressure distributions during the fan start-up were highly unsteady, and the main frequency variation of the static pressure was much smaller than the fan rotational frequency. The time-dependent pressure measurement when the fan operated at a constant speed showed that the magnitude of the blade pressure variation with time and the main variation frequency was much smaller than the fan rotational frequency. The pressure variations that were related to the vortex shedding were slightly smaller than the fan rotational frequency. The strain gages were used to measure the blade stress and the results were compared with FEA results.
TL;DR: In this article, the authors investigated the driving mechanism of major thermohydraulic instabilities simulated in a natural circulation experimental loop, under a predetermined range of system operating pressure and inlet subcoolings.
Abstract: The purpose of the study is to experimentally investigate driving mechanism of major instabilities simulated in a natural circulation experimental loop, under a predetermined range of system operating pressure and inlet subcoolings. Pressure range of 0.1 up to 0.7 MPa, input heat flux range of 0 up to 577 kW/m 2 , and inlet subcoolings of 5, 10 and 15 K respectively, are applied in the experiments. The objective of the study is to formulate a rational startup procedure, in which major thermohydraulic instabilities can be detected and prevented. The study clarifies that four (4) kinds of thermohydraulic instability might occur even up to a higher pressure of 0.7 MPa. The instabilities' sequence is as follows: (1) geysering induced by condensation accompanied by flashing, (2) oscillation induced by hydrostatic head fluctuation, (3) density wave oscillations, and (4) flashing accompanying those instabilities. The experiments confirmed that the geysering region gets narrower and suppressed with the increased system pressure. With chimneys, natural circulation can be achieved reliably and more easily. However, the flashing in the chimney cannot be avoided at low system pressure. Stable two-phase natural circulation can be established if the system pressure is increased beyond 0.7 MPa, after the high frequency density eave oscillation thoroughly suppressed. The experiments were analyzed based on frequency domain of each instability phenomenon.
TL;DR: In this article, the relationship between the structure of a catalyst layer and the cell performance has been clarified, and the experimental results suggested that there is an optimum mass ratio of electrolyte in a catalytic layer for cell performance.
Abstract: A catalyst layer is one of the key elements in polymer electrolyte membrane fuel cells (PEMFC). Improvements in the performance of a membrane electrode assembly (MEA) for PEMFC are much influenced by an electrochemically active surface area in a catalyst layer. But the relation between the structure of a catalyst layer and the cell performance has not been clarified yet. In the present study, catalyst layers with different structure and composition were fabricated, and the structural properties of catalyst layers, such as thickness and roughness, and the polarization curves were measured. The experimental results suggested that there is an optimum mass ratio of electrolyte in a catalyst layer for the cell performance, and the thickness and roughness of a catalyst layer change significantly at the optimum mass ratio.
TL;DR: In this article, a new CFD model for the transportation of volatile organic compound (VOC) from building materials to the room air in the case with high VOC concentration in the air when Langmuir model or other nonlinear adsorption isotherms are used.
Abstract: This paper presents a new CFD model for the transportation of volatile organic compound (VOC) from building materials to the room air in the case with high VOC concentration in the air when Langmuir model or other nonlinear adsorption isotherms are used. In this model, the concentration equation solves the equivalent air-phase concentration and uses the equivalent air-phase diffusion coefficient to calculate the mass flux. A new parameter, specific mass, arising from adsorption isotherm is presented to account for the sorption and desorption effect. The present model and two previous models are applied to the steady and unsteady problems. Results show that the present model correctly simulates the emission of VOC from building materials. And the present model is easy to use with commercial CFD software.
TL;DR: In this article, a computational model for flows in plain-orifice atomizers with chamfered or rounded orifice inlets is established, and the discharge coefficients and the spray angles for the atomizers are also compared.
Abstract: A computational model for flows in the plain-orifice atomizers with chamfered or rounded orifice inlets is established. The volume of fluid (VOF) method with finite volume formulation was employed to capture the liquid/gas interface. A continuum Surface Force (CSF) model was adopted to model the surface tension. The body-fitted coordinate system was used to facilitate the configuration of the atomizers. The evolution of the fluid/air interface and the velocity vector plots for the atomizers are discussed. The discharge coefficients and the spray angles for the atomizers are also compared. The result is explained by the profile of the axial velocity component at the atomizer exit and the evolution of the pressure drop. It is found that the discharge coefficient decreases very rapidly at the early stage of atomization, while the pressure drop has an abrupt rise at that time. With the same Reynolds number based on the orifice diameter and the mean axial velocity at the atomizer exit, the atomizer with a rounded orifice inlet has a larger discharge coefficient and a larger spray angle. For the conditions investigated in the present study, the atomizer with a rounded orifice inlet is beneficial for better atomization of the liquid jet.
TL;DR: In this article, the location of the control flow nozzle was determined based on the physical consideration of the drag generation, and the position of the output velocity measurement was changed within a limited region near the front windshield.
Abstract: The present paper deals with a fundamental study of aerodynamic drag reduction for a vehicle with a feedback flow control. As the first step, two-dimensional calculation was performed for a flow around a simplified vehicle model. The mechanism of unsteady drag was investigated in relation to the vortex shedding from the model. The location of the control flow nozzle was so determined that the control flow influences the drag most effectively. The key in designing the present feedback control is the definition of the output signal. Based on the physical consideration of the drag generation, the location of the output velocity measurement was changed within a limited region near the front windshield. A systematic calculation revealed that the output signal defined in a small region results in a significant drag reduction of 20% with respect to the case without control. The present feedback flow control is generally applicable to the drag reduction of the bluff body for which the drag is generated under the same mechanism of essentially two-dimensional vortex shedding.