bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Pressure and Steam Dilution on NOx and CO Emissions in a Premixed Natural Gas Flame
Authors: Göke Sebastian , Schimek Sebastian , Terhaar Steffen , Reichel Thoralf , Göckeler Katharina , Krüger Oliver , Fleck Julia , Griebel Peter , Paschereit Christian Oliver
Published: November 2013

In the current study, the influence of pressure and steam on the emission formation in a premixed natural gas flame is investigated at pressures between 1.5 bar and 9 bar.

A premixed, swirl-stabilized combustor is developed that provides a stable flame up to very high steam contents. Combustion tests are conducted at different pressure levels for equivalence ratios from lean blowout to near-stoichiometric conditions and steam-to-air mass ratios from 0% to 25%.

A reactor network is developed to model the combustion process. The simulation results match the measured NOx and CO concentrations very well for all operating conditions. The reactor network is used for a detailed investigation of the influence of steam and pressure on the NOx formation pathways.

In the experiments, adding 20% steam reduces NOx and CO emissions to below 10 ppm at all tested pressures up to near-stoichiometric conditions. Pressure scaling laws are derived: CO changes with a pressure exponent of approximately −0.5 that is not noticeably affected by the steam. For the NOx emissions, the exponent increases with equivalence ratio from 0.1 to 0.65 at dry conditions. At a steam-to-air mass ratio of 20%, the NOx pressure exponent is reduced to −0.1 to +0.25.

The numerical analysis reveals that steam has a strong effect on the combustion chemistry. The reduction in NOx emissions is mainly caused by lower concentrations of atomic oxygen at steam-diluted conditions, constraining the thermal pathway.
Effect of Variable Properties Within a Boundary Layer With Large Freestream to Wall Temperature Differences
Authors: Greiner Nathan J. , Polanka Marc D. , Robertson Jacob R. , Rutledge James L.
Published: November 2013

Modern aviation combustors run at high fuel-air ratios to achieve high turbine inlet temperatures and higher turbine efficiencies. To maximize turbine durability in such extreme temperatures, the blades are fitted with film cooling schemes to form a layer of cool air between the blade and the hot core flow. Two terms that are utilized to evaluate a cooling scheme are the heat transfer coefficient (h) and the local driving temperature, namely, the adiabatic wall temperature (Taw). The literature presents a method for calculating these two parameters by assuming the heat flux (q) is proportional to the difference in freestream and wall temperatures (T∞ − Tw). Several researchers have shown the viability of this approach by altering the wall temperature over a finite range in low temperature environment. A linear trend ensues where the slope is h and the q = 0 intercept is adiabatic wall temperature. This technique has proven valuable since constant h is known to be a valid assumption for constant property flow.

The current study explores the validity of this assumption by analytically predicting and experimentally measuring the h and q at high T∞ and low Tw characteristic of a modern combustor. Both a reference temperature method and temperature ratio method were applied to model the effects of variable properties within the boundary layer. To explore the linearity of the heat transfer with driving temperature, the analysis determined the apparent h and Taw which would be measured over small ranges of Tw by the linear method discussed in the literature. This study shows that, over large Tw ranges, property variations play a significant role. It is also shown that the linear trend technique is valid even at high temperature conditions but only when used in small temperature ranges. Finally, this investigation shows that the apparent Taw used in the linear convective heat transfer assumption is a valid driving temperature over small ranges of Tw but cannot always be interpreted literally as the temperature where q(Taw) = 0.
Reduction of Rayleigh Losses in a High G-Loaded Ultra Compact Combustor
Authors: Wilson Jacob D. , Polanka Marc D.
Published: November 2013

There is a current need to reduce turbine engine system size and weight. An Ultra Compact Combustor (UCC) is a system that attempts to accomplish this by integrating the compressor, combustor, and turbine stages. The main feature of this system is a high gravity-loaded combustor that burns the fuel in the circumferential direction around a hybrid vane row. One consequence of this design is that the UCC needs the combustion process to complete within the vane passage. A second issue is that combining the compressor and turbine results in large flow swirl and high Mach number in the core flow. Combined, these two issues lead to the potential for high Rayleigh losses for this design. This investigation used MATLAB to develop an area ruling inside the passage that allowed the flow to slow to a reasonable Mach number during the heat release process and then accelerate to the desired turbine rotor inlet Mach once combustion has finished. Results showed that it was possible to reduce Rayleigh losses in this manner, but equilibrium between Rayleigh losses and aerodynamic losses must be met. This model was incorporated into a design of a test rig. Reductions of more than 5% in Rayleigh losses in the experimental rig are expected.
Characterisation of Lean Burn Module Air Blast Pilot Injector With Laser Techniques
Authors: Meier U. , Freitag S. , Heinze J. , Lange L. , Magens E. , Schroll M. , Willert C. , Hassa C. , Bagchi I. K. , Lazik W. , Whiteman M.
Published: November 2013

For lean burn combustor development in low emission aero-engines, the pilot stage of the fuel injector plays a key role with respect to stability, operability, NOx emissions, and smoke production. Therefore it is of considerable interest to characterize the pilot module in terms of pilot zone mixing, fuel placement, flow field and interaction with the main stage.

