bluuebunny/crossref_metadata_2025 huggingface dataset:
Interface Reduction in Craig-Bampton Component Mode Synthesis by Orthogonal Polynomial Series
Authors: Carassale Luigi , Maurici Mirko
Published: August 2017

The component mode synthesis based on the Craig-Bampton method has two strong limitations that appear when the number of the interface degrees of freedom is large. First, the reduced-order model obtained is overweighed by many unnecessary degrees of freedom. Second, the reduction step may become extremely time consuming. Several interface reduction techniques addressed successfully the former problem, while the latter remains open. In this paper we tackle this latter problem through a simple interface-reduction technique based on an a-priory choice of the interface modes. An efficient representation of the interface displacement field is achieved adopting a set of orthogonal basis functions determined by the interface geometry. The proposed method is compared with other existing interface reduction methods on a case study regarding a rotor blade of an axial compressor.
Planar Velocity Measurements at 100 kHz in Gas Turbine Combustors With a Continuous Laser Source
Authors: Mazur Marek , Scouflaire Philippe , Richecoeur Franck , Cunha Caldeira Mesquita Léo , Vie Aymeric , Ducruix Sébastien
Published: August 2017

This work aims at presenting a novel approach to measure planar velocity in gas turbine combustors at very high sampling frequencies. For this purpose, a continuous wave laser is used in order to illuminate particles that are seeded into the flow. The Mie scattering images are acquired with a high-speed camera at 100 kHz with a constant time between each frame. The velocity fields are then obtained by applying classical PIV algorithms on successive particle scattering images. While this approach has been recently used in other research fields, such as aerodynamics or hydrodynamics, it is relatively new in combustion studies, where pulsed laser systems with higher power levels are usually preferred. The proposed technique is an economical and ergonomic solution to determine velocity fields at very high sampling frequencies. It is highly portable and safe and convenient to use and align. The main drawback is the long image exposure duration due to the low laser energy. This leads to a smearing effect of the captured particles and acts as a low-pass filter. It has the consequence that the PIV algorithm does not determine the displacement of “dots”, but of “traces”.

The measurement technique is tested experimentally on a model gas turbine combustor at a laboratory scale. The test is performed in three steps: (1) The instantaneous velocity fields are analysed in order to verify, whether the flame topology is represented correctly. (2) The mean and RMS velocity fields that are obtained with the present technique are compared with those obtained by classic low speed PIV. (3) Instantaneous synthetic Mie scattering fields are generated from a large eddy simulation (LES) on a similar combustor to test the algorithms. The planar velocity fields are calculated from these images and compared for the two techniques. Finally, possible error sources of the new technique are discussed.
Application of Proper Orthogonal Decomposition to High Speed Imaging for the Study of Combustion Oscillations
Authors: Gadiraju Siddhartha , Park Suhyeon , Gomez-Ramirez David , Ekkad Srinath V. , Lowe K. Todd , Moon Hee-Koo , Kim Yong , Srinivasan Ram
Published: August 2017

The flame structure and characteristics generated by an industrial low emission, lean premixed, fuel swirl nozzle were analyzed for understanding combustion oscillations. The experimental facility is located at the Advanced Propulsion and Power Laboratory (APPL) at Virginia Tech. The experiments were carried out in a model optical can combustor operating at atmospheric pressures. Low-frequency oscillations (<100 Hz) were observed during the reaction as opposed to no reaction, cold flow test cases. The objective of this paper is to understand the frequency and magnitude of oscillations due to combustion using high-speed imaging and associate them with corresponding structure or feature of the flame. Flame images were obtained using a Photron Fastcam SA4 high-speed camera at 500 frames per second. The experiments were conducted at equivalence ratios of 0.65, 0.75; different Reynolds numbers of 50K, 75K; and three pilot fuel to main fuel ratios of 0%, 3%, 6%. In this study, Reynolds number was based on the throat diameter of the fuel nozzle. Since the time averaged flame images are not adequate representation of the flame structures, proper orthogonal decomposition (POD) was applied to the flame images to extract the dominant features. The spatiotemporal dynamics of the images can be decomposed into their constituent modes of maximum spatial variance using POD so that the dominant features of the flame can be observed. The frequency of the dominant flame structures, as captured by the POD modes of the flame acquisitions, were consistent with pressure measurements taken at the exit of the combustor. Thus, the oscillations due to combustion can be visualized using POD. POD was further applied to high-speed images taken during instabilities. Specifically, the instabilities discussed in this paper are those encountered when the equivalence ratio is reduced to the levels approaching lean blowout (LBO). As the equivalence ratio is reduced to near blowout regime, it triggers low-frequency high amplitude instabilities. These low-frequency instabilities are visible as the flapping of the flame. The frequencies of the dominant POD modes are consistent with pressure measurements recorded during these studies.
Effect of Inlet Distortion Features on Transonic Fan Rotor Stall
Authors: Page James H. , Hield Paul , Tucker Paul G.
Published: August 2017

The effect of inlet distortion from curved intake ducts on jet engine fan stability is an important consideration for next generation passenger aircraft such as the boundary layer ingestion (BLI) Silent Aircraft. Highly complex inlet flows which occur can significantly affect fan stability. Future aircraft designs are likely to feature more severe inlet distortion, pressing the need to understand the important factors influencing design.

