bluuebunny/crossref_metadata_2025 huggingface dataset:
Manifold influences of phylogenetic structure on a plant–herbivore network
Authors: Bergamini Leonardo Lima , Lewinsohn Thomas M. , Jorge Leonardo R. , Almeida‐Neto Mário
Published: September 2016

Ecologists are increasingly aware of the interplay between evolutionary history and ecological processes in shaping current species interaction patterns. The inclusion of phylogenetic relationships in studies of species interaction networks has shown that closely related species commonly interact with sets of similar species. Notably, the degree of phylogenetic conservatism in antagonistic ecological interactions is frequently stronger among species at lower trophic levels than among those at higher trophic levels. One hypothesis that accounts for this asymmetry is that competition among consumer species promotes resource partitioning and offsets the maintenance of dietary similarity by phylogenetic inertia. Here, we used a regional plant–herbivore network comprised of Asteraceae species and flower‐head endophagous insects to evaluate how the strength of phylogenetic conservatism in species interactions differs between the two trophic levels. We also addressed whether the asymmetry in the strength of the phylogenetic signal between plants and animals depends on the overall degree of relatedness among the herbivores. We show that, beyond the previously reported compositional similarity, closely related species also share a greater proportion of counterpart phylogenetic history, both for resource and consumer species. Comparison of the patterns found in the entire network with those found in subnetworks composed of more phylogenetically restricted groups of herbivores provides evidence that resource partitioning occurs mostly at deeper phylogenetic levels, so that a positive phylogenetic signal in antagonist similarity is detectable even between closely related consumers in monophyletic subnetworks. The asymmetry in signal strength between trophic levels is most apparent in the way network modules reflect resource phylogeny, both for the entire network and for subnetworks. Taken together, these results suggest that evolutionary processes, such as phylogenetic conservatism and independent colonization history of the insect groups may be the main forces generating the phylogenetic structure observed in this particular plant–herbivore network system.
A simulation‐based approach to understand how metacommunity characteristics influence emergent biodiversity patterns
Authors: Sokol Eric R. , Brown Bryan L. , Barrett J. E.
Published: September 2016

To understand controls over biodiversity, it is necessary to take a multi‐scale approach to understand how local and regional factors affect the community assembly processes that drive emergent patterns. This need is reflected in the growing use of the metacommunity concept to interpret multi‐scale measures of biodiversity, including metrics derived from diversity partitioning (e.g. α, β and γ diversity) and variation partitioning (e.g. spatial and environmental components of compositional turnover) techniques. However, studies have shown limited success using these metrics to characterize underlying community assembly dynamics. Here we demonstrate how a metacommunity simulation package (MCSim) can be used to evaluate when and how biodiversity metrics can be used to make inferences about metacommunity characteristics. We examined a wide range of parameter settings representing ecologically relevant scenarios. We used artificial neural networks (ANNs) to assess the sensitivity of diversity and variation partitioning metrics (calculated from simulation outcomes) to metacommunity parameter settings. In the scenarios examined in this study, the niche‐neutral gradient strongly influenced most biodiversity metrics, metacommunity size exhibited a marginal influence over some metrics, and dispersal dynamics only affected a subset of variation partitioning outcomes. Variation partitioning response curves along the niche‐neutral gradient were not monotonic; however, simulation outcomes suggest other biodiversity metrics (e.g. dissimilarity saturation) can be used in combination with variation partitioning metrics to make inferences about metacommunity properties. With the growing availability of archived ecological data, we expect future work will apply simulation‐based techniques to better understand links between biodiversity and the metacommunity characteristics that are presumed to control the underlying community assembly processes.
A New Scale for Assessing the Severity of Uncomplicated Diaper Dermatitis in Infants: Development and Validation
Authors: Buckley Brian S. , Mantaring Jacinto Blas , Dofitas Rodney B. , Lapitan Marie Carmela , Monteagudo Aceleen
Published: September 2016

Background

One methodologic challenge in conducting research relating to diaper dermatitis (DD) is the absence of a reliable, objective, validated scale for assessing severity. The aim of this study was to develop and validate such a scale.

Methods

Scale development was based on experience of DD assessment and clinical and photographic data collected during the early stages of a randomized controlled trial of two DD treatments. The severity score is the sum of scores of four domains: severity of erythema and irritation, area with any DD, papules or pustules, and open skin. Possible scores range from 0 (clear skin) to 6 (extensive DD including intense erythema, papules or pustules, and open skin with damage to the dermis). Assessors used the scale to attribute severity scores using high‐definition photographs of infants and babies with DD. Interrater reliability (IRR), internal consistency, and test–retest reliability were considered using intraclass correlation coefficients (ICCs), Cronbach's α, and Cohen κ statistics.

