bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow Characteristics Inside Circular Injection Holes Normally Oriented to a Crossflow: Part II — Three Dimensional Flow Data and Aerodynamic Loss
Authors: Lee Sang Woo , Joo Seong Kuk , Lee Joon Sik
Published: August 2014

Presented are three-dimensional mean velocity components and aerodynamic loss data inside circular injection holes. The holes are normally oriented to a crossflow and each hole has a sharp square-edged inlet. Because of their importance to flow behavior, three different blowing ratios of M = 0.5, 1.0 and 2.0, and the hole length-to-diameter ratios of L/D = 0.5, 1.0 and 2.0 are investigated. The entry flow is characterized by a separation bubble, meanwhile the exit flow by the direct interaction with the crossflow. The uniform on-coming flow at the inlet undergoes a strong acceleration and a subsequent gradual deceleration along a converging-diverging flow passage formed by the inlet separation bubble. After passing the throat of the converging-diverging passage, the potential-core flow which is nearly axisymmetric decelerates on the windward side, but tends to accelerate on the leeward side. The presence of the crossflow thus reduces the discharge of the injectant on the windward side, but enhances its efflux on the leeward side. This trend is greatly accentuated at M = 0.5. In general, there are strong secondary flows in the inlet and exit planes of the injection hole. The secondary flow within the injection hole, on the other hand, is found relatively weak. The inlet secondary flow is characterized by a strong inward flow toward the injection-hole center. However, it is not completely directed inward since the crossflow effect is superimposed on it. Past the throat, the secondary flow is observed such that the leeward velocity component induced by the crossflow is superimposed on the diverging flow. Short L/D usually results in the exit discharging flow with a steep velocity gradient as well as a strong deceleration on the windward side, as low M does. The aerodynamic loss inside the injection hole is originated from the inlet separation bubble, wall friction and interaction of the injectant with the crossflow. The first one is considered as the most dominant source of loss, even in the case of L/D = 2.0. At L/D = 0.5, the first and third sources are strongly coupled with each other. Regardless of L/D, the mass-averaged aerodynamic loss coefficient has an increasing tendency with increasing M.
Measurements in Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions
Authors: Volino Ralph J. , Hultgren Lennart S.
Published: August 2014

Detailed velocity measurements were made along a flat plate subject to the same dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil. Reynolds numbers based on wetted plate length and nominal exit velocity were varied from 50, 000 to 300, 000, covering cruise to takeoff conditions. Low and high inlet free-stream turbulence intensities (0.2% and 7%) were set using passive grids. The location of boundary-layer separation does not depend strongly on the free-stream turbulence level or Reynolds number, as long as the boundary layer remains non-turbulent prior to separation. Strong acceleration prevents transition on the upstream part of the plate in all cases. Both free-stream turbulence and Reynolds number have strong effects on transition in the adverse pressure gradient region. Under low free-stream turbulence conditions transition is induced by instability waves in the shear layer of the separation bubble. Reattachment generally occurs at the transition start. At Re = 50, 000 the separation bubble does not close before the trailing edge of the modeled airfoil. At higher Re, transition moves upstream, and the boundary layer reattaches. With high free-stream turbulence levels, transition appears to occur in a bypass mode, similar to that in attached boundary layers. Transition moves upstream, resulting in shorter separation regions. At Re above 200,000, transition begins before separation. Mean velocity, turbulence and intermittency profiles are presented.
Effects of Reynolds Number and Free-Stream Turbulence on Boundary Layer Transition in a Compressor Cascade
Authors: Schreiber Heinz-Adolf , Steinert Wolfgang , Küsters Bernhard
Published: August 2014

An experimental and analytical study has been performed on the effect of Reynolds number and free-stream turbulence on boundary layer transition location on the suction surface of a controlled diffusion airfoil (CDA). The experiments were conducted in a rectilinear cascade facility at Reynolds numbers between 0.7 and 3.0×106 and turbulence intensities from about 0.7 to 4%. An oil streak technique and liquid crystal coatings were used to visualize the boundary layer state.

For small turbulence levels and all Reynolds numbers tested the accelerated front portion of the blade is laminar and transition occurs within a laminar separation bubble shortly after the maximum velocity near 35–40% of chord. For high turbulence levels (Tu > 3%) and high Reynolds numbers transition propagates upstream into the accelerated front portion of the CDA blade. For those conditions, the sensitivity to surface roughness increases considerably and at Tu = 4% bypass transition is observed near 7–10% of chord. Experimental results are compared to theoretical predictions using the transition model which is implemented in the MISES code of Youngren and Drela. Overall the results indicate that early bypass transition at high turbulence levels must alter the profile velocity distribution for compressor blades that are designed and optimized for high Reynolds numbers.
Separation Bubbles Under Steady and Periodic-Unsteady Main Flow Conditions
Authors: Lou Weiliang , Hourmouziadis Jean
Published: August 2014

Based on an experimental investigation carried out in a low speed test facility at the Berlin University of Technology, this paper describes the formation of separation bubbles under steady and periodic-unsteady main flow conditions. The aim of the investigation was to understand the mechanism of separation, transition and reattachment and the effect of main flow unsteadiness on it.

