bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Jet Impingement Configurations and Aerothermal Variables on Film Cooling
Authors: Yang Xing , Liu Zhao , Liu Zhansheng , Wu Bin , Feng Zhenping
Published: September 2016

In this study, the effects of impingement with various configurations at different aerothermal conditions on film cooling are investigated. The detailed adiabatic film cooling effectiveness distributions are obtained by solving steady three dimensional Reynolds-averaged Navier-Stokes equations with SST k-ω turbulence model closure. The influence of impingement on film cooling effectiveness is revealed by comparing the results from two cases: one where coolant is directly fed from a plenum (baseline case) and the other where the film coolant is extracted from the post-impingement flow on spherical dimples. For the latter case with post-impingement flow, the variations of the jet impingement configurations are considered at separation distances (H/Dj) from jet plate to target surface of 1, 2, 4 and 6, and eccentricities (F/Dj) between dimple center and film hole center of 0, 2, and 4. Besides, the effects of target wall heating the post-impingement flow on the external adiabatic film cooling performance are examined. The temperature ratios of the target surface to main flow at the inlet are set at 0.6, 0.7 and 0.8. The results are presented for four various averaged jet Reynolds numbers, which correspond to blowing ratios ranging from 0.5 to 2.0. It is observed that the impingement through the jet plate brings out pressure re-distributions on the target plate with film holes, and the dominant effect is on the flow structures in the supply chamber and near the entrance of the film hole. At the lowest blowing ratio of 0.5, film cooling with post-impingement air on dimples is reduced in comparison with the baseline case, while at higher blowing ratios, the effect of the impingement configuration on film cooling all depends on the flow conditions. In addition, the heating effect of target wall on the post-impingement flow could lower the coolant-to-mainstream density ratio, and then reduces the adiabatic film cooling performance.
High-Pressure Turbine Rim Seal Design for Increased Efficiency
Authors: Chilla M. , Hodson H. P. , Pullan G. , Newman D.
Published: September 2016

In high-pressure turbines, compressor air is used to purge the disc space in an effort to protect the blade roots and the turbine disc from overheating and failure. The purge air exits the disc space through a rim seal at the hub of the main annulus and is subsequently entrained in the rotor hub endwall flows. The introduction of the purge air into the turbine main stream causes additional losses and therefore reduced turbine efficiency. For a given rim sealing mass flow rate, the rim seal geometry has to be designed in a way that reduces the detrimental impact of the sealing flow on turbine performance.

In this study, the rim seal of a generic high-pressure turbine, representative of modern large civil aero-engines, is redesigned under consideration of the pressure field upstream of the rotor. Unsteady numerical simulations of the turbine stage are used to compare the aerodynamic impact of three different rim seal designs. The numerical simulations predict an increase in the time-averaged turbine stage efficiency of over 0.2% for the stage configuration with the final redesigned rim seal compared to the configuration with the original baseline rim seal geometry at the nominal sealing mass flow rate.
High-Frequency Thermoacoustic Modulation Mechanisms in Swirl-Stabilized Gas Turbine Combustors: Part Two — Modeling and Analysis
Authors: Hummel Tobias , Berger Frederik , Hertweck Michael , Schuermans Bruno , Sattelmayer Thomas
Published: September 2016

This paper deals with high-frequency thermoacoustic instabilities in swirl-stabilized gas turbine combustors. Driving mechanisms associated with periodic flame displacement and flame shape deformations are theoretically discussed, and corresponding flame transfer functions are derived from first principles. These linear feedback models are then evaluated by means of a lab-scale swirl-stabilized combustor in combination with part one of this joint publication. For this purpose, the models are used to thermoacoustically characterize a complete set of operation points of a this combustor facility. Specifically, growth rates of the first transversal modes are computed, and compared against experimentally obtained pressure amplitudes as an indicator for thermoacoustic stability. The characterization is based on a hybrid analysis approach relying on a frequency domain formulation of acoustic conservation equations, in which non-uniform temperature fields and distributed thermoacoustic source terms / flame transfer functions can be straightforwardly considered. The relative contribution of flame displacement and deformation driving mechanisms — i.e. their significance with respect to the total driving — is identified. Furthermore, promoting/ inhibiting conditions for the occurrence of high frequency, transversal acoustic instabilities within swirl-stabilized gas turbine combustors are revealed.
Analytic Assessment of an Embedded Aircraft Propulsion
Authors: Pelz Peter F. , Cloos Ferdinand-J. , Sieber Jörg
Published: September 2016

This paper investigates analytically the advantage of the embedded propulsion compared to a state of the art propulsion of an aircraft. Hereby, we are applying the integral method of boundary layer theory and potential theory to analyse the boundary layer thickness and the impact of the flow acceleration due to the embedded propulsion. The aircraft body is treated as a flat plate. The engine is treated as a momentum disc but there is a trade off, since the engine efficiency is effected by the boundary layer. The outcome of the energetic assessment is the following: the propulsion efficiency is increased by the embedded propulsion and the drag of the aircraft body is reduced. The optimized aircraft engine size depending on Reynolds number is given.
Effects of Inlet Swirl on Pressure Side Film Cooling of Neighboring Vane Surface
Authors: Li Yifei , Zhang Yang , Yuan Xin
Published: September 2016

