bluuebunny/crossref_metadata_2025 huggingface dataset:
Integrated Control Design for a Partially Turboelectric Aircraft Propulsion System
Authors: Simon Donald L. , Bianco Santino J. , Horning Marcus A.
Published: September 2023

Electrified Aircraft Propulsion (EAP) holds great potential for reducing aviation emissions and fuel burn. A variety of EAP architectures have been proposed including partially-turboelectric configurations that combine turbofan engines with motor-driven propulsors. Such architectures exhibit coupling between subsystems and thus require an integrated control solution. To address this need, this paper presents an integrated control design strategy for a commercial single-aisle partially-turboelectric aircraft concept consisting of two wing-mounted turbofan engines and an electric motor driven tailfan propulsor. Within this architecture the turbofans serve the dual purpose of generating thrust and supplying mechanical offtake power used to generate electricity for the tailfan motor. The propulsion control system is tasked with coordinating turbofan and tailfan operation under both steady-state and transient scenarios. The paper introduces a linear state-space representation of the architecture reflecting the coupling between the turbofan and tailfan subsystems along with loop transfer functions reflecting open- and closed-loop system dynamics. Also discussed is an applied strategy for scheduling the tailfan setpoint command based on the average sensed fan speed of the two turbofans. This approach ensures synchronized operation of the turbofan and tailfan subsystems while also allowing the turbofan fuel control design to be simplified. Performance of the integrated control design is assessed through a real-time hardware-in-the-loop test conducted at the NASA Electric Aircraft Testbed. During this test a scaled version of the electrical system and turbomachinery shaft dynamics were implemented in electrical machine hardware and evaluated under closed-loop control. Results from this facility test are presented to illustrate the efficacy of the applied integrated control design approach under steady-state and transient scenarios including a full-flight mission profile.
A Numerical Investigation of the Design Point Performance for a Centrifugal Compressor With a Parameterized Volute
Authors: McLaughlin Laura , Stuart Charles , Spence Stephen , Rusch Daniel , Geron Marco , So Kwok Kai , Kreiselmaier Erich
Published: September 2023

The volute component of a centrifugal compressor is a complex three-dimensional geometry used to collect the diffuser outflow and discharge it to a single point of delivery. Volute design has a significant impact on the overall dimensions of the entire compressor stage and it’s performance. By reducing the extent of the compressor housing in the radial direction, without sacrificing performance, the compressor will be less expensive to manufacture, lighter in weight and could be suitable for a wider range of applications. The volute can only be designed to achieve optimal performance at one operating point per compressor constant speed line. Away from this operating point, the flow will either accelerate or decelerate through the volute component and will generate increased losses.

The current investigation considered an existing state-of-the-art high pressure ratio, centrifugal compressor coupled with a new vaned diffuser component. The new diffuser incorporated a diverging endwall that reduced its length by 15% in the radial direction and consequently discharged a significantly different flow field to the volute inlet. Hence, the existing volute was no longer optimally designed for the diffuser outflow. This study targeted the design of a new volute that took advantage of the reduced diffuser diameter and matched the outlet flow field of the new diffuser. This study considered the compressor design operating point and implemented a fixed 15% radial reduction to the radial extent of the volute component. The volute spiral was carefully parameterized and a Design of Experiments investigation was set up to quantify the impact of these parameters on the performance of the volute and the upstream components. The resulting analysis generated understanding of the underlying flow physics responsible for the performance changes.

The study found that a 15% reduction could be applied to the volute geometry while achieving 99.9% of the baseline compressor total-total efficiency at design point. Detailed flow field analysis has shown the impact of different volute geometry parameters on compressor performance. For example, the area under the volute tongue was demonstrated to significantly impact the losses generated in the volute as well as the circumferential flow uniformity at the diffuser outlet. Although this investigation considered design point only, the improvement to circumferential flow uniformity suggests that the highest efficiency radially reduced design may achieve an extension to the compressor operating map width. Overall, the combination of the diverging endwall diffuser and the new volute design achieved a 15% reduction in the radial extent of the compressor stage downstream of the impeller (from the impeller trailing edge radius to the outer radius of the volute casing).
Implementation of a Central Upwind Scheme in Unsteady Compressible Flow Simulation of Multi-Stage Axial-Centrifugal Compressor
Authors: Jing Zhenhao , Neumeier Yedidia , Prasad J. V. R. , James Darrell K.
Published: September 2023

This work investigates the merits of converting the KT central scheme into a central-upwind scheme for simulating the compressible, unsteady flow in a multi-stage axial-centrifugal compressor. The conversion from a pure central scheme to an upwind scheme is very simple, and it follows the basic idea of the original KT scheme. The paper investigates the quality of shock capturing, artificial mass generation, and execution time. The results suggest that, compared to the KT scheme, the central-upwind scheme produces a thinner shock structure, reduced numerical error even for cases involving drastic variations of flow field geometry, and a higher tolerance for CFL number > 0.5.
Direct Numerical Simulation of Riblets Applied to Gas Turbine Compressor Blades at On- and Off-Design Incidences
Authors: Kozul Melissa , Nardini Massimiliano , Przytarski Pawel J. , Solomon William , Shabbir Aamir , Sandberg Richard D.
Published: September 2023