Separation bubbles for various main flow conditions were generated over a large flat plate, which experienced a similar pressure distribution to that on the suction surface of blades in turbomachines. The pressure distribution was generated by a contoured wall opposite the plate. Aimed at separating the effect of the velocity and the turbulence wake, this paper considers only the influence of the velocity wake. To this effect, a rotating flap was mounted downstream of the test section to produce periodic oscillations of the main flow.

The overall flow field under steady main flow conditions was obtained by hot-wire measurements. Pressure taps were used to measure the pressure distribution over the plate. The Reynolds number effects were determined and compared to the measurement results in the literature. Results for periodic-unsteady separation bubbles are shown using different Strouhal numbers, oscillation amplitudes and Reynolds numbers. Ensemble averaged mean velocity profiles and the Ensemble averaged rms velocity profiles are used to demonstrate the development of the periodic boundary layer. Time-space diagrams are plotted to show the development of the periodic-unsteady boundary layers. The characteristic instability frequencies in the free shear layer are identified. The impact of the major parameters, Strouhal number and amplitude, on the bubble formation are discussed.
Studies on Turbulence Structure of Boundary Layers Disturbed by Moving Wakes
Authors: Funazaki K. , Aoyama Y.
Published: August 2014

An attempt is made to measure wake-affected boundary layers on a flat plate by means of a split-film probe. Although a number of studies have been made to reveal the transitional behavior of boundary layers subjected to periodic wake passing, much is still unknown on the turbulence structure of those boundary layers. It is because most of the studies measured the boundary layers by single hot-wire probe. This measurement therefore aims at the determination of the turbulent structure of the incoming wakes as well as the disturbed boundary layers mainly in terms of ensemble-averaged Reynolds shear stress and turbulence intensity. Unsteady velocity vectors associated with the wakes and wake-induced turbulence patch (a number of turbulent spots) are also determined in detail. In comparison to a conventional how-wire probe, the probe used in this study is less sensitive to fluctuations of high frequency and less accurate in the near-wall measurement due to its relatively large diameter of its sensor. However, new and important information is obtained on the structure of wake-induced turbulence patch, which provides more insight into the wake-induced boundary layer transition than before. In addition, the measured velocity field following the turbulent patch indicates a clear image of calmed region.
Flow and Heat Transfer in a Rotating Cavity With a Stationary Stepped Casing
Authors: Jaafar Abdul A. , Motallebi Fariborz , Wilson Michael , Owen J. Michael
Published: August 2014

In this paper, new experimental results are presented for the flow in a co-rotating disc system with a rotating inner cylinder and a stationary stepped outer casing. The configuration is based on a turbine disc-cooling system used in a gas turbine engine. One of the rotating discs can be heated, and cooling air is introduced through discrete holes angled inward at the periphery of this disc. The cooling air leaves the system through axial clearances between the discs and the outer casing. Some features of computed flows, and both measured and computed heat transfer, were reported previously for this system. New velocity measurements, obtained using Laser Doppler Anemometry, are compared with results from axisymmetric, steady, turbulent flow computations obtained using a low-Reynolds-number k-ε turbulence model.

The measurements and computations show that the tangential component of velocity is invariant with axial location in much of the cavity, and the data suggest that Rankine (combined free and forced) vortex flow occurs. The computations fail to reproduce this behaviour, and there are differences between measured and computed details of secondary flow recirculations. Possible reasons for these discrepancies, and their importance for the prediction of associated heat transfer, are discussed.
A Method for Correlating the Influence of External Crossflow on the Discharge Coefficients of Film Cooling Holes
Authors: Rowbury D. A. , Oldfield M. L. G. , Lock G. D.
Published: August 2014

An empirical means of predicting the discharge coefficients of film cooling holes in an operating engine has been developed. The method quantifies the influence of the major dimensionless parameters, namely hole geometry, pressure ratio across the hole, coolant Reynolds number, and the freestream Mach number.

The method utilises discharge coefficient data measured on both a first-stage high-pressure nozzle guide vane from a modern aero-engine and a scale (1.4 times) replica of the vane. The vane has over 300 film cooling holes, arranged in 14 rows.

Data was collected for both vanes in the absence of external flow. These non-crossflow experiments were conducted in a pressurised vessel in order to cover the wide range of pressure ratios and coolant Reynolds numbers found in the engine. Regrettably, the proprietary nature of the data collected on the engine vane prevents its publication, although its input to the derived correlation is discussed.

Experiments were also conducted using the replica vanes in an annular blowdown cascade which models the external flow patterns found in the engine. The coolant system used a heavy foreign gas (SF6/Ar mixture) at ambient temperatures which allowed the coolant-to-mainstream density ratio and blowing parameters to be matched to engine values. These experiments matched the mainstream Reynolds and Mach numbers and the coolant Mach number to engine values, but the coolant Reynolds number was not engine representative (Rowbury et al., 1997 and 1998).

