bluuebunny/crossref_metadata_2025 huggingface dataset:
A Numerical Investigation of Rotating Instability in Steam Turbine Last Stage
Authors: Zhang L. Y. , He L. , Stu¨er H.
Published: May 2012

In the present study, the unsteady flow phenomenon (identified as rotating instability) in the last stage of a low-pressure model steam turbine operated at very low mass flow conditions is studied through numerical investigations. This kind of instability has been observed previously in compressors and is believed to be the cause of high stress levels associated with the corresponding flow-induced blade vibrations. The overall purpose of the study is to enhance the understanding of the rotating instability in steam turbines at off design conditions. A numerical analysis using a validated unsteady nonlinear time-domain CFD solver is adopted. The 3D solution captures the massively separated flow structure in the rotor-exhaust region and the pressure ratio characteristics around the rotor tip of the test model turbine stage, which compare well with those observed in the experiment. A computational study with a multi-passage whole annulus domain on two different 2D blade sections is subsequently carried out. The computational results clearly show that a rotating instability in a turbine blading configuration can be captured by the 2D model. The frequency and spatial modal characteristics are analyzed. The simulations seem to be able to predict a rotating fluid dynamic instability with the similar characteristic features to those of the experiment. In contrast to the previous observations and conventional wisdom, the present work reveals that the formation and movement of the disturbance can occur without 3D and tip-leakage flows, even though a quantitative comparison with the experimental data can only be expected to be possible with full 3D unsteady solutions.
Experimental Characterization of the Influence of Auxiliary Devices on the Noise Generated by Industrial Centrifugal Fan and Correlation to the Geometrical and Fluid Dynamic Parameters
Authors: Cadorin M. , Pinelli M. , Podeschi E. , Pompoli F. , Zanardi A.
Published: May 2012

In recent years, the aerodynamic noise generated by centrifugal fans is receiving increasing attention because of strict environmental noise level restrictions and customer demands. The noise generated by fans is due to aerodynamic sources and to other several sources, such as, for instance, by the fan drive, by bearings and gearing, and, when present, by the inverter. Additional noise sources can be also due to structural resonance effects induced by periodic forces associated with the blade passing frequency or vortex shedding. Usually, these additional noise sources are dominated by aerodynamic noise generated by the fan, in particular when the intake and outlet of the fan are free. On the other side, if fan intake and outlet are ducted, the additional sources can relevantly contribute to overall sound generation. In this paper, an experimental characterization of the noise generated by industrial centrifugal fans when both inlet and outlet are ducted is presented. To do this, an experimental facility has been design and set up, and the sound power measured by means of the procedures outlined in the ISO 3746 international standard. A number of different type of centrifugal fan (straight-, forward- and backward blade) in different working conditions were tested, resulting in 133 different runs. These amount of data were then processed and a general formula for fan noise estimation obtained as a function of the geometrical and fluid dynamic parameters is derived. Moreover, specific coefficients with respect to blade geometry for the determination of the A-weighted frequency spectrum are presented. Finally, auxiliary devices or other features, such as inverter, thickness of the casing, acoustic insulation, electric motor shaft, are analyzed and some general rules to estimate their influence on sound power level quantified.
Compressor Fouling Modeling: Relationship Between Computational Roughness and Gas Turbine Operation Time
Authors: Melino Francesco , Morini Mirko , Peretto Antonio , Pinelli Michele , Spina Pier Ruggero
Published: May 2012

Gas turbine axial compressor performance is heavily influenced by blade fouling; as a result, the gas turbines efficiency and producible power output decrease. Performance degradation of an axial compressor stage due to fouling can be analyzed by means of simulation through Computational Fluid Dynamics (CFD) codes. Usually these methods reproduce the deteriorated blades by increasing their surface roughness and/or thickness [1]. Another approach is the scaling of compressor stage performance maps. A model based on stage-by-stage techniques was presented in a previous work. This model is able to estimate the modifications of the overall compressor performance map as a function of the operating hours [2]. The aim of the present study is to combine these two different approaches in order to relate the increase of blade computational surface roughness with compressor operating hours.
The Use of Mathcad Functions in Teaching an Undergraduate Aerospace Propulsion Course
Authors: Macfarlan Kevin H. , McClain Stephen T.
Published: May 2012

This paper investigates the educational efficacy of using a set of Mathcad functions in teaching a typical undergraduate course in Aerospace Propulsion. This approach runs counter to two other methods often utilized in this type of course. The first, a traditional, time-consuming technique, requires students to solve problems by hand using tabular property data. The second is to supply students with a computer algorithm which calculates engine performance in a “black-box” fashion. The approach in this study attempted to bridge the divide between these two methods, incorporating the best features of each. The inclusion of Mathcad functions reduced the time required in performing mindless interpolations while still requiring the students to master the basic Thermodynamic principles required when modeling modern gas turbine engines, as verified by in-class testing. Feedback from students is presented, as well as some unexpected lessons learned from the study. Modifications to the functions are also provided.
Elastomer Compatibility Test of Alternative Fuels Using Stress-Relaxation Test and FTIR Spectroscopy
Authors: Liu Yue , Wilson Christopher W. , Blakey Simon , Dolmansley Tim
Published: May 2012