Results

IRR was very good between assessors familiar with the scale (ICC = 0.949, p < 0.001) and between assessors unfamiliar with the scale (ICC = 0.850, p < 0.001). Test–retest reliability at 2 weeks was good (κ = 0.603, p < 0.001). Cronbach's α for internal consistency was 0.702. Collation of photographs according to severity score revealed a visible continuum of DD severity, suggesting good construct validity.

Conclusion

The newly developed scale appears to be easy to use, reliable, and effective in detecting increasing or lessening DD severity.
Life Settlement Funds: Current Valuation Practices and Areas for Improvement
Authors: Braun Alexander , Affolter Sarah , Schmeiser Hato
Published: September 2016

We analyze the prevailing valuation practices in the life settlement industry based on a sample of 11 funds that cover a large portion of the current market. The most striking result is that a majority of asset managers seem to substantially overvalue their portfolios relative to the prices of comparable transactions that have recently been closed. Drawing on market‐consistent estimates with regard to medical underwriting, it is possible to trace back the observed discrepancies to inadequately low model inputs for life expectancies and discount rates. The main consequences are a dissimilar treatment of investor groups in open‐end funds structures as well as an unduly high compensation for managers and third parties. To address this predicament, we suggest defining life settlements as level 2 assets in the fair value hierarchy of IFRS 13, improving transparency and disclosure requirements, and developing new incentive‐compatible fee schedules.
Tolerance Design and Kinematic Calibration of a Four-Degrees-of-Freedom Pick-and-Place Parallel Robot
Authors: Huang Tian , Bai Pujun , Mei Jiangping , Chetwynd Derek G.
Published: September 2016

This paper presents a comprehensive methodology for ensuring the geometric pose accuracy of a 4DOF high-speed pick-and-place parallel robot having an articulated traveling plate. The process is implemented by four steps: (1) formulation of the error model containing all possible geometric source errors; (2) tolerance design of the source errors affecting the uncompensatable pose accuracy via sensitivity analysis; (3) identification of the source errors affecting the compensatable pose accuracy via a simplified model and distance measurements; and (4) development of a linearized error compensator for real-time implementation. Experimental results show that a tilt angular accuracy of 0.1/100 and a volumetric/rotational accuracy of 0.5 mm/±0.8 deg of the end-effector can be achieved over the cylindrical task workspace.
Distributed Multidisciplinary Optimization of a Turbine Blade Regarding Performance, Reliability and Castability
Authors: Buske Clemens , Krumme Alexander , Schmidt Thomas , Dresbach Christian , Zur Sascha , Tiefers Rüdiger
Published: September 2016

Modern aero-engine blades are optimized for high performance and long service life, but manufacturing requirements are not considered adequately during the design process. Thus, time-consuming, iterative re-designs become necessary until a producible component evolves. The multidisciplinary design optimization method presented in this paper addresses not only the aerodynamic efficiency and structural reliability of a new turbine blade, but also ensures the castability of the design and thereby accelerates the entire design process and reduces the time-to-production. Because real casting process simulations are very time-intensive, they were substituted by checks of experimentally and numerically validated geometrical constraints. Different engineering tools were assembled in a joint process chain using an integration framework, which manages and distributes the calculations and hence the workload in a shared network. Based on a preliminary design of a new turbine section, the selected initial low pressure turbine blade was neither castable nor reliable. The multidisciplinary optimization achieved a blade design that satisfies the requirements for a successful casting process, has a low failure probability and, although not as high as from a pure aerodynamic optimization, exhibits an efficiency improvement.
An Overview of Low-Emission Combustion Research at NASA Glenn
Authors: Reddy Dhanireddy R. , Lee Chi-Ming
Published: September 2016

An overview of research efforts at NASA Glenn Research Center (GRC) in low-emission combustion technology that have made a significant impact on the nitrogen oxides (NOx) emission reduction in aircraft propulsion is presented. The technology advancements and their impact on aircraft emissions are discussed in the context of NASA’s Aeronautics Research Mission Directorate (ARMD) high-level goals in fuel burn, noise and emission reductions. The highlights of the research presented here show how the past and current efforts laid the foundation for the engines that are flying today as well as how the continued technology advancements will significantly influence the next generation of aviation propulsion system designs.
Innovative Shallow Sump Customizations for Aero-Engine Bearing Chambers
Authors: Chandra Budi , Simmons Kathy
Published: September 2016

Aero-engines incorporate various bearing chambers and these typically contain bearings, seals, rotating shafts, stationary walls and struts, and sometimes gears. Oil is supplied for lubrication and cooling and is removed (scavenged) from the sump region of the chamber (note that in some parts of the world the entire bearing chamber is referred to as the sump). Depending on the location and function of the bearing chamber, the sump region may be deep or shallow. Effective oil removal is essential as unnecessary working of the oil can lead to excessive heat generation and reduced overall efficiency. Therefore the design of the scavenge region in a bearing chamber, as well as the ability to assess its performance is very important.

