bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessment of the Dynamical Behaviour of Hydrogen Combustors With Recurrence Quantification Analysis (RQA)
Authors: Alshahrani Ali , Talibi Midhat , Ezenwajiaku Chinonso , Beita Jadeed , Picciani Mark , Balachandran Ramanarayanan , Ducci Andrea
Published: September 2023

Hydrogen has immense potential as a future energy vector for power and propulsion applications. However there are several challenges, including flame stability, flashback and emissions, that impede its widespread use in modern energy systems. Hydrogen systems are required to be operated at fuel-lean conditions to avoid high NOx emissions and flashback issues, however lean operation makes systems more susceptible to thermoacoustic instabilities and lean blow-off (LBO). Understanding these dynamical behaviours is key to develop robust operational strategies.

Recurrence Quantification Analysis (RQA) is an effective tool to study nonlinear dynamics, and has been applied in this study to characterise hydrogen flame behaviour in industry-relevant laboratory-scale combustion systems. RQA was conducted on two different configurations — hydrogen lean direct injection (LDI) combustor and a swirl-stabilised combustor operated on methane-hydrogen blends. RQA was carried out on time-resolved data of (i) pressure measurements in the combustor and (ii) integrated OH* chemiluminescence signals. These commonly used measurements in combustion diagnostics were selected to develop a tool which can be applied easily to other industrial systems.

The effect of H2 blend, bulk velocity, and global equivalence ratio (ϕg) were investigated in this study, and four quantifiable parameters such as Shannon entropy, recurrence rate, determinism, and laminarity were extracted from the RQA, to determine different dynamical behaviours — stable, intermittent, and limit-cycle. High values of Shannon entropy and recurrence rate indicated stable conditions for the swirl-stabilised combustor, while a combination of Shannon entropy and determinism were found to best capture the hydrogen flame dynamics in the LDI combustor. The outcome of this work will allow development of efficient predictive tools for nonlinear data analysis.
Stabilization and Dynamics of Pure Hydrogen Swirling Flames Using Cross-Flow Injection
Authors: Vaysse Nicolas , Durox Daniel , Vicquelin Ronan , Candel Sébastien , Renaud Antoine
Published: September 2023

Hydrogen combustion is currently being considered as a possible means of decarbonization of power generation and air transportation. However, hydrogen raises difficult issues including that of NOx emissions linked to the high flame temperature and that of flashback caused by its high burning velocity. Innovative injection schemes are required allowing operation under lean conditions by enhancing mixing to avoid hotspots. In addition, the flame needs to be stabilized at a distance from solid boundaries to reduce thermal loads and should offer a low sensitivity to disturbances and instability. These issues are investigated with systematic experiments carried out in a single injector laboratory-scale combustor operating at atmospheric pressure, a configuration that was already used to examine the dynamics of spray flames. The atomizer is replaced by a special unit (CFI-X) that allows gaseous hydrogen injection in crossflow and mixing with the air stream set in rotation by a radial swirler. The flame behavior formed by this device is examined as a function of thermal power and global equivalence ratio. The injection head recess is also varied to change the level of mixing in the system and see how it influences thermo-acoustic instabilities. Flame structures visualized through OH* light emission images are used to classify the various flame patterns and distinguish domains where the flame is detached from the injector outlet. Variations of the injection head recess with respect to the outlet, swirl number and chamber length provide a comprehensive view of the operating regimes. It is found that the flames remain attached to the injector at low power and that detached flames are obtained in the high-power range. Dynamical phenomena leading to oscillations coupled by longitudinal chamber modes are observed in an intermediate thermal power range. The corresponding data are interpreted with a stability analysis that defines instability bands for the product of a time delay by the frequency of oscillation. It is found that unstable flames belong to the first of these bands. It is also observed that detached flames that are the most interesting in terms of thermal loads and mixing may also belong to the band of instability but do not give rise to oscillations and are therefore less sensitive to instability.
Is Blue Hydrogen a Better Alternative Than Post-Combustion Carbon Capture for Combined Cycle Gas Turbines? A Thermodynamic Point of View
Authors: Mendoza Morales Maria José , Verhaeghe Antoine , Bricteux Laurent , Blondeau Julien , De Paepe Ward
Published: September 2023