Global efforts to reduce CO2 emissions and to tackle the problem of depleting petroleum resources have stimulated the exploitation of alternative fuels in the aviation industry. One crucial aspect amongst others is to investigate the compatibility of alternative fuels with elastomeric materials currently used in gas turbine engines. However, little knowledge about this has been understood so far for commercial aircrafts under real engine conditions. This study combines Stress-Relaxation test and FTIR spectroscopy techniques to look at the effect of alternative fuels on O-rings made from different materials; such as nitrile, fluorocarbon and fluorosilicone. Results indicated that after immerged in fuels for a period of time while simultaneously being compressed at certain temperatures, the fluorocarbon O-rings showed minimum change in the eight types of fuels tested. This meant they are compatible with these fuels, with the nitrile O-rings changing the most. Furthermore, FT-SPK+20% hexanol caused the biggest relaxation in fluorosilicone O-rings but had the smallest effect on nitrile ones while all fuels presented similar behavior in fluorocarbon. FTIR spectrum analysis showed molecular composition changes are dependent on the reactions between different materials and fuels. For fluorosilicone O-rings, the absorbance reduction from 1150 to 1050 cm−1 was caused by the breakdown of (Si-O-Si) while the formation of new O-H bonds enhanced the intensity from 3000 to 2800 cm−1. For fluorocarbon O-rings, obvious increase in absorption could be found in the region from 1400 to 650 cm−1, while from 3000 to 2800 cm−1, the absorption decreased. Multiple linear regression analysis indicated a highly correlated relationship between the chemical structure changes and the force relaxation.
Intelligent Operation of Siemens (SGT-300) DLE Gas Turbine Combustion System Over an Extended Fuel Range With Low Emissions
Authors: Bulat Ghenadie , Liu Kexin , Brickwood Gavin , Sanderson Victoria , Igoe Brian
Published: May 2012

The use of an innovative, intelligent control algorithm applied to the Siemens SGT-300 DLE engine is described. The algorithm ensures stable operation and minimises emissions over a wide variation in fuel composition. The Siemens 8MW class SGT-300 gas turbine has been in operation at the University of New Hampshire (USA) since 2006. As well as operating on natural gas or diesel, the engine also operates on a gas processed from a landfill. These gases have a variable Wobbe Index (WI) covering the range 29.7 to 49 MJ/m3. No modifications have been required to the standard DLE combustion hardware. Introduction of the intelligent control algorithm has been instrumental in achieving this tri-fuel capability. Accumulation of more than 10 000 hours running on non-standard fuel has been achieved. The intelligent control algorithm exploits knowledge of the stable operating window through continual modification of the fuel schedule to avoid both lean blow out and high metal temperatures. Operationally, this results in a reduction in the NOx emissions, through controlling the unmixedness, and higher engine reliability, through the response of the algorithm to flame stability. Combining these advantages the control algorithm can deliver reliable engine operation on variable composition fuels when using standard combustion hardware achieving single digit NOx emissions not only on natural gas but also on processed landfill gas. This paper describes the control algorithm and presents results of the development from high pressure combustion rig and engine development test to field operation with both natural gas and processed landfill gas.
Experimental Characterization of Synchronous Vibrations of Blades
Authors: Jauregui Juan C. , Sen Mihir , Lopez-Cajun Carlos S.
Published: May 2012

Blades in a turbine rotor synchronize their individual vibrations after a certain period of time. Each blade has a slightly different natural frequency due to manufacturing errors, and it responds individually to external excitations. Nevertheless, after a period of time the blades synchronize and vibrate at the same frequency due to weak interactions between them. In this work, experiments are reported that identify blade synchronization. Individual blade vibrations were measured simultaneously under different conditions. Measurements were made with accelerometers attached to the tip of each blade which were very light in comparison with the mass of the blade. The blades were first excited by an impact force and the natural frequencies were identified. Then, the blades were excited by airflow and individual blade-tip vibrations were recorded at several subsequent time intervals. Synchronization is analyzed through the correlation between the time responses of every blade. The correlation was calculated at each of these intervals and the results were plotted in maps. It was found that the synchronization evolves as a function of time; it is high after a medium time period and reduced at longer time periods. Thus blade synchronization presents a long wave variation that could be a source of a very low frequency vibration. It was also possible to say that synchronization is dominated by the structure of the rotor.
Life Assessment by Fracture Mechanics Analysis and Damage Monitoring Technique on Combustion Liners
Authors: Altunlu A. Can , van der Hoogt Peter , de Boer Andre´
Published: May 2012

