bluuebunny/crossref_metadata_2025 huggingface dataset:
High-Speed Velocity Measurements of Film Cooling Applications at High-Turbulence Main Flow Conditions
Authors: Straußwald Michael , Sander Tobias , Bakhtiari Amirabas , Pfitzner Michael
Published: August 2018

High-turbulence flow conditions in gas turbine combustors create a rough environment for film cooling flows. Fundamental knowledge of the interaction between main flow fluctuations and cooling jet is essential to develop more efficient cooling geometries. To gain detailed insight into the turbulent flow structures, high-speed PIV measurements have been carried out in a closed loop, thermal wind tunnel facility. Velocity fields with a temporal resolution of 6 kHz and a vector spacing of 0.5 mm have been extracted from the recorded data. Measurements have been performed for a density ratio of DR = 1.2 and momentum ratios of I = 2, I = 8 and I = 15. A Makita style active turbulence grid has been used to create a main flow turbulence intensity of Tu = 18.5% and Tu = 21.5%. Mean flow fields and dominant flow structures that are present in the time-resolved velocity fields are analyzed and discussed. Finally, results for high-turbulence main flow conditions are compared with results from low-turbulence (Tu = 1%) thermographic PIV data, carried out in previous measurements. It can be observed that low-momentum jets are highly fragile. Both small and large vortices affect the jet trajectory. Depending on the current flow situation, the jet lifts off the surface or gets pushed towards the wall. The same is true for high momentum ratios and large eddies. On the other hand, small eddies cannot significantly affect high-momentum jets. The vortex gets deflected by the jet and its intensity is reduced while interacting with the high-turbulence shear layer fluctuations. All in all, turbulence can have a positive effect on the cooling film by steering the jet trajectory towards the surface. On the other hand, however, vortices can enhance the jet detachment and thus reduce the cooling efficiency of the film.
Aeroelastic Optimization of an Industrial Compressor Rotor Blade Geometry
Authors: Vanti Federico , Pinelli Lorenzo , Arnone Andrea , Schneider Andrea , Astrua Pio , Puppo Enrico
Published: August 2018

This paper describes a multidisciplinary optimization procedure applied to a compressor blade-row. The numerical procedure takes into account both aerodynamic (efficiency) and aeromechanic (flutter-free design) goals nowadays required by turbo-machinery industries and is applied to a low pressure compressor rotor geometry provided by Ansaldo Energia S.p.A.. Some typical geometrical parameters have been selected and modified during the automatic optimization process in order to generate an optimum geometry with an improved efficiency and, at the same time, a safety flutter margin. This new automatic optimization procedure, which now includes a flutter stability assessment, is an extension of an existing aerodynamic optimization process, which randomly perturbs a starting 3D blade geometry inside a constrained range of values, build the fluid mesh and run the CFD steady analysis. The new implementation provides the self-building of the solid mesh, the FEM analysis and finally the unsteady uncoupled aeroelastic analysis to assess the flutter occurrence. After simulating a wide range of geometries, a database with all the constraint parameters and objective functions is obtained and then used to train a neural network algorithm. Once the ANN validation error is converged, an optimization strategy is used to build the Pareto front and to provide a set of optimum geometries redesigning the original compressor rotor. The aim of this paper is to show the opportunity to also take into account the aeroelastic issues in optimization processes.
Development and Operation of Supercritical Carbon Dioxide Power Cycle Test Loop With Axial Turbo-Generator
Authors: Cho Junhyun , Shin Hyungki , Cho Jongjae , Ra Ho-Sang , Roh Chulwoo , Lee Beomjoon , Lee Gilbong , Choi Bongsu , Baik Young-Jin
Published: August 2018

In order to overcome reported failure problems of turbomachinery for the supercritical carbon dioxide power cycle induced by the high rotational speed and axial force, an axial impulse-type turbo-generator with a partial admission nozzle was designed and manufactured to reduce the rotational speed and axial force. The turbine wheel part was separated by carbon ring-type mechanical seals to use conventional oillubricated tilting-pad bearings. A simple transcritical cycle using a liquid CO2 pump was constructed to drive the turbogenerator. A 600,000 kcal/h LNG fired thermal oil boiler and 200 RT chiller were used as a heat source and heat sink. The target turbine inlet temperature and pressure were 200°C and 130 bar, respectively. Two printed circuit heat exchangers were manufactured for both sides of the heater and cooler. A leakage make-up system using a reciprocating CO2 compressor; CO2 supply valve-train to the main loop and mechanical seal; and an oil cooler for the bearings, load bank, and control systems were installed. Prior to the turbine power-generating operation, a turbine bypass loop was operated using an air-driven control valve to determine the system mass flow rate and create turbine inlet conditions. Then, 11 kW of electric power was obtained under 205°C and 100 bar turbine inlet conditions, and the continuous operating time was 45 min.
Direct Integration of Axial Turbomachinery Preliminary Aerodynamic Design Calculations in Engine Performance Component Models
Authors: Kolias Ioannis , Alexiou Alexios , Aretakis Nikolaos , Mathioudakis Konstantinos
Published: August 2018

In the context of an engine design calculation, isentropic or polytropic efficiencies of turbomachinery components are assumed at the outset of the cycle analysis and their values are updated or validated following the aerodynamic design of the components. In the present paper, aerodynamic design calculations of axial-flow compressors and turbines are directly integrated into the corresponding performance component models. This creates a consistent, single-step preliminary design and performance modelling process using a relatively small number of physical and geometric inputs.

