bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlates of Condom Use Among Male High School Students in Nairobi, Kenya
Authors: Kabiru Caroline W. , Orpinas Pamela
Published: August 2009

BACKGROUND: Correct and consistent condom use is an effective strategy to reduce the risk of sexually transmitted infections (STIs). This study examines sociodemographic, behavioral, and psychosocial characteristics of 3 groups of adolescent males: consistent, sporadic, and non‐condom users.

METHODS: The sample consisted of 931 sexually experienced male high school students in Nairobi, Kenya. Cross tabulations and general linear models were used to test the relation between each explanatory variable and condom user group.

RESULTS: Fifty‐three percent of respondents reported having used condoms. Compared with nonusers and sporadic users, consistent users engaged in sexual intercourse for the first time at an older age and reported higher condom use self‐efficacy, more positive peer attitudes toward safe sex, and stronger attitudes toward condom use. Compared with sporadic and consistent users, nonusers reported fewer sexual partners, less substance use, and more frequent intercourse with non‐romantic partners. Sporadic users, compared with the other 2 groups, had more sexual partners and stronger attitudes that support that males and females have separate roles. The 3 groups did not differ in HIV/AIDS knowledge. On average, respondents responded correctly to two thirds of the questions. All groups perceived their risk of contracting an STI as low, and this perception did not differ by condom use. Differences in participants' age by group were minimal.

CONCLUSIONS: The study supports the need for interventions that target specific condom use groups.
Assessment of Nutrient Stability in Foods from the Space Food System After Long‐Duration Spaceflight on the ISS
Authors: Zwart S.R. , Kloeris V.L. , Perchonok M.H. , Braby L. , Smith S.M.
Published: August 2009

ABSTRACT:  Maintaining an intact nutrient supply in the food system flown on spacecraft is a critical issue for mission success and crew health. Ground‐based evidence indicates that some vitamins may be altered and fatty acids oxidized (and therefore rendered useless, or even dangerous) by long‐term storage and by exposure to radiation, both of which will be issues for long‐duration exploration missions in space. In this study, the stability of nutrients was investigated in food samples exposed to spaceflight on the Intl. Space Station (ISS). A total of 6 replicates of 5 different space food items, a multivitamin, and a vitamin D supplement were packaged into 4 identical kits and were launched in 2006 on the space shuttle. After 13, 353, 596, and 880 d of spaceflight aboard the ISS, the kits were returned to Earth. Nine replicates of each food item and vitamin, from the same lots as those sent into space, remained in an environmental chamber on Earth to serve as controls at each time point. Vitamins, hexanal, oxygen radical absorbance capacity, and amino acids were measured in identical‐lot food samples at each time point. After 596 d of spaceflight, differences in intact vitamin concentrations due to duration of storage were observed for most foodstuffs, but generally, nutrients from flight samples did not degrade any faster than ground controls. This study provided the 1st set of spaceflight data for investigation of nutrient stability in the food system, and the results will help NASA design food systems for both ISS and space exploration missions.
Multiple paternity: determining the minimum number of sires of a large brood
Authors: ERIKSSON A. , MEHLIG B. , PANOVA M. , ANDRE C. , JOHANNESSON K.
Published: August 2009

We describe an efficient algorithm for determining exactly the minimum number of sires consistent with the multi‐locus genotypes of a mother and her progeny. We consider cases where a simple exhaustive search through all possible sets of sires is impossible in practice because it would take too long to complete. Our algorithm for solving this combinatorial optimization problem avoids visiting large parts of search space that would not result in a solution with fewer sires. This improvement is of particular importance when the number of allelic types in the progeny array is large and when the minimum number of sires is expected to be large. Precisely in such cases, it is important to know the minimum number of sires: this number gives an exact bound on the most likely number of sires estimated by a random search algorithm in a parameter region where it may be difficult to determine whether it has converged. We apply our algorithm to data from the marine snail, Littorina saxatilis.
The diffusion approximation overestimates the extinction risk for count‐based PVA
Authors: Kendall Bruce E.
Published: August 2009

Simple population models are increasingly being used to predict extinction risk using historical abundance estimates. A very simple model, the stochastic exponential growth (SEG) model, is surprisingly robust. Extinction risk is commonly computed for this model using a mathematical approximation (the “diffusion approximation”) that assumes continuous breeding throughout the year, an assumption that is violated by many species. Here I show that, for an organism with seasonal breeding, the diffusion approximation systematically overestimates the extinction risk. I demonstrate the conditions generating large bias (high environmental variance, intermediate extinction risk), and reanalyze 100 populations of conservation concern. Analyzing several policy applications, I find that the bias may be most important when classifying the risk status of species. The SEG model is still sound, but associated risk estimates should be calculated by performing stochastic simulations (as with all other population viability models) rather than by evaluating the diffusion approximation.
Gas Turbine Plant Thermal Performance Degradation Assessment
Authors: Mirzamoghadam Alexander V.
Published: August 2009