Previous work, much of which was conducted at the University of Nottingham Technology Centre in Gas Turbine Transmission Systems (UTC) suggests that oil often does not flow cleanly into the off-take due to a combination of several factors: oil momentum, windage, three-dimensional air flow that blocks the off-take flow or transports oil away from the off-take, and pooling because of separated air flow that acts on the oil once oil momentum is dissipated.

Experimental research at the UTC found that scavenge performance is highly affected by the sump geometry, especially its depth. Variations of shallow sumps, although some are better than the others, cannot offer the same level of performance as a deep one. However space limitation in an engine often only allows for a shallow sump. This paper presents some experimental exploration on new design ideas. They are in the form of various inserts and attachments that were designed to improve scavenge performance of a shallow sump. These “custom” sumps were tested on the UTC’s scavenge test facility at various flow settings (wall film/flying droplets, liquid flow rates, scavenge ratios, shaft speeds). The residence volumes were measured and compared to a baseline configuration with reduction in residence volume desirable.

The inserts tested were a Grille Cover, a Stepped Spillway, a Perforated Plate and a Porous Insert. Both the Porous Insert and the Perforated Plate showed reduced residence volumes in the demanding droplet/windage dominated flow condition with the Perforated Plate offering the best improvement over baseline.
Investigation of Vaned Diffuser Splitters on the Performance and Flow Control of High Pressure Ratio Centrifugal Compressors
Authors: He Xiao , Zheng Xinqian , Wei Jie , Zeng Hanxuan
Published: September 2016

High pressure ratio centrifugal compressors with vaned diffusers are required in modern gas turbine engines. A potential method to further improve the performance of such compressors is to apply splitters to the vaned diffuser. This paper numerically investigates design choices of splitters in a vaned diffuser of a high pressure ratio centrifugal compressor. The splitters are derived from the main vane but with different chord length. The total vane number has been adjusted to maintain the same diffuser flow capability relative to the impeller. When the splitter is designed in the vaned region of the datum diffuser, the flow separation has been successfully suppressed with increasing relative splitter chord length from 0.10 to 0.70. At high mass flow rate conditions, the stage efficiency and the diffuser pressure recovery coefficient increases by 1.0% and 0.04 respectively due to the suppression of the flow separation. However, at design mass flow rate, the stage efficiency and the diffuser pressure recovery coefficient drops slightly by 0.3% and 0.01 respectively because the enhanced friction loss of vane surfaces is now dominant. When the splitter vane reaches the semi-vaneless region by further increasing its chord length from 0.8 to 0.95, the shock structure changes from two parallel shocks to two sequential shocks at first, but turns to the datum shock structure at last. The stage efficiency and the diffuser pressure recovery coefficient of the sequential shock structural case significantly drops by 0.7% and 0.03 respectively due to the increased shock strength and thus the higher shock loss. It is indicated that the application of splitter vanes in the vaned region of the diffuser is beneficial when the induced friction loss from splitter surfaces is less significant or the reduced loss from the suppressed flow separation is more dominant, which is the case that the flow separation happens relatively downstream or the case operating at large incidence conditions.
A Fractal Contact Friction Model and Nonlinear Vibration Response Studies of Loosely Assembled Blade With Dovetail Root
Authors: Bo Shangguan , Feilong Yu , Jingyao Duan , Song Gao , Junfeng Xiao , Xu Zili
Published: September 2016

To investigate the friction damping effect of a loosely assembled blade with dovetail root, a fractal contact friction model is proposed to describe the friction force. In the proposed model, the friction contact interface is discretized to a series of friction contact pairs and each of them can experience stick, slip, or separate. Fractal geometry is used to simulate the topography of contact surfaces. The contact stiffness, which is related to the parameters of contact interfaces including normal load, roughness, Young’s modulus, and Poisson’s ratio, is calculated using Hertz contact theory and fractal geometry. The nonlinear vibration response of loosely assembled blade with dovetail root is predicted using the proposed model, the multiharmonic balance method and Newton iterative algorithm. The effect of centrifugal force, friction coefficient and exciting force on the forced response of a loosely assembled blade with dovetail root is studied. The numerical vibration responses are compared to the experimental results. It will verify the reliability of the numerical method and provide theoretical basis for structure design of the loosely assembled blade with dovetail root.
Investigation of Penny Leakage Flows of Variable Guide Vanes in High Pressure Compressors
Authors: Wolf Hannes , Franke Matthias , Halcoussis Alexander , Kleinclaus Christophe , Gautier Sébastien
Published: September 2016

In this paper, the modelling of leakages through a compressor stator penny cavity, and their effect on the aerodynamics within the compressor are studied. The penny, sometimes also referred to as ‘button’, is the cylindrical platform feature of a variable stator normally found between a vane’s airfoil and spindle. The pennies nominally lie recessed into the compressor endwalls at hub and casing, with a surrounding clearance to ensure the vane’s stagger angle can be adjusted. RANS-simulations, with these clearances included, have shown a significant impact from the penny cavity leakages on compressor efficiency and surge line. Neglecting this secondary flow path through the penny cavities results in an under prediction of the losses close to the endwalls.

