bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Flexoelectricity on Band Structures of One-Dimensional Phononic Crystals
Authors: Liu Chenchen , Hu Shuling , Shen Shengping
Published: November 2013

As a size-dependent theory, flexoelectric effect is expected to be prominent at the small scale. In this paper, the band gap structure of elastic wave propagating in a periodically layered nanostructure is calculated by transfer matrix method when the effect of flexoelectricity is taken into account. Detailed calculations are performed for a BaTiO3-SrTiO3 two-layered periodic structure. It is shown that the effect of flexoelectricity can considerably flatten the dispersion curves, reduce the group velocities of the system, and decrease the midfrequency of the band gap. For periodic two-layered structures whose sublayers are of the same thickness, the width of the band gap can be decreased due to flexoelectric effect. It is also unveiled from our analysis that when the filling fraction is small, wider gaps at lower frequencies will be acquired compared with the results without considering flexoelectric effect. In addition, the band gap structures will approach the classical result as the total thickness of the unit cell increases. Our results indicate that the scaling law does not hold when the sizes of the periodic structures reach the nanoscale dimension. Therefore, the consideration of flexoelectric effect on the band structure of a nanosized periodic system is significant for precise manipulation of elastic wave propagation and its practical application.
Adhesive Contact on Randomly Rough Surfaces Based on the Double-Hertz Model
Authors: Zhang Wei , Jin Fan , Zhang Sulin , Guo Xu
Published: November 2013

A cohesive zone model for rough surface adhesion is established by combining the double-Hertz model (Greenwood, J. A., and Johnson, K. L., 1998, “An Alternative to the Maugis Model of Adhesion Between Elastic Spheres,” J. Phys. D: Appl. Phys., 31, pp. 3279–3290) and the multiple asperity contact model (Greenwood, J. A., and Williamson, J. B. P., 1966, “Contact of Nominally Flat Surfaces,” Proc. R. Soc. Lond. A, 295, pp. 300–319). The rough surface is modeled as an ensemble of noninteracting asperities with identical radius of curvature and Gaussian distributed heights. By applying the double-Hertz theory to each individual asperity of the rough surface, the total normal forces for the rough surface are derived for loading and unloading stages, respectively, and a prominent adhesion hysteresis associated with dissipation energy is revealed. A dimensionless Tabor parameter is also introduced to account for general material properties. Our analysis results show that both the total pull-off force and the energy dissipation due to adhesive hysteresis are influenced by the surface roughness only through a single adhesion parameter, which measures statistically a competition between compressive and adhesive forces exerted by asperities with different heights. It is also found that smoother surfaces with a small adhesion parameter result in higher energy dissipation and pull-off force, while rougher surfaces with a large adhesion parameter lead to lower energy dissipation and pull-off force.
Stress Intensity Factor for Repaired Circumferential Cracks in Pipe With Bonded Composite Wrap
Authors: Benyahia F. , Albedah A. , Bachir Bouiadjra B.
Published: November 2013

The use of composite systems as a repair methodology in the pipeline industry has grown in recent years. In this study, the analysis of the behavior of circumferential through cracks in repaired pipe with bonded composite wrap subjected to internal pressure is performed using three-dimensional finite element analysis. The fracture criterion used in the analysis is the stress intensity factor (SIF). The obtained results show that the bonded composite repair reduces significantly the stress intensity factor at the tip of repaired cracks in the steel pipe, which can improve the residual lifespan of the pipe.
Searching Feasible Design Space by Solving Quantified Constraint Satisfaction Problems
Authors: Hu Jie , Aminzadeh Masoumeh , Wang Yan
Published: November 2013

In complex systems design, multidisciplinary constraints are imposed by stakeholders. Engineers need to search feasible design space for a given problem before searching for the optimum design solution. Searching feasible design space can be modeled as a constraint satisfaction problem (CSP). By introducing logical quantifiers, CSP is extended to quantified constraint satisfaction problem (QCSP) so that more semantics and design intent can be captured. This paper presents a new approach to formulate searching design problems as QCSPs in a continuous design space based on generalized interval, and to numerically solve them for feasible solution sets, where the lower and upper bounds of design variables are specified. The approach includes two major components. One is a semantic analysis which evaluates the logic relationship of variables in generalized interval constraints based on Kaucher arithmetic, and the other is a branch-and-prune algorithm that takes advantage of the logic interpretation. The new approach is generic and can be applied to the case when variables occur multiple times, which is not available in other QCSP solving methods. A hybrid stratified Monte Carlo method that combines interval arithmetic with Monte Carlo sampling is also developed to verify the correctness of the QCSP solution sets obtained by the branch-and-prune algorithm.
Microstructural Modeling of P91 Martensitic Steel Under Uniaxial Loading Conditions
Authors: Golden B. J. , Li D. F. , O'Dowd N. P. , Tiernan P.
Published: November 2013

