bluuebunny/crossref_metadata_2025 huggingface dataset:
Passive Control of Combustion Instability in Lean Premixed Combustors
Authors: Steele Robert C. , Cowell Luke H. , Cannon Steven M. , Smith Clifford E.
Published: July 2002

A Solar fuel injector that provides lean premixed combustion conditions has been studied in a combined experimental and numerical investigation. Lean premixed conditions can be accompanied by excessive combustion driven pressure oscillations which must be eliminated before the release of a final combustor design. In order to eliminate the pressure oscillations the location of fuel injection was parametrically evaluated to determine a stable configuration. It was observed that small axial changes in the position of the fuel spokes within the premix duct of the fuel injector had a significant positive effect on decoupling the excitation of the natural acoustic modes of the combustion system. In order to further understand the phenomenon, a time-accurate 2D CFD analysis was performed. 2D analysis was first calibrated using 3D steady-state CFD computations of the premixer in order to model the radial distribution of velocities in the premixer caused by non-uniform inlet conditions and swirling flow. 2D time-accurate calculations were then performed on the baseline configuration. The calculations captured the coupling of heat release with the combustor acoustics, which resulted in excessive pressure oscillations. When the axial location of the fuel injection was moved, the CFD analysis accurately captured the fuel time lag to the flame-front, and qualitatively matched the experimental findings. [S0742-4795(00)01103-0]
Detailed Experimental Studies of Flow in Large Scale Brush Seal Model and a Comparison With CFD Predictions
Authors: Chen L. H. , Wood P. E. , Jones T. V. , Chew J. W.
Published: July 2002

A five times scale model of an engine brush seal has been manufactured. The bristle stiffness and pressure were chosen to satisfy close similarity of the relevant non-dimensional parameters, and the choice of parameters is described. The comparison of flow characteristics for the model seal and an engine seal confirmed the non-dimensional similarity. Detailed pressure measurements were performed within the bristle pack by employing hollow bristles. This novel measurement allowed insight to be obtained into the operation of both clearance and interference seals. In particular, the measured pressure variation in the region of the bristle tips was significant. The deflection of the bristles was determined by comparing the bristle tip pressures with the static pressures along the shaft. Hence the compaction of the pack in this region was found directly. A numerical modeling of brush seals employing anisotropic flow resistance has been developed. Predictions were compared with the measured pressure distributions within the pack. This enabled sensible selection of the pack resistance distribution to be made. Although uniform anisotropic resistance throughout the pack gave reasonable flow rate characteristics, the pressure distribution was not reproduced. A variation of resistance coefficient consistent with the observed compaction was required to give a solution comparable with the experiments. [S0742-4795(00)01703-8]
Synthesis and Optimization for Rotor Profiles in Twin Rotor Screw Compressors
Authors: Su Shyh-Haur , Tseng Ching-Huan
Published: July 2002

This paper proposes a systematic synthesis method for twin-screw rotor profiles for compressors. Both “original generating curves” and “generated curves” are distributed on each rotor profile, and all the geometric parameters of these curves can be determined with satisfying the conditions of continuity in tangency by given only several specific parameter values. The contact lines on rotor surfaces and the blowhole area calculation are also presented. Three cases of optimization problems are shown in this paper and both contact-line length and blowhole area are reduced when letting the contact-line length be the object function with a constraint of blowhole area. [S1050-0472(00)01103-X]
Finite Element Analysis of Stress Singularities in Attached Flip Chip Packages
Authors: Xu A. Q. , Nied H. F.
Published: July 2002

Cracking and delamination at the interfaces of different materials in plastic IC packages is a well-known failure mechanism. The investigation of local stress behavior, including characterization of stress singularities, is an important problem in predicting and preventing crack initiation and propagation. In this study, a three-dimensional finite element procedure is used to compute the strength of stress singularities at various three-dimensional corners in a typical Flip-Chip assembled Chip-on-Board (FCOB) package. It is found that the stress singularities at the three-dimensional corners are always more severe than those at the corresponding two-dimensional edges, which suggests that they are more likely to be the potential delamination sites. Furthermore, it is demonstrated that the stress singularity at the upper silicon die/epoxy fillet edge can be completely eliminated by an appropriate choice in geometry. A weak stress singularity at the FR4 board/epoxy edge is shown to exist, with a stronger singularity located at the internal die/epoxy corner. The influence of the epoxy contact angle and the FR4 glass fiber orientation on stress state is also investigated. A general result is that the strength of the stress singularity increases with increased epoxy contact angle. In addition, it is shown that the stress singularity effect can be minimized by choosing an appropriate orientation between the glass fiber in the FR4 board and the silicon die. Based on these results, several guidelines for minimizing edge stresses in IC packages are presented. [S1043-7398(00)00904-X]
1999 Turbomachinery Committee Best Paper Award: Development of Advanced Compressor Airfoils for Heavy-Duty Gas Turbines— Part I: Design and Optimization
Authors: Ko¨ller Ulf , Mo¨nig Reinhard , Ku¨sters Bernhard , Schreiber Heinz-Adolf
Published: July 2002