The prediction of the penny cavity effect on the stator row is based on a Reynolds-Averaged-Navier-Stokes (RANS) study, using a hybrid structured-unstructured mesh to provide adequate resolution of the local flow phenomena.

The complex geometry and pressure field result in flows that are unevenly distributed within the penny cavity. The outflow or leakage is focused in a concentrated area leading to a high local velocity that strongly impacts the stator losses and turning. Since such geometries lie beyond the normal validated cases, the modelling uncertainties are discussed and the plausibility of the results is checked. In order to provide an experimental database and validate the turbulent mixing of leakage and main flow, which is seen as the main contributor to loss production, a validation test case — ‘Jet-In-Crossflow’ was chosen. As well as the standard RANS code, this validation case was run as a time-accurate high-order Lattice Boltzmann (LBM) simulation (PowerFLOW), using Very-Large-Eddy-Simulation (VLES) turbulence modelling. The LBM simulation showed significant unsteady flow features and was considerably closer to the test data than the RANS calculations.

A future test campaign, currently being prepared at the annular cascade test facility of the Institute of Jet Propulsion and Turbomachinery (IST) at RWTH Aachen university, will be briefly presented. This focuses on investigating the penny flows in a typical engine design.
Reacting Pressurized Spray Combustor Dynamics: Part 1 — Fuel Sensitivities and Blowoff Characterization
Authors: Rock Nicholas , Chterev Ianko , Smith Travis , Ek Hanna , Emerson Benjamin , Noble David , Seitzman Jerry , Lieuwen Tim
Published: September 2016

Blowoff sets important operational limits on a combustor system. While blowoff is intrinsically a system-dependent phenomenon, it is also dependent on the chemical and physical properties of the fuel. This paper describes an experimental study of the lean blowout limits of eight liquid fuels in a swirl-stabilized combustor, with data for both a pressure atomizer and an airblast atomizer. Three of the fuels were traditional jet fuels (an average Jet-A, JP-5, and JP-8) and the remaining five fuels spanned a range of physical and kinetic properties. These experiments were performed at a combustor pressure of 345 kPa and an air temperature of 450 K. In addition to some sensitivity of blowoff conditions to the thermal state of the combustor, results also clearly show sensitivities to fuel composition. Strong correlations were observed for pressure atomizer blowoff with fuel physical properties, particularly for boiling point temperature, indicating that fuels less easily atomized and vaporized are harder to blow off. These results are consistent with the idea that delaying atomization and/vaporization, and therefore reducing the level of premixing that drives the local fuel-air ratio towards the very lean global fuel/air ratio, is advantageous in order to promote regions of locally elevated flame temperatures. We suggest that this behavior occurs when the air preheat temperature is above the fuel flashpoint, as a similarly good correlation with boiling point temperature but with the opposite trend, has been previously reported in a study obtained for preheat temperatures below the fuel flashpoint. In contrast, the airblast results do not show strong correlations with fuel physical properties. Rather, the best correlation of the airblast atomizer results is with the percentage of iso-paraffins in the fuel. We speculate that this reflects a sensitivity to kinetic properties of the fuel, as the superior atomization characteristics of the airblast atomizer may de-emphasize the importance of physical properties.
Numerical Study on Effects of Axial-Slot Casing Treatment on Peak Efficiency of Axial Compressors
Authors: Ma Ning , Nan Xi , Lin Feng
Published: September 2016

Axial compressors can obtain substantial improvement on stall margin by using axial-slot casing treatments. However, this type of casing treatment usually yields large peak efficiency penalty due to the interaction between the slots and rotor tip region where the tip leakage flow plays an important role. Therefore, as a main factor that influences the peak efficiency, the tip leakage loss was examined in this paper with a variety of slot geometries. Unsteady numerical simulations were performed on both low speed and transonic compressors with axial skewed slot casing treatments with different geometric parameters. In addition, an equation which can be applied to evaluate the tip leakage loss under casing treatment cases was derived from Denton’s leakage mixing model. The leakage loss can be expressed in terms of the cube of the tip leakage flow rate. Combined with the simulation results, the effects of the number, depth and width of the slots on both the leakage loss and peak efficiency deficit were investigated. For the transonic compressor, the impacts of shock wave and its interaction with the tip leakage flow /vortex were assessed as well. Lastly, two axial-slot casing treatments with an isosceles-trapezoid shaped opening were designed to reduce the loss in the rotor tip region. It was shown that the newly designed axial-slot casing treatments were capable of improving the peak efficiency of both compressors.
Near Stall Behavior of a Transonic Compressor Rotor With Casing Treatment
Authors: Brandstetter Christoph , Holzinger Felix , Schiffer Heinz-Peter , Stapelfeldt Sina , Vahdati Mehdi
Published: September 2016