The changing face of power generation and the increasingly severe conditions experienced by power plant materials require an improved understanding of the deformation and failure response of power plant materials. Important insights can be obtained through computational studies, where the material microstructure is explicitly modeled. In such models, the physical mechanisms of deformation and damage can be represented at the microscale, providing a more accurate prediction of material performance. In this paper, two approaches are examined to represent the microstructure of a martensitic power plant steel (P91). In one approach, the model is based on a “measured microstructure” with electron backscatter diffraction (EBSD) employed to obtain the orientation of the martensitic grain structure of the steel. The alternative approach is to use a “numerically simulated” model where the microstructure is generated using the Voronoi tessellation method. In both cases, the microstructural model is incorporated within a representative volume element (RVE) in a finite-element analysis. The material constitutive response is represented by a nonlinear, rate dependent, finite strain crystal plasticity model, with the microstructural orientation specified at each finite-element integration point by the microstructural model. The predictions from the two approaches are compared. The stress distributions are observed to be very similar, though some differences are seen in the strain variation within the RVE.
Decomposition Templates and Joint Morphing Operators for Genetic Algorithm Optimization of Multicomponent Structural Topology
Authors: Zhou Zebin , Hamza Karim , Saitou Kazuhiro
Published: November 2013

This paper presents a continuum-based approach for multi-objective topology optimization of multicomponent structures. Objectives include minimization of compliance, weight, and cost of assembly and manufacturing. Design variables are partitioned into two main groups: those pertaining to material allocation within a design domain (base topology problem), and those pertaining to decomposition of a monolithic structure into multiple components (joint allocation problem). Generally speaking, the two problems are coupled in the sense that the decomposition of an optimal monolithic structure is not always guaranteed to produce an optimal multicomponent structure. However, for spot-welded sheet-metal structures (such as those often found in automotive applications), certain assumptions can be made about the performance of a monolithic structure that favor the adoption of a two-stage approach that decouples the base topology and joint allocation problems. A multi-objective genetic algorithm (GA) is used throughout the studies in this paper. While the problem decoupling in two-stage approaches significantly reduces the size of the search space and allows better performance of the GA, the size of the search space can still be quite enormous in the second stage. To further improve the performance, a new mutation operator based on decomposition templates and localized joints morphing is proposed. A cantilever-loaded structure is used to study and compare various setups of single and two-stage GA approaches and establish the merit of the proposed GA operators. The approach is then applied to a simplified model of an automotive vehicle floor subject to global bending loading condition.
A Maximum Confidence Enhancement Based Sequential Sampling Scheme for Simulation-Based Design
Authors: Wang Zequn , Wang Pingfeng
Published: November 2013

A maximum confidence enhancement (MCE)-based sequential sampling approach is developed for reliability-based design optimization (RBDO) using surrogate models. The developed approach employs the ordinary Kriging method for surrogate model development and defines a cumulative confidence level (CCL) measure to quantify the accuracy of reliability estimation when Monte Carlo simulation is used based on the developed surrogate model. To improve the computational efficiency, an MCE-based sequential sampling scheme is developed to successively select sample points for surrogate model updating based on the defined CCL measure, in which a sample point that produces the largest CCL improvement will be selected. To integrate the MCE-based sequential sampling approach with RBDO, a new sensitivity analysis approach is developed, enabling smooth design sensitivity information to be accurately estimated based upon the constructed surrogate model without incurring any extra computational costs, thus greatly enhancing the efficiency and robustness of the design process. Two case studies are used to demonstrate the efficacy of the developed approach.
Creep Characterization of Ni-Based Superalloy IN-792 Using the 4- and 6-θ Projection Method
Authors: Sajjadi S. A. , Berahmand M. , Rezaee-Bazzaz A.
Published: November 2013

This paper studies the accuracy of a technique which is capable of predicting and modeling a wide range of creep life in Ni-based superalloys. The θ-projection method was applied to characterize the creep behavior of the Ni-based superalloy IN-792 at 800 °C. Constant load creep tests have been carried out over a wide range of loads at the constant operating temperature. Creep curves were fitted using either 4-θ or 6-θ equation by the use of a nonlinear least-square technique. The results showed that both 4- and 6-θ projection parameters revealed a good linearity as a function of stress. Comparison of experimental creep curves with those predicted using both of the utilized θ-projection techniques showed that the techniques fit the experimental data at high strain values very well while the 6-θ approach describes much better the creep curves at low strain region.
Compressor Fouling: A Comparison of Different Fault Distributions Using a “Stage-Stacking” Technique
Authors: Rodríguez C. , Sánchez D. , Chacartegui R. , Muñoz A. , Martínez G. S.
Published: November 2013

