ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V006T00A001. doi:10.1115/GT2016-NS6.

This online compilation of papers from the ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition (GT2016) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster


2016;():V006T02A001. doi:10.1115/GT2016-56280.

3D four-directional braided composites are becoming widely used in the aeroengines due to their excellent transverse properties such as stiffness, strength, fracture toughness and damage resistance. In spite of great achievements in composite materials, the model of 3D four-directional braided composites gives rise to considerable challenge in establishing interior fiber bundle structure which is curved and twisted in jamming condition. Original circular cross-section of fiber bundle is squeezed into oval shape ellipse in manufacturing process of the jamming action. Thus, a novel mesoscopic modeling approach for 3D four-directional braided composites was proposed in this study, which considered the fiber bundle cross-section’s deformation. Firstly, an analytic equation to describe the transformation of fiber bundle cross-section was established based on the equal area of the ellipse and circle. Secondly, the parameters of this equation were achieved using the Matlab simulation. It was concluded that the compacted, non-interfered fiber bundle model constructed was in good agreement with actual structure. This paper provides the mathematical relationship between braiding parameters and geometric dimensions of unit cell model. Numerical results showed that the value of braiding pitch length has a relative calculation error less than 4% compared with test data. The modeling technique lays a foundation for further mesomechanics investigation on 3D four-directional braided composites.

Commentary by Dr. Valentin Fuster
2016;():V006T02A002. doi:10.1115/GT2016-56507.

Electrical resistance (ER) is a relatively new approach for real-time monitoring and evaluating damage in SiC/SiC composites for a variety of loading conditions. In this study, ER of woven silicon carbide fiber-reinforced silicon carbide composite systems in their pristine and impacted state were measured under cyclic loading conditions at room and high temperature (1200C). In addition, modal acoustic emission (AE) was also monitored, which can reveal the occasion of matrix cracks and fiber. ER measurement and AE technique are shown in this study to be useful methods to monitor damage and indicate the failure under cyclic loading. Based on the slope of the ER evolution, an initial attempt has been made to develop a method allowing a critical damage phase to be identified. While the physical meaning of the critical point is not yet clear, it has the potential to allow the failure to be indicated at its early stage.

Commentary by Dr. Valentin Fuster
2016;():V006T02A003. doi:10.1115/GT2016-56614.

The structural reliability of composite parts for aircraft is established through the “building block” approach, which is a series of tests that are conducted using specimens of various levels of complexity. In this approach, the failure modes and criteria are validated step by step with tests and analysis at coupon, element, sub-component, and component levels.

IHI is developing ceramic matrix composite (CMC) components for aircraft engines to realize performance improvement and weight reduction. We conducted the concept design of CMC low pressure turbine (LPT) blade with the building block approach. In this paper, we present the processes and results of the design, which was supported by a series of tests.

Typical low pressure turbine blade has dovetail, airfoil and tip shroud. Each element has different function and characteristic shape. In order to select the configuration of CMC LPT blade, we conducted screening tests for each element. The function of dovetail is to sustain the connection with blade and disk against centrifugal force. The failure modes and strength of dovetail elements were examined by static load tests and cyclic load tests. The configuration of airfoil was selected by modal tests. The function of tip shroud is forming gas passage and reducing the leakage flow, therefore this portion needs to sustain the shape against the centrifugal force and the rubbing force. The feasibility of tip shroud was verified by spin tests and rubbing tests. The initial CMC LPT blades were designed as combination of the selected elements by these screening tests. Prototype parts were made and tested to check the manufacturability and the structural feasibility. The static strength to the centrifugal force was examined by spin test. The durability to vibration was examined by HCF test.

Commentary by Dr. Valentin Fuster
2016;():V006T02A004. doi:10.1115/GT2016-57323.

Ceramic matrix composites (CMC) offer the potential of increased service temperatures and are thus an interesting alternative to conventional combustor alloys. Tubular combustor liner demonstrators made of an oxide/oxide CMC were developed for a lean combustor in a future aero-engine in the medium thrust range and tested at engine conditions. During the design various aspects like protective coating, thermo-mechanical design, development of a failure model for the CMC as well as design and test of an attachment system were taken into account. The tests of the two liners were conducted at conditions up to 80% take-off. A new protective coating was tested successfully with a coating thickness of up to t=1 mm. Different inspection criteria were derived in order to detect crack initiation at an early stage for a validation of the failure model. With the help of detailed pre- and post-test computer tomography scans to account for the micro structure of the CMC the findings of the failure model were in reasonable agreement with the test results.

Commentary by Dr. Valentin Fuster
2016;():V006T02A005. doi:10.1115/GT2016-58058.

Foreign object damage (FOD) behavior of an N720/alumina oxide/oxide ceramic matrix composite (CMC) was characterized at ambient temperature by using spherical projectiles impacted at velocities ranging from 100 to 350 m/s. The CMC targets were subject to ballistic impact at a normal incidence angle while being loaded under different levels of tensile loading in order to simulate conditions of rotating aeroengine airfoils. The impact damage of frontal and back surfaces was assessed with respect to impact velocity and load factor. Subsequent post-impact residual strength was also estimated to determine quantitatively the severity of impact damage. Impact force was predicted based on the principles of energy conservation.

Commentary by Dr. Valentin Fuster
2016;():V006T02A006. doi:10.1115/GT2016-58076.

ASTM test standards for CMC’s Crack Growth Resistance (CGR) may exhibit a zig-zag (wavy) crack path pattern, and fiber bridging. The experimental parameters that may contribute to the difficulty can be summarized as: specimen width and thickness, interface coating thickness, mixed mode failure evolution, and interlaminar defects. Modes I crack growth resistances, GI were analytically determined at ambient temperature using wedge test, a modified double cantilever beam (DCB). Several Finite Element (FE) based Multi-scale modeling potential techniques were investigated: a) Multi-scale progressive failure analysis (MS-PFA); b) Virtual Crack Closure Technique (VCCT). Advantages and disadvantages of each were identified. The final modeling algorithm recommended was an integrated damage and fracture evolution methodology using combined MS-PFA and VCCT.

The material tested in this study was a slurry-cast melt-infiltrated SiC/SiC composite with Tyranno ZMI fibers (Ube Industries, Kyoto, Japan) and a BN interphase. The fiber architecture consisted of eight plies of balanced 2-D woven five-harness satin. The total fiber volume fraction was about 30% with half of the fibers in the 0° direction and half in the 90° direction. All specimens had a nominal thickness of 4 mm. An alumina wedge with 18° head angle (2α) was used. In this method, a splitting force is created by inserting a vertically-moving wedge in a notch causing the arms to separate and forcing an interlaminar crack at the sharpest end of the notch The MS-PFA numerical model predicted the damage and fracture evolution and utilized the GENOA UMAT (User Material Subroutine) for Damage and FEM (Finite Element Model) stress intensity and LEFM (Linear elastic Fracture Model), Cohesive Model for Fracture. The analysis results (Fracture energy vs. crack length, Fracture energy vs. load, Fracture energy vs. crack opening displacement) matched the Mode I coupon tests and revealed the following key findings. Mode I-Wedge specimen exhibits: 1) failure mode is due to interlaminar tension (ILT) only in the interface section and a zig-zag pattern observed; 2) VCCT crack growth resistance is well matched to the test data; and 3) failure mode is a mixed mode behavior of Interlaminar tension (ILT) to interlaminar shear (ILS). The final Wedge test specimen configuration optimization includes the sensitivity of design parameters to CGR: a) wedge contact coefficient of friction; b) lever arms thickness, and c) inclined head angle, distance between the initial crack and wedge tip.

Commentary by Dr. Valentin Fuster
2016;():V006T02A007. doi:10.1115/GT2016-58086.

SiC-based ceramic matrix composites (CMC) in turbine engine applications must sustain fatigue residual life after foreign object impacts that might occur in services. Experiments, nondestructive evaluations (NDE), and simulations have illustrated good correlations between impact energy and foreign object damage (FOD) and fatigue life after impact at room and 1200°C temperatures. Flat and curved five-harness satin (5HS) woven CMCs specimens, consisting of Hi-Nicalon Type S (Goodrich) and Hi-Nicalon (Rolls Royce) in MI SiC matrix, were tested and simulated. Tests measured electrical resistivity (ER), acoustic emission (AE), and microscopy. Simulations used a building block validation strategy and the Multi-Scale Progressive Failure Analysis (MS-PFA) method. Simulations complemented experiments in understanding and predicting the damage states, of impact, and fatigue residual strength after impact of CMCs to form a more complete understanding of the damage mechanisms involved in such events. The GENOA software developed by Alpha STAR Corporation [1, 2, 3] is capable of Durability and Damage Tolerance (D&DT), life, and reliability predictions by means of multi-scale progressive failure analysis (damage and fracture evolution). In general, CMCs are modeled using effective fiber, matrix, and interface constitutive behaviors, from which the lamina stiffness, strengths, and the strain rate effect can be derived. Similarly, the fatigue strength and stiffness degradation, and the effect of defects in a matrix micro crack density, voids, as well as fibers waviness, and damages after impact can be characterized. The final simulation is static loading and impact on a generic CMC SiC/SiC (Sylramic MI 5HS) blade which is to be used in future blade optimization based on minimizing damage incurred.