Today, the impact of global warming is more tangible than ever and every sector of society needs to engage in a low-carbon plan of action. For the last few years, increasing the share of variable renewable energy (VRE) has been the strategy of the energy sector to cut CO2 emissions. It requires the energy system to enhance its flexibility, voltage and frequency control, and firm capacity. To maintain the desired stability, combined cycle gas turbine (CCGT) plants come as the ideal candidates. However, if natural gas (NG) remains the primary fuel, the gas turbines (GT) will not comply with carbon emission limits in the future. Decarbonization must, therefore, be implemented in CCGTs. Original equipment manufacturers (OEM) are turning toward hydrogen-fueled GTs. Nevertheless, the production of H2 remains a challenge. In the long run, with a surplus of renewables and the development of more powerful electrolyzers, green or zero-carbon hydrogen could be achievable on a large scale. For the moment, the most mature low-carbon method is steam methane reforming (SMR) combined with carbon capture (CC), producing the so-called blue-H2. Alternatively, post-combustion CC presents another low-carbon solution for CCGTs. However, there is quite some debate is in the literature on the most effective pathway toward CCGT decarbonization. Using multiple Aspen Plus models of an HA-class GT combined with a three-level pressure heat recovery steam generator (HRSG), a CC unit, and an SMR process, the study presented in this paper seeks to assess the performance of both solutions. Simulation results show that for the blue-H2 scenario, the production of blue-H2 has an energy penalty of 24% on the CCGT. On the other hand, post-combustion CC implies an energy penalty of 10%, making the CC-only case the most efficient pathway under the studied conditions.
Flow Control of Turbine Vane Aerodynamics for Gas Turbine Operability
Authors: Dhyani Priyank A. , Jacobs Gustaaf B. , Burnes Dan , Ferguson Tyson
Published: September 2023

An active control of the flow in a turbine is investigated with the objective of an optimal throughflow mass reduction. An array of jets placed at the suction side of a nozzle vane is intended to enhance the effective camber of the vane, thereby reducing the throat area and corresponding throughflow mass. The effect of the jet array’s location is studied parametrically. The jet array’s configuration is also investigated by spatially perturbing the array with harmonic modes. Steady-state nozzle flow simulations at engine conditions are conducted with ANSYS CFX. The injector jet flows are modeled through source points. The jet flow normal to the vane surface leads to a recirculation region that displaces the inviscid flow outside of the viscous, near-wall region. For a jet location upstream of 37% of the chord length, the recirculation region reattaches downstream on the nozzle vane. For a more downstream array location, the flow separates globally. The throat area and throughflow mass reduction is larger for locations near the throat, but relative to the losses measured through the nozzle loss coefficient, the upstream jet array location is more effective and preferred. A jet array placed at the pressure side has an effect similar to a Gurney flap and modifies the Kutta condition. A jet array that is spatially perturbed with a single mode such that the array’s midspan location is more upstream as compared to the location near the end-walls, homogenizes the flow effect of the jet array, i.e., the recirculation region is more uniform.
Role of Turbulence in Modifying the Coherent Heat Release Response of Acoustically Excited Rod-Stabilized Flames
Authors: Karmarkar Ashwini , O’Connor Jacqueline
Published: September 2023

Modern power generation gas turbine engines operate in primarily fuel-lean conditions in order to mitigate harmful emissions. Lean combustion systems, however, are susceptible to combustion instabilities, which arise from a resonant coupling between coherent oscillations in the unsteady heat release rate and the acoustic modes of the combustor. The occurrence of combustion instabilities can severely limit engine performance and operability. To design effective instability control mechanisms, it is critical to understand the response of flames to coherent fluctuations in the velocity and pressure fields of the flow. In addition to coherent oscillations, combustor flow fields also experience high levels of perturbations arising from turbulence, which can significantly alter the flow and flame dynamics. In this work, we study the coherent heat release response of a rod-stabilized flame subjected to longitudinal acoustic forcing at two levels of in-flow turbulence. We systematically vary the amplitude of acoustic excitation at two frequencies — the natural frequency of vortex shedding and its first harmonic — at both turbulence levels. Our results show that the amplitude of the coherent heat release response is strongly dependent on the turbulence intensity. Additionally, the impact of turbulence on the coherent heat release response is a function of the acoustic forcing frequency. These results provide insight into the interaction between coherent and turbulent motions in the flow and their impact on unsteady heat release oscillations in gas turbine combustors.
A Piston Theory-Based Aeroelastic Stability Prediction Toolbox for Radial Turbomachinery
Authors: Iskandar Vincent , Fellows David W. , Bodony Daniel J. , Kang Sang-Guk , Pope Aaron J. , Kweon Chol-Bum
Published: September 2023

Aircraft intermittent combustion engines often incorporate turbochargers adapted from ground-based applications to improve their efficiency and performance. These turbochargers operate at off-design conditions and experience blade failures brought on by aerodynamically-induced blade vibrations. A previously-developed reduced-order model (ROM) leveraging piston theory to compute the stability of general fluid-structural configurations is first presented and summarized. The ROM has been applied to the high-pressure turbine of a dual-stage turbocharger and the results are reviewed as a baseline for new predictions considered in this work. For each operating condition that is investigated, a computational fluid dynamic (CFD) simulation must be performed to inform the fluid loading predicted by piston theory. Interpolation-based approaches are considered to minimize the numerical expense associated with this requirement. The Gaussian-based Kriging interpolation method is presented and explored. The method provides more accurate estimates for the non-linear behavior of the quantities of interest. Kriging also estimates uncertainty and provides confidence levels as part of the interpolation process. A graphical user interface (GUI) that automates the ROM prediction is presented. The GUI presents a rapid means to alter the turbomachine of interest, predict the aeroelastic response associated with a user-specified flight condition and quantify the uncertainty associated with the prediction.
Accounting for Firing Temperature in Combustion Tuning Activities
Authors: Angello Leonard , Perullo Christopher , Nicholson Craig , Harper Jim , Noble David
Published: September 2023

As gas turbines are operated and maintained, it is key to understand engine firing temperatures at cyclical and variable load operations, and more specifically, the tradeoff between engine performance, repair costs and reliability. It is crucial to ensure performance is maintained without sacrificing the life of critical parts.

