bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotordynamics Performance of Hybrid Foil Bearing Under Forced Vibration Input
Authors: Kim Daejong , Nicholson Brian , Rosado Lewis , Givan Garry
Published: August 2017

Foil bearings (FB) are one type of hydrodynamic air/gas bearings but with a compliant bearing surface supported by structural material that provides stiffness and damping to the bearing. The hybrid foil bearing (HFB) in this paper is a combination of a traditional hydrodynamic foil bearing with externally pressurized air/gas supply system to enhance load capacity during the start and to improve thermal stability of the bearing. The HFB is more suitable for relatively large and heavy rotors where rotor weight is comparable to the load capacity of the bearing at full speed and extra air/gas supply system is not a major added cost. With 4448–22,240 N thrust class turbine aircraft engines in mind, the test rotor is supported by HFB in one end and duplex rolling element bearings (REB) in the other end. This paper presents experimental work on HFB with diameter of 102 mm performed at the U.S. Air force Research Laboratory (AFRL). Experimental works include: measurement of impulse response of the bearing to the external load corresponding to rotor's lateral acceleration of 5.55 g, forced response to external subsynchronous excitation, and high-speed imbalance response. A nonlinear rotordynamic simulation model was also applied to predict the impulse response and forced subsynchronous response. The simulation results agree well with the experimental results. Based on the experimental results and subsequent simulations, an improved HFB design is also suggested for higher impulse load capability up to 10 g and rotordynamics stability up to 30,000 rpm under subsynchronous excitation.
Basic Study on Calculation of Cutting Forces Useful for Reducing Vibration in Skiving
Authors: Tachikawa Tomokazu , Iba Daisuke , Kurita Nobuaki , Nakamura Morimasa , Moriwaki Ichiro
Published: August 2017

In order to improve the accuracy of skived gears by means of reducing vibrations that are often observed during the cutting process, a simple model for calculating the cutting forces of skiving process is presented and also its effectiveness was discussed. The model is characterized by simple geometrical calculations, and the cutting forces were assumed as sum of vectors that represent the penetration of cutting edges. In the model, multiple cutting edges that are simultaneously meshing with the workpiece were considered. Distinguished oscillating frequencies of the calculated cutting forces and the natural frequencies of the clamped workpiece and of the cutter were carefully analyzed in order to predict the cutter rotation speed that was most likely to reduce undesired vibrations. Processing experiments conducted at several cutter rotation speeds showed that the predicted cutter rotation speed which could significantly reduce undesired vibrations was very effective and enabled the quality of a skived gear to be improved. Consequently, the proposed calculation model was enough effective and useful for operation conditions such as cutter rotation speed being determined.
Exploring Effective Change Propagation in a Product Family Design
Authors: Ullah Inayat , Tang Dunbing , Wang Qi , Yin Leilei
Published: August 2017

Product family (PF) design is a widely used strategy in the industry, as it allows meeting diverse design requirements. Change propagation in any PF is difficult to predict. Consequently, while numerous design change management methodologies presently exist, their application is restricted to a single artifact. This issue is overcome in the present study. The proposed framework explores effective change propagation paths (CPPs) by considering the risks associated with design changes in the PF with the aim of minimizing the overall redesign cost. The propagated risk, which would result in rework, is quantified in terms of change impact and propagation likelihood. Moreover, a design structure matrix (DSM) based mathematical model and an algorithm for its implementation are proposed to investigate the change propagation across the PF. Finally, to demonstrate their effectiveness, a PF of electric kettles is examined in a case study. The study findings confirm that the proposed technique is appropriate for evaluating different CPPs in PF.
Effects of Loading Conditions and Skull Fracture on Load Transfer to Head
Authors: Zhang Timothy G. , Thompson Kimberly A. , Satapathy Sikhanda S.
Published: August 2017

This study focuses on the effect of skull fracture on the load transfer to the head for low-velocity frontal impact of the head against a rigid wall or being impacted by a heavy projectile. The skull was modeled as a cortical–trabecular–cortical-layered structure in order to better capture the skull deformation and consequent failure. The skull components were modeled with an elastoplastic with failure material model. Different methods were explored to model the material response after failure, such as eroding element technique, conversion to fluid, and conversion to smoothed particle hydrodynamic (SPH) particles. The load transfer to the head was observed to decrease with skull fracture.
Sensitivity Analysis of the One-Control Volume Bulk-Flow Model for a 14 Teeth-on-Stator Straight-Through Labyrinth Seal
Authors: Cangioli Filippo , Pennacchi Paolo , Riboni Giacomo , Vannini Giuseppe , Ciuchicchi Lorenzo , Vania Andrea , Chatterton Steven
Published: August 2017

Since the 80s, academic research in the rotordynamics field has developed mathematical treatment for the prediction of the dynamic coefficients of sealing components. Dealing with the straight-through labyrinth seal, Iwatsubo [1], at a first stage, and Childs [2], later on, have developed the one-control volume bulk flow model. The model allows evaluating the surrounding fluid forces acting on the rotor, analyzing the fluid dynamics within the seal: the continuity, circumferential momentum and energy equations are solved for each cavity. To consider axial fluid dynamics, correlations, aiming to estimate the leakage and the pressure distribution, are required. Several correlations have been proposed in the literature for the estimation of the leakage, of the kinetic energy carry-over coefficient (KE), of the discharge coefficient and of the friction factor.