Any realizable increase in gas turbine efficiency has significant potential to reduce fuel burn and environmental impact. Streamwise micro-groove surfaces (‘riblets’) are well-known as a passive surface treatment to reduce drag, which may be useful in the context of increasing overall gas turbine efficiency. This paper presents the first direct numerical simulation of potentially performance-enhancing riblets on an axial flow high pressure compressor blade, where the micro-geometry of the riblets is fully resolved. The midspan section of a NACA6510 profile is considered at an engine-relevant true chord Reynolds number of 700,000 and Mach number 0.5 based on inlet conditions. Fixed triangular (or sawtooth) riblets are considered in the present numerical campaign. The current high-fidelity computational method permits the extraction of data such as the wall shear stress directly from the riblet surface. At the design incidence, the riblets tend to promote earlier transition to a turbulent flow over the suction side, yet significantly reduce the skin friction over the entire downstream chord to the trailing edge. The riblets reduce the viscous force over the blade by up to 18% at this nominal inflow incidence. Thus the current dataset permits new insight into the action of the riblets, since most studies of riblets on turbomachinery blades have been conducted experimentally where direct measurements of skin friction are not possible. The riblets are also able to reduce the skin friction over the high pressure compressor blade at off-design incidences, a promising result given axial flow compressors must cope with variable operating conditions.
Constitutive Model Based Efficient Creep-Fatigue Damage Computation Technique for Steam Turbine Rotors to Enhance Flexible Operational Capabilities
Authors: Sen Suvadeep , Almstedt Henning
Published: September 2023

In the new era of renewable energy, flexible operation of conventional power plants is inevitable, causing high creep-fatigue life consumption. Conventional life assessment methods are deemed to be conservative to address the current requirements. The use of unified constitutive models for the analysis of damage evolution in steam turbine rotors in the past decade has shown promising results and closely relate to experimental data. However, identification of large number of material constants and high computational efforts hinder the widespread use of such models. In this paper, the latter is addressed using a novel representative input cycle (RIC) concept together with the non-iterative asymptotic numerical method (ANM). Various start-up shutdown sequences of a typical steam turbine rotor are studied using a unified constitutive model based on Chaboche’s kinematic hardening including the damage parameter, Chaboche-Rousselier’s isotropic hardening model including the damage parameter, Norton’s viscoplastic flow model, Lemaitre’s damage potential function and Kachanov-Rabotnov’s creep damage law. First, a conventional FEM technique is used and then the proposed RIC method is used to study the evolution of inelastic variables. The reduction in computational efforts and the compromise in accuracy using the proposed method is studied.
Effects of Fuel Composition on Azimuthal Combustion Instabilities in an Annular Combustor Equipped With Spray Injectors
Authors: Latour Véranika , Durox Daniel , Rajendram Soundararajan Preethi , Renaud Antoine , Candel Sébastien
Published: September 2023

In a context where the aviation sector is actively looking into replacing standard jet fuels with sustainable aviation fuels (SAFs), examining the effects of fuel composition on combustion dynamics is of current interest. There are indeed indications that the chemical composition may notably impact the location of the unstable operating points and the amplitude and frequency of thermo-acoustic oscillations. The fuel composition effect is hence investigated in this article by considering the case of binary fuel mixtures composed of heptane and dodecane, two fuels presenting very different volatilities (Tboil = 380 K for heptane and Tboil = 480 K for dodecane). The blends composition is varied between pure heptane and pure dodecane, and the corresponding dynamical behavior is observed in an annular combustor equipped with swirl spray injectors. It is shown that the composition changes the location, extent, and geometry of the unstable domain. The frequency of the instability also depends on fuel composition. The differences in behavior observed for the different blends are then characterized by measuring flame describing functions (FDFs). These functions are determined in a separate burner representing a sector of the annular chamber in which the air flow is modulated from the upstream side. It is found that the FDF gain and phase curves change with the fuel blend. These data, combined with a reduced-order thermo-acoustic model, are used to interpret the experimental observations. From a more practical standpoint, this study indicates that it is important to examine the dynamical behavior of combustors that will use novel fuels like SAFs.
Experimental Vibroacoustic Measurements of a Centrifugal Compressor Stage for Oil & Gas Applications
Authors: Santoni Andrea , Zuccherini Martello Nicolò , Grimaldi Roberto , Mori Francesco , Fausti Patrizio , Chiari Simone , Carmicino Carmine , Ferrini Marcello , Rossin Stefano
Published: September 2023

