Showing papers in "Jsme International Journal Series B-fluids and Thermal Engineering in 2002"

Unstable Cavitation Behavior in a Circular-Cylindrical Orifice Flow

Abstract: The cavitation characteristics were experimentally investigated about long circular-cylindrical orifices of various throat lengths including the orifice with a trip wire. Especially, the shedding process of separated vortex cavity was examined in detail with the observation using high-speed photography. The cavitation impact was measured with an accelerometer. As a result, it is found that the periodic shedding of the cloud-like cavity can be observed characteristically in the transition cavitation stage and is dependent on the formation and coalescence of micro-vortex cavities on the separated shear layer as well as the reentrant motion after the shedding and collapse. It is also pointed out that it forms a feedback loop in the self-exciting mechanism. The estimation determined from two velocity ratios is shown on the Strouhal number of cloud-like cavity. The clear shedding of cloud-like cavity appears near the cavitation impact-peak region and remains almost constant, irrespective of the difference in orifice length. The transition from the throat cavitation to the jet cavitation downstream of the orifice throat can also produce the other impact-peak stage.

Information and the Study of Turbulence and Complex Flow

Abstract: The growth of computer information capacity has profoundly changed the conduct of research in fluid mechanics. Properly executed, a direct numerical simulation can be a valid and rich source of ‘data’ for the study of certain aspects of turbulence. Multi-point experiments contain less information than direct numerical simulations, but they are ‘real’, and they are still far richer in information than classical single-point experiments. To make significant advances in understanding complex flows, especially turbulence, new practices must be used that exploit the richness of these databases. These include stricter adherence to the principal of using concepts rooted in theoretical formulations to define the quantities to be measured, quantitative concepts based on new methods of kinematic analysis for three-dimensional visualization, more extensive use of statistical analysis based on multi-point correlations and related methods, and tightly coupled combined experimental/computer simulation studies. Lastly, dissemination of the information requires new approaches.

Alternate Blade Stall and Rotating Stall in a Vaned Diffuser

Abstract: Flow instability in a vaned diffuser with an even number of blades was examined experimentally and analytically. In the experiments, an alternate blade stall, an asymmetric stall, and two types of rotating stalls (backward/forward rotating stall) were observed depending on the impeller/diffuser clearance. For narrow clearance with strong impeller/diffuser interaction, the alternate blade stall and backward rotating stall mainly occurred. With increasing the clearance, the forward rotating stall also occurred, and the onset of rotating stall shifted toward the higher flow rate corresponding to the pressure performance in the vaned diffuser. Simple 2D stability analysis showed that the impeller/diffuser clearance affects the speed and direction of the stall propagation, and the slope of the diffuser pressure performance vs. flow rate curve affects fundamentally the onset of the flow instability within the diffuser.

Inner Structure of Cloud Cavity on a Foil Section

Abstract: Collapsing stage of cloud cavity and resulting violent shock pressure have been investigated by many researchers, because the knowledge of cavitation erosion is of practical importance and the dynamics is one of the most challenging problems in the field of two-phase flow. When a researche Collapsing stage of cloud cavity and resulting violent shock pressure have been investigated by many researchers, because the knowledge of cavitation erosion is of practical importance and the dynamics is one of the most challenging problems in the field of two-phase flow. When a researcher intends to investigate the phenomenon numerically or experimentally, he or she must know the structure of the cloud cavity. The authors investigate cloud cavity, employing an off-axis laser holography system. This paper is a brief report concerning the inner structure of cloud cavity on a foil section.

Advances in Hierarchical, Parallel Evolutionary Algorithms for Aerodynamic Shape Optimisation

Abstract: The paper presents the recent developments in Hierarchical Parallel Evolutionary Algorithms to speed up optimisation of aerodynamic shapes. One is the implementation of different models in different layers of a Parallel Genetic Algorithm. The other is Asynchronous Hierarchical Evolution Strategy. These methods are employed to reconstruct a one-dimensional transonic nozzle and a two-dimensional aerofoil shape. Considerable speed up is achieved as a result.