The aerodynamic and aeroelastic performance of an advanced axial slot casing treatment (CT) was investigated on a modern one and a half stage transonic compressor test rig. It is generally accepted that a well designed CT can extend the aerodynamic stability range of a compressor to lower mass flows. The extension of stall margin of the compressor rotor blades by using CT has been the subject of numerous research articles but much less attention has been paid to the behavior of the compressor in direct vicinity of the stability limit. For the compressor investigated here, two different phenomena were repeatedly observed near stall: 1) self-excited blade vibration, and 2) low engine order fluctuations developing into rotating stall. The current investigation firstly aims to identify the triggers for each of these phenomena. It then focusses on the aerodynamic and aeromechanical mechanism which lead to the formation of low engine order fluctuations shortly before stall.

In order to measure the unsteady and transient effects, the system was instrumented with unsteady wall pressure transducers, a capacitive tip-timing system and strain gauges on the rotor blades. The flow structure in the blade tip region was measured via Particle Image Velocimetry underneath the CT-Cavities. Measurements showed a strong correlation between CT activity and the development of the low frequency oscillations with associated blade vibrations. Using numerical simulations, presented and validated in this paper, this correlation was attributed to an aerodynamic coupling between rotor passages through the recirculation of fluid inside the cavities.
Validation of Surface Temperature Measurements on a Combustor Liner Under Full-Load Conditions Using a Novel Thermal History Paint
Authors: Krewinkel Robert , Färber Jens , Lauer Martin , Frank Dirk , Orth Ulrich , Gonzalez Alvaro Yañez , Pilgrim Christopher , Feist Jörg , Saggese Raffaele , Berthier Stéphane , Araguas Silvia
Published: September 2016

The ever-increasing requirements on gas turbine efficiency, which are at least partially met by increasing firing temperatures, and the simultaneous demand for reduced emissions, necessitate much more accurate calculations of the combustion process and combustor wall temperatures.

Thermocouples give locally very accurate measurements of these temperatures, but there is a practical limit to the amount of measurement points. Thermal paints are another established measurement technique, but are toxic and at the same time require dedicated, short-duration tests. Thermal History Paints (THPs) provide an innovative alternative to the aforementioned techniques, but so far only a limited number of tests has been conducted under real engine conditions.

THPs are similar in their chemical and physical make-up to conventional thermographic phosphors which have been successfully used in gas turbine applications for on-line temperature detection before. A typical THP comprises of oxide ceramic pigments and a water based binder. The ceramic is synthesized to be amorphous and when heated it crystallizes, permanently changing the microstructure. The ceramic is doped with lanthanide ions to make it phosphorescent. The lanthanide ions act as atomic level sensors and as the structure of the material changes, so do the phosphorescent properties of the material. By measuring the phosphorescence the maximum temperature of exposure can be determined through calibration, enabling post operation measurements at ambient conditions.

This paper describes a test in which THP was applied to an impingement-cooled front panel from a combustor of an industrial gas turbine. Since this component sees a wide range of temperatures, it is ideally suited for the testing of the measurement techniques under real engine conditions. The panel was instrumented with a thermocouple and thermal paint was applied to the cold side of the impingement plate. THP was applied to the hot-gas side of this plate for validation against the other measurement techniques and to evaluate its resilience against the reacting hot gas environment.

The durability and temperature results of the three different measurement techniques are discussed. The results demonstrate the benefits of THPs as a new temperature profiling technique. It is shown that the THP exhibited greater durability compared to the conventional thermal paint. Furthermore, the new technology provided detailed measurements down to millimeters indicating local temperature variations and global variations over the complete component.
Numerically Derived Design Guidelines of Self Recirculation Casing Treatment for Industrial Centrifugal Compressors
Authors: Jung Sewoong , Pelton Robert
Published: September 2016

Casing treatments are a well-known method to extend operating range of centrifugal compressors. A common casing treatment configuration consists of a passage that allows flow from the impeller shroud to be bleed back to the inlet of the stage. This type of casing treatment is used frequently in some applications, including automotive turbochargers, but is rarely used in industrial compressors. An effective casing treatment must be developed specifically to match a given stage. For high volume production products this is practical. For the wide variety of industrial compressor designs, which are produced in low volumes, it is not often cost effective to design a casing treatment to match each application. To help reduce the design effort associated with developing a new casing treatment design, a simple set of design guidelines were developed. These guidelines are based on the results of a computational fluid dynamic (CFD) study of several different class impellers. These guidelines can be used to correctly locate and size the key geometric features of a self-recirculating casing treatment, including slot width, position and cavity vane profile.

