bluuebunny/crossref_metadata_2025 huggingface dataset:
Prevalence of Atrial Fibrillation in Patients with High CHADS2‐ and CHA2DS2VASc‐Scores: Anticoagulate or Monitor High‐Risk Patients?
Authors: TISCHER TINA S. , SCHNEIDER RALPH , LAUSCHKE JÖRG , NESSELMANN CATHARINA , KLEMM ANKE , DIEDRICH DOREEN , KUNDT GÜNTHER , BÄNSCH DIETMAR
Published: July 2014

Background

In patients with known atrial fibrillation (AF) different scores are utilized to estimate the risk of thromboembolic events and guide oral anticoagulation. Diagnosis of AF strongly depends on the duration of electrocardiogram monitoring. The aim of this study was to use established scores to predict the prevalence of AF.

Methods

The CHADS2‐ (Congestive Heart failure, hypertension, Age >75 years, Diabetes, Stroke [doubled]) and CHA2DS2VASc‐score (Congestive Heart failure, hypertension, Age ≥75 years [doubled], Diabetes, Stroke [doubled], Vascular disease, Age 65–74 years, Sex category [female sex]) was calculated in 150,408 consecutive patients, referred to the University Hospital of Rostock between 2007 and 2012. All factors constituting these scores and a history of AF were prospectively documented with the ICD‐10 admission codes.

Results

Mean age of our study population was 67.6 ± 13.6 years with a mean CHADS2‐score of 1.65 ± 0.92 and CHA2DS2VASc‐score of 3.04 ± 1.42. AF was prevalent in 15.9% of the participants. The prevalence of AF increased significantly with every CHADS2‐ and CHA2DS2VASc‐score point up to 54.2% in CHADS2‐score of 6 and 71.4% in CHA2DS2VASc‐score of 9 (P < 0.001).

Conclusion

The prevalence of AF increases with increasing CHADS2‐ and CHA2DS2VASc‐score. In intermediate scores intensified monitoring may be recommended. In high scores, thromboembolic complications occurred irrespective of the presence of AF and anticoagulant therapy may be initiated irrespective of documented AF.
Modelling approach for the quantitative variation of sex expression in monoecious hemp (Cannabis sativa L.)
Authors: Faux Anne‐Michelle , Bertin Pierre
Published: July 2014

Sex expression is of primary importance for the genetic improvement and production of monoecious hemp: masculinized phenotypes are associated with higher fungal sensitivity, and feminized phenotypes with higher seed yields. However, sex expression varies quantitatively among plants and nodes and with time. Here, we developed eight variables that characterize the sex expression in monoecious hemp to dissect its genetic determinism. The monoecy degree (MD), ranging from 1 (mostly male flowers) to 5 (mostly female flowers), was recorded for each node of 167 plants, at 6 times at 1‐week intervals. Two types of longitudinal variables were constructed: ‘synthesis’ (mean MD and percentages of nodes of each MD) and ‘structure’. The latter consisted of the parameters of a logistic curve describing MD as a function of the node position. An r‐square of 0.97 was obtained between the estimated and observed MD values, and the logistic parameters were weakly correlated with each other and with the synthesis variables. Therefore, we conclude that the present modelling approach is relevant for characterizing the sex expression in monoecious hemp.
Mismeasurement of Distance Effects: The Role of Internal Location of Production
Authors: Yilmazkuday Hakan
Published: July 2014

The estimated effects of distance in empirical international trade regressions are unrealistically high. Using state‐and‐sector level US exports data, this paper shows analytically and proves empirically that ignoring the internal location of production (of international exports), which leads to the overestimation of distance effects by about twofold, is a possible explanation. This overestimation is mostly attributed to the mismeasurement of the distance elasticity of trade costs when internal locations of production are ignored. A corrective distance index is proposed to avoid such mismeasurements and is shown to work well for the median sector. The results are robust to the consideration of alternative estimation methodologies and data sets.
The Metaphysical Implications of Conjoined Twinning
Authors: Olson Eric T.
Published: July 2014

Conjoined twinning is said to show that the number of human people—the number of us—can differ from the number of human organisms, and hence that we are not organisms. The paper shows that these arguments either assume the point at issue, rely on dubious and undefended assumptions, or add nothing to more familiar arguments for the same conclusion.
Secondary structure estimation and properties analysis of stretched Asian and Caucasian hair
Authors: Zhou A. J. , Liu H. L. , Du Z. Q.
Published: July 2014

Background

In this previous work, we investigated the secondary structure changes of stretched yak hairs by deconvolution, secondary derivation, and curve fitting and determined the number of bands and their positions in order to resolve the protein spectrum of Raman spectroscopy.

Objectives

The secondary structure estimation and properties analysis of stretched Asian and Caucasian hair were investigated by Fourier transform infrared spectroscopy, tensile curves, and measurement of density.

Methods

The hairs were stretched, dried, and baked at ratios 20%, 40%, 60%, 80% and 100%.