After decades of research in the field of seal dynamics, the bulk-flow model has been confirmed as the most popular code in the industries, however, it is not clear which is the best set of correlations for the prediction of seal dynamic coefficients.

This paper allows identifying the most accurate combination of correlations to be implemented in the bulk-flow model. The correlations are related to: the leakage formula, the flow coefficient, the KE and the friction factor. Investigating the results of several models (32 models), which consider different sets of correlations, in comparison to the experimental data (performed by General Electric Oil & Gas), it is possible to observe the dependence, of the model correlations, on the operating conditions.

The experimental results, performed using a 14 teeth-on-stator labyrinth seal, investigate several operating conditions of pressure drop.
Inhomogeneous Strain Distribution in SiCf/SiC Coupons Under Tensile Loading
Authors: Newton Christopher D. , Jones Jonathan P. , Chamberlain Adam L. , Bache Martin R.
Published: August 2017

The complex structural architecture and inherent processing artefacts within ceramic matrix composites combine to induce inhomogeneous deformation and damage prior to ultimate failure. Bulk measurements of strain via extensometry or even localised strain gauging will fail to characterise such inhomogeneity when performing conventional mechanical testing on laboratory scaled coupons. The current research project has, therefore, applied digital image correlation (DIC) techniques to the room temperature axial assessment of a SiCf/SiC composite under static and ratchetted loading.

As processed SiCf/SiC panels were subjected to detailed X-ray computed tomography (XCT) inspection prior to specimen extraction and subsequent mechanical testing. In situ DIC strain measurements were taken throughout the period of room temperature monotonic and ratchet style tensile tests. Contemporary acoustic emission (AE) signals were also recorded to indicate significant damage events and the onset of ultimate failure. Data from these separate monitoring techniques were correlated to indicate the sensitivity or otherwise to pre-existing artefacts within the as received CMC panels.
Theoretical Model of Buoyancy-Induced Heat Transfer in Closed Compressor Rotors
Authors: Tang Hui , Owen J. Michael
Published: August 2017

The cavities between the rotating compressor discs in aeroengines are open, and there is an axial throughflow of cooling air in the annular space between the centre of the discs and the central rotating compressor shaft. Buoyancy-induced flow occurs inside these open rotating cavities, with an exchange of heat and momentum between the axial throughflow and the air inside the cavity. However, even where there is no opening at the centre of the compressor discs — as is the case in some industrial gas turbines — buoyancy-induced flow can still occur inside the closed rotating cavities. The closed cavity also provides a limiting case for an open cavity when the axial clearance between the cobs — the bulbous hubs at the centre of compressor discs — is reduced to zero.

Bohn and his co-workers at the University of Aachen have studied three different closed-cavity geometries, and they have published experimental data for the case where the outer cylindrical surface is heated and the inner surface is cooled. In this paper, a buoyancy model is developed in which it is assumed that the heat transfer from the cylindrical surfaces is analogous to laminar free convection from horizontal plates, with the gravitational acceleration replaced by the centripetal acceleration. The resulting equations, which have been solved analytically, show how the Nusselt numbers depend on both the geometry of the cavity and its rotational speed. The theoretical solutions show that compressibility effects in the core attenuate the Nusselt numbers, and there is a critical Reynolds number at which the Nusselt number will be a maximum. For the three cavities tested, the predicted Nusselt numbers are in generally good agreement with the measured values of Bohn et al. over a large range of Raleigh numbers up to values approaching 1012. The fact that the flow remains laminar even at these high Rayleigh numbers is attributed to the Coriolis accelerations suppressing turbulence in the cavity, which is consistent with recently-published results for open rotating cavities.
High-Fidelity Simulations of a Linear HPT Vane Cascade Subject to Varying Inlet Turbulence
Authors: Pichler Richard , Sandberg Richard D. , Laskowski Gregory , Michelassi Vittorio
Published: August 2017

The effect of inflow turbulence intensity and turbulence length scales have been studied for a linear high-pressure turbine vane cascade at Reis = 590,000 and Mis = 0.93, using highly resolved compressible large-eddy simulations employing the WALE turbulence model. The turbulence intensity was varied between 6% and 20% while values of the turbulence length scales were prescribed between 5% and 20% of axial chord. The analysis focused on characterizing the inlet turbulence and quantifying the effect of the inlet turbulence variations on the vane boundary layers, in particular on the heat flux to the blade. The transition location on the suction side of the vane was found to be highly sensitive to both turbulence intensity and length scale, with the case with turbulence intensity 20% and 20% length scale showing by far the earliest onset of transition and much higher levels of heat flux over the entire vane. It was also found that the transition process was highly intermittent and local, with spanwise parts of the suction side surface of the vane remaining laminar all the way to the trailing edge even for high turbulence intensity cases.
Prediction of Profile Losses by Means of Large-Eddy Simulation Including Turbulence Modeling Assessment
Authors: Hasselmann Karsten , aus der Wiesche Stefan
Published: August 2017