Designing adequate noise mitigations on centrifugal compressors for Oil & Gas applications to meet the noise regulation requirements for industrial plants requires an accurate prediction of the radiated noise. Further investigation of centrifugal compressors’ noise emission is still needed to develop predictive sound models, which require a thorough understanding of all noise generation mechanisms involved. This paper presents the results of an experimental vibroacoustic analysis conducted on a centrifugal compressor equipped with one closed impeller. Vibroacoustic measurements were performed in three different operating conditions at various working points, aiming to characterize the noise emission of the compressor stage. The results identified the main tonal components of the compressor stage and the noise generated by auxiliary machinery under different working conditions. Consistent with findings from other studies from the literature, a strong influence of the rotational impeller speed on the radiated sound pressure is observed. Moreover, the results propose an experimental comparison between vaned and vaneless diffuser configurations, highlighting higher sound pressure levels radiated with a vaneless impeller design. The paper concludes by highlighting the difficulties related to the experimental testing of such machinery and the significant influence of noise emitted by auxiliary equipment.
Probabilistic Life Estimation of Turbine Shaft Bolted Joints Considering the Effect of Nonlinear Stress Gradients on Crack Growth Rates
Authors: Tümergin Özge , Ertekin Şevket , Kara Aliyye , Giz Ayşe Su , Kula Topak Emel , Ozturk Umud Esat , Muğan Ata
Published: September 2023

Crack growth in mechanical components is determined by initial crack size and fracture toughness that have probabilistic distributions. With a focus on a turbine shaft bolted joint made of Inconel 718, fracture damage was examined during a test cycle while crack size and fracture toughness were random variables. A finite element model of a turbine was created to calculate the stress field in turbine components. Following, stress distribution around the potential crack zones was included in calculating the crack growth. Proposed approach was verified by making comparisons with the results of Nasgro. Probabilistic distribution of crack depth and length, expected life and probability of detecting a crack was included in crack growth analyses. Probability of failure of the stub shaft was calculated via Monte-Carlo simulations. Comparisons were made with the crack growth results of Nasgro to verify the developed methodology. Effect of inspection period on probability of detecting the cracks before failure was determined.
Investigation on the Influence of Surface Roughness of Rotating Microchannel on Flow Conditions
Authors: Rusin Krzysztof , Wróblewski Włodzimierz , Rulik Sebastian , Pahlavanzadeh Mohammadsadegh , Hasani Malekshah Emad
Published: September 2023

This paper presents numerical and experimental investigations of the flow in a rotating microchannel whose walls’ roughness covers a substantial part of the channel height. The roughness can intensify the momentum and heat transfer but also increase the drag force. An intentionally designed roughness may contribute to the improvement in the effectiveness of different processes, especially in turbomachinery, where additional effects that accompany the rotation are of great importance. The methods of assessing the roughness impact in the rotating microchannels constitute a scientific gap and are discussed in this research.

The experimental part was carried out on the Tesla turbine, whose rotor consists of multiple disks spaced by a sub-millimetre distance. The investigations were performed for two sets of disks. In the first case, the surfaces of the disks were polished to obtain a hydraulically smooth flow regime, and in the second case, they were roughened with the use of a tracing wheel.

In the numerical part of the study, the impact of the roughness on the momentum transfer was modelled with the use of the turbulence boundary conditions imposed on the surfaces of the disks. The numerical and experimental results were compared and the validity of the method was assessed.
Cooling Effectiveness of Additive-Manufactured Internal Structure Within a Double Wall Cooling System
Authors: Song Ho Seop , Park Hee Seung , Kim Taehyun , Choi Seungyeong , Moon Hee-Koo , Cho Hyung Hee
Published: September 2023

An internal structure that additively manufactured (AM) within a double-wall cooling system for gas turbine applications was proposed and evaluated for its ability to increase cooling efficiency. Polymer AM specimens were fabricated using a Biot number similarity approach with actual gas turbine materials. Three types of double wall specimens were manufactured: a standard double wall cooling (baseline) case, a double wall cooling with pin-fin structure (pin), and a double wall cooling with a cube lattice structure(cube). Flow pattern analysis was conducted to examine the effect of the cube lattice on the internal structure of the double wall cooling system, and static pressure was measured in each chamber. An infrared thermography method was used to measure overall cooling effectiveness on the specimen surface, and each case was tested at blowing ratios (M = 0.4, 0.6, and 0.8). Compared to the baseline case, the cube lattice structure improved cooling effectiveness by 3.36% at M = 0.4, the most significant improvement of 4.33% at M = 0.6, and 2.71% at M = 0.8. These findings suggest that the use of a cube lattice structure that produced by AM can effectively enhance the cooling performance in double wall cooling system for gas turbine hot parts.
Learning RANS Model Parameters From LES Using Bayesian Inference
Authors: Hemchandra Santosh , Datta Anindya , Juniper Matthew P.
Published: September 2023