A new family of subsonic compressor airfoils, which are characterized by low losses and wide operating ranges, has been designed for use in heavy-duty gas turbines. In particular the influence of the higher airfoil Reynolds numbers compared to aeroengine compressors and the impact of these differences on the location of transition are taken into account. The design process itself is carried out by the combination of a geometric code for the airfoil description, with a blade-to-blade solver and a numerical optimization algorithm. The optimization process includes the design-point losses for a specified Q3D flow problem and the off-design performance for the entire operating range. The family covers a wide range of inlet flow angle, Mach number, flow turning, blade thickness, solidity and AVDR in order to consider the entire range of flow conditions that occur in practical compressor design. The superior performance of the new airfoil family is demonstrated by a comparison with conventional controlled diffusion airfoils (CDA). The advantage in performance has been confirmed by detailed experimental investigations, which will be presented in Part II of the paper. This leads to the conclusion that CDA airfoils that have been primarily developed for aeroengine applications are not the optimum solution, if directly transferred to heavy-duty gas turbines. A significant improvement in compressor efficiency is possible, if the new profiles are used instead of conventional airfoils. [S0889-504X(00)02102-4]
Analysis of Sandwich Plates with Isotropic Face Plates and a Viscoelastic Core
Authors: Wang Gang , Veeramani Sudha , Wereley Norman M.
Published: July 2002

Experimental and analytical validations of a Galerkin assumed modes analysis of sandwich plates are presented in this paper. The 3-layered sandwich plate specimen consists of isotropic face-plates with surface bonded piezo-electric patch actuators, and a viscoelastic core. The experimental validation is conducted by testing sandwich plates that are 67.31 cm (26.5 in.) long, 52.07 cm (20.5 in.) wide and nominally 0.16 cm (1/16 in.) thick. The analysis includes the membrane and transverse energies in the face plates, and shear energies in the core. The shear modulus of the dissipative core is assumed to be complex and variant with frequency and temperature. The Golla-Hughes-McTavish (GHM) method is used to account for the frequency dependent properties of the viscoelastic core. Experiments were conducted on symmetric and asymmetric sandwich plates with aluminum face-plates under clamped boundary conditions to validate the model for isotropic face-plates. The maximum error in damped natural frequency predictions obtained via the assumed modes solution is <11 percent. Analytical studies on the influence of the number of assumed modes in the Galerkin approximation have been conducted. Error in the first plate bending mode is 112 percent when only a single in-plane mode is used; error reduces to 3.95 percent as the number of in-plane modes is increased to 25 in each of the in-plane directions. [S0739-3717(00)00703-0]
Flowfield Measurements in the Endwall Region of a Stator Vane
Authors: Kang M. B. , Thole K. A.
Published: July 2002

A first-stage stator vane experiences high heat transfer rates, particularly near the endwall, where strong secondary flows occur. In order to improve numerical predictions of the complex endwall flow at low-speed conditions, benchmark quality experimental data are required. This study documents the flowfield in the endwall region of a stator vane that has been scaled up by a factor of nine while matching an engine exit Reynolds number of Reex=1.2×106. Laser Doppler velocimeter (LDV) measurements of all three components of the mean and fluctuating velocities are presented for several flow planes normal to the turbine vane. Measurements indicate that downstream of the minimum static pressure location on the suction surface of the vane, an attenuated suction side leg of the horseshoe vortex still exists. At this location, the peak turbulent kinetic energy coincides with the center of the passage vortex location. These flowfield measurements were also related to previously reported convective heat transfer coefficients on the endwall showing that high Stanton numbers occur where the passage vortex brings mainstream fluid toward the vane surface. [S0889-504X(00)00803-5]
The Influence of “Shell Behavior” on Load Distribution for Thin-Walled Conical Joints
Authors: Bruschelli L. , Latorrata V.
Published: July 2002

This article presents a new analytical method with a numerical solution to calculate load distribution in threaded connections. Our departure model was that suggested by D. G. Sopwith who has proposed the most recent and most tested theory. Our research consists in the introduction of conicity and, above all, in the development of the influence of boundary geometry (i.e. the nonthreaded section) on load distribution. Pipe joints are analyzed in special detail, supplying us with useful finite element method comparative results. [S0021-8936(00)02002-X]
Strength Analysis of Spherical Indentation of Piezoelectric Materials
Authors: Giannakopoulos A. E.
Published: July 2002

The present paper deals with theoretical and computational analysis of quasi-static, normal indentation of a transversely isotropic, linear elastic, piezoelectric half-space by a rigid spherical indenter. The contact is axisymmetric, nonconforming, monotonically advancing with load, frictionless and adhesionless. The indenter was modeled either as perfect conductor or as perfect insulator. The mechanical and electrical fields below the surface were examined. The issues of mechanical and dielectric strength due to indentation were examined using Weibull statistics of surface imperfections. The particular cases of PZT-4, PZT-5A, BaTiO3, and Ba0.917Ca0.083TiO3 indented by rigid punches having either zero electrical potential or zero electric charge were solved with finite element analysis. [S0021-8936(00)02502-2]
A Numerical Three-Dimensional Model for the Contact of Layered Elastic/Plastic Solids With Rough Surfaces by a Variational Principle
Authors: Peng Wei , Bhushan Bharat
Published: July 2002