This work compares different approaches (i.e. fault distribution patterns) that aim to explain how a gas turbine compressor gets fouled and how its corresponding performance map deteriorates. To do so, a series of such maps is first developed based on an updated “stage-stacking” methodology. Then, the maps so developed are affected by fouling using the aforementioned approaches. The results evidence that the lack of consensus with respect to how fouling proceeds is bond to yield engine performances that differ by large. In other words, there is still a lot of work to be done if a general approach to anticipate the quantity and quality of compressor fouling is to be developed. For the moment, all the systematic analyses are specific to the particular site where they have been applied and hence have no generality that can be extended elsewhere. Moreover, the methodologies available allow that a fouled compressor be identified and the distribution be predicted a posteriori by means of adaptive modelling and compressor zooming utilising measurements from the whole engine or by using compressor specific measurements.
Experimental Evaluation of Hydrodynamic Bearings for a High Speed Turbocharger
Authors: Kirk Robley G.
Published: November 2013

Many high speed turbochargers are known to operate with limit cycle vibration as a result of fluid-film instability. The goal of this research was to achieve a stable synchronous response with a minimum of self-excited non-synchronous contribution. Those vibration components excited by the engine harmonics and exhaust pressure pulsations were not the target of this research. This paper will review the experimental results of the fixed geometry fluid-film bearing designs selected to replace the standard stock floating-ring design. In addition, the paper documents a novel radial tilting pad bearing concept that was designed to replace the fixed geometry bearings, with a minimum of modification to the stock bearing housing. A summary of the on-engine testing over the past seven years is documented in this paper.
Analysis and Comparison of Primary Droplet Characteristics in the Near Field of a Prefilming Airblast Atomizer
Authors: Gepperth S. , Koch R. , Bauer H.-J.
Published: November 2013

As the dominating parameters influencing the Sauter mean diameter of the spray produced by a prefilming airblast atomizer, the air velocity, liquid surface tension and atomizing edge thickness could be identified.

Correlations for the prediction of the droplet sizes produced by prefilming airblast atomizers are compared to droplet sizes measured close to the atomizing edge. The measurements were performed using three geometrical variants of a planar atomizer over a wide range of operating conditions. The diagnostics are based on a particle and ligament tracking technique, that enables simultaneous measurement of the liquid blobs and ligaments formed at the atomizing edge and the droplets in the primary breakup region of the atomizer.

The comparison between measured and calculated droplet diameter indicates, that most of the correlations are capable of reproducing the correct tendency. However, since the measurement data of most correlations were collected in a region where secondary atomization effects can obscure the initial droplet sizes in the primary breakup region, the droplet sizes are generally predicted too small.
Prediction of Unsteady Force for Axial Turbine Buckets (Effects of Nozzle-Bucket Axial Gap Length and Blade Count Ratio)
Authors: Nakajima Tomomi , Shikano Yoshio , Yamashita Yutaka
Published: November 2013

The purpose of this study is to investigate the unsteady force acting on steam turbine buckets which is induced by a potential field and a wake. The authors have already proposed a new method that can separate the unsteady force into a potential field interaction and a wake interaction from the unsteady viscous flow computation. It is understood from the previous work that simple expressions may be written for the relationships between the nozzle-bucket axial gap length and the unsteady force, which means the amplitude and the phase of the unsteady force with the nozzle passing frequency component.

This paper adopts the blade count ratio as the factor to study the unsteady force of the bucket sections with different bucket heights in the same stage. Four points are seen from the results; points (a)–(c) do not depend on the blade count ratio. (a) The amplitudes of the unsteady force of each interaction can be approximated by an exponential or a power function of axial gap length. (b) The amplitudes of each interaction tend to become large when bucket turning angle is large. (c) The phase of the unsteady force of each interaction with the nozzle passing frequency component has a linear relationship with axial gap length. (d) The amplitudes of each interaction become the maximum value when the blade count ratio is 1.5.
High-Frequency Instabilities in Cylindrical Flame Tubes: Feedback Mechanism and Damping
Authors: Schwing Joachim , Sattelmayer Thomas
Published: November 2013

Thermoacoustic instabilities are a major concern in gas turbine combustion chambers today. In the last decades research interest in thermoacoustic instabilities has focused on low frequencies. The feedback mechanisms related to longitudinal modes are for the most part understood. Transverse modes, though, have not been studied to a large extent in the past. However, interest has been rising in the last few years. But little is known about the thermoacoustic feedback of high-frequency instabilities. Our previous publications characterized the flow and the flame at the eigenfrequency of high-frequency instabilities. There, a feedback mechanism was derived from the experimental results and discussed: the acoustic velocity leads to a periodic displacement of the flame resulting in a positive contribution to the Rayleigh criterion. Thus, the thermoacoustic feedback couples to the acoustic velocity, but not to the pressure or a periodic vortex formation. Different means can be derived from the model to influence high-frequency instabilities: Helmholtz dampers are used to shift the onset of instabilities to increased thermal power. With loudspeakers naturally stable operating points are excited. Stopping the excitation and evaluating the signal, decay rates are analyzed. Decay rates — i.e. stability margins — are compared for different operating conditions. Switching from perfect premixing to technical premixing, the radial profile of the fuel-to-air ratio can be changed. The influence of a lean core flow compared to a homogeneous mixture on the feedback is investigated and its impact on the instabilities is compared to the model. The observations reflect, what is predicted by the model. Velocity coupling, at least a significant part of the feedback mechanism for transverse high-frequency instabilities, is supported by the experimental results.
Numerical Investigation of Condenser Pressure Effect on Last Stage Operation of Low Pressure Wet Steam Turbines
Authors: Stanciu Mugurel , Marcelet Meryem , Dorey Jean-Marc
Published: November 2013