Knowing the sources behind degradation of gas turbine power and heat rate and the performance of other power plant equipment are critical to equipment manufacturers for guaranteeing their respective performances over the life cycle as well as to utility/plant owners for cost reasons. Many power companies are trying to improve equipment reliability by focusing on Performance Monitoring and the application of advanced diagnostic technologies. The first part of the paper describes a method to monitor gas turbine compressor efficiency, forecast an efficiency decay rate in terms of operating hours, introduce the logic leading to an optimum schedule pertaining on-line/off-line water washing, correct measured real-time gas turbine power and heat rate with respect to the new and clean reference point, and then deduce the respective turbine and compressor power contributions to gas turbine net power degradation. An example is used to aid the planning of a rigorous but effective schedule in compressor washing. The second part focuses on assessing degradation at the plant level (simple or combined cycle). The derived gas turbine degradation methodology is integrated with the overall plant performance degradation, and as a result, the maintenance effectiveness of the most recent outage can be measured. The proposed methodology is applied to a hypothetical combined cycle plant, allowing the identification of possible sources of plant deterioration with recommendations for corrective actions to improve overall power and heat rate.
Energetic and Economic Analyses of Integrated Biogas-Fed Energy Systems
Authors: Bettocchi R. , Pinelli M. , Spina P. R. , Venturini M. , Cadorin M. , Cenci G. , Morini M.
Published: August 2009

The process which includes production, collection, carriage and transformation of biomass into renewable fuels and then into energy (both electrical and thermal) involves a large number of decisions to select the most efficient plant layout. In order to identify the optimal solutions, models which simulate the whole process represent a useful and practical tool. In this paper, the energetic and economic analysis of the entire process from biomass to energy production is presented. Among the different transformation processes, the thermophilic batch anaerobic digestion is considered in this paper. A sensitivity analysis on system profitability is carried out with respect to the mass of biomass, number of batch digesters and retention time of the biomass inside each digester. Moreover, two different types of biomass (ensiled corn and organic fraction of municipal solid wastes) and two different energy systems (Micro Gas Turbine and Internal Combustion Engine) are considered.
Compressor Casing Preliminary Design Based on Features
Authors: Bretschneider Stefan , Rothe Frank , Rose Martin G. , Staudacher Stephan
Published: August 2009

Structural parts, such as casings, have a significant share of the overall turbo-machinery mass of modern aero-engines. Therefore preliminary design studies must aim to include the effect of such structures. For this paper compressor casings of commercial aero-engines have been investigated in terms of their design philosophy. It is shown that compressor casings have very similar designs from the preliminary design perspective, even though they appear as very complex structures in reality. The study identified design similarities from which generalized and simplified casing structures have been derived. The casing is divided into geometrically similar basic structures. Such generalized parts are each individually characterized by features. Through simplified physical design algorithms the features are then dimensioned based on blade containment conditions, pressures and temperatures. Finally a generalized form of compressor casing design is derived from the assembly of the individual parts. The derived preliminary design method of casings is no longer dependent on a known representative casing thickness. When increasing compressor characteristics such as blade numbers, diameters or revolution speed the casing design responds directly while still maintaining a characteristic shaping philosophy. Thus a scaling method based on physics rather than only geometrical identity is achieved. The method was integrated into an existing high pressure compressor preliminary design tool. The examination of the developed methodology is carried out against existing compressor designs. Results are presented and discussed.
Planar Laser Diagnostics at High Repetition Rates: Acquisition and Analysis of Transient Combustion Processes
Authors: Heeger Christof , Bo¨hm Benjamin , Boxx Isaac , Meier Wolfgang , Ahmed Samer F. , Mastorakos Epaminondas , Dreizler Andreas
Published: August 2009

This contribution highlights recent advances in laser diagnostics at high repetition rates. Based on recent improvements in all-solid-state, diode-pumped laser and CMOS camera technology, well known methods such as Mie scattering, particle image velocimetry and planar laser-induced fluorescence are adapted and extended to high repetition rates in the kHz-regime and applied simultaneously to a turbulent opposed jet burner. High temporal resolution enables one to track transient events such as flame extinction and ignition in turbulent combustion. New perspectives into turbulent combustion are thus possible by quasi-4D imaging or multi-dimensional conditioning on transient phenomena.
Experimental Investigations and Numerical Validation of an Outlet Guide Vane With an Engine Mount Recess
Authors: Hja¨rne Johan , Chernoray Valery , Larsson Jonas
Published: August 2009