A new numerical model for the three-dimensional contact analysis of a layered elastic–perfectly plastic half space with another rough surface is presented. The model is based on a variational principle in which the real area of contact and contact pressure distribution are those which minimize the total complementary potential energy. A quasi-Newton method is used to find the minimum. The influence coefficients matrix is determined using the Papkovich–Neuber potentials with fast Fourier transformation. The model is extended to elastic–perfectly plastic contacts in dry and wet conditions. Contact analyses have been performed to predict contact statistics of layered elastic/plastic solids with rough surfaces using this model. The effects of the stiffness of the layer and the substrate, layer thickness, as well as normal load are studied. Optimum layer parameters are identified to provide low friction/stiction and wear.
Elastohydrodynamic Lubrication Between Rolling and Sliding Ceramic Cylinders: An Experimental Investigation1
Authors: Sperrfechter T. , Haller R.
Published: July 2002

The present work focuses on the investigation of the influence of bulk ceramic materials on the behavior of elastohydrodynamically (EHD) lubricated line contacts. The materials alumina Al2O3, zirconium oxide ZrO2 and aluminum nitride (AIN) are used. Comparative measurements were taken with steel disks made of 42CrMo4. Of primary importance are the material parameters Young’s modulus and thermal conductivity. The experimental variables pressure, temperature and oil film thickness in the EHD contact of a two disk test rig were measured with the aid of evaporated thin film sensors. As the results show, an increase in the Young’s modulus causes a clear increase of the pressure level. The oil film thickness distributions show a decline of the flattening width and of the constriction occurring at the contact outlet. The influence of the material with respect to its thermal conductivity dominates, above all, in the region of the load transmitting contact zone. The transition from a good to a bad conductor of heat causes a rise in temperature, more prominent for materials with lower thermal conductivities. This leads to viscosity decrease causing clearly reduced oil film thicknesses in the contact. [S0742-4787(00)01404-1]
Tribology of Flexible Seals for Reciprocating Motion
Authors: Kaneta M. , Todoroki H. , Nishikawa H. , Kanzaki Y. , Kawahara Y.
Published: July 2002

The dynamic behavior of oil films formed between a band-shaped nitrile rubber specimen with a D-shaped or lip-shaped cross section and an oscillating glass plate is directly observed by means of the mono-chromatic optical interferometry technique. The friction force is also measured. The effects of the cross sectional shape of the seal and the length of the stroke on the film formation and friction characteristics are discussed together with the effects of surface defects. On the basis of analyses of experimental data, the conditions which bring about the film collapse are clarified. It is also claimed that the distinction of lubrication modes by the Stribeck curve might be reconsidered. [S0742-4787(00)02304-3]
A Numerical Study of Secondary Flow in Axial Turbines With Application to Radial Transport of Hot Streaks
Authors: Prasad Dilip , Hendricks Gavin J.
Published: July 2002

The flow field in a low-speed turbine stage with a uniform inlet total pressure is studied numerically. A circular hot streak is superposed on the vane inlet flow. In agreement with previous experimental and numerical work, it is observed that while the streak passes through the vane unaltered, significant radial transport occurs in the rotor. Furthermore, despite the unsteady nature of the flow field, the steady theory of Hawthorne (1974) is found to predict the radial transport velocity well. Making use of this theory, it is shown that the secondary vorticity in the rotor may be attributed to the effects of density stratification, the spatial variation of the vane exit flow angle, and the relative eddy. It then follows that the extent of radial transport in the rotor may be influenced by altering the vane exit flow angle distribution. The present study examines one means by which this may be effected, viz., varying the vane twist across the span. It is shown that a “reverse” twist, wherein the flow angle at the vane exit is larger near the tip than it is at midspan, reduces the secondary flow (and consequently, radial transport) in the blade passage. On the other hand, “positive” twist, in which the vane exit flow angle decreases with span, is found to worsen the radial transport in the blade markedly. It is to be noted that varying the vane twist is but one method to obtain the desired exit flow angle; possibilities for altering other aspects of the vane geometry also exist. [S0889-504X(00)00104-5]
Geometric and Mechanical Properties of Human Cervical Spine Ligaments
Authors: Yoganandan Narayan , Kumaresan Srirangam , Pintar Frank A.
Published: July 2002

