bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantification of the Impact of Uncertainties in Operating Conditions on the Flame Transfer Function With Non-Intrusive Polynomial Chaos Expansion
Authors: Avdonin Alexander , Polifke Wolfgang
Published: August 2018

Non-intrusive polynomial chaos expansion (NIPCE) is used to quantify the impact of uncertainties in operating conditions on the flame transfer function of a premixed laminar flame. NIPCE requires only a small number of system evaluations, so it can be applied in cases where a Monte Carlo simulation is unfeasible. We consider three uncertain operating parameters: inlet velocity, burner plate temperature, and equivalence ratio. The flame transfer function (FTF) is identified in terms of the finite impulse response from CFD simulations with broadband velocity excitation. NIPCE yields uncertainties in the FTF due to the uncertain operating conditions. For the chosen uncertain operating bounds, a second-order expansion is found to be sufficient to represent the resulting uncertainties in the FTF with good accuracy. The effect of each operating parameter on the FTF is studied using Sobol indices, i.e. a variance-based measure of sensitivity, which are computed from the NIPCE. It is observed that in the present case uncertainties in the finite impulse response as well as in the phase of the FTF are dominated by the equivalence-ratio uncertainty. For frequencies below 150 Hz, the uncertainty in the gain of the FTF is also attributable to the uncertainty in equivalence-ratio, but for higher frequencies the uncertainties in velocity and temperature dominate. At last, we adopt the polynomial approximation of the output quantity, provided by the NIPCE method, for further UQ studies with modified input uncertainties.
Thermodynamic Performance Enhancement of Marine Gas Turbine by Using Detonation Combustion
Authors: Zhao Ningbo , Zheng Hongtao , Wen Xueyou , Xiao Dongming
Published: August 2018

As a prospective pressure gain combustion technology, detonation combustion has obvious potential for greatly increasing the thermodynamic performance of marine gas turbine due to its advantage in low entropy generation, fast heat release and self-pressurization. In this paper, a thermodynamic cycle model of detonation combustion based marine gas turbine is established considering the variable specific heat capacity. On this basis, a comparative analysis is investigated to discuss the effects of different factors on the performance enhancement of marine gas turbine by using detonation combustion. The results demonstrate that compared to the conventional deflagration combustion, detonation combustion can significantly improve the thermodynamic performance of marine gas turbine under various condition. As far as the present study is concerned, the thermal cycle efficiency can be increased to 42.97∼46.76%. Besides, it is found that the effects of pressure ratio on performance enhancements of marine gas turbine are higher than those of atmospheric temperature and temperature ratio. When pressure ratio is ranged from 13 to 30, both thermal cycle efficiency and specific power enhancements are about 20∼27%.
Investigations on Transient Amplitude Amplification by Applying Intentional Mistuning
Authors: Schlesier Klaus-Dieter , Panning-von Scheidt Lars , Wallaschek Jörg
Published: August 2018

Due to the changing demands in the energy production sector which leads to higher numbers of run ups and run downs, the importance of transient processes increase. Further information on the occurring blade vibration amplitudes during these processes could be used to explore new potential for longer lifetime and efficiency. Recent publications have shown that the amplitude of a mistuned blisk during a resonance passage can exceed the stationary response of the same system [1]. This underlines the importance of a better understanding of what happens during resonance passage. The effect is described as transient amplitude amplification of mistuned structures (TAMS). In this paper the effect will be investigated theoretically as well as in experiments. The main cause of the described effect seems to be the splitting of double eigenfrequencies due to mistuning. Therefore, the focus lies on methods to deliberately influence and seperate eigenfrequencies. First, simulations are done to predict parameter values which tend to show the TAMS effect. It is then shown how a significant transient amplitude amplification can be created on a test rig depending on the intentional mistuning. For realization an experimental setup with contactless measurement and acoustic excitation is deployed on a simplified blisk.
Background Oriented Schlieren of Fuel Jet Flapping Under Thermoacoustic Oscillations in a Sequential Combustor
Authors: Weilenmann Markus , Xiong Yuan , Bothien Mirko , Noiray Nicolas
Published: August 2018

This study deals with thermoacoustic instabilities in a generic sequential combustor. The thermoacoustic feedback involves two flames: the perfectly premixed swirled flame anchored in the first stage and the sequential flame established downstream of the mixing section, into which secondary fuel is injected in the vitiated stream from the first stage. It is shown that the large amplitude flapping of the secondary fuel jet in the mixing section plays a key role in the thermoacoustic feedback. This evidence is brought using high-speed Background Oriented Schlieren (BOS). The fuel jet flapping is induced by the intense acoustic field at the fuel injection point. It has two consequences: first, it leads to the advection of equivalence ratio oscillations toward the sequential flame; second, it modulates the residence time of the ignitable mixture in the mixing section, which periodically triggers autoignition kernels developing upstream of the chamber. In addition, the BOS images are processed to quantify the flow velocity in the mixing section and these results are validated using PIV. This study presents a new type of thermoacoustic feedback mechanism which is peculiar to sequential combustion systems. In addition, it demonstrates how BOS can effectively complement other diagnostic techniques that are routinely used for the study of thermoacoustic instabilities.
The Present Challenge of Transonic Compressor Blade Design
Authors: Hergt A. , Klinner J. , Wellner J. , Willert C. , Grund S. , Steinert W. , Beversdorff M.
Published: August 2018