The flow in nuclear steam turbines is mainly affected in normal operating conditions by the condenser pressure, which depends on the heat sink temperature variations. This has an effect not only on the performance, but can also lead to undesired flow regimes and become related to safety issues.

The flow in the low pressure last stages is characterized by two main features: unsteadiness due to the rotation and asymmetry due to the exhaust hood. The effect of the condenser pressure on the stage flow is numerically analyzed with steady and full unsteady approaches. Three distinct flow regimes can be identified at nominal, low and high condenser pressures with different types of unsteady behavior.

A detailed investigation of the steam flow and possible consequences for the turbine safety is carried out at nominal and critical operating points. Comparisons of steady/unsteady approaches and the effect of the hood design are also presented.
Computational Analysis of Abradable Seal: Part 1
Authors: Pandit Rajeev Kumar , Innocenti Luca
Published: November 2013

The usage of abradable seals in the secondary flow path of centrifugal compressors is very high whenever higher efficiency is the main requirement. This is due to the fact that when compared with other sealing technologies the rubbing of teeth into the abradable material allows for the formation of grooves that result in very tight clearances between static and rotating components.

Flow physics and mass flow leakage through abradable seals are strongly dependent on the presence of these grooves and their respective shapes. For cases where no grooves are formed, the seal leakage is mainly a function of inlet pressure, pressure ratio across the seal and tooth radial clearance. Once grooves are formed, the flow physics and seal leakage are also a function of groove dimensions and tooth axial position inside the groove.

The scope of this paper is to present results obtained from an extended numerical campaign that was performed to understand the effect of groove formation and teeth position on seal leakages. Computation Fluid Dynamics (CFD) was used to study these effects. To achieve higher accuracy of numerical results, all key parameters in CFD such as mesh topology, grid independence, y+ criterion and turbulence model were studied in great detail.

In cases where no grooves are present, the vena contracta (the narrowest point of the carry-over jet) is observed across the first tooth and its effect on flow physics at higher tooth gap is highlighted in this paper. For cases where grooves are present, it is seen that the flow field strongly depends on the following parameters:

1. Groove dimensions (e.g. depth and width),

2. Tooth radial clearance and

3. Tooth axial position inside groove.

The impact of all the above parameters on seal leakage have been discussed and described in this paper. Finally, seal leakages are compared for grooved geometry with respective non-grooved geometry cases (in terms of tooth radial clearance).

In the second part of the paper, the accuracy of CFD results is verified with available experimental data. For judicious comparison, geometrical specifications of all components and boundary conditions in CFD model and experimental setup were maintained very closely.
Experimental and Numerical Investigation of the Tip Clearance Noise of an Axial Fan
Authors: Zhu Tao , Carolus Thomas H.
Published: November 2013

The aerodynamic and aeroacoustic performance of axial fans are strongly affected by the unavoidable tip clearance. Three identical fan impellers but with different tip clearance ratio were investigated. Details of the time averaged tip flow were analysed by a numerical RANS-simulation. Unsteady wall pressure fluctuations in the tip region of the rotating blades and on the interior wall of the duct type shroud and the overall sound radiated were measured. As the tip clearance is increased, overall fan pressure rise and efficiency drop, the onset of the rotating stall moves to higher flow rates and broadband noise is found to become dominant in the spectrum. The RANS simulation revealed a vortex system consisting of three different vortices in the tip region. Their strength and trajectories are controlled by the size of the tip clearance and the fan’s operating point. Measurements showed that these vortices impose tonal and — more pronounced — broadband pressure fluctuations on stationary and rotating walls in the vicinity of the blade tip. The tip vortex system is the main driver of the unsteady flow in the tip region. As tip clearance is increased the unsteady wall pressure becomes more developed and with it the sound radiated by the fan. Hence it is concluded that these vortex induced pressure fluctuations form the dipole source mechanism of the noise observed. This hypothesis is supported by a preliminary correlation analysis.
Development of New High-Performance Labyrinth Seal Using Aerodynamic Approach
Authors: Kuwamura Yoshihiro , Matsumoto Kazuyuki , Uehara Hidekazu , Ooyama Hiroharu , Tanaka Yoshinori , Nishimoto Shin
Published: November 2013

As key technologies to improve the performance of steam turbines, various types of high performance seal, such as active clearance control (ACC) seals and leaf seals, have been developed by Mitsubishi Heavy Industries, LTD (MHI). Moreover, a new seal concept has also been developed, which remarkably reduces the leakage flow through the blade tip/base clearance by using an aerodynamic approach.