The study found that the bleed slot position and width were the primary factors controlling performance of the casing treatment. In general, when the slot width is wider and the bleed position is moved further downstream, the range increases but the efficiency falls. The optimal slot width is found to be when the slot area is 23% of the area of inducer eye and positioning the slot near the impeller throat gives a good balance of increased range with minimal efficiency loss. A well designed casing treatment is expected to result in approximately a 25% increase in range while keeping the drop in efficiency less than 0.4 point for the cases considered in this study. In addition, the rise to surge increased more than 60% and turndown almost doubled value with the optimized design.
Numerical Investigation of the Blade Tip and Overtip Casing Aerothermal Performance in a High Pressure Turbine Stage
Authors: Du Kun , Li Zhigang , Li Jun
Published: September 2016

In modern transonic gas turbine engines, the blade tip and overtip casing endures high thermal load, therefore these components are always subjected to thermal failures due to large unsteady heat flux. The unsteadiness is induced by the interaction of the rotor blades and periodic upstream wake of the vanes. The present study adopts a typical high pressure gas turbine stage (GE-E3 engine), and the computational domain consists of 1 high pressure stator vane and 2 rotor blades. The rotor blade in question has a squealer tip with a clearance gap about 1% of the blade height. This study focuses on the physics of the heat transfer characteristic of the blade tip and overtip casing regions. The present simulations were conducted using three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) commercial code at the real engine conditions ( Mexit = 1.07, n = 8450rpm ). The standard k–ω turbulence model was utilized to model the turbulence. The accuracy of CFD predictions has been validated by comparison with the experimental data. The steady, unsteady and time-averaged results on the blade tip and overtip casing have been observed and discussed. Results indicate that the depth of the cavity has great influence on the blade tip and overtip casing. The averaged heat transfer coefficient on the blade tip is reduced with the increase of the cavity depth, however, the thermal load on the blade tip presents a contrary tendency. Moreover, the largest unsteadiness was observed for the case with D = 3.0 among the cases investigated, especially near the suction side squealer. In addition, the variation of the cavity depth has little effect on the heat transfer coefficient and thermal load on the overtip casing.
Effect of Surface Roughness on Conjugate Heat Transfer of a Turbine Vane
Authors: Li Hongyang , Zheng Yun
Published: September 2016

For the purpose of researching the effect of surface roughness on boundary layer transition and heat transfer of turbine blade, a roughness modification approach for γ-Reθ transition model was proposed based on an in-house CFD code. Taking surface roughness effect into consideration, No. 5411 working condition of Mark II turbine vane was simulated and the results were analyzed in detail. Main conclusions are as follows: Surface roughness has little effect on heat transfer of laminar boundary layer, while has considerable effect on turbulent boundary layer. Compared with smooth surface, equivalent sand roughness of 100μm increases the temperature for about 28.4K on suction side, reaching an increase of 5%. Under low roughness degree, effect of shock wave dominants on boundary layer transition process on suction side, while above the critical degree, effect of surface roughness could abruptly change the transition point.
Large-Eddy Simulation of a High-Pressure Turbine Vane With Inlet Turbulence
Authors: Jee Solkeun , Joo Jongwook , Medic Gorazd
Published: September 2016

It is important to be able to predict the state of the boundary layer on a high-pressure turbine vane with incoming turbulence. The laminar-to-turbulent transition significantly changes the heat transfer on the blade, which impacts the turbine performance and durability assessment. In this study, an experimental cascade with a high-pressure turbine vane was simulated with large-eddy simulation (LES). To incorporate the impact of turbulence from the freestream on the boundary layer, appropriate turbulence data sets were generated in separate direct-numerical simulation and fed into the LES inlet. The state of the boundary layer is reasonably well predicted for the test condition of the turbulence intensity of Tu=4%. Streamwise vortical structures and spanwise two-dimensional waves are observed in the transitional boundary layer. Discrepancy for the case with the higher turbulence intensity Tu=6% is discussed with the focus on the need for more detailed information about the freestream disturbances.
Fan Blade Design Methods: Cascade Versus Isolated Airfoil Approach — Experimental and Numerical Comparison
Authors: Castegnaro Stefano
Published: September 2016

Both cascade and isolated airfoil methods are considered valid in axial fan blade design, for high (σ≳1) and low (σ≲0.7) solidities respectively. For bladings that feature intermediate solidities the modified isolated approach is commonly employed. This method uses isolated airfoil data, with proper adjustments to take into account multiplane interference effects. Contrarily, the literature does not refer about modifications of the cascade approach to design medium solidity fans. Such method would use cascade data, properly adjusted for the blade sections at lower solidities. Thus, with the aim of comparing these two opposite design approaches (modified cascade versus modified isolated) for medium solidity blades, two free-vortex blading were designed for a 315 mm rotor-only axial fan and experimentally tested. CFD analyses were performed as well to obtain the local flow features. NACA-65 series airfoils were employed, as both cascade and isolated data are available for chord Reynolds numbers typical of axial fans applications. Results highlight the differences between the two approaches. Finally, a mixed approach that employs both isolated and cascade data is suggested as the most accurate one. Moreover, results also show the detrimental effects of the low chord Reynolds numbers on the performance of the blades. This effect should be taken into account in blade design for small-to-medium size machines.
Impingement Jet Cooling With Ribs and Pin Fin Obstacles in Co-Flow Configurations: Conjugate Heat Transfer Computational Fluid Dynamic Predictions
Authors: El-Jummah Abubakar M. , Andrews Gordon E. , Staggs John E. J.
Published: September 2016