The test cascades are based on the profile of General Electric Energy Efficient Engine (GE-E3) with the inlet Mach number of 0.1 and Reynolds number of 1.48×105. The scale ratio of the test vanes is 1.95 and the film cooling effectiveness is tested with Pressure Sensitive Painting (PSP) technology. Nitrogen is used to simulate the coolant which can provide a density ratio near to 1.0. The two adjacent passages are investigated simultaneously by which the film cooling effectiveness can be compared in the same case with pressure side and suction side respectively. The rotating flow is simulated by an upstream swirler at the inlet, fixed at 5 different positions along the pitchwise direction at the inlet. They are Blade 1 aligned, Passage 1 aligned, Blade 2 aligned, Passage 2 aligned and Blade 3 aligned. According to the experimental results, the inlet rotating flow can dominate the film cooling effectiveness distribution at Pressure Side. The averaged film cooling effectiveness changes significantly with the change in swirler position. The effect of the rotating flow at the Pressure Side region is mainly interacted with the main flow to cause the change in incidence angle. The influence of the inlet rotating flow is more obvious on the pressure side upstream part. Meanwhile the downstream part is not so sensitive to rotating flow as pressure side.
Fouling Detection in Low Speed Fan Using Near-Field DIY Sound Pressure Sensors
Authors: Corsini Alessandro , Tortora Cecilia
Published: September 2016

This work investigates the dynamics of rotating stall of a low speed axial fan in presence of fouling on the blades. Rotating stall is an aerodynamic issue of recognized importance in turbomachinery. The combination of rotating stall and presence of particles of dust and dirt from the surrounding environment, may lead to further issues in terms of performance, stall limit and blades life.

In this paper the identification of the rotating stall pattern is carried out using time-resolved sound measurements in the far field region by means of a condenser microphone. The experimental tests are carried out with various geometries of fouling in order to evaluate the system ability to detect acoustically fouling and rotating stall.

The results have been validated against state of the art techniques described in the literature. The acquired signals have been analysed using frequency domain analysis, and time domain analysis using a phase space reconstruction inspired technique. Both of the approaches demonstrate a modification of the stall dynamics in the low speed fan and allow the identification of diverse stall precursors and fouling presence.
Temperature Effects on Nozzle Guide Vane Deposition in a New Turbine Cascade Rig
Authors: Lundgreen Ryan , Sacco Craig , Prenter Robin , Bons Jeffrey P.
Published: September 2016

A new turbine cascade has been constructed that is designed to investigate the performance of actual nozzle guide vane hardware at temperatures representative of modern gas turbine engines. The facility is designed to investigate internal and external deposition, analyze the effectiveness of new cooling techniques, characterize material systems such as metal substrates or coatings, and assess the aerodynamic performance of a vane. The results presented here are the first results obtained in this new facility. External deposition on cooled CFM56 nozzle guide vanes has been explored at inlet temperatures of 1090° C, 1265° C, and 1350° C. Results at 1090° C have been compared to similar results in a previous facility. External deposition tests at temperatures greater than 1100° C on actual turbine hardware have not been reported publicly prior to this paper. These results show that deposition is concentrated at the stagnation line at all three inlet conditions. The amount of deposition on the vane pressure surface increased with increasing inlet temperatures.
Micro Gas Turbine/Renewable Hybrid Power System for Distributed Generation: Effects of Ambient Conditions on Control Strategy
Authors: Shixi Ma , Zhou Dengji , Zhang Huisheng , Lu Zhenhua
Published: September 2016

Hybrid power systems are becoming popular for remote areas due to lower operating cost and green gas emission. Most of these systems are used in remote or harsh environments, so the effect of ambient conditions on system operation is an important factor that should not be ignored. In this paper, the system referred is a domestic hybrid power system including a renewable energy conversion device (Photovoltaic, PV), a traditional energy conversion device (Micro Gas Turbine, MGT) and an electrochemical energy storage unit (batteries). A numerical model, which considers the effect of ambient conditions on the whole system, has been developed. Model Predictive Control (MPC) strategy has been applied to the analysis of power management. The control strategy includes the objective of minimizing system costs, while considering real operational constraints of the plants. Performances attainable with the MPC strategy have been evaluated in comparison with a standard Rule Based Control logic (RBC), by means of costs and efficiency parameters of the system. The effects of ambient conditions on system operation based on MPC-based strategy are evaluated. The simulation has been carried out for the summer and winter periods in four places with different climate in China. Results show that a lower cost of primary fossil energy is found by using the MPC strategy. This is mainly due to the increased use of renewable energy sources by considering the future load. An obvious effect of ambient conditions on control process is observed. A significant improvement for the whole year in efficiency of the system, especially in high latitude cold regions with larger temperature difference from the design condition, is achieved by considering the ambient conditions. The highest reduction of fuel consumption reaches to 4% during the winter. As a result, the effect of the ambient conditions in some areas must be taken into account for control system design.
Experimental Investigation on the Time-Space Evolution of a Laminar Separation Bubble by POD and DMD
Authors: Lengani D. , Simoni D. , Ubaldi M. , Zunino P. , Bertini F.
Published: September 2016

