bluuebunny/crossref_metadata_2025 huggingface dataset:
The Use of Strain Energy and Fracture Correlation to Determine Life Expectancy for Notched Components
Authors: Scott-Emuakpor Onome , George Tommy , Cross Charles , Shen M.-H. Herman
Published: February 2010

An energy-based method for predicting fatigue life of half-circle notched specimens, based on the nominal applied stress amplitude, has been developed. This developed method is based on the understanding that the total strain energy dissipated during a monotonic fracture and a cyclic process is the same material property, where the density of each can be determined by measuring the area underneath the monotonic true stress-strain curve and measuring the sum of the area within each Hysteresis loop in the cyclic process, respectively. Using this understanding, the criterion for determining fatigue life prediction of half-circle notched components is constructed by incorporating the stress gradient effect through the notch root cross-section. Though fatigue at a notch root is a local phenomenon, evaluation of the stress gradient through the notch root cross-section is essential for incorporating this method into finite element analysis minimum potential energy process. The validation of this method was carried out by comparison with both notched and unnnotched experimental fatigue life of Aluminum 6061-T6 (Al 6061-T6) specimens under tension/compression loading at the theoretical notch fatigue stress concentration factor of 1.75. The comparison initially showed a slight deviation between prediction and experimental results. This led to the analysis of strain energy density per cycle up to failure, and an improved Hysteresis representation for the energy-based prediction analysis. With the newly developed Hysteresis representation, the energy-based prediction comparison shows encouraging agreement with unnotched experimental results and a theoretical notch stress concentration value.
The Design of Highly Loaded Axial Compressors
Authors: Dickens Tony , Day Ivor
Published: February 2010

Increasing compressor pressure ratios (thereby gaining a benefit in cycle efficiency), or reducing the number of stages (to reduce weight, cost, etc.), will require an increase in pressure rise per stage. One method of increasing the pressure rise per stage is by increasing the stage-loading coefficient and it is this topic which forms the focus of the present paper. In the past, a great deal of effort has been expended in trying to design highly loaded blade rows. Most of this work has focused on optimizing a particular design rather than looking at the fundamental problems associated with high loading. This paper looks at the flow physics behind the problem, makes proposals for a new design strategy and explains sources of additional loss specific to highly loaded designs. Detailed experimental measurements of three highly loaded stages (Δh0/U2 ≈ 0.65) have been used to validate a CFD code. The calibrated CFD has then been used to show that as the stage loading is increased the flow in the stator passages breaks down first. This happens via a large corner separation which significantly impairs the stage efficiency. The stator can be relieved by increasing stage reaction, thus shifting the burden to the rotor. Fortunately, the CFD calculations show that the rotor is generally more tolerant of high loading than the stator. Thus, when stage loading is increased, it is necessary to increase the reaction to achieve the optimum efficiency. However, the design exercise using the calibrated CFD also shows that the stage efficiency is inevitably reduced as the stage loading is increased (in agreement with the experimental results). In the second part of the paper, the role profile loss plays in the reduction in efficiency at high stage loading is considered. A simple generic velocity distribution is developed from first principles to demonstrate the hitherto neglected importance of the pressure surface losses in highly loaded compressors.
Passive Control of Rotating Stall in Vaneless Diffuser With Radial Grooves: Detailed Numerical Study
Authors: Gao Chuang , Gu Chuangang , Wang Tong , Yang Bo
Published: February 2010

According to experiments described in the literature, radial grooves in vaneless diffuser walls are simple and powerful devices for suppressing rotating stall. To understand the mechanism behind the grooves and find some guidelines for diffuser designers, a detailed numerical study based on Computational Fluid Dynamics (CFD) was carried out. Not only the flow field variation caused by the grooves but also a simple model graphing the underlying nature was established. Also, the classic boundary layer integral method widely used in practical design procedure was adopted to calculate the diffuser flow distribution to verify the model. The CFD analysis indicated that the effectiveness of the grooves increases the flow angle thus delaying the diffuser wall flow reversals. The recommended placement of the grooves was in the region with reversed flow. Such locally fixed groove could effectively delay the stall without too much pressure loss. Also, a combined variable, representing the overall geometry of grooves was established and verified. The detailed study given in this paper gives guidelines for using grooves as a stall delay method.
Analysis of Rotor-Stator-Interaction and Blade-to-Blade Measurements in a Two Stage Axial Flow Compressor
Authors: Ernst Marco , Michel Armin , Jeschke Peter
Published: February 2010