We propose a formal mathematical approach to assimilate LES data into values of RANS model parameters combined with some prior knowledge of the expected RANS parameter values. This is achieved using Bayesian inference to determine parameter values that maximize their posterior probability and is known as maximum a posteriori (MAP) estimation. We apply this approach to a premixed turbulent methane-air round jet flame using unburnt mixture equivalence ratio and bulk flow velocity as design parameters. The k-ε model is used for turbulence closure and the eddy dissipation concept (EDC) model is used to model combustion. Three dimesional LES data for six design cases are computed and upto three of these are used for MAP estimation. The likelihood of RANS solutions is evaluated using flow field statistics from LES at training data points. The results show significant improvement in agreement between LES and RANS solutions, computed using MAP estimate parameters for species mass fraction and temperature fields at design points not in the training set. Marginal improvement is observed for velocity fields. This is most likely due to the absence of production terms in the RANS model that capture the three-dimensional nature of the flow being modelled. The marginal likelihood of the RANS model when assimilating both k-ε and EDC model parameters is significantly higher than the case that leaves out the EDC model parameters. This suggests that the former approach is more likely to yield reliable RANS parameters. These results demonstrate the viability of MAP estimation as a means to improving the reliability of turbulent reacting flow RANS simulations for engineering design and optimization applications.
An Apparatus for Tensile and Fatigue Testing of Metallic Materials Involving Miniature Specimens
Authors: Pal Ritam , Garikipati Shashank , Banks Andrew , Basak Amrita
Published: September 2023

Mechanical testing under tensile and fatigue loading is ubiquitous. Such testing often requires expensive equipment and peripherals. During the development stages of any new material or manufacturing processes, it is often desired to perform quick evaluations to optimize the material composition or the manufacturing process parameters to achieve the desired properties. To address this requirement, this paper designs, develops, and deploys a low-cost custom testing apparatus capable of performing both tensile and fatigue testing to investigate the failure characteristics of a candidate material system, e.g., cold-rolled Al7075-T6. The apparatus consists of a linear actuator, a load cell, a polycarbonate sleeve, and a data acquisition system. The apparatus is additionally integrated with a high-speed video imaging system to investigate the failure evolution. The experiments are performed through a custom MATLAB® code. The resulting data are processed to extract the relevant mechanical properties such as the yield strength, the ultimate tensile strength, and the fatigue life. The results demonstrate that the apparatus can reliably characterize the tensile and fatigue properties of Al7075-T6. Throughout its use, the apparatus is found to be robust, resilient, and easy to assemble/disassemble. In conclusion, this apparatus can serve as a low-cost testing equipment to understand tensile and fatigue failure characteristics of metallic specimens during the qualification stages.
Effect of Leading-Edge Inlet Models on the Performance of Gas Bearings Operating in Highly Compressible Flow Regimes
Authors: Prabhakar Arvind , Kim Daejong
Published: September 2023

Reynolds equation has been widely used in modelling lubrication flow and load carrying performance prediction of fluid flow within gas bearings for a number of years. However, the underlying assumption of negligible fluid inertia is not a reasonable design approach philosophy as the operating speeds of such bearings become extremely high. In this paper, A 3D thermos hydrodynamic (THD) model is presented to show effects of leading-edge inlet models on pressure profile predictions of gas films for a rigid 3d bearing film, operating in high speed and highly compressible flow regimes.

The model solves full 3-dimensional Navier-Stokes equations in laminar form coupled along with the gas film energy equation for gas temperature throughout the gas film. Results are also compared with a classical Reynolds equation-based 3D thermos-hydrodynamic model to show impact of choice of boundary condition treatments on performance and pressure profile predictions of such bearings, along with impacts of including inlet dead volume (IDV) region modeling on gas film properties. Further, some more results are discussed from solutions of 3-dimensional Navier-Stokes equations coupled with Peng-Robinson equation of state (EOS), to illustrate impacts on flows and pressure profiles when considering highly compressible real gas effects for typical high density refrigerant flows within such bearings.
Windage Torque Reduction in Low-Pressure Turbine Cavities Part 2: Experimental and Numerical Results
Authors: Jackson Richard , Lock Gary D. , Sangan Carl M. , Scobie James A. , Li Zhihui , Christodoulou Loizos , Jefferson-Loveday Richard , Ambrose Stephen
Published: September 2023

Minimizing the losses within a low-pressure turbine (LPT) system is critical for the design of next-generation ultra-high bypass ratio aero-engines. The stator-well cavity windage torque can be a significant source of loss within the system, influenced by the ingestion of mainstream annulus air with a tangential velocity opposite to that of the rotor.

This paper presents experimental and numerical results of three carefully designed Flow Control Concepts (FCCs) — additional geometric features on the stator surfaces, which were optimized to minimize the windage torque within a scaled, engine-representative stator-well cavity. FCC1 and FCC2 featured rows of guide vanes at the inlet to the downstream and upstream wheel-spaces, respectively. FCC3 combined FCC1 and FCC2. Superposed flows were introduced to the upstream section of the cavity, which modelled the low radius coolant and higher radius leakage between the rotor blades. In addition to torque measurements, total and static pressures were collected, from which the cavity swirl ratio was derived. Additional swirl measurements were collected using a five-hole aerodynamic probe, which traversed radially at the entrance and exit of the cavity.