Tip Leakage Vortex Cavitation from the Tip Clearance of a Single Hydrofoil

Abstract: Focusing on the tip leakage vortex cavitation, experimental and numerical studies were carried out as the first step of the investigation of cavitations in tip leakage flow. For a single hydrofoil with a tip clearance, tip leakage vortex cavitations were observed for various cavitation numbers and angles of attack. To simulate the tip leakage vortex cavitation, a simple calculation of 2-D unsteady flow based on the slender body approximation with taking into account the effects of cavity growth (Watanabe et al., 2001) was made. The results of calculations show qualitative agreement with the experimental results with respect to the location and size of the cavity. The influences of the cavitation number, angle of attack, blade loading, and the size of tip clearance were simulated reasonably well.

Turbulent Burning Velocities of Two-Component Fuel Mixtures of Methane, Propane and Hydrogen

Abstract: In order to clarify the turbulent burning velocity of multi-component fuel mixtures, both lean and rich two-component fuel mixtures, in which methane, propane and hydrogen were used as fuels, were prepared while maintaining the laminar burning velocity approximately constant. A distinct difference in the measured turbulent burning velocity at the same turbulence intensity is observed for two-component fuel mixtures having different addition rates of fuel, even the laminar burning velocities are approximately the same. The burning velocities of lean mixtures change almost constantly as the rate of addition changes, whereas the burning velocities of the rich mixtures show no such tendency. This trend can be explained qualitatively based on the mean local burning velocity, which is estimated by taking into account the preferential diffusion effect for each fuel component. In addition, a model of turbulent burning velocity proposed for single-component fuel mixtures may be applied to two-component fuel mixtures by considering the estimated mean local burning velocity of each fuel.

Observations of oscillating cavitation on a flat plate hydrofoil

Abstract: An experimental investigation was made to clarify the characteristics of oscillating cavitation on a flat plate hydrofoil in a water tunnel. Dynamic the behavior of oscillating cavitation is discussed from the unsteady pressure measurements at the upstream of the blade and the visual observations of cavitation phenomena using high-speed video recording. It was found that the mean cavity length characterizes the fundamental characteristics of cavity oscillation. The cavity oscillations are categorized into two types, i.e. the transitional cavity oscillation and the partial cavity oscillation.

Study on Convective Mixing for Thermal Striping Phenomena. Experimental Analyses on Mixing Process in Parallel Triple-Jet and Comparisons between Numerical Methods.

Abstract: A quantitative evaluation on the thermal striping, in which temperature fluctuation due to convective mixing imposes thermal fatigue on structures, is of importance for reactor safety. As for convective mixing, a water experiment was performed on vertical, parallel triple-jet : a cold jet at the center and hot jets in both sides. Three kinds of calculations based on the finite difference method for the experiment were carried out. Two types of turbulence models were used in the calculations, namely k-e two-equation turbulence model (k-e Model) and low Reynolds number turbulence stress and heat flux equation models (LRSFM). Furthermore, a quasi-direct numerical simulation (DNS) was performed. The DNS could simulate the time-averaged temperature field. The prominent frequency in temperature fluctuation obtained by the LRSFM was in good agreement with that in the experiment. The profile of power spectrum density of temperature fluctuations calculated by the DNS was close to the experimental results.

Influence of Cross-sectional Configuration on the Synchronization of Kármán Vortex Shedding with the Cylinder Oscillation

Abstract: The influence of cross-sectional configuration of a cylindrical body on the lock-in phenomenon of Karman vortex shedding was investigated using a mechanical oscillator for cross-flow oscillation of the cylinder. A circular, a semi-circular and a triangular cylinder with an equal height were used as test cylinders to see the effect of the movement of the separation point. The lock-in criteria accounting for the spanwise coherency of the Karman vortex were discussed based on the experimental data under a fixed Reynolds number of around 3500. The lock-in region on the plane of non-dimensional cylinder frequency and the non-dimensional amplitude was almost the same for all of the cylinders in spite of differences in the range of the separation point movement. The minimum value of non-dimensional threshold amplitude for lock-in was much smaller than the value of 0.05 which was reported so far. Results obtained in this work imply that the movement of the separation point is a result of the lock-in phenomenon, rather than an essential cause.