Results

The analysis of the amide I band indicated that the transformation from α‐helix to β‐pleated structure occured during the stretching process, which could be verified from the tensile analysis. The cysteine oxide in S–O vibration area exhibited that stretching led to the breakage of the disulfide bonds. When the stretching ratio of Caucasian hair was more than a certain ratio, the fiber macromolecular structure was destroyed because Caucasian hair had finer diameter and less medulla than Asian hair. The β turn was easier to retract compared with other conformations, resulted in the content increase. The density measurements revealed that the structure of Caucasian hair was indeed more destroyed than that of Asian hair. The cuticles characterization indicated the length of scales was stretched longer and the thickness became thinner. Caucasian hair tended to collapse to form small fragments at the early stage of stretching. With the increase in stretching ratio, the scales of Caucasian hair lifted up, then flaked off and the scale interval increased accordingly. Asian hair was more easily peeled off than Caucasian hair cuticles with the increase in stretching ratio.

Conclusions

The secondary structure of Caucasian hair was destroyed more easily than that of Asian hair.
Design of Thermoelectric Modules for High Heat Flux Cooling
Authors: Ranjan Ram , Turney Joseph E. , Lents Charles E. , Faustino Virginia H.
Published: July 2014

Thermoelectric (TE) coolers work on the Seebeck effect, where an electrical current is used to drive a heat flux against a temperature gradient. They have applications for active cooling of electronic devices but have low coefficients of performance (COP < 1) at high heat fluxes (>10 W/cm2, dT = 15 K). While the active elements (TE material) in a TE cooling module lead to cooling, the nonactive elements, such as the electrical leads and headers, cause joule heating and decrease the coefficient of performance. A conventional module design uses purely horizontal leads and vertical active elements. In this work, we numerically investigate trapezoidal leads with angled active elements as a method to improve cooler performance in terms of lower parasitic resistance, higher packing fraction and higher reliability, for both supperlattice thin-film and bulk TE materials. For source and sink side temperatures of 30 °C and 45 °C, we show that, for a constant packing fraction, defined as the ratio of active element area to the couple base area, trapezoidal leads decrease electrical losses but also increase thermal resistance. We also demonstrate that trapezoidal leads can be used to increase the packing fraction to values greater than one, leading to a two times increase in heat pumping capacity. Structural analysis shows a significant reduction in both tensile and shear stresses in the TE modules with trapezoidal leads. Thus, the present work provides a pathway to engineer more reliable thermoelectric coolers (TECs) and improve their efficiency by >30% at a two times higher heat flux as compared to the state-of-the-art.
A New Analytical Method for Calculating Maximum Junction Temperature of Packaged Devices Incorporating the Temperature Distribution at the Base of the Substrate
Authors: Ling J. H. L. , Tay A. A. O.
Published: July 2014

All current analytical methods for calculating junction temperature of field effect transistor (FET) and monolithic microwave integrated circuits (MMIC) devices have assumed a constant uniform temperature at the base of the substrate. In a packaged device, however, where the substrate is attached to a carrier, finite element thermal analyses have shown that the temperature distribution along the base of the substrate is not uniform but has a bell-shaped distribution. Consequently, current analytical methods which attempt to predict the junction temperature of a packaged MMIC device by assuming a constant uniform temperature at the base of the substrate have been found to be inaccurate. In this paper, it is found that the temperature distribution along the base of a substrate can be well approximated by a Lorentz distribution which can be determined from a few basic parameters of the device such as the gate length, gate pitch, number of gates, and length of substrate. By incorporating this Lorentz temperature distribution at the base of the substrate with a new closed-form solution for the three-dimensional temperature distribution within the substrate, a new analytical method is developed for accurately calculating the junction temperature of MMIC devices. The accuracy of this new method has been verified with junction temperatures of MMIC devices measured using thermoreflectance thermography (TRT) as well as those calculated using finite element analysis (FEA).
Synthesis and Characterization of ZrO2 Coatings on Micro End Mills With Sol–Gel Processing
Authors: Morrow Justin D. , Tejedor-Anderson M. Isabel , Anderson Marc A. , Ruotolo Luis A. M. , Pfefferkorn Frank E.
Published: July 2014

The objective of this paper is to develop a coating method for placing ultrathin zirconia films, fabricated using sol–gel methods, onto micro end mills. Sol–gel synthesis is investigated because of its potential as a low energy and inexpensive method for depositing a variety of wear resistant and chemically stable oxide coatings that could potentially extend tool life. Two sol–gel based deposition methods are investigated: dip-coating and electrophoretic deposition (EPD). The coatings are deposited on 300-μm-diameter micro end mills. Initial results suggest that sol–gel coating methods can produce the required coverage and conformity in the cutting zone. However, preliminary findings show that the coating is removed in the cutting zone after a short time of machining (6 s). The tool pretreatment and extent of sintering, which affect surface adhesion coating mechanical properties, require further study and optimization.
Use of DME as a Gas Turbine Fuel
Authors: Basu Arun , Gradassi Mike , Sills Ron , Fleisch Theo , Puri Raj
Published: July 2014