OEMs traditionally have not provided much information about how tuning activities can impact performance. The current practice of tuning to meet emissions and combustion dynamics, or pulsations, can directly impact output, efficiency, and hot gas path (HGP) component life. A good understanding how and when this can happen can empower users to ensure they get the best performance and understand tradeoffs between performance, operability, and emissions.

To better explain the concept of ‘tuning’, this paper discusses and provides basic tuning guidance common for all gas turbines regarding how tuning steps can impact gas turbine firing temperature, performance, and emissions tradeoffs. A three-step process is described for; 1) assessing if there is evidence of temperature control curve changes; 2) performing a cursory analysis using existing gas turbine instrumentation and operating data to see if there is potential impact on firing temperature / turbine inlet temperature; and 3) examining for evidence of temperature change, whereby a means to analyze time series data using a physics-based model (EPRI gas turbine digital twin) to estimate delta changes in firing temperature pre and post outage is discussed. This process uses software tools available to the authors but is applicable to any trusted analysis toolset.

The paper aims to enhance industry understanding of what is completed during combustion tuning; it provides information about tuning activities impact on gas turbine performance to connect the dots between tuning and performance; and it describes how to conduct a simplified digital twin heat balance analysis to better quantify these impacts.
Turbine Operation and Measurement at DLRs Next Generation Turbine Test Facility
Authors: Pahs Andreas , Söhngen Anna-Samira , König Franz-Xaver , Wolf Torsten
Published: September 2023

Test facilities are important to validate new developments before bringing them into aero engines. The challenge is to have a deep understanding of the individual facility to be able to state measurement uncertainties, boundary conditions and stability to ensure high quality data which allow for comparison with predictions mostly done by simulations. Especially in today’s environment with the big challenge to bring disruptive technologies into service to meet the climate goals.

One large test facility is the Next Generation Turbine test facility (NG-Turb) situated at the German Aerospace Center in Göttingen. Together with Rolls-Royce Deutschland a turbine test rig with the option to use a combustor simulator was operated.

The turbine rig was designed as TRL 6 technology demonstrator with representative cooling scheme, realistic Mach and Reynolds number to determine the turbine characteristics like capacity and efficiency but also to get insight into local flow phenomena. A necessity is the determination of all boundary conditions with mass flow rate, total temperature and total pressure. These must be deduced from different measurements but they are discrete in place and time. A major tool to determine the flow conditions are pneumatic probes which need to be traversed to cover a minimum number of positions for averaging. During that time the operation point of the turbine must be kept as constant as possible. This paper presents the operation of the test rig within the facility including reached stability of operation conditions and thermal behaviour of facility, test rig and flow characteristics.

It is shown that very good stability of boundary conditions is reached but that there are also challenges mainly belonging to thermal gradients which are always present. By improving the instrumentation thermal gradients are resolved and can be taken into account when evaluating the gained turbine characteristics.
Damping Prediction Method for Asynchronous Vibration Modes and Verification by Electromagnetic Excitation for Full Scale Turbine Blade
Authors: Goto Toshiro , Watanabe Toshio , Baba Takeo , Momma Kazuhiro , Miyajima Keiichiro
Published: September 2023

The blades at the end stage of low-pressure turbine (LP-End) become thinner and longer to increase the efficiency and power of steam turbines even under severe conditions. Integral shrouded blades are often used in the low-pressure turbine to increase blade stiffness and get frictional damping to reduce the blade vibration. It is important to predict the frictional damping characteristics of such shrouded blades analytically during the design phase, and many analytical methods have been vigorously proposed.

However, the behavior of the friction damping is complex due to nonlinearities such as contact and sliding, it is difficult to make a high-fidelity model to express the friction. For example, it is important to consider micro-slip phenomena, to initiate sliding due to small excitation force in localized areas, and formulating the contact stiffness. In this paper, with the aim of incorporating damping prediction into the design process, reasonable micro-slip model was developed by predicting the contact conditions during operation in advance, and calculating the damping considering the contact areas of shroud and stub, as well as the distribution of contact forces.