In order to improve unsteady CFD analysis for turbomachinery applications there is still the need for high quality experimental data for validation. The first aim of this paper is to represent a unique work in providing unsteady data for the purpose of a CFD test case. Secondly, a detailed analysis of rotor-stator-interaction in a multi-stage compressor is performed by means of accompanying frequency analysis of the blading and the tip leakage flow for different indexing positions. The transonic two stage axial flow compressor is equipped with integrally bladed rotors and vanes with hub and tip shrouds with regard to a modern high pressure jet engine compressor. For detailed flow measurements both intrusive and non-intrusive measurement techniques were used. Beside pressure probes mounted with a semiconductor and micro strain gauges applied to the blades and vanes, a 3 Component Doppler Laser-Two-Focus Velocimeter was used. The results of the time resolved pressure traverse measurements in the axial gaps as well as the analysis of the tip leakage flow show the propagation of the wakes and the potential upstream influence which can be clearly detected by the blade passing frequencies as well. Additionally, the results of the blade-to-blade measurements impart an idea of the intra-rotor-transport of the fluid and the flow phenomena within the rotor. Beside detailed flow analysis, the preferential aim of this paper is to establish a method for the detection of flow phenomena due to blade-row-interaction by means of a frequency analysis.
Role of Tip Leakage in Stall of a Transonic Centrifugal Impeller
Authors: Hazby Hamid R. , Xu Liping
Published: February 2010

The objective of the current paper is to employ numerical simulations to identify flow features, which could lead to the breakdown of stable operation in transonic centrifugal impellers at near-stall operating conditions. Steady state three-dimensional viscous flow calculations are used to investigate the flow inside a transonic impeller representative of state of the art automotive turbocharger technology. The detailed impeller flow field is compared at different operating conditions. It is observed that the interaction of the relative total pressure deficit coming from the main blade tip region and the adverse pressure gradient in the splitter passage results in a breakdown of the flow in the tip region. These low relative total pressure fluids originate from the main blade tip leakage flow and/or the transported boundary layers. Effects due to the splitter wall shear stress and the turbulence model on the flow are also investigated and addressed in the paper.
Improving Intermediate Pressure Turbine Performance by Using a Non-Orthogonal Stator
Authors: Yoon Sungho , Denton John , Curtis Eric , Longley John , Pullan Graham
Published: February 2010

Intermediate Pressure (IP) turbines in high bypass ratio civil aeroengines are characterized by a significant increase in radius and a low aspect ratio stator. Conventional aerodynamic designs for the IP turbine stator have had leading and trailing edges orthogonal to the hub and casing end walls. The IP turbine rotor, however, is stacked radially due to stress limits. These choices inevitably lead to a substantial gap between the IP stator and rotor at the outer diameter in a duct that is generally diffusing the flow due to the increasing radius. In this low Mach number study, the IP stator is redesigned, incorporating compound sweep, so that the leading and trailing edges are no longer orthogonal to the end walls. Computational investigations showed that the non-orthogonal stator reduces the flow diffusion between the stator and rotor which yields two benefits: the stator trailing edge velocity was reduced, as was the boundary layer growth on the casing endwall within the gap. Experimental measurements confirm that the turbine with the non-orthogonal stator has an increased efficiency (by 0.49%), whilst also increasing the work output (by 4.6%), at the design point.
Experimental Study of Supersonic Jet Noise Reduction With Microjet Injection
Authors: Watanabe Toshinori , Okada Ryuichi , Uzawa Seiji , Himeno Takehiro , Oishi Tsutomu
Published: February 2010

Experimental study was conducted concerning active control of supersonic jet noise with a microjet injection technique. The microjets were injected into a rectangular main jet with Mach number up to 1.49. The nozzle lip of the main jet was equipped with 44 injection holes of the microjets, whose angles against the main jet were changed as 60 and 90 degrees. From far-field sound pressure data, a significant reduction of the jet noise by several dB was found in the cases with 60 and 90 degrees of injection angles. The microjet was found to affect all components of supersonic jet noise, namely, turbulent mixing noise, shock-associated broadband noise and screech tone noise. In the results of FFT analysis, the effect of the microjet was observed in the sound pressure level of the shock-associated broadband noise, the pressure level and frequency of the screech tone noise, and average level of the turbulent mixing noise. Schlieren visualization was also made for the jet flow, and the microjet was seen to change the shock structure and shear layer behavior of the supersonic jet.
Rotor-Blade Coupled Vibration Analysis by Measuring Modal Parameters of Actual Rotor
Authors: Okabe Akira , Kudo Takeshi , Yoda Hideo , Sakurai Shigeo , Matsushita Osami , Shiohata Koki
Published: February 2010

The designers of rotor shafts and blades for a traditional turbine-generator set typically employed their own models and process by neglecting the coupled torsional effect. The torsional coupled umbrella mode of recent longer blades systems designed for higher output and efficiency tends to have nearly doubled the frequency of electric disturbance (i.e., 100 or 120 Hz). In order to precisely estimate the rotor-blade coupled vibration of rotating shafts, the analysis must include a process to identify the parameters of a mathematical model by using a real model. In this paper we propose the use of a unique quasi-modal technique based on a concept similar to that of the modal synthesis method, but which represents a unique method to provide a visually reduced model. An equivalent mass-spring system is produced for uncoupled umbrella mode and modal parameters are measured in an actual turbine rotor system. These parameters are used to estimate the rotor-blade coupled torsional frequencies of a 700-MW turbine-generator set, with the accuracy of estimation being verified through field testing.
Evaluation of Friction Damping in Dovetail Root Joints Based on Dissipation Energy on Contact Surfaces
Authors: Asai Kunio , Sakurai Shigeo , Kudo Takeshi , Ozawa Norihiko , Ikeda Taizo
Published: February 2010