A new, ultra-clean fuel for gas turbines — a blend consisting primarily of dimethyl ether (DME) with lesser amounts of methanol and water — has been identified by BP. This fuel, containing no metals, sulfur or aromatics, burns like natural gas and it can be handled like LPG. The turbine-grade DME fuel can be manufactured from natural gas, coal and other hydrocarbon or biomass feedstocks. High-purity DME, manufactured from methanol, is currently used as an aerosol propellant due to its environmentally benign characteristics. Fuel-grade DME is used commercially as a LPG-substitute in China. BP initiated key programs to test various fuel mixtures containing DME in General Electric test combustors with equivalent electricity production of nearly 16 MW. Later, BP collaborated with EPDC (Electric Power Development Corporation, Japan) to conduct additional follow-up tests. These tests show that DME is an excellent gas turbine fuel with emissions properties comparable to natural gas. BP is currently working with the Indian Oil Corporation (IOCL), the Gas Authority of India Limited (GAIL) and the Indian Institute of Petroleum to evaluate the potential of DME as a multi-purpose fuel for India. In June 2000, the India Ministry of Power issued a notification permitting the use of DME as a fuel for power generation subject to its meeting all the environmental and pollution regulations. This paper presents key gas turbine combustor test results and discusses how DME can be used as a fuel in gas turbines.
Application of Fuzzy Logic for Fault Isolation of Jet Engines
Authors: Ganguli Ranjan
Published: July 2014

A fuzzy logic system is developed for gas turbine module fault isolation. Inputs to the fuzzy logic system are measurement deviations of gas path parameters from a “good” baseline engine. The gas path measurements used are exhaust gas temperature, low and high rotor speed and fuel flow. These sensor measurements are available on most jet engines. The fuzzy logic system uses rules developed from performance influence coefficients to isolate the module fault while accounting for uncertainty in the gas path measurements. Tests with simulated data show the fuzzy system isolates module faults with accuracy of over 95 percent. In addition, the fuzzy logic system shows good performance even with poor quality data. Additional pressure and temperature measurements between the compressor and before the burner help to increase the accuracy of fault isolation at high levels of uncertainty and when modeling assumptions weaken.
Dynamic Simulation of Full Start-Up Procedure of Heavy Duty Gas Turbines
Authors: Kim J. H. , Song T. W. , Kim T. S. , Ro S. T.
Published: July 2014

A simulation program for transient analysis of the start-up procedure of heavy duty gas turbines for power generation has been constructed. Unsteady one-dimensional conservation equations are used and equation sets are solved numerically using a fully implicit method. A modified stage-stacking method has been adopted to estimate the operation of the compressor. Compressor stages are grouped into three categories (front, middle, rear), to which three different stage characteristic curves are applied in order to consider the different low-speed operating characteristics. Representative start-up sequences were adopted. The dynamic behavior of a representative heavy duty gas turbine was simulated for a full start-up procedure from zero to full speed. Simulated results matched the field data and confirmed unique characteristics such as the self-sustaining and the possibility of rear-stage choking at low speeds. Effects of the estimated schedules on the start-up characteristics were also investigated. Special attention was paid to the effects of modulating the variable inlet guide vane on start-up characteristics, which play a key role in the stable operation of gas turbines.
Global Nonlinear Modelling of Gas Turbine Dynamics Using NARMAX Structures
Authors: Chiras Neophytos , Evans Ceri , Rees David
Published: July 2014

This paper examines the estimation of a global nonlinear gas turbine model using NARMAX techniques. Linear models estimated on small-signal data are first examined and the need for a global nonlinear model is established. A nonparametric analysis of the engine nonlinearity is then performed in the time and frequency domains. The information obtained from the linear modelling and nonlinear analysis is used to restrict the search space for nonlinear modelling. The nonlinear model is then validated using large-signal data and its superior performance illustrated by comparison with a linear model. This paper illustrates how periodic test signals, frequency domain analysis and identification techniques, and time-domain NARMAX modelling can be effectively combined to enhance the modelling of an aircraft gas turbine.
A Multiple Harmonic Open Loop Controller for Hydro/Aerodynamic Force Measurements in Rotating Machinery Using Magnetic Bearings
Authors: Baun Daniel O. , Maslen Eric H. , Knospe Carl R. , Flack Ronald D.
Published: July 2014

