bluuebunny/crossref_metadata_2025_split huggingface dataset:
Evaluation of Minimum NOx Emission From Ammonia Combustion
Authors: Gubbi Srujan , Cole Renee , Emerson Ben , Noble David , Steele Robert , Sun Wenting , Lieuwen Tim 
Published: September 2023

Abstract
Ammonia (NH3) is being explored as a hydrogen carrier with no carbon emissions. However, if burned directly as NH3, rather than being completely decomposed back to N2/H2, the fuel-bound nitrogen comes with a potentially significant NOx emissions penalty. Indeed, several existing studies are showing ammonia combustion NOx emissions that exceed current natural gas fueled, DLN technologies by one to two orders of magnitude. Therefore, it is important to establish the theoretical minimum NOx emissions for an ammonia combustor, to determine how much NOx levels can be reduced via further technology development. This work determines this minimum NOx level for a two-stage combustor system for a given combustor exit temperature and residence time, with a constraint on unburned fuel levels. As expected, the optimum configuration is an RQL combustor with a rich front end to burn and crack ammonia with significant H2 production, followed by a lean second stage that consumes the remaining H2. The optimum residence time and stoichiometry of each zone is determined in the fast mixing limit. These results show minimum NOx levels are in the 195–400 ppm range at 1 bar, but drop to levels of ∼25 ppm at 20 bar. These NOx emissions are dominated by NOx production in the primary stage which relaxes to equilibrium levels quite slowly. As processes controlling NOx relaxation to equilibrium in the primary stage dominate overall NO emission levels, combustor NOx sensitivities are essentially opposite that of natural gas fired, DLN systems. Specifically, NOx values drop with increased combustor residence time, increased pressure, and increased combustor exit temperature. These results also suggest that promising routes for research are kinetic means to accelerate primary zone NOx relaxation rate to equilibrium. They also show that combustors specifically designed for ammonia will have residence times that are one to two orders of magnitude larger than current natural gas systems. By doing so, NOx levels below 10 ppm may be achievable.
Numerical Investigations of Near Surge Operating Conditions in a Two-Stage Radial Compressor With Refrigerant Gas
Authors: Cravero Carlo , Marsano Davide , Sishtla Vishnu , Halbe Chaitanya , Cousins William T. 
Published: September 2023

Abstract
Modern compressor design targets require high performance and a wide operating range in order to reduce the environmental impact. To understand the fluid dynamics mechanisms that trigger instability, studying the system at the stability limit is required. In this work, a two-stage back-to-back centrifugal compressor for refrigerant applications has been simulated with CFD techniques using unsteady calculations in different operating points close to surge. These models have been validated by comparing numerical performance with experimental data. An in-depth fluid dynamics analysis combined with the monitoring of several pressure signals, post-processed with FFT, identified different flow phenomena in the two stages towards the surge limit. The key role of the volute that induces a stronger upstream counter-pressure in the first stage has been highlighted. This effect causes the formation of high entropy (low momentum) rotating cells in the diffuser that involve a higher channel portion with respect to the flow structure in the second diffuser. This phenomenon affects the upstream flow conditions at the impeller. In addition, the interaction between the IGV and the inducer has been analysed, observing that in the second stage, due to the flow non-uniformity after the ICP, non-negligible separations occur. Starting from the peaks detected in the FFT analysis of the pressure signals all the above flow mechanisms have been detected and discussed.
Desktop-DNS: An Open Toolkit for Turbomachinery Aerodynamics
Authors: Wheeler Andrew P. S. 
Published: September 2023