This paper presents the findings from a large CFD investigation into which aspects of inlet distortion cause the most significant reductions in stall margin and, therefore, which flow patterns should be targeted by mitigating technology. The study considers the following aspects of distortion commonly observed in intakes: steady vortical distortion due to secondary flow, unsteady vortical distortion due to vortex shedding and mixing, static pressure distortion due to curved streamlines, and low momentum endwall flow due to thickened boundary layers or separation. Unsteady CFD was used to determine the stall points of a multipassage transonic rotor geometry with each of the inlet distortion patterns applied. Interesting new evidence is provided which suggests that low momentum flow in the tip region, rather than distortion in the main body of the flow, leads to damaging instability.
Effect of Internal Crossflow Velocity on Film Cooling Effectiveness: Part I — Axial Shaped Holes
Authors: McClintic John W. , Anderson Joshua B. , Bogard David G. , Dyson Thomas E. , Webster Zachary D.
Published: August 2017

The effect of feeding shaped film cooling holes with an internal crossflow is not well understood. Previous studies have shown internal crossflow reduces film cooling effectiveness from axial shaped holes, but little is known about the mechanisms governing this effect. It was recently shown that the crossflow-to-mainstream velocity ratio is important, but only a few of these crossflow velocity ratios have been studied. This effect is of concern because gas turbine blades typically feature internal passages that feed film cooling holes in this manner. In this study, film cooling effectiveness was measured for a single row of axial shaped cooling holes fed by an internal crossflow with crossflow-to-mainstream velocity ratio varying from 0.2–0.6 and jet-to-mainstream velocity ratios varying from 0.3–1.7. Experiments were conducted in a low speed flat plate facility at coolant-to-mainstream density ratios of 1.2 and 1.8. It was found that film cooling effectiveness was highly sensitive to crossflow velocity at higher injection rates, while it was much less sensitive at lower injection rates. Analysis of the jet shape and lateral spreading found that certain jet characteristic parameters scale well with the crossflow-to-coolant jet velocity ratio, demonstrating that the crossflow effect is governed by how coolant enters the film cooling holes.
Fault Detection Through Model Based Diagnostics of AE-T100 Micro Gas Turbine
Authors: Mahmood Mariam , Martini Alessio , Traverso Alberto
Published: August 2017

Micro gas turbines (mGT) are emerging power sources for distributed generation facilities considering their environment friendliness, high fuel flexibility and efficient cogeneration of heat and power (CHP). Numerical simulation and diagnostic tools are essential for cycle optimization of mGT and prediction of performance degradation.

This work is focused on fault diagnostics of T100 mGT through the application of AE-T100 model, a simulation tool, which has been developed within a collaboration between the University of Genoa (Unige) and Ansaldo Energia. This model has been developed for steady-state simulation of mGT in off-design conditions. Leveraging on previous efforts for model development, tuning, first phase of validation through Ansaldo Enegia test rig (AE-T100) and diagnostic application of the model, the present work deals with further utilization of the diagnostic capability of the model for mGT cycles. For this purpose, it was used in real operating conditions through the AE-T100 test rig for the diagnostic aim of some unexpected machine behavior. The model results indicated high deviation from the actual field data in terms of fuel flow and efficiency, and so justified the diagnostic capability of the AE-T100 tool.

Afterwards, based on the experimental observation of some bypass leakage from the burner in the same test rig, AE-T100 model was applied to model such leakage through the variation of AS ratio, leakage from the recuperator outlet to the ambient, combustor pressure drop and turbine section modification, and carried out the sensitivity analysis of these parameters. Sensitivity analysis has verified that the accurate impact of this leakage on overall mGT performance cannot be modeled with the help of AE-T100 tool in its current capacity. Therefore, some other investigations like analysis of compressor maps must be carried out to explain such a performance deviation in case of leakage. Afterwards, the analysis of compressor maps resulted from the operating conditions of the test conducted on the same machine after leakage repair, has highlighted the change in compressor operating point and thus, more efficient compressor functioning, which results in higher net power. Hence, this analysis has provided a more comprehensible explanation of this leakage impact on the mGT performance.
Design and Analysis of an Intentional Mistuning Experiment Reducing Flutter Susceptibility and Minimizing Forced Response of a Jet Engine Fan
Authors: Figaschewsky Felix , Kühhorn Arnold , Beirow Bernd , Nipkau Jens , Giersch Thomas , Power Bronwyn
Published: August 2017