This contribution focusses on the investigation of soot formation during pilot-only operation. Optical test methods were applied in an optically accessible single sector rig at engine idle conditions. Using planar laser-induced incandescence (LII), the distribution of soot and its dependence on air/fuel ratio, as well as geometric injector parameters, was studied. The data shows that below a certain air/fuel ratio, an increase of soot production occurs. This is in agreement with smoke number measurements in a standard single sector flame tube rig without optical access. Reaction zones were identified using chemiluminescence of OH radicals. In addition, the injector flow field was investigated with PIV. A hypothesis regarding the mechanism of pilot smoke formation was made based on these findings. This along with further investigations will form the basis for developing strategies for smoke improvement at elevated pilot only conditions.
Experimental Investigation of the Effect of Purge Flow on Film Cooling Effectiveness on a Rotating Turbine With Non-Axisymmetric Endwall Contouring
Authors: Rezasoltani M. , Schobeiri M. T. , Han J. C.
Published: November 2013

The impact of the purge flow injection on aerodynamics and film cooling effectiveness of a three-stage high pressure turbine with non-axisymmetric endwall contouring has been experimentally investigated. As a continuation of the previously published work involving stator-rotor gap purge cooling, this study investigates film cooling effectiveness on the first stage rotor contoured platform due to a coolant gas injection. Film cooling effectiveness measurements are performed on the rotor blade platform using a pressure sensitive paint (PSP) technique. The present study examines, in particular, the film cooling effectiveness due to injection of coolant from the rotor cavity through the circumferential gap between the first stator followed by the first rotor. Efficiency and performance experiments were conducted with and without cooling injection to show (a) the impact of endwall contouring on the turbine efficiency and (b) the impact of film cooling injection in association with the endwall contouring.

The experimental investigation is carried out in a three-stage turbine facility at the Turbomachinery Performance and Flow Research Laboratory (TPFL) at Texas A&M University. Its rotor includes non-axisymmetric endwall contouring on the first and second rotor row [1]. The turbine has two independent cooling loops. Film cooling effectiveness measurements are performed for three coolant-to-mainstream mass flow ratios of 0.5%, 1.0% and 1.5%. Film cooling data is also obtained for three rotational speeds, 3000 rpm (reference condition), 2550 rpm and 2400 rpm and compared with non-contoured endwall data.
Avoiding Compressor Surge During Emergency Shut-Down Hybrid Turbine Systems
Authors: Pezzini Paolo , Tucker David , Traverso Alberto
Published: November 2013

A new emergency shutdown procedure for a direct-fired fuel cell turbine hybrid power system was evaluated using a hardware-based simulation of an integrated gasifier/fuel cell/turbine hybrid cycle (IGFC), implemented through the Hybrid Performance (Hyper) project at the National Energy Technology Laboratory, U.S. Department of Energy (NETL). The Hyper facility is designed to explore dynamic operation of hybrid systems and quantitatively characterize such transient behavior. It is possible to model, test and evaluate the effects of different parameters on the design and operation of a gasifier/fuel cell/gas turbine hybrid system and provide means of quantifying risk mitigation strategies.

An open-loop system analysis regarding the dynamic effect of bleed air, cold air by-pass and load bank is presented in order to evaluate the combination of these three main actuators during emergency shut-down. In the previous Hybrid control system architecture, catastrophic compressor failures were observed when the fuel and load bank were cut-off during emergency shut-down strategy. Improvements were achieved using a non-linear fuel valve ramp down when load bank was not operating. Experiments in load bank operation show compressor surge and stall after emergency shut-down activation. The difficulties in finding an optimal compressor and cathode mass flow for mitigation of surge and stall using these actuators are illustrated.
Measurement and Modelling of Blade Tip Cooling in Gas Turbines
Authors: Teloxa Julio , Narzary Diganta , Sierra F. Z. , Lei Jiang , Carrillo Francisco , Han Je-Chin
Published: November 2013

Blade tip cooling effectiveness simulations and measurements are reported. Measurements of metal blade surface temperature distributions were obtained in the laboratory using temperature sensitive paint technique, TSP, for mainstream Reynolds number, ReM = 200,000, and cooling Reynolds number, ReC = 10,000. The blade tip region consists of a cavity with thin walls on pressure and suction sides known as squealer rim. TSP technique is used to investigate the temperature demonstrating its applicability as cooling effectiveness measurement method on a blade tip region. Several film cooling holes placed along the airfoil camber line discharge coolant and spread over the tip cavity. Besides, conjugate cooling flow was numerically simulated, considering blade metal and mainstream contact under the effects of leakage through blade tip clearance using a finite volume program, Fluent, with good agreement between data and numerical results. Cooling effectiveness is investigated as function of blowing ratio and tip clearance.
Comparison of the Accuracy of Time-Domain Measurement Methods for Combustor Damping
Authors: Wagner Michael , Jörg Christoph , Sattelmayer Thomas
Published: November 2013