The GENOA software platform supports FAA recommended ASTM standard Building-Block Validation Strategy with reduced tests conducting: 1) Material Calibration and Qualification, and 2) FEM Verification, Validation, and 3) Blind Predictions (Accreditation). The simulation and test comparisons performed included the damage size for both the CMC (fracture) and the steel impactor (plastic deformation), rebound velocities, SN curves for fatigue of pristine and impacted specimens at room and high temperatures. All simulations showed good correlation. The MS-PFA tool demonstrated a great potential for CMC post FOD fatigue life for part certification supported with reduced tests.

Commentary by Dr. Valentin Fuster

Controls, Diagnostics and Instrumentation

2016;():V006T05A001. doi:10.1115/GT2016-56007.

An annular pulsed detonation combustor basically consists of a number of detonation tubes which are firing in a predetermined sequence into a common downstream annular plenum. Fluctuating initial conditions and fluctuating environmental parameters strongly affect the detonation. Operating such a set-up without misfiring is delicate. Misfiring of individual combustion tubes will significantly lower performance or even stop the engine. Hence, an operation of such an engine requires a misfiring detection.

Here, a supervised data driven machine learning approach is used for the misfiring detection. The features used as inputs for the classifier are extracted from measurements incorporating physical knowledge about the given set-up. To this end, a neural network is trained based on labeled data which is then used for classification purposes, i.e., misfiring detection. A surrogate, non-reacting experimental set-up is considered in order to develop and test these methods.

Commentary by Dr. Valentin Fuster
2016;():V006T05A002. doi:10.1115/GT2016-56166.

The new energy mix places greater demands on power gas turbine operation; precision combustion monitoring, therefore has become a major issue. Unforeseen events such as combustion instabilities can occur and represent a danger to the integrity of the hot parts and also lead to a limitation of the output power. This is usually accompanied by an increase in maintenance costs. The enlarged off-design operating envelope of gas turbines to adapt to a fast-changing grid has made this issue even more acute, necessitating an expansion of the operating envelope into areas that were — for many engines — not foreseen in the original combustor design process. A good understanding of what happens within the gas turbine combustor is crucial. Complex and costly full-field measurements such as laboratory optical instrumentation in precision combustion diagnostics are not suitable for permanent fleet deployment. For practical and financial reasons, the monitoring should ideally be achieved with a limited amount of discrete sensors. If installed and interpreted correctly, fast response measurement chains could lead to a better gas turbine combustion management, possibly yielding considerable savings in terms of operating and maintenance costs. The firm Meggitt Sensing System (MSS), assisted by Combustion Bay One (CBOne), initiated an applied research programme dedicated to this topic — with MSS providing the instrumentation and CBOne providing the facility and test conditions. The objective was to investigate realistic combustion phenomena in a precisely controlled and reproducible way and to document the individual readings of the heat-resistant fast pressure transducers mounted on the combustor casing, as well as the accelerometers mounted on the outer surface of the machine. Particular attention was paid to the correlation between these two types of sensor readings. This paper reports on the monitoring of the flame using piezoelectric dynamic pressure sensors and accelerometers in a number of different situations that are relevant to the safe and efficient operation of gas turbines. Discussed are single events such as flame ignition, lean blow-out and flash-back, as well as longer test sequences observing the effect of warming-up or the presence of flame instability. The measurement chains and processing techniques are discussed in detail. The atmospheric test rig used for this purpose and the different testing configurations required for each of these situations are also illustrated in detail. The results and recommendations for their implementation in an industrial context conclude this paper.

Commentary by Dr. Valentin Fuster
2016;():V006T05A003. doi:10.1115/GT2016-56242.

Performance of the compressors deteriorates due to detrimental effects of fouling on the aerodynamic flow characteristic. The compressors need periodic clean up services to re-gain the designed performance. Apart from the operating time, the ambient and the operating conditions affect the fouling phenomenon making accurate scheduling for predictive maintenance very difficult. In this work, the symptoms of compressor fouling are captured through the evolution of the compressor map in terms of loss of isentropic efficiency and mass flow decrease. Compressor mass flow and the rate of humidity condensation at the inlet of the compressor are identified as the effective factors on the fouling rate. Humidity condensation has a competing effect on fouling rate; increment of the condensed humidity up to a certain level accelerates the fouling rate, while additional mist has an inverse effect. The complex effect of the condensed humidity along with the air mass flow is extracted through training an adaptive neuro-fuzzy inference system. The resulting model reveals how the efficiency and the mass flow of the compressor map vary as a result of fouling development, given the mass flow and the humidity condensation history. The methodology is verified using data from a similar compressor commissioned at a different period.

Topics: Compressors
Commentary by Dr. Valentin Fuster
2016;():V006T05A004. doi:10.1115/GT2016-56300.

Observers are usually used in model-based FDI system for aero engines, especially for control system sensors. However, when a bank of traditional observers is used to detect gas path faults, it seems difficult to locate or isolate them, which is not a problem for sensor FDI. As a solution, a modified design method for a kind of robust observer called UIO (Unknown Input Observer) is proposed in this paper. An augmented linear model of a big turbofan is built at first. The degradations of compressor and HP turbine efficiencies are integrated as model inputs. Then some functions reflecting the FDI performance of UIO for these degradations can be obtained. DE (Differential Evolution) algorithm is introduced to solve these equations, whose best solution means the residual of observer is only sensitive for the specified gas path fault and not effected by other faults and unknown inputs. Some simulation results for UIOs designed by modified method are given to verify their FDI performance for compressor and HP turbine efficiencies.

Commentary by Dr. Valentin Fuster
2016;():V006T05A005. doi:10.1115/GT2016-56304.

Sensors installed on gas turbine gas path are used to obtain gas path measurement parameters for control and condition monitoring purpose. These sensors are prone to degradation or failure due to hostile working environment around them. Most gas path sensor diagnostic research is based on an assumption that the power setting sensor, a sensor used by engine control system to control engine power output, has no fault so engine measurement data can always be obtained at desired operating conditions. However in practice, power setting sensor may also be faulty, which may result in misleading measurement data and diagnostic results.

In this paper, an artificial neural network based gas path diagnostic approach for engine power setting sensor fault detection and quantification has been introduced. Nested artificial neural networks (ANN) are used to detect power setting sensor fault and ensure prediction accuracy. Measurement noise is also considered in the training and testing samples to ensure the robustness of the diagnostic system.

The developed power setting sensor diagnostic approach has been applied to a model 2-shafts industrial gas turbine engine similar to a GE LM2500+G4 engine to test the effectiveness of the approach. The selected power setting parameter is the shaft power output measured by a power setting sensor. An engine performance model is produced using Cranfield University’s gas turbine performance and diagnostics software, Pythia. Training samples with the consideration of sensor faults were simulated with the engine model assuming one of the sensors, either the power setting sensor or other gas path sensors may be faulty. In the nested neural network for sensor fault diagnostics, the system separately performs sensor fault detection, sensor fault identification and sensor fault quantification.

Results show that the developed nested neural network diagnostic system is able to identify the power setting sensor fault and correctly predict the magnitude of the fault. This would allow the engine control system correct its control schedule and accommodate the power setting sensor fault.

Commentary by Dr. Valentin Fuster
2016;():V006T05A006. doi:10.1115/GT2016-56330.

Extending disc life through online health monitoring has been a proven method of minimising engine downtime and maintenance costs. To properly monitor the disc requires a robust model of the disc’s non-linear thermal dynamics. A model can be improved by filtering the output using a measurement of the disc in real time. The damage models can then be computed with higher statistical confidence leading to increased safe life prediction. Recently, a model of disc temperature has been developed based on the proper orthogonal decomposition of simulated data. The model produced detailed thermal gradients for use in damage calculation and life assessment. This paper presents the development and implementation of a Kalman filter to augment that model. The location of the measurement has been assessed in order to select the most appropriate target for instrumentation. Points all around the front and back of the disc have been assessed, and the best practice result is found to be near the centre of the disc neck. Matching temperatures at this point represents a compromise between the fast dynamic response of the rim, with the slower response of the cob. The new model has been validated against an independent flight simulation that had previously been excluded from any training process. The addition of the Kalman filter allows the model to match aircraft dynamics outside the regular training trajectories. The accuracy is approximately ±30K, and there is a root-mean-square error of only 2K over the whole model at any one point in time.

Commentary by Dr. Valentin Fuster
2016;():V006T05A007. doi:10.1115/GT2016-56365.

High OPR engine cycles for reduced NOx emissions will generate new aggravated requirements and boundary conditions by implementing low emission combustion technologies into advanced engine architectures. Lean burn combustion systems will have a significant impact on the temperature and velocity traverse at the combustor exit. Lean burn fuel injectors dominate the combustor exit conditions. This is due to the fact that they pass a majority of the total combustor flow, and to the lack of mixing jets like in a conventional combustor. With the transition to high pressure engines it is essential to fully understand and determine the high energetic interface between combustor and turbine to avoid excessive cooling, which has a detrimental impact on turbine and overall engine efficiency.