The flow through a transonic compressor cascade shows a very complex structure due to the occuring shock waves. In addition, the interaction of these shock waves with the blade boundary layer inherently leads to a very unsteady flow behaviour.

The aim of the current investigation is to quantify this behaviour and its influence on the cascade performance as well as to describe the occuring transonic flow phenomena in detail. Therefore, an extensive experimental investigation of the flow in a transonic compressor cascade has been conducted within the transonic cascade wind tunnel of DLR at Cologne. In this process, the flow phenomena were thoroughly examined for an inflow Mach number of 1.21. The experiments investigate both, the laminar as well as the turbulent shock wave boundary layer interaction within the blade passage and the resulting unsteady behaviour. The experiments show a fluctuation range of the passage shock wave of about 10 percent chord for both cases, which is directly linked with a change of the inflow angle and of the operating point of the cascade.

Thereafter, RANS simulations have been performed aiming at the verification of the reproducibility of the experimentally examined flow behavior. Here it is observed that the dominant flow effects are not reproduced by a steady numerical simulation. Therefore, a further unsteady simulation has been carried out in order to capture the unsteady flow behaviour. The results from this simulation show that the fluctuation of the passage shock wave can be reproduced but not in the correct magnitude. This leads to a remaining weak point within the design process of transonic compressor blades, because the working range will be overpredicted.

The resulting conclusion of the study is that the use of scale resolving methods such as LES or the application of DNS is necessary to correctly predict unsteadiness of the transonic cascade flow and its impact on the cascade performance.
Direct Assessment of the Acoustic Scattering Matrix of a Turbulent Swirl Combustor by Combining System Identification, Large Eddy Simulation and Analytical Approaches
Authors: Merk Malte , Silva Camilo , Polifke Wolfgang , Gaudron Renaud , Gatti Marco , Mirat Clément , Schuller Thierry
Published: August 2018

This study assesses and compares two alternative approaches to determine the acoustic scattering matrix of a pre-mixed turbulent swirl combustor:

1) The acoustic scattering matrix coefficients are obtained directly from a compressible Large Eddy Simulation (LES). Specifically, the incoming and outgoing characteristic waves f and g extracted from the LES are used to determine the respective transmission and reflection coefficients via System Identification techniques.

2) The flame transfer function (FTF) is identified from LES time series data of upstream velocity and heat release rate. The transfer matrix of the reactive combustor is then derived by combining the FTF with the Rankine-Hugoniot relations across a compact heat source and a transfer matrix of the cold combustor, which is deduced from a linear network model. Linear algebraic transformation of the transfer matrix consequently yields the combustor scattering matrix.

A cross-comparison study that includes comprehensive experimental data shows that both approaches successfully predict the scattering matrix of the reactive turbulent swirl combustor.
Synchronization of Thermoacoustic Modes in Sequential Combustors
Authors: Bonciolini Giacomo , Noiray Nicolas
Published: August 2018

Sequential combustion constitutes a major technological step-change for gas turbines applications. This design provides higher operational flexibility, lower emissions and higher efficiency compared to today’s conventional architectures. Like any constant pressure combustion system, sequential combustors can undergo thermoacoustic instabilities. These instabilities potentially lead to high-amplitude acoustic limit cycles, which shorten the engine components’ lifetime and therefore reduce their reliability and availability. In case of a sequential system, the two flames are mutually coupled via acoustic and entropy waves. This additional inter-stages interaction markedly complicates the already challenging problem of thermoacoustic instabilities. As a result, new and unexplored system dynamics are possible. In this work, experimental data from our generic sequential combustor are presented. The system exhibits many different distinctive dynamics, as function of the operation parameters and of the combustor arrangement. This paper investigates a particular bifurcation, where two thermoacoustic modes synchronize their self-sustained oscillations over a range of operating conditions. A low-order model of this thermoacoustic bifurcation is proposed. This consists of two coupled stochastically driven non-linear oscillators, and is able to reproduce the peculiar dynamics associated with this synchronization phenomenon. The model aids in understanding what the physical mechanisms that play a key role in the unsteady combustor physics are. In particular, it highlights the role of entropy waves, which are a significant driver of thermoacoustic instabilities in this sequential setup. This research helps to lay the foundations for understanding the thermoacoustic instabilities in sequential combustion systems.
Prediction of Boundary Layer Flashback Limits of Laminar Premixed Jet Flames
Authors: Hoferichter Vera , Hirsch Christoph , Sattelmayer Thomas
Published: August 2018

Preventing flame flashback into the premixing section is one of the major challenges in premixed combustion systems. For jet flames, the flame typically propagates upstream inside the low velocity region close to the burner wall, referred to as boundary layer flashback.