In this contribution, a Large-Eddy Simulation (LES) analysis was carried out, to get detailed information about the unsteady flow behavior and loss generation in a turbine cascade at moderate Reynolds numbers. A comprehensive comparison study with experimental data was conducted to validate the LES results. Compared to Reynolds averaged Navier-Stokes (RANS) results, the LES shows a much better agreement with the measured values of the profile loss coefficient, downstream velocity profile, and blade pressure distribution. The unsteady separation and reattachment process was covered well by the LES approach. The power spectral density (PSD) profiles at several positions of the downstream wake were compared and analyzed. Although the results of the LES show mainly a good agreement with the experimental values, there are still some deviations at high frequency. In summery the present case study indicates the high potential of LES especially in case of moderate Reynolds numbers with flow separation.
Numerical Investigation of the Effects of Part-Span Shrouds on Aerodynamic and Aeroelastic Characteristics of a Transonic Fan Rotor
Authors: Zhou Di , Lu Zhiliang , Guo Tongqing
Published: August 2017

In this paper, the aerodynamic and aeroelastic effects of part-span shrouds are both investigated by means of numerical simulations. Based on a well-publicized NASA Rotor 67, a modified transonic fan rotor with the inclusion of a fictitious but typical part-span shroud, is selected to be studied. First, by CFD technique, three-dimensional steady-state calculations are performed for aerodynamic study. Numerical results show that the flow blockage and associated flow losses are caused and would become more severe as the operating condition approaches to the stall margin. It is also found that the presence of part-span shrouds could delay the occurrence of stall and extends the operating range. Further, the pressure distributions at different sections of the unshrouded and shrouded blades are compared and relevant analysis is presented. On the other hand, for aeroelastic study, by directly coupling CFD and CSD computations in the time domain, an effective fluid-structure coupled system is constructed. Considering the speciality of the shrouded blade, some special issues on both aerodynamic and structural modeling are discussed. Particularly, respectively corresponding to situations of in-phase and anti-phase vibrations, a single passage model and a double passage model are used. Comparative study shows that the presence of part-span shrouds greatly improves the convergence rates of all modes, numerically verifying their positive effects on aeroelastic stability. Also it is shown that the blade vibrating in opposite phase has a better aeroelastic stability. Besides, since the aeroelastic effect of part-span shrouds is a combined result of aerodynamic and structural factors, this paper also solely assesses the influence of aerodynamic factor by ignoring the forces on the contact plane. The numerical results show that its effect may be negative but negligibly small.
Investigation on Effect of Curvilinear Element Blades on Centrifugal Impeller Performance
Authors: Hiradate Kiyotaka , Kobayashi Hiromi , Nishioka Takahiro
Published: August 2017

This study experimentally and numerically investigates the effect of application of curvilinear element blades to fully-shrouded centrifugal compressor impeller on the performance of centrifugal compressor stage. Design suction flow coefficient of compressor stage investigated in this study is 0.125.

The design guidelines for the curvilinear element blades which had been previously developed was applied to line element blades of a reference conventional impeller and a new centrifugal compressor impeller with curvilinear element blades was designed. Numerical calculations and performance tests of two centrifugal compressor stages with the conventional impeller and the new one were conducted to investigate the effectiveness of application of the curvilinear element blades and compare the inner flowfield in details.

Despite 0.5% deterioration of the impeller efficiency, it was confirmed from the performance test results that the compressor stage with the new impeller achieved 1.7% higher stage efficiency at the design point than that with the conventional one. Moreover, it was confirmed that the compressor stage with the new impeller achieved almost the same off-design performance as that of the conventional stage.

From results of the numerical calculations and the experiments, it is considered that this efficiency improvement of the new stage was achieved by suppression of the secondary flows in the impeller due to application of negative tangential lean. The suppression of the secondary flows in the impeller achieved uniformalized flow distribution at the impeller outlet and increased the static pressure recovery coefficient in the vaneless diffuser. As a result, it is thought that the total pressure loss was reduced downstream of the vaneless diffuser outlet in the new stage.
Effect of Holes Shape on Cooling Performance of Trailing Edge of Gas Turbine Blade With Perforated Blockages With Inclined Holes
Authors: Ye Yi , Li Xueying , Ren Jing , Jiang Hongde
Published: August 2017

In this paper, we conduct an experimental and computational study about trailing edge cooling of a gas turbine blade with perforated blockages with inclined holes. The internal cooling passage is modeled as a wide square channel with 2 parallel blockages. There are 7 inclined holes in each blockages. The holes have an inclined angle of 30°, and the adjacent impingement holes have opposite orientations with each other, so that both up and down wall could be cooled. There are 2 patterns of configurations. The basic one has round shape holes with a diameter of 20 mm (D). The second one has oval shape hole which is made up of two parallel lines with the length of 20mm and two opposite semicircle with a diameter of 20 mm. This design is aimed at enlarging the opening area of the blockages. The height of the cooling channel is 2D and the width of the cooling channels is 20D. The space between each hole is 3D. The distance between two blockages is 5D. The main purpose of this paper is to study the effect of holes shape on heat transfer performance, both heat transfer coefficient and friction factor are evaluated. Heat transfer coefficient is measured using transient liquid crystal method. And pressure dissipation is measured by three holes probe.