The aerodynamic design for establishing a component’s overall efficiency is accomplished through a mean-line, stage-by-stage approach where the stagewise isentropic efficiency is calculated employing either loss or semi-empirical correlations. From this process, the stagewise flow annulus radii are also obtained and are used to axially size the component stages assuming the blade aspect ratio and axial gapping distributions. The component flowpath geometry is then produced by simply “stacking” axially the component stages.

The developed method is validated against publicly available data for a high-pressure compressor and a low-pressure turbine. Finally, the effectiveness of the method is demonstrated by considering the multi-point design of a High Bypass Ratio Geared Turbofan Engine with bypass Variable Area Nozzle.
Measurement Quality Assessment of an On-Wing Engine Thrust Measurement System
Authors: Bauer Marc , Friedrichs Jens , Wulff Detlev , Werner-Spatz Christian
Published: August 2018

Aircraft engine maintenance is performed on an on-condition basis. Monitoring the engine condition during operation is important to provide an efficient maintenance. Engine Condition Monitoring has thus become a standard procedure during operation. However, one of the most important parameters, the engine thrust, is not directly measured and can therefore not be monitored, which makes it difficult to distinguish whether deteriorating trends e.g. in fuel comsumption must be attributed to the engine (e.g. due to thermodynamic wear) or to the aircraft (e.g. due to increased drag). Being able to make this distinction would improve troubleshooting and maintenance planning and thus help to reduce the cost of ownership of an aircraft. This paper describes the development and quality assessment of a system for direct engine thrust measurement during the normal engine operation. The system was designed, calibrated and validated with engine test runs. After the necessary certification of the whole system a flight test campaign to validate the system, when installed on an aircraft, was started. In the presented work an assessment of the quality of measured data from the first period of the ongoing flight test is presented.
Design, Flow Simulation, and Performance Test for a Partial-Admission Axial Turbine Under Supercritical CO2 Condition
Authors: Cho Jongjae , Shin Hyungki , Cho Junhyun , Baik Young-Jin , Choi Bongsu , Roh Chulwoo , Ra Ho-Sang , Kang Youngseok , Huh Jaesung
Published: August 2018

The development of a 60-kWe turbo generator that uses supercritical carbon dioxide (sCO2) cycle technology at the lab scale is described herein. The design concept for the turbo generator involved using commercially available components to reduce the developmental time and to increase the reliability of the machine. The developed supercritical partial-admission CO2 turbine has a single-stage axial-type design with a 73-mm rotor mean diameter. The design of the sCO2 turbine uses impulse and partial admission to reduce the axial force and rotational speed. We simulated the flow of the designed sCO2 turbine. To increase the simulation accuracy, a real gas property table is coupled with the flow solver. The turbine performance test apparatus and test results are described; then, the turbine is continuously operated for 44 min. The maximum turbine power is 25.4 kW, and the maximum electric power is 10.3 kWe.
Advanced Medium-Order Modelling for the Prediction of the Three-Dimensional Wake Shed by a Vertical Axis Wind Turbine
Authors: Saverin Joseph , Marten David , Pechlivanoglou George , Paschereit Christian Oliver , Persico Giacomo , Dossena Vincenzo
Published: August 2018

A recently formulated model for the treatment of the evolution of the wake of aerodynamic bodies has been implemented into the wind turbine simulation software QBlade with the aim of modelling near and far wake behavior with a so-called medium order model. The paper first presents the vortex particle treatment of the wake. Shed and trailing vortex elements generated by a lifting line model of the turbine blade are allowed to freely convect under the action of the freestream, body and wake influence. Induced velocities are calculated with use of a regularized Biot-Savart kernel. The method is validated against experiments carried out in the large-scale wind tunnel of the Politecnico di Milano on a H-type turbine architecture. Wake velocities and periodic unsteadiness are predicted relatively well by the method for two tip speed ratios. It is observed that higher order effects such as vortex stretching and viscous interaction must be implemented into the model in order to accurately predict wake evolution. A recently developed vortex particle multilevel multi-integration method has been implemented which approximates the far-field influence of the particles and reduces significantly the computational expense. The paper also reports on the implementation of higher order effects into this optimization framework to account for evolution of vortex particle strength and the inclusion of viscous effects into the model, which are shown to be particularly relevant for vertical axis wind turbines.
Analysis of Unsteady Flow Structures in a Radial Turbomachine by Using Proper Orthogonal Decomposition
Authors: Witte Matthias , Torner Benjamin , Wurm Frank-Hendrik
Published: August 2018