A correlation for discharge coefficients in the absence of external crossflow has been derived from this data and other published data. An additive loss coefficient method is subsequently applied to the cascade data in order to assess the effect of the external crossflow. The correlation is used successfully to reconstruct the experimental data. It is further validated by successfully predicting data published by other researchers.

The work presented is of considerable value to gas turbine design engineers as it provides an improved means of predicting the discharge coefficients of engine film cooling holes.
Application of Evaporative Coolers for Gas Turbine Power Plants
Authors: McNeilly J. Douglas
Published: August 2014

The functioning of evaporative coolers is based on the thermodynamic process called adiabatic saturation. It involves spraying a mist or fog of water into the air-stream entering a gas turbine; thereby saturating the air with water vapor before it enters the compressor. The evaporation of these water droplets takes energy out of the air thus reducing the inlet temperature to approximately the ambient wet bulb temperature. Gas turbines generally loose power when the inlet air temperature increases. Because of the temperature reduction caused by the saturation process, the use of evaporative coolers can increase power output of gas turbine power plants. The proper application of these devices requires the correct understanding of a) the basis of the site capacity requirement, b) the functioning of evaporative coolers, c) the use of statistical weather data, and d) site weather variations on both a daily and long term basis. It is frequently thought that evaporative coolers are not useful unless the site has near desert weather conditions. This is far from the truth. Depending on the contractual requirements to provide capacity, evaporative coolers can be justified for relatively humid sites as well as for the drier locations. Average or high temperatures in combination with an average or high relative humidity are frequently used in evaluating the application of evaporative coolers, even though they may lead to a very misleading combination of weather parameters. The value of evaporative coolers is discussed as well as a method for correctly evaluating the use of evaporative coolers with examples at four locations around the world.
Inlet Fogging of Gas Turbine Engines: Part A — Theory, Psychrometrics and Fog Generation
Authors: Meher-Homji Cyrus B. , Mee Thomas R.
Published: August 2014

Gas Turbine output is a strong function of the ambient air temperature with power output dropping by 0.3–0.5 % for every 1°F rise in ambient temperature. This loss in output presents a significant problem to utilities, cogenerators and IPPs when electric demands are high during the hot months. In the petrochemical and process industry, the reduction in output of mechanical drive gas turbines curtails plant output. One way to counter this drop in output is to cool the inlet air. The paper contrasts the traditional evaporative cooling technique with direct inlet fogging. The state of the art relating to fog generation and psychrometrics of inlet fogging are described.
Real-Time On-Line Performance Diagnostics of Heavy-Duty Industrial Gas Turbines
Authors: Gülen S. Can , Griffin Patrick R. , Paolucci Sal
Published: August 2014

This paper describes the results of real-time, on-line performance monitoring of two gas turbines over a period of five months in 1997. A commercially available software system is installed to monitor, analyze and store measurements obtained from the plant’s distributed control system. The software is installed in a combined-cycle, cogeneration power plant, located in Mass., USA, with two Frame 7EA gas turbines in April 1997.

Vendor’s information such as correction and part load performance curves are utilized to calculate expected engine performance and compare it with measurements. In addition to monitoring the general condition and performance of the gas turbines, user-specified financial data is used to determine schedules for compressor washing and inlet filter replacement by balancing the associated costs with lost revenue. All measurements and calculated information are stored in databases for real-time and historical trending and tabulating. The data is analyzed ex post facto to identify salient performance and maintenance issues.
Inverted Brayton Cycle Employment for Low Temperature Cogenerative Applications
Authors: Bianchi M. , Negri di Montenegro G. , Peretto A.
Published: August 2014

The employment of cogeneration plants for thermal and electric power production is constantly increasing especially for low power requirements. In most cases, to match these low power needs, the cogeneration plant is built up with diesel or gasoline engine or with gas turbine units.

In this paper, the performance, in terms of the most utilized cogenerative indexes, of an Inverted Brayton Cycle working with the gas exhausted by the open power plant have been evaluated.

Subsequently, the analysis of a cogenerative gas turbine equipped with IBC was carried out and the benefits numerically calculated.

It resulted that the IBC employment may increase of about 5 percentage points the plant electric efficiency, making this solution particularly attractive for cogenerative applications.
Performance Prediction of Small-Scale Tri-Generation Plants Based on Integrated SOFC and Microturbine Systems
Authors: Campanari Stefano , Macchi Ennio
Published: August 2014

The combined adoption of solid oxide fuel cells and microturbines has been recently considered for the development of small advanced plants for high efficiency distributed electric power generation. This paper investigates the application of this technology to an independent, small-capacity power plant, which operates in conjunction with a compression heat-pump/chiller system and eventually with an absorption chiller, cogenerating heating, cooling and electric power for a single user like a medium-size commercial building.