The physical mechanism of boundary layer flashback of laminar flames is mainly influenced by flame-wall interaction and flame quenching. Flashback is initiated if the burning velocity at some wall distance is higher than the local flow velocity. Since the burning velocity drops towards the wall due to heat losses, the wall distance of flashback can be defined at the location closest to the wall where the burning velocity still is sufficiently high. The well-established critical gradient concept of Lewis and von Elbe to predict flashback limits of laminar flames represents these assumptions but neglects the important influence of flame stretch on the burning velocity close to the wall. For that reason, a modified prediction model is developed in this work based on similar assumptions as in the critical gradient concept, but including the effect of flame stretch. A validation for hydrogen-air and methane-air flames highlights its advantages compared to the critical gradient concept and shows good prediction accuracy.
A Top-Down Approach for Quantifying the Contribution of High Pressure Compressor Deterioration Mechanisms to the Performance Deterioration of Turbofan Engines
Authors: Vogel Helena , Kando André , Schulte Holger , Staudacher Stephan
Published: August 2018

Maintenance costs are a substantial contributor to airline operating costs. In this context, understanding, analyzing, and predicting engine performance deterioration is crucial. While diagnostic methods for analyzing the current module and overall engine condition are established in state-of-the-art engine condition monitoring (ECM) systems, deterioration modeling and prognosis are still fields of research. The identification of the contribution of deterioration mechanisms, such as fouling, erosion, and abrasion, to the in-service deterioration poses a key challenge in this area. This paper focuses on a top-down approach for the high pressure compressor (HPC) module. The selected approach is to quantify the contribution of individual deterioration mechanisms to the overall HPC efficiency deterioration based on in-flight measurements. This is accomplished by first using the in-flight measurements to analyze the HPC efficiency loss. Then, the resulting time series of the analyzed HPC efficiency loss are preprocessed. Finally, models of the deterioration mechanisms are fitted to the preprocessed time series. The deterioration models are chosen based on literature references to the respective deterioration mechanisms. As multiple influencing factors affect the deterioration mechanisms, a fleet analysis is conducted to select the model inputs. The fitting process involves a parametric nonlinear regression problem. The outcome is an estimation of the evolution of the deterioration mechanisms over time. This methodology is used to evaluate all available in-service engines of the same type and fleet and to define a fleet model. In the final step, benefits and limitations of the fleet model are investigated.
Effect of Mixtures on Compressor and Cooler in Supercritical Carbon Dioxide Cycles
Authors: Vesely Ladislav , Manikantachari K. R. V. , Vasu Subith , Kapat Jayanta , Dostal Vaclav , Martin Scott
Published: August 2018

With the increasing demand for electric power, the development of new power generation technologies is gaining increased attention. The supercritical carbon dioxide (S-CO2) cycle is one such technology, which has relatively high efficiency, compactness, and potentially could provide complete carbon capture. The S-CO2 cycle technology is adaptable for almost all of the existing heat sources such as solar, geothermal, fossil, nuclear power plants, and waste heat recovery systems. However, it is known that, optimal combinations of: operating conditions, equipment, working fluid, and cycle layout determine the maximum achievable efficiency of a cycle. Within an S-CO2 cycle the compression device is of critical importance as it is operating near the critical point of CO2. However, near the critical point, the thermo-physical properties of CO2 are highly sensitive to changes of pressure and temperature. Therefore, the conditions of CO2 at the compressor inlet are critical in the design of such cycles. Also, the impurity species diluted within the S-CO2 will cause deviation from an ideal S-CO2 cycle as these impurities will change the thermodynamic properties of the working fluid. Accordingly the current work examines the effects of different impurity compositions, considering binary mixtures of CO2 and: He, CO, O2, N2, H2, CH4, or H2S; on various S-CO2 cycle components. The second part of the study focuses on the calculation of the basic cycles and component efficiencies. The results of this study will provide guidance and defines the optimal composition of mixtures for compressors and coolers.
Experimental and Analytical Characterization of Alternative Aviation Fuel Sprays Under Realistic Operating Conditions
Authors: Corber Andrew , Rizk Nader , Chishty Wajid Ali
Published: August 2018

