bluuebunny/crossref_metadata_2025 huggingface dataset:
A Turbo-Oriented Data-Driven Modification to the Spalart-Allmaras Turbulence Model
Authors: He Xiao , Zhao Fanzhou , Vahdati Mehdi
Published: October 2022

The Spalart-Allmaras turbulence model is one of the most popular models applied to compressors, but it often over-predicts compressor blockage size and hence under-predicts the compressor stall margin. In this paper, a novel modification to the SA model is proposed to improve the prediction of compressor near-stall flows. The modification is based on the dimensionless vortical pressure gradient, which identifies blockage cells featured by 3D swirling low-momentum flows under adverse pressure gradients. It unblocks the compressor passage by enhancing the eddy viscosity in the identified blockage cells; whereas in canonical 2D flows the modification is automatically switched off. The model coefficients are calibrated via Bayesian inference, which considers the uncertainties involved in experiments and CFD. The rotor exit radial profile data of NASA Rotor 67 at the peak efficiency (PE) and near stall (NS) conditions at the design speed are used for calibration. Validation on the NASA Rotor 67 case shows that the proposed model not only predicts a better agreement with the measured near-tip profiles compared to the original SA model, but also predicts more accurate stall margins at all operating speeds. Further validations are conducted on the TUDa-GLR-OpenStage transonic axial compressor and the low-speed BUAA Stage B rotor, where stall is driven by the rotor tip blockage cell. Results show that the proposed model predicts a more accurate stall margin at all operating speeds of TUDa-GLR-OpenStage and a more accurate tip blockage size of BUAA Stage B rotor. Finally, the proposed model is validated on the LMFA NACA65 cascade featured by corner separation. In combination with the quadratic constitutive relation (QCR), significant improvement in predicting the static pressure rise coefficient, the total pressure loss coefficient, the flow angle and the blade loading is achieved, indicating the model predicts a more accurate corner separation size. The proposed model, termed as SA-PGω in this work, is a promising engineering tool for future RANS simulations of compressor near-stall flows.
Multi-Objective Aerodynamic and Aeroelastic Coupled Design Optimization Using a Fully Turbulent Discrete Adjoint Harmonic Balance Method
Authors: Wu Hangkong , Wang Dingxi , Huang Xiuquan
Published: October 2022

With increasing requirements for high-loading and high-efficiency turbomachines, blades become thinner and thinner and thus design optimizations considering both aerodynamic performances and aeroelastic stability become more and more necessary. In this study, a fully turbulent discrete adjoint harmonic balance (HB) solver is developed using algorithmic differentiation (AD), validated by a discrete linear solver, and then adopted to perform multi-disciplinary coupled design optimizations. To this end, a framework of multi-objective adjoint design optimizations is developed to improve both aerodynamic performances and aeroelastic stability of turbomachinery blades. This framework is divided into two steps including the aeroelastic design initialization and aerodynamic Pareto front determination. First, the blade profiles are optimized to improve the aeroelastic stability only and constrain the change of aerodynamic performances. Second, the optimized blade profiles in the first step are used as the initial ones and then further optimized with the objective function of aerodynamic parameters and the constraints of aeroelastic parameters. The effectiveness of the multi-objective design optimization method is demonstrated using the transonic NASA Rotor 67 subjected to a hypothetical vibration mode. The results show that the multi-objective adjoint design optimization method can improve not only aerodynamic performances but also flutter stabilization.
Detection of the Onset of Trip Symptoms Embedded in Gas Turbine Operating Data
Authors: Losi Enzo , Venturini Mauro , Manservigi Lucrezia , Bechini Giovanni
Published: October 2022

Nowadays, the importance of gas turbine monitoring and diagnostics pushes OEM operators to exploit historical data to search for early indicators of incipient failures.

One of the most disrupting events that affect GT operation is gas turbine trip, since its occurrence causes a reduction of equipment remaining useful life as well as revenue loss because of business interruption. Thus, early detection of incipient symptoms of gas turbine trip is crucial to ensure efficient operation and lower operation and maintenance costs.

This paper presents a data-driven methodology aimed at investigating and disclosing the onset of trip symptoms. The goal of the methodology is the identification of the time point at which trip symptoms are triggered, by exploring multiple scenarios characterized by different trigger positions. For each scenario, a time window of the same length is considered before and after the trigger time point. For classification purposes, the former is supposed to be representative of normal operation and thus is labeled as “No trip”, whereas the latter is labeled as “Trip”. A Long Short-Term Memory (LSTM) neural network is employed as the classification model. A predictive model is first trained for each scenario and subsequently tested on new observations (i.e., trip events) by considering the whole available timeframe. Finally, trips are clustered into homogeneous groups according to their most likely trigger position, which identifies the time point of onset of trip symptoms.

The methodology is applied to two real-world case studies composed of a collection of trips, of which the causes are different, taken from various fleets of Siemens gas turbines. Data collected from multiple sensors during three days of operation before trip occurrence are employed and analyzed.