A time resolved Particle Image Velocimetry (TR-PIV) system has been employed to investigate a laminar separation bubble which is induced by a strong adverse pressure gradient typical of Ultra-High-Lift lo pressure turbine blades. Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are described and applied within the paper. These techniques allow reducing the degrees of freedom of complex systems producing a low order model ranked by the energy content (POD) or by the modal contribution to the dynamics of the system itself (DMD), useful to highlight the dominant dynamics.

The time-space evolution of the laminar separation bubble is characterized by rollup vortices shed in the surround of the bubble maximum displacement as a consequence of the Kelvin-Helmholtz instability process as well as by a low frequency motion of the separated shear layer. The decomposition techniques proposed allow the identification of these coherent structures and the characterization of their modal properties (e.g. temporal frequency, spatial wavelength and growth rate).

The POD separates the different dynamics that induce velocity fluctuations at different frequencies and wavelength looking at their contribution to the overall kinetic energy. The DMD provides complementary information: the unstable spatial frequencies are identified with their growth (or decay) rates. DMD modes associated with the Kelvin-Helmholtz instability and the corresponding vortex shedding phenomenon clearly dominate the unsteady behavior of the laminar separation bubble, being characterized by the highest growth rate. Modes with longer wavelength describe the low frequency motion of the laminar separation bubble and are neutrally stable. Results reported in the paper prove the ability of the present methods in extracting the dominant dynamics from a large dataset, providing robust and rapid tools for the in dept analysis of transition and separation processes.
High Frequency Thermoacoustic Modulation Mechanisms in Swirl-Stabilized Gas Turbine Combustors: Part One — Experimental Investigation of Local Flame Response
Authors: Berger Frederik M. , Hummel Tobias , Hertweck Michael , Kaufmann Jan , Schuermans Bruno , Sattelmayer Thomas
Published: September 2016

This paper presents the experimental approach for determination and validation of non-compact flame transfer functions of high frequency, transverse combustion instabilities observed in a generic lean premixed gas turbine combustor. The established non-compact transfer functions describe the interaction of the flame’s heat release with the acoustics locally, which is necessary due to the respective length scales being of the same order of magnitude. Spatio-temporal dynamics of the flame are measured by imaging the OH* chemiluminescence signal, phase-locked to the dynamic pressure at the combustor’s front plate. Radon transforms provide a local insight into the flame’s modulated reaction zone. Applied to different burner configurations, the impact of the unsteady heat release distribution on the thermoacoustic driving potential, as well as distinct flame regions that exhibit high modulation intensity are revealed. Utilizing these spatially distributed transfer functions within thermoacoustic analysis tools (addressed in this joint publication’s part two) allows then to predict transverse linear stability of gas turbine combustors.
Numerical Investigation of Flow Instability in Centrifugal Pump Based on Energy Gradient Method
Authors: Zheng Lulu , Dou Hua-Shu , Chen Xiaoping , Zhu Zuchao , Cui Baoling
Published: September 2016

Simulation of turbulent flow in a pump is carried out with the RANS equations and the RNG k-epsilon turbulence model. Numerical simulation has been compared with the experimental data. The results show that separating vortex is firstly produced at the pressure side of the impeller passage near the tongue. Then it spreads to the inlet and outlet of the impeller passages and moved to the centre region of impeller passages from the boundaries. Finally, it almost occupies all the impeller passages and multiple vortices exist in impeller passages at small flow rate. It is found that the tongue has large effect on the flow in the impeller passage approaching to it. The impeller passage near the tongue is easily tending to be unstable comparing with others passages. The energy gradient theory is used to analyze the flow stability in the impeller. The region with larger value of energy gradient function K means the bigger turbulence intensity and poor flow stability. At small flow rate the regions with large value of K are enlarged and are mainly located at both sides of blade pressure and suction surfaces where the flow is easily tending to be unstable.
A 3D Time-Accurate CFD Simulation of the Flow Field Inside a Vaneless Diffuser During Rotating Stall Conditions
Authors: Marconcini Michele , Bianchini Alessandro , Checcucci Matteo , Ferrara Giovanni , Arnone Andrea , Ferrari Lorenzo , Biliotti Davide , Rubino Dante Tommaso
Published: September 2016

An accurate characterization of rotating stall in terms of inception modality, flow structures, and stabilizing force is one of the key goals for high-pressure centrifugal compressors. The unbalanced pressure field that is generated within the diffuser can be in fact connected to a non-negligible aerodynamic force and then to the onset of detrimental sub-synchronous vibrations which can prevent the machine from operating beyond this limit. An inner comprehension on how the induced flow pattern in these conditions affects the performance of the impeller and its mechanical stability can therefore lead to the development of a more effective regulation system able to mitigate the effects of the phenomenon and extend the left-side margin of the operating curve.