The main concept of this technology is to control and utilize the vortex structure in the cavities of the labyrinth seal by optimizing the cavity geometry. In the optimized geometry, locally-controlled flow on the upstream side of the fin tip causes a strong contraction of the leakage flow and reduces the discharge coefficient significantly. This concept allows for a remarkably reduced leakage flow, whilst keeping clearance at the same level as that of conventional set-up, and thus the risk of contact between rotating and stationary parts low. Therefore, this technology realizes reliable and durable seals with high performance. In addition, it is possible to keep manufacturing cost at the same level as conventional seals, since the structure of this seal is basically the same as the conventional one.

In the development of this technology, a parametric study using Computational Fluid Dynamics (CFD) was carried out to optimize the cavity geometry. The verification test was carried out for the optimized geometry under the real steam conditions. From the results, we confirmed that the optimized geometry reduced the discharge coefficient by up to 30%, compared to conventional seals.
Numerical Modelling of Slot Film Cooling Using a Wall Function
Authors: Shalash Karim M. , El-Gabry Lamyaa A. , Abo El-Azm Mohamed M.
Published: November 2013

CFD modeling of gas turbine film cooling remains a challenge for the computational arena due to the lack of robust accurate turbulence model or numerical technique to solve this highly complex problem. Modeling the exact behavior of the coolant jet is computationally expensive due to the complexity of the jet mainstream interaction, such as vortex generation, and separation. This paper, validation progress is presented using experimental data executed by the second author [GT2011-46491] and Thurman et-al [GT2011-46498] for blowing ratios of 1.0 and 2.0, and density ratio of 1.0. A wall function approach is chosen for a robust computation, and aiming for CPU time reduction. The in-house CFD code EOS is used to solve the RANS equations. A simple flow over flat plate validation problem was executed using experimental data of Klebanoff and El-Tahry as a code validation evidence. The computational results of the flow field were in agreement with the experimental measurements, with a slight over estimation of the thermal field due to over prediction of dissipation, resulting in less diffusion and mixing between the coolant jet and the mainstream. The wall function approach is shown to have great potentials for a robust accurate solution. The use of the continuous slot injection is known to be the best film cooling technique when compared to the other conventional circular coolant tubes, with the critical drawback of lowering the mechanical integrity of the blade. One way to overcome this problem is the use of discrete slots, which will optimize for better film cooling effectiveness, while maintaining the structural strength of the blade. Rectangular slots showed an increase in film cooling effectiveness for the same mass flow rate of coolant and blowing ratio when compared to the standard circular hole, which is due to the minor counter rotating vortex pair effect. An oval slot design showed higher film cooling effectiveness and more spreading, covering more surface area.
Effect of Geometry Changes in an Aggressive Turbine Exhaust Delivery System
Authors: Dobhal Shweta , Pradeep A. M. , Roy Bhaskar
Published: November 2013

The paper reports numerical studies carried out on an aggressive turbine exhaust delivery system. A typical industrial turbine exhaust system has been used as the baseline configuration. Several geometry modifications of the diffuser system were attempted to study their effect on the diffuser performance. The geometrical modifications used were 1) Blended shapes — a blended strut-hub geometry; 2) Repositioning of the struts; and 3) contoured hub end. The performance of the diffusers has been quantified in terms of total pressure loss and static pressure rise coefficient across the diffuser. The last stage turbine tip leakage flow has been simulated by using annular casing injectors applied ahead of annular diffuser. The effect of varying the rate of injection has also been analyzed. In addition, the effect of shortening the length of the diffuser was studied. Modification-2, as stated above, has been applied on the shortened aggressive diffuser and it was observed that with injection the performance of shortened aggressive diffuser is better than that of the baseline configuration.
Experimental Measurements of Ingestion Through Turbine Rim Seals: Part 4 — Off-Design Conditions
Authors: Scobie James A. , Sangan Carl M. , Teuber Roy , Pountney Oliver J. , Owen J. Michael , Wilson Michael , Lock Gary D.
Published: November 2013

This paper describes results obtained from an experimental facility which models ingress through the rim seal into the upstream wheel-space of an axial-turbine stage. The experimental rig included 32 nozzle guide vanes and 41 symmetrical turbine blades, and the paper presents measurements of ε (the sealing effectiveness) for single- and double-clearance seals for both over-speed (where the blades rotate faster than at the design point) and under-speed conditions. The design flow coefficient was CF = 0.538, and tests were conducted for 0 < CF < 0.9, which is larger than the range experienced in engines. The measured values of ε were correlated by the ‘effectiveness equations’ for rotationally-induced (RI) and externally-induced (EI) ingress.