The Reynolds Number ranges from 10000 to 20000. The result shows that round shape structure has the highest average heat transfer coefficient as well as the highest friction factor. And oval shape structure has the lowest average heat transfer as well as the lowest friction factor. The main reason of this phenomenon is opening area ratio. Oval shape structure has bigger opening area and lower flow resistance. Round shape structure has a lower opening area ratio which result in stronger impingement flow. To investigate the flow characteristic of these structures in detail, numerical simulations was conducted. The mesh for each structure contains approximately 4 million cells. The result shows a clear picture of flow inside the inclined holes.
Implementation of a Surface Roughness-Based Transition Onset Correction in the γ-Rθt~ Transition Model
Authors: Minot Alexandre , Marty Julien , Perraud Jean , Casalis Grégoire
Published: August 2017

This paper presents an extension of the γ-Rθt~ transition model of Menter et al. (2006) that aims to take into account the effect of a specific type of surface roughness on the transition location. This was done by implementing the roughness transition onset correction of Stripf et al. (2009) inside the γ-Rθt~ model. Stripf et al. have developed a correlation that takes into account the height and spacing of distributed roughness elements to correct the transition Reynolds number predicted over a smooth surface. As the transition Reynolds number takes multiple forms in the γ-Rθt~ model, different implementations of the Stripf et al. correction were tested and are discussed here. We find that it is best to correct the value of the critical Reynolds number Rθc. Computation results on a rough, low-pressure turbine vane are presented and compared to experimental results of the Karlsruhe Institute of Technology. We find that there is a good agreement on the transition location between the computations and experiments. In particular, the model displays accurate sensitivity to the roughness height, though the influence of the roughness spacing, which is of second order in these experiments, is not as well captured. We therefore conclude that the roughness transition onset correction of Stripf et al. is well suited for use in the γ-Rθt~ model.
Comparative Analysis of Methodologies for Uncertainty Propagation and Quantification
Authors: Cuneo Alessandra , Traverso Alberto , Shahpar Shahrokh
Published: August 2017

In engineering design, uncertainty is inevitable and can cause a significant deviation in the performance of a system. Uncertainty in input parameters can be categorized into two groups: aleatory and epistemic uncertainty. The work presented here is focused on aleatory uncertainty, which can cause natural, unpredictable and uncontrollable variations in performance of the system under study. Such uncertainty can be quantified using statistical methods, but the main obstacle is often the computational cost, because the representative model is typically highly non-linear and complex. Therefore, it is necessary to have a robust tool that can perform the uncertainty propagation with as few evaluations as possible. In the last few years, different methodologies for uncertainty propagation and quantification have been proposed. The focus of this study is to evaluate four different methods to demonstrate strengths and weaknesses of each approach. The first method considered is Monte Carlo simulation, a sampling method that can give high accuracy but needs a relatively large computational effort. The second method is Polynomial Chaos, an approximated method where the probabilistic parameters of the response function are modelled with orthogonal polynomials. The third method considered is Mid-range Approximation Method. This approach is based on the assembly of multiple meta-models into one model to perform optimization under uncertainty. The fourth method is the application of the first two methods not directly to the model but to a response surface representing the model of the simulation, to decrease computational cost. All these methods have been applied to a set of analytical test functions and engineering test cases. Relevant aspects of the engineering design and analysis such as high number of stochastic variables and optimised design problem with and without stochastic design parameters were assessed. Polynomial Chaos emerges as the most promising methodology, and was then applied to a turbomachinery test case based on a thermal analysis of a high-pressure turbine disk.
Sensitivity Analysis of Heat Transfer in a Honeycomb Acoustic Liner to Inlet Conditions With Large Eddy Simulation
Authors: Duchaine Florent
Published: August 2017