The National Jet Fuel Combustion Program (NJFCP) is an initiative, currently being led by the Office of Environment & Energy at the FAA, to streamline the ASTM jet fuels certification process for alternative aviation fuels. In order to accomplish this objective, the program has identified specific applied research tasks in several areas. The National Research Council of Canada (NRC) is contributing to the NJFCP in the areas of sprays and atomization and high altitude engine performance. This paper describes work pertaining to atomization tests using a reference injection system. The work involves characterization of the injection nozzle, comparison of sprays and atomization quality of various conventional and alternative fuels, as well as use of the experimental data to validate spray correlations. The paper also briefly explores the application viability of a new spray diagnostic system that has potential to reduce test time in characterizing sprays. Measurements were made from ambient up to 10 bar pressures in NRC’s High Pressure Spray Facility using optical diagnostics including laser diffraction, phase Doppler anemometry (PDA), LIF/Mie Imaging and laser sheet imaging to assess differences in the atomization characteristics of the test fuels. A total of nine test fluids including six NJFCP fuels and three calibration fluids were used. The experimental data was then used to validate semi-empirical models, developed through years of experience by engine OEMs and modified under NJFCP, for predicting droplet size and distribution. The work offers effective tools for developing advanced fuel injectors, and generating data that can be used to significantly enhance multi-dimensional combustor simulation capabilities.
Large Eddy Simulation to Predict Flame Front Position for Turbulent Lean Premixed Jet Flame at High Pressure
Authors: Yunoki Keita , Murota Tomoya
Published: August 2018

To predict the flame front position, we adopt the large eddy simulation (LES) and incorporate into it a combustion model, the hybrid turbulent combustion model (HTC model), applicable to any flame mode. We take previously obtained test results for the axisymmetric jet lean premixed flame in a cylindrical chamber at high pressure and investigate the effects of equivalence ratio variation on flame front positions. Full details of these tests are available in the literature. We find the simulation results of the axial positions of the flame front points, defined by the inflection point of the OH concentration distributions, show good agreement with test results (less than 4% difference between them). Therefore, we conclude that the HTC model is capable of capturing the actually observed tendency when changing the equivalence ratios and that the combination of the HTC model with LES suggests that flame stretch effect is important to predict the flame front position for lean premixed combustion.
Study on the Effect of Jets Geometry in Liquid Atomization Based on Nonlinear Stability Analysis
Authors: Jafari Sajad , Nehzati Taravatsadat
Published: August 2018

In this paper, nonlinear instability of an elliptical jet is investigated by considering the impacts of orifice geometry variations using regular perturbation method. In the breakup mechanisms, created disturbances on the jet interfaces will grow owing to the nonlinear dynamics of fluid. In this study, a scrutiny of nonlinear cylindrical jet breakup is done initially. Next, Cosserat equations as a low order form of Navies-Stokes equations are solved on the nonlinear form to exert the impacts of orifice deformation on various aspect ratios. These nonlinear equations, Cosserat equations, are linearly solved in the past papers. As a result, the dispersion equation is derived to find the most unstable wavelength as well as the breakup length. Results reveal that the elliptical jet with low aspect ratio is more unstable rather than cylindrical jet with aspect ratio of one. Furthermore, the nonlinear equations can predict the break up length of elliptical jet more accurate than linear equations. The predicted results are shown to be in good agreement with the experimental results.
Integration of Helicopter Annular Combustion Chamber Rig in Propulsion Systems Course for Graduate Students
Authors: Al-Asmi I. , Vandel A. , Cabot G. , Grisch F. , Moureau V. , Savary N. , Richard S. , Renou B.
Published: August 2018

The integration of graduate research in the training of engineering students has demonstrated a significant increase in learning efficiency, by giving them a practical experience with real industrial issues. The department of Energy and Propulsion of National Institute of Applied Sciences in Normandy, a French Engineering School, continues to implement the latest fully instrumented facilities in their field to initiate the students to inquiry-based education courses. In this type of education, they are carrying out a series of tests, learning how to handle equipment, control and monitor tests, extract results and ultimately analyze and present them in technical reports. This paper addresses how the Helicopter Annular Combustion Chamber test facility has been integrated in the Propulsion systems engineering course sequence. The Annular Combustion Chamber kindly provided by SAFRAN Helicopter Engines was progressively incorporated and instrumented in a dedicated test facility by the students themselves along the last 8 years. Now, this laboratory practical work offers the students the possibility to interactively learn about the operation of a combustion chamber inside a helicopter engine at various air/fuel flow rates. Students learn how to determine the limits of ignition/non-extinction as a function of the entry air-flow rate. In addition, this facility is equipped with high-level instrumentation that allows to measure the different flow rates, pressure, temperature inside and outside the annular chamber, and the pollutant emissions at the exit. Results provided by students help to build a comprehensive knowledge base of combustion phenomena inside a turbojet engine. It is to be mentioned that this educational facility is unique in its category, from the point of view of results accuracy, instrumentation level and realistic operating conditions.
Correlations for the Prediction of Intermittency and Turbulent Spot Production Rate in Separated Flows
Authors: Dellacasagrande M. , Guida R. , Lengani D. , Simoni D. , Ubaldi M. , Zunino P.
Published: August 2018