Detailed results are presented for both the application of a SOFC+microturbine system and a simple microturbine system, in terms of annual energy balances; they indicate, specially for the SOFC+microturbine systems, high primary energy savings and reduction in emissions of carbon dioxide. A preliminary economic analysis is presented, showing that, in absence of incentives, specific SOFC investment costs as low as 1200 $/kW are required to make economically attractive the adoption of this highly efficient and environmentally benign component.
Digital Controls and Modern Fuel Systems for Marine Prime Movers
Authors: Bellio Philip R.
Published: August 2014

Digital controls and modern fuel systems for marine prime movers are beginning to populate the waterways and oceans, but long after industrial facilities recognized their benefits and standardized on them. Additionally, it is somewhat ironic that many marine prime movers, particularly aeroderivative gas turbines, still utilize aircraft or aircraft-derivative hydromechanical controls, yet require significant engine room manning.

Digital controls, electrically actuated fuel and air valves, shipboard networks, and CRT-based operator interfaces have all been proven, and are available for every marine prime mover application. It appears to be more of a mindset than anything else preventing the move to electronic controls and modern fuel systems. Additional roadblocks to change may be “comfort with the old stuff”, marine certifications, and high availability of older hardware. As demands for improved efficiency, decreased emissions, and reduced operating expense continue there will be more pressure to upgrade existing older controls and fuel systems, as well as build ships with prime movers equipped with current control technology. A modern electronic control designed for prime movers which interfaces to not only the prime mover, can also provide data to shipboard systems, as well as be configured for satellite links to off-shore facilities.

Other advantages of a digital control and modern fuel system include:

• Software configurable/dependent

• Self-diagnostics and monitoring

• Condition-based maintenance

• Elimination of hydraulics

• Reduced manning requirements

• Precise air and fuel metering

• Current technology
Model-Based Detection of Leaks and Blockages in Pipes
Authors: Narendra Anju , Down John , Mathews Kirk , Rajamani Ravi , Leone Sal , Thatcher Jonathan , Norman Bruce
Published: August 2014

This paper deals with a model-based strategy for detecting leaks and blockages in a network of pipes. The fault detection and isolation (FDI) system uses a Kalman filter and a model of the piping system to decide whether the system is operating in a normal or failed state, and to distinguish the type of fault. While the idea of using Kalman filters is quite old, its application in the present case is novel, as is the formulation of the FDI system that uses only one model. The theory is backed by experimental validation on a test rig.
Component-Mode-Based Reduced Order Modeling Techniques for Mistuned Bladed Disks: Part I — Theoretical Models
Authors: Bladh Ronnie , Castanier Matthew P. , Pierre Christophe
Published: August 2014

Component mode synthesis (CMS) techniques are widely used for dynamic analyses of complex structures. Significant computational savings can be achieved by using CMS, since a modal analysis is performed on each component structure (substructure). Mistuned bladed disks are a class of structures for which CMS is well suited. In the context of blade mistuning, it is convenient to view the blades as individual components, while the entire disk may be treated as a single component. Individual blade mistuning may then be incorporated into the CMS model in a straightforward manner. In this paper, the Craig-Bampton (C-B) method of CMS is formulated specifically for mistuned bladed disks, using a cyclic disk description. Then a novel secondary modal analysis reduction technique (SMART) is presented: a secondary modal analysis is performed on a C-B model, yielding significant further reduction in model size. In addition, a straightforward non-CMS method is developed in which the blade mistuning is projected onto the tuned system modes. Though similar approaches have been reported previously, here it is generalized to a form that is more useful in practical applications. The theoretical models are discussed and compared from both computational and practical perspectives. It is concluded that using SMART, based on a C-B model, has tremendous potential for highly efficient, accurate modeling of the vibration of mistuned bladed disks.
Mode Localization in a Grouped Bladed Disk
Authors: Kuang J. H. , Huang B. W.
Published: August 2014

The effect of local blade crack on the mode localization in a rotating turbo disk with a group-blade assembly was studied. Periodically coupled Euler-Bernoulli beams were used to approximate the grouped and shrouded turbo blades. The cracked blade, regardedas a local disorder, was modeled using a two-span beam. A spring was imposed on this two-span beam to characterize the local crack. The Galerkin method was applied to formulate the localization equations of the mistuned system. Numerical results indicate that the blade crack, crack distribution and rotation speed in a rotating group-blades disk may affect the localization phenomenon significantly.
Forcing Function Measurements and Predictions of a Transonic Vaneless Counter Rotating Turbine
Authors: Weaver Matthew M. , Manwaring Steven R. , Abhari Reza S. , Dunn Michael G. , Salay Michael J. , Frey Kuk K. , Heidegger Nathan
Published: August 2014