It is necessary to increase and estimate friction damping at contact interfaces to reduce vibratory stresses in turbines. The hysteresis behavior between tangential contact force and relative displacement should be precisely estimated to improve the accuracy of fiction-damping estimates. There is a difficulty in establishing a general model of hysteresis because tangential contact stiffness depends on many parameters, such as normal contact force, contact geometry, surface roughness, and wear status. We discuss a procedure to empirically calculate friction damping in dovetail root joints using the tangential contact stiffness estimated from measured natural frequencies and the micro-slip model whose coefficients were experimentally obtained from special fretting tests. Instead of the multi-harmonic balance methods, we calculated the friction damping on the basis of the energy dissipation at contact surfaces to discuss the effects of the tangential contact stiffness on several physical values, i.e., tangential and normal contact forces, natural frequency, and micro-slip. In our model, the linear forced response analysis was conducted by taking into consideration the non-linearity between the tangential contact force and the relative displacement by defining the actual and imaginary tangential contact stiffness. We confirmed that the numerically calculated damping ratios are quantitatively in very good agreement with the measured ones under different contact angles, input gravity levels, and contact forces. This indicates that if the tangential contact stiffness is accurately estimated, friction damping with our method can be precisely estimated under different test conditions. We also showed that the estimated tangential contact stiffness for dovetail root joints are smaller than those obtained by the fretting tests at high input gravity. This is probably because the contact interface partially separates during a cyclic loading in the former case; this results in the decrease of the contact area and contact stiffness.
Online Stall Control With the Digital Signal Processing Method in an Axial Compressor
Authors: Tong Zhiting , Li Lin , Nie Chaoqun , Lin Bin , Cui Yaoxin , Qi Wei
Published: February 2010

The present work develops an online stall control system combining a correlation measure, Digital Signal Processing (DSP) and tip micro injection with the current results of unsteady flow, the improved modern control theory and the advanced control technology. An online scheme is established by computing a correlation measure with signal multiplication and integration. When a correlation measure drops below a preset threshold, the DSP control system sends an electrical signal to an electromagnetic valve that turns on a micro injection system. In this paper, the results of the tests conducted in a research compressor show that the correlation scheme is able to sense an impending stall in advance. The experiments results in the compressor show that the correlation measure goes up accordingly with micro-injection. A new method for online stall control combining the modern Digital Signal Processing (DSP) technology and micro air injection is presented and discussed. A successful demonstration of the capability with the online DSP method in detecting the distribution function of the correlation measure and triggering the micro injection system is presented.
Extension of the Stator Vane Upstream Across the 180° Bend for a Multistage Radial Compressor Stage
Authors: Aalburg Christian , Simpson Alexander , Carretero Jorge , Nguyen Tue , Michelassi Vittorio
Published: February 2010

The design, analysis and optimization of a new stator concept for multistage centrifugal compressors using numerical methods is presented. The first objective was to further improve the performance of a well-optimized stage with a short vaneless diffuser, see Aalburg et al [1]. The second objective was to achieve a significant increase in the flow turning in the stator part. In order to achieve these goals an extension of the return channel vane upstream, over the U-turn bend, was considered. This design poses challenges that are quite different from those encountered for a conventional design. For example, a conventional vane angle distribution leads to lean angles across the bend that are not feasible from a manufacturing and aerodynamic perspective. In addition, conventional design tools for geometry generation were found to have limited applicability for this concept. To address these issues a geometry generator was developed that facilitated the design of three-dimensional across-the-bend type vanes with unconventional vane angle distributions. The geometry generator was based on an analytical design procedure similar to that outlined by Veress and Braembussche [2]. This procedure allows a desired loading distribution to be specified. In this paper the vane concept will be introduced, the development of the geometry generator will be outlined and the effect of varying design parameters will be considered. An optimized design will then be presented that outperformed the reference conventional design in terms of efficiency by up to one point across the operating range. This improvement was achieved despite a significantly higher vane loading.
Analytical Modelling of a Fluid Coupling
Authors: Kruisbrink A. Ch. H. , Pickering S. J.
Published: February 2010