The correlated effectiveness curves were used to determine Φmin′ (the value of the sealing flow parameter when ε = 0.95), and the variation of Φmin′ with CF was in mainly good agreement with the theoretical curve for CI (combined ingress), which covered the transition from RI to EI ingress. Departure of the measured values of Φmin′ from the CI curve occurred at very low values of CF for all the seals tested; this was attributed to the effects of separation of the mainstream flow over the turbine blades at large ‘deviation angles’ between the flow and the blades.

The measurements are expected to be qualitatively similar to but quantitatively different from those experienced in engines.
Effect of Coolant Density on Leading Edge Showerhead Film Cooling Using PSP Measurement Technique
Authors: Li Shiou-Jiuan , Yang Shang-Feng , Han Je-Chin
Published: November 2013

The density ratio effect on leading edge showerhead film cooling has been studied experimentally using the pressure sensitive paint (PSP) mass transfer analogy method. Leading edge model is a blunt body with a semi-cylinder and an after body. There are two designs: seven-row and three-row of film cooling holes for simulating vane and blade, respectively. The film holes are located at 0 (stagnation row), ±15, ±30, and ±45 deg for seven-row design, and at 0 and ±30 for three-row design. Four film holes configurations are used for both test designs: radial angle cylindrical holes, compound angle cylindrical holes, radial angle shaped holes, and compound angle shaped holes. Coolant to mainstream density ratio varies from DR = 1.0, 1.5, to 2.0 while blowing ratio varies from M = 0.5 to 2.1. Experiments were conducted in a low speed wind tunnel with Reynolds number 100,900 based on mainstream velocity and diameter of the cylinder. The mainstream turbulence intensity near leading edge model is about 7%. The results show the shaped holes have overall higher film cooling effectiveness than cylindrical holes, and radial angle holes are better than compound angle holes, particularly at higher blowing ratio. Larger density ratio makes more coolant attach to the surface and increases film protection for all cases. Radial angle shaped holes provides best film cooling at higher density ratio and blowing ratio for both designs.
Numerical Investigation on the Flow and Heat Transfer Characteristics of the Superheated Steam in the Wedge Duct With Pin-Fins
Authors: Liao Gaoliang , Wang Xinjun , Bai Xiaowei , Zhu Ding , Yao Jinling
Published: November 2013

By using the CFX software, the three-dimensional flow and heat transfer characteristics in the cooling duct with pin-fin in the blade trailing edge were numerically simulated. The effects of pin-fin arrangements, Reynolds number, steam superheat degrees, streamwise pin density and convergence angle of the wedge duct on the flow and heat transfer characteristics were analysed. The results show that the Nusselt number on the endwall and pin-fin surfaces as well as the pin-fin row averaged Nusselt number increase with the increasing of Reynolds number, while it decreased with the with the increasing of X/D. The pressure drop increases with the increasing of Reynolds number while decreases with the increasing of X/D in the wedge duct. The degree of superheat has little effect on the pressure loss in the wedge duct. A comprehensive analysis and comparison show that the highest thermal performance is reached in the wedge duct when the value of X/D is 1.5.
Slow-Roll of Natural Gas Compressors in a 420 MW Cogeneration Plant
Authors: Ingistov Steve , Milos Michael
Published: November 2013

This Paper describes the efforts required to operate two steam turbine-driven centrifugal natural gas compressors at significantly reduced rotative speeds. Natural gas is used to power a 420 MW combined cycle cogeneration plant. Normally, the plant uses natural gas fuel at supply line pressure and the two compressors are on idled standby. The goal of the project is to reduce motive steam consumption in the steam turbine driving the natural gas compressors. The present operating mode, or idle mode, is where both natural gas compressors recycle compressed gas via dedicated intercoolers. This mode of operation occurs about 95% of the time. The remaining 5% of the time is when natural gas compressors boost the trunk line pressure in order to deliver fuel to the gas turbine combustors. Both natural gas compressor trains are designed for rapid increase of rotative speed in case one natural gas compressor is shut down. In order to put a natural gas compressor on hot standby slow-roll condition, a special, dedicated slow roll line is proposed for the fine control of the rotative speed.
Evaluation of Massively-Parallel Spectral Element Algorithm for LES of Film-Cooling
Authors: Duggleby Andrew , Camp Josh L. , Laskowski Greg
Published: November 2013

A blind Large-Eddy Simulation (LES) of film-cooling heat transfer is performed on a canonical cylindrical cooling hole geometry using a massively-parallel, geometrically-flexible, open-source spectral element solver NEK5000. The simulation is for a blowing-ratio of 1.0, density-ratio of 1.5, and Reynolds-number Reθ = 4,300 based on boundary layer momentum thickness and ReD = 32,000 based on hole diameter. A low-Mach ideal gas formulation is used to match the density ratio. A spectral-damping LES subgrid model is used which does not restrict time-stepping, allowing CFL numbers of 5–10 through characteristics time-integration. The numerical mesh resolves the boundary layer and coarsens to acceptable LES sizing in the free stream, resulting in 88 million grid points (410,464 elements at 5th order polynomial). For this blowing ratio, the coolant hole Mach number is too large for the low-Mach formulation (> 0.3). This results in faster hole velocities as opposed to fluid compression, effectively changing the momentum ratio leading to coolant lift-off as compared to experiment. The film-cooling effectiveness along centerline and spanwise locations of x/D = 2 and 8 are lower than experiment. Ideal parallel scaling is shown up to 256 processors and estimated to continue at ideal scaling to 2048 processors.
Heat Transfer in Rotating Serpentine Coolant Passage With Ribbed Walls at Low Mach Numbers
Authors: Yang Shang-Feng , Han Je-Chin , Azad Salam , Lee Ching-Pang
Published: November 2013