Velocity distributions and their fluctuations at the combustor exit for lean burn are of special interest as they can influence the efficiency and capacity of the turbine. Within the EU project LEMCOTEC, a lean burn single sector combustor was designed and built at DLR, providing optical access to its rectangular exit section. The sector was operated with a fuel staged lean burn injector from Rolls-Royce Deutschland. Measurements were performed under various operating conditions, covering idle and cruise operation. Two techniques were used to perform velocity measurements at the combustor exit in the demanding environment of highly luminous flames under elevated pressures: Particle Image Velocimetry (PIV) and Filtered Rayleigh Scattering (FRS). The latter was used for the first time in an aero-engine combustor environment. In addition to a conventional signal detection arrangement, FRS was also applied with an endoscope for signal collection, to assess its practicality for a potential future application in a full annular combustor with restricted optical access. Both measurement techniques are complementary in several respects, which justified their respective application and comparative assessment. PIV is able to record instantaneous velocity distributions and is therefore capable to deliver higher velocity moments, in addition to temporal averages. Applied in two orthogonal traversable light sheet arrangements, it could be used to map all three velocity components across the entire combustor cross section, and obtain data on velocity variances, cross-correlations and turbulence intensities. FRS is limited to measurements of average velocities, as long sampling times are required due to the weak physical process of Rayleigh scattering. However, FRS has two advantages: It requires no particle seeding, because it is based on the measurement of a molecular Doppler shift, and it can provide temperature information simultaneously. This contribution complements a second paper (GT2016-56370) focusing on the measurement of temperature distributions at the same combustor exit section by laser-based optical methods.

Commentary by Dr. Valentin Fuster
2016;():V006T05A008. doi:10.1115/GT2016-56370.

High OPR engine cycles for reduced NOx emissions will generate new aggravated requirements and boundary conditions by implementing low emission combustion technologies into advanced engine architectures. Lean burn combustion systems will have a significant impact on the temperature and velocity traverse at the combustor exit. The highly swirling flow from the lean burn fuel injector interacts with the combustor wall cooling before exiting the combustor. As a large portion (up to 80%) of the total flow passes through the fuel injector, the combustor exit flow and temperature field is dominated by the fuel injector. With the transition to high pressure engines it is essential to fully understand and determine the high energetic interface between combustor and turbine to avoid excessive cooling with detrimental impacts to turbine and overall engine efficiency.

In this context the knowledge of temperature distributions at the combustor exit is of special importance. A lean burn single sector combustor was designed and built at DLR, providing optical access to the exit section. The sector was operated with a staged lean burn injector from Rolls-Royce Deutschland. Spatially resolved temperatures were measured at different operating conditions using planar laser-induced fluorescence of OH (OH-PLIF) and Filtered Rayleigh Scattering (FRS), the latter being used in a combustor environment for the first time. Apart from a conventional signal detection arrangement, FRS was also applied with an endoscope for signal collection, to assess its feasibility for future application in a full annular combustor with restricted optical access.

Both techniques are complementary in several respects, which justified their combined application and comparative assessment in this specific environment. OH-PLIF allows instantaneous measurements and therefore enables local temperature statistics, but is limited to relatively high temperatures. On the other hand FRS can also be applied at low temperatures, which makes it particularly attractive for measurements in cooling layers. However, due to the weak physical process of Rayleigh scattering, FRS requires long sampling times and therefore can only provide temporal averages. When applied in combination, the accuracy of both techniques could be improved by each method helping to overcome the other’s shortcomings.

In an accompanying paper, additional experiments are described which characterize the flow field at the combustor exit; the combined data provide comprehensive information on combustor exit conditions.

Commentary by Dr. Valentin Fuster
2016;():V006T05A009. doi:10.1115/GT2016-56467.

A significant challenge in improving the regeneration process of jet engines is the reduction of engine down-time during inspection. As such, early defect detection without engine disassembly will speed up the regeneration process. Defects in the engines hot-gas path (HGP) influence the density distribution of the flow and lead to irregularities in the density distribution of the exhaust jet which can be detected with the optical Background-Oriented Schlieren (BOS) method in a tomographic set-up.

The present paper proposes a combination of tomographic BOS measurements and supervised learning algorithms to develop a methodology for an automatic defect detection system. In a first step, the methodology is verified by analyzing the exhaust jet of a swirl burner array with a non-uniform fuel-supply of single burners with tomographic BOS measurements. The measurements are used to implement a Support Vector Machine (SVM) pattern recognition algorithm.

It is shown that the reconstruction quality of tomographic BOS measurements is high enough to be combined with pattern recognition algorithms. The results strengthen the hypothesis, that it is possible to automatically detect defects in jet engines with tomographic BOS measurements and pattern recognition algorithms.

Topics: Exhaust systems
Commentary by Dr. Valentin Fuster
2016;():V006T05A010. doi:10.1115/GT2016-56481.

There exist many approaches for gas turbine engine condition monitoring and fault diagnosis. Among them, gas path analysis depends on the relations between deviations of performance parameters and deviations of measurements, such as pressure, temperature, at some positions in the flow path. In the first author’s previous study, a dynamic tracking filter is combined with a nonlinear gas turbine model to form a fault diagnosis system. The dynamic tracking filter is composed with multiple negative feedback control loops in which the residuals between model outputs and measurements are driven to zero by adjusting the performance parameters. In the present study, an interaction analysis technique, named the Relative Gain Analysis (RGA), is introduced to design more convincing and formal tracking filter for a heavy-duty gas turbine diagnostic problem. The basic concept of the RGA method is introduced in this paper with a simple blending example. Then a gas turbine model built using the Simscape language and environment from the MathWorks Co. is presented. The effects of secondary air system on the performance of compressor and turbine are considered in this gas turbine model. The linear influence coefficient matrix for four performance parameters and four measurement parameters is obtained from the steady state simulation with proper disturbance of performance parameters. Then the relative gain matrix (RGM) is obtained from matrix operation. To evaluate the pairing rule proposed in the RGA method, four tracking loops are built up to form a tracking filter for the four performance parameters selected. Deviations of performance parameters are implanted into the gas turbine model by adjusting the scaling factors of performance maps; and then simulation results are taken as measurements needed for the tracking filter to run. Tracking results of performance parameters in different cases are given to show the tracking capability for isolated performance deviations and concurrent performance deviations.

Commentary by Dr. Valentin Fuster
2016;():V006T05A011. doi:10.1115/GT2016-56544.

The accurate design, control and monitoring of the running gaps between static and moving components is vital to preserve the mechanical integrity and ensure the correct functioning of any compact rotating machinery. Throughout engine service, the rotor tip clearance undergoes large variations due to installation tolerances or as the result of different thermal expansion rates of the blades, rotor disk and casing during speed transients. Hence, active tip clearance control concepts and engine health monitoring systems rely on precise real-time gap measurements. Moreover, this tip gap information is crucial for engine development programs to verify the mechanical and aerothermal design, and validate numerical predictions.

This paper presents an overview of the critical design requirements for testing engine-representative blade tip flows in a rotating turbine facility. The manuscript specifically focuses on the challenges related with the design, verification and monitoring of the running tip clearance during a turbine experiment.

In the large-scale turbine facility of the von Karman Institute, a rainbow rotor was mounted for simultaneous aerothermal testing of multiple blade tip geometries. The tip shapes are a selection of high-performance squealer-like and contoured blade tip designs. On the rotor disc, the blades are arranged in seven sectors operating at different clearance levels from 0.5 up to 1.5% of the blade span.

Prior to manufacturing, the blade geometry was modified to compensate for the radial deformation of the rotating assembly under centrifugal loads. A numerical procedure was implemented to minimize the residual unbalance of the rotor in rainbow configuration, and to optimize the placement of every single airfoil within each sector. Subsequently, the rotor was balanced in-situ to reduce the vibrations and satisfy the international standards for high balance quality. The single-blade tip clearance in rotation was measured by three fast-response capacitive probes located at three distinct circumferential locations around the rotor annulus. Additionally, the minimum running blade clearance is captured with wear gauges located at five axial positions along the blades chord. The capacitance probes are self-calibrated using a multi-test strategy at several rotational speeds. The in-situ calibration methodology and dedicated data reduction techniques allow the accurate measurement of the distance between the turbine casing and the local blade tip features (rims and cavities) for each rotating airfoil separately. General guidelines are given for the design and calibration of a tip clearance measurement system that meets the required measurement accuracy and resolution in function of the sensor uncertainty, nominal tip clearance levels and tip seal geometry.

Commentary by Dr. Valentin Fuster
2016;():V006T05A012. doi:10.1115/GT2016-56624.

The subject of this paper is a flow-adaptive measurement grid algorithm developed for 1D and 2D flow field surveys with pneumatic probes in turbomachinery flows. The algorithm automatically determines the distribution and the amount of measurement points needed for an approximation of the pressure distribution within a predefined accuracy.

The algorithm is based on transient traverses, conducted back and forth in the circumferential direction. The dynamic response of the pressure-measuring system is disregarded during the traverses, which serve to detect changes in the pressure field. In contrast to previous investigations by the authors, a correction of the dynamic response is applied by deconvolving the transient measurement data using the information embedded in both transient measurements. In consequence, the performance of the algorithm is — to a large extent — independent of the transient traversing speed and the geometry of the pressure-measuring system. Insertion and removal strategies are incorporated in order to reduce measurement points and increase robustness towards differing flow field conditions. By approximation of the pressure distribution, the flow-adaptive measurement data is suited for the application of post-processing corrections without any constraints. The performance of the algorithm is demonstrated for 2D flow field surveys with a pneumatic 5-hole probe in an annular cascade wind tunnel. Compared to conventional techniques for data sampling, e.g., uniform measurement grids, the measurement grid points are automatically adjusted so that a consistent resolution of the flow features is achieved within the measurement domain. Furthermore, the application of the algorithm shows a significant reduction in the number of measurement points. Compared to the measurement duration based on uniform grids, the duration is reduced by at least 7%, while maintaining a high accuracy of the measurement.