Optical Measurement of the Deformation, Motion, and Generated Force of the Wings of a Moth, Mythimna Separata (Walker)

Abstract: Motion and deformation of the wings of a moth Mythimna Separata (Walker) tethered to a steel beam were measured by using an optical method that uses fringe pattern projection Simultaneously, the vertical force due to aerodynamic force generated by the wings was estimated by measuring the bending deformation of the beam during flapping motion of the moth The force was estimated by subtracting the vertical forces due to inertial and centrifugal forces acting on the wings, which were estimated from the measured flapping motion, from the measured vertical force Both the measured motion and deformation of the wing and the estimated vertical aerodynamic force generated by the wings suggest that the feathering angles are mainly affected by the inertial moment generated by the flapping motion and that the wing camber is mainly affected by the aerodynamic force generated by the wing

On the collapsing behavior of cavitation bubble clusters

Abstract: The present paper reports observation results of collapsing cavity bubbles on a two-dimensional foil section by a high-speed video camera, together with impulsive force measurement. Results of numerical simulations of the behavior of bubble cluster corresponding to the above condition are also shown. With these materials the authors discuss the mechanism of generation of the impulsive force due to cavitation collapse.

Thermal Performance Analysis of Small Hermetic Refrigeration and Air-Conditioning Compressors

Abstract: A simple physical model for small hermetic reciprocating, rotary and scroll compressors has been developed based on thermodynamic principles and large data sets from the compressor calorimeter and in situ tests. Pressure losses along the refrigerant path are neglected and a compression process is assumed isentropic. A mass flow rate model reflects clearance volumetric efficiency and simulates suction gas heating using an effectiveness method. Compressor work is calculated using the compressor efficiency represented by only two empirical parameters. A linear relationship between the discharge and shell temperatures is extracted from the large data sets and applied to the model for calculating the discharge temperature. Another experimental observation indicates that the specific volumes at the suction and discharge ports of the cylinder have linear relationships with the specific volumes at the compressor suction and discharge. Those relationships can be used for the compressor model and the possibility is reported. The models developed using physical principles and experimental observations can predict the mass flow rate and power consumption within ±3.0% accuracy.

A Basic Behavior of CNG DI Combustion in a Spark-Ignited Rapid Compression Machine

Abstract: A basic characteristics of compressed natural gas direct-injection (CNG DI) combustion was studied by using a rapid compression machine. Results show that comparing with homogeneous mixture, CNG DI has short combustion duration, high pressure rise due to combustion, and high rate of heat release, which are considered to come from the charge stratification and the gas flow generated by the fuel injection. CNG DI can realize extremely lean combustion which reaches 0.03 equivalence ratio, φ. Combustion duration, maximum pressure rise due to combustion and combustion efficiency are found to be insensitive to the injection modes. Unburned methane showed almost the same level as that of homogeneous mixture combustion. CO increased steeply with the increase in φ when φ was greater than 0.8 due to the excessive stratification, and NOx peak value shifted to the region of lower φ. Combustion inefficiency maintains less than 0.08 in the range of φ from 0.1 to 0.9 and increases at very low φ due to bulk quenching and at higher φ due to excessive stratification. The combustion efficiency estimated from combustion products shows good agreement with that of heat release analysis.

Magnetorheological Fluids: From Basic Physics to Application

Abstract: Magnetorheological (MR) fluids are suspensions of magnetizable micrometer particles in a liquid The rheological properties of the fluids can be changed rapidly, reversibly and repeatedly from that of a liquid to that of a solid when an external magnetic field is applied This tunability of the fluids comes from the microscopic structure of the particles Different microscopic structures are observed, quantified and correlated with rheological properties The yield stress and viscosity of MR fluids depends on the particle size and volume fraction, sample cell geometry, magnetic field strength and application rate The application of MR fluids in cancer therapy is discussed In-vitro experiments show the blood flow can be blocked leading to a tumor by injecting a dilute MR fluid in a blood vessel and placing a magnet on top of the tumor Without blood supply, tumor necrosis occurs shortly

A review of study on thermal energy transport system by synthesis and decomposition reactions of methanol