A methodology has been developed and tested including a multi-disciplinary framework towards integrated analysis of gas turbine combustors. The sub-elements consist of combustion dynamics, stress and modal analysis, fracture mechanics and structural health monitoring have been interlinked indicating the damage evaluation to life assessment. The interaction between the interrelated combustion driven flame dynamics, acoustic pressure fluctuations and liner wall vibration has been investigated in the laboratory combustor test system. During the operation, the combustion, acoustics and wall vibrations have been coupled together. The dynamic combustion process generates high amplitude pressure oscillations resulting in vibration of the liner structure at about constant elevated temperature in base load operation. The thermo-acoustic instabilities have a significant destructive impact on the life of the liner material due to high cyclic vibration levels at high temperature. A structural health monitoring (SHM) method has been established to identify the damage, detect the flaw existence and determine the location, severity and progress of the damage for the combustion liners. Vibration-based and acoustic emission (AE) techniques have been applied in the test system to assess the structural behavior. The applicability of the technique has been tested by examining the dynamic modal parameters of the structure. The method enables a reliable assessment on the liner specimen at elevated temperatures by means of non-destructive evaluation under continuous operation of the combustor. The combustion liner specimen material has been assessed by calculating the near-tip fields at the crack tip by finite element based stress and fracture mechanics analysis. An algorithm based on J-Integral has been utilized to analyze the crack growth behavior under various loading conditions considering both linear and non-linear elastic fracture mechanics concepts. The location and the direction of the cracking on the liner specimen have been predicted. The presented work interrelates the different mechanisms in gas turbine combustors and the applicability of the concepts has been verified and validated in the test systems.
Viscous Controlled Vortex Design of a 1.5-Stage Axial Subsonic Test Turbine
Authors: Deng Qingfeng , Zheng Qun , Liu Chunlei , Zhang Hai , Luo Mingcong
Published: May 2012

This paper presents a Viscous Controlled Vortex (VCV) design practice of an experimental turbine. The Controlled Vortex (CV) design method was modified to take the local viscous losses into account. The method began with the analysis of initial turbine geometry of a 1.5-stage turbine, which was designed by a developed CV design approach based on a prescribed pressure distribution to resolve the circulation requirements. Then the loss corrections were combined into the terms of the CV design equations and the pressure distributions are refined. The design and optimization of the 1.5-stage axial test turbine demonstrates the effectiveness of this technique. A reduction of secondary flows and a corresponding increase of stage efficiency have achieved.
A Summary of Stall Warning and Suppression Research With Micro Tip Injection
Authors: Feng Lin , Tong Zhiting , Geng Shaojuan , Zhang Jingxuan , Chen Jingyi , Nie Chaoqun
Published: May 2012

Micro tip injection is a stall control technique in which engineers inject small-flowrate yet high-velocity jets into the tip region of axial compressor rotor blades to extend the stable operation range, while keeping the entire compressor characteristic line nearly unchanged. This paper summarizes the related research performed in Chinese Academy of Sciences in the past several years, ranging from understanding the fundamental mechanisms to demonstrating the technology in a laboratory compressor. A brief review in tip injection research indicates that for each compressor, there exists a critical injection momentum ratio, below which the injection flows may work differently from those over the critical value. Detailed casing unsteady static pressure measurements and the related sophisticated data processing proved the existence of self-induced unsteady tip leakage flow, which was also confirmed by unsteady CFD simulations. It is showed that this kind of unsteadiness in tip leakage flow might be responsible to make the auto-correlation coefficient drop significantly as the compressor operated at lower flowrate close to stall limit. This warning signal, which was first successfully used by other research groups with little care taken on its flow mechanism, appears much earlier than the classical modal-wave or spike stall precursors. Micro tip injection was effective if it interacts with the unsteady tip leakage flow long before the stall precursor emerges. As a technology demonstration, a DSP board with a build-in auto-correlation early-stall-warning algorithm and an on-off controller of injection valve worked well to drive a micro tip injection system and successfully delayed the stall in a low-speed compressor with much less jet flows than the continuous micro tip injection.
Heat Transfer Enhancement in a Rectangular (AR=3:1) Channel With V-Shaped Dimples
Authors: Jordan C. Neil , Wright Lesley M.
Published: May 2012