Inherent in the construction of many experimental apparatus designed to measure the hydro/aerodynamic forces of rotating machinery are features that contribute undesirable parasitic forces to the measured or test forces. Typically, these parasitic forces are due to seals, drive couplings, and hydraulic and/or inertial unbalance. To obtain accurate and sensitive measurement of the hydro/aerodynamic forces in these situations, it is necessary to subtract the parasitic forces from the test forces. In general, both the test forces and the parasitic forces will be dependent on the system operating conditions including the specific motion of the rotor. Therefore, to properly remove the parasitic forces the vibration orbits and operating conditions must be the same in tests for determining the hydro/aerodynamic forces and tests for determining the parasitic forces. This, in turn, necessitates a means by which the test rotor’s motion can be accurately controlled to an arbitrarily defined trajectory. Here in, an interrupt driven multiple harmonic open loop controller was developed and implemented on a laboratory centrifugal pump rotor supported in magnetic bearings (active load cells) for this purpose. This allowed the simultaneous control of sub-harmonic, synchronous and super-harmonic rotor vibration frequencies with each frequency independently forced to some user defined orbital path. The open loop controller was implemented on a standard PC using commercially available analog input and output cards. All analog input and output functions, transformation of the position signals from the time domain to the frequency domain, and transformation of the open loop control signals from the frequency domain to the time domain were performed in an interrupt service routine. Rotor vibration was attenuated to the noise floor, vibration amplitude ≈ 0.2 μm, or forced to a user specified orbital trajectory. Between the whirl frequencies of ¼ and 2 times running speed, the orbit semi-major and semi-minor axis magnitudes were controlled to within 0.5% of the requested axis magnitudes. The ellipse angles and amplitude phase angles of the imposed orbits were within 0.3° and 1.0°, respectively, of their requested counter parts.
Industrial Trent Dry Low Emissions Gas Fuel Control System
Authors: Gessaman Jeff , Goetz Andreas
Published: July 2014

Regulations are requiring that the emissions from land-based gas turbines be tightly controlled. To limit emissions, the Industrial Trent turbine controls flame temperature through a premixed multi-stage combustor. This requires a multi-path fuel delivery system that provides accurate and repeatable metering of the fuel. There are also requirements that the fuel system quickly stop the flow of fuel and vent appropriate lines in the case of a load rejection or an emergency shutdown.

The gas system is a four-path fuel metering system that incorporates shutoff and vent capabilities in addition to high accuracy multi-path flow metering. One of the fuel paths is dedicated to the ignition torch and is significantly smaller and flow accuracies are less demanding than for the three main metering legs.

The concentration of work was dedicated to providing highly accurate and repeatable flow metering for the three main fuel legs. There was also a drive to minimize cost and provide commonality of parts between each of the three legs. Current fuel schedules require metering through a 20:1 turn down ratio.

Flow through each metering leg is measured in the same manner as flow measurement is performed across an orifice flow meter. The metering valve is a primary element for which the flow characteristics are well known under a large number of valve positions and flow conditions. Analysis and testing has been conducted to define flow accuracy and repeatability. Further work has been conducted to minimize pressure drop across the system and to reduce the number of sensors required.
On-Line Combustion Turbine Reliability Monitoring System Field Evaluation Results
Authors: Pollard Michael A. , Angello Leonard , Christiansen Tom
Published: July 2014

This paper describes the initial application of a reliability and condition-assessment monitoring system to Carolina Power and Light’s (CP&L) Asheville Unit 3, a General Electric (GE) 7FA+ combustion turbine (CT) rated at 165 MW. The system provides a non-intrusive means of interfacing with plant control systems to obtain real-time data for transformation and display as reliability, performance, condition assessment, and life management information. The paper describes application of this monitoring platform and development of state-of-the-art condition assessment models for compressor efficiency, turbine blade-coating life, and turbine blade thermal mechanical fatigue life. The paper discusses the use of this monitoring platform and these condition assessment models to allow data related to the life and reliability of CT parts to be collected and tracked. In addition, the paper briefly discusses two other installations on Westinghouse 501F and GE 7FA+ CTs at CP&L’s Monroe and Lee stations that have been in operation for approximately six months.
Three Dimensional Thermoacoustic Oscillation in a Premix Combustor
Authors: Akamatsu S. , Dowling A. P.
Published: July 2014

A theory is developed to describe high frequency three-dimensional thermoacoustic waves in a simplified geometry representing a typical premix combustor. The theory considers linear modes of frequency ω and circumferential mode number m i.e. proportional to eiωt+imθ. The radial and axial dependence is determined for a cylindrical combustor.

Simple geometries are investigated systematically to analyze the effect of different inlet boundary conditions to the combustion chamber on the frequency of oscillation and on the susceptibility to instability, both near and away from the cut-off frequencies.

The model includes a one-dimensional mean flow, radial mode coupling and idealized combustion processes, which are added in stages to build up an understanding of the complicated acoustics of the premix combustor geometry.

It is demonstrated that the flow through the premix ducts provides a frequency-dependent boundary condition at combustor inlet and causes modal coupling.

Generalized linear equations of conservation of mass, momentum and energy, together with boundary conditions, are solved to identify the eigenfrequencies, ω, of the total system. Then Real ω determines the frequency of the oscillation, while Imaginary ω indicates the growth rate of the disturbance.