This article presents a parametric numerical study to analyze the sensitivity of wall heat fluxes on an academic acoustic liner to inlet conditions. Large Eddy Simulation (LES) is used to simulate an array of 20 aligned honeycomb cells on a flat plate with 10% porosity characteristic of installed liners. The computational domain is periodic in the span direction comprising 2 honeycomb cells. The operating conditions are representative of cruise with a Mach number of 0.5 at ambient pressure and temperature. Comparisons of heat fluxes obtained on a none perforated flat plate with the honeycomb liner are proposed with different inlet conditions: steady laminar boundary layer profile, turbulence injection and acoustic perturbation injection at different frequencies. Results show that for the operating condition and the boundary layer thickness used, large differences are observed on the first cells of the liners resulting from different transition to turbulence processes. A first important difference exists from laminar and turbulent conditions where turbulent conditions exhibits higher heat fluxes as expected. Then, case pulsed at the resonant frequency of the honeycomb shows higher heat fluxes than other frequencies. Finally, after a given number of cells, the heat fluxes reach an asymptotic behavior at the same level which seems to be controlled by the turbulence generated by the interaction of the flow and the perforations whatever the inlet conditions.
Experimental Investigations on the Efficiency of Active Flow Control in a Compressor Cascade With Periodic Non-Steady Outflow Conditions
Authors: Staats Marcel , Nitsche Wolfgang
Published: August 2017

We present results of experiments on a periodically unsteady compressor stator flow of the type which would be expected in consequence of pulsed combustion. A Reynolds number of Re = 600000 was used for the investigations. The experiments were conducted on the two-dimensional low-speed compressor testing facility in Berlin. A choking device downstream the trailing edges induced a periodic non-steady outflow condition to each stator vane which simulated the impact of a pressure gaining combuster downstream from the last stator. The Strouhal number of the periodic disturbance was Sr = 0.03 w.r.t. the stator chord length. Due to the periodic non-steady outflow condition, the flow-field suffers from periodic flow separation phenomena, which were managed by means of active flow control. In our case, active control of the corner separation was applied using fluidic actuators based on the principle of fluidic amplification. The flow separation on the centre region of the stator blade was suppressed by means of a fluidic blade actuator leading to an overall time-averaged loss reduction of 11.5%, increasing the static pressure recovery by 6.8% while operating in the non-steady regime. Pressure measurements on the stator blade and the wake as well as PIV data proved the beneficial effect of the active flow control application to the flow field and the improvement of the compressor characteristics. The actuation efficiency was evaluated by two figures of merit introduced in this contribution.
Wall Pressure and Temperature Distribution in Bent Oblong Exhaust Ejectors
Authors: Maqsood Asim , Birk A. M.
Published: August 2017

In aerospace industry ejectors are employed to reduce infrared signatures of hot exhaust gases and ducts. The ambient air, entrained from the surrounding, acts as a cushion between hot exhaust and the ejector walls. This reduces the temperature and hence the infrared signature of exhaust ducts. In many applications the ejectors are bent upward to hide the hot engine from heat seeking missiles. Due to the bend, the hot gases from the turbine hit the side walls leading to hot spots on the ejector walls. This study was aimed to see the effectiveness of a series of bent oblong ejectors as infrared signature suppressors.

Wall temperatures were measured with infrared thermal imaging camera and pressures were measured with static wall taps. The wall static pressure shows rise in pressure along the length of the ejector. It also identifies areas of flow separation and the areas where the primary flow hits the ejector walls and produces hot spots. Wall temperature distribution shows that the oblong nozzle has a detrimental effect by creating hot spots on the ejector surface. Wall temperature of the ejectors increased with the degree of bend. The normalized maximum wall temperature (T*w(max)) of 67.5° bent ejector was 45% higher than the straight ejector. The swirl in the primary flow also increased the wall temperature. On a straight ejector the T*w(max) with the 30° swirl was 35% higher than the no swirl case.
Optimization of the Operation Characteristic of a Highly Stressed Centrifugal Compressor Impeller Using Automated Optimization and Metamodeling Methods
Authors: Geller Marius , Schemmann Christoph , Kluck Norbert
Published: August 2017

The continuously rising global demand for energy together with simultaneously decreasing resources has made the topic of energy efficiency — and therefore optimization — one of the fundamental questions of our time. Turbomachinery is one of the most important parts of the process chain in nearly every case of energy conversion. This makes the turbomachine a promising approach point for optimizations. The special relevance of this topic in regard to the global challenge of climate change can be illustrated by a simple calculation: If the efficiency of a turbo compressor with a power consumption of 15MW is improved by one percent, approximately 2t CO2 per day or over 760t CO2 per year can be saved.

This work describes the optimization of the operation characteristic of a highly stressed centrifugal compressor impeller with regard to the size of the operation range and the efficiency in the operation point. The base impeller used for this optimization has already been pre-optimized by classical engineering methods utilizing analytical and empirical models. Due to the high mechanical stress in these kind of turbo impellers, each design has to be checked for compliance with the structural constraints in addition to the fluid dynamic computations. This context results in a highly complex, multicriterial, high dimensional optimization problem. The main subjects of the presented work are a robust geometry generation and grid generation, a highly automated workflow for the computation of the operation characteristic and the mechanical results and the representation of the operation characteristic by scalar parameters. Utilizing these tools a DOE is performed and based on its results a metamodel is created. The optimization is carried out on the metamodel using a Particle Swarm algorithm.