The purpose of this paper is to demonstrate the performance of measurement grids adapted to local flow field conditions. Consequently, valuable measurement time can be saved without a loss in quality of the data obtained.

Commentary by Dr. Valentin Fuster
2016;():V006T05A013. doi:10.1115/GT2016-56626.

The subject of this paper is a statistical method for the accurate evaluation of the uncertainties for pneumatic multi-hole probe measurements. The method can be applied to different types of evaluation algorithms and is suitable for steady flowfield measurements in compressible flows. The evaluation of uncertainties is performed by a Monte Carlo method (MCM), which is based on the statistical law of large numbers. Each input quantity, including calibration and measurement quantities, is randomly varied on the basis of its corresponding probability density function (PDF) and propagated through the deterministic parameter evaluation algorithm. Other than linear Taylor series based uncertainty evaluation methods, MCM features several advantages. On the one hand, MCM does not suffer from lower-order expansion errors and can therefore reproduce nonlinearity effects. On the other hand, different types of PDFs can be assumed for the input quantities and the corresponding coverage intervals can be calculated for any coverage probability.

To demonstrate the uncertainty evaluation, a calibration and subsequent measurements in the wake of an airfoil with a 5-hole probe are performed. MCM is applied to different parameter evaluation algorithms. It is found that the MCM approach presented cannot be applied to polynomial curve fits, if the differences between the calibration data and the polynomial curve fits are of the same order of magnitude compared to the calibration uncertainty.

Since this method has not yet been used for the evaluation of measurement uncertainties for pneumatic multi-hole probes, the aim of the paper is to present a highly accurate and easy-to-implement uncertainty evaluation method.

Commentary by Dr. Valentin Fuster
2016;():V006T05A014. doi:10.1115/GT2016-56628.

This paper investigates how the external geometry of a five-hole probe affects its accuracy and how internal geometry affects its settling time. An analytical model, which predicts settling time, is used to design an accurate, fast-settling, millimetre-scale probe. The paper has three components:

First, results are presented from a series of area traverses performed with five-hole probes which range in head diameter from 0.99 to 2.67 mm. It is found that the smallest probe gives the greatest accuracy when traversing the shear layers in blade wakes. However, it takes 3.4 times longer to complete this traverse than compared to the largest probe. This is because traverses with small probes require more time to allow the pressure readings to settle between each traverse position.

Second, an analytical model is developed which predicts settling time based upon the internal geometry of the probe. The approach adopted is capable of modeling any number of connected tubes with different lengths and diameters. It is validated against experimental measurements and is shown to give good agreement. This model can be used to ensure that probes are designed with acceptable settling times.

Finally, the analytical model is used to design an optimised five hole probe. Use of the model highlights two important results which are required to reduce settling time: First, the length of the smallest diameter tubes, i.e. the ones in the probe head, should be minimised. Second, the volume of tubing downstream of the head should be minimised. Applying these principles to a new probe design cuts the total traverse time by 71%, whilemaintaining the highest value of accuracy.

Topics: Probes
Commentary by Dr. Valentin Fuster
2016;():V006T05A015. doi:10.1115/GT2016-56817.

Current maintenance, having a great impact on the safety, reliability and economics of gas turbine, becomes the major obstacle of the application of gas turbine in energy field. An effective solution is to process Condition based Maintenance (CBM) thoroughly for gas turbine. Maintenance of high temperature blade, accounting for most of the maintenance cost and time, is the crucial section of gas turbine maintenance. The suggested life of high temperature blade by Original Equipment Manufacturer (OEM) is based on several certain operating conditions, which is used for Time based Maintenance (TBM). Thus, for the requirement of gas turbine CBM, a damage evaluation model is demanded to estimate the life consumption in real time. A physics-based model is built, consisting of thermodynamic performance simulation model, mechanical stress estimation model, thermal estimation model, creep damage analysis model and fatigue damage analysis model. Unmeasured parameters are simulated by the thermodynamic performance simulation model, as the input of the mechanical stress estimation model and the thermal estimation model. Then the stress and temperature distribution of blades will be got as the input of the creep damage analysis model and the fatigue damage analysis model. The real-time damage of blades will be evaluated based on the creep and fatigue analysis results. To validate this physics-based model, it is used to calculate the lifes of high temperature blade under several certain operating conditions. And the results are compared to the suggestion value of OEM. An application case is designed to evaluate the application effect of this model. The result shows that the relative error of this model is less than 10.4% in selected cases. And it can cut overhaul costs and increase the availability of gas turbine significantly. Therefore, the physical-based damage evaluation model proposed in this paper, is found to be a useful tool to tracing the real-time life consumption of high temperature blade, to support the implementation of CBM for gas turbine, and to guarantee the reliability of gas turbine with lowest maintenance costs.

Commentary by Dr. Valentin Fuster
2016;():V006T05A016. doi:10.1115/GT2016-57120.

In this paper, common faults in main components of industrial three-shaft gas turbine are simulated through the nonlinear steady-state model, and the effects of these faults on the performance of all five main components and main measurement parameters are analyzed in both part and full load. Meanwhile, the sensitivity of these parameters to the location and type of fault and the fault severity in both part and full load is analyzed. In this study, in order to improve the accuracy of the non-linear steady-state model, the variable specific heat is considered with fitting formulas. In this model, the faults simulation is performed by changing the flow capacity and isentropic efficiency of each component via modification of the compressors and turbines characteristic curve. Meanwhile, a Genetic Algorithm (GA) based non-linear gas turbine steady-state performance simulation approach has been presented to best estimate the unknown component parameters. The fault simulation results in both part and full-load conditions indicate that by increasing the faults severity, the component performance parameters and the measurement parameters will deviate almost linearly from clean condition. Moreover, the sensitivity of these parameters varies with the location and type of the fault and also varies with the operating condition. Therefore, it can be used as the basis for selection of key parameters to identify the location and type of the fault. Furthermore, it also shows that considering the effects of load variations in design of a gas turbine diagnosis system is needed.

Commentary by Dr. Valentin Fuster
2016;():V006T05A017. doi:10.1115/GT2016-57206.

During last decades the scientific community focused its attention on energy efficiency solutions. Materials have been developed or improved to avoid energy dispersions and new strategies have been developed in order to exploit at best renewable and non renewable energy sources. Other paths are currently followed to pursue the same goal, for instance, the introduction of innovative monitoring system and techniques. In this framework, the concept of virtual sensing reveals itself as one of the most powerful tools. Several physical quantities are relevant to define the health status of the system. Unfortunately, many of these quantities cannot be directly measured in many practical applications due to lack of specific sensor or to the environment in which the system is operating. As a consequence, state estimation techniques come out as an extremely helpful tool. These techniques allow combining the world of measurements with the one of numerical modeling leading to the estimation of the desired unmeasurable system quantities. In the current research a Kalman Filter based algorithm is applied to investigate and assess the health condition of a rotor system. The system under investigation is a classical example in rotordynamics. The attention is focused on the estimation of one of the most common malfunction in rotating machinery: the unbalance.

Topics: Rotordynamics
Commentary by Dr. Valentin Fuster
2016;():V006T05A018. doi:10.1115/GT2016-57341.

For aero-engines, the throttle command is often changed dramatically, which will perturb the control error (defined as actual rotor speed minus desired rotor speed) of the primary set-point controller. When the perturbation is amplified by the gain, the controller will output unreasonable values of control variables, i.e. the mass of fuel flow, and cause abnormal engine operation. For this reason, limit protection controllers are applied to constrain the controlled variables at a safe level. Besides, the transient control modes are required to provide smooth, stall free operation of the engine.

The schedule-based approach, which is the traditional transient control mode, is easy to implement but the performance of acceleration and deceleration will suffer from degradation or manufacturing errors. With the development of digital control system, N-dot control mode has been adopted in some modern aero-engines, which focuses on rotor acceleration rather than rotor speed. To some extent, this method can overcome the obstacles of the schedule-based approach.

In terms of N-dot control mode, there are two main methods: direct control and indirect control. The former one suggests using a differentiator to get the actual N-dot value, then minus it by the desired N-dot value to get the error of N-dot. When the error is reduced to zero by a controller, the actual N-dot value follows the desired N-dot value. The latter one suggests inputting the desired N-dot value to an integrator for a rotor speed value, which essentially transforms the N-dot command to the speed command. With this transformation, the familiar set-point controller can be used to control the engine following the N-dot command indirectly.

This paper presents implementation schemes of the two types of N-dot control, and focuses on a comparative study of them. To avoid integral windup issue when the indirect method switches controllers, such as from N-dot controller to set-point controller, we have introduced a logic to determine whether the integrator is operational. This design allows flexible switchings.

After frequency domain analysis, we find out that the essential difference between the two schemes lies in the magnitude of crossing frequency. The direct N-dot control, with a higher crossing frequency, has faster responses but is sensitive to noise. While the indirect N-dot control, with a lower crossing frequency, has slower responses but can suppress noise. When the dynamic nature of sensor and actuator is considered, the direct N-dot control with a higher crossing frequency may cause the close-loop system unstable.

Using a reliable aero-engine mathematical model, we designed a set of simulations to test the two N-dot schemes. The simulation results showed that the direct N-dot control performed better than the indirect one under ideal situation. When noise or dynamic nature of sensor and actuator was taken into consideration, however, the indirect N-dot control was more robust, which confirmed the analysis above.

Commentary by Dr. Valentin Fuster
2016;():V006T05A019. doi:10.1115/GT2016-57368.