Tonalities in hydro and airborne noise emission are a known problem of turbomachines, wherein the tonalities in the noise spectrum are associated with the different orders of the blade passing frequency (BPF). The proper orthogonal decomposition (POD) method was utilized to find the relationship between the fluctuations in the pressure field at the BPF orders which are the origin of the noise emission and the correlated fluctuations in the turbulent velocity field in terms of coherent, periodic flow structures. In order the provide the input data for the POD analysis, a URANS k-ω-SST scale adaptive simulation (SAS) of the turbulent flow field in a single stage radial pump under part load conditions was performed. Compared to traditional two equation turbulence models this approach is less dissipative and allows the development of small scale turbulence structures and is therefore an appropriate method for this study. In order to compute the POD correlation matrix Sirovich’s “Methods of Snapshots” was applied to the unsteady pressure and velocity fields from the CFD simulation.

The discrimination of coherent, periodic flow structures and the incoherent, chaotic turbulence was carried out by analyzing the POD eigenvalue distributions, the POD mode shapes and the spectral properties of the POD time coefficients. Five coupled POD mode pairs were identified in total, which were strictly correlated with the 1st, 2nd, 3rd, 4th and 5th order of the BPF and therefore responsible for the noise emission at these discrete frequencies. The coherent structures were explored on the basis of the spatial POD velocity und pressure mode shapes and in terms of vortical structures after an additional phase averaging. The scope of this study is to introduce an enhanced collection of post processing techniques which are capable of analyzing highly unsteady flow fields from numerical simulations in a better way than is possible by just using traditional techniques like the evaluation of integral or time averaged quantities. The identified coherent flow structures and their associated pressure fluctuations are key elements for a proper comprehension of the internal dynamics of the turbulent flow field in a turbomachine and therefore essential for the understanding of the noise generation processes and the optimization of such machines.
Numerical Study of a Linear Cascade With Upstream Cavity Using Various Rim-Seal Geometries and Purge Rates
Authors: Fiore Maxime , Gourdain Nicolas , Boussuge Jean-François , Lippinois Eric
Published: August 2018

This paper introduces numerical investigation of a low-speed linear cascade rig with upstream cavity at Reynolds number commonly observed in modern low-pressure turbine stage. This configuration was tested experimentally during the EU project MAGPI (2007–2011) focused on the impact of secondary air systems on gas turbine performance. Three different purge mass flow rate have been tested numerically using three different rim-seal geometry (axial clearance, simple and double radial overlap). The mechanisms and influence of these two parameters on loss generation for the main annulus flow is investigated. The ability of high-fidelity numerical methods to deal with such kind of configuration is assessed by comparing several unsteady codes over the axial geometry at three purge mass flow rate available. Two Large-Eddy Simulation (LES) solvers based respectively on structured and unstructured meshes and a LES-LBM approach in which equations discretization is based upon a Lattice-Boltzmann Method (LBM) and a Sub-Grid Scale (SGS) model from LES developments are used. The comparison against MAGPI experiments and previous Reynolds Averaged Navier-Stokes (RANS) simulation show that despite a variety of flow dynamics modelling, discretization and numerical parameters, the different unsteady codes are well able to recover aerodynamic quantities into the mainstream passage in which purge flow blows at various rate and different rim-seal geometry. Further results obtained from such high-fidelity methods exhibit strong interaction of separated hub boundary at rim-seal interface with nonuniform pressure field imposed by downstream blade leading to a strong in-depth of mainstream flow into the cavity for the axial clearance. Simple and double overlap damper this phenomenon due to localized recirculation zone into the rim-seal. In addition, hub passage vortex and blade suction side unsteadiness are shown to be strongly related to the vortex shedding process occurring at rim-seal interface.
Adiabatic Effectiveness on High Pressure Turbine Nozzle Guide Vanes Under Realistic Swirling Conditions
Authors: Bacci T. , Becchi R. , Picchi A. , Facchini B.
Published: August 2018