The workflow presented in this work utilizes in-house preprocessing tools as well as the tools of the ANSYS Workbench. The operation characteristics are computed using an in-house tool to control the ANSYS CFX-Solver. The statistical and stochastic pre- and post-processing as well as the metamodeling are carried out in optiSLang.
Effect of Free-Stream Velocity Definition on Boundary Layer Thickness and Losses in Centrifugal Compressors
Authors: Tiainen Jonna , Jaatinen-Värri Ahti , Grönman Aki , Turunen-Saaresti Teemu , Backman Jari
Published: August 2017

The estimation of boundary layer losses requires the accurate specification of the free-stream velocity, which is not straightforward in centrifugal compressor blade passages. This challenge stems from the jet-wake flow structure, where the free-stream velocity between the blades cannot be clearly specified. In addition, the relative velocity decreases due to adverse pressure gradient. Therefore, the common assumption of a single free-stream velocity over the blade surface might not be valid in centrifugal compressors.

Generally in turbomachinery, the losses in the blade cascade boundary layers are estimated e.g. with different loss co-efficients, but they often rely on the assumption of a uniform flow field between the blades. To give guidelines for the estimation of the mentioned losses in highly distorted centrifugal compressor flow fields, this paper discusses the difficulties in the calculation of the boundary layer thickness in the compressor blade passages, compares different free-stream velocity definitions, and demonstrates their effect on estimated boundary layer losses. Additionally, a hybrid method is proposed to overcome the challenges of defining a boundary layer in centrifugal compressors.
Two-Way Hybrid LES/CAA Approach Including Acoustic Feedback Loop for the Prediction of Thermoacoustic Instabilities in Technical Combustors
Authors: Klenke Timo , Lo Presti Federico , Lackhove Kilian , di Mare Francesca , Sadiki Amsini , Janicka Johannes
Published: August 2017

Due to the reduction of fuel consumption and new global emission limits, especially for the pollutant emissions of NOx, improvements to lean combustion technologies in aeroengine combustors are unavoidable. Near to the lean limits, combustion tends to be unstable. A geometry related coupling between unsteady heat release and acoustic perturbations leads to thermoacoustic instabilities, which show an undesirable impact on pressure, velocity and heat release in the combustor. Such instabilities occur when the unsteady heat release fluctuations are in phase with the acoustic pressure fluctuations. The aim of this study is to find an industrially applicable, three-dimensional numerical model for the prediction of combustion noise, which can also provide insight in thermoacoustic instabilities and acoustic effects in a responsive environment in enclosed, technical combustion systems. The turbulent reacting flow in a realistic gas turbine combustor has been computed by means of Large Eddy Simulation coupled to a tabulated chemistry approach based on the Flamelet Generated Manifold ansatz. The reactive LES provides very well suited method to study the impact of unsteady heat release as a major source of acoustic noise in combustion. For the simultaneous treatment of the reacting flow and its acoustic features, a Computational Aero Acoustics (CAA) solver has been coupled with the LES solver following a hybrid approach. In this work the acoustic wave propagation is calculated by the Linearized Euler Equations (LEE). The interface between both codes is optimized for the realisation of an acoustic feedback loop in order to obtain a suitable representation of acoustically self-excited oscillations. To demonstrate the prediction capability of the hybrid LES/CAA approach, geometry-dependent thermoacoustic instabilities in a generic half-dump combustor, for which experimental data are available, are investigated. The numerical results are compared to measured pressure fluctuations under both thermoacoustically stable and unstable conditions.
Blowout Sensitivities in a Liquid Fueled Combustor: Fuel Composition and Preheat Temperature Effects
Authors: Rock Nicholas , Chterev Ianko , Emerson Benjamin , Seitzman Jerry , Lieuwen Tim
Published: August 2017

The objective of this paper is to identify the influence of fuel composition on blowoff limits in a liquid fueled combustor. In premixed, gaseous systems, blowoff is a kinetically limited phenomenon, possibly with additional heat loss effects. In liquid fueled systems, the situation is far more complex, as a variety of processes can influence blowoff, including kinetics, atomization, vaporization, mixing, and heat transfer. Which one of these processes is controlling is a function of fuel and air preparation and premixing, approach flow temperature and pressure, and fuel physical and kinetic properties. This paper extends our prior work on this problem by presenting blowoff results from ten liquid fuels at two air inlet temperatures, 450 K and 300 K. At 450 K, blowoff appears to be limited by kinetics and/or radiation losses, while it seems vaporization limited at 300 K. Specifically, strong negative correlations were observed between blowoff limits and the cetane number of the fuel at the 450 K conditions. Similarly good correlations are observed with the fuel smoke point, and its percentage of aromatics. This supports two different hypotheses: (1) it is the fuels with the shortest ignition delay, and therefore the fastest reaction rates, that are the most resistant to blowoff, or (2) it is the fuels with the greatest radiation losses (presumably vaporizing/preheating the approach flow fuel droplets) that are the most blowoff resistant. Additional measurements with other fuels that decouple ignition and radiative characteristics are needed to differentiate these effects. At air inlet temperatures of 300 K, the governing physics seem quite clear from the data. Strong positive correlations were observed with LBO and boiling point temperature across the entire distillation range. As long as the air inlet temperature is above the fuel flash point, the easiest to vaporize fuels are the hardest to blowoff. It is suspected that difficult to vaporize fuels blowoff easily due to locally non-flammable regions in this low temperature regime.
Numerical Investigation of a Laser-Drilled Cooling Hole
Authors: Cerantola D. J. , Birk A. M.
Published: August 2017