This paper presents experiments and CFD calculations of a Low Pressure Turbine/Outlet Guide Vane (LPT/OGV) equipped with an engine mount recess (a bump) tested in the Chalmers linear LPT/OGV cascade. The investigated characteristics include performance for the design point in terms of total pressure loss and turning as well as a detailed description of the downstream development of the secondary flow field. The numerical simulations are performed for the same inlet conditions as in the test-facility with engine-like properties in terms of Reynolds number, boundary-layer thickness and inlet flow angle. The objective is to validate how accurately and reliably the secondary flow field and losses can be predicted for an LPT/OGV equipped with a bump. Three different turbulent models as implemented in FLUENT, the k-ε realizable model, the kω-SST model and the RSM are validated against detailed measurements. From these results it can be concluded that the kω-SST model predicts both the secondary flow field and the losses most accurately.
Impact of Continuous Inlet Fogging and Overspray Operation on GE 5002 Gas Turbine Life and Performance
Authors: Brun Klaus , Gonzalez Luis Eduardo , Platt John P.
Published: August 2009

The usage of an industrial inlet fogging and overspraying system on BP Colombia’s fleet of GE 5002 gas turbines was intended to provide additional shaft power output and improved efficiency. However, operating experience has shown less than anticipated power increase and almost no efficiency change, while the gas turbines have experienced more rapid degradation. Consequently, a detailed study was undertaken to identify the principal degradation mechanisms and quantify their relative influence on the gas turbine’s performance and life reduction. This study included a field assessment; review and analysis of the installation and operating data from the historical trend monitoring system; inspection of a rotor for fouling, corrosion, and pitting; materials analysis of the fouling deposits, rotor surface pitting, and inlet filter media; review of the function and effects of inlet fogging and overspray; assessment of the effectiveness of the current on-line/off-line compressor washing program and its compatibility with the overspraying operation; and an analysis of the overall gas turbine efficiency to determine levels of performance degradation. Results from this study identified the principal gas turbine degradation mechanisms, such as blade erosion, corrosion, fouling tip clearance widening, their causes and their relative influence on the overall performance. For example, the study showed that the total power and efficiency degradation of the units exceeded 10% at the time of the rotor overhaul which is well above what is expected for this type of gas turbine. About 70% of this degradation was due to blade erosion and rotor clearance widening. These were attributed to the water overspray operation of the gas turbines. Surface fouling and pitting also contributed about 20% to the total performance degradation. For the given site conditions, the fogging and overspray system provided a gas turbine performance boost of approximately 2–5% in power and less than 0.5% in efficiency. Of this performance gain, saturation fogging accounted for about 85%, while overspray only provided 15%. The principal findings of this study showed that, while the fogging worked, the performance degradation due to water overspray negated most performance gains after only about 24,000 hours of operation. More detailed findings are included in the paper.
Effect of Wakes and Secondary Flow on Re-Attachment of Turbine Exit Annular Diffuser Flow
Authors: Kluß David , Wiedermann Alexander , Stoff Horst
Published: August 2009

In this paper numerical results of wake and secondary flow interaction in diffuser flow fields are discussed. The wake and secondary flow are generated by a rotating wheel equipped with 30 cylindrical spokes with a diameter of 10 mm as a first approach to the turbine exit flow environment. The apex angle of the diffuser is chosen such that the flow is strongly separated according to the well-known performance charts of Sovran and Klomp [1]. This configuration has been tested in an experimental test rig at the Leibniz University Hannover [2]. According to these experiments, the flow in the diffuser separates as free jet for low rotational speeds of the spoke-wheel as expected by theory. However, if the 30 spokes of the upstream wheel rotate beyond the value of 500 rpm the measurements indicate that the flow remains attached to the outer diffuser wall. It will be shown by the present numerical analysis with the commercial solver ANSYS CFX-10.0 that only an unsteady approach using the elaborate SAS-SST turbulence model is capable of predicting the stabilizing effect of the rotating wheel to the diffuser flow at larger rotational speeds. The favourable comparison with the experimental data suggests that the mixing effect of wakes and secondary flow pattern is responsible for the reattachment. As a result of our studies it can be stated that the considerably higher numerical costs associated with unsteady calculations must be accepted in order to increase the understanding of the physical flow phenomena in turbine exit flow and its interaction with the downstream diffuser.
Boundary Layer Control in a Compressor Cascade Using Distributed Suction
Authors: Chakraborthy Utpal , Pradeep A. M.
Published: August 2009