In this paper an analytical model is presented for fluid couplings in aero-engine applications. It can be used to predict the performance of fully filled as well as partially filled couplings in terms of a transmitted torque. As such it will lead to a predictive tool for design purposes. The model makes use of toroidal coordinates. This allows for the assessment of the mass flux and angular momentum flux within the entire toroid halves, formed by the driving unit (pump) and driven unit (turbine). In previous work only the fluxes at the coupling plane (between pump and turbine) could be evaluated, since cylindrical coordinates were used. The Euler equations in toroidal coordinates are used to obtain approximate solutions for the 2D pressure field within these toroid halves. Assuming that the pressure within an air cavity of a partially filled coupling is constant, the air-oil interface, flow regime (annular, stratified) and fill status are obtained from contours of constant pressure. In previous work the pressure distribution is not considered, except in criteria for the flow regimes, based on the centrifugal and vortex head in the coupling plane. The analytical model is validated. It shows a good agreement with torque measurements on a fully filled coupling, after a Reynolds dependency for the power loss coefficients is introduced. It shows a reasonable, qualitative agreement with CFD simulations on a partially filled coupling, after the solution for the pressure distribution is corrected for the variable vortex speed and radial velocity component. The analytical approach is efficient compared to CFD, which is very expensive in terms of CPU times, in particular for the two-phase flow in partially filled fluid couplings.
Calculations of High-Lift Cascades in Low Pressure Turbine Conditions Using a Three-Equation Model
Authors: Pacciani Roberto , Marconcini Michele , Fadai-Ghotbi Atabak , Lardeau Sylvain , Leschziner Michael A.
Published: February 2010

A three-equation model has been applied to the prediction of separation-induced transition in high-lift low-Reynolds-number cascade flows. Classical turbulence models fail to predict accurately laminar separation and turbulent reattachment, and usually over-predict the separation length, the main reason for this being the slow rise of the turbulent kinetic energy in the early stage of the separation process. The proposed approach is based on solving an additional transport equation for the so-called laminar kinetic energy, which allows the increase of the non-turbulent fluctuations in the pre-transitional and transitional region to be taken into account. The model is derived from that of Lardeau and Leschziner, which was originally formulated to predict bypass transition for attached flows, subject to a wide range of free-stream turbulence intensity. A new production term is proposed, based on the mean shear and a laminar eddy-viscosity concept. After a validation of the model for a flat-plate boundary layer, subjected to an adverse pressure gradient, the T106 and T2 cascades, recently tested at the von Ka´rma´n Institute, are selected as test cases to assess the ability of the model to predict the flow around high-lift cascades in conditions representative of those in low-pressure turbines. Good agreement with experimental data, in terms of blade-load distributions, separation onset, reattachment locations and losses, is found over a wide range of Reynolds-number values.
Influence of Inlet Velocity Ratio on the Outlet Flow Uniformity of a Fan-Shaped Film Cooling Hole
Authors: Porter James S. , Henderson Alan D. , Walker Gregory J.
Published: February 2010

Literature regarding the influence of inlet conditions on cooling hole flows is reviewed. A general failure to fully quantify inlet conditions and an inconsistent terminology for describing them is noted. This paper argues for use of an inlet velocity ratio (IVR) defined as the ratio of the coolant passage velocity to the jet velocity, together with additional parameters required to define the velocity distribution in the coolant supply passage. Large scale experimental investigations of the internal flow field for a laterally expanded 50 times scale fan-shaped hole are presented, together with a computational investigation of the flow, for three inlet velocity ratios. Inlet lip separation causes a jetting effect that extends throughout the length of the cooling hole. A low velocity region of separated fluid exists on the downstream wall of the diffuser which deflects the jetting fluid towards the upstream side of the hole. This effect is most pronounced at low IVR values. The exit velocity profiles and turbulence distributions are highly dependent on the IVR.
Unsteady Flow Effects in a High-Speed Multistage Axial Compressor
Authors: Yamagami Mai , Kodama Hidekazu , Kato Dai , Tsuchiya Naoki , Horiguchi Yasuo , Kazawa Junichi
Published: February 2010

As a first step in practical application of virtual rig test, in present work, a full unsteady multistage simulation was performed for a 6-stage transonic axial compressor at design point. A steady multistage simulation was also performed and compared with the results of the unsteady simulation to clarify the unsteady flow effects. Comparison of overall performance, stage matching, and inter stage performance with measured test data showed that the unsteady multistage simulation could predict the performance reasonably. It was also demonstrated that, at design point, the steady multistage simulation could predict the stage matching at the same level as the unsteady multistage simulation. To clarify the effects of the unsteady flow in a multistage compressor, the pressure loss by unsteady wake interaction was investigated specifically at the inter-blade passage or down stream blade rows. Some issues in the predictive ability of the unsteady multistage simulation were newly found in the spanwise mixing phenomena.
Effect of Scaling of Blade Row Sectors on the Prediction of Aerodynamic Forcing in a Highly-Loaded Transonic Compressor Stage
Authors: Mayorca Mari´a A. , De Andrade Jesu´s A. , Vogt Damian M. , Ma˚rtensson Hans , Fransson Torsten H.
Published: February 2010