In modern lean burn aero-engine combustors, highly swirling flow structures are adopted to control the fuel-air mixing and to provide the correct flame stabilization mechanisms. Aggressive swirl fields and high turbulence intensities are hence expected in the combustor-turbine interface. Moreover, to maximize the engine cycle efficiency, an accurate design of the high pressure nozzle cooling system must be pursued: in a film cooled nozzle the air taken from last compressor stages is ejected through discrete holes drilled on vane surfaces to provide a cold layer between hot gases and turbine components. In this context, the interactions between the swirling combustor outflow and the vane film cooling flows play a major role in the definition of a well performing cooling scheme, demanding for experimental campaigns at representative flow conditions. An annular three-sector combustor simulator with fully cooled high pressure vanes has been designed and installed at THT Lab of University of Florence. The test rig is equipped with three axial swirlers, effusion cooled liners and six film cooled high pressure vanes passages, for a vortex-to-vane count ratio of 1:2. The relative clocking position between swirlers and vanes has been chosen in order to have the leading edge of the central airfoil aligned with the central swirler. In this experimental work, adiabatic film effectiveness measurements have been carried out in the central sector vanes, in order to characterize the film-cooling performance under swirling inflow conditions. The Pressure Sensitive Paint technique, based on heat and mass transfer analogy, has been exploited to catch highly detailed 2D distributions. Carbon dioxide has been used as coolant in order to reach a coolant-to-mainstream density ratio of 1.5. Turbulence and five hole probe measurements at inlet/outlet of the cascade have been carried out as well, in order to highlight the characteristics of the flow field passing through the cascade and to provide precise boundary conditions. Results have shown a relevant effect of the swirling mainflow on the film cooling behaviour. Differences have been found between the central airfoil and the adjacent ones, both in terms of leading edge stagnation point position and of pressure and suction side film coverage characteristics.
Non-Linear k-ε-ζ-f Model Sensitized to Rotation for Blade Turbine Internal Cooling
Authors: Borello Domenico , Salvagni Alessandro
Published: August 2018

The need to study flow and heat transfer in turbine blade cooling design calls to develop appropriate modelling approaches able to return accurate predictions at a reduced computational costs. Here we propose and scrutinize a quadratic version of the well-known k-ε-ζ-f RANS turbulence models, aiming at sensitizing the model to the effect of rotation in configurations mimicking the flow in turbine internal cooling. Starting from the evidence that rotation modified turbulent flow through a turbulence suppression (enhancement) on the stabilized (destabilized) surface, we modified the Cμ coefficient present in the formulation of turbulent viscosity introducing a dependence on the strain and vorticity tensors, the latter explicitly including solid body rotation. The proposed model was tested on plane channel and square-sectioned duct flows, and then used for simulating a rib-duct rotating channel. Results are assessed against DNS literature data and properly developed LES computations, by examining flow variables, heat transfer and turbulence budgets. We demonstrate that, as for the channel flows, the proposed quadratic model is able to accurately reproduce velocity, temperature and turbulent variables at various angular velocity regimes. In the duct flow the flow is subjected to the mutual influence vorticity induced by rotation and turbulence anisotropy developing close the walls. In particular, the non-linear rotation-sensitized model is able to reproduce the near-wall turbulent kinetic energy distribution close to the suction side, returning a zero value in the mid-span and a small peak close to the wall on the suction side. Turbulent kinetic energy and temperature budgets analysis demonstrates the capabilities of the model in describing all the terms in the equations. Also if some tuning of the model is required, these analysis showed very encouraging results. In fact if the basic mechanisms of turbulence and heat transfer are properly predicted, then it can be expected that the model can be successfully applied to a set of different cases. For such reason, the model was applied to the analysis of flow and heat transfer in a rotating ribduct with reasonably results.
Numerical Investigation of Flame Structure and Soot Formation in a Lab-Scale Rich-Quench-Lean Burner
Authors: Giusti Andrea , Gkantonas Savvas , Foale Jenna M. , Mastorakos Epaminondas
Published: August 2018

The understanding of the processes involved in soot formation and oxidation is a critical factor for a reliable prediction of emissions in aero-engines, particularly as legislation becomes increasingly stringent. This work studies the flame structure and soot formation in a lab-scale burner, which reproduces the main features of a Rich-Quench-Lean (RQL) combustor, using high-fidelity numerical simulations. The investigated burner, developed at the University of Cambridge, is based on a bluff-body swirl-stabilised ethylene flame, with air provided in the primary region through two concentric swirling flows and quenching enabled by means of four dilution jets at variable distance downstream. Measurements for different air split between the two inlet swirling flows and dilution ports, and different height of the dilution jets, indicate noticeable differences in the soot tendency. Numerical simulations have been performed using Large-Eddy Simulation with the Conditional Moment Closure combustion model and a two-equation model for soot, allowing a detailed resolution of the mixing field and to directly take into account the effect of turbulent transport on the flame structure, which has been shown to have an important effect on the soot formation and evolution. The main objective of this work is to study the flow field and mixing characteristics in the burner’s primary region, in order to improve the understanding of the mechanisms leading to the soot behaviour observed in the experiment at different operating conditions. Results show the key role of mixing in determining the level of soot in the burner, with the soot production mainly related to the extension of the flame zone characterized by a rich mixture, with pyrolysis products and soot precursors. The presence of additional dilution air seems to improve the oxidation and leads to a leaner mixture in the primary combustion region whereas the air added through the outer swirl stream seems to have less impact on the mixture formation in the primary region. Analysis of the solution in mixture fraction space shows the importance of residence time for the soot formation and highlights the existence of a range of values of mixture fraction, between 0.1 and 0.2, where the soot production terms are maximum. High residence times and local air-to-fuel ratio in the range of high soot production should be avoided to decrease the level of soot mass fraction in the burner.
Effects of Downstream Vane Bowing and Asymmetry on Unsteadiness in a Transonic Turbine
Authors: Clark John P. , Anthony Richard J. , Ooten Michael K. , Finnegan John M. , Johnson P. Dean , Ni Ron-Ho
Published: August 2018