The advent of tip-timing systems makes it possible to assess turbomachinery blade vibration using non-contact systems. The most widely used systems in industry are optical. However, these systems are still only used on developmental gas turbine engines, largely because of contamination problems from dust, dirt, oil, water etc. Further development of these systems for in-service use is problematic because of the difficulty of eliminating contamination of the optics. Eddy current sensors are found to be a good alternative and are already being used for gas turbine health monitoring in power plants. Experimental measurements have been carried out on three different rotors using an eddy current sensor developed in a series of laboratory and engine tests in-house to measure rotor blade arrival times. A new tip-timing algorithm for eddy current sensors based on integration has been developed and is compared with two existing tip-timing algorithms: peak-to-peak and peak-and-trough. Among the three, the integration method provided the most promising results in the presence of electrical noise interference. The main aim of this work is to develop an algorithm that can be used to build a simple, robust, real-time and low cost analogue electronic circuit for use in-service health monitoring of engines.

Commentary by Dr. Valentin Fuster
2016;():V006T05A020. doi:10.1115/GT2016-57650.

Currently fast response aerodynamic probes are widely used for advanced experimental investigations in turbomachinery applications. The most common configuration is a virtual three hole probe. This solution is a good compromise between probe dimension and accuracy. Several authors have attempted to extend the capabilities of these probes in terms of bandwidth and operating conditions. Even though differences exist between the solutions in the literature, all of the designs involve the positioning of a dynamic pressure sensor close to the measurement point. In general terms, the higher the frequency response, the more the sensor is exposed to the flow. This physical constraint puts a limit on the probe applicability since the measurement conditions have to comply with the maximum allowed operating conditions of the sensor. In other applications, when the conditions are particularly harsh and a direct measurement is not possible, a waveguide probe is commonly used to estimate the local pressure. In this device the sensor is connected to the measurement point through a transmitting duct which guarantees that the sensor is operating in a less critical condition. Generally, the measurement is performed through a pressure tap and particular attention must be paid to the probe design in order to have an acceptable frequency response function. In this study, the authors conceived, developed and tested a probe which combines the concept of a fast response aerodynamic pressure probe with that of a waveguide probe. Such a device exploits the benefits of having the sensor far from the harsh conditions while maintaining the capability to perform an accurate flow measurement.

Topics: Pressure , Probes , Waveguides
Commentary by Dr. Valentin Fuster
2016;():V006T05A021. doi:10.1115/GT2016-57671.

The growing environmental impacts and dwindling supply of conventional fuels have led to the development of more efficient and clean energy systems. Micro gas turbines (mGT) have emerged as energy conversion technology, which offer promising features like high fuel flexibility, low emissions level, and efficient cogeneration of heat and power (CHP). Numerical simulation is a vital tool to predict the off-design performance of mGT cycles, and it also helps in cycle optimization. Starting from a model available at Ansaldo Energia, for steady state simulation of mGT T100 cycles based on user requirements, within the cooperation between University of Genova (Unige) and Ansaldo Energia, a new more comprehensive simulation tool has been developed through the incorporation of additional components, features, and involving a more detailed mathematical approach. The most important upgrades involved a number of different air path flows and the power electronics, which takes into account the power consumption from auxiliary components as well as the generator and inverter efficiencies.

Once the model has been verified against the existing tools, it was used in real operating conditions at the Ansaldo Energia test rig. The mGT performance has been assessed for different power levels, starting from 100 kW (nominal power) to 60 kW and then back to 100 kW, with 10 kW steps. The two tests at 100 kW operating conditions have been carried out with two different ambient temperatures: 20°C and 25°C, respectively. Data have been acquired under stable operating conditions, considering the recuperator cold outlet temperature as the stability indicator.

Finally the new model AE-T100 has been used also for diagnosis of the whole mGT cycle. The model has been successfully applied to a special mGT equipped with an on-purpose damaged recuperator, identifying the causes of performance degradation.

Commentary by Dr. Valentin Fuster
2016;():V006T05A022. doi:10.1115/GT2016-57722.

The paper presents a thorough analysis of historical data and results acquired over a period of two years through an online real-time monitoring system installed at a Combined Heat and Power (CHP) plant. For gas turbine health and performance assessment, a Gas Path Analysis tool based on the adaptive modeling method is integrated into the system. An engine adapted model built through a semi-automated method is part of a procedure which includes a steam/water cycle simulation module and an economic module used for power plant performance and economic assessment. The adaptive modeling diagnostic method allowed for accurate health assessment during base and part load operation identifying and quantifying compressor recoverable deterioration and the root cause of an engine performance shift. Next the performance and economic assessment procedure was applied for quantifying the economic benefit accrued by implementing daily on-line washing and for evaluating the financial gains if the off-line washings time intervals are optimized based on actual engine performance deterioration rates.

The results demonstrate that this approach allows continuous health and performance monitoring at full and part load operation enhancing decision making capabilities and adding to the information that can be acquired through traditional analysis methods based on heat balance and base load correction curves.

Commentary by Dr. Valentin Fuster
2016;():V006T05A023. doi:10.1115/GT2016-57847.

Defects in combustion chambers of aircraft engines might have an impact on the reliability of the downstream turbine and the machine’s performance. Detecting failures in the combustion chamber of an aircraft engine during operation may improve the resource management and the availability of the system. Aim of the ongoing research project is to find an approach to evaluate the state of the jet engine by analyzing the temperature and emissions field in the exhaust jet.

This investigation is part of the collaborative research center SFB 871. The SFB 871 deals with the improvement of the regeneration process of complex capital goods such as aircraft engines. Maintenance, repair, and overhaul processes would be more efficient if the internal status of the engine would be known while still on the wing before it is disassembled.

The feasibility of this approach is investigated for a pilot scaled model combustor, which provides optical access and allows the selection of “defined errors” in the combustor. It consists of an atmospheric tubular combustor with an array of eight premixed swirl burners with a maximum output of 160 kW. The operating conditions of one of the eight burners concerning power and air-fuel ratio, can be controlled. A power distribution between the burners is typical fault in an aircraft combustor and will be investigated in this study. It is observed that it is possible to determine small deviations by measuring density profiles applying a tomographic background-oriented schlieren (BOS) technique behind the combustor. Additionally, particle image velocimetry is used to measure differences in the velocity field of the exhaust gases. This study shows that a minimum power deviation of one burner in an array of a total of eight burners is detectable in the exhaust plane with the above mentioned measurement techniques.

Commentary by Dr. Valentin Fuster
2016;():V006T05A024. doi:10.1115/GT2016-57916.

In diagnostic applications, data acquired from a unit in operation is often compared to predictions generated from a reference model. The equipment condition is often assessed via residual analysis, which compares the running data to model predictions. Often, the reference model may take the form of a high-fidelity, first principles physics model. Here, we seek to capture the dominant features of the turbine engine, using parameters typically instrumented in field applications using reduced rank linear models that are trained on data generated from the high fidelity design models. The reduced rank linear models are well suited to diagnostic applications. Specifically, a modified Principle Component Regression is applied to the reference data to obtain our reduced rank model. We then use real measurement data input into the reduced rank model to produce predictions and correspondingly, residuals whose statistical properties can characterized with respect to the high-fidelity model. This requires characterization of the expected measurement errors which are user input. The model is capable of working with a complete or reduced set of measurements. In the case of the redundant measurements, we can perform an analysis on the fidelity of the measurements. We show how to calculate a measure of agreement between a given set of measurements and the underlying model. Departures of the given, real data from the models predictions indicate possible faults in the operating variables. In addition, if a certain sensor is suspected of being fouled, we can leverage the ability of the model to predict with a reduced set of inputs and then compare the new predictions measure of agreement with the underlying model.

Commentary by Dr. Valentin Fuster
2016;():V006T05A025. doi:10.1115/GT2016-58101.

Following three decades of research in short duration facilities, Purdue University has developed an alternative turbine facility in view of the modern technology in computational fluid mechanics, structural analysis, manufacturing, heating, control and electronics. The proposed turbine facility can perform both short transients and long duration tests, suited for precise heat flux, efficiency and optical measurement techniques to advance turbine aero-thermo-structural engineering. The facility has two different test sections, linear and annular, to service both fundamental and applied research. The linear test section is completely transparent for visible spectra, aimed at TRL 1 and 2. The annular test section was designed with optical access to perform proof of concepts as well as validation of turbine components at the relevant non-dimensional parameters in small engine cores, TRL 3 to 4. The large mass flow (28 kg/s) combined with a minimum hub radius to tip radius of 0.85 allows high spatial resolution. The Reynolds (Re) number extends from 60,000 to 3,000,000, based on the vane outlet flow with an axial chord of 0.06 m and a turning angle of 72 deg. The pressure ratio can be independently adjusted, allowing for testing from low subsonic to Mach 3.2. To ensure that the thermal boundary layer is fully developed the test duration can range from milliseconds to minutes. The manuscript provides a detailed description of the sequential design methodology from zero-dimensional to three-dimensional unsteady analysis as well as of the measurement techniques available in this turbine facility.

Topics: Design , Turbines
Commentary by Dr. Valentin Fuster
2016;():V006T05A026. doi:10.1115/GT2016-58102.