Recent demands for a reduction of specific fuel consumption of jet engines have been opposed by increasing propulsive efficiency with higher bypass ratios and increased engine sizes. At the same time the challenge for the engine development is to design safe and efficient fan blades of high aspect ratios. Since the fan is the very first rotor stage, it experiences significant distortions in the incoming flow depending on the operating conditions. Flow distortions do not only lead to a performance and stall margin loss but also to remarkable low engine order (LEO) excitation responsible for forced vibrations of fundamental modes. Additionally, fans of jet engines typically suffer from stall flutter, which can be additionally amplified by reflections of acoustic pressure waves at the intake. Stall flutter appears before approaching the stall line on the fan’s characteristic and limits its stable operating range. Despite the fact that this “flutter bite” usually affects only a very narrow speed range, it reduces the overall margin of safe operation significantly. With increasing aspect ratios of ultra-high bypass ratio jet engines the flutter susceptibility will probably increase further and emphasizes the importance of considering aeromechanical analyses early in the design phase of future fans. This paper aims at proving that intentional mistuning is able to remove the flutter bite of modern jet engine fans without raising issues due to heavily increased forced vibrations induced by LEO excitation. Whereas intentional mistuning is an established technology in mitigating flutter, it is also known to amplify the forced response. However, recent investigations considering aeroelastic coupling revealed that under specific circumstances mistuning can also reduce the forced response due to engine order excitation. In order to allow a direct comparison and to limit costs as well as effort at the same time, the intentional mistuning is introduced in a non-destructive way by applying heavy paint to the blades. Its impact on the blade’s natural frequencies is estimated via finite element models with an additional paint layer. In parallel, this procedure is experimentally verified with painted fan blades in the laboratory. A validated SNM (subset of nominal system modes) representation of the fan is used as a computational model to characterize its mistuned vibration behavior. Its validation is done by comparing mistuned mode shape envelopes and frequencies of an experimental modal analysis at rest with those obtained by the updated computational model. In order to find a mistuning pattern minimizing the forced response of mode 1 and 2 at the same time and satisfying stability and imbalance constraints, a multi-objective optimization has been carried out. Finally, the beneficial properties of the optimized mistuning pattern are verified in a rig test of the painted rotor.
Aeromechanical Design and Test of a Modern Highly Loaded Fan
Authors: Nipkau Jens , Power Bronwyn , Jordan Matthew
Published: August 2017

The paper describes the aeromechanic design of a modern highly loaded fan blisk. The state of the art fan has been designed in a wholistic approach involving input from the disciplines of aerodynamics, structural dynamics, impact, design/make and aeroelasticity. This holistic approach puts large demands for the predictive capability of the involved disciplines. However it will be shown that the final design matches the predictions very well with even exceeding some of the targeted key parameters. The fan was designed aerodynamically to exhibit very high loading and yet was also required to have improved performance and stability characteristics relative to the current state-of-practice in fan design. Integrity requirements necessitated that the previously-stated aerodynamic design goals be achieved with increased blade thicknesses relative to state-of-practice designs. Despite the high loading at the aerodynamic design point the fan was also designed for a maximum operating range at part speed conditions. At these conditions flutter limits the operating range and needs to be avoided at any possible operating condition within the flight envelope. During design phase the part speed flutter phenomenon has been addressed by a combination of using simple aeromechanical design criteria in the early stages aided by CFD analysis in the later stages to verify the flutter behaviour of intermediate designs. The combination of low fidelity criteria and advanced CFD analysis lead to a fast convergence into a final design which met all requirements. The final design has been extensively tested in order to capture aerodynamic parameters and establish the boundaries of the safe operating range. The test results show that the fan met the key performance parameters and even exceeds some of the requirements. In terms of flutter margin the predicted values could largely be confirmed.
Egress Interaction Through Turbine Rim Seals
Authors: Scobie James A. , Hualca Fabian P. , Sangan Carl M. , Lock Gary D.
Published: August 2017

Engine designers require accurate predictions of ingestion (or ingress) principally caused by circumferential pressure asymmetry in the mainstream annulus. Cooling air systems provide purge flow designed to limit metal temperatures and protect vulnerable components from the hot gases which would otherwise be entrained into disc cavities through clearances between rotating and static discs. Rim seals are fitted at the periphery of these discs to minimise purge. The mixing between the efflux of purge (or egress) and the mainstream gases near the hub end-wall results in a deterioration of aerodynamic performance.