It is found that strong resonant peaks in the pressure waves exist close to the cut-off condition for acoustic waves and that the relationship between the unsteady rate of heat release and the flow significantly influences the instability of oscillation.
Instability of a Premix Burner With Non-Monotonic Pressure Drop Characteristic
Authors: Polifke Wolfgang , Fischer Andreas , Sattelmayer Thomas
Published: July 2014

Thermo-acoustic combustion instabilities have frequently been reported to occur when slight changes in operating conditions lead to significant and abrupt changes in flame shape or flame position, i.e. changes in the mode of flame stabilization. The present paper offers an explanation and mathematical model of this observation. The analysis rests on the assumption that changes in the mode of flame stabilization are accompanied by a significant variation of the pressure drop across burner and flame, such that the pressure drop - flow rate characteristic locally displays a negative slope. In the limit of low frequencies (Helmholtz mode), it is then straightforward to show that an oscillatory instability can result from such behavior. A stability criterion is derived, relating the non-dimensionalized gradient of the pressure drop characteristic to the Helmholtz number of the burner. The physics of the instability mechanism is explained, and it is observed that the Rayleigh criterion need not be satisfied for this kind of instability to occur. The analysis is then extended to higher frequencies and the transfer matrix for a burner with non-monotonic pressure drop is derived in the limit of low Mach number and negligible fluctuations of the rate of heat release. Experimental results obtained with an externally premixed swirl burner are presented. The pressure drop characteristic, the observed onset of instability and the instability frequency match very well the analytical predictions.
Thermoacoustic Oscillations in an Annular Combustor
Authors: Stow Simon R. , Dowling Ann P.
Published: July 2014

Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. Acoustic waves produce fluctuations in heat release, for instance by perturbing the fuel–air ratio or flame shape. These heat fluctuations will in turn generate more acoustic waves and in some situations self-sustained oscillations can result.

A linear model for thermoacoustic oscillations in LPP combustors is described. A thin annular combustor is assumed and so circumferential modes are included but radial dependence is ignored. The geometry consists of straight ducts joined by short regions of area change. Perturbations to the flow can be thought of as a combination of acoustic, entropy and vorticity waves. The development of these waves along the straight ducts is found using a propagation matrix approach. At the entrance to the combustion chamber, a flame model is used in which the unsteady heat release is related to fluctuations in fuel–air ratio. Various possible inlet and outlet conditions are described. The model is then applied to a simplified example based on a sector rig. The resonant modes are found numerically and compared with the frequencies that occurred in experiments.
On the Use of Helmholtz Resonators for Damping Acoustic Pulsations in Industrial Gas Turbines
Authors: Bellucci Valter , Paschereit Christian Oliver , Flohr Peter , Magni Fulvio
Published: July 2014

In modern gas turbines operating with premix combustion flames, the suppression of pressure pulsations is an important task related to the quality of the combustion process and to the structural integrity of engines. High pressure pulsations may occur when the resonance frequencies of the system are excited by heat release fluctuations independent of the acoustic field (“loudspeaker” behavior of the flame). Heat release fluctuations are also generated by acoustic fluctuations in the premixed stream. The feedback mechanism inherent in such processes (“amplifier” behavior of the flame) may lead to combustion instabilities, the amplitude of pulsations being limited only by nonlinearities.

In this work, the application of Helmholtz resonators for damping low-frequency pulsations in gas turbine combustion chambers is discussed. We present a nonlinear model for predicting the acoustic response of resonators including the effect of purging air. Atmospheric experiments are used to validate the model, which is employed to design a resonator arrangement for damping low-frequency pulsations in an ALSTOM GT11N2 gas turbine. The predicted damper impedances are used as the boundary condition in the three-dimensional analysis of the combustion chamber. The suggested arrangement leads to a significant extension of the low-pulsation operating regime of the engine.
Acoustic Nearfield Characteristics of a Premixed Flame in a Longitudinal Acoustic Field
Authors: Lee Doh-Hyoung , Lieuwen Tim C.
Published: July 2014

The occurrence of self excited, combustion driven oscillations pose significant problems in lean, premixed gas turbine combustors. The interactions between flames and longitudinal acoustic oscillations play a key role in many of these instabilities. This paper analyzes the acoustic field in the vicinity of a premixed flame front in order to clarify these interactions. Specifically, it describes a numerical analysis of the acoustic characteristics of a premixed flame that is excited by longitudinal disturbances. Solutions are determined from an integral formulation of the acoustic wave equation that is solved via boundary element techniques. Analyses of these results are performed to characterize the deviations of the acoustic field from one-dimensionality. First, as can be anticipated from quasi one-dimensional considerations, the flame’s reflection coefficient is lower than that predicted by purely one-dimensional calculations. The results also show that the acoustic pressure is nearly one-dimensional in the vicinity of the flame. Finally, they show that the velocity oscillations are strongly multidimensional, even in acoustically compact flames. At very low frequencies, these local velocity oscillations are in phase with each other, while at higher frequencies, their phase may change significantly along the flame. The results of this study show that the multidimensional nature of the acoustic velocity oscillations in the near field of the flame must be taken into consideration in analyses of the interactions between acoustic waves and flames.
Numerical Simulation of High-Frequency Flow Instabilities Near an Airblast Atomizer
Authors: Noll B. , Schütz H. , Aigner M.
Published: July 2014