Reducing the vibratory stress due to the forced response excitation of turbomachinery blades is an important engineering challenge facing designers. Detailed knowledge of the unsteady forces, the damping within the system, and the structural stiffness is required to predict the vibrational response and hence the high cycle fatigue life of a component. This study is focused on understanding the physical parameters influencing the unsteady forces causing the blade excitation in a transonic vaneless counter-rotating turbine, consisting of a vane row, a High Pressure (HP) spool, and a Low Pressure (LP) spool. Time averaged and time resolved measurements of the unsteady surface pressures on the HP and LP rotor blades are presented for a full scale rotating rig, using the actual engine components. Measurements were made and analyses performed at three different engine corrected aerodynamic conditions and with reduced frequencies (based on half blade chord) of approximately 10 for the unsteady aerodynamics. By varying the high-pressure rotor exit Mach number (1.44, 1.20, 1.05), the effects of varying the shock excitation to the LP blade row was studied. Extensive comparisons with CFD codes were obtained to determine flow-modeling requirements for the flow regimes studied. Comparison shows that for steady loading on the LP blade, 2D, single blade row Euler solvers are sufficient to achieve engineering accuracies. For the 1st harmonic unsteady loading, this level of modeling is adequate in the mid and lower half of the blade, but in the outer diameter region, three-dimensional effects require 3D modeling. The inclusion of nonlinear/viscous modeling shows moderately improved predictions.
Sensitivity Analysis of Blade Mode Shape on Flutter of Two-Dimensional Turbine Blade Sections
Authors: Tchernycheva Olga V. , Regard Sébastien , Moyroud François , Fransson Torsten H.
Published: August 2014

A parametric study on the flutter stability of a turbine cascade as a function of the torsion axis position, the bending direction and the reduced frequency is presented. In this process two different unsteady flow models are used in order to minimize the uncertainties of numerical modeling on the physical conclusions of the study. Comparisons are performed against available experimental data.

It was found that the comparison of the global aerodynamic damping between numerical results and experimental data was reasonably good. It was observed that the stability was more sensitive to changes in the mode shape than in the reduced frequency. Comparisons of the local unsteady pressures showed similar tendencies for the numerical models and the experimental data, while discrepancies on the blade suction surface between the models were observed around the trailing edge for the subsonic flow and close to shock location for the transonic flow. The results indicated interesting agreement of the mode shape stability maps with results obtained on a largely different low-pressure turbine blade.
A Coupled Fluid-Structure Analysis for 3D Flutter in Turbomachines
Authors: Gnesin Vitaly , Rządkowski Romauld , Kolodyazhnaya Luba
Published: August 2014

A three-dimensional nonlinear time-marching method and numerical analysis for aeroelastic behaviour of oscillating blade row has been presented. The approach is based on the solution of the coupled fluid-structure problem in which the aerodynamic and structural equations are integrated simultaneously in time. In this formulation of a coupled problem, the interblade phase angle at which a stability (or instability) would occur, is a part of the solution. The ideal gas flow through multiple interblade passage (with periodicity on the whole annulus) is described by the unsteady Euler equations in the form of conservative laws, which are integrated by use of the explicit monotonous second order accurate Godunov-Kolgan volume scheme and a moving hybrid H-H (or H-O) grid. The structure analysis uses the modal approach and 3D finite element model of the blade. The blade motion is assumed to be a linear combination of modes shapes with the modal coefficients depending on time. The influence of the natural frequencies on the aerodynamic coefficient and aeroelastic coupled oscillations for the Fourth Standard Configuration is shown. The stability (instability) areas for the modes are obtained. It has been shown that interaction between modes plays an important role in the aeroelastic blade response. This interaction has essentially nonlinear character and leads to blade limit cycle oscillations.
A Navier-Stokes Analysis of the Stall Flutter Characteristics of the Buffum Cascade
Authors: Weber Stefan , Platzer Max F.
Published: August 2014

Numerical stall flutter prediction methods are highly needed as modern jet engines require blade designs close to the stability boundaries of the performance map.

A Quasi-3D Navier-Stokes code is used to analyze the flow over the oscillating cascade designed and manufactured by Pratt & Whitney, and studied at the NASA Glenn Research Center by Buffum et al. The numerical method solves for the governing equations with a fully implicit time-marching technique in a single passage by making use of a direct-store, periodic boundary condition. For turbulence modeling the Baldwin-Lomax model is used. To account for transition, the criterion to predict the onset location suggested by Baldwin and Lomax is incorporated.

Buffum et al. investigated two incidence cases for three different Mach numbers. The low-incidence case at a Mach number of 0.5 exhibited the formation of small separation bubbles at reduced oscillation frequencies of 0.8 and 1.2. For this case the present approach yielded good agreement with the steady and oscillatory measurements. At high-incidence at the same Mach number of 0.5 the measured steady-state pressure distribution and the separation bubble on the upper surface was also found in good agreement with the experiment. But computations for oscillations at high-incidence failed to predict the negative damping contribution caused by the leading edge separation.
Synchronous Instability Problems in High Performance Turbomachines During Steady State Full Load Operation
Authors: Haq Inam U. , Kumar Chittineni V. , Basheer Adel H.
Published: August 2014