And then, full-scale testing of turbine blades with the electromagnetic excitation system was conducted to validate the analytical method, and vibratory stress and damping of the blades were measured by strain gauges in the test facility. As the result, it was confirmed that the analytical predictions matched well with the measured damping of the high nodal diameter (ND) of the 1st mode by asynchronous excitation.
Short Ducted Fan Diffusers With Integral Splitter Blades
Authors: Hine Rory D. , Farman Judith R. , Grimshaw Sam D. , Taylor James V.
Published: September 2023

To maximise the range of vertical take-off and landing (VTOL) aircraft, the hover efficiency of ducted fan propulsors should be optimised. Diffusers at the outlet of ducted fans increase thrust, for a given power input, by converting otherwise wasted kinetic energy into a static pressure rise. The weight, and therefore length, of diffusers must be minimised to maximise the net thrust added. However, reducing length increases the adverse, axial pressure gradient which increases flow separation and stagnation pressure losses. In this paper, circumferential splitter blades are used to control loading distributions and reduce the size of separations by modifying pressure gradients. The aim is to increase the pressure recovered by short diffusers.

Using 3D computational fluid dynamics (CFD) simulations and pattern search optimisation, the performance and geometric trends of optimised diffusers are mapped as non-dimensional length (L/h) and splitter count (Ns) vary. Two different pressure maximising strategies are identified for diffusers with two splitters (Ns = 2). The first minimises stagnation pressure loss by positioning a splitter near to the hub to inhibit endwall separation. The second maximises the pressure recovered through diffusion by modifying the loading distribution between splitter divided sub-passages. These strategies guide the development of short diffusers for different operational requirements.

To validate simulations, an experimental rig is used to study the inlet and outlet flow fields of short, optimised, Ns = 0 and Ns = 2 diffusers. The experimentally measured stagnation pressure loss suffered by the optimised Ns = 2 diffuser is half that of the Ns = 0 design. This is primarily due to the alleviation of casing endwall separations. Flow mechanisms are identified using CFD simulated limiting surface streamlines and experimental flow visualisation.
Development and Evaluation of Generic Test Pieces for Creep Property Assessment of Laser Powder Bed Fusion Components
Authors: Megahed Sandra , Krämer Karl Michael , Heinze Christoph , Konterman Christian , Udoh Annett , Weihe Stefan , Oechsner Matthias
Published: September 2023

Metal Laser Powder Bed Fusion (PBF-LB/M) allows for high degrees of design freedom and the manufacture of high temperature Ni-based materials, such as IN738LC. The PBF-LB/M microstructure is dependent on several factors, including process parameters, component geometry, build orientation and post-processing steps (e.g. heat treatment). The correlation between the resulting microstructure and these parameters is material specific and not yet fully understood.

In this study, the development of a Specimen Extraction Cube (SEC), based on a generic component with design aspects related to turbomachinery applications, is presented. The SEC allows for the extraction of three samples, one for each of the build orientations: 0° (perpendicular to build direction), 45° (diagonal) and 90° (parallel to build direction).

Specimens extracted from the SEC are mechanically tested and compared to witness samples manufactured in 0°, 45° and 90° build orientation. Particular focus is placed on correlating measured properties and their variations with heat treated microstructures. Creep testing was performed using 240 MPa and a temperature of 850 °C. Microstructural differences and hence differences in mechanical properties are found in extracted and witness samples.
A Computationally Efficient Method for the Thermo-Structural Analysis of a Turbocharger Turbine Wheel Under Engine Transient Operation
Authors: Catalani Iacopo , Balduzzi Francesco , Bianchini Alessandro , Ferrara Giovanni , Pini Nicola , Mosciaro Luigi , Mattiello Daniele , Rabbolini Silvio , Hoffer Pierre-Alain
Published: September 2023

The accurate evaluation of thermally-induced stresses is of particular importance for life-cycle analysis of modern turbochargers. Conjugated heat-transfer (CHT), steady-state calculations represent the state-of-the-art, whose temperature distribution is usually imported into a structural solver as a thermal load for carrying out thermo-structural analyses. In the case of transient analyses, however, issues arise. Indeed, due to the different timescales of convective and conductive heat-transfer mechanisms, unsteady CHT simulations are burdened by high computational costs. Moreover, in the specific case of turbocharged engines, the timescales of engine load transients and turbocharger response may differ by a factor of up to 105.

The present study proposes an approach for carrying out transient thermo-structural analyses of the solid domain only, characterized by a physical time in the order of 101 s, without the need for unsteady fluid flow calculations, which would imply the use of a solver timestep in the order of 10−5 s. In the proposed methodology, the effects of the fluid flow in the heat-transfer process are modelled by imposing adequate boundary conditions to wet surfaces of the solid domain with minimum computational effort. Indeed, by performing two steady-state CHT calculations and one steady-state computational fluid dynamics (CFD) calculation, time-dependent boundary conditions can be derived in terms of convective heat transfer coefficient and near-wall fluid temperature. The case study used for model development is a high-performance turbocharger turbine for automotive applications, responding to engine load transients of tenths of seconds. Initially, steady-state CFD and CHT models of the turbine stage are validated against experimental data. Then, transient analyses on the turbine wheel are carried out and their results are imported as thermal loads into an unsteady structural solver, allowing for accurate transient thermo-structural analyses at sustainable computational cost.
Numerical Study on the Influence of Geometrical Parameters on the Performance of a Turbine Exhaust Hood
Authors: Fomina Anastasia , Fuhrer Christopher , Vogt Damian M. , Willeke Tobias
Published: September 2023

Complying with the current environmental requirements, modern turbine designs must be robust, highly efficient and able to operate at flexible operating conditions simultaneously. An improvement of the pressure recovery in the turbine exhaust hood and especially in its diffuser can increase the overall turbine performance at multiple operating conditions.