In the present study, a 3D-unsteady CFD approach was applied to the simulation of a radial stage model including the impeller, the vaneless diffuser and the return channel. Simulations were carried out with the TRAF code of the University of Florence. The tested rotor was an industrial impeller operating at high peripheral Mach number, for which unique experimental pressure measurements, including the spatial reconstruction of the pressure field at the diffuser inlet, were available. The comparison between experiments and simulations showed a good matching and corroborated the CFD capabilities in correctly describing also some of the complex unsteady phenomena taking place in proximity of the left margin of the operating curve.
Improvement of Laminar-Turbulent Transition Modeling Within a Low-Pressure Turbine
Authors: Minot A. , Salah El-Din I. , Barrier R. , Boniface J.-C. , Marty J.
Published: September 2016

The flow within turbomachines is intrinsically complex and involves boundary layer transition, separation and vortices such as the tip leakage vortex and wakes. In a low-pressure turbine, as the Reynolds number can be small, the flow over the suction side is likely to separate leading to the formation of a laminar (or transitional) separation bubble. This flow mechanism can be predicted using Large-Eddy Simulation. However the computation is still very expensive in a design framework. Thus, Reynolds-Averaged Navier-Stokes (RANS) method is used in the present investigation to simulate the flow over the low-pressure turbine airfoil T106C. The laminar-turbulent transition is modeled with the γ-Rθt~ model of Menter and Langtry. Following the work of Minot et al. in which the CFD setup was deeply investigated, the present study aims at evaluating the sensitivity to uncertainties relative to experimental values (freestream turbulence, Reynolds number, incidence flow angle and exit isentropic Mach number) and at improving this model regarding the calibration of several functions using optimization process. The uncertainty study highlights the parameters which mainly influence the isentropic Mach number and loss distributions. The new calibration of the Menter-Langtry model improves significantly the flow prediction over the suction side, except for the open bubble configuration.
Experimental Characterisation of a Humidified T100 Micro Gas Turbine
Authors: Montero Carrero Marina , De Paepe Ward , Magnusson Jan , Parente Alessandro , Bram Svend , Contino Francesco
Published: September 2016

Despite the potential of micro Gas Turbines (mGTs) for Combined Heat and Power (CHP), this technology still poses limitations that curb its widespread adoption, especially for applications with a variable heat demand. In fact, whenever the user heat demand is low, mGTs are generally shut down. Otherwise, the high temperature exhaust gases have to be blown off and the resulting electrical efficiency is not high enough to sustain a profitable operation. If, instead of released, the heat in the exhaust gases is re-inserted in the cycle — by injecting hot water and transforming the mGT into a micro Humid Air Turbine (mHAT) — the electrical efficiency can be increased during periods of reduced heat demand, thus improving the economics of the technology.

Although the enhanced performance of the mHAT cycle has been thoroughly investigated from a numerical point of view, results regarding the experimental behaviour of this technology remain scarce. In this paper, we present the experimental characterisation of the mHAT located at the Vrije Universiteit Brussel (VUB): based on the T100 mGT and equipped with a spray saturation tower. These are the first experimental results of such an engine working at nominal load with water injection. In addition, the control system of the unit has been modified so that it can operate either at constant electrical power output (the default setting) or at constant rotational speed. The latter option allowed to better assess the effect of water injection.

Eperimental results demonstrate the patent benefits of water injection on mGT performance: at fixed rotational speed, the power output of the mHAT increases by more than 30%while the fuel consumption rises only by 11%. Overall, the electrical efficiency in wet operation increases by up to 4.2% absolute points. Future work will involve further optimising the current facility to reduce pressure losses in the air and water circuits. In addition, we will carry out transient simulations and experiments in order to further characterise the facility.
Uncertainty Quantification of Growth Rates of Thermoacoustic Instability by an Adjoint Helmholtz Solver
Authors: Silva Camilo F. , Runte Thomas , Polifke Wolfgang , Magri Luca
Published: September 2016

The objective of this paper is to quantify uncertainties in thermoacoustic stability analysis with a Helmholtz solver and its adjoint. Thermoacoustic combustion instability may be described by the Helmholtz equation combined with a model for the flame dynamics. Typically, such a formulation leads to an eigenvalue problem in which the eigenvalue appears under nonlinear terms, such as exponentials related to time delays that result from the flame model. Consequently, the standard adjoint sensitivity formulation should be augmented by first- and second-order correction terms that account for the nonlinearities. Such a formulation is developed in the present paper, and applied to the model of a combustion test rig with a premix swirl burner. The uncertainties considered concern plenum geometry, outlet acoustic reflection coefficient, as well as gain and phase of the flame response. The nonlinear eigenvalue problem and its adjoint are solved by an in-house adjoint Helmholtz solver, based on an axisymmetric finite volume approach. In addition to first-order correction terms of the adjoint formulation, which are often used in literature, second-order terms are also taken into account. It is found that one particular second-order term has significant impact on the accuracy of results. Finally, the Probability Density Function of the growth rate in the presence of uncertainties in input paramters is calculated with Monte Carlo simulations. It is found that the second-order adjoint method, while giving quantitative agreement, requires far less compute resources than Monte Carlo sampling for the full nonlinear eigenvalue problem.
Technique for Adequate CFD-Modeling of the Pump With Hydro-Drive of the Low-Pressure Stage
Authors: Zubanov Vasilii M. , Shabliy Leonid S. , Krivcov Alexander V. , Matveev Valeriy N.
Published: September 2016