This study characterized the geometry and mechanical properties of the cervical ligaments from C2–T1 levels. The lengths and cross-sectional areas of the anterior longitudinal ligament, posterior longitudinal ligament, joint capsules, ligamentum flavum, and interspinous ligament were determined from eight human cadavers using cryomicrotomy images. The geometry was defined based on spinal anatomy and its potential use in complex mathematical models. The biomechanical force-deflection, stiffness, energy, stress, and strain data were obtained from 25 cadavers using in situ axial tensile tests. Data were grouped into middle (C2–C5) and lower (C5–T1) cervical levels. Both the geometric length and area of cross section, and the biomechanical properties including the stiffness, stress, strain, energy, and Young’s modulus, were presented for each of the five ligaments. In both groups, joint capsules and ligamentum flavum exhibited the highest cross-sectional area p<0.005, while the longitudinal ligaments had the highest length measurements. Although not reaching statistical significance, for all ligaments, cross-sectional areas were higher in the C5–T1 than in the C2–C5 group; and lengths were higher in the C2–C5 than in the C5–T1 group with the exception of the flavum (Table 1 in the main text). Force-deflection characteristics (plots) are provided for all ligaments in both groups. Failure strains were higher for the ligaments of the posterior (interspinous ligament, joint capsules, and ligamentum flavum) than the anterior complex (anterior and posterior longitudinal ligaments) in both groups. In contrast, the failure stress and Young’s modulus were higher for the anterior and posterior longitudinal ligaments compared to the ligaments of the posterior complex in the two groups. However, similar tendencies in the structural responses (stiffness, energy) were not found in both groups. Researchers attempting to incorporate these data into stress-analysis models can choose the specific parameter(s) based on the complexity of the model used to study the biomechanical behavior of the human cervical spine. [S0148-0731(00)01006-2]
Effect of Tool Wear on Force and Quality in Dam-Bar Cutting of Integrated Circuit Packages
Authors: Cheung C. F. , Lee W. B. , Chiu W. M.
Published: July 2002

Dam-bar cutting is an essential trimming process in which the dam-bars in between the leads of Integrated Circuit (IC) packages are removed after encapsulation and deflashing. There are stringent requirements imposed on the quality of the sheared dam-bar edges so as to avoid the post-process failure of the package during assembly on to printed circuit boards. In this paper, a detailed analysis of the wear characteristics of the dam-bar cutting tools and their effects on the force and quality of dam-bar cutting process is reported. The correlation between the peak dam-bar cutting force and the tool wear was also studied under various combinations of wear states of the punch and die. Based on the experimental findings, relationships have been built to correlate the states of wear of the punch and die to the protrusion and burr height of the dam-bar sheared edge. Hence, revised tool life criteria were proposed for the quality control of dam-bar cutting process. The results of the analysis provide an important means for the on-line monitoring of tool wear and edge quality of dam-bar cutting process in the IC packaging industry.
Formation of Steady Dimples in Point TEHL Contacts
Authors: Yang P. , Qu S. , Kaneta M. , Nishikawa H.
Published: July 2002

Experimental results of steady dimples measured in elliptical glass-steel contact under pure sliding conditions are presented. It is found that two dimples connected with a shallower furrow are generated, each near an end of the major radius of the contact ellipse. The complete solution of the corresponding thermal elastohydrodynamic lubrication (TEHL) problem is calculated numerically. Good agreement is obtained between the experimental and theoretical results. This agreement can be explained by the temperature-viscosity wedge mechanism. Correctness of this mechanism is demonstrated using additional experiments with ceramic balls in contact with glass and sapphire disks.
Geometric Design of Linkages
Authors: McCarthy, J. Michael
Published: July 2002

[S1050-0472(00)03204-9]
Identification of Mistuning Characteristics of Bladed Disks From Free Response Data—Part I
Authors: Mignolet M. P. , Rivas-Guerra A. J. , Delor J. P.
Published: July 2002

The focus of the present two-part investigation is on the estimation of the dynamic properties, i.e., masses, stiffnesses, natural frequencies, mode shapes and their statistical distributions, of turbomachine blades to be used in the accurate prediction of the forced response of mistuned bladed disks. As input to this process, it is assumed that the lowest natural frequencies of the blades alone have been experimentally measured, for example, in a broach block test. Since the number of measurements is always less than the number of unknowns, this problem is indeterminate in nature. In this first part of the investigation, two distinct approaches will be investigated to resolve the shortfall of data. The first one relies on the imposition of as many constraints as needed to ensure a unique solution to this identification problem. Specifically, the mode shapes and modal masses of the blades are set to their design/tuned counterparts while the modal stiffnesses are varied from blade to blade to match the measured natural frequencies. The second approach, based on the maximum likelihood principle, yields estimates of all the structural parameters of the blades through the minimization of a specified “cost function.” The accuracy of these two techniques in predicting the forced response of mistuned bladed disks will be assessed on simple dynamic models of the blades.
Analysis and Modeling of the Fluid-Dynamic Effects in Branched Exhaust Junctions of ICE
Authors: Payri F. , Reyes E. , Galindo J.
Published: July 2002