Accurate predictions of unsteady forcing on turbine blades are essential for the avoidance of high-cycle-fatigue issues during turbine engine development. Further, if one can demonstrate that predictions of unsteady interaction in a turbine are accurate, then it becomes possible to anticipate resonant-stress problems and mitigate them through aerodynamic design changes during the development cycle. A successful reduction in unsteady forcing for a transonic turbine with significant shock interactions due to downstream components is presented here. A pair of methods to reduce the unsteadiness was considered and rigorously analyzed using a three-dimensional, time resolved Reynolds-Averaged Navier Stokes (RANS) solver. The first method relied on the physics of shock reflections itself and involved altering the stacking of downstream components to achieve a bowed airfoil. The second method considered was circumferentially-asymmetric vane spacing which is well known to spread the unsteadiness due to vane-blade interaction over a range of frequencies. Both methods of forcing reduction were analyzed separately and predicted to reduce unsteady pressures on the blade as intended. Then, both design changes were implemented together in a transonic turbine experiment and successfully shown to manipulate the blade unsteadiness in keeping with the design-level predictions. This demonstration was accomplished through comparisons of measured time-resolved pressures on the turbine blade to others obtained in a baseline experiment that included neither asymmetric spacing nor bowing of the downstream vane. The measured data were further compared to rigorous post-test simulations of the complete turbine annulus including a bowed downstream vane of non-uniform pitch.
On the Simulation of Unsteady Turbulence and Transition Effects in a Multistage Low Pressure Turbine: Part I — Verification and Validation
Authors: Ashcroft Graham , Frey Christian , Kersken Hans-Peter , Kügeler Edmund , Wolfrum Nina
Published: August 2018

This is the first part of a series of three papers on the simulation of turbulence and transition effects in a multistage low pressure turbine. In this first part, the extension, verification and validation of a Harmonic Balance (HB) method recently proposed by the authors to fully include established turbulence and transition models in the method is presented. As an alternating frequency/time-domain type method the implemented HB solver has the advantage of being able to utilize models (e.g. boundary conditions or residual functions) formulated in either the frequency- or time-domain. On the one hand this allows highly accurate nonreflecting boundary conditions formulated in the frequency-domain to be used along entry, exit or interface boundaries, and on the other hand complex nonlinear terms formulated in the time-domain to be used to describe nonlinear effects. Nevertheless, the wish to minimize the number of harmonics used to describe a given time-periodic unsteady flow, coupled with the often highly nonlinear nature of turbulence and transition models makes the full inclusion of such models in the HB method challenging.

In this work the integration of Menter’s SST two-equation k–ω turbulence model along with Menter and Langtry’s two-equation γ–Reθ transition model in the context of a general framework for transport equations in the CFD solver TRACE is described in detail. Following the basic verification of the underlying transport equation framework, the implemented models are used to compute the well known high lift, low pressure turbine airfoil T106C and results are compared with the available experimental data as well as results from more conventional time-domain simulations. Alongside the basic validation of the models this testcase is furthermore used to investigate the importance of including higher harmonics, as opposed to only the zeroth harmonic, of the turbulence and transition models for the accurate prediction of the time-mean flow.
Optimization of a 900 mm Tilting-Pad Journal Bearing in Large Steam Turbines by Advanced Modeling and Validation
Authors: Mermertas Ümit , Hagemann Thomas , Brichart Clément
Published: August 2018

Modernization of steam turbine components can extend the life of a power plant, decrease maintenance costs, increase service intervals and improve operational flexibility. However, this can also lead to challenging demands for existing components such as bearings, e.g., due to increased rotor weights. Therefore, a careful design and evaluation process of bearings is of major importance. This paper describes the advanced modeling methods applied for the optimization of a novel 900 mm three-pad tilting pad journal bearing followed by validation results that showed a high bearing temperature sensitivity to the fresh oil supply temperature during operation. The bearing was especially developed to cope with increased rotor weights within the framework of low pressure steam turbine modernizations at two similar 1000 MW nuclear power plants. With a static bearing load of approximately 2.7 MN at a rotor speed of 1500 rpm, it represents one of the highest loaded applications for tilting pad journal bearings in turbomachinery worldwide. After identification of the reasons for the sensitivity, advanced modeling methods were applied to optimize the bearing. For this purpose, a more comprehensive bearing model was developed taking into account the direct lubrication at the leading edge of the pads and the thermo-mechanical pad deformation. For the latter, a co-simulation between the bearing computation code and structural mechanics software was performed. The results of the entire analyses indicated modifications of bearing and pad clearance, pad pivot position, circumferential and axial pad length as well as pad thickness. Furthermore, the oil distribution into the pads was optimized by modifying the orifices within the bearing.