Combustors of modern gas turbines for power generation and mechanical drive are predominantly operated in premixed mode, which is sensitive to coupling between flame dynamics and combustor acoustics. In practice, combustor flames tend to drive instabilities at certain eigenfrequencies of the systems, according to the classical Rayleigh criterion. In order to guarantee combustor stability in the entire operation range of the engine it has to be avoided under all circumstances that the flame excites the system beyond its damping potential. One option to accomplish this is to provide sufficient damping capabilities of the combustor system so that the decay of acoustic energy inside the system always exceeds the excitation provided by the flame. Experimental methods for the determination of combustor damping rates therefore may become a valuable tool for combustor design in the future. In the past, methods with different accuracy, complexity and capabilities have been developed to gain experimental access to decay rates of the acoustic energy inside combustor systems.

In this study we compare accuracy and capabilities of three different time-domain methods that allow the determination of pressure decay rates from experimental dynamic pressure traces: A simple exponential fit to the measured dynamic pressure, a method based on the decay of acoustic energy and a newly developed statistical method are examined. In the first step, the methods are tested using artificially generated test signals. The simple signals decaying exponentially with the known rate α are of pure sinusoidal shape and have discrete frequencies. As practical dynamic pressure traces are in general corrupted with noise, in the second step of the analysis a certain amount of random noise is added to the test signal. The last step of the analysis involves realistic pressure signals obtained from a simple duct with throughflow. The results obtained from the different methods are compared with each other and differences regarding performance, accuracy, robustness as well as computational costs are presented.
Aerodynamic Design Optimization of Nacelle and Intake
Authors: Albert Moritz , Bestle Dieter
Published: November 2013

Design of nacelle and intake for an aero engine is highly complex and always a compromise between contradicting design goals. The intake has to provide air for the engine with minimal pressure loss at all flight conditions like take-off, cruise, approach, or crosswind. Further the airflow has to be evenly distributed to allow satisfying operation of the fan, and the nacelle should produce as less drag as possible. Taking this into account, the ideal lip shape alternates between thin to minimize drag during cruise and thick contour to cover the requirements at off-design conditions.

To efficiently deal with these contradicting geometric design demands, a multi-objective optimization problem in 2D is defined. The major aim is to simultaneously minimize inner peak Mach number at off-design and outer peak Mach number at cruise conditions, i.e., two different flight conditions are taken into account which are evaluated in parallel. Three different parametric models are introduced to describe design changes, a first model, using superellipses and polynomials, a second one based on Class-Shape-Transformation, and a B-spline approach. To check the robustness of the design process, design of experiments for all three parameterization approaches have been performed. Based on these results proper design bounds are derived to ensure reliable CAD model generation, meshing and CFD analysis.

Optimization results from a multi-objective genetic algorithm demonstrate feasibility of the proposed approach. Compared to an existing design based on human experience, the designs found are better with respect to both objectives.
The Effect of Cooling Air on the Air Fuel Distribution of a Silo Combustor
Authors: Seidel V. , Marosky A. , Sattelmayer T. , Geng W. , Magni F.
Published: November 2013

Lean blow out (LBO) has a big impact on emission formation at part load of gas turbines, where flame temperature is low and flame stabilization is an issue. With improved combustion behavior at LBO conditions the operation flexibility of a silo gas turbine can be increased within the scope of retrofitting. In multi burner arrangements a part of the preheated air designated for combustion is used for impingement cooling of the burner front panel and subsequently injected into the primary combustion zone. In this region of flame stabilization air and unburned fuel as well as burned products are mixed to sustain stable combustion. The object of this study is to determine the level of dilution of the flow field by the cooling air with the focus on the conditions below LBO that can impair flame stability. The question addressed in this paper is how mixing of the front panel cooling air with the incoming reactants and the combustion products in multi burner arrangements can be computed in a numerically efficient way. As test case for the methodology the local distribution of cooling air in a silo combustor is presented. In this numerical study mixing processes of air-fuel mixture and cooling air as well as aerodynamic interaction of adjacent burners in a multi burner systems are investigated using isothermal Reynolds Averaged Navier Stokes (RANS) simulations. Former published single burner water channel experiments and Large Eddy Simulations (LES) [1] serve as a baseline. Single burner RANS simulations are done and compared to measurement and LES to validate the velocity and scalar fields. A Schmidt number variation is used to modify the mixing process in the RANS single burner calculations. Based on the LES the single burner is modified to address the multi burner conditions and calculated with LES and RANS. Finally the multi burner system is computed with the settings applied in the single burner configuration. Using the symmetry of the investigated burner matrix an efficient methodology is implemented that allows computation of one sixth of a silo combustor. The results expose a strong burner-burner interaction of the recirculation zones and in contrast to the single burner configuration regions of concentrated cooling air.
Numerical Study of Incidence Angles and Gap Heights in Turbine Cascades and Rotors on Tip Clearance Losses
Authors: Buske Clemens , Ullrich Wolfram C. , Roehle Ingo
Published: November 2013