In the paper it is shown that statistical averaging of transport equations (URANS = Unsteady Reynolds Averaged Navier Stokes) imposes no inherent restriction concerning the ability to predict periodic or other deterministic transient flow processes. This even holds for periodic oscillations at relatively high frequencies lying in the spectral range of the inertial sub-range of flow turbulence. As an application, the unsteady behaviour of an isothermal swirling air flow through and behind an airblast-atomizer of a design typical for modern aeroengine combustors is treated. This flow exhibits self-excited oscillations at a frequency of 2.8 kHz. Computations of this flow behaviour based on the numerical solution of the unsteady statistically averaged Navier-Stokes equations are presented. The turbulence model employed in the computations is a k,ε-model modification for swirling flows. The transport equations are discretized by a Finite-Volume method on a curvilinear grid. Calculated mean velocity profiles as well as the predicted dynamic flow behaviour at the nozzle exit agree very well with appropriate LDV- and microphone-measurements.
Liquid Fuel Spray Characterization for the Design of the Dual-Fuel PGT10B Combustor
Authors: Tap F. A. , Modi R. , Van Buijtenen J. P.
Published: July 2014

The Dry Low NOx (DLN) silo combustor of the Nuovo Pignone PGT10B gas turbine is being redesigned to meet Dual-Fuel capability. A prototype with specially designed fuel injectors, placed on airfoil-shaped elements, was tested at cold conditions (using water instead of Diesel fuel) to map the spray mass distribution at the premixer exit. The resulting profile showed high concentrations of liquid near the premixer centerline and on the premixer wall.

Parallel to this test, a small-scale experimental and numerical study was made of a single atomizer of the fuel system, placed in cross flow position. This small-scale study was launched in order to gain insight in the behavior of the spray, as well as to assess the relative importance of spray modeling parameters. The PDPA experiments and 2D CFD simulations of these experiments showed fair agreement on the average drop size distribution and drop size-velocity correlation. The flow visualization also revealed liquid film formation on the surface of the airfoil, behind the injector, due to the low atomization pressure differential at cold conditions.

Using this modeling experience, the spray patternation test with the prototype combustor has been modeled using an existing 3D CFD model of the premixer. The model also showed high liquid concentration on the wall, but not near the centerline. From the results of the small-scale study it is concluded that the measured high concentration near the premixer centerline is not a result of the flow field. It is assumed that in the complete assembly the liquid film from the injector vanes accumulates on the center body, resulting in a high liquid concentration downstream on the premixer centerline.

Overall, the application of CFD analyses on the tests performed proved to be a very useful tool to evaluate the test results. The modeling experience identified the important factors in modeling the fuel spray in a gas turbine environment, but further evolution of computer resources is required before large-scale test results will be reproducible with CFD models.
A Combined Eulerian and Lagrangian Method for Prediction of Evaporating Sprays
Authors: Burger M. , Klose G. , Rottenkolber G. , Schmehl R. , Giebert D. , Schäfer O. , Koch R. , Wittig S.
Published: July 2014

Polydisperse sprays in complex three dimensional flow systems are important in many technical applications. Numerical descriptions of sprays are used to achieve a fast and accurate prediction of complex two-phase flows.

The Eulerian and Lagrangian methods are two essentially different approaches for the modeling of disperse two-phase flows. Both methods have been implemented into the same CFD - package which is based on a 3D body-fitted Finite Volume method. Considering sprays represented by a small number of droplet starting conditions, the Eulerian method is clearly superior in terms of computational efficiency. However, with respect to complex polydisperse sprays, the Lagrangian technique gives a higher accuracy. In addition, Lagrangian modeling of secondary effects such as spray-wall interaction enhances the physical description of the two-phase flow. Therefore, in the present approach the Eulerian and the Lagrangian methods have been combined in a hybrid method. The Eulerian method is used to determine a preliminary solution of the two-phase flow field. Subsequently, the Lagrangian method is employed to improve the accuracy of the first solution using detailed sets of initial conditions. Consequently, this combined approach improves the overall convergence behavior of the simulation.

In the final section, the advantages of each method are discussed when predicting an evaporating spray in an intake manifold of an IC-engine.
CFD Simulation of Humid Air Premixed Flame Combustion Chamber for Evaporative Gas Turbine Cycles
Authors: Bianco M. , Camporeale S. M. , Fortunato B.
Published: July 2014

Evaporative cycles, such as Recuperated Water lnjected (RWI) cycle, Humid Air Turbine (HAT) cycle, Cascaded Humidified Advanced Turbine (CHAT) offer the attractive possibility to increase plant efficiency without the use of a steam turbine, necessary for gas-steam combined cycles, appearing, therefore, as an interesting solution for industrial power applications such as electric utilities and independent power producers.

It is expected that water addition may contribute to reduce NOx emissions in premixed flame combustors. In order to analyse this solution, a lean-bum combustor, fed with an homogeneous mixture formed by methane and humid air, has been analysed through CFD simulations, in order to predict velocity field, temperatures and emissions. The study has been carried out under the hypothesis of a two-dimensional, axisymmetric combustion chamber assuming, as set of operation conditions, atmospheric pressure, inlet temperature of 650 K, fuel-air equivalence ratio of the methane-air mixture ranging from 0.5 to 0.7 and water-air mass ratio varying from 0% to 5%. In the simulation, the presence of turbulence in the flow has been taken into account using a RNG k-ε model, whilst the chemical behaviour of the system has been described by means of a five-step global reduced mechanism including the oxidation mechanism and the NOx formation mechanism.