Conjugate heat transfer (CHT) computational fluid dynamics (CFD) predictions were carried out for impingement heat transfer with obstacle (fins) walls on the target surface midway between the impingement jets and aligned in the direction of the crossflow (direction of outflow of the impingement cooling air) to minimise the pressure loss increase due to the fins. A single sided flow exit was used in a geometry that was applicable to reverse flow cooling of low NOx combustors, but was also relevant to turbine blade and nozzle cooling. A 10 × 10 row of impingement jet holes (hole density n of 4306 m−2) was used, which had ten rows of holes in the cross-flow direction. One heat transfer enhancement obstacle per impingement jet was investigated and compared with previously published experimental results, for Nimonic 75 metal walls, for flow pressure loss and surface averaged heat transfer coefficients. Two different shaped obstacles were investigated with an impingement gap, Z, of 10mm: a continuous rectangular rib 4.5mm high (H) and 3.0 mm thick and a rectangular pin-fin rib with ten 8mm high (H) pins that were 8.6mm wide and 3.0 mm thick. The obstacles were equally spaced on the centreline between each row of impingement jets aligned with the crossflow. The impingement jet pitch to diameter X/D and gap to diameter Z/D ratios were kept constant at 4.66 and 3.06 for X, Z and D of 15.24, 10.00 and 3.27 mm, respectively. The two obstacles investigated had obstacle height to diameter ratio H/D of 1.38 and 2.45. The computations were carried out for three different air coolant mass fluxes G of 1.08, 1.48 and 1.94 kg/sm2bar. The pressure loss ΔP/P and surface average heat transfer coefficient (HTC) h predictions for all three G showed good agreement with the experimental results. The predicted results were also compared with the impingement jet single exit flow, for a smooth target wall of the same impingement hole configuration. A significant increase in the overall surface averaged heat transfer was predicted and measured for the co-flow configuration with rectangular pin-fins. This was a 20% improvement at low coolant flow rates for the rectangular pin fin obstacles and 15% for the ribs. At high coolant flow rates the improvement was smaller at 5% for the rectangular pin fins and 1% for the rectangular ribs.
Experimental Assessment of Noise Generation and Transmission in a High-Pressure Transonic Turbine Stage
Authors: Knobloch Karsten , Neuhaus Lars , Bake Friedrich , Gaetani Paolo , Persico Giacomo
Published: September 2016

The noise originating from the core of an aero-engine is usually difficult to quantify and the knowledge about its generation and propagation is less advanced than that for other engine components. In order to overcome the difficulties associated with dynamic measurements in the crowded core region, dedicated experiments have been set up in order to investigate the processes associated with the generation of noise in the combustor, its propagation through the turbine and the interaction of these two components, which may produce additional — so-called indirect combustion — noise. In the current work, a transonic turbine stage installed at the Laboratorio di Fluidodinamica delle Macchine of the Politechnico di Milano was exposed to acoustic, entropic, and vortical disturbances. The incoming and outgoing sound fields were analyzed in detail by two large arrays of microphones. The mean flow field and the disturbances were carefully mapped by several aerodynamic and thermal probes. The results include transmission and reflection characteristics of the turbine stage, latter one was found to be much lower than usually assumed. The modal decomposition of the acoustic field in the upstream and downstream section show beside the expected rotor-stator interaction modes additional modes. At the frequency of entropy or respectively vorticity excitation, a significant increase of the overall sound power level was observed.
Dynamic Analysis of a Recuperated mGT Cycle for Fuel Cell Hybrid Systems
Authors: Larosa Luca , Traverso Alberto , Massardo Aristide F.
Published: September 2016

This work presents the dynamic behaviour of a new recuperated micro gas turbine (mGT) coupled with a large volume. Such system, called “emulator”, has been purposely designed for the future upgrade into a fuel cell mGT hybrid system. The tests, carried out by LG Fuel Cell Systems (LGFCS), aimed both at understanding the dynamic behaviour of the system and validating the dynamic simulation tool.

Within the wide experimental campaign, a subset of data has been selected to identify the key transient phenomena and characterise the dynamic behaviour of the system: in this respect, the focus is on start-up, warm-up and shutdown phases.