Synchronous instability problems in high speed and large throughput turbomachinery are experienced and realized as a serious potential threat to mechanical dynamics and structural integrity of machines and, consequently, to plant continuous production. Four different cases are being reported where shaft vibrations cropped up at constant speed and full load operation. Three cases are pertained to the steam turbines and one is related to the high pressure centrifugal compressor. Synchronous vibration characteristics of the turbomachines are found distinct and are classified on the basis of phase angle change. The successive increase and decrease of synchronous amplitudes, in two cases, were always accompanied by the simultaneous shift in the phase angle whereas, in the rest two cases, the phase angle remained at steady state level. The excitation sources, responsible for generation of synchronous vibrations, have been identified through practical investigations and possible explanations are furnished in view of the infantile theory of synchronous rotordynamics instability.
Dynamics and Stability of Rotor/Stator Systems With Rubs
Authors: Jiang Jun , Ulbrich Heinz
Published: August 2014

This paper carries out an analytical stability study on the solutions of externally excited, symmetrical rotor/stator systems with full annular rubs. The obtained analytical stability conditions contribute to a better understanding of the dynamical jump phenomena, which correspond to the occurrence of the saddle-node bifurcation of the full annular rub solutions, and the transition between full annular rubs and partial rubs, which corresponds to the Hopf bifurcation. A systematic investigation of the influence of the system parameters on these phenomena is performed. It is also found by means of numerical simulations that the Hopf bifurcation boundaries of the full annular rub solutions can serve as good approximations or conservative boundaries for the dry whip instability of the considered rotor system.
Identification of Bearing Coefficients of Flexible Rotor-Bearing Systems
Authors: Yang Tachung , Chaung Wei-Ching
Published: August 2014

The accuracy of stiffness and damping coefficients of bearings is critical for the rotordynamic analysis of rotating machinery. However, the influence of bearings depends on the design, manufacturing, assembly, and operating conditions of the bearings. Uncertainties occur quite often in manufacturing and assembly, which causes the inaccuracy of bearing predictions. An accurate and reliable in-situ identification method for the bearing coefficients is valuable to both analyses and industrial applications.

The identification method developed in this research used the receptance matrices of flexible shafts from FEM modeling and the unbalance forces of trial masses to derive the displacements and reaction forces at bearing locations. Eight bearing coefficients are identified through a Total Least Square (TLS) procedure, which can handle noise effectively. A special feature of this method is that it can identify bearing coefficients at a specific operating speed, which make it suitable for the measurement of speed-dependent bearings, like hydrodynamic bearings. Numerical validation of this method is presented. The configurations of unbalance mass arrangements are discussed.
Enhancements to AMB Force Measurement Procedures for Application to a Rocket Thrust Measurement System
Authors: Imlach Joe , Kasarda Mary E. F. , Balaji P. A.
Published: August 2014

Active Magnetic Bearings (AMBs) can be used concurrently as support bearings and as load cells for the measurement of support forces. This paper discusses the preliminary design for a test rig to simulate a full-scale rocket thrust measurement system utilizing AMBs and procedures that have been developed to enhance the use of AMBs in this application. These enhancements include the development of a model of the effect of fringing on force measurements and an on-line calibration procedure. The fringing effect model is based on actuator geometry and has been verified through finite element analyses. On-line calibration procedures for a magnetic thrust bearing arrangement have also been developed and verified through finite element analyses. Since the rocket thrust measurement system is not rotating, planar magnetic bearings will also be used for vertical support and will utilize the same calibration procedure, resulting in force and torque measurements in all six degrees of freedom.
Analysis of Aerodynamically Induced Whirling Forces in Axial Flow Compressors
Authors: Spakovszky Z. S.
Published: August 2014

A new analytical model to predict the aerodynamic forces in axial flow compressors due to asymmetric tip-clearance is introduced. The model captures the effects of tip-clearance induced distortion (i.e. forced shaft whirl), unsteady momentum-induced tangential blade forces and pressure induced forces on the spool. Pressure forces are shown to lag the tip-clearance asymmetry, resulting in a tangential (i.e. whirl-inducing) force due to spool pressure. This force can be of comparable magnitude to the classical Alford force. Prediction and elucidation of the Alford force is also presented.

In particular, a new parameter denoted as the blade loading indicator is deduced. This parameter depends only on stage geometry and mean flow and determines the direction of whirl tendency due to tangential blade loading forces in both compressors and turbines.

All findings are suitable for incorporation into an overall dynamic system analysis and integration into existing engine design tools.
Nozzle Cracking in Gas Turbines
Authors: Shirmohamadi Manuchehr , Bratt Shawn , Wang Difei , Ganji Ahmad , Neville Rob
Published: August 2014

Cracking of 1st stage nozzles occurs routinely in industrial gas turbines. Such cracking, which sometimes is observed after only a few operating cycles, leads to short intervals between overhauls and expensive repairs or replacements of the nozzle sections. Despite the enormous economic costs associated with nozzle cracking; mechanisms, root causes, and optimum mitigation of such cracking has not been convincingly determined. In fact, some original equipment manufacturers (OEM) consider nozzle cracking as a “fact of life” and have only issued recommendations for their monitoring and replacements.