An alternative to ribs for internal heat transfer enhancement of gas turbine airfoils is dimpled depressions. Relative to ribs, dimples incur a reduced pressure drop, which can increase the overall thermal performance of the channel. This experimental investigation measures detailed Nusselt number ratio distributions obtained from an array of V-shaped dimples (δ / D = 0.30). Although the V-shaped dimple array is derived from a traditional hemispherical dimple array, the V-shaped dimples are arranged in an in-line pattern. The resulting spacing of the V-shaped dimples is 3.2D in both the streamwise and spanwise directions. A single wide wall of a rectangular channel (AR = 3:1) is lined with V-shaped dimples. The channel Reynolds number ranges from 10,000–40,000. Detailed Nusselt number ratios are obtained using both a transient liquid crystal technique and a newly developed transient temperature sensitive paint (TSP) technique. Therefore, the TSP technique is not only validated against a baseline geometry (smooth channel), but it is also validated against a more established technique. Measurements indicate that the proposed V-shaped dimple design is a promising alternative to traditional ribs or hemispherical dimples. At lower Reynolds numbers, the V-shaped dimples display heat transfer and friction behavior similar to traditional dimples. However as the Reynolds number increases to 30,000 and 40,000, secondary flows developed in the V-shaped concavities further enhance the heat transfer from the dimpled surface (similar to angled and V-shaped rib induced secondary flows). This additional enhancement is obtained with only a marginal increase in the pressure drop. Therefore, as the Reynolds number within the channel increases, the thermal performance also increases. While this trend has been confirmed with both the transient TSP and liquid crystal techniques, TSP is shown to have limited capabilities when acquiring highly resolved detailed heat transfer coefficient distributions.
Multi-Row Inverse Method Based on the Adjoint Optimization
Authors: Yang Jinguang , Huang Xiuquan , Wu Hu
Published: May 2012

The conventional inverse method possesses several disadvantages: experience-dependence, inconvenience for multi-row application and lots of human intervention. This hinders it from becoming a routine design tool. In the present paper, those mentioned shortcomings are conquered by combining the inverse method with a multi-row adjoint method, which could give an automatic and optimized design in a multi-row enviroment. The principles of the inverse method and adjoint method are first introduced. Then a derivation for the adjoint equation based on the Euler inverse method is conducted, and the corresponding adjoint boundary conditions are deeply discussed. After that, the developed inverse method is validated by recovering a turbine stator from a different initial shape. And the validity of the proposed adjoint based inverse method are illustrated by the redesign of a 1-1/2 turbine stage. Finally a comparison is made between the inverse method, the adjoint method based inverse method and the adjoint method based direct method.
PIV Study on the Dimple Mid-Plane of a Narrow Rectangular Channel With Dimples Applied to One Wall
Authors: Tran Lucky V. , Valentino Michelle I. , Saha Abhishek , Slabaugh Carson D. , Ricklick Mark , Kapat J. S. , Basu Saptarshi
Published: May 2012

This paper presents an investigation of the fluid flow in the fully developed portion of a rectangular channel (Aspect Ratio of 2) with dimples applied to one wall at channel Reynolds numbers of 20,000, 30,000, and 40,000. The dimples are applied in a staggered-row, racetrack configuration. Results for three different dimple geometries are presented: a large dimple, small dimple, and double dimple. Heat transfer and aerodynamic results from preceding works are presented in Nusselt number and friction factor augmentation plots as determined experimentally. Using particle image velocimetry, the region near the dimple feature is studied in detail in the location of the entrainment and ejection of vortical packets into and out of the dimple; the downstream wake region behind each dimple is also studied to examine the effects of the local flow phenomenon that result in improved heat transfer in the areas of the channel wall not occupied by a feature. The focus of the paper is to examine the secondary flows in these dimpled channels in order to support the previously presented heat transfer trends. The flow visualization is also intended to improve the understanding of the flow disturbances in a dimpled channel; a better understanding of these effects would lead the development of more effective channel cooling designs.
Combustion Characteristics of Fuel Staged Combustor for Aeroengines at LTO Cycle Conditions
Authors: Yamamoto Takeshi , Shimodaira Kazuo , Kurosawa Yoji , Yoshida Seiji
Published: May 2012

In JAXA, combustion technologies have been developed with an aggressive target that is an 80% NOx reduction of CAEP/4. For the drastic reduction of NOx emissions, a fuel nozzle was designed based on the lean staged combustion concept. This paper describes single-sector combustor tests of a fuel staged combustor at ICAO LTO cycle conditions of an assumed engine with rated output of 40 kN and overall pressure ratio of 25.8. The results showed that the combustor enables a 77% reduction of the NOx standard of CAEP/4.
The Effect of Transpiration on Discrete Injection for Film Cooling
Authors: Natsui G. , Johnson P. L. , Torrance M. C. , Ricklick M. A. , Kapat J. S.
Published: May 2012