An investigation of the sensitivity of a geometrical scaling technique on the blade forcing prediction and mode excitability has been performed. A stage of a transonic compressor is employed as test object. A scaling ratio is defined which indicates the amount of scaling from the original geometry. Different scaling ratios are selected and 3D Navier Stokes unsteady calculations completed for each scaled configuration. A full annulus calculation (non-scaled) is performed serving as reference. The quantity of interest is the generalized force, which gives a direct indication of the mode excitability. In order to capture both up- and downstream excitation effects the mode excitability has been assessed on both rotor and stator blades. The results show that first harmonic excitation can be predicted well for both up- and downstream excitation using moderate amount of scaling. On the other hand, the predictions of second harmonic quantities do show a higher sensitivity to scaling for the investigated test case.
Numerical Computation of the Pulsatile Flow in a Turbocharger With Realistic Inflow Conditions From an Exhaust Manifold
Authors: Hellstrom Fredrik , Fuchs Laszlo
Published: February 2010

The combined effect of different secondary perturbations at the turbine inlet and the pulsatile flow on the turbine performance was assessed and quantified by using Large Eddy Simulation. The geometrical configuration consists of a 4-1 exhaust manifold and a radial turbine. At the inlet to each port of the manifold, engine realistic pulsatile mass flow and temperature fields are specified. The turbine used in this numerical study is a vaneless radial turbine with 9 blades, with a size that is typical for a turbocharger mounted on a 2.0 liters IC engine of passenger cars. The flow field is investigated and the generated vortices are visualized to enable a better insight into the unsteady flow field. Correlations between the turbine inflow conditions, such as mass flow rate, strength of secondary flow components, and the turbine performance have also been studied. The results show that the flow field entering the turbine is heavily disturbed with strong secondary flow components and disturbed axial velocity profile. Between the inlet to the turbine and the wheel, the strength of the secondary flow and the level of the disturbances of the axial flow decrease which gives large losses in this region. Even though the magnitude of the disturbances decrease, the flow entering the wheel will still be disturbed, resulting in a perturb inlet flow to the wheel which affects the shaft power output from the turbine.
Rotor Casing Contouring in High Pressure Stages of Heavy Duty Gas Turbine Compressors With Large Tip Clearance Heights
Authors: Kro¨ger Georg , Cornelius Christian , Nicke Eberhard
Published: February 2010

Clearance leakage losses of axial compressor rotors and stators have a major impact on the overall compressor performance. The clearance heights in the last stages (high pressure stages) of a gas turbine compressor are very large in comparison to the low pressure stages due to mechanical constraints and small blade heights. The reduction of clearance leakage losses in a high pressure stage still holds an important potential for the overall performance improvement at design point conditions. In the following work, a method for tip clearance loss reduction by circumferential casing contouring above a high pressure stage rotor with a constant clearance height is presented. The subsonic compressor blade provides Siemens HPA-Family [1, 2, 3] airfoils. Starting over with a 3D-Optimization of the mentioned rotor casing the work additionally refers to the aerodynamic effects and the off design performance of the optimized geometry. It has been found that an optimized casing and blade tip contour lead to a smaller overall clearance mass flow and lower pressure loss coefficient of the clearance flow so that the endwall blockage is reduced and the stage performance is improved by about 0.35% at design point conditions. Furthermore it was found that the performance improvement drops with increasing exit pressure to about 0.1% close to stall conditions. At lower exit pressure values the optimized geometry provides an additional performance improvement in comparison to the baseline configuration.
Acoustic Eigenmodes in the Side Cavities of Centrifugal Compressors
Authors: Ko¨nig Sven
Published: February 2010

The current paper presents results of an extended research program to gain insight into a better understanding of excitation mechanisms in centrifugal compressors. The focus is on acoustic eigenmodes in the respective side cavities. If those eigenmodes are excited, the resonances may lead to high levels of noise, and, more importantly, vibration amplitudes which may, under certain conditions, ultimately lead to machine failures. Modeling issues arise from the correct modeling of the boundary conditions and a possible mode coupling with connecting cavities. Those issues have been addressed by evaluating different computational domains. By means of finite element analysis it could be shown that a large variety of acoustic modes establishes in the side cavities — some of them with frequencies and mode orders in a frequency range relevant for impeller excitation. For the calculated frequency range it was found that the modes in the hub and shroud side cavities are fully coupled, leading to eigenfrequencies well away from the eigenfrequencies of the local cavity modes.
Rotordynamic Characteristics of a 5 Pad, Rocker-Pivot, Tilting Pad Bearing in a Load-on-Pad Configuration: Comparisons to Predictions and Load-Between-Pad Results
Authors: Childs Dara W. , Carter Clint R.
Published: February 2010