The design and optimization of turbines demands the use of fast low-fidelity tools. To obtain adequate results, loss correlations simplifying the complex turbine throughflow are implemented. Accounting for modern turbine designs and flow conditions, revisions of profile and secondary loss correlations were primarily focused upon, while improvements of the tip clearance loss correlations are difficult to achieve. Realistic engine-like conditions concerning variations of the tip clearance, blade loading and solidity are time- and cost-intensive to investigate.

This paper is focused on an extensive numerical study, intending to support experiments on tip clearance loss correlations. The losses of a high pressure axial turbine rotor are analyzed for different tip clearance gap heights and incidence angles at cruise condition. The results are contrasted with a cascade having comparable tip profile and gap heights. The cascade’s flow is comparable to the rotor, but with respect to experimental restrictions concerning inlet and outlet conditions.

Steady 3D calculations in the stationary and rotating frame were performed applying DLR’s turbomachinery CFD code TRACE using Menter’s SST k – ω turbulence model. The tip clearance loss coefficients were extracted from the flow field by post-processing data of an outlet plane and as massflow averaged global values. The findings are discussed referencing previous publications about the leakage flow system and tip clearance loss. Finally, a comparison to results from tip clearance loss correlations of Ainley-Mathieson and Dunham-Came is presented.
Cross Flow Effects on Endwall Heat Transfer in Film-Cooled Turbine Nozzle Guide Vanes
Authors: Reviol Rebecca , Franze Roman , Böhle Martin , Takeishi Kenichiro , Wiedermann Alexander
Published: November 2013

Film cooling effects on endwalls in the stagnation point region are of special interest since the heat transfer is influenced drastically by secondary flows. Additionally, a complex vortex structure exists along the stagnation streamline which influences heat transfer on the endwall. The flow phenomenon is described and discussed in the open literature but it is still difficult to predict the heat transfer on the endwall and the turbine profile by CFD methods with sufficient accuracy.

In this paper it is examined how the flow field in the stagnation region should be simulated using CFD. The effect of meshes with various grid resolutions and turbulence models as k-ε-, k-ω-SST- and DES-turbulence models have been investigated. The CFD-data are compared with the experimental results obtained by Naphthalene Sublimation Method, Pressure Sensitive Paint, Laser Induced Fluorescence and Particle Image Velocimetry. Three cases, namely film cooling on a flat plate, the endwall flow near a symmetrical airfoil and the symmetrical airfoil with endwall film cooling, are examined in detail.
How to Upgrade Gas Turbines to Meet Customer Requirements and Be Fast to Market
Authors: Irmisch Stefan , Bauer Andreas , Ferber Jörgen , Kappis Wolfgang , Soumine Anton
Published: November 2013

This paper summarizes the techniques and methods available today to allow for significant improvements of existing and previously upgraded Gas Turbine fleets, while keeping time to market and investment to the absolute minimum. This allows for flexibility to react to market changes and to extend the utilization of the customer’s assets. The successful application of these techniques is described using the example of the latest service upgrade of Alstom’s GT13E2 heavy duty gas turbine also showing first validation results.

In contrast to previous improvements, mostly focusing on an increase in power-output, this upgrade development followed a different strategy: As customers are demanding a reduction in Cost of Electricity, the focus this time was on increasing CC-efficiency and elongating service interval lengths. The design challenge has been to come up with an upgrade package, which limits the additional power-output while maximizing efficiency and lifetime at the same time. Limiting the additional power output is crucial to be retrofit-able into existing plants with a minimum in investment costs, as some plant components like Generators and Transformers, especially for previously upgraded units, may already be close to their capacity limits.

The paper describes in detail how the upgrade scope and design concept has been derived based on customer input. Among other measures, it is demonstrated how the consequent application of 3D design methods, the utilization of state-of the art airfoil profiling and clearance optimization in combination with an optimized thermodynamic cycle have been used to maximize the component and GT efficiencies. Furthermore it is shown how development time and risks have been minimized by a consequent application of a combined concurrent design & manufacturing approach and rapid prototyping methods for early design validation. Finally, short- and mid-term validation results after the first implementation and testing in spring 2012 are presented, confirming that the design targets have been met.
The Relationship Between Energy Separation and Base Drag in Turbine Blade Wakes
Authors: Gostelow J. P. , Carscallen W. E. , Kurosaka M. , Mahallati A.
Published: November 2013