Abstract
The prevailing view is that high fidelity simulation, particularly DNS (direct numerical simulation), is not something for the practical turbomachinery aerodynamicist — requiring too much computational and personal effort to make it worth it. The aim of the ‘Desktop-DNS’ toolkit described in this paper is to change this by greatly lowering the barrier to entry for running DNS. The paper shows how, using an efficient high-order Navier-Stokes computer code, it is becoming increasingly possible to solve testcases of industry relevance with high fidelity LES and DNS, making use of the latest advances in single compute node performance. This is achievable using both efficient algorithms and GPU acceleration. The paper will use a compressor blade testcase to illustrate how, in some cases, high-fidelity simulations can be performed at relatively low costs on a small number of computer nodes. This raises the possibility of a much more widespread use of DNS to inform early design choices, enhance or benchmark current models, and to potentially reveal new physical mechanisms. An increasing uptake of DNS into mainstream CFD use has profound consequences in terms of data-processing and the training needs for practitioners. The paper therefore provides guidance on some practical ways in which high fidelity data can be exploited for turbomachinery design, as well as a step-by-step guide and set of codes to enable a turbomachinery aerodynamicist to explore DNS.
Assessment of Flamelet Generated Manifold Approach With Inclusion of Stretch Effects of Pure Hydrogen Flames
Authors: Amerighi Matteo , Nassini Pier Carlo , Andreini Antonio , Orsino Stefano , Verma Ishan , Yadav Rakesh , Patil Sunil 
Published: September 2023

Abstract
Hydrogen provides the most significant potential to meet the net-zero carbon target for the power generation and aviation propulsion, since it can be directly used in gas turbines. Its high energy density, higher auto-ignition temperatures, and ability to lean-burn are well suited for consumption in gas turbines. On the other hand, the high adiabatic flame temperature and reactivity need special attention for limiting the NOx emissions and the flashback risks. Therefore, the development of numerical models capable of predicting its behavior is essential to speed up the design phase of a new aero-engines combustor operating with pure hydrogen fuel. The present work aims to study through high-fidelity simulations a laboratory-scale rig investigated at the University of Berlin (TUB) featuring a swirl stabilized, premixed hydrogen flame. A tabulated chemistry approach based on the Flamelet Generated Manifold (FGM) is adopted for all the simulations, implementing key improvements to the canonic formulation. Specifically, a cost-efficient strategy to include the stretch and heat loss effects on flame reactivity is developed and tested to capture the peculiar behavior of hydrogen flames. Additionally, a hybrid approach based on FGM is proposed in which turbulence chemistry interaction is modeled through flame thickening. The model predictions are then analyzed in detail to highlight their differences, and a comparison with the available experimental data was performed to assess the most accurate model.
Auto-Ignition Test Results of Hydrogen and Natural Gas Fuels at Atmospheric And Elevated Pressures for Gas Turbine Safety
Authors: Loving Christopher , Mastantuono Garrett , Terracciano Anthony C. , Vasu Subith S. , Pigon Travis , Hernandez Angel , Cloyd Scott 
Published: September 2023

Abstract
The success of the global Energy Transition hinges on meeting the world’s growing demand for power, while at the same time reducing greenhouse gas (GHG) emissions. Achieving this will require significant growth in electricity generation from clean and carbon-free energy sources. Several energy providers [1,2] have already begun the transition from traditional carbon-based fuels to cleaner alternatives, such as hydrogen (H2) and hydrogen enriched natural gas (HENG). However, there are still many technical questions/challenges that must be addressed when applying these fuels in gas turbines.
The application of H2 or H2/natural gas (NG) blends to advanced class gas turbines, which have higher operating pressures and temperatures, has raised concerns about the potential for leakages or fuel sequencing operations where flammable mixtures of fuel and air could auto-ignite. Public information on the auto-ignition of H2 in air at atmospheric pressure is well documented. Such data shows auto-ignition temperature (AIT) of H2 is ∼100°C lower than that of methane (CH4). Studies also show that as pressure increases, methane’s AIT decreases. However, there was insufficient information in the published literature to characterize the influence of pressure on auto-ignition for hydrogen fuel applications.
This study describes the test methodology used to evaluate conditions where auto-ignition occurs for various fuel-air mixtures operating at different pressures (1–30 atm) and temperatures. Testing was completed with 100% H2 and multiple H2/NG blends at various equivalence ratios (φ). Testing was similarly performed for 100% NG to validate the test and data collection methods cited in prior published literature. Results indicate that, at atmospheric pressures, an increase in H2 concentration results in a reduced AIT. However, at 30 atm, the AIT of H2 increased. Variations of auto-ignition delay times (AIDT) were also observed during the testing and are compared to modeling predictions, providing insight into auto-ignition characteristics.
A Systematic Probe Into the Starting Torque Characteristics of a Bio-Inspired Orange Sea-Pen Bladed Savonius Wind Rotor
Authors: Rathod Umang H. , Nalavade Chetan S. , Saha Ujjwal K. , Kulkarni Vinayak 
Published: September 2023