This paper presents an intelligent monitoring and fault diagnosis approach for rotating machinery by utilizing artificial neural networks and fuzzy logic expert systems (FLES). A recurrent neural network (RNN) is introduced to filter the input signal before they are forwarded to the expert system. The RNN is trained based on existing operational data so that it can adapt to a specific machine’s configurations and conditions. The RNN is able to generate proper baseline signal even if the machine is not under the exact same condition. A fuzzy logical expert system is then used for diagnosis based on the residual signal generated by the RNN. The system is incorporated into an existing comprehensive roto-dynamics software package named LVTRC.

Commentary by Dr. Valentin Fuster


2016;():V006T07A001. doi:10.1115/GT2016-56071.

This 5 day-course is offered to undergraduate students at the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technology. Goal of the course is to give students an holistic education and to train a “understanding of systems function as wholes”. Within this course students are designing an axial turbine stage from the very beginning and apply all the theory learned in separate courses before. At first the students design the annulus and the blades. They determine the inlet and outlet velocity triangles for several channel heights and define the camber line of the profile before a thickness distribution is superimposed. Secondly, a 3d CAD model including a disk design is created to get a realistic blade root. Then the students perform a finite element analysis (FEA) of the rotor blade and evaluate mechanical stresses in distinct sections of the blade. Also, natural frequencies are determined and a Campbell diagram is calculated. If the students have proven that there design is free from mechanical problems a steady state simulation of the flow through one passage is performed. Due to the fact that there is one numerical simulation platform for FEA as well as for CFD, a coupling of the fluid structure interaction (FSI) can be realised as final step. In a 1 way FSI analysis the students evaluate basic aeroelastic characteristics. After finishing that theoretical part an experimental part follows in which the students measure the static wall pressure distribution on the suction and pressure surface of that particular mid span profile in a subsonic wind tunnel. Natural frequencies are also experimentally determined using laser vibrometers. At the end of the course students will “produce” their own rotor blade with a 3d-printer. The blade can be kept as a souvenir. This paper describes the content of the course in detail and presents some results of last year’s class and reports the feedback of the students.

Commentary by Dr. Valentin Fuster
2016;():V006T07A002. doi:10.1115/GT2016-56186.

A laboratory based undergraduate course focused on energy systems and energy efficiency was developed at the Lucerne University of Applied Sciences. The course is a practice-oriented introduction to fluid / energy systems, turbomachinery, and energy production. It is offered to the mechanical engineering as well as to the “Energy Systems” program students in the first semester. Main parts of the course are experiments and mini projects carried out in the fluid mechanics, thermodynamics, and turbomachinery laboratories. After an introduction of (1) the basic theory of the mass- and energy conservation equations as main governing laws for energy systems and (2) measurement techniques during the first two weeks, the students carry out experiments in small groups on different test rigs with the help of an instructor for about four weeks (each week a different test). The experiments and test rigs are selected considering different aspects of turbomachinery and energy systems. They cover hydraulic turbines, small wind turbines, a water pump, compressor, heat pump, combined heat and power system, fuel cell, solar energy, and combustion engine. Following a two weeks theory wrap up, the students start with mini projects again in small groups in the laboratory. The students are requested to do a new design or to carry out a design change or modification at an existing machine or test rig. They also need to test their new design or design modification. At the end the students have to do a presentation about the mini project results and write a short report.

The objectives of the course are first to introduce the laboratory environment to the students from the beginning of the curriculum. Further the experimental investigations on laboratory test rigs make the students familiar with the fundamentals, working principles, characteristics, operation, and application of turbomachinery and energy systems. They also learn the basics of energy conversion, conservation, and the importance of relevant performance parameters such as efficiency. The mini projects have direct practical relevance to real world problems and applications. From an educational viewpoint the students are introduced to team-work during the projects, project planning with execution, and they are learning by doing or by experiential learning. The students are actively involved in the mini projects and they can reflect their experience at the end of the course during the presentations. The implementation of the course is time consuming and costly but the feedback from the students is very positive although they are challenged by confronting real world energy systems and problems in the first semester.

Commentary by Dr. Valentin Fuster
2016;():V006T07A003. doi:10.1115/GT2016-56200.

Improving the quality and productivity of academic teaching is a continuous challenge. To respond to it, the study platform of a graduate level course on design of turbomachinery was changed from blended education to problem based learning (PBL). The main idea behind PBL is to let the students define the solved problem themselves and study what they need to know before the problem can be solved. The learning is connected to a real-life problem-solving environment which reflects real working situations as much as possible. This study presents the results from the first four years of utilizing PBL as a study platform of a graduate level turbomachinery course. The new design and implementation is presented first and is compared with the original one. The performed analysis indicated good student feedback and learning results overall, but also highlighted the importance of teachers’ competence to utilize PBL. In conclusion, the use of PBL is recommended for turbomachinery courses if the challenges in explaining the principles of PBL and tutoring the students are taken into account while organizing the course.

Commentary by Dr. Valentin Fuster
2016;():V006T07A004. doi:10.1115/GT2016-56225.

Mechanical Engineers graduating from the Hellenic Air Force Academy (HAFA), are initially appointed in a military squadron, to support and manage the aircraft maintenance workload. Throughout their career, they will at some point, be appointed in a procurement or in a research department, where they will need to have an integral understanding of how a certain aircraft in conjunction with its propulsion system, can meet prescribed operational needs. The syllabus of the Mechanical Engineering degree offered at the HAFA, encompasses several modules related to Aircraft Design, Material Science and Propulsion Systems. The Aircraft Design Project (ADP) presented herein, aims to stimulate the cadets in applying a diverse field of knowledge on a single application, building thus soon enough the missing communication link between those who deal with the power plant and those who deal with the aircraft design. The assignment input is only confined in a short description of the operational profile of an Unmanned Air Vehicle (UAV) to be designed. A number of teams are formed which act competitively and develop a design proposal, each one for its own sake. As part of the project, they also have to print a 3D mock-up and do a testing in the wind tunnel operated within the HAFA. Finally, each team has the obligation to defend its design in front of an audience consisting of HAF military officers, HAFA academics and delegates from industries. The proposed exercise constitutes a novel conception of coursework type, extending over one year, engaging three Academic Sectors and aiming to achieve the following educational targets: a) Learn to work in a team within a competitive environment. Each cadet has to collaborate with his teammates and compete with the members of the other team(s). b) Combine and apply knowledge acquired from various scientific fields. c) Learn how to “sell” a product to a diverse audience being interested in engineering excellence (academia) cost effectiveness (industry) and degree of compliance with operational needs (military).

Commentary by Dr. Valentin Fuster
2016;():V006T07A005. doi:10.1115/GT2016-56231.

This paper describes an innovative, three-day, turbomachinery research project for Japanese and British high-school students. The project is structured using modern teaching theories which encourage student curiosity and creativity. The experience develops team-work and communication, and helps to break-down cultural and linguistic barriers between students from different countries and backgrounds. The approach provides a framework for other hands-on research projects which aim to inspire young students to undertake a career in engineering.

The project is part of the Clifton Scientific Trust’s annual UK-Japan Young Scientist Workshop Programme. The work focuses on compressor design for jet engines and gas turbines. It includes lectures introducing students to turbomachinery concepts, a computational design study of a compressor blade section, experimental tests with a low-speed cascade and tutorials in data analysis and aerodynamic theory. The project also makes use of 3D printing technology, so that students go through the full engineering design process, from theory, through design, to practical experimental testing.

Alongside the academic aims, students learn what it is like to study engineering at university, discover how to work effectively in a multinational team, and experience a real engineering problem. Despite a lack of background in fluid dynamics and the limited time available, the lab work and end of project presentation show how far young students can be stretched when they are motivated by an interesting problem.

Commentary by Dr. Valentin Fuster
2016;():V006T07A006. doi:10.1115/GT2016-57469.

The work presented in this paper concerns a useful method for axial fans preliminary design based on the “Derivative Design” concept. The emphasis is, on one side, on education and, on the other, on the practical help that such method can provide in the early preliminary design process.

A complete data set of an axial fan measured with ISO 5801 standards is the start point for the investigation and the prediction of the multiple possible performance that different fan configurations can provide, in terms of dimensionless duty coefficients. In particular, configurations with different number of blades, and hence of solidity, are studied. The typical options of derivative design are explored and relations for performance prediction are presented.

A detailed description of the derivative design methodology is followed by tests and validation. The tools employed are a fully three dimensional code, the Advanceded Actuator Disk Mode (AADM), and two other in-house codes, the Meanline Axisymmetric Calculation (MAC) and Axisymmetric Laboratory (AXLAB).

Results of the derivative design method are reported, showing a good accuracy against the AADM data. The MAC and AXLAB ensure still acceptable results when increasing the solidity of the machine. On the contrary, a decrease of solidity leads to higher relative errors in the prediction of the load coefficient.

In conclusion, an exploration of the possible fields of operation of a blade profile can be carried out by a correct prediction of the stage diffusion factor.

Topics: Design
Commentary by Dr. Valentin Fuster
2016;():V006T07A007. doi:10.1115/GT2016-57601.

A new, simple model for the thermodynamic properties of dry air and combustion gases is presented. The model has been developed mainly for educational purposes with the focus on gas turbine cycle calculations included in university courses. The equations used are short and easy to include in a student computer program. This ideal gas model is based on well-known correlations from the open literature and each equation is an approximated fit to an appropriate expression from the original reference. In the new concept, each relation is expressed using a simple algebraic formulation that has an explicit inverse function. Therefore, the numerical result of an inverse function can be obtained directly, without the involvement of any iterative procedure. This simplifies the programming of auxiliary subroutines for thermodynamic properties. It becomes an easy task and the use of complicated models, in basic university courses, is avoided. Long and complex subroutines which are treated as “black boxes” are now excluded from the code, and a program for thermodynamic cycle calculation is then completely written by a student, starting from the very beginning.