The objective of this work was to identify the mechanisms and the root causes for the 1st stage nozzle cracking of a Frame-3 gas turbine. In this project, we measured, monitored, modeled, and analyzed the temperature and stress response of the nozzles during actual operating practices. This work concluded that the primary mechanism of such cracking is low cycle fatigue caused by high cyclic stresses. The major source of these high stresses was determined to be related to thermal and pressure shocks caused by transonic events which occur during the unit startups. This work offers recommendations to mitigate such cracking and a testing program to verify the root cause.
A Transport Model for the Deterministic Stresses Associated With Turbomachinery Blade Row Interactions
Authors: van de Wall Allan G. , Kadambi Jaikrishnan R. , Adamczyk John J.
Published: August 2014

The unsteady process resulting from the interaction of upstream vortical structures with a downstream blade row in turbomachines can have a significant impact on the machine efficiency. The upstream vortical structures or disturbances are transported by the mean flow of the downstream blade row, redistributing the time-average unsteady kinetic energy (K) associated with the incoming disturbance. A transport model was developed to take this process into account in the computation of time-average multistage turbomachinery flows.

The model was applied to compressor and turbine geometry. For compressors, the K associated with upstream 2-D wakes and 3-D tip clearance flows is reduced as a result of their interaction with a downstream blade row. This reduction results from inviscid effects as well as viscous effects and reduces the loss associated with the upstream disturbance. Any disturbance passing through a compressor blade row results in a smaller loss than if the disturbance was mixed-out prior to entering the blade row.

For turbines, the K associated with upstream 2-D wakes and 3-D tip clearance flows are significantly amplified by inviscid effects as a result of the interaction with a downstream turbine blade row. Viscous effects act to reduce the amplification of the K by inviscid effects but results in a substantial loss. 2-D wakes and 3-D tip clearance flows passing through a turbine blade row results in a larger loss than if these disturbances were mixed-out prior to entering the blade row.
Investigation of the Flowfield in the Transonic VKI BRITE EURAM Turbine Stage With 3D Steady and Unsteady N-S Computations
Authors: Laumert Björn , Mårtensson Hans , Fransson Torsten H.
Published: August 2014

This paper presents the results from three-dimensional (3D) steady and unsteady Navier-Stokes computations, performed on the transonic VKI BRITE EURAM test turbine stage. The work aimed at giving deeper insight in the aerodynamics of the turbine stage. The analysis has been carried out with the nominal stator trailing edge ejection slot geometry and cooling flow ejection. Additionally a simplified rounded stator trailing edge was employed. The results from the unsteady computations were compared with measured pressure perturbation traces at 22 locations around the rotor blade at midspan. Computations with both the ejection slot and the rounded stator trailing edge geometry were in good agreement with the measurements on the pressure side and half chord of the rotor blade’s suction side. Measurements and computations showed less good agreement downstream a weak shock on the suction side of the rotor blade. The measured pressure double peak in the rotor blade leading edge region is only observed in the computations with the ejection slot geometry.
Numerical and Experimental Investigation of Unsteady Flow Interaction in a Low Pressure Multistage Turbine
Authors: Höhn Wolfgang , Heinig Klaus
Published: August 2014

The paper presents results of unsteady viscous flow calculations and corresponding cold flow experiments of a three stage low pressure turbine. The investigation emphasize the study of unsteady flow interaction. A time accurate Reynolds averaged Navier-Stokes solver is applied for the computations. Turbulence is modeled using the Spalart-Allmaras one equation turbulence model and the influence of modern transition models on the unsteady flow predictions is investigated. The integration of the governing equations in time is performed with a four stage Runge-Kutta scheme, which is accelerated by a two grid method in the viscous boundary layer around the blades. At the inlet and outlet non-reflecting boundary conditions are used. The quasi 3D calculations are conducted on a stream surface around midspan allowing a varying stream tube thickness. In order to study the unsteady flow interaction a three stage low pressure turbine rig of a modern commercial jet engine is built up. Besides the design point, the Reynolds number, the wheel speed and the pressure ratio are varied in the tests. The numerical method is able to capture important unsteady effects found in the experiments, i.e. unsteady transition as well as the blade row interaction. In particular, the flow field with respect to time averaged and unsteady quantities such as surface pressure, entropy and skin friction is compared with the experiments conducted in the cold air flow test rig.
Effects of Tip Clearance on Hot Streak Migration in a High-Subsonic Single-Stage Turbine
Authors: Dorney Daniel J. , Sondak Douglas L.
Published: August 2014