During annular cascade testing of a highly-loaded turbine stage of aggressive design, the nozzle blading experienced a redistribution of the downstream total temperature field. In this ostensibly adiabatic arrangement, the central regions of the vane wakes exhibited a significant decrease in total temperature and their edges showed an unexpected increase. To resolve these anomalous results and obtain detailed information over the Mach number range, the mid-span section of the nozzle was tested in a large scale transonic planar cascade. At high subsonic speeds, vortex shedding created energy redistribution in the wake. This was measured using an 80 kHz bandwidth temperature probe, making it possible to investigate wake total temperature fluctuations in addition to fluctuations in total pressure, and hence entropy. ‘Hot spots’ of increased total temperature were found to be located at the edge of the wake and ‘cold spots’ of decreased total temperature were located close to the wake center line. The results from the turbine cascade were consistent with the phenomenon of energy separation behind bluff bodies. High base pressure losses were observed and were also related to the vortex shedding. The blade had a thick trailing edge and the high base pressure loss condition coincided with the peak of energy separation in the wake. The analysis indicates that in the subsonic speed range the phenomena of energy separation and of base pressure deficit are inextricably linked to, and are caused by, vortex shedding. A strategy for minimizing the related adverse impacts on performance is outlined.
Investigation of the Turbulent Flame Speed for Natural Gas and Natural Gas/Hydrogen Mixtures at High Turbulence Levels and Volumetric Heat Release Rates
Authors: Zajadatz Martin , Zarzalis Nikolaos , Leuckel Wolfgang
Published: November 2013

In gas turbine combustion application, there is a strong tendency towards high volumetric heat release rates without compromising ignition stability and the requirement of low emission concentrations of NOx, CO and unburned hydrocarbons. In order to meet these demands for industrial gas turbines the lean premixed combustion concept has been developed.

In the scope of this paper fundamental experimental work, which has been carried out in order to analyze the important topic of turbulence/chemistry interaction on a semi-technical scale, will be reported. The turbulent intensity and length scales have been varied by a generic burner system, which consists of four geometrically scaled burners. At atmospheric pressure conditions more than 700 Bunsen type flames in a Reynolds number range from 21000 to 128000 have been investigated. Gas/air mixture preheating has been included in the tests as a typical boundary condition for combustion in gas turbines. The natural gas was blended with 25 vol. % and 50 vol. % hydrogen in order to alter the kinetics of the fuel gas.

The influence of the aforementioned parameters on the turbulent flame speed were assessed and compared with existing correlations for the turbulent flame speed. Special emphasis has been taken on the influence of gas/air mixture preheating and kinetics.
Helicopter Mission Analysis for a Regenerated Turboshaft
Authors: Fakhre Ali , Pachidis Vassilios , Goulos Ioannis , Tashfeen Mahmood , Pilidis Pericles
Published: November 2013

The aim of the study presented in this paper, is to compare helicopters employing simple cycle turboshaft engines, with helicopters employing novel regenerated turboshafts. Two existing helicopter configurations, a Twin Engine Light and a Twin Engine Medium are compared against regenerated configurations. The reference installed engines of both helicopters are notionally optimized by incorporating a heat exchanger, which enables heat transfer between the exhaust gas and the compressor delivery air to the combustion chamber. This process leads to a lower fuel input requirement as well as higher overall thermal efficiency compared to the reference simple cycle engine.

The benefits arising from the adoption of the heat exchanger for both configurations are firstly presented by conducting part-load performance analysis for each optimized engine against its reference simple cycle engine. The obtained results suggest substantial reduction in specific fuel consumption for a major part of the operating power range with respect to both helicopter configurations. The results also demonstrate that the heat exchanger effectiveness is a critical parameter in achieving further reductions in specific fuel consumption.

The study is further extended to investigate mission fuel burn saving limits for both helicopter configurations under the simulated part-load performance conditions by conducting a heat exchanger tradeoff study. The weight estimation correlation for the heat exchanger is adopted from the previously reported studies of similar fashion and is simulated accordingly for both helicopter configurations. A multi-disciplinary simulation tool with an integrated range of capabilities applicable to helicopter performance evaluation and mission analysis is adopted to simulate various types of missions, targeting wide range of helicopter operations. The results of the mission analysis suggest that the regenerated counterpart configurations are capable of achieving significant reductions in mission fuel burn. However, the level of gain from mission fuel burn savings is dependent on the selected helicopter mission profile, the recuperator design effectiveness as well as the overall evaluation criteria. The results also conclude that while the amount of benefit is dependent on various parameters, there is always an optimum “saving” region for each mission that justifies the need for regeneration.
Capacitive Film Thickness Measurements in a Ventless Aero-Engine Bearing Chamber: Influence of Operating Conditions and Offtake Design
Authors: Kurz Wolfram , Dullenkopf Klaus , Bauer Hans-Jörg
Published: November 2013

The paper presents results from film thickness measurements in a bearing chamber test rig. The measurements were performed at different operating conditions and with two offtake designs. A discussion of the measurement technique using in-situ calibrated capacitive sensors shows good accuracy and repeatability.