The optimized bearing was then implemented on both units and proved its excellent operational behavior at increased fresh oil supply temperatures of up to 55°C. In addition, inspections during scheduled outages after 18 months of operation and subsequent restarts with reproducible bearing behavior confirmed the robustness of the optimized bearing. In conclusion, the application of advanced modeling methods proved to be the key success factor in the optimization of this bearing, which represents an optimal solution for large steam turbine and generator rotor train applications.
Computational Fluid Dynamics Simulations of Windage Loss in a Spur Gear
Authors: Aktas Murat K. , Yavuz Mehmet A. , Ersan Ali K.
Published: August 2018

Windage losses arise due to the viscous dissipation of air -oil mist flow driven by the rotating faces, sides of the gears in a gearbox. The windage losses can consume up to 3% of the transmitted power. In the open literature, theoretical investigations mostly focused on the single phase (air) flow. In the present study we attempt to elucidate the complex flow physics between the gears by considering the effects of oil particles in the air flow. The goals of this work is to develop a physical understanding of the aerodynamics of gear windage loss. A spur gear geometry was considered, for which experimental and theoretical data are available. A commercial Computational Fluid Dynamics algorithm (CFD++) was utilized for the numerical simulations. The first group of simulations were performed for a single phase (air) flow in order to validate the numerical approach. The results showed good agreement with the experiment. Various shrouding configurations and free spinning cases are studied at different rotation speeds of the gear. The effects of the oil droplets on the windage losses were quantified.
Methodology for Evaluating Efficiency Benefits of Hydrophobic Coatings in Steam Turbine Applications
Authors: Melas Mauro , Sigg Roland , Bühlmann Stefan , Limacher Daniela , Manyoky Thomas
Published: August 2018

Performance of steam turbines operating under wet steam is known to worsen proportionally to the increase in wetness at turbine exhaust. Numerous strategies have been attempted in the past to minimize the efficiency loss and mechanical damage associated with liquid phase formation, mostly with limited success and poor commercial viability due to increased complexity.

The application of hydrophobic coatings is a simple economically favorable strategy and is expected to partially inhibit the formation of water films on blade surfaces and associated consequences.

This paper addresses a need of turbine manufacturers: the development of a methodology to test, rapidly and economically, performance impact and endurance of coatings to allow a systematic selection. The resulting methodology employs specialized equipment to measure the fog rejection effectiveness of coatings and to generate droplet impingement and thermal loads comparable to real applications, all under laboratory conditions. Measurement results indicated that different materials cause different wetness losses and turbine efficiency can be improved with the coating of blades. Transfer functions used to generate the correspondence between laboratory and representative turbine conditions enabled the coating selection for turbine application. Finally, tests in a model turbine allowed validating the prediction of efficiency improvement, confirming the suitability of the developed methodology.

This work demonstrates that this technology delivers an important improvement in turbine efficiency.
Investigation of Vanadium Carbide Reinforced CMC for Gas Turbine Stator
Authors: Kumar S. Ram , Ramanan Vikram
Published: August 2018

Gas turbine, or the jet engines have seen a rise in thrust output by increasing the chamber temperatures, advancement in compressor staging etc. On due of increased temperature limits, the turbine has particularly been subjected to higher thermomechanical stresses on the first stage high pressure stators, caused by the hot combustion product gases at high velocities. Thermal strains also get induced due to non-uniform temperature distribution from the hub to tip of the vanes. Improvements in material technologies to curb for higher temperatures have brought ceramic matrix composites into implementation. These are lighter in weight and have higher melting points than the conventionally used metal super alloys, with silicon carbide SiC/SiC matrix being specially used as the material for turbine stage of the engine. This consists of long unidirectional sic fibers as reinforcement, embedded in sic matrix phase, with fibre volume fractions in the range 30–40 percentages. But with further increase in thrust requirements, the turbine inlet temperature is also increased, affecting the material and the performance life of the component. Hence furthermore innovation in materials with higher strengths and melting points has to be incorporated. This work is on the use of vanadium carbide fibers as the reinforcement phase of silicon carbide matrix composite for the high pressure turbine stator. Structural analysis is carried out for the blade for both the composites, in ansys software using time stepped increment in temperature upto 1600°c. Results are compared to that for Sic/Sic composites. Thus the comparative analysis between the two composites is carried out on the thermo-structural aspect to predict their performance in the elevated temperatures.
Rotating Instabilities Versus Rotating Stall in a High-Speed Centrifugal Compressor
Authors: Grondin Julissa , Trébinjac Isabelle , Rochuon Nicolas
Published: August 2018