Rotordynamic data are presented for a rocker-pivot tilting-pad bearing in load-on-pad (LOP) configuration for (345–3101 kPa) unit loads and speeds from 4k to 13k rpm. The bearing was direct lubricated through a leading-edge groove with 5 pads, .282 preload, 60% offset, 57.87° pad arc angle, 101.587 mm (3.9995 in) rotor diameter, 0.1575 mm (.0062 in) diametral clearance, and 60.325 mm (2.375 in) pad length. Measured results were reported for this bearing by Carter and Childs in 2008 in the load-between-pad (LBP) configuration. Results for the LOP are compared to predictions from a bulk-flow Navier-Stokes model (as utilized by San Andres in 1991) and to the prior LBP results. Frequency effects on the dynamic-stiffness coefficients were investigated by applying dynamic-force excitation over a range of excitation frequencies. Generally, the direct real parts of the dynamic-stiffness coefficients could be modeled as quadratic functions of the excitation frequency and accounted for by adding a mass matrix to the conventional [K][C] model to produce a frequency-independent [K][C][M] model. Measured added mass terms in the loaded direction approached 60 kg. The static load direction in the tests was y. The direct-stiffness coefficients Kyy and Kxx depend strongly on the applied unit load, more so than speed. They generally increased linearly with load, shifting to a quadratic dependence at higher unit loads. At lower unit loads, Kyy and Kxx increase monotonically with running speed. The experimental results were compared to predictions from a bulk-flow CFD analysis. Stiffness orthotropy was apparent in test results, significantly more than predicted, and it became more pronounced at the heavier unit loads. Measured Kyy values were consistently higher than predicted, and measured Kxx values were lower. Comparing the LOP results to prior measured LBP results for the same bearing, at higher loads, Kyy is significantly larger for the LOP configuration than LBP. Measured values for Kxx are about the same for LOP and LBP. At low unit loads, stiffness orthotropy defined as Kyy / Kxx is the same for LOP and LBP, progressively increasing with increasing unit loads. At the highest unit load, Kyy / Kxx = 2.1 for LOP and 1.7 for LBP. Measured direct damping coefficients Cxx and Cyy were insensitive to changes in either load or speed in contrast to predictions of marked Cyy sensitivity for changes in the load. Only at the highest test speed of 13 krpm were the direct damping coefficients adequately predicted. No frequency dependency was observed for the direct damping coefficients.
Nonparametric Stochastic Modeling of Uncertainty in Rotordynamics
Authors: Murthy Raghavendra , Mignolet Marc P. , El-Shafei Aly
Published: February 2010

A systematic and rational approach is presented for the consideration of uncertainty in rotordynamics systems, i.e. in rotor mass and gyroscopic matrices, stiffness matrix, and bearing coefficients. The approach is based on the nonparametric stochastic modeling technique which permits the consideration of both data and modeling uncertainty. The former is induced by a lack of exact knowledge of properties such as density, Young’s modulus, etc. The latter occurs in the generation of the computational model from the physical structure as some of its features are invariably ignored, e.g. small anisotropies, or approximately represented, e.g. detailed meshing of gears. The nonparametric stochastic modeling approach, which is briefly reviewed first, introduces uncertainty in reduced order models through the randomization of their system matrices (e.g. stiffness, mass, and damping matrices of nonrotating structural dynamic systems). Here, this methodology is first extended to permit the consideration of uncertainty in symmetric and asymmetric rotor dynamic systems. Its application is next demonstrated on a symmetric rotor on linear bearings and uncertainties on the rotor stiffness (stiffness matrix) and/or mass properties (mass and gyroscopic matrices) are introduced that maintain the symmetry of the rotor. The effects of these uncertainties on the Campbell diagram, damping ratios, mode shapes, forced unbalance response, and oil whip instability threshold are analyzed. The generalization of these concepts to uncertainty in the bearing coefficients is achieved next. Finally, the consideration of uncertainty in asymmetric rotors is addressed and exemplified.
Influence of Surface Roughness on the Performance of a Compressor Blade in a Linear Cascade: Experiment and Modeling
Authors: Back Seung Chul , Jeong In Cheol , Sohn Jeong Lak , Song Seung Jin
Published: February 2010

Compressor blades experience significant surface degradation with service. Elevated levels of surface roughness reduce compressor efficiency and mass flow rate. This paper presents measurement and a new model of compressor blade performance degradation due to blade surface roughness. Performance tests have been conducted in a low-speed, linear cascade with roughened compressor blades. Equivalent sandgrain roughnesses of 12, 180, 300, 425, and 850 microns have been tested. These roughness values are representative of compressor blade roughnesses found in actual gas turbines in service. Flow angle, flow rate, and loss have been measured. For the tested roughnesses of 180, 300, 425, and 850 microns, the axial velocity ratio decreases by 0.1, 2.1, 2.5, and 5.4%, respectively. For the same cases, the deviation increases by 24, 38, 51, and 70%, respectively. Finally, the loss increases by 12, 44, 132, and 217%, respectively. Thus, among the three parameters, the loss responds most sensitively to changes in compressor blade roughness. Furthermore, a new mean-line model based on the assumption of 50% reaction stages has been developed to estimate the effects of roughness on the performance of a multi-stage compressor. The data from the cascade data are used as input to predict the performance of a single compressor stage. Subsequently, a stage-stacking method is used to enable prediction for a multi-stage compressor. According to the model, the pressure ratio, and mass flow rate are significantly influenced by the blade surface roughness.
Experimental Study for the Effect of Vaned Diffuser Leading Edge on the Noise Generated From a Centrifugal Fan
Authors: Zhang Wei , Gong Wu Qi , Fan Xiao Hua , Xi Guang
Published: February 2010