A segment of permeable wall is installed near a row of cylindrical film holes, parallel to the flow and inclined at 35 degrees. Coolant is forced through both the permeable wall and the film holes resulting in a downstream film composed of both transpired and discretely injected coolant. The permeable wall extends 1.5 cylindrical hole diameters in the flow direction. The effects on the aerodynamic performance and cooling downstream of the row of cylindrical holes in the presence of transpiration is studied numerically with a procedure validated by hot-wire anemometer and temperature sensitive paint measurements. The hydrodynamic boundary layer in the presence of film and adiabatic film cooling effectiveness downstream of single and coupled film sources are compared with numerical predictions. The performance of the coolant film is predicted in order to understand the sensitivity of cooling and aerodynamic losses on the relative positioning of the two sources at each blowing ratio. The results indicate that a coupling of the two sources allows a more efficient use of coolant by generating a more uniform initial film. With careful optimization the discrete holes can be placed farther apart laterally and operate at a lower blowing ratio with a transpiration segment making the large deficits in cooling effectiveness mid-pitch less severe, overall minimizing coolant usage. Comparisons of linear superposition predictions of the two independent sources with the corresponding coupled scenario indicate the two films positively influence one another and surpass additive predictions of cooling. All relative placements have an overall beneficial effect on the cooling seen by the protected wall. Some cases show an increase in area-averaged film cooling effectiveness of 300% along with a 50% increase in aerodynamic loss coefficient by injecting an additional 10% coolant. In this study the downstream transpiration placement is found to perform best of the three geometries tested while considering cooling, aerodynamic losses, local uniformity and manufacturing feasibility. With further study and optimization this technique can potentially provide more effective thermal protection at a lower cost of aerodynamic losses and spent coolant.
Design and Experimental Study of Organic Rankine Cycle (ORC) and Radial Turbine
Authors: Kang Seok Hun , Chung Dae Hun
Published: May 2012

The Organic Rankine Cycle (ORC), which generates electric power by using a low temperature heat source, is developed and an experimental study is conducted. A radial turbine and high speed DC generator are also designed and developed in this study. R245fa is adopted as a working fluid, considering the operation conditions of the cycle and its environmental friendly characteristics. Experiments are conducted to analyze the operation characteristics and performance of the developed ORC. The efficiencies of the cycle and turbine, electric power and temperature versus entropy diagram of the developed ORC with respect to the operation condition are investigated through experiments. The factors that influence the efficiency of the developed ORC are analyzed and discussed.
Rotor Dynamics Analysis and Testing of a Turbomolecular Pump Rotor-Bearing System
Authors: Hsu Chih-Neng , Chiang Hsiao-Wei D. , Huang Yu-Meng , Fu Ying-Chia , Hsu Kuo-Hsun , Li Chao-Huan
Published: May 2012

This study is focused on the dynamic characteristics of a vertical turbomolecular pump (TMP) rotor-bearing system. The research methods can be divided into two parts, which are numerical analysis and experimental measurements. In numerical analysis, we use the finite element analysis software DyRoBeS and ANSYS to construct a two- and three-dimensional models of the rotor-bearing system. In the analysis process, by using the pump system assembly testing data, we can verify the rotor-bearing system finite element models under different boundary conditions. Next, we calculate the Campbell diagram to study the dynamic characteristics of the rotor-bearing system, and to compare with the experimental results to verify the models. Finally, we found the relationship between the rotor critical speed and the bearing stiffness in order to study the design of the molecular pump rotor and the bearing system. Experimental measurements were divided into two parts: static modal tests and dynamic measurements. Static modal tests can provide the natural frequencies of the rotor-bearing system. Waterfall diagrams of the dynamic tests can measure the pump system critical speed from zero speed up to the working speed crossing, and to insure that the pump working speed is far from the critical speed of at least 10% in the safe margin. In summary, the results of the experimental measurements and numerical analysis can provide the basis for the design tool for turbomolecular pump rotor-bearing system in order to identify and prevent pump vibrations.
A Method for Theoretical Prediction of Frequency Scatter Ranges for Freestanding Forged Steam Turbine Blades
Authors: Gro¨nsfelder Thomas , Hofbauer Alexander , Richter Christoph H.
Published: May 2012

Large steam turbine end stage rotating blades are commonly manufactured by forging and machining to the final geometry. As in every manufacturing process certain geometric tolerances have to be granted. In particular, the allowed tolerances on the airfoil geometry do have a significant influence on the natural frequencies of the final blades. The resulting frequency scatter is appreciated in terms of mistuning the whole ring of blades, as an adequate mistuning has shown advantages under unstalled flutter conditions. An excessively large band is not acceptable, due to the fact that the blade frequencies are tuned to not-coincide with harmonic multiples of the rotor speed under stationary operation. This paper describes a theoretical method for prediction of a manufactured blade design frequency scatter, based only on nominal geometric information about the blade. Therefore, it is suited to be used during the development of a blade without having a prototype produced. The method is divided into three different steps. First, a numerical experiment is performed creating a number of geometrically modulated FE models. These models are used in a calculation of natural frequencies. Second, these frequencies serve as input for an identification of a simple algebraic representation of the frequencies. This allows a fast calculation by interpolation without the need to process the FE models. Third, the identified simplified equation is used in conjunction with different optimization algorithms for analysis of the specific design characteristics. The validity of the chosen matrix equation is shown by comparison to the FE calculations, before different blade types are investigated. Characteristics and options of the implemented optimization routines are discussed. Finally, the comparison of differently tuned blade types are used to demonstrate the capabilities of the described algorithm.
Optimization Strategy for a Coupled Design of the Last Stage and the Successive Diffuser in a Low Pressure Steam Turbine
Authors: Musch Christian , Stu¨er Heinrich , Hermle Georg
Published: May 2012