This article describes the technique for CFD-modeling of a powerful two-stage pump with the following main parameters: main rotor speed is 13,300 rpm, inlet pressure is 0.2 MPa, pressure head is more than 3,000 meters with mass flow of 250 kg/s. The main feature of investigated pump is the hydro-drive of the low-pressure stage of turbine with variable rotational speed. There are two highlights in this work in comparison with the previous ones. The first one is how to choose the rotating speed of hydro-turbine. The second one is the CFD-modeling of cavitation processes. The core part of proposed technique is the determination of rotational speed during CFD-simulation by special methodology. Another feature is the cavitation modeling to be sure that there is no cavitation in pre-pump at quite low inlet pressure and variable rotor speed. Also, recommendations about program tools (ANSYS CFX, NUMECA AutoGrid5, ANSYS ICEM CFD) are a significant part of the discussed technique, as well as modeling features (fluid domain restriction, meshing, turbulence models choosing, convergence checking, post-processing). The adequacy of CFD-model was evaluated by comparing predicted characteristics of the pump with the experimental ones derived from the test rig. The differences amounted to less than 10%. The obtained technique can be used in the future research for performance improving and efficiency increasing of pumps with hydro-drive of the low-pressure stage by CFD-tools.
Investigation of Different Solar Hybrid Gas Turbines and Exploitation of Rejected Sun Power
Authors: Kalathakis C. , Aretakis N. , Roumeliotis I. , Alexiou A. , Mathioudakis K.
Published: September 2016

Solar hybrid gas turbine performance is studied through consideration of four engine configurations: a) single shaft, b) recuperated single-shaft, c) twin-shaft and d) two-spool three-shaft, intercooled, recuperated. For each configuration and for the same design point, the performance is obtained for two hybridization schemes: Fuel only engines Retrofitted for Solar operation (FRS) and engines designed with Solar only operation at the Design Point (SDP). In an attempt to further improve the benefits of hybridization, the concept of a Dual Fluid Receiver for exploiting the rejected solar power, during sunny hours with high irradiation, is demonstrated. Steam is produced by focusing the defocused mirrors of the heliostat field to a second receiver and injected into the combustion chamber.

For the cases examined, it can be concluded that FRS engines show better performance than SDP ones, since they exhibit higher thermal fuel efficiency and higher specific power. Concerning the configurations, an annual fuel saving of ∼35% and an annual output reduction, ranging from 4% for the recuperated single-shaft configuration to 9% for the twin shaft configuration compared to the corresponding fuel-only engines is demonstrated. The inclusion of a Dual Fluid Receiver in an FRS engine removes the power penalty while it maintains the fuel saving benefit.
Leveraging Correlation to Reduce Uncertainty in Efficiency Measurements for Low Pressure Ratio Fans
Authors: Ferrar Anthony M. , Schneck William C. , O’Brien Walter F.
Published: September 2016

This paper presents the often-neglected correlation terms for uncertainty propagation and shows their importance for rigorous uncertainty analysis. It shows that these terms can be leveraged, often through simple experimental design changes to substantially reduce uncertainty. The efficiency equation is particularly suited to this approach, with order of magnitude uncertainty reductions possible. The paper will present this effect from both a theoretical study and from data measured behind a transonic fan during a distortion response program.

The efficiency equation is particularly suited to improving uncertainty by leveraging the correlation terms because the temperature uncertainty contributes most of the overall uncertainty. Systematic error sources usually dominate thermocouple measurement uncertainty because the slow time response leads to small random errors. The partial derivatives with respect to inlet and outlet total temperature have opposite signs. Correlations between these systematic errors are relatively easy to force through using a single data acquisition system and in-situ calibration standard. This paper presents performance comparisons of these effects for thermocouples. For a 1.35 pressure ratio, 85% efficiency fan, the uncertainty can be reduced by correlation from 85% ± 6.7% to 85% ± 0.48% for typical K-type thermocouples.
Analysis of a LPT Rotor Blade for a Geared Engine: Part II — Characterization of the Time-Varying Flow Field in a Single Stage Research Turbine
Authors: Infantino Daniele , Satta Francesca , Simoni Daniele , Ubaldi Marina , Zunino Pietro , Bertini Francesco
Published: September 2016