This paper studies a novel vaned diffuser with inclined leading edge to reduce the fan noise. The aerodynamic performances and sound pressure level (SPL) of the centrifugal fan with five different diffusers were measured in an anechoic chamber, and the inclined angles of the five diffusers are 45°, 60°, 120°, 135° and 150°, respectively. The flow coefficient of the fan varies from 0.037 to 0.14, the rotational speed ranges from 20000rpm to 35000rpm. The test results show that, by using diffuser vanes with inclined leading edge, the overall noise of the fan is reduced in the experimental conditions, and it has the minimum value at the flow rate coefficient of 0.084. The overall noise reduction of 5.3dB was made with the diffuser leading edge inclined angle of 150°.
Validation of Turbine Airfoil Leading Edge Pressure and Skin Friction Modeling and Measurement
Authors: Holley Brian M. , Langston Lee S.
Published: February 2010

New pressure and skin friction measurements on a cylinder were made and compared with analytical and computational modeling. The measurements were made in what we call the stagnation region, such as exists on the leading edge of a turbine airfoil. The new skin friction measurements were made with an oil film interferometry technique that has been used in earlier turbine cascade studies, and the accuracy of the technique is demonstrated by comparisons with the analytical and computational modeling. The analytical model is an exact solution for laminar stagnation point or Hiemenz flow, and it is based solely on a reference velocity and a length scale, both which describe the free stream acceleration and pressure gradient in the stagnation region. Comparisons were made between the analytical model and measurements of preasure and skin friction from other cylinder studies in the open literature. These comparisons were used to demonstrate that the analytical model accounts for variations in the free stream pressure gradient and acceleration of the flow due to blockage, separation location, and compressibility. The simplicity of the analytical model allows one to quickly characterize stagnation region aerodynamics based only on a pressure profile, which can be used, for example, to assess the computational grid density in the stagnation region of a turbine airfoil. This basic cylinder experiment along with the analytical modeling are shown to be particularly effective for the validation of skin friction techniques, and this study serves to validate the OFI method used in earlier cascade studies.
Evaluation of Q-Values of a Rotor-Bearing System Using a Modal Open Loop Transfer Function
Authors: Fujiwara Hiroyuki , Fukushima Yasuo , Takahashi Naohiko , Oyama Hiroto , Matsushita Osami
Published: February 2010

As indicated by ISO standard 10816, Q-value estimation is very important for rotating machinery commission and/or machine safety operation. Impulse tests and resonance curve measurements are commonly contributed to Q-value estimation. Instead of two methods, we are developing a new method using open-loop transfer function (Go) at real operational condition. In this paper, we present a theoretical background on how to measure the open loop transfer function. Our key technique is the measurement of the modal coordinates with a combination of several displacement sensors. Concerning the experimental measurement of Go, we use multiple sensors and only one active magnetic bearing (AMB) in a rotor system supported by oil-film/ball bearing. This Go is converted to damping ratio. In fact, a smaller damping ratio is recognized just before oil-whip instability by our open loop measurement. In addition, we set a dynamic damper which has been developed for suppression of oil whip instability, and an increasing damping ratio is also recognized by our method.
Numerical Simulation of Flow Fields of Variable Geometry Turbine With Spherical Endwalls or Nonuniform Clearance
Authors: Yue Guoqiang , Yin Shengqi , Zheng Qun
Published: February 2010

To rotate the vane of variable geometry turbine, the end walls contours of the turbine have to be spherical. If the end walls are cylinders, there will be nonuniform clearances between the ends of the vane and the end walls. In these two cases, the flow fields of the rotated vanes are simulated and compared. Special attentions are paid to the flows in the spherical clearances and nonuniform clearances under four rotating angles. The leakage flow and the loss are investigated and analyzed in this paper.
The Influence of Rotor-Stator Spacing on the Loss in One-Stage Transonic Compressor
Authors: Li Wan’ai , Sun Yutao , Ren Yu-Xin , Fu Song , Wadia Aspi R.
Published: February 2010

The influence of axial spacing on the unsteady flow field and loss generation in the axial compressor is an important and interesting problem. In this paper, this influence is studied numerically by solving the 3D unsteady flows in a one-stage compressor consisting of NASA Rotor 35 and Stator 37 with three axial gaps in different sizes. The effect of axial spacing is discussed in terms of the adiabatic efficiency and the irreversibility. Numerical results indicate that smaller axial gap is favourable in reducing the irreversibility in the compressor of the present test case. A possible mechanism of the axial gap effects is presented.
Similarity Design of Rotary Machine’s Bearing Parameters
Authors: Su Shenjian , Wang Liming , Li Ping , Zuo Jianli
Published: February 2010