An experimental study in a low speed compressor cascade was carried out to investigate the effect of distributed suction (aspiration) on the cascade performance. Unlike suction used conventionally, distributed suction requires lower mass flow rates and is achieved by suction through holes of very small diameter distributed over the surface. A set of 5 NACA - 65(18)10 blades were used in the experiments that were carried out at a Reynolds number of 1.6 × 105. Detailed measurement of surface static pressure, total pressure loss in the wake of the blades and boundary layer thicknesses were taken at incidence angles in the range −10 to +6 degrees. Significant effect of suction was observed on the total pressure loss distribution at the trailing edge of the blades. The mean total pressure loss coefficient reduced in the range 14 to 36 percent for the various configurations tested. Higher performance improvement was observed at negative incidence angles. Boundary layer measurements revealed that the effect of suction was prominent in the mid-span of the blades. Reduction in boundary layer momentum thickness in the range 8 to 20 percent was observed for the various configurations. The calculated diffusion factor also showed improving trends in line with the observations of total pressure loss and boundary layer thicknesses. The mass flow ratio for the best configuration was only 0.15 percent of the primary mass flow. This experimental study demonstrates the effectiveness of distributed suction (using only a fraction of the primary mass flow) on compressor cascade performance.
A Study of Unsteady Secondary Flow in a Water Flow Axial Turbine Model
Authors: Kasper Christian , Rose Martin G. , Staudacher Stephan , Gier Jochen
Published: August 2009

The influence of secondary flows on the performance of turbines has been investigated in great detail in the last decades. The interaction of vortices with following blade rows has been identified to be one of the loss mechanisms within a turbo-machine. This paper presents for the first time detailed flow visualization photographs of the interaction of the vane passage vortex with the rotor. The appearance vortex breakdown could be identified before and within the rotating passage of the turbine. The measurements were taken in a vertical water channel. Water is used instead of air because the flow visualization can be realised very easily with injected ink. For different relative positions of rotor to stator a series of photographs were taken. With an image editing process the average and the pixel RMS were calculated for each relative position. The pixel RMS is a useful indicator to identify highly turbulent regions in the flow field. The photographs of the vortex breakdown show spots of high pixel RMS which are associated with very high turbulence and therefore can be regarded as sources of loss. Insight is gained into the nature of the passage vortex breakdown mechanisms as follows: first the pressure wave of the rotor stretches the vortex causing a spiral vortex instability, then the vortex interacts with the leading edge as it attempts to cut the vortex. In the stagnation region of the blade a bubble type instability forms, expands and then convects through the rotor. The absolute trajectory of the vortex fluid reveals that it exchanges no work with the rotor.
Analysis of the Biomass Integrated Combined Cycles With Two Different Structures of Gas Turbines
Authors: Lepszy Sebastian , Chmielniak Tadeusz
Published: August 2009

Biomass integrated gasification combined cycles (BIGCC) are an interesting solution for electricity production. In relation to other biomass conversion technologies, BIGCC is characterized by relative high energy efficiency. For the sake of high complexity of such systems, one of crucial tasks is evaluation and comparison of the different technological structures of BIGCC. The article shows models and results of simulations of gas steam cycles integrated with biomass gasification. All models and simulations are preformed with Aspen Plus computer program. In the paper the main comparison is made between systems with simple gas turbine and gas turbine with regeneration. Simple gas turbine model based on LM2500 gas turbine parameters, Mercury 50 gas turbine parameters are used for model of gas turbine with regeneration. The model of gas generator consists of two equilibrium reactors. The use of two reactors led to more precise simulations of the flue gas composition, than the model with one reactor. Systems used for study include low-temperature gas cleaning system. Steam cycle consists of 1-pressure heat recovery steam generator (HRSG) and a condensing steam turbine. The main results of the work are: comparison of energy efficiency between system with gas turbine with regeneration and simple gas turbine, sensitive analysis of the impact of pressure in HRSG on energy efficiency, comparison of energy efficiency and heat and mass streams for different configurations of heat exchangers.
Model Based Active Combustion Control Concept for Reduction of Pulsations and NOx Emissions in Lean Premixed Gas Turbine Combustors
Authors: Schneider Ernst , Staudacher Stephan , Schuermans Bruno , Ye Haiwen
Published: August 2009

Strict environmental regulations demand gas turbine operation at very low equivalence ratios. Premixed gas turbine combustors, operated at very lean conditions, are prone to thermoacoustic instabilities. Thermoacoustic instabilities cause significant performance and reliability problems in gas turbine combustors, so their prevention is a general task. Splitting the fuel mass flow between different burner groups, i.e. using a burner group fuel staging technique, is a possibility to control the thermoacoustic instabilities. The resulting combustion perturbations have also effects on the gas turbine NOx emissions making it necessary to find an optimum balance between pulsations and emissions. This paper presents a model based active combustion control concept for the reduction of pulsations and emissions in lean premixed gas turbine combustors. The model is integrated in an observer structure derived from a Luenberger observer. The control logic is based on a PID algorithm allowing either the direct command of the pulsation level with a continuous monitoring and a potential limit setting of the NOx emission level or vice versa. The gas turbine pulsations and emissions are modelled using Gaussian Processes. - Gaussian Processes are stochastic processes related to Neural Networks that can approximate arbitrary functions. Based on measured gas turbine data they can be implemented in an easy and straightforward manner. The model provides the control system with real time data of the outputs resulting from settings of the staging ratio that is the actuating variable of the system. A model based control concept can significantly alleviate the effects of time delays in the system. The model based control concept allows for fast adaptation of the burner group staging ratio during static and transient operations to achieve an optimum of the pulsation and emission levels. During tests the model based control concept gave good results and proved to be robust even at high disturbance levels.
Computation of the Flow Field of Transonic Axial Compressor by Steady Arbitrary Lagrangian Eulerian Formulation
Authors: Ghenaiet Adel , Djeghri Nouredine
Published: August 2009