Commentary by Dr. Valentin Fuster

Manufacturing Materials and Metallurgy

2016;():V006T21A001. doi:10.1115/GT2016-56120.

Nozzle guide vanes (NGV) of gas turbine engines are the first components to withstand the impingement of hot combustion gas, and therefore often suffer thermal fatigue failures in service. A lifing analysis is performed for the NGV of a gas turbine engine using the integrated creep-fatigue theory (ICFT). With the constitutive formulation of inelastic strain in terms of mechanism-strain components such as rate-independent plasticity, dislocation glide-plus-climb, and grain boundary sliding, the dominant deformation mechanisms at the critical locations are thus identified quantitatively with the corresponding mechanism-strain component. The material selection scenarios are discussed with regards to damage accumulated during take-off and cruise. The interplay of those deformation mechanisms in the failure process are elucidated such that an “optimum” material selection solution may be achieved.

Commentary by Dr. Valentin Fuster
2016;():V006T21A002. doi:10.1115/GT2016-56475.

Facing a high demand for aircraft engines over the next decades in combination with new challenging materials, aircraft engine manufacturers are striving for new manufacturing processes. The manufacturing of profiled grooves for the mounting of the turbine blades on the disc is a bottle neck process today due to the exclusive use of High Speed Steel (HSS) tools in broaching. Because of the limited hot hardness of HSS, the applied cutting speeds are low compared to other conventional machining processes, i.e. 2–5 m/min. Furthermore, the broaching process has some more drawbacks regarding flexibility, capital commitment for machinery and tools as well as costs. Nevertheless, broaching offers outstanding properties regarding surface finish, manufacturing accuracy and is still a productive process due to the many cutting edges applied. There are some alternative process chains which are not yet in industrial use, which are able to substitute and/or complement the HSS-broaching process.

In this paper, results are presented on two different roughing strategies for the manufacturing of profiled grooves in Nickel Based Alloys Allvac718plus and Inconel718. On the one hand, rough broaching with cemented carbide tools using indexable inserts was investigated at different cutting speeds, which are up to five times higher than the applied cutting speeds in industrial applications with HSS-tools. Two different carbide grades were investigated varying the cobalt content and the grain size. Cemented carbide is not state of the art in broaching Nickel Based Alloys due to its low fracture toughness. Different cutting edge inclination angles were applied and their effect on cutting forces, wear and tool chipping tendency were analyzed. On the other hand, rough side milling with ceramic cutting tools was investigated. Ceramic cutting tools excel in high hot hardness and thus can be used at very high cutting speeds i.e. up to 1000 m/min in Nickel Based Alloys. However, being very brittle and sensitive to alternating loads and thermal shock, machining processes with ceramic tools require extensive process design. In side milling experiments, Whisker reinforced as well as SiAlON and Oxide ceramic were investigated. In a first step, a window for machining parameters was identified. Then, tool life tests were conducted varying the feed at a fixed cutting speed of 1000 m/min. Subsequent to the experiments, the rim zone of the roughed grooves was investigated depending on the wear state of the used tools. The condition of the rim zone is a major criterion for the assessment of the adequacy of the roughing processes, because it can affect the subsequent finishing process. In further work, the interdependencies between the investigated roughing processes and finishing will be addressed.

Topics: Nickel , Alloys , Grinding
Commentary by Dr. Valentin Fuster
2016;():V006T21A003. doi:10.1115/GT2016-56594.

Low emission combustion is one of the most important requirements for Industrial gas turbines. Siemens Industrial gas turbines SGT-800 and SGT-700 use DLE (Dry Low Emission) technology and are equipped with 3rd generation of DLE burners. These burners demonstrate high performance and reliable operation for the duration of their design lifetime. The design and shape of the burner tip is of great importance in order to achieve a good fuel/ air mixture and at the same time a resistance to the fatigue created by heat radiation input. This gives a requirement for a tip structure with delicate internal channels combined with thicker structure for load carrying and production reasons. It was found that the extension of the burner lifetime beyond the original design life could be accomplished by means of repair of the burner tip.

Initially the tip repair has been done by conventional methods — i.e. cutting off the tip and replacing it with a premanufactured one. Due to the sophisticated internal structure of the burner the cuts have to be made fairly high upstream to avoid having the weld in the delicate channel area.

Through the use of AM (Additive Manufacturing) technology it has been possible to simplify the repair and only replace the damaged part of the tip.

Special processes have been developed for AM repair procedure, including:

a) machining off of the damaged and oxidized tip,

b) positioning the sintered model on the burner face,

c) sintering a new tip in place,

d) quality assurance and inspection methods,

e) powder handling,

f) material qualification including bonding zone,

g) development of methods for mechanical integrity calculation,

h) qualification of the whole repair process.

This paper describes how we have developed and qualified SGT-800 and SGT-700 DLE burners repair with the help of additive manufacturing technology and our research work performed. In addition, this paper highlights the challenges we faced during design, materials qualification and repair work shop set-up.

Commentary by Dr. Valentin Fuster
2016;():V006T21A004. doi:10.1115/GT2016-57015.

Since its introduction in 2003, alloy 718Plus™ spurred a lot of interest owing to its increased maximum service temperature over conventional Inconel 718 (704°C vs 650°C), good formability and weldability together with its moderate cost. Understanding the high temperature deformation characteristics and microstructural evolution is still of interest to many. It is known that the service performance and hot-flow behavior of this alloy is a strong function of the microstructure, particularly the grain size. To develop precise microstructure evolution models and foresee the final microstructure, it is important to understand how and under which forming conditions softening and precipitation processes occur concurrently. In this work, the softening behavior, its mechanisms and the precipitation characteristics of 718Plus™ were investigated in two parallel studies. While cylindrical compression tests were employed to observe the hot-flow behavior, the precipitation behavior and other microstructural phenomena such as particle coarsening were tracked via hardness measurements. A PTT diagram was reported and modeling of the flow curves via hyperbolic sine model were discussed in the light of the PTT behavior. Both “apparent” approach and “physically based” approach are implemented and two different sets of parameters were reported for the latter. Finally, recovery and recrystallization kinetics are described via Estrin-Mecking and Bergstrom, and Avrami kinetics, respectively.

Commentary by Dr. Valentin Fuster
2016;():V006T21A005. doi:10.1115/GT2016-57262.

Additive manufacturing and in particular Selective Laser Melting (SLM) are manufacturing technologies that can become a game changer for the production of future high performance hot gas path parts. SLM radically changes the design process giving unprecedented freedom of design and enabling a step change in part performance. Benefits are manifold, such as reduced cooling air consumption through more efficient cooling schemes, reduced emissions through better mixing in the combustion process and reduced cost through integrated part design. GE is already making use of SLM for its gas turbine components based on sound experience for new part production and reconditioning.

The paper focuses on:

a) Generic advantages of rapid manufacturing and design considerations for hot gas path parts

b) Qualification of processes and additive manufacturing of engine ready parts

c) SLM material considerations and properties validation

d) Installation and validation in a heavy duty GT

Additive Manufacturing (AM) of hot gas path components differs significantly from known process chains. All elements of this novel manufacturing route had to be established and validated. This starts with the selection of the powder alloy used for the SLM production and the determination of essential static and cyclic material properties.

SLM specific design features and built-in functionality allow to simplify part assembly and to shortcut manufacturing steps. In addition, the post-SLM machining steps for engine ready parts will be described.

As SLM is a novel manufacturing route, complementary quality tools are required to ensure part integrity. Powerful nondestructive methods, like 3D scanning and X-ray computer tomography have been used for that purpose. GE’s engine validation of SLM made parts in a heavy duty GT was done with selected hot gas path components in a rainbow arrangement including turbine blades with SLM tip caps.

Although SLM has major differences to conventional manufacturing the various challenges from design to engine ready parts have been successfully mastered. This has been confirmed after the completion of the test campaign in 2015. All disassembled SLM components were found in excellent condition. Subsequent assessments of the SLM parts including metallurgical investigations have confirmed the good part condition.

Commentary by Dr. Valentin Fuster
2016;():V006T21A006. doi:10.1115/GT2016-57425.

Continuously increasing hot gas temperatures in heavy duty gas turbines lead to increased thermal loadings of the hot gas path materials. Thermal barrier coatings are used to reduce the superalloys temperature and cooling air needs. Until now 6–8 wt% yttria stabilized zirconia is the first choice material for such coatings, but it is slowly reaching its maximum temperature capability due to the phase transformation at high temperature and sintering.

New thermal barrier coating material with increased temperature capability enable the next generation of gas turbine with >60% combined cycle efficiency. Such material solutions have been developed through a multi-stage selection process. In a first steps, critical material performance requirements for thermal barrier coating performance have been defined based on the understanding of standard TBC degradation mechanisms. Based on these requirements, more than 30 materials were a pre-selected and evaluated as potential coating materials. After carefully reviewing their properties both from literature data and laboratory test results on raw materials, five materials were selected for coating manufacturing and laboratory testing.

Based on the coating manufacturing trials and laboratory test results, two materials have been selected for engine testing, in a first step in GT26 Birr Test Power Plant and afterwards in customer engines. For such tests the original coating thickness has been increased such to achieve coating surface temperature ∼100K higher than with a standard thermal barrier coating. Both coatings performed as predicted in both GT26 Birr Test Power Plant and customer engines.