A cavity windage torque reduction of 55% on the baseline (which has no flow control) was measured for FCC3, at the design condition with superposed flow. For this concept, an increase in the cavity swirl in both the upstream and downstream wheel-spaces was demonstrated experimentally and numerically. With increasing superposed flow, the contribution of FCC1 surpassed FCC2, due to more mass flow entering the downstream wheel-space across the rotor fins (passing FCC1), and less ingestion from the annulus into the upstream wheel-space (passing FCC2). The torque changes from the concepts are explained using the fluid dynamic evidence from experimental swirl measurements and computational simulations. The simulations allow translation to engine-operating conditions and practical information to the engine designer.
Hysteresis Effect of Volute on Compressor Performance Under Pulsating Flow Condition
Authors: Hayashi Yoshihiro , Parikh Aakeen , Barrera-Medrano Maria Esperanza , Martinez-Botas Ricardo
Published: September 2023

Centrifugal compressors for turbochargers are affected by pressure pulsations, generated by the engine valvetrain. It results in a hysteresis loop on a compressor performance map, and the time-averaged performance deviates from the one in constant pressure condition. Since regulations of greenhouse gas emissions are tightening, achieving higher turbocharger efficiency in real-operating conditions is essential. Therefore, understanding of loss mechanisms in pulsating pressure conditions is crucial.

A high number of studies far have contributed to revealing the effects of pulsation on compressor performance. However, numerical investigations of unsteady flow structure in a volute are still lacking, and there are only some validated examples by measurement.

This paper presents a comprehensive experimental and numerical study that sheds light on the pulsating flow field in the volute. An instantaneous and time-averaged pressure signal inside a volute has been captured by measurement. A numerical simulation with Unsteady Reynolds Averaged Navier-Stokes equations has been carried out. Pressure distributions from the simulated results has shown good consistency with experimental results.

Leading off from the validated simulation results, the instantaneous velocity distribution and energy loss profile have been obtained. It can be seen that the flow direction is switched around the volute tongue when the volume flow rate reaches the maximum value. This phenomenon leads to different flow patterns depending on whether the flow rate increases or decreases. When the volume flow rate turns to decrease from the maximum value, the switched flow direction leads a leak flow from the tongue to the volute exit. The total pressure of this flow significantly decreases when it passes the tongue, and it finally appears as the low-energy fluid at the volute exit. It causes a notable blockage, and consequently leads to an additional friction loss. Due to these effects, the energy loss of volute increases from the steady flow condition.
Influence of Variable Swirl on Emissions in a Non-Premixed Fuel-Flexible Burner at Elevated Ambient Conditions
Authors: Pugh Daniel , Bowen Philip , Navaratne Rukshan , Goktepe Burak , Giles Anthony , Valera Medina Agustin , Morris Steven , Vivoli Robin
Published: September 2023

As alternative fuels are designated for future energy applications, flexible combustor designs require considerable development to ensure stable operation with reduced NOx emissions. A non-premixed variable swirl burner was used to experimentally appraise changes in NO production pathways, with CH4 NH3, and H2 flames, alongside intermediate fuel blends. Maintaining an equivalent thermal power and flame temperature between fuels, preheated reactants (500 K) were supplied to the burner, with parametric changes made to pressure (1–6 bara) and swirl number (0.8–2.0). NO production was characterized, alongside variations in flame structure and topology, with a correlation demonstrated for exhaust emissions. NO production was shown to be sensitive to combustor pressure, providing an expected increase for CH4 and H2 flames. Emission profiles from both NH3 and H2 flames are shown to be significantly augmented by a change in swirl number. As NH3 fractions were increased in the H2 blend, a decaying trend in NO emissions was observed with an increase in pressure, and as a function of mixture ratio. However, this behaviour was markedly augmented by a change in swirl number and suggests that further reductions may be possible at increased pressure. At the low swirl/high pressure condition the NH3/H2 blend outperformed pure H2, providing lower NO concentrations. Emissions data were normalised using the traditional dry/O2 correction, alongside mass scaled by thermal power, with a comparison provided. The corresponding differences in emission formation pathways were investigated, alongside high-speed OH* chemiluminescence to further elucidate findings.
Experimental Measurements of Flow-Averaged Toroidal Vortices in Buoyancy-Dominated Rotating Cavities
Authors: Fox Emma C. , Puttock-Brown Mark R.
Published: September 2023

The flow structures and heat transfer within rotating cavities of aero-engine axial compressors influences the thermal expansion of the rotor discs, and consequently the blade-tip clearances. To investigate the flow field at the bore and lower cavity region, experimental measurements have been acquired in an engine representative test facility. Axial, tangential and radial velocities were measured using a miniature five-hole probe at a range of axial and radial positions. Time-averaged results from 28 tests carried out at non-dimensional parameters comparable to engine conditions: 1.3 × 104 ≤ Rez ≤ 8.2 × 104, 3.0 × 105 ≤ Reθ ≤ 3.2 × 106, 0.11 ≤ Ro ≤ 3.24, 0.14 ≤ βΔT ≤ 0.37 are presented in this paper.

