bluuebunny/crossref_metadata_2025_split huggingface dataset:
Unsteady Pre-Stall Behavior in a Centrifugal Compressor With Vaned Diffuser
Authors: Suzuki Yutaro , Fujisawa Nobumichi , Ohta Yutaka 
Published: September 2023

Abstract
Unsteady pre-stall behavior in a centrifugal compressor with a vaned diffuser was investigated by experimental and numerical analysis. The pre-stall disturbances occurred at a slightly higher flow coefficient at the stall point in the diffuser region. Five disturbances occurred in the circumferential direction, and each rotated at approximately 1.7%N at this flow coefficient. Numerical analysis showed that five stall cells rotated at approximately 2.0%N within the diffuser passage. To understand this pre-stall phenomenon, we focused on the rotation mechanism and initiation process of the five-cell rotating stalls. Each of the five-cell stalls was found to rotate by the following mechanism. When the preceding low-velocity region moved to an adjacent passage, the high-velocity region was circumferentially pushed by the low-velocity area and reached the following passage. The incoming flow collided with the back flow around the throat area, and the flow bent at the diffuser inlet of the passage. Consequently, the incidence angle toward the adjacent passage increased, and a separation was induced at the leading edge of the succeeding diffuser vane. Subsequently, the mass flow rate of the succeeding passage started to decrease. These phenomena occurred sequentially, causing the five-cell stalls to rotate. Five stationary low-velocity regions that did not rotate were observed before the initiation of the five-cell rotating stalls. When the outlet mass flow rate decreased, a one-cell rotating stall appeared within the diffuser passage. It provided a low-energy fluid to the diffuser passages where the low-velocity regions existed. Subsequently, five lowvelocity regions were clearly formed, which started rotating according to the rotating mechanism explained above.
Combined Joule-Humphrey-Recuperator Cycle: Performance and Parametric Analysis Evaluation Towards More Efficient Air Transportation
Authors: Saavedra Jorge , Cadarso Luis 
Published: September 2023

Abstract
A three-stream combined Joule-Humphrey cycle that employs a heat recovery stream to function as a recuperator is presented. Based on an in-house developed thermodynamic performance tool, the operation of a modified dual-shaft turbofan engine is proposed. The engine core is modified by adding an intercooler and a re-heating chamber to approach isothermal compression and expansion processes. A fraction of the primary flow is introduced into a re-heat chamber that uses Rotating Detonation Combustion (RDC) technology. The outflow of the RDC is then merged with the rest of the nucleus current before being discharged to the next turbine stage.
The overall system behavior is captured by means of a nonlinear mathematical model featuring eight decision variables, including mass flow rates and compression ratios. A parametric analysis identifies the operational and performance envelope of the proposed engine concept. Ultimately, the model is endowed with an objective function, which includes global efficiency and thrust looking for an operation regime that boosts the thermodynamic performance. A Generalized Reduced Gradient based algorithm is used to solve the nonlinear model, where each iteration solves a linearly constrained subproblem to generate a search direction. The performance and operational envelope presented here could be used as guidance for others considering the implementation of any of the discussed Joule cycle modifications or assessing the cost-effective balance of their use.
Insights Into Frictional Brush Seal Hysteresis
Authors: Phan Hien M. , Pekris Michael J. , Chew John W. 
Published: September 2023