Commentary by Dr. Valentin Fuster
2016;():V006T21A007. doi:10.1115/GT2016-57458.

Additive manufacturing radically changes the design process giving unprecedented freedom of design and enabling a step change in part performance.

The general idea of selecting appropriate candidates within the Alstom hot gas parts portfolio is described in this paper. The strategy as used in the selection process is to identify parts with either a high (internal) complexity or to choose parts with a high number of sub-parts and the necessary high number of assembly steps, which are needed with the current manufacturing methods (e.g. precision casting or weld assemblies).

The current reheat burner front panel of the heavy duty gas turbine is today produced from three metal layers. The layer facing the hot gas contains the complex near-wall cooling channel system. The metal sheet facing the cold side features the acoustic damping volumes. An intermediate metal sheet separated the two systems. Brazing is used to assemble the sup parts into the final front panel. With the identified strategy to use additive manufacturing, the design is adapted to allow the production of a single component that includes all the functionality and avoids heavy machining and assembly steps.

The paper focuses on the design changes and challenges that were required and observed during the adaption of the GT26 (Rating 2011 onwards) reheat burner front panel. Also the necessary adaptions of the qualification process and the lifetime assessments are described in detail.

Finally, the additive manufacturing version of the reheat burner front panel was subjected to a heavy duty test program. Engine tests were completed. The disassembled SLM component was found in excellent condition. Subsequent material investigations have confirmed the good part condition from a metallurgical point of view.

Commentary by Dr. Valentin Fuster
2016;():V006T21A008. doi:10.1115/GT2016-57461.

Compared to conventional manufacturing processes, additive manufacturing offers a degree of freedom that has the potential to revolutionize the turbine components supply chain. Additive manufacturing facilitates the design and manufacture of highly complex components in high performance materials with features that cannot currently be realized with other processes. In addition, shorter development and manufacturing lead-times are possible due to the flexibility of 3D based processing and the absence of expensive, complicated molds or dies.

Having been confined for many years to rapid prototyping or R&D applications, additive manufacturing is now making the move to the factory floor. However, a dearth of manufacturing experience makes the development effort and risk of costly mistakes a deterrent for many organizations that would otherwise be interested in exploring the benefits of additive manufacturing.

A former manufacturer of industrial gas turbines recently established an additive manufacturing workshop designed to deliver highly complex engine-ready components that can contribute to increased performance of the gas turbine. A strong emphasis on process validation and implementation of the organization’s best practice Lean and Quality methodologies has laid solid foundations for a highly capable manufacturing environment.

This paper describes the approach taken to ensure that the workshop achieves a high level of operational excellence. Process development topics explored in the paper include the following:

• Planning of process flow and cell layout to permit the maximum lean performance

• Strategy used to determine machine specification and selection method.

• Assessment of process capability

• Influence of design for manufacture on process efficiency and product quality

• Experience gained during actual production of first commercial components

Commentary by Dr. Valentin Fuster
2016;():V006T21A009. doi:10.1115/GT2016-57627.

Designing a state-of-the-art combustor requires an iterative process where mechanical design solutions in the early concept phase are continuously assessed using analytical cooling and lifetime assessments, which are later backed up with experimental investigations and validation measures.

This paper describes the integrated design process in terms of design, cooling, manufacturing, aerodynamic and mechanical integrity for the transition duct of the GT13E2 combustor, which houses the flame development region of the combustor.

The objective was to develop a retrofit design with reduced lifecycle costs, with no impact on the combustor-turbine interface and no impact on the overall engine performance. The major challenges were the on-site welding of the two half-shells as well as the increasing demand for cyclic operation of the engine.

During the development process, the focus was on selecting reliable and robust cooling schemes on the inner and outer shell to reduce mechanical deflections and thus to reduce the overall stress distribution.

Key features of the GT13E2 combustor zone 2 are the following:

• Drastically reduced lifecycle cost with no performance penalty (retrofit)

• Separation planes at 3 and 9 o’clock position with mechanical locking connections (bridges) and a recess weld with film cooling

• Bellmouth shaped inlet of cooling channel with improved impingement and convective cooling

• Unique membrane seals with great mechanical flexibility around the turbine inlet outer diameter including mitigation against hot gas ingress (bow wave effect)

• Improved cooling during transient conditions to reduce mechanical deflections

• Reduced reconditioning effort due to innovative manufacturing methods

The cooling schemes were successfully validated against laboratory experiments and integrated into a stable manufacturing process. Increased dimensional stability was achieved through higher rigidity and led to an improved on-site welding process for the separation planes. All the aforementioned features result in a marked improvement of the cyclic lifetime with no performance penalty. This is proven by the fact that the GT13E2 with annular combustor transition duct has now accumulated more than 30,000 operating hours with 1,200 starts.

Commentary by Dr. Valentin Fuster
2016;():V006T21A010. doi:10.1115/GT2016-57903.

As a mainstream dynamic dry classifer, the turbo air classifier is widely used in powder preparation industries for its adjustable cut size, controllable product granularity and high classification performance. As an important indicator for evaluating the classification performance of a turbo air classifier, cut size is often predicted in advance to evaluate classification effect so that the operation parameters can be adjusted suitably according to the production requirement. There are two common ways to obtain cut size of turbo air classifiers. One is based on a theoretical formula; another is based on an experimentally derived formula. There are a few problems with the aforementioned ways of predicting cut size. The theoretical analysis often has some large deviations from the actual values. Analysis based on empirical formula can obtain an accurate predicted cut size at the cost of a large number of training samples. In this paper, a new strategy is introduced to determine the cut size based on numerical simulation of gas-solid two-phase flow in the turbo air classifier using ANSYS® CFX, Release 15.0. The three-dimensional Reynolds-averaged Navier-Stokes equations along with the k-ε turbulence model are adopted to describe the gas flow, and the Lagrangian particle tracking technique is used to calculate the particle trajectory. According to its definition, cut size can be obtained by means of analyzing the particle trajectory. The effects of rotor cage rotational speed and particle density on cut size are also obtained based on analysis of the change of cut size. The simulation results are validated against the experimental data. Numerical simulation provides a new way to obtain the cut size of a turbo air classifier and serves as a method to regulate the operating parameters for classification. It also provides a reference method to study the cut size of various types of classifier.

Commentary by Dr. Valentin Fuster
2016;():V006T21A011. doi:10.1115/GT2016-57922.

Current elevated turbine inlet temperatures were made possible by the development of blade thermal barrier coatings (TBCs). However the effectiveness of these coatings could be compromised by erosive particles ingested into the engine with the incoming air or generated by the combustion of heavy and synthetic fuels. Reliable test facilities are essential to characterize their erosion resistance in increased test temperatures. This paper provides a detailed description of an advanced high temperature erosion tunnel capable of testing at temperature of 1370 °C (2500 °F) that has been recently constructed and installed in the University of Cincinnati Gas Turbine Erosion Lab. The paper also presents an overview of both theoretical and experimental investigations dealing with the new high temperature erosion tunnel design optimization and validation with comparisons to our legacy erosion tunnels. Results are presented for tested standard plasma sprayed 7 wt% Yttria stabilized Zirconia (7YSZ) TBC coated samples’ erosion rates at different temperatures, particle impact velocities and impingement angles.

Commentary by Dr. Valentin Fuster
2016;():V006T21A012. doi:10.1115/GT2016-58080.

The limits of gas turbine technology are heavily influenced by materials and manufacturing capabilities. Inconel alloys remain the material of choice for most hot gas path components in gas turbines, however recent increases in turbine inlet temperature (TIT) are associated with the development of advanced convective cooling methods and ceramic thermal barrier coatings (TBC). Increasing cycle efficiency and cycle specific work are the primary drivers for increasing TIT. Lately, incremental performance gains responsible for increasing the allowable TIT have been made mainly through innovations in cooling technology, specifically convective cooling schemes. An emerging manufacturing technology may further facilitate the increase of allowable maximum TIT, thereby impacting cycle efficiency capabilities. Laser Additive Manufacturing (LAM) is a promising manufacturing technology that uses lasers to selectively melt powders of metal in a layer-by-layer process to directly manufacture components, paving the way to manufacture designs that are not possible with conventional casting methods. This study investigates manufacturing qualities seen in LAM methods and its ability to successfully produce complex features found in turbine blades. A leading edge segment of a turbine blade, containing both internal and external cooling features, along with an engineered-porous structure is fabricated by laser additive manufacturing of superalloy powders. Various cooling features were incorporated in the design, consisting of internal impingement cooling, internal lattice structures, and external showerhead or transpiration cooling. The internal structure was designed as a lattice of intersecting cylinders in order to mimic that of a porous material. Variance distribution between the design and manufactured leading edge segment are carried out for both internal impingement and external transpiration hole diameters. Through a non-destructive approach, the presented geometry is further analyzed against the departure of the design by utilizing x-ray computed tomography (CT). Employing this non-destructive evaluation (NDE) method, a more thorough analysis of the quality of manufacture is established by revealing the internal structures of the porous region and internal impingement array. Flow testing was performed in order to characterize the uniformity of porous regions and flow characteristics across the entire article for various pressure ratios (PR). Discharge coefficient of internal impingement arrays and porous structure are quantified. The analysis yields quantitative data on the build quality of the LAM process, providing insight as to whether or not it is a viable option for manufacture of micro-features in current turbine blade production.

Commentary by Dr. Valentin Fuster

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