A dynamic model of the emulator was developed, based on the original software TRANSEO. The model was used to characterise the mGT performance and identify a performance gap in the expander. For this purpose, the machine was upgraded and substituted. Final results show that, after refinement of input data, the model is capable to predict accurately the overall system transient behaviour.
Modeling Flame Describing Functions Based on Hydrodynamic Linear Stability Analysis
Authors: Oberleithner Kilian , Paschereit Christian Oliver
Published: September 2016

Combustion instabilities crucially affect the operational range of modern lean premixed gas turbine combustors and must be avoided or kept at low amplitudes. The main uncertainty of current prediction models is the flame describing function (FDF) that characterizes the flame response to high amplitude acoustic forcing. In this work, we present a new FDF model based on linear hydrodynamic stability analysis. This work is in continuation of an earlier study, where the frequency dependence and saturation of the FDF gain of a perfectly premixed flame was linked to the growth rates of the Kelvin–Helmholtz (KH) instability. In this work, we report on FDF measurements in a newly designed swirl-stabilized combustor. We identify two independent mechanisms that determine the flame response. The first stems from swirl-fluctuations that are generated in the swirler and the second stems from the KH instability. The swirl-fluctuations are approximated by a convective time lag model. The KH instability is predicted from linear hydrodynamic stability analysis based on the time-mean flow measured via PIV. A combination of both models leads to a good quantitative agreement with the measured FDF. Besides the practical advantages of predicting the FDF from stationary flow data, the model reveals the mechanisms driving the saturation of the FDF and guides the way out from the black-box treatment of the nonlinear flame response.
A Comparative Study on N-Dot Acceleration Technique
Authors: Yang Shubo , Wang Xi
Published: September 2016

For aero-engines, the throttle command is often changed dramatically, which will perturb the control error (defined as actual rotor speed minus desired rotor speed) of the primary set-point controller. When the perturbation is amplified by the gain, the controller will output unreasonable values of control variables, i.e. the mass of fuel flow, and cause abnormal engine operation. For this reason, limit protection controllers are applied to constrain the controlled variables at a safe level. Besides, the transient control modes are required to provide smooth, stall free operation of the engine.

The schedule-based approach, which is the traditional transient control mode, is easy to implement but the performance of acceleration and deceleration will suffer from degradation or manufacturing errors. With the development of digital control system, N-dot control mode has been adopted in some modern aero-engines, which focuses on rotor acceleration rather than rotor speed. To some extent, this method can overcome the obstacles of the schedule-based approach.

In terms of N-dot control mode, there are two main methods: direct control and indirect control. The former one suggests using a differentiator to get the actual N-dot value, then minus it by the desired N-dot value to get the error of N-dot. When the error is reduced to zero by a controller, the actual N-dot value follows the desired N-dot value. The latter one suggests inputting the desired N-dot value to an integrator for a rotor speed value, which essentially transforms the N-dot command to the speed command. With this transformation, the familiar set-point controller can be used to control the engine following the N-dot command indirectly.

This paper presents implementation schemes of the two types of N-dot control, and focuses on a comparative study of them. To avoid integral windup issue when the indirect method switches controllers, such as from N-dot controller to set-point controller, we have introduced a logic to determine whether the integrator is operational. This design allows flexible switchings.

After frequency domain analysis, we find out that the essential difference between the two schemes lies in the magnitude of crossing frequency. The direct N-dot control, with a higher crossing frequency, has faster responses but is sensitive to noise. While the indirect N-dot control, with a lower crossing frequency, has slower responses but can suppress noise. When the dynamic nature of sensor and actuator is considered, the direct N-dot control with a higher crossing frequency may cause the close-loop system unstable.

Using a reliable aero-engine mathematical model, we designed a set of simulations to test the two N-dot schemes. The simulation results showed that the direct N-dot control performed better than the indirect one under ideal situation. When noise or dynamic nature of sensor and actuator was taken into consideration, however, the indirect N-dot control was more robust, which confirmed the analysis above.
Numerical Investigation of the First Booster Stage Tone Noise of a High Bypass Ratio Turbofan
Authors: Rossikhin Anton , Pankov Sergey , Mileshin Victor
Published: September 2016

The results of the first booster stage tone noise numerical investigation for a model of low pressure compressor are presented. The investigation was performed using the frequency domain numerical method of multistage turbomachines tone noise simulation, developed in CIAM (Central Institute of Aviation Motors) and implemented in the 3DAS (3 Dimensional Acoustics Solver) in-house solver. The model under consideration included high bypass ratio fan, stator, booster and bypass duct. Calculation was performed at the approach operating conditions. Far field directivities for two tones in the forward hemisphere were obtained. One tone corresponded to the blade passing frequency of the first stage rotor, the other - to the sum of this frequency with the blade passing frequency of the fan. The results of the computation were compared with the experimental data obtained in the CIAM C-3A acoustic test facility. In general satisfactory correspondence between calculation and experiment was obtained.