Abstract
Brush seals offer a superior sealing effectiveness compared to labyrinth seals. However, widespread use of brush seals is constrained by deleterious behaviors such as pressure-stiffening and hysteresis. For the latter, the bristles bend during the shaft incursion process and do not fully recover during the shaft retraction process. An opening gap is created, which increases seal leakage unless the pressure load drops to a certain level. In the present work, analytical and numerical models based on a single bristle are proposed to capture the seal’s response to shaft displacement with and without pressure loading. The models are validated using static stiffness tests at an unpressurized condition from literature. The main results show that modeling of the backing ring friction is essential to capture the bristle hang-up behavior. Shaft friction dominates at unpressurized conditions, while backing ring friction dominates at high pressure loading. An expression for shaft hang-up displacement has been derived. A sensitivity study shows that seals with shallow lay angle, short bristle length, and large bristle diameter are less prone to hang-up problems. The models developed in the present framework have been shown to qualitatively capture the pressure stiffening, hysteresis, bristle hang-up, and shaft rotation effects.
Intermittency of Flame Structure and Thermo-Acoustic Behavior in a Staged Multipoint Injector Using Liquid Fuel
Authors: Truffot Marie , Renaud Antoine , Zimmer Laurent , Richecoeur Franck , Cayre Alain , Méry Yoann 
Published: September 2023

Abstract
This study investigates the impact of the staging factor, the ratio between the fuel injected through the pilot stage and the multipoint injection, on the flame dynamic. The BIMER combustor is an atmospheric pressure rig equipped with two co-rotating swirling air injections (a fixed amount of around 87% of the air goes inside the multipoint stage) and two fuel injection paths for staged combustion. Liquid dodecane is injected with air preheated at 437K with a global equivalence ratio of 0.6 and a thermal power around 72 kW. The change of the staging factor from 100% (pilot-only injection) toward 0% (multipoint-only injection) generates changes in the flame-shape which bifurcates from an anchored V-flame into a lifted flame. This flame shape bifurcation appears at a staging factor around 25%.
Around this staging factor, one can witness multistable flames where the flame structure transits randomly between five different states. Processing microphone signals recorded in the chamber provides an understanding of the flame dynamics. The attached flame presents limited pressure fluctuations level at 270 Hz, while the lifted flame features high-pressure fluctuations at 323 Hz. The intermittency between the five states (including the two stable states) is investigated.
Influence of Steam and Elevated Ambient Conditions on N2O in a Premixed Swirling NH3/H2 Flame
Authors: Pugh Daniel , Bowen Philip , Goktepe Burak , Giles Anthony , Mashruk Syed , Valera Medina Agustin , Morris Steven 
Published: September 2023

Abstract
Renewably generated NH3 has the potential to support future energy demand, however combustor designs and strategies require considerable development to ensure reduced NOx emissions. Expanding on previous work, a turbulent swirl burner was used to appraise potential NOx reduction pathways, both experimentally and numerically, with a premixed NH3/H2/air flame. With a 100-year global warming potential ∼265 times that of CO2, particular emphasis was given to N2O. Maintaining a constant thermal power, reactants were supplied at elevated temperature, with parametric changes made to pressure and humidity. Favourable agreement was demonstrated between exhaust emission measurements and simulations performed using a chemical reactor network model. NO and N2O emissions were shown to be sensitive to operational equivalence ratio, increasing by several orders of magnitude across the experimental range. An increase in combustor pressure was experimentally shown to reduce exhaust N2O concentrations with this globally lean fuel mixture, alongside NO. Steam injection was also explored in detail for the first time and shown to provide contrasting trends, with a reduction in NO, and a rise in N2O, as water loading was increased. Both pressure increase and steam injection were combined to give optimal NOx performance for the evaluated dataset. Changes in chemical kinetic pathways were investigated in detail, and compared to high-speed OH*, NH2*, and NH* chemiluminescence.
A Critical Analysis on the Impact of External Losses on the Performance of a Centrifugal Compressor Stage – Experimental and Multi-Fidelity Numerical Assessment
Authors: Baroni Alberto , Catalani Iacopo , Romani Luca , Bicchi Marco , Balduzzi Francesco , Marconcini Michele , Bianchini Alessandro , Arnone Andrea , Ferrara Giovanni , Toni Lorenzo , Biliotti Davide 
Published: September 2023