In a current study, an axial-radial diffuser is investigated, which represents a typical compact design for industrial turbine exhaust hoods. Since this diffuser-exhaust hood system turns the flow direction from axial to radial direction over a short distance, a characteristic complex three-dimensional asymmetric and unsteady flow field results. Due to this flow topology, local flow separations occur in regions with stronger adverse pressure gradients limiting the maximum overall pressure recovery of the exhaust hood. This paper presents the influence of geometrical diffuser parameters on the diffuser-exhaust hood system performance in terms of pressure recovery, pressure loss, and flow-field asymmetry based on numerical flow simulations. For this purpose, a Design of Experiment (DoE) analysis is performed on a parametrized model of a diffuser-exhaust hood system for a newly developed diffuser test rig, whereby the diffuser geometry is modified asymmetrically. A sensitivity study of the geometrical parameters is performed for two operating conditions.

The simulation results indicate that the overall pressure recovery as well as the pressure loss of the diffuser-exhaust hood system can be influenced through asymmetric diffuser-shape modifications. Besides, it is demonstrated, that the aerodynamic origins of these sensitivities emerge from the local diffuser-loading and flow distribution.
Flashback of Hydrogen-Methane Mixtures in a Fixed-Vane, Low-Swirl Burner at Atmospheric Pressure
Authors: Searle Matthew , Strakey Peter
Published: September 2023

Equivalence ratio at flashback, ϕFB, of low swirl injectors for 60%–95% (H2, by mole) hydrogen-methane mixtures was determined experimentally in a < 100 kW atmospheric pressure burner. Pre-mixer velocities equaled 5, 7.5, and 10 m/s, with the reactants entering the burner at 294 K. The swirler design was selected following a literature review, and the test articles were additively manufactured. The test articles had two turning angles, 26° and 33°, with center-body hole diameters equaling 1.08 and 1.16 mm. A test article was built with each parameter combination, which yielded four test articles with measured swirl numbers varying from 0.43 to 0.49. The results showed agreement with prior work. ϕFB increased with increasing pre-mixer velocity and decreasing percent hydrogen. ϕFB was also observed to increase with swirl angle and decrease with increasing center-body hole diameter. The data was correlated using a semi-empirical quenching approach, with Cquench* = 0.01, where the chemical time scale was calculated using a perfectly stirred reactor (PSR) model. Two different chemical kinetics mechanisms independently yielded the same results. The correlation makes the present small-scale laboratory results relevant to large, high-pressure gas turbines. The flashback data set is available for computational fluid dynamics (CFD) validation.
A Toolbox for the Preliminary Aerodynamic Design of Highly Loaded Annular S-Shaped Transition Ducts
Authors: Veyrat A. , Carrotte J. F. , Walker A. D. , Simpson H. G.
Published: September 2023

The drive towards net-zero carbon emissions is favoring lighter, more compact compressors. For the connecting S-shaped ducts, a larger radial offset must be accommodated within the shortest length ensuring no boundary layer separation and minimum losses. The duct aerodynamic design is however significantly affected by highly complex flow field interactions requiring relatively advanced and time-consuming Computational Fluid Dynamics (CFD) simulations. To limit their use at a preliminary design stage, the Agile TooL for Aerodynamic ductS (ATLAS) is currently being developed for the industrial partner. After shaping a baseline annulus, affine transformations can be carried out to change its bulk dimensions and, for example, enforce a desirable dimensionless curvature distribution. The design space can thus be mapped with ducts sharing similar geometric properties. Candidate designs can next be identified by fast exploration via Ordinary Kriging interpolations of minimum axial length and stagnation pressure loss in a CFD database. An even sample mapping by Delaunay triangulations is used to promote a high-quality semivariance model. In the vast area ratio vs hub radius ratio space investigated here, cross-validation showed that the minimum axial length is predicted with an averaged error of 2.2% at separation and 2.1% at the recommended margin from detachment while stagnation pressure loss is approximated with a mean discrepancy of 4.8% and 2.6% respectively. These capabilities are sufficiently accurate to inform a compressor toolbox about the duct performance and therefore enable a holistic preliminary design.
Analytical Modelling of Flame-Dynamics Within Auto-Ignition Hydrogen and Methane Flames in Mach Stabilized Combustion
Authors: Biagioli Fernando , Freitag Ewald , Syed Khawar J.
Published: September 2023