The analysis of the results shows that the moisture in the premixed flow reduces both NOx and CO emissions at constant equivalence ratio; moreover the lean blow-out limit is shifted toward higher equivalence ratio. The main effect of the water seems to be the increase of the specific heat the mixture which causes a reduction in flame temperature, slowing the chemical reactions responsible of NOx formation.

The reasonable agreement has been found between the simulation results concerning NOx emissions and recent experimental results carried out on premixed flamed with humid air. A discussion is also provided about the adopted turbulence models and their influence on the emission results.
Application of an Advanced CFD-Based Analysis System to the PW6000 Combustor to Optimize Exit Temperature Distribution: Part I — Description and Validation of the Analysis Tool
Authors: Malecki Robert E. , Rhie Chae M. , McKinney Randal G. , Ouyang Hong , Syed Saadat A. , Colket Meredith B. , Madabhushi Ravi K.
Published: July 2014

This paper is the first of two parts that describes an advanced CFD-based analysis system, developed at Pratt & Whitney, which has been used to optimize the PW6000 combustor exit temperature distribution. It utilizes a CFD calculation through the entire combustor domain to predict temperature distribution at the combustor exit. In this part, all components of the analysis system are presented, including the CAD and grid generation approach used to represent the complex combustor geometry, the core CFD solver, the Lagrangian fuel spray model, and the combustion model. In addition, the predictive capability of the system is established by comparing calculated exit temperature profiles to full annular rig test data for three aircraft gas turbine engine combustors: PW4090, PW4098, and a low-emissions technology development combustor. Comparisons of combustor airflow distribution and pressure drop are also presented to verify the accuracy of the tool. The paper demonstrates that the CFD-based analysis system is capable of calculating exit temperature distribution for a range of combustor configurations, and thus can be utilized as a predictive design tool. Part II demonstrates this predictive capability by applying the analysis tool to optimize the PW6000 combustor exit temperature distribution for turbine durability and life.
Application of an Advanced CFD-Based Analysis System to the PW6000 Combustor to Optimize Exit Temperature Distribution: Part II — Comparison of Predictions to Full Annular Rig Test Data
Authors: Snyder Timothy S. , Stewart Jesse F. , Stoner Matthew D. , McKinney Randal G.
Published: July 2014

Pratt & Whitney is developing a 107 kN (24,000 pound) thrust PW6000 engine for the 100-seat aircraft market. The combustor for this engine has been designed by combining the TALON emissions concept demonstrated on the PW4000 engine family with an advanced CFD-based analysis system to optimize the combustor exit temperature distribution. The design objective is to provide a low cost highly reliable engine, which produces low emissions.

This paper is the second of two parts, which describe an advanced CFD-based analysis system used to optimize the combustor exit temperature distribution for turbine life. The analysis system applied the identical Allstar solver, which is described and validated in part I, to the PW6000 combustor. All calculations in this paper were completely predictive in nature. The effect of dilution hole pattern changes on the exit temperature profile was determined by solving the flowfield from the prediffuser inlet to the combustor exit. Results from the study were used to understand the physical processes taking place inside the prediffuser and combustor that impact the exit temperature profile and from this understanding a hole pattern configuration was identified. Full annular rig measurements of the pressure drop and airflow distribution throughout the model along with exit temperature profile measurements agreed very well with CFD predictions. A second target exit temperature profile was defined based upon engine testing and the analysis tool demonstrated the ability to define a second dilution hole pattern that met the target profile to optimize turbine life. An annular rig test again confirmed the CFD predictions.

Parametric studies were also performed on the prediffuser inlet pressure profile to predict how the turbine inlet temperature profile would change. These studies were used to desensitize the combustor temperature profile to prediffuser inlet profile changes that may occur over the life of the engine. The predictive capability of this CFD-based analysis tool has significantly reduced experimental development costs and has optimized the combustor exit temperature profile to meet PW6000 design objectives.
Numerical Analysis of NOx Formation in a Diffusion Flame Combustor Based on a Flamelet Model
Authors: Volkov Dmitry V. , Belokon Alexandr A. , Lyubimov Dmitry A. , Zakharov Vladimir M. , Opdyke George
Published: July 2014