The influence of exhaust junction geometry on flow-dynamics of exhaust gas is analyzed. The authors propose an experimental characterization method based on the measurement of the instantaneous pressure in the junction operating with engine exhaust flow and solving the problems posed for the accurate instantaneous pressure measurements under the adverse temperature condition. In this paper, the method is applied to two “Y” type junctions, with a reed being the unique difference between them, to determine the influence of this element on the junction behavior. The analysis of the experimental results denotes two major differences: the characteristics of the wave reflected at the junction, and the energy of the pulse transmitted to the lateral branch of the junction. The results of the analysis are introduced in the junction modeling used in a one-dimensional gas dynamic model with an important improvement in the agreement of the modeled predictions with the experimental measurements.
An Experimental Investigation of the Effects of Common-Rail Injection System Parameters on Emissions and Performance in a High-Speed Direct-Injection Diesel Engine
Authors: Tennison P. J. , Reitz R.
Published: July 2002

An investigation of the effect of injection parameters on emissions and performance in an automotive diesel engine was conducted. A high-pressure common-rail injection system was used with a dual-guided valve covered orifice nozzle tip. The engine was a four-valve single cylinder high-speed direct-injection diesel engine with a displacement of approximately 12 liter and simulated turbocharging. The engine experiments were conducted at full load and 1004 and 1757 rev/min, and the effects of injection pressure, multiple injections (single vs pilot with main), and pilot injection timing on emissions and performance were studied. Increasing the injection pressure from 600 to 800 bar reduced the smoke emissions by over 50 percent at retarded injection timings with no penalty in oxides of nitrogen NOx or brake specific fuel consumption (BSFC). Pilot injection cases exhibited slightly higher smoke levels than single injection cases but had similar NOx levels, while the single injection cases exhibited slightly better BSFC. The start-of-injection (SOI) of the pilot was varied while holding the main SOI constant and the effect on emissions was found to be small compared to changes resulting from varying the main injection timing. Interestingly, the point of autoignition of the pilot was found to occur at a nearly constant crank angle regardless of pilot injection timing (for early injection timings) indicating that the ignition delay of the pilot is a chemical delay and not a physical (mixing) one. As the pilot timing was advanced the mixture became overmixed, and an increase of over 50 percent in the unburned hydrocarbon emissions was observed at the most advanced pilot injection timing.
Steady Performance Measurements of a Turbofan Engine With Inlet Distortions Containing Co- and Counterrotating Swirl From an Intake Diffuser for Hypersonic Flight
Authors: Schmid Norbert R. , Leinhos Dirk C. , Fottner Leonhard
Published: July 2002

The influence of distorted inlet flow on the steady and unsteady performance of a turbofan engine, which is a component of an air-breathing combined propulsion system for a hypersonic transport aircraft, is reported in this paper. The performance and stability of this propulsion system depend on the behavior of the turbofan engine. The complex shape of the intake duct causes inhomogeneous flow at the engine inlet plane, where total pressure and swirl distortions are present. The S-bend intakes are installed axisymmetrically left and right into the hypersonic aircraft, generating axisymmetric mirror-inverted flow patterns. Since all turbo engines of the propulsion system have the same direction of rotation, one distortion corresponds to a corotating swirl at the low pressure compressor (LPC) inlet while the mirror-inverted image counterpart represents a counterrotating swirl. Therefore the influence of the distortions on the performance and stability of the ‘CO’ and ‘COUNTER’ rotating turbo engine are different. The distortions were generated separately by an appropriate simulator at the inlet plane of a LARZAC 04 engine. The results of low-frequency measurements at different engine planes yield the relative variations of thrust and specific fuel consumption and hence the steady engine performance. High-frequency measurements were used to investigate the different influence of CO and COUNTER inlet distortions on the development of LPC instabilities.
Influence of Crossflow-Induced Swirl and Impingement on Heat Transfer in a Two-Pass Channel Connected by Two Rows of Holes
Authors: Pamula Gautam , Ekkad Srinath V. , Acharya Sumanta
Published: July 2002

Detailed heat transfer distributions are presented inside a two-pass coolant square channel connected by two rows of holes on the divider walls. The enhanced cooling is achieved by a combination of impingement and crossflow-induced swirl. Three configurations are examined where the crossflow is generated from one coolant passage to the adjoining coolant passage through a series of straight and angled holes and a two-dimensional slot placed along the dividing wall. The holes/slots deliver the flow from one passage to another. This is typically achieved in a conventional design by a 180 deg U-bend. Heat transfer distributions will be presented on the sidewalls of the passages. A transient liquid crystal technique is applied to measure the detailed heat transfer coefficient distributions inside the passages. Results for the three-hole supply cases are compared with the results from the traditional 180 deg turn passage for three channel flow Reynolds numbers ranging between 10,000 and 50,000. Results show that the new feed system, from first pass to second pass using crossflow injection holes, produces significantly higher Nusselt numbers on the second pass walls. The heat transfer enhancements in the second pass of these channels are as much as two to three times greater than that obtained in the second pass for a channel with a 180 deg turn. Results are also compared with channels that have only one row of discharge holes.
Characterization of Laser Cleaning of Copper for Soldering Processes
Authors: Lee J. M. , Watkins K. G. , Steen W. M.
Published: July 2002