The sequential combustion concept is characterized by two combustor stages. In the first stage a fraction of the fuel is premixed into the air flow before burning in a standard lean premixed flame. In the second stage, which is the focus of the present contribution, the remaining part of the fuel is injected into the hot products arriving from the first stage and combustion takes place in an autoignition-dominated flame. We analyse here a situation where auto-ignition in the second stage takes place in a flow subject to a positive static temperature gradient. This concept allows very short combustor residence times ans is well suited for incorporation into an integrated combustor-nozzle guide vane. In order to set up the static temperature gradient, the mixture of reactants flows through a converging-diverging channel, first accelerated to a relatively high Mach number, ca. M = 0.6, and then decelerated in the diffusing section where the flame is located. A fully analytical model describing the thermo-acoustics behaviour of the converging-diverging combustor is developed. The critical part of this exercise is given by modelling the frequency response of the heat release rate concentrated at the position of the auto-igniting flame. It is shown here how this is closely related to the response of the displacement of the flame for which a novel analytical model, free from empirical assumptions, is provided. The model is finally applied to compare hydrogen and methane flames in terms of their thermo-acoustic behaviour.
Analysis of Flutter Mechanisms and Unsteady Aerodynamics of a Transonic Fan Blade Near Stall
Authors: Zoepke-Sonntag Roger , Hill George , Stapelfeldt Sina
Published: September 2023

This paper investigates the onset of aeroelastic instability of a modern low-pressure ratio transonic fan using numerical simulations, validated against experimental measurements. Different domain configurations are compared in terms of fan characteristic and radial profiles. It is shown that flutter can be predicted over the investigated speed range using a reduced computational domain. The benefit in computational speed justifies simplifications made and allows a more detailed unsteady analysis. During the unsteady analysis, it is shown that the transition from stable to aeroelastic unstable along a constant speed characteristic results from increased negative aerodynamic damping on the pressure side of the blade, accompanied by a drop of damping on the suction side near the tip. The shock and the radial migration have a stabilizing effect on the blade, while the region downstream of the shock tends to destabilize the blade. The drop in stability occurs in multiple low nodal diameters, independent of acoustic propagation conditions. While previous work tried to identify unique mechanisms that drive flutter, this research concludes that the change from stable to unstable is subtle and that it is a combination of many drivers. In addition, the tip region, especially the region downstream of the shock, tends to have a more significant impact than assumed, while the shock shows only small variations near the tip.
The Impact of Thermal Load and Turbulent Flow on Minimum Film Thickness of a Large Tilting-Pad Journal Bearing
Authors: Vetter Daniel , Hagemann Thomas , Schwarze Hubert
Published: September 2023

Thermal loads induced by internal fluid friction significantly influence the operating behavior of high-speed journal bearings. These loads increase on average if critical Reynolds numbers are exceeded. This paper presents a detailed thermo-elasto-hydrodynamic (TEHD) analysis of a large five-pad tilting-pad journal bearing operating up to surface speeds of 94 m/s. The structure mechanical analysis is conducted by a FE-Code that is implemented as a sub-module of the bearing code to improve computational efficiency. The non-linear structure model includes the pads, their aligning ring and the housing while the journal’s thermal expansion is approximated analytically. The contact of pads and ring is modeled with a contact algorithm. Validation with test data from literature indicates the relevance of structure model’s complexity. Besides the pad deflection, in particular, the consideration of the ring and housing thermal expansion is decisive to predict the effective clearance properly. Furthermore, comparisons between measurement and simulation show that the experimentally identified non-linear speed dependent characteristic of minimum film thickness that features an increase due to turbulent flow is well predicted by the theoretical model. Generally, results provide an insight on the impact of the different effects dominating the behavior of the bearing in the respective speed ranges, and therefore, improve the understanding of complex bearing systems. Finally, the quality of results of approximation formulas that are easier to implement and more time efficient than the complex FE-code is investigated.
A Model of Mass and Heat Transfer for Disc Temperature Prediction in Open Compressor Cavities
Authors: Nicholas Thomas E. W. , Scobie James A. , Lock Gary D. , Tang Hui
Published: September 2023

Accurate prediction of heat transfer in compressor cavities is crucial to the design of efficient and reliable aircraft engines. The heat transfer affects the thermal expansion of the compressor rotor and, in turn, the tip clearance of the compressor blades. This paper presents a novel, physically-based predictive theoretical model of heat transfer and flow structure in an open compressor cavity, which can be used to accurately calculate disc temperatures. The radially higher region of the cavity is dominated by buoyancy effects created by the temperature difference between the hot mainstream flow and the axial throughflow used to cool the turbine. Strong interaction between the air in the cavity and this throughflow creates a mixing region at low radius. For a given geometry, the heat transfer and flow physics are governed by four parameters: the rotational Reynolds number Reϕ, the buoyancy parameter βΔT, the compressibility parameter χ, and the Rossby number Ro. The model quantifies both the buoyancy- and throughflow-induced mass and heat transfer, producing a reliable prediction of the disc and air temperatures. The model takes into account a two-fold effect of the throughflow: being entrained into the cold radial plumes directly and creating a toroidal vortex in the radially lower region of the cavity. The exchange of mass between the cavity and throughflow is related to the mass flow rate in the radial plumes in the buoyancy-induced region, considering the effect of flow reversal at low Ro. The model is validated using data collected in the Bath Compressor Cavity Rig and can be incorporated in engine design codes to robustly compute the thermal stress and expansion of the compressor rotor, contributing to more efficient engine designs.
Towards Best Practice for Predicting a Lifted Hydrogen Diffusion Flame Using a Flamelet Generated Manifold Approach
Authors: van Bruygom Ashley , Garmory Andrew , Walker A. Duncan
Published: September 2023