This paper experimentally investigates the effect of rotation on heat transfer in typical turbine blade serpentine coolant passage with ribbed walls at low Mach numbers. To achieve the low Mach number (around 0.01) condition, pressurized Freon R-134a vapor is utilized as the working fluid. The flow in the first passage is radial outward, after the 180° tip turn the flow is radial inward to the second passage, and after the 180° hub turn the flow is radial outward to the third passage. The effects of rotation on the heat transfer coefficients were investigated at rotation numbers up to 0.6 and Reynolds numbers from 30,000 to 70,000. Heat transfer coefficients were measured using the thermocouples-copper-plate-heater regional average method. Heat transfer results are obtained over a wide range of Reynolds numbers and rotation numbers. An increase in heat transfer rates due to rotation is observed in radially outward passes; a reduction in heat transfer rate is observed in the radially inward pass. Regional heat transfer coefficients are correlated with Reynolds numbers for non-rotation and with rotation numbers for rotating condition, respectively. The results can be useful for understanding real rotor blade coolant passage heat transfer under low Mach number, medium-high Reynolds number and high rotation number conditions.
Influence of Pressure on Markstein Number Effect in Turbulent Flame Front Propagation
Authors: Vukadinovic Vlade , Habisreuther Peter , Zarzalis Nikolaos , Suntz Rainer
Published: November 2013

In gas turbine operation a turbulent flame is employed. Thus, better understanding of the turbulent flame propagation is the key for further optimisation of turbine combustors and reduction of the environmental footprint. As turbulent flames are exposed to stretch, the effect of flame-stretch interaction must be better understood especially at higher pressures.

In present study, turbulent burning velocity of two mixtures, hydrogen/air and propane/air, with negative and positive Ma, respectively are experimentally investigated in fan-stirred explosion vessel. For the investigation an optical laser method is employed based on the Mie-scattering of the laser light by smoke particles. Within this study the influence of initial parameters as initial pressure and turbulence intensity on the flame front propagation is investigated by giving special attention on influence of Ma variation. The experiments were performed at three different pressures 1, 2, 4 bar. The RMS fluctuation velocity was varied in the range of 0–2.77 m/s. The observed results are compared and discussed in detail.
Investigations on the Effect of Inflow Distortion on the Performance of a High Aspect Ratio, Low Speed Contra Rotating Fan Stage
Authors: Mistry Chetan S. , Pradeep A. M.
Published: November 2013

In this paper, results from an experimental study on the effect of circumferential inflow distorted on the performance and flow behavior of a high aspect ratio, low speed contra rotating fan, are reported. The total pressure at the inlet is artificially distorted by means of 90° mesh sector having porosity of 0.70. The performance of contra rotating fan was studied under variable speed combinations of rotors under clean and distorted inflow conditions. Detailed flow analyses were conducted under design and off-design conditions. In order to understand the extent of inlet distortion, the distortion sector was rotated circumferentially at intervals of 15° to cover the entire annulus. Detailed measurements of total pressure, velocity components and flow angles were carried out at the inlet of the first rotor, between the two rotors and at the exit of the second rotor. The study reveals a few interesting aspects on the effect of inflow distortion on the performance of a contra-rotating stage. It is observed that a higher rotational speed of the second rotor has a beneficial effect on the overall performance.
CFD Investigation of Turbulent Premixed Flame Response to Transverse Forcing
Authors: Lee C. Y. , Cant R. S.
Published: November 2013

Screech is a high frequency oscillation that is usually characterized by instabilities caused by large-scale coherent flow structures in the wake of bluff-body flameholders and shear layers. Such oscillations can lead to changes in flame surface area which can cause the flame to burn unsteadily, but also couple with the acoustic modes and inherent fluid-mechanical instabilities that are present in the system.

In this study, the flame response to hydrodynamic oscillations is analyzed in a controlled manner using high-fidelity Computational Fluid Dynamics (CFD) with an unsteady Reynolds-averaged Navier-Stokes approach. The response of a premixed flame with and without transverse velocity forcing is analyzed. When unforced, the flame is shown to exhibit a self-excitation that is attributed to the anti-symmetric shedding of vortices in the wake of the flameholder. The flame is also forced using two different kinds of low-amplitude out-of-phase inlet velocity forcing signals. The first forcing method is harmonic forcing with a single characteristic frequency, while the second forcing method involves a broadband forcing signal with frequencies in the range of 500–1000 Hz.