This paper presents a multi-block solver dealing with an inviscid three dimensional compressible flow through a transonic compressor blading. For efficient computations of the 3D time dependant Euler equations, this solver that we have developed has been cast within a stationary ALE ‘Arbitrary Lagrangian Eulerian’. The main contribution of this paper is by consolidating this ALE formulation, to alleviate the shortcomings linked to rotation effects and the mixed relative subsonic–supersonic inlet flow conditions, which are now simply implemented through an absolute subsonic flow velocity. The finite volume based solver is using the central differencing scheme known as JST (Jameson-Schmidt-Turkel). The explicit multistage Runge-Kutta algorithm is used as a pseudo time marching to the steady-state, coupled with two convergence accelerating techniques; the variable local time-stepping and the implicit residual smoothing procedure. The adaptive implicit residual smoothing has extended the stability range of this explicit scheme, and proved to be successful in accelerating the rate of convergence. This code is currently being extended to include viscous effects, where fluxes are discretized based on Green’s theorem. To support this solver, an H type grid generator based on algebraic and elliptic methods has been developed. The segmentation of the complete domain into smaller blocks has provided full topological and geometrical flexibilities. The code was used to compute the flow field of a transonic axial compressor NASA rotor 37, and comparisons between the calculations and some available experimental data under the design speed and part speed, show qualitatively good agreement.
Influence of Force Field Direction on Pressure Sensors Calibrated at up to 12000g
Authors: Uffrecht Wieland , Kaiser Erwin
Published: August 2009

The measurement of pressure within both stationary and rotating frames of reference are a fundamental requirement when studying the flow field through turbomachinery blading. Measurement of pressure within the rotating frame presents a particular challenge, as centrifugal acceleration of the sensor can have a significant impact on sensor calibration, and therefore accuracy of the resulting measurements. In this paper the telemetric calibration of pressure sensors at up to 12,000g is described, and the impact on calibration of membrane size, sensor body shape and sensor mounting direction discussed. The program of work reported in this paper focuses on experimental issues associated with rotating pressure measurement. The combined effect of centrifugal load and pressure on integrally temperature compensated silicon pressure sensors is presented. Experimental results are given that provide insight into the influence of acceleration on pressure readings. Implementation of acceleration into sensor calibration is presented. Supplementary finite-element calculations enable impact of sensor body shape to be taken into account during the evaluation of sensor acceleration-to-pressure sensitivity ratio. Different sensors with varied membrane sizes and acceleration force directions are examined and compared.
Performing Heat Transfer Experiments in Blade Cooling Circuits Using a Transient Technique With Thermochromic Liquid Crystals
Authors: Poser Rico , von Wolfersdorf Jens , Lutum Ewald , Semmler Klaus
Published: August 2009

Blades and vanes in the first stages of modern gas turbines are exposed to high thermal loads. As the resulting temperatures exceed the temperature stability of the material, it is necessary to validate the internal cooling capability of these parts experimentally, before proving their reliability in service. In the present state of the art a commonly used experimental method for evaluating heat transfer coefficients is given by the transient technique using thermochromic liquid crystals (TLCs). Though it is easily applicable for short single-pass and simple-shaped cooling channels, additional aspects must be considered for small engine-representative 3D cooling circuits that have a complex multi-pass system. Mass flow splits and space-time-dependent fluid temperatures may be noted as examples. To point out and suggest solutions for these characteristics, we therefore present a procedure for conducting transient heat transfer experiments using TLCs with respect to engine-representative, complex 3D cooling circuits in particular. General design and dimensioning guidelines are given by means of exemplary geometries. Furthermore, an approach for the experimental setup, preparation of models, and test procedure is discussed. The experiments were conducted with an engine-representative Reynolds number and Mach number and the same heat flux direction as in a real blade. We evaluated the present method and performed an uncertainty analysis where the technique demonstrated robustness for a variety of investigated geometries.
Thermoacoustic Modeling and Transfer Functions Determination for a Matrix Burner Using Unsteady CFD
Authors: Bohn Dieter , Willie James F. , Ohlendorf Nils
Published: August 2009