In this study, an effective yet numerically simple approach for a coupled design of the last stage running blade and diffuser is presented. The method applied uses a 2-dimensional streamline curvature code combined with a boundary layer solver for the prediction of flow separation within the diffuser. An accurate representation of the diffuser flow is vital for the assessment of the overall performance. Thus the major influences from the turbine stage on the diffuser flow, i.e. the tip leakage jet and the swirl of the flow, are taken into account. Secondary effects like blade wakes are neglected. The basic capability of the method to correctly represent the flow is demonstrated by a comparison with 3-dimensional CFD simulations of a sample configuration. Solid correlation can be found between both cases. For the optimization process, a genetic algorithm is used. Optimization parameters include the blade exit angle and the diffuser contour. The results of the optimization are again scrutinized with the assistance of 3-dimensional CFD simulations.
Lowering Equivalence Ratio Limit for Stable Combustion in Gas Turbines by Injection of Free Radicals
Authors: Levy Yeshayahou , Erenburg Vladimir , Sherbaum Valery , Ovcharenko Vitali , Rosentsvit Leonid , Chudnovsky Boris , Herszage Amiel , Talanker Alexander
Published: May 2012

Lean premixed combustion is one of the widely used methods for NOx reduction in gas turbines (GT). When this method is used combustion takes place under low Equivalence Ratio (ER) and at relatively low combustion temperature. While reducing temperature decreases NOx formation, lowering temperature reduces the reaction rate of the hydrocarbon–oxygen reactions and deteriorates combustion stability. The objective of the present work was to study the possibility to decrease the lower limit of the stable combustion regime by the injection of free radicals into the combustion zone. A lean premixed gaseous combustor was designed to include a circumferential concentric pilot flame. The pilot combustor operates under rich fuel to air ratio, therefore it generates a significant amount of reactive radicals. The experiments as well as CFD and CHEMKIN simulations showed that despite of the high temperatures obtained in the vicinity of the pilot ring, the radicals’ injection from the pilot combustor has the potential to lower the limit of the global ER (and temperatures) while maintaining stable combustion. Spectrometric measurements along the combustor showed that the fuel-rich pilot flame generates free radicals that augment combustion stability. In order to study the relevant mechanisms responsible for combustion stabilization, CHEMKIN simulations were performed. The developed chemical network model took into account some of the basic parameters of the combustion process: ER, residence time, and the distribution of the reactances along the combustor. The CHEMKIN simulations showed satisfactory agreement with experimental results.
150 kW Class Two-Stage Radial Inflow Condensing Steam Turbine System
Authors: Hayashi Kazutaka , Shiraiwa Hiroyuki , Yamada Hiroyuki , Nakano Susumu , Tsubouchi Kuniyoshi
Published: May 2012

A prototype machine for a 150 kW class two-stage radial inflow condensing steam turbine system has been constructed. This turbine system was proposed for use in the bottoming cycle for 2.4 MW class gas engine systems, increasing the total electrical efficiency of the system by more than 2%. The gross power output of the prototype machine on the generator end was 150kW, and the net power output on the grid end which includes electrical consumption of the auxiliaries was 135kW. Then, the total electrical efficiency of the system was increased from 41.6% to 43.9%. The two-stage inflow condensing turbine system was applied to increase output power under the supplied steam conditions from the exhaust heat of the gas engines. This is the first application of the two-stage condensing turbine system for radial inflow steam turbines. The blade profiles of both high- and low-pressure turbines were designed with the consideration that the thrust does not exceed 300 N at the rated rotational speed. Load tests were carried out to demonstrate the performance of the prototype machine and stable output of 150 kW on the generator end was obtained at the rated rotational speed of 51,000 rpm. Measurement results showed that adiabatic efficiency of the high-pressure turbine was less than the design value, and that of the low-pressure turbine was about 80% which was almost the same as the design value. Thrust acting on the generator rotor at the rated output power was lower than 300 N. Despite a lack of high-pressure turbine efficiency, total thermal efficiency was 10.5% and this value would be enough to improve the total thermal efficiency of a distributed power system combined with this turbine system.
Autoignition Limits of Hydrogen at Relevant Reheat Combustor Operating Conditions
Authors: Fleck Julia , Griebel Peter , Steinberg Adam M. , Sto¨hr Michael , Aigner Manfred , Ciani Andrea
Published: May 2012