Modern aeroengines utilized effusion cooling technology to further protect the components from degrading at the operating temperatures. Most studies did not address the influence of the manufacturing process used to form the cooling holes on the flow physics where percussion laser drilling was a common technique that produced irregularly shaped holes with roughened surfaces. The investigated as-drilled hole surface was statistically homogeneous, non-isotropic, and generally composed of gradually transitioning plateaus that had imperfections with an average height of 0.32 hole diameters. A conjugate heat transfer CFD study was completed on cylindrical, conical nozzle, and as-drilled holes, all yielding the same hole mass flow rates, with the realizable k-ε turbulence model at representative engine conditions. The cylindrical hole had higher film cooling effectiveness due to lower effluent velocity, and better in-hole heat transfer performance due to higher on-average in-hole flow velocities. The as-drilled hole had nominally better film cooling than the conical nozzle hole due to the higher in-hole turbulence production caused by the roughened surface texture. Ultimately, the hole area profile more significantly influenced the averaged metal temperature.
High-Speed Imaging and Measurements of Ignition Delay Times in Oxy-Syngas Mixtures With High CO2 Dilution in a Shock Tube
Authors: Barak Samuel , Pryor Owen , Lopez Joseph , Ninnemann Erik , Vasu Subith , Koroglu Batikan
Published: August 2017

In this study, syngas combustion was investigated behind reflected shock waves in order to gain insight into the behavior of ignition delay times and effects of the CO2 dilution. Pressure and light emissions time-histories measurements were taken at a 2cm axial location away from the end wall. High-speed visualization of the experiments from the end wall was also conducted. Oxy-syngas mixtures that were tested in the shock tube were diluted with CO2 fractions ranging from 60% – 85% by volume. A 10% fuel concentration was consistently used throughout the experiments. This study looked at the effects of changing the equivalence ratios (ϕ), between 0.33, 0.5, and 1.0 as well as changing the fuel ratio (θ), hydrogen to carbon monoxide, from 0.25, 1.0 and 4.0. The study was performed at 1.61–1.77 atm and a temperature range of 1006–1162K. The high-speed imaging was performed through a quartz end wall with a Phantom V710 camera operated at 67,065 frames per second. From the experiments, when increasing the equivalence ratio, it resulted in a longer ignition delay time. In addition, when increasing the fuel ratio, a lower ignition delay time was observed. These trends are generally expected with this combustion reaction system. The high-speed imaging showed non-homogeneous combustion in the system, however, most of the light emissions were outside the visible light range where the camera is designed for. The results were compared to predictions of two combustion chemical kinetic mechanisms: GRI v3.0 and AramcoMech v2.0 mechanisms. In general, both mechanisms did not accurately predict the experimental data. The results showed that current models are inaccurate in predicting CO2 diluted environments for syngas combustion.
Development of a Simulation Model of Transient Operation of Micro-CHP Systems in a Microgrid
Authors: Ippolito Francesco , Venturini Mauro
Published: August 2017

This paper presents the development of a simulation tool for modeling the transient behavior of micro-CHP systems, equipped with both thermal and electric storage units and connected with both electric and district heating grid.

The prime mover considered in this paper is an internal combustion reciprocating engine, which is currently the only well-established micro-CHP technology. Different users, characterized by different demands of electric and thermal energy, both in terms of absolute value and electric-to-thermal energy ratio, are analyzed in this paper. Both summer and winter hourly trends of electric and thermal energy demand are simulated by using literature data.

The results present a comprehensive energy analysis of all scenarios on a daily basis, in terms of both user demand met and energy share among system components. The transient response of the prime mover and the thermal energy storage is also analyzed for the two scenarios with the lowest and highest daily energy demand, together with the trend over time of the state of charge of both thermal and electric energy storage.
Numerical Prediction of Centrifugal Compressor Stability Limit
Authors: Carretta Mauro , Cravero Carlo , Marsano Davide
Published: August 2017

The compressor operating range is a crucial performance parameter for the design and development of such a gas turbine component. During the design phase it is of utmost importance to be able to predict or estimate the limit mass flow rate, for a given speed line, that will form the surge line and that will influence the overall operating range and matching capabilities of the compressor. CFD simulations are routinely used for the compressor analysis and strongly embedded into the design process. Nevertheless, the ability of a CFD model to accurately predict the surge limit without the need of a massively expensive full 3D unsteady simulation of the centrifugal compressor stage is still an open issue. Moreover in a design process it is very cumbersome to require series of computationally intensive simulations for a 3D geometry that is still under development. In the present work, the use of a CFD approach for the numerical prediction of the stability limit of centrifugal compressors is presented. The proposed approach is applied to different centrifugal stages with vaned diffusers and the obtained surge lines are compared to available experimental datasets. An in-depth analysis of the compressor stage stability limits due to its different components is presented and the use of fully 3D unsteady simulations for the complete stage are discussed to give an additional insight into the phenomenon.
A Methodology for a Detailed Loss Prediction in Low Pressure Steam Turbines
Authors: Grübel Marius , Dovik Robin M. , Schatz Markus , Vogt Damian M.
Published: August 2017