The present paper is the second part of a two-part paper focused on the design and the analysis of an optimized rotor blade for a geared open rotor engine. This part is focused on the experimental investigation of the three-dimensional unsteady flow field at the exit plane of a rotor row installed in a large scale single-stage low speed research turbine. The investigation is aimed at in depth characterizing the wake-boundary layer and the vortex-vortex interaction processes induced by the rotor-stator relative motion. Measurements have been carried out at a typical aeroengine cruise condition Reynolds number. The rotor blade aerodynamic loadings at different blade spans have been measured. A five-hole probe has been used to assess the row efficiency and detailed hot-wire phase-locked ensemble-averaged data have been analyzed to characterize the three-dimensional time-dependent flow field at the rotor exit plane. Results clearly highlight a significant distortion of the rotor blade wake and tip vortex during the migration of the high turbulence regions (wake and secondary flows) associated with the upstream stator. The unsteady interaction between the stator secondary flows and the rotor passage vortex provokes a time dependent movement of the low momentum area at the hub surface, sensibly modifying the penetration of the rotor secondary flows in an incoming stator wake passage period. The comparison of deterministic and random velocity fluctuations also allows the distinction between the structures generated by the stator and those due to the rotor.
Particle Image Velocimetry Investigations of a Subsonic Compressor Tandem Cascade
Authors: Hertel Charlotte , Bode Christoph , Kožulović Dragan , Schneider Tim
Published: September 2016

A highly loaded, optimized compressor tandem cascade for low speed application (Ma1 = 0.175) [1, 2] was investigated numerically and experimentally using the standard particle image velocimetry measurement technique. The investigated regions are divided in the inlet, the overlap and the outlet region and are investigated for three different inlet angles.

For all inlet angles investigated the results show a homogeneous velocity distribution. The analysis of the measured inlet angle shows a strong suction effect of the cascade starting far in front of the leading edge of the front blade. The acceleration in the overlap region and the change of the velocity distribution is measured for all inlet angles investigated and clarifies the conclusions resulting of former blade surface static pressure measurements [1]. Although the used PIV technique is only capable to resolve two velocity components, the results of the spanwise measured outlet flow fields allow a good analysis of the secondary flow and complete the standard measurement results [2]. The comparison of the experimental flow field results with the numerical results reveals the known numerical limits of secondary flow prediction regarding the extension and intensity [2].
Effects of Rotor Tip Blade Loading Variation on Compressor Stage Performance
Authors: Tiralap Aniwat , Tan Choon S. , Donahoo Eric , Montgomery Matthew , Cornelius Christian
Published: September 2016

Changes in loss generation associated with altering the rotor tip blade loading of an embedded rotor-stator compressor stage are assessed with unsteady three-dimensional computations, complemented by control volume analyses. Tip-fore-loaded and tip-aft-loaded rotor blades are designed and assessed to provide variation in rotor tip blade loading distributions for determining if aft-loading rotor tip would yield a stage performance benefit in terms of a reduction in loss generation. Aft-loading rotor blade tip delays the formation of tip leakage flow resulting in a relatively less mixed-out tip leakage flow at the rotor outlet and a reduction in overall tip leakage mass flow, hence a lower loss generation; however, the attendant changes in tip flow angle distribution are such that there is an overall increase in the flow angle mismatch between tip flow and main flow leading to higher loss generation. The latter outweighs the former so that rotor passage loss from aft-loading rotor tip is marginally higher unless a constraint is imposed on tip flow angle distribution so that associated induced loss is negligible; a potential strategy for achieving this is proposed. Tip leakage flow, which is not mixed-out at the rotor outlet, enters the downstream stator, where it can be recovered. The tip leakage flow recovery process yields a higher benefit for a relatively less mixed-out tip leakage flow from aft-loading a rotor blade tip. These characterizing parameters together determine the attendant loss associated with rotor tip leakage flow in a compressor stage environment. A revised design hypothesis is thus as follows: rotor should be tip-aft-loaded and hub-fore-loaded while stator should be hub-aft-loaded and tip-fore-loaded with tip/hub leakage flow angle distribution such that it results in no additional loss. For the compressor stage being assessed here, an estimated 0.15 points enhancement in stage efficiency is possible from aft-loading rotor tip only. In the course of assessing the benefit from unsteady tip leakage flow recovery in the downstream stator, it was determined that tip clearance flow is inherently unsteady with a time-scale distinctly different from the blade passing time. The disparity between the two timescales: (i) defines the periodicity of the unsteady rotor-stator flow, which is an integral multiple of blade passing time; and (ii) causes tip leakage vortex to enter the downstream stator at specific pitchwise locations for different blade passing cycles, a tip leakage flow phasing effect. Despite the inherent unsteadiness from tip leakage flow, the recovery process is demonstrated to be beneficial on a time-averaged basis.
Detailed Examination of Two-Staged Micro Gas Turbine Combustor
Authors: Schwärzle A. , Monz T. O. , Aigner M.
Published: September 2016