Abstract: The study on thermal energy transport system by synthesis and decomposition reactions of methanol was reviewed. To promote energy conservation and global environment protection, a two-step liquid-phase methanol synthesis process, which starts with carbonylation of methanol to methyl formate, then followed by the hydrogenolysis of the formate, was studied to recover wasted or unused discharged heat from industrial sources for the thermal energy demands of residential and commercial areas by chemical reactions. The research and development of the system were focused on the following three points. ( 1 ) Development of low-temperature decomposition and synthetic catalysts, ( 2 ) Development of liquid phase reactor (heat exchanger accompanying chemical reaction), ( 3 ) Simulation of the energy transport efficiency of entire system which contains heat recovery and supply sections. As the result of the development of catalyst, promising catalysts which agree with the development purposes for the methyl formate decomposition reaction and the synthetic reaction are being developed though some studies remain for the methanol decomposition and synthetic reactions. In the fundamental development of liquid phase reactor, the solubilities of CO and H 2 gases in methanol and methyl formate were measured by the method of total pressure decrease due to absorption under pressures up to 1 500 kPa and temperatures up to 140°C. The diffusivity of CO gas in methanol was determined by measuring the diameter and solution time of single CO bubbles in methanol. The chemical reaction rate of methanol synthesis by hydrogenolysis of methyl formate was measured using a plate-type of Raney copper catalyst in a reactor with rectangular channel and in an autoclave reactor. The reaction characteristics were investigated by carrying out the experiments at various temperatures, flow rates and at various catalyst development conditions. We focused on the effect of Raney copper catalyst thickness on the liquid-phase chemical reaction by varying the development time of the catalyst. Investigation results of the catalyst such as surface area, pore radius, lattice size, and photographs of scanning electron microscope (SEM) were also given. In the simulation of energy transport efficiency of this system, by simulating the energy transfer system using two-step liquid phase methanol decomposition and synthetic reactions, and comparing with the technology so far, it can be expected that an innovative energy transfer system is possible to realize.

Slip Flow over a Smooth Platinum Surface

Abstract: The molecular dynamics method for the interaction of gas molecules with a solid wall together with the Monte-Carlo method for the motion of gas molecules are applied to analyze the behavior of a slightly rarefied gas between two walls. The walls consist of platinum molecules, and the gas is taken to be xenon or argon. The Couette flow and the thermal problem for which two walls have different temperatures are considered. The slip and jump coefficients, tangential momentum and energy accommodation coefficients are obtained at the wall whose temperature is 300K. The tangential momentum accommodation coefficient of argon is as low as 0.19. The distribution functions of the reflected molecules are also obtained, and it is found that the Maxwell-type boundary condition describes well the distribution function of the reflected molecules.

Development of Mitsubishi High Thermal Performance Grid

Abstract: Mitsubishi has developed a new zircalloy grid spacer for PWR fuel with higher thermal performance. Computational Fluid Dynamics (CFD) evaluation method has been applied for designing of the new lower pressure loss and higher Departure from Nucleate Boiling (DNB) benefit grid spacer. Reduction of pressure loss of grid structures has been examined by CFD. Also, CFD has been developed as a design tool to predict the coolant mixing ability of vane structures, which is to compare the relative peak spot temperatures around fuel rods at the same heat flux condition. Prototype grids were manufactured and several tests, which were pressure loss measurements, cross-flow measurements and freon DNB tests, were conducted to verify CFD predictions. It is concluded that the applicability of the CFD evaluation method for the thermal hydraulic design of the grid is confirmed.

Conjugated heat transfer on a horizontal surface impinged by circular free-surface liquid jet

Abstract: Analytical research was conducted to study the heat transfer from horizontal surfaces to normally impinging circular free-surface jets under arbitrary-heat-flux conditions. General expressions of heat transfer coefficients and recovery factor were obtained in the stagnation region and boundary layer region as well as viscous similarity region by the integral method. Then by using the analytical results, the conjugated heat transfer between a water jet and the impinged wall was numerically solved. Finally, an experimental study was also performed to characterize conjugated heat transfer coefficient on a thick copper target base impinged normally by circular free-surface jet. The present predictions of the mean conjugated heat transfer coefficient were good agreement with the experimental data.