This paper presents experimental results using a turbine test rig with wheel-spaces upstream and downstream of a rotor disc. Ingress and egress was quantified using a CO2 concentration probe, with seeding injected into the upstream and downstream sealing flows. The probe measurements have identified an outer region in the wheel-space and confirmed the expected flow structure. For the first time, asymmetric variations of concentration have been shown to penetrate through the seal clearance and the outer portion of the wheel-space between the discs. For a given flow coefficient in the annulus, the concentration profiles were invariant with rotational Reynolds number. The measurements also reveal that the egress provides a film-cooling benefit on the vane and rotor platforms. Further, these measurements provide unprecedented insight into the flow interaction, and provide quantitative data for CFD validation, which should help reduce the use of purge and improve engine efficiency.
Mistuned Higher-Order Mode Forced Response of an Embedded Compressor Rotor: Part I — Steady and Unsteady Aerodynamics
Authors: Li Jing , Aye-Addo Nyansafo , Kormanik Nicholas , Matthews Douglas , Key Nicole , Kielb Robert
Published: August 2017

This paper is the first part of a two-part paper that presents a comprehensive study of the higher-order mode mistuned forced response of an embedded rotor blisk in a multistage axial research compressor. The resonant response of the second-stage rotor (R2) in its first chordwise bending (1CWB) mode due to the second harmonic of the periodic forcing of its neighboring stators (S1 and S2) is investigated computationally and experimentally at three steady loading conditions in the Purdue Three-Stage Compressor Research Facility. State-of-the-art numerical methods applicable in an industrial design environment are used to construct a 1.5-stage stator/rotor/stator configuration for the prediction of the aerodynamic forcing function of the rotor. The time-averaged component of these simulations provides a good prediction of the compressor performance, rotor tip leakage flow (TLF), and characteristics of the stator aerodynamic disturbances. The contribution of the rotor TLF on the rotor forcing function is small, responsible for less than 5% of the total modal force in amplitude. Moreover, the individual contributions of the upstream and downstream stators to the rotor modal force are separated via a linear forcing decomposition approach. It is shown that the upstream stator provides the dominant forcing function with an amplitude almost 6 times that of the downstream stator, and is mostly due to the impulse-like appearance of the upstream stator wakes which have significant higher-harmonic (including the second-harmonic) contents. An excellent prediction of the tuned 1CWB resonant response amplitudes is achieved with only 35%, 4%, and 7% difference to the measured values at three loading conditions.
Pressure and Temperature Effects on Particle Deposition in an Impinging Flow
Authors: Lundgreen Ryan K.
Published: August 2017

Particle deposition is a significant problem in gas turbine engines. Internal cooling passages are of particular interest because deposition build up is observed at far lower temperatures than it is for external flows. Computational fluid dynamics were employed to investigate how changes in the particle Reynolds number affected deposition in an impinging flow. Three-dimensional, steady Reynolds-Averaged Navier-Stokes equations were solved for a single impinging jet that had a jet to wall spacing of H/D = 2. Pressure ratios of 1.015 and 1.03 were considered at three different discharge pressures, 0.1, 1 , and 3 MPa. Three different flow temperatures were also considered, 300, 700, and 1000 K. Five different particle diameters ranging from 0.5 – 10 μm were tracked in each solution. The aerodynamic lensing focal point of the particle tracks, particle impact velocities, particle impact angles, and particle impact locations were all characterized well by the effective Stokes number. The effective Stokes number adjusts the Stokes number by the non-Stokes drag correction factor, which is a function of the particle Reynolds number.
Numerical and Experimental Comparison of Forced Response of Free-Standing and Single-Connected Last Stage Blades
Authors: Piraccini Francesco , Rice Tim , Auciello Jury , Evtushenko Alexey , Mossom Michael
Published: August 2017

In this paper a comparison is made between numerical forced response simulations and experimental measurements for two popular Steam Turbine Last Stage Blade (LSB) architectures: a free standing LSB and a single-connected LSB with a mid-span wing (also known as a ‘snubber’). These variants share the same aerodynamic design in operation, i.e. they have the same ‘operating’ geometry. The focus of this study is the level of vibration response induced on the blades by resonance with a non-synchronous excitation. This can reduce the maximum condenser pressure safely attainable by a Steam Turbine during low volume flow (LVF) operation.