This paper examines a lifted turbulent H2/N2 diffusion flame in a vitiated co-flow, using a Flamelet Generated Manifold (FGM) combustion model. FGM is commonly used in combusting simulations in the gas turbine industry as it is cheaper than solving detailed chemistry as part of the simulation. The aim of this paper is to examine the ability of the FGM model to reproduce experimental results, and its sensitivity to modelling choices, using an implementation within the commercial code, Simcenter STAR-CCM+. Hence, the current study systematically examines the impact of turbulence modelling, inlet conditions, the flamelet model used to generate the FGM table and the chemical mechanisms. Both Reynold Averaged Navier Stokes (RANS) and Large Eddy Simulations (LES) are presented with focus on the effect of inlet conditions, three different flamelet generation methods and two chemical mechanisms. The flame lift-off height is shown to be highly sensitive to chemical mechanism and flamelet table generation method, with the 0D flamelet giving a higher lift-off than the 1D flamelets. The inlet velocity profile was found to have less of an impact on LES results. This work shows that care should be taken when attempting to identify an accurate simulation methodology and that modelling choices are not considered in isolation as the apparent superiority of a modelling choice may be an artefact of other modelling choices.
A Two-Step Approach for Centrifugal Compressor Performance Mapping Based on a Reduced-Order Model and an Evolutionary Algorithm
Authors: Bicchi Marco , Biliotti Davide , Toni Lorenzo , Marconcini Michele , Grimaldi Angelo , Arnone Andrea
Published: September 2023

Due to their good efficiency, robustness, and wide operating range, centrifugal compressors are pivotal in several industrial and civil applications. In this context, predicting the performance of a centrifugal compressor stage is essential for its aerodynamic design and selection during customer orders. Therefore, the present research focuses on performance mapping once the aerodynamic design is complete. Indeed, once the final stage geometry is achieved, experiments and computational fluid dynamics (CFD) analyses are reliable methods for performance map predictions. However, these methods are expensive and time-consuming, in contrast to typical limitations of the industrial scenario. To limit their cost, a two-step approach based on a reduced-order model leveraging CFD analyses and an evolutionary algorithm is proposed. The main goal is to decrease performance mapping efforts while maintaining a high level of accuracy. Specifically, the theoretical contribution of this research is to investigate the joint use of CFDs with reduced-order models to generate accurate performance maps with low computational effort. While on a practical level, a business-friendly tool for performance mapping is provided. As an example case study, the approach is applied on centrifugal compressor stages for high-head low-Mach number applications. Results show good agreement with CFD and experimental data. The present research lays the foundations for future studies aiming to further reduce efforts in performance mapping and time-to-market of centrifugal compressor stages.
Effect of Incoming Wakes on the Passage Flow of an Axial Compressor Rotor
Authors: Koppe Björn , Lange Martin , Mailach Ronald
Published: September 2023

A multistage setup has become indispensable in current turbomachinery rendering the rotor-stator interaction (RSI) to one of the interested fields of modern research. Its inherent complexity originating in its highly unsteady, three-dimensional and turbulent characteristic aggravates the thorough understanding of the underlying phenomena. In an attempt to simplify this interaction an experimental study has been carried out on a single axial rotor of a low speed research compressor (LSRC) utilizing two different wake generators (WG) to simulate incoming disturbances in the flow. Here, cylindrical bars and radially extracted symmetrical profiles generated to some extent isolated wakes without the complexity of secondary flows typically found downstream of a stator row. The rotor was examined at two different tip clearances (tip gap height/channel height = 1.25 %|4.17 %) for its design operating point. Various measuring techniques were conducted to capture steady and unsteady flow features in planes up- and downstream of the rotor. The influence of the wakes on the losses produced by the rotor are discussed including the unsteady effects on the underlying secondary flow phenomena. Here, in particular the tip leakage vortex (TLV) and the suction side separation of the flow near the hub endwall are examined.
Experimental Characterization of Surge Cycles in a Centrifugal Compressor Under Wet Gas Operation
Authors: Serena Alberto , Hundseid Øyvind , Bakken Lars Eirik
Published: September 2023

This investigation focuses on instabilities in turbomachinery operation due to variations in fluid properties and composition, as with the injection of water droplets. The main objective of this study is to experimentally document compressor behaviour and characteristics under off-design condition in the unstable area between surge inception point to zero flow; flow visualization is used at this scope.