The laser cleaning of copper surfaces with a Nd:YAG Q-switched laser pulse for the improvement of solder quality on printed circuit boards (PCBs) has been monitored and characterized by sensing the acoustic emission during the process and one-dimensional mathematical model analysis. It was found that selective removal of copper oxides from the surface was achieved by the laser operation, which was described theoretically by the model and was confirmed experimentally by the acoustic monitoring. The acoustic monitoring provided not only threshold laser fluence and optimal process window for the cleaning of copper but also the clear possibility for real-time surface monitoring of the process. Different features at the two laser wavelengths used (1064 nm and 532 nm) were observed in the surface morphology, i.e., the laser pulse with 532 nm wavelength produced a lightly sputtered wide area around the laser crater. From the theoretical and experimental investigation of laser cleaning mechanisms at different wavelengths, it was found that the mechanical effect induced by the intense shock waves was much more dominant at 532 nm wavelength than at 1064 nm.
On Crack Initiation Mechanisms in Fretting Fatigue
Authors: Yang B. , Mall S.
Published: July 2002

By using the crack analogue model of rigid flat-ended contact, crack initiation in fretting fatigue is analyzed. The coefficient of friction at the edge of contact, which characterizes the asymptotic stress field, is considered as the primary controlling parameter in the process. Meanwhile, the maximum tangential stress criterion and the maximum shear stress criterion are used to predict opening-mode and shear-mode crack initiations, respectively. By examining the model prediction and comparing it with experimental observations, it is shown that the observed microcracks at the small angles to a fretting surface were nucleated in shear mode in the early stage of tests with a smooth initial surface, while the microcracks at the large angles were nucleated in opening mode in the later stage with a rough worn surface. This understanding may help to establish the sequential damage mechanisms in the complex process of fretting fatigue.
Unsteady Flow Pattern Characteristics Downstream of a Forward-Curved Blades Centrifugal Fan
Authors: Velarde-Sua´rez Sandra , Ballesteros-Tajadura Rafael , Santolaria-Morros Carlos , Gonza´lez-Pe´rez Jose´
Published: July 2002

The results of an experimental investigation of the flow at two exit radial locations of a forward-curved blades centrifugal fan are presented. Hot wire techniques were used to obtain steady velocity components and velocity unsteadiness levels (rms value of the components of velocity fluctuation) for different operating conditions. Globally speaking, the data reveal a strong flow asymmetry, with considerable changes in both magnitude and direction along the different circumferential positions. Particularly, big differences appear between the circumferential positions closer to the volute tongue and the other ones. The periodic character of the velocity signals due to the passing of the blades, clearly observed around the impeller, is missed in the vicinity of the volute tongue, where the main contribution to the velocity fluctuations appears to be random. Based on the measured velocity signals, velocity unsteadiness of the flow is determined analyzing the main contributions as a function of the flow rate and the measurement position. High levels of velocity unsteadiness were observed near the volute tongue, mainly at low flow rates.
Three-Dimensional Tube Geometry Control for Rotary Draw Tube Bending, Part 1: Bend Angle and Overall Tube Geometry Control
Authors: Lou Huazhou , Stelson Kim A.
Published: July 2002

Traditional trial-and-error springback compensation methods have the problems of high scrap rate, low efficiency, high cost of fixtures and operator experience dependency. The method presented in this paper uses on-line measured springback data from the same batch to predict and compensate for springback. Because there are no springback data for the first bend, bend-rebend control is used to make the first bend to eliminate trial tubes. In addition to springback, relaxation and radial growth are also estimated and compensated for to make a bend more accurate. A process control method is developed to optimize the overall control strategy such that the overall tube error is minimized without increasing the required hardware accuracy. The optimal process control strategy has significantly higher accuracy than the traditional trial-and-error method. The details of statistical analysis of tube tolerance and adaptive bend correction algorithm are presented in Part 2 of the paper.
Nonlinear Model Predictive Control Experiments on a Laboratory Gas Turbine Installation
Authors: van Essen H. A. , de Lange H. C.
Published: July 2002

Results on the feasibility and benefits of model based predictive control applied to a gas turbine are presented. For a laboratory gas turbine installation, the required dynamic simulation model and the real-time (nonlinear) model predictive control (MPC) implementation are discussed. Results on both model validation and control performance are presented. We applied a nonlinear MPC configuration to control the laboratory gas turbine installation and succeeded in a real-time implementation. Although the available computation time for prediction and optimization of the model limits the sample time, the advantages of MPC, i.e. constraint handling, and anticipation to future (set-point) changes are fully reached, and the control performance is good. Special attention is paid to the performance of the applied filter that compensates for inevitable mismatches between model and process measurements. In general, the opportunities of model based control of turbomachinery are promising.
Buckling Criteria for Torispherical Heads Under Internal Pressure
Authors: Miller Clarence D.
Published: July 2002