Abstract
The study presents a combined experimental and numerical analysis of the impact of external losses in defining the real performance of a model stage. This is of particular importance in the early-validation phase, where novel solutions going beyond the state of the art need to be verified for the first time. Due to the number of simulations to be carried out during preliminary design, in fact CFD calculations often focus on main flow path analyses and neglect these losses. If optimization is carried out within a known design space, this does not create any problem and the different solutions can be analyzed in a comparative way. On the other hand, when a new design solution comes into play, possible changes in the impact of losses can lead to unfair comparison between numerical predictions and test-rig experimental data.
In particular, the results of detailed experimental tests carried out in the test rig developed by the University of Florence in support of its industrial partner Baker Hughes are presented here. In the test, different seals at the impeller’s eye are compared. Results are then compared to numerical simulations of the stage that include cavities and resolve leakage flow, and of those of a newly-developed integrated and computationally-efficient tool that combines CFD simulations of only the main flow path with engineering corrections for loss sources. Results show that indeed a comprehensive CFD model that resolves cavity leakage is able to correctly replicate experimental results, although this is obtained with a notable calculation cost, which is often not affordable at industrial level. On the other hand, almost the same accuracy is reached with the combination of a main flow path-only CFD model and correction method, thus proving the interest for this type of approach in the industrial context.
Compressor Water Extraction
Authors: Farman Judith R. , Day Ivor J. , Dickens Tony M. J. , Taylor James V. , Hussain Zahid 
Published: September 2023

Abstract
The effect of putting a bleed slot behind the rotor or stator of a modern representative IP compressor stage has been investigated using a purpose built test rig. The first part of the paper, goes over the construction and operation of a rig, which for the first time allows for accurate measurement of water extraction and rapid testing of a variety of bleed slot geometries. Using this test rig it was found that the use of a bleed slot behind the rotor increases water extraction by up to 40% compared to placing it behind the stator. The overall extraction efficiency for the rotor exit was above 70% for all bleed rates considered from 0% to 30% and is relatively constant. Whilst rotor exit bleed might be advantageous for water extraction, further improvements in extraction efficiency are required for future engines with low fan speed and low blade solidity. The final part of the paper, looks at the effect of a bleed lip protrusion to improve water extraction, as this can be retrofitted in engines. An extra 5% can be achieved by doing this without a penalty operability. This has been explained using 3D CFD by examining the flow separation topology.
Loss Analysis in Radial Inflow Turbines for Supercritical CO2 Mixtures
Authors: Aqel Omar , White Martin , Sayma Abdulnaser 
Published: September 2023

Abstract
Recent studies have indicated the potential of CO2-mixtures to lower the cost of concentrated solar power plants. Based on aerodynamic and cost considerations, radial inflow turbines (RIT) can be a suitable choice for small to medium sized sCO2 power plants (about 100 kW to 10 MW). The aim of this paper is to quantify the effect of doping CO2 on the design of RITs. This is achieved by comparing the 1D mean-line designs and aerodynamic losses of pure sCO2 RITs with those of three sCO2 mixtures containing tetrachloride (TiCl4), sulphur dioxide (SO2), and hexaflourobenzene (C6F6).
Results show that the optimal turbine designs for all working fluids will have similar rotor shapes and velocity diagrams. However, factors such as the clearance-to-blade-height ratio, turbine pressure ratio, and the difference in the viscosity of the fluids cause variations in the achievable turbine efficiency. Once the effects of these factors are eliminated, differences in the total-to-static efficiency amongst the fluids may become less than 0.1%. Moreover, if rotational speed limits are imposed, then greater differences in the designs and efficiencies of the turbines emerge amongst the fluids. It was found that limiting the rotational speed reduces the total-to-static efficiency in all fluids; the maximum reduction is about 15% in 0.1 MW CO2 compared to the 3% reduction in CO2/TiCl4 turbines of the same power.
Among the mixtures studied, CO2/TiCl4 achieved the highest performance, followed by CO2/C6F6, and then CO2/SO2. For example, 100 kW turbines for CO2/TiCl4, CO2/C6F6, CO2/SO2, and CO2 achieve total-to-static efficiencies of 80.0%, 77.4%, 78.1%, and 75.5% respectively. Whereas, the efficiencies for 10 MW turbines are 87.8%, 87.3%, 87.5%, and 87.2%, in the same order.
Supercritical CO2 Compressor and Expander Design for Industrial Waste-Heat Valorization
Authors: Toni Lorenzo , Bisio Valentina , Milani Alberto , Biliotti Davide , Bellobuono Ernani Fulvio , Valente Roberto , Dozzini Matteo , Bigi Manuele , Generini Giulio 
Published: September 2023