Experimental data describing laminar separation bubbles developing under strong adverse pressure gradients, typical of Ultra-High-Lift turbine blades, have been analyzed to define empirical correlations able to predict the main features of the separated flow transition. Tests have been performed for three different Reynolds numbers and three different free-stream turbulence intensity levels. For each condition, around 4000 Particle Image Velocimetry (PIV) snapshots have been acquired. A wavelet based intermittency detection technique, able to identify the large scale vortices shed as a consequence of the separation, has been applied to the large amount of data to efficiently compute the intermittency function for the different conditions. The transition onset and end positions, as well as the turbulent spot production rate are evaluated. Thanks to the recent advancements in the understanding on the role played by Reynolds number and free-stream turbulence intensity on the dynamics leading to transition in separated flows, guest functions are proposed in the paper to fit the data. The proposed functions are able to mimic the effects of Reynolds number and free-stream turbulence intensity level on the receptivity process of the boundary layer in the attached part, on the disturbance exponential growth rate observed in the linear stability region of the separated shear layer, as well as on the nonlinear later stage of completing transition. Once identified the structure of the correlation functions, a fitting process with own and literature data allowed us to calibrate the unknown constants. Results reported in the paper show the ability of the proposed correlations to adequately predict the transition process in the case of separated flows. The correlation for the spot production rate here proposed extends the correlations proposed in liter-ature for attached (by-pass like) transition process, and could be used in γ–Reϑ codes, where the spot production rate appears as a source term in the intermittency function transport equation.
Quantification and Propagation of Uncertainties in Identification of Flame Impulse Response for Thermoacoustic Stability Analysis
Authors: Guo Shuai , Silva Camilo F. , Ghani Abdulla , Polifke Wolfgang
Published: August 2018

The thermoacoustic behavior of a combustion system can be determined numerically via acoustic tools such as Helmholtz solvers or network models coupled with a model for the flame dynamic response. Within such a framework, the flame response to flow perturbations can be described by a Finite Impulse Response (FIR) model, which can be derived from LES time series via system identification. However, the estimated FIR model will inevitably contain uncertainties due to e.g., the statistical nature of the identification process, low signal-to-noise ratio or finite length of time series. Thus, a necessary step towards reliable thermoacoustic stability analysis is to quantify the impact of uncertainties in FIR model on the growth rate of thermoacoustic modes. There are two practical considerations involved in this topic. First, how to efficiently propagate uncertainties from the FIR model to the modal growth rate of the system, considering it is a high dimensional uncertainty quantification (UQ) problem? Second, since longer CFD simulation time generally leads to less uncertain FIR model identification, how to determine the length of the CFD simulation required to obtain satisfactory confidence? To address the two issues, a dimensional reduction UQ methodology called “Active Subspace approach” is employed in the present study. For the first question, Active Subspace approach is applied to exploit a low-dimensional approximation of the original system, which allows accelerated UQ analysis. Good agreement with Monte Carlo analysis demonstrates the accuracy of the method. For the second question, a procedure based on Active Subspace approach is proposed, which can serve as an indicator for terminating CFD simulation. The effectiveness of the procedure is verified in the paper.
An Introduction Into the Clearance Management of Ansaldo GT36 From Development to Validation
Authors: Boeller S. , Feuillard B. , Filkorn G. , Olmes S. , Prou F. , Robson C. , Santos R.
Published: August 2018

The optimization and evaluation of blading clearance is important for gas turbine efficiency and performance. The Ansaldo GT36 gas turbine offers high efficiency together with outstanding flexibility across a large load range. Active management of engine clearances during the complete development process followed by a thorough validation on the Ansaldo test plant facility in Birr, Switzerland enables the GT to attain ambitious clearance targets. The clearance at baseload must be minimized but is limited by the pinch point clearance during cold, warm and hot start-ups — including normal and fast ramp-up and/or shutdown. Therefore transient analysis is necessary for covering the different operating conditions. A well-established process of 2d finite element modelling of the whole engine model (WEM) comprised of axis-symmetric and plane stress elements was used during the design process from concept to detailed design to optimize the clearances. It delivers the transient stator and rotor deformation and together with the compressor and turbine airfoil deformation based on 3D models the basic clearance evaluation process is defined. The GT engine design was significantly influenced, starting with a simplified version of the WEM for identification of the most promising variants. Subsequently a detailed WEM was developed which is fully validated against measurements on the test engine. Different 3D effects are considered separately at identified critical transient conditions and overlaid on the 2d clearances which lead to the final optimized clearances. In addition to this, limitations from each step of the manufacturing process were identified and improved to reduce tolerances and uncertainties to their minimum. The results of the calculation and clearance prediction process are compared against clearance measurements during all kinds of GT operation and cooldown. Passive clearance indicators showing the remaining gap till rubbing would occur and rub marks, in areas that tolerate it, further validate the clearances and clearance prediction process.
A Highly Flexible Approach on the Steady-State Analysis of Innovative Micro Gas Turbine Cycles
Authors: Krummrein Thomas , Henke Martin , Kutne Peter
Published: August 2018