Transient Heat Transfer for Forced Convection Flow of Helium Gas

Abstract: The knowledge of forced convection transient heat transfer at various periods of exponential increase of heat input to a heater is important as a database for understanding the transient heat transfer process in a high temperature gas cooled reactor (HTGR) due to an accident in excess reactivity. The transient heat transfer coefficients for forced convection flow of helium gas over a horizontal cylinder were measured using a forced convection test loop. The platinum heater with a diameter of 1.0 mm was heated by electric current with an exponential increase of Q 0 exp(t/r)

Growth-Morphologies in Solidification and Hydrodynamics

Abstract: The evolution of interfacial patterns during growth processes is shortly summarized. The analogy between hydrodynamic dewetting patterns and diffusion controlled growth patterns is demonstrated. A phase-field method for the treatment of hydrodynamic flow with free interfaces is presented and illustrated with some examples.

Numerical study of the effect of the leading edge shape on cavitation around inducer blade sections

Abstract: A numerical study of the cavitation behaviour of two-dimensional hydrofoils simulating a section of an inducer blade is presented Two leading edge shapes were chosen to approach rocket engine inducer designs They was tested with respect to the development of sheet cavitation The numerical model of cavitating flows is based on the 3D code FINE/TURBOTM, developed by NUMECA International The cavitation process is taken into account by using a single fluid model, which considers the liquid vapour mixture as a homogeneous fluid whose density varies with respect to the static pressure Numerical results are compared with experimental ones, obtained in the CREMHyG large cavitation tunnel [Reboud et al 1996] Pressure distributions along the foil suction side and the tunnel walls were measured for different cavity lengths Total pressure measurements along the foil suction side allow characterizing the effects of cavitation on the liquid flow Influence of the leading edge shape on the cavitation behaviour and comparison between experiments and numerical predictions are discussed

Prediction of Bubble Behavior in Subcooled Pool Boiling Based on Microlayer Model

Abstract: Bubble behaviors of subcooled nucleate pool boiling are investigated both experimentally and theoretically. The analytical study based oil the dynamic microlayer model predicts that one cycle of an individual bubble experiences four stages, i.e., the initial growth with a semi-spherical shape, the final growth with a spherical segment geometry, the condensation process and the waiting time. Also, vapor bubbles on the Pt wire are experimentally observed by a high-speed camera at the speed of 10 000 frames/sec in the nucleate pool boiling of subcooled water. The predicted four stages of an individual bubble are clarified experimentally. Relatively good agreement is shown between the present predictions of the total periods of individual bubbles and experimental data.

A Rotating Stall Type Phenomenon Caused by Cavitation in LE-7A LH2 Turbopump

Abstract: The cause of 350 Hz large-amplitude rotor vibration of the H-2A rocket's LE-7A fuel turbopump was investigated by wideband oscillating-pressure measurement. Measurement was successfully conducted by quartz-type pressure sensors in full-load liquid-hydrogen tests. The phase difference of pressure oscillations between two pressure measurement ports under conditions of pump cavitating operation revealed a phenomenon similar to rotating stall in turbomachinery which had not been previously observed. The rotating speed of a cell was 350 Hz, about a half the rotor speed. This phenomenon is different from rotating cavitation in which the cavitation pattern rotates around the periphery of an impeller faster than shaft rotating speed. Based on these findings, it was concluded that this phenomenon cased the large-amplitude 350 Hz vibration of the LE-7A fuel turbopump. Results of FFT analysis of measured oscillating pressure are presented.

Flow Measurement in Helical Inducer and Estimate of Fluctuating Blade Force in Cavitation Surge Phenomena

Abstract: An attachment of inducer is a powerful method to improve the cavitation performance of a turbo-pump. Cavitation surge phenomena, occurring under the severe suction pressure at a partial flow rate, is focused in the present paper. Flow measurements were carried out at the inlet and outlet sections of a flat-plate helical inducer with the solidity of 2.0 and tip blade angle of 11° by using a total-head yaw-meter with a phase locked sampling method in one period of the shaft rotation or the cavitation oscillation. Time variation of the flow distributions during the oscillation is clarified with observed cavitation behaviors. After these results are compared with those in conditions just before and after the oscillations, the fluctuating blade forces are estimated from the blade to blade distributions of casing wall pressures measured in these conditions.