The results of 190 model tests and failures of five torispherical heads in service are analyzed in this paper. Seventy-two of the heads failed by buckling of the knuckle and 17 of the heads were pressurized until the heads failed by bursting or by rupture of the crown. The test pressures were compared with the design pressures obtained from ASME Section VIII, Divs. 1 and 2 rules which are limited to heads with L/t⩽500, ASME Code Cases 2260 and 2261 rules which are limited to heads with L/t⩽500, the French Code CODAP rules which apply to heads with D/t ratios up to 1000, and the ECCS rules which apply to heads with D/t ratios up to 1250. The margins of safety were found to vary widely for all of the rules. A new set of rules has been developed for design of torispherical heads under internal pressure. The rules are applicable to heads with L/t or D/t ratios up to 2500 and apply to all ASME Code materials. The rules are based on three criteria: failure of the knuckle by buckling, limiting the maximum compressive strain in the knuckle to prevent gross distortions, and failure of the crown by bursting. The proposed rules provide a more accurate prediction of the buckling pressure and a more consistent upper limit on the compressive stress in the knuckle than any of the other rules considered in this paper.
A Neural Network Simulator of a Gas Turbine With a Waste Heat Recovery Section
Authors: Boccaletti C. , Cerri G. , Seyedan B.
Published: July 2002

The objective of the paper is to assess the feasibility of the neural network (NN) approach in power plant process evaluations. A “feed-forward” technique with a back propagation algorithm was applied to a gas turbine equipped with waste heat boiler and water heater. Data from physical or empirical simulators of plant components were used to train such a NN model. Results obtained using a conventional computing technique are compared with those of the direct method based on a NN approach. The NN simulator was able to perform calculations in a really short computing time with a high degree of accuracy, predicting various steady-state operating conditions on the basis of inputs that can be easily obtained with existing plant instrumentation. The optimization of NN parameters like number of hidden neurons, training sample size, and learning rate is discussed in the paper.
A BEM for the Prediction of Unsteady Midchord Face and/or Back Propeller Cavitation
Authors: Young Yin L. , Kinnas Spyros A.
Published: July 2002

A boundary element method (BEM) is used for the numerical analysis of sheet cavitation on a propeller subjected to the non-axisymmetric wakes of marine vehicles. This method is extended in order to treat mixed partial and supercavity patterns on both the face and back of the blades with searched cavity detachment. The convergence of the method is studied. The predicted cavity shapes and forces by the present method agree well with experiments and with those predicted by another numerical method.
Lyapunov Exponents and Moment Lyapunov Exponents of a Two-Dimensional Near-Nilpotent System
Authors: Xie W.-C.
Published: July 2002

The Lyapunov exponents and moment Lyapunov exponents of a near-nilpotent system under stochastic parametric excitation are studied. The system considered is the linearized system of a two-dimensional nonlinear system exhibiting a pitchfork bifurcation. The effect of stochastic perturbation in the vicinity of static pitchfork bifurcation is investigated. Approximate analytical results of Lyapunov exponent are obtained. The eigenvalue problem for the moment Lyapunov exponent is converted to a two-point boundary value problem, which is solved numerically by the method of relaxation.
Hydraulic Performance of a Mixed-Flow Pump: Unsteady Inviscid Computations and Loss Models
Authors: van Esch B. P. M. , Kruyt N. P.
Published: July 2002

The hydraulic performance of an industrial mixed-flow pump is analyzed using a three-dimensional potential flow model to compute the unsteady flow through the entire pump configuration. Subsequently, several additional models that use the potential flow results are employed to assess the losses. Computed head agrees well with experiments in the range 70 percent–130 percent BEP flow rate. Although the boundary layer displacement in the volute is substantial, its effect on global characteristics is negligible. Computations show that a truly unsteady analysis of the complete impeller and volute is necessary to compute even global performance characteristics; an analysis of an isolated impeller channel and volute with an averaging procedure at the interface is inadequate.
Tip Clearance Effects in a Turbine Rotor: Part II—Velocity Field and Flow Physics
Authors: McCarter Andrew A. , Xiao Xinwen , Lakshminarayana Budugur
Published: July 2002

A comprehensive experimental investigation was undertaken to explore the flow field in the tip clearance region of a turbine rotor to understand the physics of tip leakage flow. Specifically the paper looks at its origin, nature, development, interaction with the secondary flow, and its effects on performance. The experimental study was based on data obtained using a rotating five-hole probe, Laser Doppler Velocimeter, high-response pressure probes on the casing, and static pressure taps on the rotor blade surfaces. The first part of the paper deals with the pressure field and losses. Part II presents and interprets the vorticity, velocity, and turbulence fields at several axial locations. The data provided here indicates that the tip leakage vortex originates in the last half chord. The leakage vortex is confined close to the suction surface corner near the blade tip by the relative motion of the blade and the casing, and by the secondary flow in the tip region. The tip leakage flow clings to the blade suction surface until midchord then lifts off of the suction surface to form a vortex in the last 20 percent of the blade chord. The relative motion between blades and casing leads to the development of a scraping vortex that, along with the secondary flow, reduces the propagation of the tip leakage flow into the mainflow. The rotational effects and coriolis forces modify the turbulence structure in the tip leakage flow and secondary flow as compared to cascades.
Numerical Simulation of Tip Leakage Flows in Axial Flow Turbines, With Emphasis on Flow Physics: Part I—Effect of Tip Clearance Height
Authors: Tallman J. , Lakshminarayana B.
Published: July 2002