The subject of the paper is a high speed unshrouded centrifugal compressor in which rotating instabilities have been measured near the stage stall point. The impeller is studied numerically by means of unsteady RANS simulations, and the results are compared to experimental measurements. Instead of rotating instabilities, the numerical results directly capture a rotating stall pattern in which a tornado-like separation vortex is shed due to a separation at the impeller leading edge, and propagates around the circumference. The vortex has one end attached to the casing and the other end attached to the pressure side of the blade. Its propagation speed is within 10% from the measured speed of the rotating instabilities. Because of the high pressure gradient the tip leakage flow crosses the impeller front and thus convects the vortex in front of the adjacent blade. The spillage of the vortex in the adjacent channel convects radial and azimuthal vorticity onto the next blade. This triggers the inception of a new vortex, and induces the propagation of the rotating stall cells.
Analytical and Experimental Results of a Novel Single-Stage Centrifugal Compressor With Economizer Injection
Authors: Cousins William T. , Yu Lei , Sishtla Vishnu , Shen Feng
Published: August 2018

Centrifugal compressors are used in large water-cooled chillers for Heating, Ventilating, and Air Conditioning (HVAC) applications. A water-cooled chiller is one of the largest power consumers in a building HVAC system. One way of reducing the power consumption is to use an economizer cycle. In a typical economizer cycle, the vapor from the economizer (located between the condenser and the evaporator) is injected into the inlet of the compressor second stage of a multistage machine. The added cost and complexity of a two-stage compressor can be reduced with a single-stage design. To achieve the best of a single-stage design along with the benefit of an economizer cycle, a novel single-stage compressor with an economizer gas injection into the stage is analyzed using computational fluid dynamics methods. The compressor is designed with an unshrouded impeller and a pipe diffuser. Design parameters such as injection location, angle and size were analyzed to maximize the cycle benefit and achieve a minimum loss in compressor efficiency. Experiments were carried out in a real gas test rig using R134a refrigerant to validate the analytical results. This paper presents the concept and computational results of the parametric study, along with the test rig details and results.
Vibration Based Condition Monitoring of Wind Turbine Gearboxes Based on Cyclostationary Analysis
Authors: Mauricio Alexandre , Qi Junyu , Gryllias Konstantinos
Published: August 2018

Wind industry experiences a tremendous growth during the last few decades. As of the end of 2016, the worldwide total installed electricity generation capacity from wind power amounted to 486,790 MW, presenting an increase of 12.5% compared to the previous year. Nowadays wind turbine manufacturers tend to adopt new business models proposing total health monitoring services and solutions, using regular inspections or even embedding sensors and health monitoring systems within each unit. Regularly planned or permanent monitoring ensures a continuous power generation and reduce maintenance costs, prompting specific actions when necessary. The core of wind turbine drivetrain is usually a complicated planetary gearbox. One of the main gearbox components which are commonly responsible for the machinery breakdowns are rolling element bearings. The failure signs of an early bearing damage are usually weak compared to other sources of excitation (e.g. gears). Focusing towards the accurate and early bearing fault detection, a plethora of signal processing methods have been proposed including spectral analysis, synchronous averaging and enveloping. Envelope analysis is based on the extraction of the envelope of the signal, after filtering around a frequency band excited by impacts due to the bearing faults. Kurtogram has been proposed and widely used as an automatic methodology for the selection of the filtering band, being on the other hand sensible in outliers. Recently an emerging interest has been focused on modelling rotating machinery signals as cyclostationary, which is a particular class of non-stationary stochastic processes. Cyclic Spectral Correlation and Cyclic Spectral Coherence have been presented as powerful tools for condition monitoring of rolling element bearings, exploiting their cyclostationary behaviour. In this work a new diagnostic tool is introduced based on the integration of the Cyclic Spectral Coherence along a frequency band that contains the diagnostic information. A special procedure is proposed in order to automatically select the filtering band, maximizing the corresponding fault indicators. The effectiveness of the methodology is validated using the National Renewable Energy Laboratory (NREL) wind turbine gearbox vibration condition monitoring benchmarking dataset which includes various faults with different levels of diagnostic complexity.
An Experimental Study of Using Vortex Generators As Tip Leakage Flow Interrupters in an Axial Flow Turbine Stage
Authors: Andichamy Veerandra C. , Khokhar Gohar T. , Camci Cengiz
Published: August 2018