Steady state simulations are an important method to investigate thermodynamic processes. This is especially true for innovative micro gas turbine (MGT) based cycles as the complexity of such systems grows. Therefore, steady state simulation tools are required which ensure large flexibility and computation robustness. As the increased system complexity result often in more extensive parameter studies also a fast computation speed is required.

While a number of steady state simulation tools for micro gas turbine based systems are described and applied in literature, the solving process of such tools is rarely explained. However, this solving process is crucial to achieve a robust and fast computation within a physically meaningful range. Therefore, a new solver routine for a steady state simulation tool developed at the DLR Institute of Combustion Technology is presented in detail in this paper.

The solver routine is based on Broyden’s method. It considers boundaries during the solving process to maintain a physically and technically meaningful solution process. Supplementary methods are implemented and described which improve the computation robustness and speed.

Furthermore, some features of the resulting steady state simulation tool are presented. Exemplary applications of a hybrid power plant, an inverted Brayton cycle and an aircraft auxiliary power unit show the capabilities of the presented solver routine and the steady state simulation tool. It is shown that the new solver routine is superior to the standard Simulink algebraic solver in terms of system evaluation and robustness for the given applications.
Experimental Study on Low Load Operation Range Extension by Autothermal On-Board Syngas Generation
Authors: Baumgärtner Max H. , Sattelmayer Thomas
Published: August 2018

Volatile renewable energy sources induce power supply fluctuations. These need to be compensated by flexible conventional power plants. Gas turbines in combined cycle power plants adjust the power output quickly but their turn-down ratio is limited by the slow reaction kinetics which lead to CO and unburned hydrocarbon (UHC) emissions. To extend the turn-down ratio, part of the fuel can be converted to syngas, which exhibits a higher reactivity. By an increasing fraction of syngas in the fuel, the reactivity of the mixture is increased and total fuel mass-flow and the power output can be reduced. An Autothermal On-board Syngas Generator in combination with two different burner concepts for natural gas/syngas mixtures was presented in a previous study [1]. The study at hand shows a mass-flow variation of the reforming process with mass-flows which allow for pure syngas combustion and further improvements of the two burner concepts which result in a more application-oriented operation. The first of the two burner concepts comprises a generic swirl stage with a central lance for syngas injection. Syngas is injected with swirl to avoid a negative impact on the total swirl intensity and non-swirled. The second concept includes a central swirl stage with an outer ring of jets. For this burner, syngas is injected in both stages to avoid NOx emissions from the swirl stage. Increased NOx emissions produced by natural gas combustion of the swirl pilot was reported in last year’s paper. For both burners, combustion performance is analyzed by OH*-chemiluminescence and gaseous emissions. The lowest possible adiabatic flame temperature without a significant increase of CO emissions was 170 K – 210 K lower for the syngas compared to low load pure natural gas combustion. This corresponds to a decrease of 15–20 % in terms of thermal power.
Structural Integrity Assessment of ORC Turbine Tie-Rods: An Analysis Based on Elastic Shakedown and Fracture Mechanics
Authors: Rabbolini S. , Sanvito M. , Lo Conte A. , Beretta S. , Rizzi D.
Published: August 2018

Fatigue life assessment of turbine rotating components is usually performed taking into account the presence of potential defects and their propagation, together with an accurate description of the stress/strain fields acting at components critical regions. In this work, such an approach was applied on a radial outflow turbine: the component investigated in this paper is the tie-rod. An axisymmetric FEM model of the whole turbine was implemented to evaluate the stress field acting at the root of tie-rod thread, the component most critical region. Three different preload procedures were investigated, in order to evaluate the effects of plastic strains on fatigue cycles. It was found that an initial overtightening, performed before machine assembly, induces a larger plastic zone at the root of the thread, which lowers the peak stress and lessens the stress ratio of the fatigue cycle. Stress gradients for the three different preloading conditions were the starting point of a crack propagation model, which calculated crack advancement as a function of service time. The model, which considered crack closure and stress ratio effects, allowed structural integrity assessment and showed that the initial overtightening had also the effect of crack growth rates reduction.
Cooling Effectiveness for a Shaped Film Cooling Hole at a Range of Compound Angles
Authors: Haydt Shane , Lynch Stephen
Published: August 2018