The use of highly reactive fuels in the lean premixed combustion systems employed in stationary gas turbines can lead to many practical problems, such as unwanted autoignition in regions not designed for combustion. In the present study, autoignition characteristics for hydrogen, diluted with up to 30 vol. % nitrogen, were investigated at conditions relevant to reheat combustor operation (p = 15 bar, T > 1000 K, hot flue gas, relevant residence times). The experiments were performed in a generic, optically accessible reheat combustor, by applying high-speed imaging and Particle Image Velocimetry (PIV). Autoignition limits for different mixing section (temperature, velocity) and fuel jet (N2 dilution) parameters are described. The dominant factor influencing autoignition was the temperature, with an increase of around 2% leading to a reduction of the highest possible H2 concentration without “flame-stabilizing autoignition kernels” of approximately 16 vol. %. Furthermore, the onset and propagation of the ignition kernels were elucidated using the high-speed measurements. It was found that the ability of individual autoignition kernels to develop into stable flames depends on the initial position of the kernel and the corresponding axial velocity at that position. While unwanted autoignition occurred prior to reaching the desired operating point for most investigated conditions, for certain conditions the reheat combustor could be operated stably with up to 80 vol. % H2 in the fuel.
Determination of Crack Initiation on Blade L-1 LP Steam Turbine Blades: Part 2—Computational Analyses
Authors: Kellner Josef , Kubin Zdenek , Hlous Jan , Prchlik Lubos
Published: May 2012

After ten years operation, one L-1 blade of steam turbine with large output power suddenly fell down. All 6 LP rotors (two machines) were checked and many cracks on L-1 blades were found. Due to economic reason, new blades were made with same geometry, but with better material in term of yield limit. Some L-1 stages were made from blades of original material, the rest was made from new material blades. Also the tip-timing measurement was installed on two L-1 stages to monitor blades. The investigation of blade cracks reason started. After one year of smooth operation there was an inspection. Surprisingly, the cracks were indicated again. The blades from new material had relatively more cracks than the original blades. This part describes some computational analyses which were done. For example, the modal analysis and forced vibration of mistuned bladed disk and of whole rotor train with attached L-1 blades, CFX and CFD calculations were done. The measurement of rotor torque, damping, Tip-Timing, experimental modal analysis of blades and rotor train and material test are presented in Part 1 – [7]. Application of both approaches (experimental and computational) rejected some hypotheses and revealed which hypotheses should be analyzed in a deeper way. The unstalled flutter has been identified as the most probable blade failures.
Computational Fluid Dynamics Drag Prediction and Decomposition for Propulsive System Integration
Authors: Sibilli Thierry , Savill Mark
Published: May 2012

The prediction and decomposition of drag associated to Propulsive System Integration (PSI) is investigated applying a methodology based on entropy variations in the flow and the momentum conservation theorem. This advanced prediction method can decompose the total drag in to viscous, wave, induced and spurious drag, allowing a better understanding of the flow. The spurious drag, due to numerical errors, can be eliminated reducing the dependency of the solution on the grid quality. Four applications are presented: two wing-body configurations, a wing-body-nacelle-pylon with trough-flow-nacelle, and a wing-body-nacelle-pylon with a very-high-bypass-ratio engine in power-on condition. One objective is to minimize grid resolution to enable design optimization.
Impeller–Vaned Diffuser Interaction in a Centrifugal Compressor at the Best Efficiency Point
Authors: Gaetani P. , Persico G. , Mora A. , Dossena V. , Osnaghi C.
Published: May 2012

Centrifugal compressors find a huge number of applications in industry and in aero-engines; the detailed comprehension of the complex fluid-dynamic mechanisms occurring in these machines is crucial to improve their efficiency and their operating range. The paper presents a study on the impeller–vaned diffuser interaction in a high-performance compressor stage, in the frame of a wide experimental campaign devoted to the comprehension of unsteady flows in centrifugal compressor stages. The paper focuses on the best efficiency operating point. Data were collected in the impeller–vaned diffuser gap by applying a fast response probe. The impeller runs at 12500 RPM and the peripheral Mach number is 0.77. At first, data are reduced to highlight the main flow structures released by the impeller. CFD simulations — first verified against experimental data — were also performed to get detailed information of the flow field inside the impeller. The core of the paper is the discussion of the impeller–diffuser interaction. Results evidence the effect of the diffuser on the impeller in terms of static pressure and flow velocity. Moreover, the interaction process makes the average flow rate discharged by an impeller channel and the power exchange to be unsteady.
Implementation of an Edge-Based Navier-Stokes Solver for Unstructured Grids in Graphics Processing Units
Authors: Gisbert Fernando , Corral Roque , Pastor Guillermo
Published: May 2012

The implementation of an edge-based three-dimensional RANS equations solver for unstructured grids that runs on both central processing units (CPUs) and graphics processing units (GPUs) is presented. This CPU/GPU duality is kept without double-writing the code, reducing programming and maintenance costs. The GPU implementation is based on the standard OpenCL language. The code has been parallelized using MPI. Some turbomachinery benchmark cases are presented. For all cases, an order of magnitude reduction in computational time is achieved when the code is executed on GPUs instead of CPUs.
Analysis of Gas Turbine Rotating Cavities: Estimation of Rotor Disk Pumped Mass Flow Rate
Authors: Da Soghe Riccardo , Facchini Bruno , Innocenti Luca , Micio Mirko
Published: May 2012