The study develops a common method to effectively represent the LVF excitation in FEM harmonic analysis for both free standing and single-connected LSB’s. This is valuable to LSB designers during the initial phase of a new development (when an assessment of the LVF capability is required before experimental measurements have been taken). In addition, the method can be used to evaluate the suitability of an existing LSB design to a revised environment, as it is often the case in retrofit applications.
Optimizing a Helical Groove Seal With Grooves on Both the Rotor and Stator Surfaces
Authors: Watson Cori , Wood Houston G.
Published: August 2017

Helical groove seals are non-contacting annular seals commonly used in pumps within the impeller stages to sustain a pressure differential for a given leakage. Helical groove seals have continuously cut grooves, like the threads of a screw, on the surface of the rotor, the surface of the stator, or both. The two main components of the flow within helical groove seals are axial flow and groove flow. The axial flow serves to reduce the leakage by dissipating kinetic energy as the fluid expands in the grooves and then is forced to contract within the jet stream region. The groove flow serves to reduce the leakage by acting as screw pump. The fluid within the grooves is displaced towards the high pressure region as it spins with the rotor.

Previous work has shown that seals with grooves on both the surface of the rotor and the surface of the stator can sustain higher pressure differentials for a given leakage than seals with grooves on only one surface. The goal of this study is to optimize the leakage performance of a double surface helical groove seal for a given set of operating conditions. To accomplish this goal, simulations are run in ANSYS CFX. A sufficient mesh with appropriate boundary layers is determined from the mesh independence study. The turbulence model is k-ε turbulence for water at 25°C. This is the first paper to present numerical results for the performance of helical groove seals with grooves on both the rotor and the stator.

The design parameters used in the optimization are inner (rotor) groove size, inner helix angle, outer (stator) groove size, and outer helix angle. A Kennard-Stone algorithm, which optimally spaces the simulations within the design space, is used to select the designs to be simulated. A multifactor quadratic regression is derived. Backward regression is used to reduce the performance function to only statistically significant terms. Finally, the optimal seal design is derived from the performance function and is simulated to demonstrate the predictive power of the performance function. Interaction terms for the rotor and stator design parameters will be used to explore the mechanism whereby helical groove seals with grooves on both the rotor and the stator surfaces are able to have lower leakage than helical groove seals with grooves on just one surface. The end result of this study is a seal design which minimizes leakage and therefore improve machine efficiency.
Impact of PVC Dynamics on Shear Layer Response in a Swirling Jet
Authors: Frederick Mark , Dudash Joshua , O’Connor Jacqueline , Manoharan Kiran , Hemchandra Santosh , Brubaker Brian
Published: August 2017

Combustion instability, or the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll-up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a non-reacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m = 0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m = 1 and m = −1 modal content. By comparing the relative magnitude of the m = 0 and m = −1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.
Inverted Brayton Cycle With Exhaust Gas Condensation
Authors: Kennedy Ian , Chen Zhihang , Ceen Bob , Jones Simon , Copeland Colin D.
Published: August 2017

Approximately 30% of the energy from an internal combustion engine is rejected as heat in the exhaust gases. An inverted Brayton cycle (IBC) is one potential means of recovering some of this energy, in order to improve the overall system efficiency. When a fuel is burnt, water and CO2 are produced and expelled as part of the exhaust gases. In an IBC, in order to reduce compression work, the exhaust gases are cooled before compression up to ambient pressure. If coolant with a low enough temperature is available, it is possible to condense some of the water out of the exhaust gases, further reducing compressor work.

In this study the condensation of exhaust gas water is studied. The results show that the IBC can produce an improvement of approximately 5% in BSFC at the baseline conditions chosen and for a compressor inlet temperature of 310 K. The main factors that influence the power output are heat exchanger pressure drop, turbine expansion ratio, coolant temperature and turbine inlet temperature. A lower coolant temperature significantly increases power output, particularly when condensation occurs. Larger turbine expansion ratios produce more power and slightly lower the temperature at which condensation onset occurs. The system is very sensitive to heat exchanger pressure drop, as larger pressure drops increase the compressor pressure ratio whilst leaving the turbine expansion ratio unchanged. Higher turbine inlet pressures can also increase net power, but the higher exhaust backpressures may increase engine pumping losses.

Finally, for conditions when condensation is possible, the water content of the exhaust gas has a significant influence on power output. The hydrogen to carbon ratio of the fuel has the most potential to vary the water content and hence the power generated by the system. If there is no condensation, water content has a small impact on performance. The effect on power in the condensing region is predominantly due to reduced mass flow in the compressor.
Scale-Resolving Simulations of Bypass Transition in a High-Pressure Turbine Cascade Using a Spectral-Element Discontinuous-Galerkin Method
Authors: Garai Anirban , Diosady Laslo T. , Murman Scott M. , Madavan Nateri K.
Published: August 2017