Abstract
In the pursuit of exploiting wind energy, the drag-based Savonius wind rotors have been extensively used in the low wind velocity regions. These type of rotors, as small-scale stand-alone systems, are characterized by their direction independency, ease of fabrication and the absence of yaw-mechanism. Over the past few decades, various blade profiles/shapes and augmentation techniques have been developed to improve the performance of the Savonius rotors. In recent times, in the quest of further improvement, the nature-inspired (such as golden spiral) and the biomorphic (such as fish-ridge) blades have also been evolved. Some researchers have also studied bio-inspired blade shapes (such as sea-pen) to utilize the organism’s strategy of optimized vortex induced feeding. In the present paper, the starting torque characteristics of a sea-pen bladed Savonius rotor is explored experimentally. The rotor blades are derived from the reported data of the Orange sea-pen (Ptilosarcus gurneyi). Wind tunnel experiments are conducted in the range of 5 m/s to 8 m/s. In order to have a direct comparison, the starting torque characteristics of a conventional semicircular bladed rotor is also obtained in the same range of velocities. The present investigation demonstrates the enhancement of starting torque characteristics of the sea-pen bladed wind rotor at specific rotor angles.
ECAT+: A Second-Generation High-Temperature Engine Component Aerothermal Facility at the University of Oxford
Authors: Messenger Andrew , Cartlidge James , Chowdhury Nafiz , Goenaga Frédéric , Povey Thomas 
Published: September 2023

Abstract
This paper describes the extension in capability of the Engine Component AeroThermal (ECAT) facility at the University of Oxford. The ECAT facility is an established, high technology readiness level facility for research and development of aeroengine components, particularly high-pressure turbine nozzle guide vanes (NGVs). The original facility is capable of matching Mach number, Reynolds number and coolant-to-mainstream pressure ratio to engine conditions. It is also possible to use a combustor simulator module to match combustor exit temperature and swirl profiles.
It was desired to further increase the capability of the facility to match engine mainstream-to-coolant temperature ratio (TR), achieved by increasing the mainstream inlet temperature to 600 K. This required: a new high temperature air feed and exhaust system with all the associated compressor plant and air heating systems; a new test section capable of high temperature operation; a new control system; and new high-temperature-rated instrumentation. The facility operates in a semi-transient mode, with air stored at 50 bar (g) and 600 K in three large air receivers. The air is released in a controlled manner through the test section, allowing test times over 100s in duration. This presents a particular challenge for the thermomechanical design of the test section, due to rapid heating of components leading to high transient thermal stress.
The extended facility (referred to as ECAT+) has been successfully commissioned and an initial test campaign undertaken. The facility design, results from the commissioning tests and operating envelope are described within the paper. These results demonstrate the new facility meets all of the operating requirements for high TRL research and product development. The results showed very good agreement with a low order transient system model which was used throughout the design process. It is anticipated that details of the design, and the design process itself, will be of interest to other developers of high TRL facilities.
Improvements in the Prediction of Steady and Unsteady Transition and Mixing in Low Pressure Turbines by Means of Machine-Learnt Closures
Authors: Pacciani Roberto , Marconcini Michele , Arnone Andrea , Bertini Francesco , Spano Ennio , Rosa Taddei Simone , Sandberg Richard D. 
Published: September 2023