Abstract
Within the energy transition framework, it is believed that supercritical carbon dioxide (sCO2) power cycles can significantly contribute to climate change goals. Compared to other energy conversion technologies, higher efficiency can be achieved thanks to the minimized compression work, driven by the peculiar properties of sCO2. Footprint reduction coupled with good operational flexibility and reduced water consumption are additional advantages.
The present work discusses design challenges and solutions studied for an sCO2 power system object of the CO2OLHEAT project, “Supercritical CO2 power cycles demonstration in Operational environment Locally valorizing industrial Waste Heat”, European Union Horizon 2020 funded program (grant agreement # 101022831). The project aims at demonstrating the first MW-scale waste heat recovery system employing sCO2 in the EU; the demo cycle will be installed in an existing industrial plant. Object of the authors’ work includes the design and testing of a turboexpander driving the cycle compression phase.
In this paper, the main challenges faced in the conceptual design phase of turbomachinery will be assessed. The centrifugal compressor, whose inlet conditions are in proximity to CO2 critical point to get the highest cycle efficiency, has been designed using specific equilibrium models and equation of state to both properly consider phase-change regions and strong gradients of thermodynamic properties. Regarding the expander, the paper will describe how the inlet conditions (pressure and temperature) represented specific challenges in particular for mechanical configuration and safe operation of crucial components such as the Dry Gas Seals (DGS), for which dedicated thermal analyses were conducted. The paper concludes with considerations on design choices for turbomachinery made to improve the operability of the complete cycle even in off-design conditions such as possible variations in minimum and maximum CO2 temperatures due to external factors.
Machine Learning Based Optimizer for Tilting Pad Journal Bearing Inlet Flowrate
Authors: Gheller Edoardo , Chatterton Steven , Panara Daniele , Turini Gabriele , Pennacchi Paolo 
Published: September 2023

Abstract
In view of the green economy and energy transition, the reduction of the environmental impact of the power generation sector plays a key role. Fluid film bearings are the most common bearing for industrial turbomachinery and new design requirements have a direct impact on bearings operation. In fact, to achieve higher levels of efficiency, bearings must support higher specific loads and higher peripheral speeds. Furthermore, there is great interest in reducing the oil flowrate required for the bearing operation as much as possible. In this work, an optimization strategy for reducing the flowrate fed to tilting pad journal bearings (TPJBs) is proposed. An artificial neural network (ANN) is trained to estimate the static and dynamic performance of the bearings. The training dataset is built with a Reynolds-based thermo-hydrodynamic model. The trained ANN is then used in a constrained optimization that has the goal of minimizing the oil flowrate while ensuring safe bearing operation. Predictions are compared with experimental data from Compressor Mechanical Running Tests. The proposed model is an effective tool that can help industry achieve the goals required by the energy transition and can help in the development of optimized fluid film bearings.
Neutron Characterization of 3D Printed Gas Turbine Alloy With Internal Cooling Structure
Authors: Raju Nandhini , Fouliard Quentin , Bunn Jeffrey R. , Subramanian Ramesh , Kapat Jayanta , Raghavan Seetha 
Published: September 2023