Jet-stabilized combustion is a promising technology for fuel flexible, reliable, highly efficient combustion systems. In this work, experiments have been carried out on a two-staged combustor, with a jet-stabilized main stage and a swirl-stabilized pilot stage. Both stages have been run separately to allow a more detailed understanding of the flame stabilization within the combustor and its range of stable operation. All experiments were conducted at atmospheric pressure and preheating temperatures of 650 °C. The air was fed to both stages of the combustor for all experiments. The flame was analyzed in terms of shape, length and lift-off height, using the OH* chemiluminescence signal detected by an ICCD-camera. Emission measurements for NOx, CO and UHC emissions were carried out. The pilot stage was examined at a local air number between 0.14 and 1.43, which corresponds to a global air number of 2.0 to 20.7. For lowest air numbers, the combustor works with the RQL principle with lowest emissions in pilot stage only operation. This is because the remaining fuel fed to the pilot stage mixes rapidly with the air from the main stage and reacts under lean conditions. The optimum operating range of the main stage is at global air numbers between 3 and 3.2 with a blow-off limit beyond λg = 4.0. At a global air number of λg = 2, a fuel split variation was carried out from 0 (only pilot stage) to 1 (only main stage). In combined operation and at higher fuel splits, the NOx emissions are reduced compared to the main stage only operation, while the opposing effect on NOx emissions was observed for lower fuel splits. CFD simulations of the combustor test rig showed higher residence times in the pilot stage compared to the main stage which facilitates higher NOx formation rates in the pilot stage. This could be improved by a geometry optimization. The operation of the pilot stage was beneficial at fuel splits above 90 %, especially concerning an extended operating range to higher global air numbers. In addition, the capability of the combustor to operate at higher thermal power inputs was investigated. Originally designed for the Turbec T100 micro gas turbine, the combustor was operated at 160% of the original design point. At a constant air number, this led to a decrease in NOx and to an increase in CO emissions, caused by shorter residence times in the combustion chamber at higher power input. An operation strategy of constant pilot air number increases the envelope of a stable operation regime further.
Flow Dynamics in Single and Multi-Nozzle Swirl Flames
Authors: Smith Travis , Emerson Benjamin , Chterev Ianko , Noble David R. , Lieuwen Tim
Published: September 2016

This paper describes an analysis of the unsteady flow structures in a single nozzle and triple nozzle swirl combustor (with nozzle spacing of s/D=2.9). It was motivated by a prior study by our group which compared the time averaged and unsteady features for a different swirling nozzle, and found that the single and triple nozzle flow dynamics were quite similar upstream of the jet merging region. This work is motivated by the fact that realistic hardware, whether in can or annular combustion systems, almost always contains several nozzles. However, it is common to use test facilities with a single nozzle to study flame dynamics, a key component of the combustion instability problem [1]. Simultaneous OH Planar Laser Induced Fluorescence (OH PLIF) and Stereoscopic Particle Image Velocimetry (sPIV) techniques were performed at 5 kHz on a swirl methane-air flame. Two transverse forcing configurations were applied, so that the flame nominally lies in a pressure node/transverse velocity antinode, and vice versa. The time averaged flow fields of the single and triple nozzle configurations are compared, and several key differences are identified. Most prominently, there are non-negligible differences in the recirculation zone reverse flow velocity and flame spreading angle. However, the spatial variation of the disturbance magnitudes along the shear layers exhibit quite similar growth and decay trends, and the convection speeds along the shear layer were nearly identical. These results corroborate the findings of Aguilar et al. [2] and Szedlmayer et al. [3], which show that despite differences in time average quantities, comparable flow dynamics occur in single and multi-nozzle flames. These results imply that useful insights into the dynamics of multi-nozzle systems can be gleaned from appropriately designed single-nozzle hardware, with appropriate accounting for the differences in time averaged flow/flame characteristics.
Detailed Experimental Characterization of Heat Transfer Coefficient Over the Internal Cooling Passages of an Additive Manufactured Turbine Airfoil
Authors: Siw Sin Chien , Chyu Minking K. , Um Jae Y. , Lee Ching-Pang
Published: September 2016

This report describes the detailed experimental study to characterize the local heat transfer coefficient distribution over the internal cooling passages of a simplified generic airfoil. The airfoil is manufactured through additive manufacturing based on actual geometry and dimensions (1X scale model) of row one airfoil, applicable in large gas turbine system. At the mainbody section, the serpentine channel consists of three passages without any surface features or vortex generators. Both the leading edge and trailing edge sections are subjected to direct impingement. The trailing edge section is divided into three chambers, separated by two rows of blockages. This study employs the well-documented transient liquid crystal technique, where the local heat transfer coefficient on both pressure and suction sides is deduced. The experiments were performed at varying Reynolds number, ranging from approximately 31,000–63,000. The heat transfer distribution on the pressure side and suction side is largely comparable in the first and third pass, except for the second pass. Highest heat transfer occurs at the trailing edge region, which is ultimately dominated by impingement due to the presence of three rows of blockages. A cursory numerical calculation is performed using commercially available software, ANSYS CFX to obtain detailed flow field distribution within the airfoil, which explains the heat transfer behavior at each passage. The flow parameter results revealed that the pressure ratio is strongly proportional with increasing Reynolds number.
Acoustic Damper Placement and Tuning for Annular Combustors: An Adjoint-Based Optimization Study
Authors: Mensah Georg A. , Moeck Jonas P.
Published: September 2016