Abstract
In this paper novel machine-learnt transition and turbulence models are applied to the prediction of boundary-layer transition and wake-mixing in a Low-Pressure Turbine (LPT) cascade, including unsteady inflow cases with incoming wakes. A Laminar Kinetic Energy (LKE) transport approach is employed as baseline transition framework. The application of the machine learnt EARSMs takes advantage of a zonal strategy based on a newly developed sensing function that allows automated wake demarcation. This work compares the performance of several approaches that are based on the application of improved transition models without machine learnt EARSMs, baseline transition model with EARSMs trained for improved wake mixing, and new transition and turbulence closures that are simultaneously developed in a fully coupled way and aimed at improving both transition and wake mixing predictions in LPTs. The investigation is carried out on a cascade of industrial footprint, representative of modern LPT bladings. First of all, the various modelling frameworks are evaluated, without bar wakes (steady conditions), over a wide range of Reynolds number values. URANS analyses have also been carried out to investigate the predictive capabilities of machine-learnt closures in the presence of incoming bar wakes. RANS/URANS results are scrutinized against LES calculations and experimental data. It will be demonstrated that machine learnt EARSMs are capable of producing realistic wake mixing when simply applied on top of the baseline transition models. However, the most accurate predictions will be shown to be provided by fully integrated machine-learnt transition/turbulence closures.
Effect of Rim Seal Geometry on Rotationally-Driven Ingestion
Authors: Burden Shaun , Chew John W. , Gao Feng , Marxen Olaf 
Published: September 2023

Abstract
This study investigates turbine rim seal geometry effects within the rotationally-driven ingestion regime. Computations were performed with a wall-resolved unsteady Reynolds-averaged Navier-Stokes (URANS) model and a large-eddy simulation (LES) model including near-wall boundary layer modelling, that is wall-modelled LES (WMLES). Use of simplified rim sealing models is proposed as an efficient method of ranking seal designs and investigating sensitivity to seal geometry. Four rim seal configurations, two chute seals, an axial seal and a radial seal which are representative of those used in gas turbines and in previous research were investigated. Furthermore, hybrid seals combining geometric characteristics from both the chute and radial seal were considered. Significant sensitivities of sealing performance to turbulence modelling are identified, but URANS and WMLES show similar trends in ranking of seal performance, and these are consistent with previous experimental work. The addition of an outer radial clearance section to a chute seal is effective in reducing ingestion levels.
Application of Numerical Bifurcation Tracking Strategy to Blade-Tip/Casing Interactions in Aircraft Engines
Authors: Delbé Clément , Colaïtis Yann , Batailly Alain 
Published: September 2023

Abstract
Building on the regularized-Lanczos harmonic balance method, a previously developed frequency method, this paper presents a numerical bifurcation tracking strategy dedicated to high-dimensional nonlinear mechanical systems. In order to demonstrate its applicability to industrial applications, it is here used to obtain original results in the context of blade-tip/casing interactions in aircraft engines. The emphasis is put specifically on the tracking of predicted limit point bifurcations as key parameters — such as the amplitude of the aerodynamic forcing applied on the blade, the friction coefficient or the operating clearances — vary. Overall, presented results underline that the employed frequency method is well-suited to tackle the numerical challenges inherent to such computations on high-dimensional systems. For the mechanical system of interest, the industrial fan blade NASA rotor 67, it is shown that the application of the presented strategy yields an efficient way to identify isolated branches of solutions, which may be of critical importance from a design standpoint.
Working Fluid Selection and Thermodynamic Optimization of the Novel Renewable-Energy Based RESTORE Seasonal Storage Technology
Authors: Alfani Dario , Giostri Andrea , Astolfi Marco 
Published: September 2023