Abstract
The objective of this research work is to investigate the residual strains induced by the presence of a exit hole and internal hollow geometry present in a 3D printed IN718 similar to that seen in a gas turbine blade. The specimens were manufactured with an internal hollow structure that ended with an exit hole, and fabricated by direct metal laser sintering additive manufacturing technique. Using this technique, metal powder was melted by a laser source and rapidly cooled to form the part layer by layer. Rapid heating and cooling generate large thermal gradients and residual stresses in the 3D printed material. To be aligned with industry standards, specimens were heat-treated to relieve most of the residual stresses as well as to investigate whether IN718 still preserved a nonhomogeneous stress state after the thermal treatment. Roughness and microstructure analysis was performed to understand the heat-treated sample properties. To investigate the volumetric residual strain, the neutron diffraction technique was used with a definite neutron wavelength. Bragg’s law was used to determine lattice spacing. Interplanar lattice spacing and stress-free state lattice spacings were measured to calculate the strains. Residual strains were measured in the x and y direction to understand the impact of the presence of a exit hole and internal hollow geometry. The results of volumetric residual strain in the x and y directions showed no significant variations in the distribution of the strain due to the presence of a hollow structure as well as the exit hole. Overall, no significant stress concentration was observed in the heat treated sample. Sample geometry, instrumentation for neutron diffraction, and material properties are discussed in detail in this paper.
Adjoint-Based Aeroacoustic Optimization of NASA Rotor37
Authors: Katsapoxaki Pelagia , Hottois Romain , Tran Thanh-Son , Schram Christophe , Coussement Grégory , Verstraete Tom 
Published: September 2023

Abstract
Within the past decades, numerous optimization methods have been developed to maximize the aerodynamic characteristics of various turbomachinery components. Lately, more attention is being paid to methods able to enhance their aero-acoustic performance due to the increasing demand for quiet propulsion systems for the aircraft industry. In the framework of the present work, a multi-disciplinary, multi-objective, gradient-based shape optimization is performed to minimize the shock-associated tonal noise of NASA Rotor37, while maximizing its isentropic efficiency. The rotor’s flow field is obtained through three-dimensional Reynolds averaged Navier Stokes simulations with the Spalart-Allmaras turbulence model, while the corresponding sound power level is obtained using a simple noise calculation tool based on the flow perturbations at the rotor’s inlet. An adjoint approach is used for computing the gradients of the objective functions with respect to the 41 design variables, which parametrize the geometry of the blade. A Sequential Quadratic Programming optimization algorithm is used to identify several trade-off points of the noise-efficiency Pareto front. Constraints are added to the optimization problem to keep the mass flow within a given range. Results show a substantial improvement in the overall performance of the rotor, with 2.75dB of sound power level reduction and a 1.4% increase in isentropic efficiency. Compared to a similar study that was conducted with a gradient-free algorithm, these results are obtained at a limited computational effort thanks to the adjoint-based methodology. The flow fields of the baseline and optimized designs are analyzed to understand the mechanisms behind the performance improvements.
Modelling of Combustor Non-Uniformities Evolution Through a High-Pressure Turbine Stage
Authors: Notaristefano Andrea , Gaetani Paolo 
Published: September 2023

Abstract
In modern gas turbines, the reduction of pollutant emissions can be achieved by employing lean-burn combustors. At the combustion chamber outlet, the flow is non-uniform and characterized by a residual swirl superimposed to steady (hot streak) and unsteady (entropy waves) temperature disturbances. During the transport from the combustor outlet to the turbine inlet, these disturbances are weakly dissipated and persist at the turbine inlet. Therefore, the interaction between the combustor non-uniformities and the turbine has to be deeply studied.
To study combustor-turbine interaction experimentally, a common practice is to install combustor simulators on non-reactive turbine test facilities. For this purpose, a combustor simulator was designed and installed at Politecnico di Milano turbine test facility. This device can generate a combined steady/unsteady temperature disturbance and swirl profile at the turbine inlet. Using this layout, several experimental campaigns have been carried out changing the type of injected disturbance, the injection position and the turbine operating condition. In this paper, the data collected from these experiments have been used to develop simplified models to predict the transport and dissipation of combustor perturbations through a turbine first stage. In the open literature, few attempts are discussed regarding the modelling of combustor-turbine interaction that — in authors’ opinion — represents an important tool for preliminary turbine design.