The film thickness results are first compared to measurements with the same chamber in a vented configuration. The analysis of the measurements at various operating conditions shows a strong influence of the shaft speed, the chamber pressure, and the offtake design. In contrast to that, flow rate and scavenge ratio have only minor influence. Furthermore, the momentum flux of the core air flow is proposed as a suitable parameter with which the influence of shaft speed and pressure can be correlated to the film thickness distribution.
Design for Thermo-Acoustic Stability: Modeling of Burner and Flame Dynamics
Authors: Bade Stefanie , Wagner Michael , Hirsch Christoph , Sattelmayer Thomas , Schuermans Bruno
Published: November 2013

A Design for Thermo-Acoustic Stability (DeTAS) procedure is presented, that aims at selecting a most stable burner geometry for a given combustor. It is based on the premise that a thermo-acoustic stability model of the combustor can be formulated and that a burner design exists, which has geometric design parameters that sufficiently influence the dynamics of the flame. Describing the flame dynamics in dependence of the geometrical parameters an optimization procedure involving a linear stability model of the target combustor maximizes the damping and thereby yields the optimal geometrical parameters. To demonstrate the procedure on an existing annular combustor a generic burner design was developed that features a significant variability of dynamical flame response in dependence of two geometrical parameters. In this paper the experimentally determined complex burner acoustics and complex flame responses are described in terms of physics based parametric models with excellent agreement between experimental and model data. It is shown that these model parameters correlate uniquely with the variation of the burner geometrical parameters, allowing to interpolate the model with respect to the geometrical parameters. The interpolation is validated with experimental data.
Modeling an Enclosed, Turbulent Reacting Methane Jet With the Premixed Conditional Moment Closure Method
Authors: Martin Scott , Jemcov Aleksandar , de Ruijter Björn
Published: November 2013

Here the premixed Conditional Moment Closure (CMC) method is used to model the recent PIV and Raman turbulent, enclosed reacting methane jet data from DLR Stuttgart [1]. The experimental data has a rectangular test section at atmospheric pressure and temperature with a single inlet jet. A jet velocity of 90 m/s is used with an adiabatic flame temperature of 2,064 K. Contours of major species, temperature and velocities along with velocity rms values are provided.

The conditional moment closure model has been shown to provide the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable runtimes [2]. The simplified CMC model used here falls into the class of table lookup turbulent combustion models where the chemical kinetics are solved offline over a range of conditions and stored in a table that is accessed by the CFD code. Most table lookup models are based on the laminar 1-D flamelet equations, which assume the small scale turbulence does not affect the reaction rates, only the large scale turbulence has an effect on the reaction rates. The CMC model is derived from first principles to account for the effects of small scale turbulence on the reaction rates, as well as the effects of the large scale mixing, making it more versatile than other models. This is accomplished by conditioning the scalars with the reaction progress variable. By conditioning the scalars and accounting for the small scale mixing, the effects of turbulent fluctuations of the temperature on the reaction rates are more accurately modeled.

The scalar dissipation is used to account for the effects of the small scale mixing on the reaction rates. The original premixed CMC model used a constant value of scalar dissipation, here the scalar dissipation is conditioned by the reaction progress variable. The steady RANS 3-D version of the open source CFD code OpenFOAM is used. Velocity, temperature and species are compared to the experimental data. Once validated, this CFD turbulent combustion model will have great utility for designing lean premixed gas turbine combustors.
Modeling Static Instabilities of Biogas Flames in a Stirred-Reactor Using Detailed Chemical Kinetics Simulations
Authors: Bolin Christopher D. , Engeda Abraham
Published: November 2013

Kinetic modeling of lean static stability limits of the combustion of biogas type fuels in a model of an ideal primary zone of a gas turbine combustor is presented here. In this study, CH4 is diluted with CO2 to simulate a range of gases representative of the products of anaerobic digestion of organic materials from different sources (e.g., landfill and animal waste digester). Fuels of this type are of interest for use in small gas turbines used in distributed generation applications. Predictions made by two detailed mechanisms (GRI-Mech 3.0 and San Diego) and one reduced mechanism (GRI-Mech 1.2, reduced) are employed to investigate the underlying kinetics near lean extinction. Approximate correlations to lean extinction are extracted from these results and compared to those of other fuels.
The Aerodynamic Performance of an Over-the-Rotor Liner With Circumferential Grooves on a High Bypass Ratio Turbofan Rotor
Authors: Bozak Rick , Hughes Christopher , Buckley James
Published: November 2013