Turbulent combustion of a lean premixed methane-air mixture is simulated numerically using unsteady CFD. The configuration is a matrix burner suitable for stationary gas turbine applications. The geometry consists of the following: seven slots which constitute the flame holder for stabilizing the flame, a diffuser which serves the purpose of lowering the pressure loss across the burner and a combustion chamber. A contraction ensures that a recirculation zone is created close to the exit of the flame holder for anchoring and stabilizing the flame. Fuel is injected in 112 holes, 8 along each end of the 7 slots. The injected fuel meets the in-coming high velocity air stream for mixing to begin in the premixed ducts before finally entering the combustion chamber. This paper validates the cold flow velocity field and the steady flame results from CFD with measurements and investigates combustion instability in a matrix burner, the onset of which can be attributed to changes in flow variables using URANS. Particularly, the effect of the mixture strength variation caused by fluctuations in the velocity field on the unsteady heat rate inside the combustor is investigated. The fuel inlet is assumed to be choked due to the high pressure drop across it. The time lag between the time fuel is injected and the time it reaches the flame front is estimated. Quantifying this time delay (or “flight time”) helps to characterize the burner with respect to thermo-acoustic instabilities. The flame frequency response to a white noise forcing at the air inlet is determined. This is followed by the determination of the acoustic transfer matrix linking the pressure and velocity downstream and upstream of the burner/flame. This is done by using system identification that is common in control theory. The determined flame frequency response and the time lag are used in a 1D acoustic network code for determining the longitudinal eigenmodes of the combustor of the matrix burner.
Review and Analysis of Bluff Body Flame Stabilization Data
Authors: Husain Sajjad A. , Nair Ganesh , Shanbhogue Santosh , Lieuwen Tim C.
Published: August 2009

This paper compiles and analyzes bluff body stabilized flame blowoff data from the literature. Many of these studies contain semi-empirical blowoff correlations that are, in essence, Damko¨hler number correlations of their data. This paper re-analyzes these data, utilizing various Damko¨hler number correlations based upon detailed kinetic modeling for determining chemical time scales. While the results from this compilation are similar to that deduced from many earlier studies, it demonstrates that a rather comprehensive data set taken over a large range of conditions can be correlated from “first-principles” based calculations that do not rely on empirical fits or adjustable constants (e.g., global activation energy or pressure exponents). The paper then discusses the implications of these results on understanding of blowoff. Near blowoff flames experience local extinction of the flame sheet, manifested as “holes” that form and convect downstream. However, local extinction is distinct from blowoff — in fact, under certain conditions the flame can apparently persist indefinitely with certain levels of local extinction. We hypothesize that simple Damko¨hler number correlations contain the essential physics describing this first stage of blowoff; i.e., they are correlations for the conditions where local extinction on the flame begins, but do not fundamentally describe the ultimate blowoff condition itself. However, such correlations are reasonably successful in correlating blowoff limits because the ultimate blowoff event appears to be correlated to some extent to the onset of this first stage.
Development of a Guideline for the Design of Surge Control Systems
Authors: Nored Marybeth G. , Brun Klaus , Kurz Rainer
Published: August 2009

Under the Gas Machinery Research Council 2007 program, Southwest Research Institute has developed an application guideline to be used in the design and selection of centrifugal compressor surge control systems. The surge control system guideline provides a common reference on the required functionality of the surge control system components. The guideline benefits the user, system designer, and compressor manufacturer by enhancing the understanding of surge control system design to improve performance and reduce unnecessary cost. The guideline also provides methods of evaluating surge control system performance against recommended design criteria. This allows the operating company to determine if the surge control system will meet expectations. This paper highlights the critical issues of surge control system design based on the recommendations of the GMRC guideline. Due to the competing factors in the design of the compressor station, the optimal measurement instrumentation, piping layout and recycle valve are not always available. The surge control system designer must overcome these factors by designing against the limitations of the compressor station, the selected measurement instrumentation, and the various components of the control system.
An Investigation on Dynamic Characteristics of a High Speed Rotor With Complex Structure for Microturbine Test Rig
Authors: Zhang Di , Xie Yonghui , Feng Zhenping
Published: August 2009

During the research and development of 100kW class microturbine, a test rig with complex structure rotor for turbine impeller was constructed. Due to the limitation of test rig, the test rotor has to adopt cantilever configuration, which consists of shaft, connect disk with cool air holes, turbine impeller, and tie bar that joints the previous three components together. Because the rotor has a high rotational speed of 30000rpm, the ceramic rolling bearing is adopted for the test rig. In order to understand the dynamic characteristics of the rotor, a numerical model for temperature field has been developed using three dimensional finite element method firstly; then a numerical model to analyze the dynamic characteristics of high-speed ceramic rolling bearing and a numerical model to analyze the dynamic characteristics of the rotor considering the effect of temperature, shear deflection and gyroscopics have been developed too. At last, the temperature field and the dynamic characteristics of the test rig rotor were investigated in detail, some numerical results such as the critical speeds of lateral and torsional vibration, imbalance response, accelerate transient response were obtained. The numerical result of the first critical speed of lateral vibration shows good agreement with the experimental data, so the numerical model can provide accurate results for rotor dynamic analysis of such kind of rotor. Meanwhile, the critical speeds of lateral vibration and torsional vibration are far away from the operation speed of the rotor, and the response values are also very small, so that the rotor can operate reliably.
Parametric Study of On-Line Compressor Washing Systems Using CFD Analysis
Authors: Litinetsky Alexander , Litinetski Victor , Hain Yakov , Gutman Alexander
Published: August 2009