A pressure-correction based, 3D Navier-Stokes CFD code was used to simulate the effects of turbine parameters on the tip leakage flow and vortex in a linear turbine cascade to understand the detailed flow physics. A baseline case simulation of a cascade was first conducted in order to validate the numerical procedure with experimental measurements. The effects of realistic tip clearance spacing, inlet conditions, and relative endwall motion were then sequentially simulated, while maintaining previously modified parameters. With each additional simulation, a detailed comparison of the leakage flow’s direction, pressure gradient, and mass flow, as well as the leakage vortex and its roll-up, size, losses, location, and interaction with other flow features, was conducted. Part I of this two-part paper focuses on the effect of reduced tip clearance height on the leakage flow and vortex. Reduced tip clearance results in less mass flow through the gap, a smaller leakage vortex, and less aerothermal losses in both the gap and the vortex. The shearing of the leakage jet and passage flow to which leakage vortex roll-up is usually attributed to is not observed in any of the simulations. Alternative explanations of the leakage vortex’s roll-up are presented. Additional secondary flows that are seen near the casing are also discussed.
Extraction of Free-Free Modes from Constrained Vibration Data for Flexible Multibody Models
Authors: Yi, Tong Y. , Nikravesh Parviz E.
Published: July 2002

This paper presents a method for identifying the free-free modes of a structure by utilizing the vibration data of the same structure with boundary conditions. In modal formulations for flexible body dynamics, modal data are primary known quantities that are obtained either experimentally or analytically. The vibration measurements may be obtained for a flexible body that is constrained differently than its boundary conditions in a multibody system. For a flexible body model in a multibody system, depending upon the formulation used, we may need a set of free-free modal data or a set of constrained modal data. If a finite element model of the flexible body is available, its vibration data can be obtained analytically under any desired boundary conditions. However, if a finite element model is not available, the vibration data may be determined experimentally. Since experimentally measured vibration data are obtained for a flexible body supported by some form of boundary conditions, we may need to determine its free-free vibration data. The aim of this study is to extract, based on experimentally obtained vibration data, the necessary free-free frequencies and the associated modes for flexible bodies to be used in multibody formulations. The available vibration data may be obtained for a structure supported either by springs or by fixed boundary conditions. Furthermore, the available modes may be either a complete set, having as many modes as the number of degrees of freedom of the associated FE model, or an incomplete set.
Wrinkling Analysis in Shrink Flanging
Authors: Wang Xi , Cao Jian , Li Ming
Published: July 2002

Flanging is a commonly used sheet forming operation to increase the stiffness of a sheet panel and/or to create the mating surface for subsequent assemblies. This paper presents an energy approach to predict the onset of wrinkling during a shrink flanging operation. A curved sheet model is established to obtain the critical buckling stress in terms of stress state, geometry properties and material properties. The predictions of wave number agree well with the experimental results. The results indicate that the sensitivity of wave number on the plan view radius (PVR) increases as PVR decreases, and the wave number decreases with the increase of flange length. The critical flange length is significantly influenced by PVR, sheet thickness and material work-hardening, and is not sensitive to material strength and anisotropy.
On the Simulation of Discontinuous Functions
Authors: Turner J. D.
Published: July 2002

Discontinuous function constraints arise during the calculation of surface contact, stiction, and friction effects in studies of the behavior of complex systems. These nonlinear effects are mathematically defined by inequality constraints of the form 0⩾gxt,t. The unknown in the problem is the time, t*, when the equality condition is reached. This paper presents an exact solution for t*, which is obtained by introducing a slack variable that replaces time as the independent variable, leading to an extended state-space that is noniteratively integrated to the constraint surface. Several applications are presented to demonstrate the method.
The Improved Volumetric-Efficiency of an Axial-Piston Pump Utilizing a Trapped-Volume Design
Authors: Manring Noah D. , Zhang Yihong
Published: July 2002

In this research, the volumetric efficiency of the axial-piston pump is examined as it relates to the compressibility losses of the fluid. In particular, two valve-plate geometries are compared to show that alterations in the valve-plate design can cause differences in the operating efficiency of the pump. In this paper, a standard valve-plate design which utilizes slots is compared to a trapped-volume design which eliminates the slots altogether. In the analytical result of this paper, it may be shown that the standard valve-plate design introduces a volumetric loss which may be accounted for by the uncontrolled expansion and compression of the fluid that occurs through the slots themselves. By eliminating these slots, and utilizing a trapped volume design, it may be shown that improvements in the operating efficiency can be achieved. Though this paper does not claim to provide the ideal valve-plate design for all pump applications, it does provide the theoretical reason for utilizing trapped volumes and lends general insight into the overall problem of valve-plate design.