Shaped film cooling holes are a well-established cooling technique used in gas turbines to keep component metal temperatures in an acceptable range. One of the goals of film cooling is to reduce the driving temperature for convection at the wall, the success of which is generally represented by the film cooling adiabatic effectiveness. However, the introduction of a film cooling jet-in-crossflow, especially if it is oriented at a compound angle, can augment the convective heat transfer coefficient and dominate the flowfield. The present work aims to understand the effect that a compound angle has on the flowfield and adiabatic effectiveness of a shaped film cooling hole. Five orientations of the public 7-7-7 shaped film cooling hole were tested, from a streamwise oriented hole (0° compound angle) to a 60° compound angle hole, in increments of 15°. Additionally, two pitchwise spacings of P/D = 3 and 6 were tested to examine the effect of hole-to-hole interaction. All cases were tested at a density ratio of 1.2 and blowing ratios ranging from 1.0 to 4.0. Experimental results show that increasing compound angle leads to increased lateral spread of coolant, and enables higher laterally-averaged effectiveness at high blowing ratios. A smaller pitchwise spacing leads to more complete coverage of the endwall, and has higher laterally averaged effectiveness even when normalized by coverage ratio, suggesting that hole to hole interaction is important for compound angled holes. Steady RANS CFD was not able to capture the exact effectiveness levels, but did predict many of the observed trends. The lateral motion of the coolant jet was also quantified, both from the experimental data and the CFD prediction, and as expected, holes with a higher compound angle and higher blowing ratio have greater lateral motion, which generally also promotes hole-to-hole interaction.
Fast Conjugate Heat Transfer Simulation of Long Transient Flexible Operations Using Adaptive Time Stepping
Authors: Maffulli R. , He L. , Stein P. , Marinescu G.
Published: August 2018

The emerging renewable energy market calls for more advanced prediction tools for turbine transient operations in fast startup/shutdown cycles. Reliable numerical analysis of such transient cycles is complicated by the disparity in time scales of the thermal responses in fluid and solid domains. Obtaining fully-coupled time-accurate unsteady Conjugate Heat Transfer (CHT) results under these conditions would require to march in both domains using the time step dictated by the fluid domain: typically, several orders of magnitude smaller than the one required by the solid.

This requirement has strong impact on the computational cost of the simulation as well as being potentially detrimental to the accuracy of the solution due to accumulation of round-off errors in the solid. A novel loosely coupled CHT methodology has been recently proposed, and successfully applied to both natural and forced convection cases that removes these requirements through a source-term based modelling approach of the physical time derivative terms in the relevant equations. The method has been shown to be numerically stable for very large time steps with adequate accuracy.

The present effort is aimed at further exploiting the potential of the methodology through a new adaptive time stepping approach. The proposed method allows for automatic time step adjustment based on estimating the magnitude of the truncation error of the time discretization. The developed automatic time stepping strategy is applied to natural convection cases under long (2000 s) transients: relevant to the prediction of turbine thermal loads during fast startups/shutdowns. The results of the method are compared with fully coupled unsteady simulations showing comparable accuracy with a significant reduction of the computational costs.
Demonstration of a Remotely-Controlled Swirl Generator for Simulating Aircraft Inlet Secondary Flows During Turbine Engine Ground Tests
Authors: Beale David
Published: August 2018

The development of superior combat aircraft demands the complex integration of the airframe, engine, control system, avionics, and on-board weapon systems. The integration of the engine and the inlet is tantamount to prevailing in an engagement due to the thrust required to execute combat maneuvers. For this reason, test and evaluation methods have been developed to help ensure inlet-engine compatibility by design. The most commonly used methodology characterizes inlet distortion in terms of total-pressure descriptors and correlations. The method includes ground tests employing both wind tunnel and engine test facilities, to acquire the information needed to establish inlet-engine compatibility prior to flight test. Advanced aircraft employing evolving technologies never seen in legacy systems have introduced new challenges to the methodology, and to the ground test methods employed by the methodology. One such challenge arises from the significant flow angularity, or swirl, often found in advanced inlet systems. This paper focuses on the simulation of aircraft inlet swirl during direct-connect turbine engine ground tests.

To meet the engine test challenges introduced by advanced aircraft, the Arnold Engineering Development Complex (AEDC) embarked on the development of a swirl generator capable of simulating the different types of swirl expected in future inlet systems over a wide range of swirl angles, and with the ability to remotely set steady-state or transient swirl patterns. The development progressed through a five-step process that culminated in the validation and demonstration of a fully-functional prototype.