The application of a new computational capability for accurate and efficient high-fidelity scale-resolving simulations of turbomachinery is presented. The focus is on the prediction of heat transfer and boundary layer characteristics with comparisons to the experiments of Arts et al. for an uncooled, transonic, linear high-pressure turbine (HPT) inlet guide vane cascade that includes the effects of elevated inflow turbulence. The computational capability is based on an entropy-stable, discontinuous-Galerkin spectral-element approach that extends to arbitrarily high orders of spatial and temporal accuracy. The suction side of the vane undergoes natural transition for the clean inflow case, while bypass transition mechanisms are observed in the presence of elevated inflow turbulence. The airfoil suction-side boundary layer turbulence characteristics during the transition process thus differ significantly between the two cases. Traditional simulations based on the Reynolds-averaged Navier Stokes (RANS) fail to predict these transition characteristics. The heat transfer characteristics for the simulations with clean inflow agree well with the experimental data, while the heat transfer characteristics for the bypass transition cases agree well with the experiment when higher inflow turbulence levels are prescribed. The differences between the clean and inflow turbulence cases are also highlighted through a detailed examination of the characteristics of the transitional and turbulent flow fields.
Effect of RANS Method on Stall Inception Eigenvalue Approach
Authors: Xie Zhe , Liu Yangwei , Liu Xiaohua , Lu Lipeng , Sun Xiaofeng
Published: August 2017

The effect of mesh, turbulence model and discretization scheme in Reynolds-averaged Navier-Stokes method (RANS) on a stall inception eigenvalue approach is investigated in a transonic compressor rotor. The most influencing flow structures on the result of eigenvalue approach are also identified. The compressor stall point is calculated by a recently developed eigenvalue model. Based on the 3D Navier-Stokes equations, the body-force term and small disturbance were used to transform the original equations into the eigenvalue approach. Because the eigenvalue mainly relies on the results from RANS, the sensitivity of the eigenvalue to the mesh density, turbulence model, and numerical scheme needs to be clearly identified before it is applied to engineering.

The effect of mesh density is firstly specified. Several grids with different densities and distributions are employed in RANS. The eigenvalue results indicate that the solution converges at the same grid density as RANS does. Besides, the eigenvalue approach has the ability to predict a more accurate stall point compare to RANS with a coarse computational grid. The investigation of the detailed flow field indicates that the flow structures in the vicinity of blade tip region change significantly with three different grid densities, the eigenvalue is also influenced. Two important flow mechanisms are found to be the decisive factors for the eigenvalue, namely the blockage generated by the shock-vortex interaction, the separated flow and the wake near the trailing edge. These flow patterns are consistent with the flow mechanisms of the compressor stall inception. Further investigations are conducted with four different turbulence models combined with three different spatial discretization schemes. Calculated eigenvalue proves that the turbulence model changes the eigenvalue with an over-prediction of stall point at about 1%. The spatial discretization scheme has small effect on stall point prediction using k-ε and SA models, whereas it has large effect when using SST model. The scheme shows great influence in the simulations with specific turbulence model by changing the predicted stall point at least 1.7%. The existence of blockage, the separation and the wake flow are identified as the major and secondary factor which contributes to an unstable prediction of eigenvalue approach, respectively.
Installation Effects on Highly Loaded Turboprop S-Duct Intake Proximity
Authors: Atalayer Çağlar , Friedrichs Jens , Wulff Detlev
Published: August 2017

S-duct intake of a highly loaded turboprop installed on a channel wing was investigated for the interaction effects using computational fluid dynamics. Performance effects on the aerodynamic interface plane (AIP) were compared for varying the intake proximity to propeller mid-plane and nacelle hub. A wrap-around intake with a length aspect ratio of L/D = 3 was used for the analysis. Propeller slipstream was approximated with an actuator disc model. Steady-state RANS simulations was run for cruise condition. The performance was analysed in terms of total pressure recovery and distortion coefficients on AIP. Circumferential and radial distortion intensity descriptors were used along with standard coefficients for the distortion levels. Gain in total pressure recovery was achieved when the intake distance to nacelle hub was increased, whereas it was decreased with farther distance to actuator disc. The larger gap between the nacelle hub and the S-duct intake increased both the distortion coefficient and circumferential intensity on the AIP, as opposed to the decrease in radial distortion intensity. A different trend was observed for the intakes closest to the actuator disc, which showed higher recovery, experienced lower distortion except in radial direction. Analysis of the swirl distortion coefficient on the intake AIP revealed multiple peaks along the circumference. The comparison of maximum positive peaks confirmed a decrease with increasing distance to nacelle hub as the radial distortion intensity. The negative swirl peak comparison revealed an increase with increasing distance to nacelle hub, similar to distortion coefficient. The results will serve as guidelines for the S-duct intake design process and its integration into highly loaded turboprop nacelle of the high-lift aircraft.
Comparative Analysis of Different Numerical Techniques to Analyze the Wake of a Wind Turbine
Authors: Bianchini Alessandro , Balduzzi Francesco , Gentiluomo Domenico , Ferrara Giovanni , Ferrari Lorenzo
Published: August 2017

The analysis of wind turbine wakes’ structure and interaction with other machines installed in the same array or park has become a key element in the current wind energy research due to the notable impact that wakes can have on the actual energy production of the turbines themselves.