The test rig is a single stage compressor operating with air and water. The compressor is full-sized, and the design is based on a traditional dry gas design optimized for wet gas compression. It is equipped with visualization slots enabling detection of unsteady and reversible flow.

A suitable description of the positive slope area requires unsteady mass flow measurement: gas fractions accurate measuring and control still play a key role in a proper description of the flow phenomena involved, with reverse flow detection being the major challenge to be properly identified.

The results quantify the liquid impact on performance and instability of system characteristics and flow composition, and show the development of unsteady flow regimes. This proves valuable for future system modeling tuning and validation and further understanding of the phenomenon, the development of reliable surge onset prediction models and control strategies. In addition, useful guidelines focused on compressor system design and its behaviour, rather than machine design.
Measurement of Surface Pressure Field of Turbocharger Impeller Using Pressure and Temperature Sensitive Paints
Authors: Furuya Takenori , Nakai Takumi , Imai Masato , Kakimoto Tomihiro , Kameda Masaharu , Matsuoka Yuya , Machida Kazuya , Matsuda Masaaki , Taneda Yoshio
Published: September 2023

A pressure-sensitive paint (PSP) technique is developed for measuring the pressure profile of an automotive compressor impeller at various rotational speeds. Short-exposure images from a digital high-speed camera were used for pressure measurements to prevent image blurring due to high rotational speeds. The pressure was calculated by the two-gate lifetime method using the intensity ratio of two images during PSP dye excitation. Temperature correction of the PSP signal was performed using a temperature-sensitive paint (TSP). Polyurethane resin was used as a binder to improve the heat resistance of the PSP and TSP. Pressure and temperature distributions at the impeller surface were obtained at four speeds when the efficiency was maximum at each speed. Characterization of the paint confirms that this PSP technique provides sufficiently high sensitivity and large emission intensity in the turbocharger impeller test range. Temperatures measured by TSP agree quantitatively well with thermography. The impeller surface temperature increases with increasing RPM and exceeds 100 °C at 182,000 rpm. Temperature-corrected PSP signals provide reliable surface pressure distributions consistent with CFD analysis for the same geometry and operating conditions as in the experiment.
Investigation of Power Turbine Casing Thermal Environment Using One Dimensional Thermal-Fluid Network Solver
Authors: Uyav Omer , Bedir Hasan
Published: September 2023

The gas turbine engine operates in high-temperature, instantly changing power demand and various atmospheric conditions. Therefore, turbine inlet temperature and material selection are the most critical parameters to maintain its integrity and satisfy power output, and cooling the engine components is crucial regarding the durability and reliability of both rotating and structural parts. In addition, thermal and secondary air systems design collectively play a crucial role in the successful case cooling, as they determine the heat load and cooling flow distribution. This paper aims to present a 1D network modeling methodology for any turbine geometry at the preliminary design stage. In this sense, a test campaign is carried out to obtain the temperature distribution of a power turbine casing using thermocouples and pressure sensors. In addition, a 1D thermo-fluid steady-state model is established. The results of the model are validated against test data. Furthermore, a sensitivity analysis is performed, which helps to determine critical parameters for the base model and to determine how the model is affected. Additionally, another casing geometry with a different cooling system is designed to examine temperature distribution on the casing by changing the conduction path. Finally, comprehensive comparisons of all the results are evaluated.
Hydrogen Combustor Integration Study for a Medium Range Aircraft Engine Using the Dry-Low NOx “Micromix” Combustion Principle
Authors: Funke Harald H.-W. , Beckmann Nils , Stefan Lukas , Keinz Jan
Published: September 2023

The feasibility study presents results of a hydrogen combustor integration for a Medium-Range aircraft engine using the Dry-Low-NOx Micromix combustion principle. Based on a simplified Airbus A320-type flight mission, a thermodynamic performance model of a kerosene and a hydrogen-powered V2530-A5 engine is used to derive the thermodynamic combustor boundary conditions. A new combustor design using the Dry-Low NOx Micromix principle is investigated by slice model CFD simulations of a single Micromix injector for design and off-design operation of the engine. Combustion characteristics show typical Micromix flame shapes and good combustion efficiencies for all flight mission operating points. Nitric oxide emissions are significant below ICAO CAEP/8 limits. For comparison of the Emission Index (EI) for NOx emissions between kerosene and hydrogen operation, an energy (kerosene) equivalent Emission Index is used.

A full 15° sector model CFD simulation of the combustion chamber with multiple Micromix injectors including inflow homogenization and dilution and cooling air flows investigates the combustor integration effects, resulting NOx emission and radial temperature distributions at the combustor outlet. The results show that the integration of a Micromix hydrogen combustor in actual aircraft engines is feasible and offers, besides CO2 free combustion, a significant reduction of NOx emissions compared to kerosene operation.