Compressor fouling is a significant operational problem faced by gas turbine operators. It negatively affects the overall gas turbine performance in terms of power output and heat rate. In order to recover gas turbine performance, operators use compressor washing. When the gas turbine cannot be stopped, on-line compressor washing is used. This technology utilizes water spray injection in front of the compressor inlet at full rotational speed. To provide good cleaning, water spray nozzles should be properly positioned and directed in order to allow uniform wetting of the compressor blades. At the same time, water droplets should not be collected on the air duct walls upstream of the compressor. Additionally, the water droplets should be big enough to be able to clean the compressor and, at the same time, small enough to avoid erosion of the blades. In order to provide gas turbine operators with guidelines for the proper selection and use of on-line washing systems, a numerical study has been carried out, based on typical washing systems installed on large industrial gas turbines. The study includes air flow analysis inside the air duct upstream of the compressor and calculations of the motion of water droplets injected into the air flow at different locations. The influence of the following parameters has been studied: droplet size, both injection direction and velocity, spray cone angle and location of the spray nozzles. “Fluent” CFD software was used. Effectiveness of the washing systems was evaluated by: a) amount of droplets entering the compressor (as mass percent of total injected water), b) uniformity of droplet mass distribution along the compressor first blade length and c) droplet accumulation on the surrounding surfaces (this being a negative phenomenon). The results showed that the effectiveness of the washing systems depends highly on the spray nozzle location (a total of six locations have been analyzed) and in each location — on droplet size and initial velocity. Special diagrams have been developed to illustrate the required optimal combinations of droplet size and initial velocity in each location of the spray nozzles. The general conclusion was that the proper design of any on-line washing system should be based on detailed CFD analysis of the motion of the injected droplets. The results could be helpful for gas turbine operators as well as for designers of the on-line washing systems.
A Forced Response Method for Annular Combustors Excited by Traveling Acoustic Pressure Waves
Authors: Baik Sanghum , Dede Mehmet
Published: August 2009

Recent progress in the development of an industry level tool for computing forced response of annular combustors is presented. Hereby, in addressing productivity issues caused by huge finite element model of full-wheel combustor, the theoretical framework of cyclic symmetry is introduced. The complex-variable theory, which originated for capturing natural frequency and mode shape characteristics of rotationally periodic structure, was extended for real-number-based finite element analysis (FEA) to solve forced response problem; specifically, a systematic method was developed to create cyclic domain replica of traveling pressure wave loading on full-wheel combustor. In this paper, theoretical descriptions of the physics-based, practical forced response analysis technique will be provided, and its implementation into building the tool of industrial level will be discussed. The technology developed herein will be verified using a simple cylindrical structure that is excited by acoustic pressure wave that travels in circumferential direction with a certain number of nodal diameter. In the end, a practical application to forced response prediction of a combustor component will be presented.
Measurements of Leakage and Power Loss in a Hybrid Brush Seal
Authors: San Andre´s Luis , Baker Jose´ , Delgado Adolfo
Published: August 2009

Simplicity, low cost and easy replacement make labyrinth seals the primary seal type in gas turbines. However, excessive leakage and potential for rotordynamic instability are well known issues. Brush seals effectively control leakage in air breathing engines, albeit only applied for relatively low-pressure differentials. Hybrid brush seals (HBS) are an alternative to resolve poor reliability resulting from bristle tip wear while also allowing for reverse rotation operation. The novel configuration incorporates pads contacting the shaft; which under rotor spinning; lift-off due to the generation of a hydrodynamic pressure. The ensuing gas film prevents intermittent contact; thus lowering the operating temperature and thermal distortions, and even eliminating bristle wear. The hybrid brush seal improves sealing, is more durable and reliable than conventional brush seals, and allows reverse shaft rotation without seal damage. The paper presents measurements of power loss and leakage in a hybrid brush seal (HBS) for increasing pressure differentials over a range of rotor speeds. The test HBS, Haynes-25 bristle pack [∼850 bristles/cm] and 45° lay angle, is 166.4 mm in diameter and integrates 20-arcuate pads connected with thin EDM-webs to the seal casing. The webs are designed with low radial stiffness to allow for rotor excursions and high axial stiffness to avoid pad pitching motions resulting from high pressure differentials across the seal. Measured drag power at low rotor speeds (< 11 m/s at 1,300 rpm) decreases as the pressure differential across the seal increases. At a fixed rotor speed, a significant drop in drag torque (and drag power) ensues as the supply pressure increases, thus demonstrating that a gas film separates the rotor from the seal pads. Additionally, the operating temperature measured at the rotor/seal interface remains approximately constant (∼24°C) during tests with shaft rotation (power loss and drag torque measurements) under pressurized conditions; indicating that the rotor and seal pads are not in contact. Flow rate measurements at room temperature (25°C) show an improved sealing ability with a leakage reduction of about 36%, when compared to a first generation shoed-brush seal (SBS). The HBS calculated effective clearance (∼50 μm) is approximately 70% smaller than the radial clearance (∼180 μm) of an ideal non contacting seal with similar rotor diameter. Improved brush seal technology will increase the efficiency of gas turbines while also aiding to improve the engine stability and to reduce vibrations.
A Study of Modest CFD Models for the Design of an Annular Diffuser With Struts for Swirling Flow
Authors: Feldcamp G. K. , Birk A. M.
Published: August 2009