The present frontier of the research in this field is leading to the massive use of large-eddy simulations to completely solve the flow field surrounding the rotors. By doing so, however, enormous calculation resources are needed, which are often not available in an industrial context and also generally not compatible with extended optimization analyses (e.g. for a park layout definition). In this latter case, several cases need to be solved in a reasonable amount of time and therefore more computationally efficient methods are still needed.

Within this context, the present study reports a comparative analysis between three different techniques to analyse the wind turbine wakes’ structure and their mutual interaction. In particular, a state-of-the-art 3D RANS calculation of the famous NREL Phase VI rotor was used as a benchmark for comparison with two other methods. The first one is based on the Virtual Blade Model (VBM) of the commercial solver ANSYS® FLUENT®, in which a 3D RANS calculation of the flow field is carried out for the outer domain, while the effect of the rotating blades on the fluid is simulated through a body force, which acts inside a disk of fluid with an area equal to the swept area of the turbine. The value of the body force is time-averaged over a cycle from the forces calculated by a simplified Blade Element Method. In the present study, a stall delay model was also implemented within the VBM module. The second one is instead based on the even more simple approach with an Actuator Disk Model (ADM), in which the turbine presence is actually modelled as a sink of momentum in the main flow. Cross-comparisons between the techniques are shown, both in terms of single wake description and of wake-turbine interaction, leading to the conclusion that the VBM model may represent a valuable and computationally affordable tool in many wind energy applications.
Impeller Manufacturing: Design for Machining
Authors: Cave Michael , Ji Min
Published: August 2017

Today, there are 3 primary methods Solar Turbines, Inc. uses for manufacturing shrouded centrifugal impellers for the Oil and Gas industry. Impellers can be made by an investment casting process, single piece integrally-machined using point milling, or fabricated using a flank milled, open faced impeller and brazing a shroud in place. Investment cast impellers give the aero designer the greatest flexibility in the design, since the designer doesn’t need to be concerned about tool access or other manufacturing constraints. It is common to use this process for any impeller. Single piece integrally-machined impellers are relatively straightforward for high flow coefficient impellers, as the wide flow-path provides plenty of room for tool access. For lower flow coefficients, brazed shrouded impellers can be made very precisely, but at a substantially higher cost than the other methods. Each of these manufacturing methods also has a cost and aerodynamic performance associated with them.

With the advent of high-speed machining and better cutting tools, integrally-machined impellers can offer an alternative manufacturing process over cast or fabricated impellers. However, determining the ability to integrally-machine an impeller historically is done late or even after the detailed design process. This can lead to costly redesigns to make sure the impeller can be machined, with flow-path and blade adjustments done after the aerodynamic design. Sometimes these adjustments are unsuccessful and the manufacturing process is abandoned.

In this paper, 2 medium flow coefficient impellers are redesigned. These impellers are used in pipeline applications for the transmission of natural gas. The original designs were cast. By incorporating highly customized commercial software written exclusively for defining tool paths of integrally-machined shrouded impellers into the aerodynamic design process, new impeller geometry was defined that was able to be integrally-machined, while meeting or exceeding existing impeller performance and improved design cycle time.

This paper will discuss how the machining software was used concurrently with CFD and FEA analysis during the design process. Test results from rig tests will be presented, showing measured results from both the original and redesigned stages.
Redesign of a Compressor Stage for a High-Performance Electric Supercharger in a Heavily Downsized Engine
Authors: Wang Peng , Zangeneh Mehrdad , Richards Bryn , Gray Kevin , Tran James , Andah Asuquo
Published: August 2017

Engine downsizing is a modern solution for the reduction of CO2 emissions from internal combustion engines. This technology has been gaining increasing attention from industry. In order to enable a downsized engine to operate properly at low speed conditions, it is essential to have a compressor stage with very good surge margin. The ported shroud, also known as the casing treatment, is a conventional way used in turbochargers to widen the working range. However, the ported shroud works effectively only at pressure ratios higher than 3:1. At lower pressure ratio its advantages for surge margin enhancements are very limited. The variable inlet guide vanes are also a solution to this problem. By adjusting the setting angles of VIGVs, it is possible to shift the compressor map towards the smaller flow rates. However, this would also undermine the stage efficiency, require extra space for installing the IGVs, and add costs. The best solution is therefore to improve the design of impeller blade itself to attain high aerodynamic performances and wide operating ranges. This paper reports a recent study of using inverse design method for the redesign of a centrifugal compressor stage used in an electric supercharger, including the impeller blade and volute. The main requirements were to substantially increase the stable operating range of the compressor in order to meet the demands of the downsized engine. The 3D inverse design method was used to optimize the impeller geometry and achieve higher efficiency and stable operating range. The predicted performance map shows great advantages when compared with the existing design. To validate the CFD results, this new compressor stage has also been prototyped and tested. It will be shown that the CFD predictions have very good agreement with experiments and the redesigned compressor stage has improved the pressure ratio, aerodynamic efficiency, choke and surge margins considerably.
Investigation of an Industrial Gas Turbine Combustor and Pollutant Formation Using LES
Authors: Mallouppas George , Goldin Graham , Zhang Yongzhe , Thakre Piyush , Krishnamoorthy Niveditha , Rawat Rajesh , Gosman David , Rogerson Jim , Bulat Ghenadie
Published: August 2017