Abstract
Seasonal based energy storage aimed at increasing the dispatchability of renewable energies is expected to be one of the main options for the decarbonization of the space heating sector. Technologies available today can only partially fulfill the requirements of efficiency, energy density and affordability, being based on the use of hot water. This work focuses on the analysis of a non-conventional system based on the adoption of pumped thermal energy storage concept to maximize the usage of renewable energies and waste heat excess during summer season and make them available during winter months. Organic fluid-based cycles are adopted for both the heat upgrade during hot season (heat pump) and to produce electricity and hot water during cold season (power unit). Upgraded thermal energy is used for supporting an endothermic reaction producing dehydrated solid salts that can be stored for months using inexpensive, high efficiency and high energy density (up to 2 GJ/m3) solutions. This paper focuses on the design of the thermodynamic cycles, comparing the performance attainable with several different working fluids. Two different systems are investigated: coupled systems sharing the same heat exchangers and the same fluid in both operating modes and decoupled systems. Analysis is completed with a preliminary economic assessment of the system, including a sensitivity analysis on electricity and heat cost. Cyclopentane resulted to be one of the most promising working fluids for coupled systems, reaching a competitive round-trip efficiency of around 35%, maximizing the ratio between performance and total heat transfer surface, without showing excessive turbomachinery volume ratios and volumetric flow rates. Economic analysis also highlighted that in this cogenerative application the use of working fluids with lower efficiency, but also low system capital cost, can achieve competitive payback times. On the contrary, decoupled cycles systems result to be less attractive, being able to reach only slightly higher thermodynamic performance, but requiring a higher capital cost and thus possibly being of interest only in specific applications.
Experimental and Numerical Results of a Non-DLE Aeroderivative GT Combustion System Burning Methane-Ammonia Blends at Intermediate Pressures
Authors: Ditaranto Mario , Füri Marc , Solvang Sindre , Saanum Inge 
Published: September 2023

Abstract
Ammonia is considered as a practical hydrogen carrier, as it can be stored in liquid form at moderate pressures and temperatures and is commonly produced and transported today. Ammonia could be used directly as a Gas Turbine fuel to replace current hydrocarbon-based fuels, but its low reactivity and propensity to produce high NOx will require major combustor design changes. Alternatively, ammonia can be fully or partially converted back into hydrogen and nitrogen before using it as a fuel with additional equipment and penalty on cycle performance. Another option is to replace only part of the natural gas with ammonia to minimize the changes of combustion properties and hence combustor design. Although this only partially decarbonizes the fuel, it may be a viable intermediate step to reduce the gas turbine footprint in the short term and with minimum hardware changes to the GT. This experimental and numerical study investigates combustion of methane-ammonia blends with a simplified version of the non-DLE Siemens Energy SGT-A35 turbine combustion system (Rich-Quench-Lean system, RQL) at pressures up to 8 [bar] and powers up to 100 [kW]. The effect of ammonia/methane ratios and Primary Zone Equivalence Ratio (PZER) on emissions of NOx, CO and N2O has been investigated for different pressures and power. A pressure exponent has been extracted based on this experimental data to provide an order of magnitude of NOx emissions at engine conditions. To complement these experimental results, a simple Chemical Reactor Network (CRN) has been used to support the experimental results further. The experimental results showed that even small amounts of ammonia in methane result in unacceptably high NOx emissions for all Primary Zone temperatures and pressures investigated. Although increasing pressure reduces NOx emissions (as previously reported), extrapolation to engine conditions showed that the order of magnitude of NOx emissions will remain unacceptably high. It is also shown that flame stability becomes insufficient for fuels containing more than 40–50 [% by vol.] of NH3 as blow out occurred for these fuels. Furthermore, the results showed that N2O emissions are very low for rich Primary Zone Equivalence Ratios and increase approximately exponentially with decreasing equivalence ratios, but remained below 10 [ppm] for cases at higher pressures. Finally, the CRN results reproduced the experimental trends qualitatively well for %NH3 variations but PZ ER variations and absolute values could not be matched for all cases simultaneously without re-tuning of the CRN.