Two-dimensional analysis for non-equilibrium homogeneously condensing flows through steam turbine cascade

Abstract: A computational technique for the two-dimensional non-equilibrium homogeneously condensing flows through steam turbine cascades is presented. The fundamental equations of compressible wet steam flows are descretized based on the TVD scheme and following thermodynamic assumptions. 1) The gas phase is an ideal gas. 2) A liquid phase consists of droplets whose radii are in the order of 10-6m or less, thereby, the wet steam is a homogeneous fluid. 3) A gas-liquid phases are changed by the homogeneous nucleation and growth of the existing droplets, which are described by the classical nucleation theory. In this theory, a driving force of a phase change is a degree of supercooling. The described assumptions are valid for the flow through the low-pressure stage of the steam turbines. Flow through the moving blades for a low-pressure steam turbine is calculated. The calculated pressure distributions on the blade surface agree well with experimental data. The influences of both the degree of inlet supercooling and the ratio of the inlet total pressure to outlet static pressure on the blade performance, mass flow, flow angle, energy loss are studied.

Flow around a Rotating Circular Cylinder near a Moving Plane Wall

Abstract: The flow around a rotating circular cylinder, which is placed near a moving plane wall, is investigated experimentally for high Reynolds number (Re=33000). The cylinder spanned the test section of a wind tunnel and was aligned with its axis parallel to the under wall of the test section and normal to a uniform flow. The wall of the test section under the cylinder was moved at the same speed as the uniform flow. Surface pressure measurements, velocity surveys and flow visualizations suggest the existence of three flow patterns in the near wake behind the cylinder by the change of the gap between the cylinder and the moving wall. The wake from the cylinder is characterized by the behavior of a jet passing through the gap. Furthermore, the behavior of the jet is sensitive to both the centrifugal force generated by the rotating cylinder and the inertial force of the jet.

Emission Characteristics of NOx and Unburned Carbon in Fly Ash of Sub-bituminous Coal Combustion

Abstract: Sub-bituminous coal is considered to be one substitute fuel for bituminous coal. Since sub-bituminous coal contains more than 20% moisture, there are some problems with its utilization, such as a decrease in combustibility, high NO x emission and so on. This report describes the emission characteristics of NO x and unburned carbon in fly ash of sub-bituminous coal combustion through the use of a pulverized coal combustion test furnace. On the sub-bituminous coal combustion, ignition at the burner exit worsened and the combustion flame became diffused. Then, both NO x emission and the unburned carbon concentration in fly ash became high. In order to keep stable combustion and to form an effective NO x reduction flame, the swirl vane angle of secondary air was reduced and the Air/Coal ratio was lowered. As a result, the combustion flame became moderate, and both NO x emission and unburned carbon concentration in fly ash could be reduced.

Verification and Validation in Computational Fluid Dynamics: the FLOWNET Database Experience

Abstract: The aspects of verification and validation of computational fluid dynamics must always be addressed with an emphasis on the quantification of the uncertainties due to the model assumptions (either physical or geometrical), and to the numerical and experimental approximations. The credibility of computational fluid dynamics can only be established by a rigorous process of verification and validation. Verification is the process of determining how accurate a computational simulation represents a given conceptual model. Validation establishes how accurate a simulation (i.e. the conceptual model) represents the phenomenon to be investigated. In the present paper we discuss a roadmap toward the development of a verification and validation procedure through the use of a flow library on the Web. In particular, we describe the experience of the web-based FLOWnet database and we analyze some representative test cases installed in the data base

Multiblock Navier-Stokes Solver for Wing/Fuselage Transport Aircraft

Abstract: An implicit multiblock Navier-Stokes solver, which contains the LU-SGS subiteration method and the HLLEW scheme, has been developed for numerical simulations on complex and realistic aerodynamic configurations. Two-level halo cells are used to communicate data between abutting blocks. The transfinite interpolation (TFI) and the elliptic method with boundary control are employed to generate the initial multiblock grid and to smooth the grid distribution in each block. A comparison is first done for the AGARD supercritical LANN-wing with single- and multi-block grids. Then the present method is applied to a NASA transport wing/fuselage configuration and the NAL supersonic transport (SST) model. The choice of multiblock grid topology from the view of aeroelastic calculation and the comparison of Euler and Navier-Stokes solutions are investigated