Thermoacoustic instabilities pose a major threat to modern gas turbines. The use of acoustic dampers, like Helmholtz resonators, has proven useful for the mitigation of such instabilities. However, assessing the effect of acoustic dampers on thermoacoustic modes in annular combustion chambers remains an intricate task. This results from the implicit nature of the thermoacoustic Helmholtz equation associated with the high number of possible parameter values for the positioning of the dampers and their impedance design. In the present work, the principal challenges of the effective placement and the design of the impedance of acoustic dampers in annular chambers are discussed. This includes the choice of an appropriate objective function for the optimization, the combinatorial challenges when dealing with different possible damper arrangements, and the numerical complexities when using the thermoacoustic Helmholtz equation to approach this issue. As a key aspect, the paper proposes a new adjoint-based approach to tackle these problems. The new algorithm establishes algebraic models that predict the effect of acoustic dampers on the growth rates of the thermoacoustic modes. The theory is exemplified on the basis of a generic annular combustor model with 12 burners.
Manufacturing Considerations in the Initial Design Process
Authors: Anderson Mark R. , Klein Peter L.
Published: September 2016

Many factors come into play in a successful turbomachinery design, but peak aerodynamic performance, maximum operating range, stress levels, and manufacturability are generally the dominant concerns. Depending on the application, other considerations such as acoustics, size, weight, and environmental issues can also come into the picture. The most effective designers take a holistic approach, which tries to incorporate all of these concerns at the beginning of the design process. One of the challenges facing aerodynamicists is a basic understanding of how these downstream issues can be accommodated in the initial design process. Typically, an aerodynamic engineer has minimal knowledge of manufacturing methods such as 5-axis machining. This paper will provide a basic understanding of the subsequent manufacturing impact of various decisions typically made during the preliminary and detailed aerodynamic design process. The paper will focus on radial turbomachinery, but much of the information provided is common for axial turbomachinery produced by 5-axis Computer Numerical Controlled (CNC) machining.

In the initial phase of the design, basic layout decisions generally dictate the overarching manufacturing process. For components produced with 5-axis machining, these would typically be: flank milled or point milled, open or covered impeller, and for covered impellers, integral or welded shroud. The relative costs of these processes are considered in this work along with first-order estimates of their typical impact on aerodynamic performance and stress levels. Once a general layout (and accompanying manufacturing process) is determined, other geometric parameters of the components drive secondary costs in the manufacturing process. The secondary parameters include main blade count, the presence of splitter blades, the blade length, thickness, curvatures, leading and trailing edge shapes, and fillet radii. The use of Computational Fluid Dynamics (CFD) and numerical optimization has driven even more choices into blade shapes, such as splitter blades with different shapes and offsets from the main blade, bowed blades, irregular blade patterns, and non-axisymmetric hub shapes. In many cases, the aerodynamicists and CFD analysts have pushed geometry further than manufacturing capabilities are ready to accept. These secondary costs, and the aerodynamic compromises needed to reduce them, are also considered here. This paper attempts to lay out the basic principles of cost-effective manufacturing, and how these can be considered as early in the design process as possible. Specific examples are considered, and quantitative information is provided which can help guide the designer from the beginning and avoid expensive reworks resulting from downstream revisions. This paper will provide a framework for collaboration between aerodynamic engineers and the manufacturing teams assigned to produce the parts they have designed.
Computational Simulation of Deposition in a Cooled High-Pressure Turbine Stage With Hot Streaks
Authors: Prenter Robin , Ameri Ali , Bons Jeffrey P.
Published: September 2016

Ash particle deposition in a high-pressure turbine stage was numerically investigated using steady (RANS) and unsteady (URANS) methods. An inlet temperature profile consisting of Gaussian non-uniformities (hot streaks) was imposed on the vanes, with vane cooling simulated using a constant vane wall temperature. The steady case utilized a mixing plane at the vane-rotor interface, while a sliding mesh was used for the unsteady case. Corrected speed and mass flow were matched to an experiment involving the same geometry, so that the flow solution could be validated against measurements. Particles ranging from 1 to 65 μm were introduced into the vane domain, and tracked using an Eulerian-Lagrangian tracking model. A novel particle rebound and deposition model was employed to determine particles’ stick/bounce behavior upon impact with a surface. Predicted impact and capture distributions for different diameters were compared between the steady and unsteady methods, highlighting effects from the circumferential averaging of the mixing plane. The mixing plane simulation was found to over predict impact and capture efficiencies compared with the unsteady calculation, as well as over predict particle temperature upon impact with the blade surface. Blade impact efficiencies increased with higher Stokes numbers in both simulations, with multiple rebounds occurring on the pressure surface in the mixing plane case, and on the suction surface in the unsteady case.