While liners have been utilized throughout turbofan ducts to attenuate fan noise, additional attenuation is obtainable by placing an acoustic liner over-the-rotor. Previous experiments have shown significant fan performance losses when acoustic liners are installed over-the-rotor. The fan blades induce an oscillating flow in the acoustic liners which results in a performance loss near the blade tip. An over-the-rotor liner was designed with circumferential grooves between the fan blade tips and the acoustic liner to reduce the oscillating flow in the acoustic liner. An experiment was conducted in the W-8 Single-Stage Axial Compressor Facility at NASA Glenn Research Center on a 1.5 pressure ratio fan to evaluate the impact of this over-the-rotor treatment design on fan aerodynamic performance. The addition of a circumferentially grooved over-the-rotor design between the fan blades and the acoustic liner reduced the performance loss, in terms of fan adiabatic efficiency, to less than 1% which is within the repeatability of this experiment.
Gas Turbine Compressor Dependability and Risk Mitigation Measures
Authors: Scheibel John R. , Dewey Robert P. , Angello Leonard , Barron Josh
Published: November 2013

This paper addresses recent industrial gas turbine compressor dependability issues and risk mitigation measures viewed from the end user’s perspective. Industrial reliability-availability-maintainability statistics related to power generation applications are reviewed. Several case histories with specific component issues involving blades and vanes are covered. Case histories are used to summarize field experience, engineering analysis and evaluation of related design and operating modifications as appropriate. Recent progress with setting up a field monitoring demonstration using pressure pulsations, vibration and acoustic emissions is summarized.
Ignition Delay Time Experiments for Natural Gas/Hydrogen Blends at Elevated Pressures
Authors: Brower Marissa L. , Mathieu Olivier , Petersen Eric L. , Donohoe Nicola , Heufer Alexander , Metcalfe Wayne K. , Curran Henry J. , Bourque Gilles , Güthe Felix
Published: November 2013

Applications of natural gases that contain high levels of hydrogen have become a primary interest in the gas turbine market. While the ignition delay times of hydrogen and of the individual hydrocarbons in natural gases can be considered well known, there have been few previous experimental studies into the effects of different levels of hydrogen on the ignition delay times of natural gases at gas turbine conditions. To examine the effects of hydrogen content at gas turbine conditions, shock-tube experiments were performed on nine mixtures of an L9 matrix. The L9 matrix was developed by varying four factors: natural gas higher-order hydrocarbon content of 0, 18.75, or 37.5%; hydrogen content of the total fuel mixture of 30, 60, or 80%; equivalence ratios of 0.3, 0.5, or 1; and pressures of 1, 10, or 30 atm. Temperatures ranged from 1092 K to 1722 K, and all mixtures were diluted in 90% Ar. Correlations for each mixture were developed from the ignition delay times and, using these correlations, a factor sensitivity analysis was performed. It was found that hydrogen played the most significant role in the ignition delay times of a mixture. Pressure was almost as important as hydrogen content, especially as temperature increased. Equivalence ratio was slightly more important than hydrocarbon content of the natural gas, but both were less important than pressure or hydrogen content. Comparison with a modern chemical kinetic model demonstrated that the model captures well the relative impacts of H2 content, temperature, and pressure, but some improvements are still needed in terms of absolute ignition delay times.
Effect of Film Cooling Arrangement on Impingement Heat Transfer on Turbine Blade Leading Edge
Authors: Yang Li , Kan Rui , Ren Jing , Jiang Hongde
Published: November 2013

Impingement cooling plays an important role in gas turbine blade leading edge where proper heat transfer distribution is needed for extremely high and nonuniform thermal load.

A 2/3 cylinder leading edge model with 3 arrays of film cooling holes was investigated with 8 film cooling arrangements. The impingement parameters and the jet Reynolds number were kept the same for the 8 configurations. The transient liquid crystal (TLC) measurement was applied on heat transfer coefficient on the leading edge. A 3D numerical method with the SST k-ω model was verified by experimental data, which shows a heat transfer error less than 15%.

The film suction creates both local heat transfer enhancement and limit effect to wall jets. The hole position of film cooling holes significantly affects the shape of high heat transfer area and cooling of the intermediate area. The array angle of film cooling holes affects the spread of heat transfer laterally. The Nu in stagnation zone decreases with the increase of array angle of film cooling holes. Smaller pitch of film cooling holes helps decrease the size of fountaining flow and heat transfer valley. The Nu in stagnation zone increases with the decrease of pitch of film cooling holes.

The hole position of x0/P = 0.125 is recommended for the best cooling performance in the intermediate area. The configurations with θ = 13 or P/pf = 3 work best in this study.
Squealer Tip Leakage Flow Characteristics in Transonic Condition
Authors: Li Wei , Jiang Hongmei , Zhang Qiang , Lee Sang Woo
Published: November 2013

The Over-Tip-Leakage (OTL) flow characteristics for a typical squealer tip of high pressure turbine blade, which consists of subsonic and transonic flow, have been numerically investigated in the present study, in comparison with the corresponding flat tip results. For the squealer tip employed, flow choking behavior still exists above the tip surface, even though the Mach number is lower and the transonic region is smaller than that for the flat tip. Detailed flow structure analysis shows that most of the fluid entering the squealer cavity is from the frontal leading edge region. The fluid migrates along the cavity and is ejected at various locations near the suction side rim. These fluid forms a large subsonic flow zone under the supersonic flow passing over the tip gap which reduces the OTL flow flux. The squealer design works even in the presence of choked OTL flow. Comparisons between results from two different cavity depths with and without relative casing motion suggest that the over tip leakage flow flux has much dependence upon the cavity depth for subsonic region, but is less sensitive to the depth for transonic tip flow region. Such behavior has been confirmed with and without the existence of relative casing motion.
Effects of Dilution and Nozzle Diameter on Laminar Dimethyl Ether Lifted Flames
Authors: Zhou Jie , Ai Yuhua , Kong Wenjun
Published: November 2013