The axial and tangential velocity measurements conform to previous work, while the radial velocity component provides quantitative evidence of an asymmetric toroidal vortex in the cavity gap, biased towards the downstream disc. The vortex is characterised by the local vorticity and grows in strength and size as Rossby number increases above Ro = 0.34 to 1.63. The effect of βΔT on the vortex formation is negligible compared to the influence of the tangential Reynolds number as the local circulation is suppressed by the Coriolis forces at high rotational speeds. Both the tangential and radial velocity results suggest that as Ro is increased, the proportion of air that is radially ingested and expelled from a cavity decreases.
Heat Transfer Coefficient and Adiabatic Effectiveness Measurements on a Nozzle Guide Vane With a Single Row of Cylindrical Holes
Authors: Galeotti Sofia , Bacci Tommaso , Picchi Alessio , Facchini Bruno , Cubeda Simone
Published: September 2023

Film cooling injection significantly affects the thermal behaviour of turbine vane surfaces. In addition to the beneficial effect of the film shielding the vane from the hot gas flow, alteration of the thermal boundary layer should also be taken into account. The aim of the present work is to detail the film cooling performance in terms of adiabatic effectiveness and external heat transfer coefficient on a 2D nozzle guide vane.

A single row of cylindrical holes was tested on both pressure and suction side of a literature vane, the VKI LS89, in a linear cascade. The employed measurement technique is a transient thermal method based on IR thermography, which was thoroughly described and validated in a previous work. The influence of inlet free stream turbulence and blowing ratio was evaluated, and two different injection angles were considered for both pressure and suction side. Spatially resolved distributions of adiabatic effectiveness and heat transfer coefficient on the vane surface allow to precisely quantify the above mentioned aspects and highlight qualitative differences between pressure and suction side behaviour. Details regarding the generated non-uniformities in the measured parameters could be also provided, to emphasize how average quantities are not always sufficient to characterize such complex phenomena. The impact of different reference conditions to scale HTC results was also investigated. Such effect was found not negligible on the overall performance of the film cooling system, especially on the suction side where transition plays a critial role.

Ultimately, the collected results constitutes a wide and detailed experimental database for numerical modeling validation in a well-studied environment as the LS89 configuration.
Thermoacoustic Characterization of a Swirl Premixed Flame Using Doak’s Momentum Potential Theory
Authors: D’Aniello Raffaele , Gövert Simon , Janus Bertram , Knobloch Karsten
Published: September 2023

This study proposes a novel approach, based on Doak’s Momentum Potential Theory, for the characterization of the thermoacoustic behavior of turbulent premixed flames. The novel approach has the following two advantages: firstly, an unambiguously separation of acoustic, thermal and turbulent dynamics is achieved by means of a Helmholtz decomposition of the momentum fluctuations density; secondly, the development as well as the interaction between the different dynamics are related to the fluxes of turbulent, acoustic, and thermal mean energies, which can be identified in the fluctuating stagnation enthalpy. The Momentum Potential Theory is applied here for the first time to describe the thermoacoustic behaviour of a confined turbulent flame, represented by large-eddy simulation data of a premixed CH4/Air model combustor. Interaction and energy exchange mechanisms between the fluctuations are analyzed in order to show the potential of the theory as a general framework for the characterization of thermoacoustic problems.
Best Practices for Electricity Generators and Energy Storage Optimal Dispatch Problems
Authors: Vasylyev Andriy , Vannoni Alberto , Sorce Alessandro
Published: September 2023

The growing share of renewable energy sources in the energy mix and the liberalization of electricity markets has drastically affected the operation of electricity generators. Especially, in the last decade, fossil fuel-based generators have shifted their role from providing continuous base load to covering the peak demand and providing backup capacity to stabilize the grid. At the same time, a large amount of storage capacity is foreseen to be integrated into electricity grids in the coming years to shave demand peaks, mitigate price volatility and provide services to the grid. In such a situation, in order to properly manage these crucial technologies, and thus guarantee the economic viability of the operation, it is essential to properly optimize the dispatch and define the best scheduling. This paper considers a Gas Turbine Combined Cycle and Battery Energy Storage to study the problem of dispatch optimization of both generators and storage technologies. Different optimization algorithms have been considered and Mixed Integer Linear Programming is selected for its ability to identify the global optimum and the reduced optimization time. The impact of optimization windows (i.e., the forecast horizon of electricity prices) is also investigated. It is highlighted that an increase in forecasting ability, at least up to 36h, guarantees more effective scheduling; on the other hand, it may require a significantly longer time. Subsequently, different approaches to account for the operation and maintenance costs at the optimization stage are assessed, and, finally, a sensitivity analysis is carried out with respect to market parameters (price average and variability) and technology features (conversion efficiency, cycle cost, etc.).
Study on Secondary Flow Phenomena With Variable Leading Edge Fillet and Endwall Contouring on the Low-Pressure Turbine Vane Linear Cascade
Authors: Darji Anand P. , Baloni Beena D. , Mistry Chetan S.
Published: September 2023

Detailed understanding of the secondary flow field is crucial for reducing low-pressure turbine losses. With the aid of numerical tools, it is possible to investigate, understand and develop triggering methodologies that reduce endwall losses. The present study explores passive control techniques to examine their impact on endwall losses.