An evaluation method for CFD simulations is presented, which allows an in-depth analysis of different loss mechanisms applying the approach of entropy creation proposed by Denton. The entropy creation within each single mesh element is determined based on the entropy flux through the cell faces and therefore the locations, where losses occur, can be identified clearly. By using unique features of the different loss mechanisms present in low pressure steam turbines, the losses are categorized into boundary layer, wake mixing and shock losses as well as thermodynamic wetness losses.

The suitability of the evaluation method is demonstrated by means of steady state CFD simulations of the flow through a generic last stage of a low pressure steam turbine. The simulations have been performed on streamtubes extracted from three-dimensional simulations representing the flow at 10 % span. The impact of non-equilibrium steam effects on the overall loss composition of the stator passage is investigated by comparing the results to an equilibrium steam simulation. It is shown, that the boundary layer losses for the investigated case are of similar magnitude, but the shock and wake losses exhibit significant differences.
Resistant Statistical Methodologies for Anomaly Detection in Gas Turbine Dynamic Time Series: Development and Field Validation
Authors: Ceschini Giuseppe Fabio , Gatta Nicolò , Venturini Mauro , Hubauer Thomas , Murarasu Alin
Published: August 2017

The reliability of gas turbine health state monitoring and forecasting depends on the quality of sensor measurements directly taken from the unit. Outlier detection techniques have acquired a major importance, as they are capable of removing anomalous measurements and improve data quality. To this purpose, statistical parametric methodologies are widely employed thanks to the limited knowledge of the specific unit required to perform the analysis. The backward and forward moving window (BFMW) k-σ methodology proved its effectiveness in a previous study performed by the authors, to also manage dynamic time series, i.e. during a transient. However, the estimators used by the k-σ methodology are usually characterized by low statistical robustness and resistance.

This paper aims at evaluating the benefits of implementing robust statistical estimators for the BFMW framework. Three different approaches are considered in this paper. The first methodology, k-MAD, replaces mean and standard deviation of the k-σ methodology with median and mean absolute deviation (MAD), respectively. The second methodology, σ-MAD, is a novel hybrid scheme combining the k-σ and the k-MAD methodologies for the backward and the forward windows, respectively. Finally, the bi-weight methodology implements bi-weight mean and bi-weight standard deviation as location and dispersion estimators.

First, the parameters of these methodologies are tuned and the respective performance is compared by means of simulated data. Different scenarios are considered to evaluate statistical efficiency, robustness and resistance. Subsequently, the performance of these methodologies is further investigated by injecting outliers in field data sets taken on selected Siemens gas turbines.

Results prove that all the investigated methodologies are suitable for outlier identification. Advantages and drawbacks of each methodology allow the identification of different scenarios in which their application can be most effective.
A Comprehensive Approach for Detection, Classification and Integrated Diagnostics of Gas Turbine Sensors (DCIDS)
Authors: Ceschini Giuseppe Fabio , Gatta Nicolò , Venturini Mauro , Hubauer Thomas , Murarasu Alin
Published: August 2017

Anomaly detection in sensor time series is a crucial aspect for raw data cleaning in gas turbine industry. In addition to efficiency, a successful methodology for industrial applications should be also characterized by ease of implementation and operation.

To this purpose, a comprehensive and straightforward approach for Detection, Classification and Integrated Diagnostics of Gas Turbine Sensors (named DCIDS) is proposed in this paper. The tool consists of two main algorithms, i.e. the Anomaly Detection Algorithm (ADA) and the Anomaly Classification Algorithm (ACA). The ADA identifies anomalies according to three different levels of filtering based on gross physics threshold application, inter-sensor statistical analysis (sensor voting) and single-sensor statistical analysis. Anomalies in the time series are identified by the ADA, together with their characteristics, which are analyzed by the ACA to perform their classification. Fault classes discriminate among anomalies according to their time correlation, magnitude and number of sensors in which an anomaly is contemporarily identified. Results of anomaly identification and classification can subsequently be used for sensor diagnostic purposes.

The performance of the tool is assessed in this paper by analyzing two temperature time series with redundant sensors taken on a Siemens gas turbine in operation. The results show that the DICDS is able to identify and classify different types of anomalies. In particular, in the first dataset, two severely incoherent sensors are identified and their anomalies are correctly classified. In the second dataset, the DCIDS tool proves to be capable of identifying and classifying clustered spikes of different magnitudes.