Cold flow experiments were conducted to study swirling flows in an annular diffuser with various strut configurations. Experimental results at 0°, 20°, and 40° of inlet swirl were obtained. Measured properties included detailed inlet and exit traverses using three and seven hole pressure probes, surface pressure taps on the diffuser wall, and surface flow visualization techniques. Evaluation of the diffuser and strut performance was based on pressure recovery, and detailed pressure distributions. The experimental results have been studied further using Reynolds Averaged Navier Stokes (RANS) based Computational Fluid Dynamics (CFD). These models are of modest size (less than four million volumes). Several turbulence models have been used to predict the performance of the annular diffuser with struts. Both high and low Reynolds number implementations of the turbulence models have been investigated. The results show that modest CFD models can be used with confidence to design these devices.
A Discrete Model to Consider the Influence of the Air Flow on Blade Vibrations of an Integral Blisk Compressor Rotor
Authors: Beirow Bernd , Ku¨hhorn Arnold , Schrape Sven
Published: August 2009

The influence of the aerodynamic coupling in the forced response analysis of a HPC test-blisk is studied by means of a reduced order mechanical model. In the first step this equivalent blisk model (EBM) is derived based on a finite element analysis of the disk from design and an adjustment to experimentally determined blade alone frequencies in order to consider the real blade mistuning. Applying the EBM — so far not considering the air flow influence — to carry out forced response analyses due to a rotating excitation acting on the stationary blisk, a maximum blade displacement amplification of more than 50% has been calculated comparing the tuned and the mistuned blisk. Aiming at an additional consideration of the air flow, fully coupled computations of the fluid structure interaction (FSI) are exemplarily carried out for elastically supported blades in a cascade arrangement. The results are used to calibrate simple mass-spring-damper models from which quantities of additional aerodynamic elements in terms of a consideration of co-vibrating air masses, air stiffening and aerodynamic damping are derived. Based on this information the EBM is extended to a so called advanced EBM. Aerodynamic influences are considered assigning the aerodynamic properties to each blade in dependence on the inter blade phase angle (IBPA). Forced response analyses, now including all aerodynamic influences, show that for an extreme application of a rear blisk close to the combustion chamber and under MTO conditions a strong smoothing of originally localized vibration modes occurs. The maximum blade displacement amplification due to mistuning is decreased from more than 50% to below 12% for the first blade flap mode.
Performance Comparison of More Electric Engine Configurations
Authors: Pluijms Antoon , Schmidt Klaus-Juergen , Stastny Karel , Chibisov Borys
Published: August 2009

An analytical study was undertaken to investigate the fuel burn potential of More Electric Engine (MEE) configurations using the performance model of a 2-shaft high BPR 20–30 klbf turbofan in revenue service. The 3 following power off-take configurations were compared: an HP-generator, an LP-generator, and a split-power generator (small HP starter/generator and a main LP generator). For this study, because of the small performance differences, high accuracy steady-state and transient performance models must be used. For steady-state operating conditions, the design point was modified and the off-design redline margins were calculated; ground and flight idle settings were adjusted to yield both the lowest possible fuel burn and residual thrust within the surge margin of the compressor, and the resulting short range mission fuel burn was calculated. For transient conditions, the thrust response, as well as both HPC and LPC surge margin lapse during engine acceleration and deceleration, had to maintain those of the baseline engine and fulfill certification requirements. This was achieved by modifying the idle settings and acceleration/deceleration schedules. Subsequently, the resulting short range mission fuel burn was calculated. Lastly, an introduction to the business case is provided with a simple cost-effectiveness calculation. This study was an initial investigation into MEE’s that focused primarily on the propulsion unit. For further in-depth studies, it is recommended to consider in detail the business model, aircraft weight issues, and the interaction propulsion performance and aircraft performance.