The two-section paper discusses the effect of three different fillet configurations on endwall secondary flows in the first part. The second section explores the impact of the optimized non-axisymmetric endwall (NAEW) profile and fillet structures on the secondary flow field. The study is performed for design incidence where steady-state RANS-equations are solved using an SST γ-θ turbulence model with low turbulence intensity for linear low-pressure turbine vane cascade at chord exit Reynolds number of 1.6 × 105. A successful attempt was made to reduce the passage vortex (PV) effect within the passage region. According to the study, among all three fillet alone configurations, fillet-2 was found to be the most suitable design. Furthermore, the comparative study found that optimized NAEW with fillet-3 reduced the loss coefficient by 8%, while optimized NAEW alone reduced the loss coefficient by 11.64%.
The Structure and Stability of Premixed CH4, H2, and NH3/H2 Flames in an Axially Staged Can Combustor
Authors: Ånestad Aksel , Sampath Ramgopal , Moeck Jonas , Gruber Andrea , Worth Nicholas A.
Published: September 2023

An experimental investigation of flame structure, stability, and emissions performance was conducted in a two-stage lab-scale generic combustor design operated with CH4, H2, and NH3/H2 fuel blends. The main flame zone features a premixed bluff body stabilized flame, with a secondary flame zone initiated downstream by injecting premixed air and fuel using two opposing radial jets. The total power and air flow rate are kept constant between the different fuelling cases, while the air split between stages and equivalence ratios are varied to explore conditions relevant to gas turbine operation. Given the relative novelty of the configuration, special emphasis is given to analysing the structure of the opposing jet flames in the secondary stage. In contrast to previous literature on reacting jets in cross flow, these interact significantly due to their proximity, leading to a merged flame zone at the impingement location in the centre of the combustion chamber, and some flame propagation upstream of the jet location. As the jet-to-crossflow momentum ratio increases, the merged flame zone changes shape, reaching close to the walls for the methane cases, but remaining very compact when operating with almost pure hydrogen. For the hydrogen flames, diverting more air to the second stage allows higher total thermal power conditions to be reached, while avoiding flashback, and eliminates thermoacoustic instabilities. For ammonia-hydrogen flames, air is diverted to the second stage, while a constant fuel flow is sent to the primary stage, resulting in some locally rich conditions in the primary flame. A local minima in terms of NOx occurs when the primary flame is operated at an equivalence ratio of 1.15. Analysis of the flame structure suggests that this state corresponds to almost complete combustion or pyrolysis of NH3 in the main flame, with the remaining hydrogen burned in an inverse diffusion flame in the secondary zone.
Numerical Investigation of Friction Induced Interaction of Flutter Modes in a Realistic LPT Rotor
Authors: Rodríguez-Blanco Salvador , González-Monge Javier , Martel Carlos
Published: September 2023

Flutter response of bladed disks is a quite important problem in modern designs of low pressure turbines, but, even for tuned configurations, it is not fully understood yet. Some research work suggests that, in a tuned rotor, the flutter induced vibration state that sets in consists of just a single traveling wave mode, while others show flutter states composed of several traveling waves active at the same time. Many of these studies were performed using conceptual models, such as mass-spring systems or asymptotically reduced models. In this work, a high fidelity bladed disk model is integrated in the time domain for the case of a realistic aerodynamically unstable low pressure turbine. In this configuration, the only nonlinear effect is the friction present at the blade disk attachment, which is responsible for the saturation of the flutter growth, and it is also the only mechanism that allows the interaction among the different unstable traveling wave modes. A case with multiple unstable modes of the same modal family is studied first. After that, the study is extended to a case where modes of different modal families are unstable and have similar aerodynamic instability levels. The interaction of unstable modes from different modal families is studied, we believe, for the first time, as previous flutter analyses were restricted to the interaction of unstable modes from the same family.
Hydrogen-Enriched Natural Gas Co-Firing: A Comparison of FGM and EDC Models
Authors: Zhao Rang , Igie Uyioghosa , Abbott David
Published: September 2023