An experimental variant of a commercial swirl burner for industrial gas turbine combustors operating at 3 bar is numerically investigated using high-fidelity Computational Fluid Dynamics models using STAR-CCM+ v11.06. This work presents the computational results of the SGT-100 Dry Low Emission gas turbine provided by Siemens Industrial Turbomachinery Ltd. The related experimental study was performed at the DLR Institute of Combustion Technology, Stuttgart, Germany. The objective of this work is to compare the performance of the Flamelet Generated Manifold model, which is the widely accepted combustion model in Gas Turbines with the Complex Chemistry model. In particular this work examines the flame shape and position, pollutant formation predicted by the aforementioned models with Large Eddy Simulations.

Mean and RMS quantities of the flow field, flame temperatures and major species are presented and compared with the experiments. The results show that the predictions are insensitive on the meshing strategy and at the evaluated mesh sizes of ∼10 million and ∼44 million cells. The mean and RMS errors are ∼8% compared to the reported experiments and these differences are within the measurement errors. The results show that the calculated flame positions are in very good agreement with the reported measurements and the typical M-shape flame is reproduced independent of the combustion model. Pollutant formation in the combustor predicted by two combustion models is scrutinised. The predicted NO and CO emissions levels are in agreement with the literature.
Formation of Soot in Ethylene-Air Partially Premixed Flames Over a Wide Range of Premixedness
Authors: Chakraborty Aritra , Chakravarthy Satya R.
Published: August 2017

This paper reports an investigation of soot formation in ethylene-air partially premixed flames over a wide range of premixedness. An axisymmetric co-flow configuration is chosen to establish partially premixed flames from the fully non-premixed to fully premixed conditions. Reducing the fuel flow rate as a percentage of the maximum from the core stream and supplying the same to the annular stream leads to stratification of the reactant concentrations. The thermal power, overall equivalence ratio, and the average velocity in the both streams are maintained constant under all conditions. The soot volume fraction is estimated by light attenuation method, and laser induced incandescence is performed to map the soot distribution in the flow field. The soot volume fraction is observed to exhibit a ‘S’-type trend as the conditions are traversed from near the premixed to the non-premixed regimes. That is, when traversing from the non-premixed to near-premixed regime, below 60% fuel flow rate in core, the soot volume fraction drops drastically. The onset of sooting in the partially premixed flames is clearly seen to be at the tip of the rich-premixed flame branch of their triple flame structure, which advances upstream towards the base of the flame as the premixing is reduced. The ‘S’-type variation is clearly the effect of partial premixing, more specifically due to the presence of the lean premixed flame branch of the triple flame. Laser induced incandescence intensities are insufficient to capture the upstream advance of the soot onset with decreased premixedness. So, a quick and inexpensive technique to isolate soot luminescence through flame imaging is presented in the paper involving quasi-simultaneous imaging with a 650 nm and a BG-3 filter using a normal color camera.
Part Load Behavior of the LP Part of an Industrial Gas Turbine
Authors: Petrovic Milan V. , Wiedermann Alexander , Nedeljkovic Srecko M. , Banjac Milan
Published: August 2017

The operation under off-design conditions of a two-stage LP part of a 6.5 MW industrial gas turbine was analyzed in this work. Since the turbine is able to vary the rotation speed in a wide range from 40 to 140% of the design speed, a flow with extremely large positive and negative incidence angle appears.

The flow field was calculated applying 2D through-flow code for the analysis of axial multistage turbines with cooling by air from compressor bleed. The code was developed by the authors and validated by calculation of a number of test cases with different configurations. The method is based on a stream function approach and a finite element solution procedure. In parallel, the flow in the turbine was calculated using a commercial CFD code. Based on the calculated flow field, the turbine efficiency and pressure ratio and also different stage parameters were determined for the design point and for a wide range of off-design conditions. Comparison of the predicted results and measured test data for a number of parameters showed good agreement.