This paper focuses on the prototype swirl generator and the progression from the establishment of simulation requirements through the prototype validation. Following summaries of each development step, the results of the validation test are presented. The paper also summarizes a recent application of the prototype which not only demonstrated the device in an engine test, but which provided a data set to support swirl methodology development.
FlameSheet™ Combustor Extended Engine Validation for Operational Flexibility and Low Emissions
Authors: Rizkalla Hany , Hernandez Fred , KeshavaBhattu Ramesh , Stuttaford Peter
Published: August 2018

Flexibility is key to the future success of natural gas fired power generation. As renewable energy sources continue their penetration of the global energy market, the need for reliable, flexible generation will increase. Gas turbines equipped with a fuel flexible combustion system allowing the capability to extend in-emissions-compliance turndown limit, will have a significant advantage supporting todays and future energy market demand. The FlameSheet™ combustor incorporates a novel dual zone burn system to address operational and fuel flexibility with low emissions and extended turndown. FlameSheet™ is simply retrofittable into existing installed E/F-class heavy duty gas turbines and is designed to meet the energy market drivers set forth above. The operating principle of the new combustor is briefly described, and details of implementation and extended validation results on two General Electric 7FA heavy duty gas turbines operating in a combined cycle power plant since 2015 with over 36,600hrs of uninterrupted commercial operation is discussed, with special focus on operational profile optimization to accommodate the heat recovery steam generator (HRSG), while substantially increasing the gas turbine normal operating load range. Emphasis is also provided on performance assessment, combustion and downstream hot gas path component inspection and durability assessment after 16,600 hours of operation in a 7FA gas turbine.
Improved Empirical Identification of the Inverse Model of a Squeeze-Film Damper Bearing Based on a Recurrent Neural Network
Authors: Al-Ghazal Ghaith Ghanim , Bonello Philip , Torres Cedillo Sergio G.
Published: August 2018

Most recently proposed techniques for inverse rotordynamic problems seek to identify the unbalance on a rotor using a known structural model and measurements from externally mounted sensors only. Such non-intrusive techniques are important for balancing rotors that cannot be accessed under operational conditions because of temperature or space restrictions. The presence of nonlinear bearings, like squeeze-film damper (SFD) bearings used in aero-engines, complicates the solution process of the inverse rotordynamic problem. In certain practical aero-engine configurations, the solution process requires a substitute for internal instrumentation to quantify the SFD journal vibration. This can be provided by an inverse model of the SFD bearing which outputs the time history of the relative vibration of the SFD journal relative to its housing, for a given input time history of the SFD force. This paper focuses on the inverse model of the SFD and presents an improved methodology for its identification via a Recurrent Neural Network (RNN) trained using experimental data from a purposely designed rig. The novel application of chirp excitation via two orthogonal shakers considerably improves both the quality of the training data and the efficiency of its generation, relative to an earlier preliminary work. Validation test results show that the RNNs can predict the journal displacement time history with reasonable accuracy. It is therefore expected that such an inverse SFD model would serve as a reliable component in the solution of the wider inverse problem of a rotordynamic system.
LES Investigation of Boussinesq Constitutive Relation Validity in a Corner Separation Flow
Authors: Monier Jean-François , Poujol Nicolas , Laurent Mathieu , Gao Feng , Boudet Jérôme , Aubert Stéphane , Shao Liang
Published: August 2018

The present study aims at analysing the Boussinesq constitutive relation validity in a corner separation flow of a compressor cascade. The Boussinesq constitutive relation is commonly used in Reynolds-averaged Navier-Stokes (RANS) simulations for turbomachinery design. It assumes an alignment between the Reynolds stress tensor and the zero-trace mean strain-rate tensor. An indicator that measures the alignment between these tensors is used to test the validity of this assumption in a high fidelity large-eddy simulation. Eddy-viscosities are also computed using the LES database and compared.

A large-eddy simulation (LES) of a LMFA-NACA65 compressor cascade, in which a corner separation is present, is considered as reference. With LES, both the Reynolds stress tensor and the mean strain-rate tensor are known, which allows the construction of the indicator and the eddy-viscosities. Two constitutive relations are evaluated. The first one is the Boussinesq constitutive relation, while the second one is the quadratic constitutive relation (QCR), expected to render more anisotropy, thus to present a better alignment between the tensors.

The Boussinesq constitutive relation is rarely valid, but the QCR tends to improve the alignment. The improvement is mainly present at the inlet, upstream of the corner separation. At the outlet, the correction is milder. The eddy-viscosity built with the LES results are of the same order of magnitude as those built as the ratio of the turbulent kinetic energy k and the turbulence specific dissipation rate ω. They also show that the main impact of the QCR is to rotate the mean strain-rate tensor in order to realign it with the Reynolds stress tensor, without dilating it.