The flow leaking through the gap between rotor blade tips and casing surface in a turbine stage is an important source of energy loss. The current study uses a new concept named as Tip Leakage Interrupters (TLI) to mitigate some of the adverse effects of the tip leakage flows and improve the efficiency of an axial turbine stage. The TLIs are a system of vortex generators attached onto the suction side of the turbine blade tip. The TLI design was developed in a proof of concept effort and they operate by inducing controlled vortical structures originating from strategically shaped/oriented multiple and sub-miniature vortex generators. These induced vortical structures, when properly interact with the tip leakage vortex reduce the damaging aerodynamic effects of the leakage flow. The TLIs in this investigation were mounted near the suction side corner of turbine blade tips rotating in a single-stage cold-flow turbine facility. In this investigation, three different parameters such as the mounting location of TLI on the airfoil tip region, the number of TLIs mounted and the specific orientation of TLI were varied. The TLI mounted near the minimum pressure point on the suction side of the blade generated the largest vortical structure that is counter rotating to the leakage vortex system and hence had the greatest effect in reducing the strength of the leakage vortex. Adding more TLIs on the blade suction surface was found to improve the tip leakage mitigation effort. The study showed that changing the specific orientation of the TLI with respect to the incoming flow drastically changes the rotational direction of the vortex it generates and its nature of interaction with the leakage vortex.
A New Type of Rotary Liquid Piston Pump for Multi-Phase CO2 Compression
Authors: Thatte Azam
Published: August 2018

A novel rotary liquid piston multi-phase pump that transfers pressure energy from high pressure motive fluid stream to a low pressure process fluid stream within a high speed multi-ducted rotor is presented. The multiple ducts in the rotor act like cylinders of a rotating liquid piston pump with the liquid-to-liquid interface between the working fluid and the motive fluid acting like a piston. This novel pump has promise to solve challenges typically seen in multi-phase pumping and in trans-critical and supercritical CO2 compression systems, na m el y, risks due to phase change, two-phase compression inefficiencies, rotordynamic instabilities and sealing challenges etc. In this design the entrance and exit flow angles impart momentum to the rotor and the rotor achieves a self-sustained rotation without external power. The rotational speed dictates the volumetric efficiency, travel distance of the liquid piston within the ducts and the zero-mixing effectiveness of the design. This creates a very efficient pumping/compression system with just one moving part and three stationary parts, which can handle very high pressures and temperatures typical of supercritical CO2 turbomachines and also mitigates some of the rotordynamic stability challenges typically seen in MW-scale sCO2 turbomachinery designs. Ability of the pressure exchanger to dynamically maintain micro-scale gaps between rotor and stators through intelligent pressure balancing features relaxes the need to have complex dynamic seals. In this paper, use of this novel pump for multi-phase CO2 pumping application is explored through an advanced 3D multi-scale multi-phase flow model. The model captures the phase transport, compressibility, advection & diffusion of one phase into the other using a hybrid Eulerian-Lagrangian algorithm. Using these advanced models, performance curves are developed and results for key performance parameters including phase mixing, compressibility losses, effect of inlet gas volume fractions etc. are presented. A detailed transient evolution of two-phase fluid piston interface in the rotor ducts that captures acoustic wave propagation and reflection is presented. This new technology has promise to solve challenges typically seen in multi-phase pumping/ compression, transcritical and supercritical CO2 compression systems or in applications where the traditional pumps face steep challenges like phase change, erosive/ corrosive fluids, particle laden flows with high particle loading or flows with high gas volume fractions. This technology renders itself useful to several applications including supercritical CO2 turbomachines, waste pressure recovery, applications in oil & gas extraction and carbon sequestration etc.
Large Eddy Simulations in a Transonic Centrifugal Compressor
Authors: Dombard Jérôme , Duchaine Florent , Gicquel Laurent , Staffelbach Gabriel , Buffaz Nicolas , Trébinjac Isabelle
Published: August 2018

In an attempt to validate a Large Eddy Simulation (LES) approach, computations of a transonic centrifugal compressor with a backswept, unshrouded impeller followed by radial and axial vaned diffusers are performed. A sector composed of one main blade and one splitter blade, two radial diffuser vanes and six axial diffuser vanes is simulated including all the technological effects of the experimental rig. The LES methodology to simulate the rotor/stator configuration is introduced. Emphasis is put on the best trade-off between accuracy of the simulation and affordable CPU cost. A law-of-the-wall boundary condition is used to reduce the mesh size, with a target of y+ around a hundred for all walls except in the tip leakage with y+ around five. Computation of one entire characteristic line is obtained continuously in time: the transient from the flow at rest to the converged points at blockage, peak efficiency, near surge and path to deep surge is computed increasing progressively the outlet pressure as in the experiments. First, LES results are compared to experiments and show excellent agreement both in terms of overall performance and time-averaged internal flow fields previously obtained by Laser Doppler Anemometry. Then, a focus is proposed on the complementary information LES provide in the rotor. The key findings are that contrary to previous URANS studies in this centrifugal compressor, LES capture influential details of the flow structures in the rotor: secondary structures, shock/boundary layer interaction and boundary layer separation at the tip of the impeller. Moreover, it is clearly shown that the tip leakage vortex increases in size and intensity from peak efficiency to surge and becomes much more erratic. Emphasis is put on the causes and consequences of the tip leakage spillage in the neighbouring rotor channels. Pressure fluctuations were also found to increase from peak efficiency to surge downstream the splitter blade leading edge. The whole results finally show that LES with a law-of-the-wall provides excellent results in such a complex case.