This work aimed at studying the effects of nitrogen dilution and nozzle exit inner diameter on the liftoff properties of the dimethyl ether (DME) jet diffusion flames. The liftoff properties including the liftoff position (HL), the critical liftoff velocity (Ulo) and the critical blowout velocity (Ubo) were studied experimentally. In nitrogen dilution experiments, a slowly converging nozzle was used with inner exit diameter of 0.43 mm. When mole fraction of N2 (Z) increased, a) HL increased because the dilution reduced the chemical activity of fuel, in order to achieve stoichiometric conditions, the stabilization point of the lifted flame moved downstream. b) at the critical liftoff condition, the flow rate of DME decreased with the increase of N2, while the total flow rate was almost unchanged, so the jet velocity was almost the same. c) as Z increased, the stabilization zone of the DME liftoff flames became narrow and small. In the experimental study of the effects of the nozzle diameter on the flame liftoff characteristics, six nozzles with i.d. of 0.17mm, 0.25mm, 0.386mm, 0.43mm, 0.693mm and 1.152mm were used. These nozzles had different materials and nozzle exit types. The experimental results showed that the nozzle inner diameter has a significant impact on the flame liftoff characteristics. As the nozzle diameter increased, four types of different liftoff features were observed. The flame was blown out directly with i.d. of 0.17 mm. The DME flame could only be observed liftoff by ignition at a proper position downstream with i.d. of 0.25 mm, 0.386 mm and 0.43 mm. The observations are agreed with that reported in the literatures. While it could be lifted off directly by increasing the mass flow rate of fuel/dilution with i.d. of 0.693 mm. This is the new observation in the present work. It is different from the report in the literatures that the DME flame could not be lifted off directly by increasing the jet velocity except for far field ignition at relatively low mass flow rate. When the nozzle i.d. was increased to 1.152 mm, the DME flames could be lifted off by three different methods: increasing the flow rate of fuel/dilution, decreasing the flow rate of the fuel and ignition the flame downstream. Oscillation lifted DME flames were found with i.d. of 1.152 mm when the fuel was highly diluted by nitrogen. The experimental results also showed that the critical liftoff velocity Ulo and the critical blowout velocity Ubo were strongly dependent on the inner diameter, which decreased with the increase of the nozzle diameter. When the jet velocity was kept constant, the flame liftoff height HL increased with the increase of the nitrogen mole fraction Z for all lifted flames.
Forced and Self-Excited Instabilities From Lean Premixed, Liquid-Fuelled Aeroengine Injectors at High Pressures and Temperatures
Authors: Hochgreb Simone , Dennis David , Ayranci Isil , Bainbridge William , Cant Stewart
Published: November 2013

Measurements of unsteady pressure and chemiluminescence during flow forced operation of aeroengine lean direct injection fuel spray nozzles were made, with a goal to determine the response of the flame, subject to a range of air fuel ratios, fuel flow splits between pilot and mains injectors, and cooling flows. A rotating shutter installed at the downstream choked nozzle provided the excitation for forcing the mass flow rate between 100 to 600 Hz, at normalized intensities of 0.1 to 0.7 relative to the mean velocity at the injector. The experiments were performed at inlet conditions of 800 K and 5.7 bar.

Self-excitation created by the coupling between the flame and the combustor cavity was observed, in the form of a broad peak around 275 Hz. Numerical studies indicate that the peak is associated with an entropy spot (a region of non-uniform temperature) travelling from the flame to the choked nozzle, followed by the ensuing expansion wave towards the injector and amplification of the excitation. Investigation of previous studies suggests that similar phenomena may have been present in other studies at high pressure. The main impact of the self-excitation is the significant amplification of the velocity fluctuations from 0.1 of the mean velocity away from the self-excitation frequency to around 0.7 at the peak. The flame response, represented by the ratio of the fractional fluctuations in OH* chemiluminescence to the fractional velocity fluctuations at the injector, can be determined under conditions where the self-excitation heat release contributes only a small portion of the forced heat release, based on the measured background. The flame response shows a significant dependence on both air fuel ratio and fuel splits, with a decreasing gain towards higher frequency.

The results show that it is possible to generate high amplitude fluctuations on the flow using this method, and demonstrate the role of entropy spots during normal operation in lean direct injection systems. Finally, the results suggest that there is an interaction between the forcing frequency and the self-excitation, which may behave in a non-linear manner, and which deserves further investigation.