The bearing parameters, which dominate the dynamic behavior of the rotary machine, should be optimized so that the machine can successfully pass through the critical speed and stably run at the operating speed. However, the parametric optimization usually needs complicated numerical calculation and a great deal of experimentation, especially for the high-speed vertical machine studied here, the dynamic behavior of which is even more sensitive to the parameters. Whereas, the new-style rotor is of similar structure with the old-style rotor. The difference between the two types of rotors is that the new-style rotor’s length and diameter are 1.3 times of the old-style rotor. So it is feasible that the parametric optimization of new-style bearings is carried out by the similarity principle. The similarity principle is that the ratio between the new-style rotor’s whirl energy with the old-style rotor’s equals the ratio between the new-style rotor’s dissipation energy imposed by the bottom bearing with the old-style rotor’s when they run at the operating speed. The parametric optimization involved in this paper only needs simple numerical operation and a little experimentation and makes the new-style bearing parameters design much faster and cheaper than the old-style.
Forced Response Prediction of Blades With Loosely Assembled Dovetail Attachment by HBM
Authors: Bo Shangguan , Xu Zili , Wu Qilin , Zhou XianDing , Cao ShouHong
Published: February 2010

To understand the mechanism of interfacial damping of axial loosely assembled dovetail to suppress blade vibration, a dry friction force model is presented by the Coulomb friction law and the macroslip model, and the mathematical expression of the friction force is derived. The nonlinear friction force is linearized as an equivalent stiffness and an equivalent damping through the one-term harmonic balance method. The effect of centrifugal force on the equivalent stiffness and the equivalent damping is studied. The forced response of one simplified blade with loosely assembled dovetail attachment is predicted by the harmonic balance method, in which the blade is described by the lumped mass and spring model, and the friction contact joints is simplified as a ideal friction damper. The results show that the equivalent stiffness of loosely assembled dovetail attachment increases with blade centrifugal force, gradually reaches a certain value, and there exists the maximum value for the equivalent stiffness. The equivalent damping increases at the beginning and then decreases with blade centrifugal force increasing, there exists a maximum too. The resonant frequency of blade rises with blade centrifugal force, but it no longer increases when the centrifugal force exceed a certain value. There exists a special centrifugal force on which the effect of dry friction damping is the best.
Influences of Axial Gap Between Blade Rows on Secondary Flows and Aerodynamic Performance in a Turbine Stage
Authors: Yamada K. , Funazaki K. , Kikuchi M. , Sato H.
Published: February 2010

A study on the effects of the axial gap between stator and rotor upon the stage performance and flow field of a single axial flow turbine stage is presented in this paper. Three axial gaps were tested, which were achieved by moving the stator vane in the axial direction while keeping the disk cavity constant. The effect of the axial gap was investigated at two different conditions, that is design and off-design conditions. The unsteady three-dimensional flow field was analyzed by time-accurate RANS (Reynolds-Averaged Navier-Stokes) simulations. The simulation results were compared with the experiments, in which total pressure and the time-averaged flow field upstream and downstream of the rotor were obtained by five-hole probe measurements. The effect of the axial gap was confirmed in the endwall regions, and obtained relatively at off-design condition. The turbine stage efficiency was improved almost linearly by reducing the axial gap at the off-design condition.
Pressure Loss Modeling of Non-Symmetric Gas Turbine Exhaust Ducts Using CFD
Authors: Farber Steven , Ghaly Wahid
Published: February 2010

In typical gas turbine applications, combustion gases that are discharged from the turbine are exhausted into the atmosphere in a direction that is sometimes different from that of the inlet. In such cases, the design of efficient exhaust ducts is a challenging task particularly when the exhaust gases are also swirling. A parametric Computational Fluid Dynamics (CFD) based study was carried out on non-symmetric gas turbine exhaust ducts where the effects of geometry and inlet aerodynamic conditions were examined. These exhaust ducts comprise an annular inlet, a flow splitter, an annular to rectangular transition region, and an exhaust stub. The duct geometry, which is a three-dimensional complex one, is approximated with a six-parameter model (four geometric and two aerodynamic), which was coupled with a design of experiment method to generate a relatively small number of exhaust ducts. The flow in these ducts was simulated using CFD for different values of inlet swirl and aerodynamic blockage and the numerical results were reviewed so as to assess the effects of the geometric and aerodynamic parameters on the total pressure loss in the exhaust duct. These flow simulations were used as a data base to generate a total pressure loss model that designers can use as a tool to build more efficient non-symmetric gas turbine exhaust ducts.
Flow Field Structures of the Impeller Backside Cavity and Its Influences on the Centrifugal Compressor
Authors: Sun Zhigang , Tan Chunqing , Zhang Dongyang
Published: February 2010

The impeller backside cavity is one of the unique features of the centrifugal compressors, it can affect the aerodynamic performances of a centrifugal compressor in many ways. This paper presents the researches on the coupled flow fields between a centrifugal compressor main flow passage and its impeller backside cavity. The flow field structures and features of the impeller backside cavity are presented for different leakage flow patterns, and its influences on the flow field details, axial thrust, shaft power, pressure ratio and efficiency of the centrifugal compressor have been studied. Some general conclusions are drawn for different centrifugal compressor operating conditions and impeller backside cavity leakage flow patterns.