To facilitate the transition from natural gas to a future hydrogen economy, the combustion of natural gas/hydrogen blends in gas turbines will play an important role in power generation. The influence of hydrogen content on technically premixed swirl-stabilized flame using large eddy simulation has proven to be powerful but with high computational costs. Hence, RANS-based models are useful for preliminary investigations and sensitivity studies. Flamelet Generated Manifold (FGM) and Eddy-Dissipation Concept (EDC) are two widely used RANS-based combustion models. EDC, in particular, accounts for the interaction between chemistry and turbulence using detailed chemical mechanisms, but at the cost of higher computational resources. FGM preprocesses the flamelet as a function of mixture fraction and progress variable and pre-integrates the chemistry-turbulence interaction into a Probability Density Functions (PDF) table, which makes FGM computationally inexpensive. This study aims to compare the predictions of these models with experimental data of a methane-fueled technically premixed swirl-stabilized low-pressure burner. The better-performing model is used to evaluate the influence of methane and hydrogen blends (up 40% by volume) in a higher-pressure burner also validated with experiments. The study has shown that EDC produces better agreement with the experimental data than FGM in estimating the flame temperature, flow velocity, and carbon dioxide profiles. FGM did not correctly capture the flame pattern and overestimated the reaction rate. This is possibly due to its simplified preprocessed chemistry mechanism, which could not evaluate the local thermal properties of the gas mixture properly. For the higher pressure evaluation at 5 bar, the EDC model captured the influence of hydrogen content addition on flame behaviour. As the hydrogen content increased, the chemical reaction rate increased, and the flame length indicated by OH decreased. This reduction in flame length is consistent with the experimental results. The CFD showed that at 20% H2, the change in NOx emission compared to 100% methane is negligible using the mass of NOx per unit of heat release calculation. A slight increase in NOx is shown for the same case using the concentration by volume corrected to 15% O2 approach. Nevertheless, both approaches showed NOx reductions at 40% H2. This study has shown that the behaviour of a technically premixed swirl-stabilized flame-firing methane/hydrogen blend is well represented by a non-adiabatic RANS-EDC model with low computation cost. This confirms its applicability in evaluating acceptable lean premixed burners characteristics for gas turbines.
Techno-Economic Analysis of the Solid Oxide Semi-Closed CO2 Cycle and Comparison With Other Power Generation Cycles With CO2 Capture
Authors: Martinelli Matteo , Scaccabarozzi Roberto , Gatti Manuele , Campanari Stefano , Martelli Emanuele
Published: September 2023

This work presents the techno-economic analysis of the Solid oxide semi-closed CO2 cycle (SOS-CO2), a hybrid sem-iclosed cycle with solid oxide fuel cells (SOFC) recently de-veloped by Politecnico di Milano for power generation from natural gas with high efficiency and near-zero CO2 emis-sions. All the cycles components, including the fuel cell and the turbine, have been designed and sized with the aim of assessing the performance and the capital cost. The performance and the economic key performance indicators are compared with those of two benchmark technologies for power generation with CO2 capture: the Allam cycle and a combined cycle equipped with ammines for post-combustion capture. Moreover, a sensitivity analysis is performed on the cost of the natural gas and the SOFC stacks. The results indicate that the specific total capital requirement of the SOS-CO2 cycle (2500 €/kWel) is higher than the Allam cycle (1930 €/kWel) and combined cycle with post-combustion capture (1980 €/kWel). On the other hand, the SOS-CO2 cycle benefits from its higher efficiency (74.0% vs. 53.9% of the Allam cycle and 52.8% of the combined cycle) which makes the cycle less sensitive to the fuel cost and CO2 tax. In terms of cost of electricity, the SOS-CO2 cycle turns out to be the best technology for natural gas prices above 8 €/GJ, while the Allam cycle appears to be the preferred option at lower fuel prices. For a fuel cost of 12 €/GJ, the SOS-CO2 features a cost of electricity of 106.2 €/MWh, lower than the Allam cycle (119.1 €/MWh) and the NGCC (126.9 €/MWh).
On the Shock Boundary Layer Interaction in Transonic Compressor Blading
Authors: Hergt Alexander , Klose Björn , Klinner Joachim , Bergmann Michael , Munoz Lopez Edwin Joseph , Grund Sebastian , Morsbach Christian
Published: September 2023

Shock waves and their interaction with the boundary is a significant flow effect within transonic compressor blading. The main challenge here is the unsteady nature of this interaction effect because it imposes further requirements to maintain the structural integrity of the blading. Furthermore, it has been shown that the shock oscillations limit the working range of transonic blade profiles. Therefore, it is of great interest to mitigate these effects for the next generation of aero engine designs. In order to do this, it is mandatory to identify the mechanisms that originate the oscillation. Intensive experimental studies have already been carried out in the past using unsteady PIV, as well as Shadowgraphy, which have led to new insights into the nature behind the shock oscillation. However, the measurement insight into the flow is strongly limited and allows only an extract of the entire flow field. In order to get a deeper view, scale-resolving simulations have to be carried out. In addition to the experiments performed at DLR’s Transonic Cascade Wind Tunnel, the flow through a transonic compressor cascade was simulated using a high-order accurate discontinuous Galerkin spectral element method with a finite-volume (FV) subcell shock capturing. The implicit LES scheme is applied for subgrid-scale dissipation, i.e. the dissipation is provided by the intercell flux. The content of the paper includes first a validation of the LES results based on the steady measurement data from the experiment in order to ensure the datasets are comparable. The comparison shows that the experimental operating point is almost exactly reproduced by the LES with respect to the mean inflow conditions and the resulting profile Mach number distribution. A time-frequency analysis is also shown, which was based on the evaluation of local probe data and on spectral decomposition of the simulation snapshots taken. Dominant frequencies of the shock movement and the vortex shedding of the transonic flow are extracted and used to analyze the unsteady flow behavior in a direct comparison with the experimental results. These analyses give a unique insight into the self-exciting mechanism of the shock oscillation and form the basis for the applicability of combined experimental and LES data set for a deeper analysis of the mechanism in future studies.