Experimental data have shown that combustor temperature non-uniformities can lead to the excessive heating of first-stage rotor blades in turbines. This heating of the rotor blades can lead to thermal fatigue and degrade turbine performance. The results of recent studies have shown that variations in the circumferential location, or clocking, of the first-stage vane airfoils can be used to minimize the adverse effects of the hot streaks due to the hot fluid mixing with the cooler fluid contained in the vane wake. In addition, the effects of the hot streak/airfoil count ratio on the heating patterns of turbine airfoils have been quantified. In the present investigation, three-dimensional unsteady Navier-Stokes simulations have been performed for a single-stage high-pressure turbine geometry operating in high subsonic flow to study the effects of tip clearance on hot streak migration. Baseline simulations were initially performed without hot streaks to compare with the experimental data. Two simulations were then performed with a superimposed combustor hot streak; in the first the tip clearance was set at the experimental value, while in the second the rotor was allowed to scrape along the outer case (i.e., the limit as the tip clearance goes to zero). The predicted results for the baseline simulations show good agreement with the available experimental data. The simulations with the hot streak indicate that the tip clearance increases the radial spreading of the hot fluid, and increases the integrated rotor surface temperature compared to the case without tip clearance.
Unsteady Effects in Turbine Tip Clearance Flows
Authors: Prasad Anil , Wagner Joel H.
Published: August 2014

The present study is concerned with the unsteady flow field on the blade outer air seal segments of high-work turbines; these segments are installed between the blade tip and outer casing and are usually subjected to extreme heat loads. Time-resolved measurements of the unsteady pressure on the blade outer air seal are made in a low-speed turbine rig. The present measurements indicate the existence of a separation zone on the blade tip which causes a vena contracta to form at the entrance of the tip gap. In addition, a careful comparison between the ensemble-averaged pressure measurement and the corresponding result from steady computation suggests that the pressure on the blade outer air seal can largely be described as being due to a steady flow (in the rotating frame) sweeping past a stationary probe. The ensemble deviation measurement indicates that unsteadiness (from one revolution to the next) is confined to the tip gap.
Experimental and Computational Investigation of the Time-Averaged and Time-Resolved Pressure Loading on a Vaneless Counter-Rotating Turbine
Authors: Haldeman C. W. , Dunn M. G. , Abhari R. S. , Johnson P. D. , Montesdeoca X. A.
Published: August 2014

The experimental program reported here was executed using full-scale vaneless counter-rotating engine hardware operating at nondimensionally scaled aerodynamic design point conditions. Measurements were obtained for three different pressure ratio values: design point, low pressure ratio, and high pressure ratio. For brevity, only the design point data will be presented in this paper. Time-averaged and time-resolved surface pressures on the high pressure turbine (HPT) vane, HPT blade, and low pressure turbine (LPT) blades are presented. Additionally, three-dimensional (3D) Navier-Stokes computational fluid dynamics (CFD) predictions are presented for comparison with experimental data. The results presented show that the predictions qualitatively capture the flowfield physics, but require some additional calibration to fully match experimental data quantitatively.
Inverse Design of Centrifugal Impellers by Simultaneous Modification of Blade Shape and Meridional Contour
Authors: Paßrucker H. , Van den Braembussche R. A.
Published: August 2014

A numerical procedure for the design of a centrifugal compressor impeller with prescribed isentropic Mach number distribution along the blade suction and pressure side is described. Both the blade shape and the endwalls are defined at the same time. This method is a combination of a 3D and a quasi-3D flow analysis programs with a quasi-3D geometry modification algorithm. The 3D analysis program is used for a given impeller geometry to calculate the velocity distribution along the blade. The difference between the desired velocity at pressure and suction side and the one calculated by the 3D analysis program for the actual geometry, is used to calculate the required modification of the mean velocities and swirl in the flow. These values are added to the values given by the quasi-3D analysis program to define the new quasi-3D target used for the modification of the geometry. The endwalls are designed with the transpiration method so that a prescribed distribution of mean velocity at hub and shroud is reached. The blade shape is modified until a prescribed swirl distribution is obtained. An iterative use of these programs results in a geometry for which the calculated 3D velocity is equal or as close as possible to the desired one. The practical application of this procedure to the design of centrifugal compressor impellers is illustrated by an example.
Incipient Rotating Stall Characteristics Measured by Semiconductor L2F and Pressure Transducer in Vaneless Diffusers of Centrifugal Blowers
Authors: Ishida Masahiro , Sakaguchi Daisaku , Ueki Hironobu
Published: August 2014

In order to clarify the transient behavior from the non-stall condition to the developed rotating stall in vaneless diffuser of a centrifugal blower, blade-to-blade variations of velocity and wait pressure were measured immediately upstream and downstream of the impeller exit as well as in the vaneless diffuser by means of the semiconductor laser 2-focus velocimeter and the pressure sensor respectively. The specified auto-correlation coefficient of large scale disturbances with the wavelength larger than twice of the blade spacing show that the intermittent stall occurs prior to the developed rotating stall. By analyzing the flow-fluctuation-ellipse and the correlation coefficient with respect to the radial and tangential components of velocity fluctuation, it is found that the intermittent stall as well as the developed rotating stall are triggered by the interaction between the reverse flow in the diffuser entry region and the jet-wake like flow discharged from the impeller.