The dependable design of secondary air system is one of the main tasks for the safety, reliability and performance of gas turbine engines. To meet the increasing demands of gas turbine design, improved tools in prediction of the secondary air system behaviour over a wide range of operating conditions are needed. A real gas turbine secondary air system includes several components, therefore its analysis is not carried out using a complete CFD approach. Usually, these predictions are performed using codes, based on simplified approach which allows to evaluate the flow characteristics in each branch of the air system requiring very poor computational resources and little calculation time. Generally the available simplified commercial packages allow to correctly solve only some of the components of a real air system and often the elements with a more complex flow structure cannot be studied; among such elements, the analysis of rotating cavities is very hard. This paper deals with a design-tool developed at the University of Florence for the simulation of rotating cavities. This simplified in-house code solves the governing equations for a steady one-dimensional axysimmetric flow using experimental correlations both to incorporate flow phenomena caused by multidimensional effects, like heat transfer and flow field losses, and to evaluate the circumferential component of velocity. The simplified calculation tool was designed to simulate the flow in a rotating cavity with radial outflow both with a Batchelor and/or Stewartson flow structures. Several studies have been carried out by the authors to develop suitable correlations for the discs friction coefficients and for co-rotation factor evaluation. The results of these analyses are available in the literature. In the present paper the authors develop, using CFD tools, reliable correlation for rotor disk pumped mass flow rate and provide a full 1D-code validation comparing, due to a lack of experimental data, the in-house design code predictions with those evaluated by CFD.
Terrain Effects on Wind Flow: Simulations With an Immersed Boundary Method
Authors: Jafari S. , Chokani N. , Abhari R. S.
Published: May 2012

The modelling of the wind resource over arbitrary topography is required to optimize the micrositing of wind turbines. Most solvers use classical body-fitted grid for simulations. In such an approach, to cover the wind rose using a rectangular domain, a dedicated mesh must be generated for each direction. Moreover, over complex terrain, additional numerical errors are introduced in the solver due to coordinate transformations. To overcome these challenges and to facilitate the grid generation process, an immersed boundary method is developed in connection with a RANS solver in order to simulate turbulent atmospheric flows over arbitrary topography. In this method, a Cartesian grid is used and the boundary condition on the terrain surface is modelled within the solver using a “direct forcing” approach. With the immersed boundary method a rectangular grid can be used to simulate the flow field for all wind directions and only a rotation of the digital elevation map is required. Ghost cells are used to enforce the desired boundary condition at the immersed surface. The immersed boundary method developed in this work is used to simulate the flow in connection with both Baldwin-Lomax and kω turbulence models. The performance of the method is examined for the flow over a two-dimensional hill. Results are compared with experimental data as well as a classical body-fitted grid to isolate the effect of the boundary conditions. The comparisons show good agreement among all the results. The results for the three-dimensional wind flow simulation over the Askervein Hill test case are also presented, and show the capability of the immersed boundary method in a full-scale scenario.
Dynamics of Flame Stabilized by Triangular Bluff Body in Partially Premixed Methane-Air Combustion
Authors: Santosh Kumar T. V. , Alemela P. R. , Kok J. B. W.
Published: May 2012

In the design and operational tuning of gas turbine combustors it is important to be able to predict the interaction of the flame stabilization recirculation area with the burner aerodynamics. In the present paper transient computational fluid dynamics analysis is used to study these effects. Vortex interactions with the flame play a key role in many practical combustion systems. The interactions drive a large class of combustion instabilities and are responsible for changing the reaction rates, shape of the flame and the global heat release rate. The evolution of vortex shedding in reactive flows and its effects on the dynamics of the flame are important to be predicted. The present study describes dynamics of bluff body stabilized flames in a partially premixed combustion system. The bluff body is an equilateral wedge that induces the flame recirculation zone. The wedge is positioned at one-third length of the duct, which, is acoustically closed at the bottom end and open at the top. Transient computational modeling of partially premixed combustion is carried out using the commercial ANSYS CFX code and the results show that the vortex shedding has a destabilizing effect on the combustion process. Scale Adaptive Simulation turbulence model is used to compare between non-reacting cases and combustion flows to show the effects of aerodynamics-combustion coupling. The transient data reveals that frequency peaks of pressure and temperature spectra and is consistent with the longitudinal natural frequencies and Kelvin-Helmholtz instability frequency for reactive flow simulations. The same phenomenon is observed at different operating conditions of varying power. It has also been shown that the pressure and heat release are in phase, satisfying the Rayleigh criterion and therefore indicating the presence of aerodynamic-combustion instability. The data are compared to the scarce data on experiments and simulations available in literature.