For the harmonic forcing method, the flame is perturbed only lightly about its mean position and exhibits a limit cycle oscillation that is characteristic of the forcing frequency. For the broadband forcing method, larger changes in the flame surface area and detachment of the flame sheet can be seen. Transition to a complicated trajectory in the phase space is observed. When analyzed systematically with system identification methods, the CFD results, expressed in the form of the Flame Transfer Function (FTF) are capable of elucidating the flame response to the imposed perturbation. The FTF also serves to identify, both spatially and temporally, regions where the flame responds linearly and nonlinearly. Locking-in between the flame’s natural self-excited frequency and the subharmonic frequencies of the broadband forcing signal is found to alter the dynamical behaviour of the flame.
A Novel Theoretical Approach to Passive Control of Thermo-Acoustic Oscillations: Application to Ducted Heat Sources
Authors: Magri Luca , Juniper Matthew P.
Published: November 2013

In this paper, we develop a linear technique that predicts how the stability of a thermo-acoustic system changes due to the action of a generic passive feedback device or a generic change in the base state. From this, one can calculate the passive device or base state change that most stabilizes the system. This theoretical framework, based on adjoint equations, is applied to two types of Rijke tube. The first contains an electrically-heated hot wire and the second contains a diffusion flame. Both heat sources are assumed to be compact so that the acoustic and heat release models can be decoupled. We find that the most effective passive control device is an adiabatic mesh placed at the downstream end of the Rijke tube. We also investigate the effects of a second hot wire and a local variation of the cross-sectional area but find that both affect the frequency more than the growth rate. This application of adjoint sensitivity analysis opens up new possibilities for the passive control of thermo-acoustic oscillations. For example, the influence of base state changes can be combined with other constraints, such as that the total heat release rate remains constant, in order to show how an unstable thermo-acoustic system should be changed in order to make it stable.
A Parametric Numerical Study of the Effects of Inlet Swirl Distortion on a Transonic Compressor Stage
Authors: Ben Sghaier Taieb , Mehdi Ahad , Pachidis Vassilios , MacManus David
Published: November 2013

The ingestion or manifestation of a vortical flow can have dramatic effects on an aero engine. It is therefore imperative to quantify these effects and understand their underlying mechanism. This numerical study analyses the response of a transonic compressor stage to the ingestion of different streamwise vortical distortions using steady-state CFD. The vortex is described using a number of features, which are varied and combined together in order to generate a wide range of different swirl disturbances. The initial aim of this research is to identify the vortex features which have the highest impact on compressor performance.

A numerical model of a compressor stage is generated which enables prescribed vortical flows to be imposed at the domain inlet. The method is validated against experimental data which was obtained under clean, undistorted conditions. The response of the compressor following the ingestion of a vortex is assessed both in terms of overall compressor performance parameters as well as more detailed aerodynamic characteristics.

The results show that the compressor is sensitive to the vortex magnitude, core size, polarity and radial location. Furthermore, co-rotating, high-strength vortices which are ingested in the near-hub region cause the most significant drop in pressure ratio and corrected mass flow. In contrast, counter-rotating vortices cause little change in compressor performance.

Overall, the work shows that modest swirl distortions can have a notable impact on the compressor performance and stability, and highlights the growing need to develop methods and an understanding of how this class of distortion can be evaluated during the engine design phase.
Compressor Leading Edge Sensitivities and Analysis With an Adjoint Flow Solver
Authors: Giebmanns A. , Backhaus J. , Frey C. , Schnell R.
Published: November 2013

Based on the results of a prior study about fan blade degradation, which state a noticeable influence of small geometric changes on the fan performance, an adjoint computational fluid dynamics method is applied to systematically analyze the sensitivities of fan blade performance to changes of the leading edge geometry.

As early as during manufacture, blade geometries vary due to fabrication tolerances. Later, when in service, engine operation results in blade degradation which can be reduced but not perfectly fixed by maintenance, repair and overhaul processes. The geometric irregularities involve that it is difficult to predict the blade’s aerodynamic performance. Therefore, the aim of this study is to present a systematic approach for analyzing geometric sensitivities for a fan blade.

To demonstrate the potential, two-dimensional optimizations of three airfoil sections at different heights of a transonic fan blade are presented. Although the optimization procedure is limited to the small area of the leading edge, the resulting airfoil sections can be combined to a three-dimensional fan blade with an increased isentropic efficiency compared to the initial blade.

Afterwards, an adjoint flow solver is applied to quasi-three-dimensional configurations of an airfoil section in subsonic flow with geometric leading edge variations in orders representative for realistic geometry changes. Validations with non-linear simulation results demonstrate the high quality of the adjoint results for small geometric changes and indicate physical effects in the leading edge region that influence the prediction quality.