Laminar flamelet models have demonstrated good quality predictions of NOx emission from diffusion flame type combustors. In this paper, the NOx formation process is analyzed by using a flamelet model and 3D flow calculations to take a virtual look inside a combustor. The main phenomena affecting NOx emission are turbulent mixing and the turbulence-chemistry interaction. Local scalar dissipation is the main parameter responsible for the turbulence-chemistry interaction within the flamelet model. At the same time, scalar dissipation is also related to the mixing process. On one hand, higher values of scalar dissipation correspond to higher fuel consumption rates, which decrease the volume of the high temperature zones. On the other hand, higher values of scalar dissipation lead to higher NOx formation rates. Unfortunately, scalar dissipation is not commonly used by combustion engineers because of the difficulty of the clear physical interpretation of this variable and its relationship with the usual parameters. In this paper, the influence of several design features, such as primary zone equivalence ratio and air flow distribution along the liner, is studied relative to scalar dissipation distributions in the combustion zones and to NOx formation. A real industrial diffusion flame combustor is used as an example, and the results can provide a better understanding of real combustor processes. The NOx prediction results are in reasonable agreement with test data.
Modelling of NOx Formation of a Premixed DLE Gas Turbine Combustor
Authors: Eggels R. L. G. M.
Published: July 2014

To comply with the stringent emission regulations industrial gas turbines operate under lean premixed conditions. To be able to predict emissions in an early design phase, advanced premixed combustion models are required. The rate of NOx formation is sensitive to temperature and some radical concentrations, therefore the flame predictions have to be accurate and detailed. It is well known that there are several routes (thermal, prompt, N2O mechanism) by which NOx can be formed. As lean premixed gas turbines operate at relative low flame temperatures, the contribution of the prompt NO and N2O mechanisms has to be accounted for.

The modelling of NOx formation is done in a post-processor, as the influence of nitrogen species on the main combustion characteristics is negligible. This post-processor is based on flame calculations using a Flame Generated Manifold method. This is a reduced reaction mechanism, so that only a limited number of variables have to be solved during the CFD computations.

A post-processor method has been developed to compute NO formation. Differential equations are solved for NO and N2O, other species including nitrogen are solved using a steady-state equation.

The flame and post-processor models are applied to 2-D and 3-D models of an industrial series staged gas turbine. Parameter studies for the fuel split between the two stages and inlet fuel/air ratio variations have been carried out and the data has been compared with generic engine data.
The Influence of Film Cooling on Emissions for a Low NOx Radial Swirler Gas Turbine Combustor
Authors: Andrews G. E. , Kim M. N.
Published: July 2014

An experimental investigation was undertaken of the influence on emissions of full coverage discrete hole film cooling of a lean low NOx radial swirler natural gas combustor. The combustor used radial swirler vane passage fuel injection on the centre of the vane passage inlet. The test configuration was similar to that used in the Alstom Power Tornado and related family of low NOx gas turbines. The test conditions were simulated at atmospheric pressure at the flow condition of lean low NOx gas turbine primary zones. The tests were carried out at an isothermal flow Mach number of 0.03, which represents 60% of industrial gas turbine combustor airflow through the swirl primary zone. The effusion film cooling used was Rolls-Royce Transply, which has efficient internal cooling of the wall as well as full coverage discrete hole film cooling. Film cooling levels of 0, 16 and 40% of the primary zone airflow were investigated for a fixed total primary zone air flow and reference Mach number of 0.03. The results showed that there was a major increase in the NOx emissions for 740K inlet temperature and 0.45 overall equivalence ratio from 6ppm at zero film cooling air flow to 32ppm at 40% coolant flow rate. CO emissions increased from 25ppm to 75ppm for the same increase in film cooling flow rate. It was shown that the main effect was the creation of a richer inner swirler combustion with a surrounding film cooling flow that did not mix well with the central swirling combustion. The increase in NOx and CO could be predicted on the basis of the central swirl flow equivalence ratio.
Advanced Burner Development for the Vx4.3A Gas Turbines
Authors: Streb Holger , Prade Bernd , Hahner Thomas , Hoffmann Stefan
Published: July 2014

The Vx4.3A gas turbine family has already been well received by the market. Nevertheless the market drives technology towards both increased turbine inlet temperatures and reduced emissions.

The HR3 burner was originally developed for the V4.2 and Vx4.3 fleet featuring silo combustors in order to mitigate the risk of flashback and to improve the NOx- emissions (Prade, Streb, 1996). Due to its favourable performance characteristics in the Vx4.3 family the advanced HR3 burner was adapted to the Vx4.3A series with annular combustor.

The paper reports upon the design, testing and field evaluation steps which were necessary to implement the burner for the 50 and 60 cycle gas turbines.

With CFD calculations the flow field and the mixing of natural gas and combustion air have been optimised. A number of tests in the Siemens test facilities confirmed these predictions. The atmospheric 3 burner segment combustion test rig allows to test flame interaction, stability and exhaust gas emission simultaneously.

In the Siemens Berlin Test Facility which provides a platform for full scale gas turbine testing 24 HR3-burners were implemented into a V84.3A gas turbine with a base load power output of 184 MW at ISO conditions for prototype testing before introducing this new burner generation into the bigger 50 cycle family V94.3A.

Implementation of 24 scaled HR3 burners were installed in the V94.3A of Cottam Development Centre (Great Britain) and demonstrated an excellent performance. The gas turbine reached an ISO base load output of 265 MW with NOx emissions well below 25 ppmvd.

Due to the very promising test results in Berlin and Cottam, this burner modification, which can be retrofitted to all VX4.3A gas turbines, was implemented nearly fleet wide.