The methodology provides the most likely trigger position for four clusters of trips within the two days before trip occurrence with a confidence in the range 66%–97%.
Application of Adjoint-Enhanced First Order Second Moment Method for Robust Design Optimization of a High Pressure Compressor Rotor
Authors: Dittmann Max , Schmidt Robin , Meyer Marcus
Published: October 2022

This paper applies the computationally advantageous combination of a differential analysis technique and the adjoint method in the context of uncertainty quantification to the gradient-based robust design optimization of aerofoils. First, the accuracy and feasibility of the method is evaluated using an analytical test function. The method is subsequently applied to the robust design optimization of a rotor blade under geometric uncertainty, that is based on a parametrized population of optically 3D scanned blades. A significant improvement of the robustness w.r.t. the isentropic efficiency of the rotor is reported.
The Influences of Trailing Edge Base Pressure on the Profile Loss of Low-Pressure Turbine Blades
Authors: Kodama Hidekazu , Funazaki Ken-Ichi
Published: October 2022

The knowledge of the pressure at the base of the trailing edge (base pressure) is important for the prediction of profile loss of low-pressure turbine (LPT) blades. It is known that base pressure varies with a surface velocity profile and a Reynolds number, and that a lower base pressure increases the losses, and a higher base pressure decreases the losses. However, in the airfoil design, it is difficult to predict the base pressure. This is because the physical mechanism to determine the base pressure level is not well understood. This paper aims at clarifying the mechanism to determine the base pressure level for a low speed LPT cascade and investigating the influences of the base pressure on the profile loss by using the results of numerical simulations. In the study, two kinds of LPT blade design with different levels of the base pressure are used. One is the design with the base pressure higher than the far downstream static pressure (Blade A) and another is the design with the base pressure lower than the far downstream static pressure (Blade B). Based on the findings from the study, the correlation parameter BP calculated by the numerical simulation results is proposed for the prediction of base pressure. The prediction method using the parameter BP is applied to low speed cascade test data and compared with the measured base pressure. It is concluded that the method reasonably predicts the base pressure for blades with no laminar separation and predicts the base pressure within the range of an engineering allowable error for blades with a closed separation bubble.
Autoignition Delay Time Measurements and Chemical Kinetic Modeling of Hydrogen/Ammonia/Natural Gas Mixtures
Authors: Baker Jessica B. , Rahman Ramees K. , Pierro Michael , Higgs Jacklyn , Urso Justin , Kinney Cory , Vasu Subith
Published: October 2022

In recent years, hydrogen-carrying compounds have accrued interest as an alternative to traditional fossil fuels due to their function as zero-emission fuels. As such, there is interest in investigating hydrogen-carrying compounds to improve understanding of the fuels’ characteristics for use in high-pressure systems. In the current study, the oxidation of ammonia/natural gas/hydrogen mixtures was carried out to study CO formation profiles as well as the ignition delay times behind reflected shock waves in order to refine chemical kinetic models. Experiments were carried out in the University of Central Florida’s shock tube facility by utilizing chemiluminescence to obtain OH* emission and laser absorption spectroscopy to obtain CO profiles. Experimental results were then compared with the GRI 3.0 mechanism, as well as the proprietary UCF 2022 mechanism utilizing CHEMKIN-Pro software. In general, both models were able to capture the trend in autoignition delay times and CO time histories for natural gas and ammonia mixtures. However, for ammonia-hydrogen mixtures, GRI 3.0 failed to predict ignition delay times whereas the UCF 2022 mechanism was able to capture the IDTs within the uncertainty limits of the experiments. A sensitivity analysis was conducted for different mixtures to understand the important reactions at the experimental conditions. Finally, a reaction pathway analysis was carried out to understand important ammonia decomposition pathways in the presence of hydrogen and natural gas.
Numerical Investigation of the Effects of Tip Clearance and Rotor Cavities on the Performance of a 1.5-Stage High-Work Turbine
Authors: Hansen Thorsten , Munktell Erik , Scheuerer Georg , Zwiener Kim
Published: October 2022

The paper presents CFD simulations about the effects of tip clearance and rotor cavities on the performance of a 1.5-stage high-work turbine. The experimental test case to which the simulation results are compared is the 1.5-stage unshrouded high-work turbine investigated by Behr et al. [1]. The authors performed the simulations on three homogeneously refined meshes. The results on the three meshes were used to quantify discretisation errors applying Richardson extrapolation. The mesh-related discretisation errors for the mass flow rate, total pressure ratio, and mechanical efficiency were below 0.2 % on the finest mesh. The simulation results agreed well with the experimental data. In particular, the loss topologies induced by the hub and shroud passage vortices and the tip-leakage vortex compared well to the measured contours. The simulations showed a consistent and realistic response to the addition of the rotor cavities, with increased mass flow rates and reduced efficiency, shifting the hub passage vortices upwards. Increasing the tip gap decreased the efficiency and strengthened the tip-leakage vortex; decreasing the tip gap had the opposite effect.
Breaking 70% Net Electric Combined Cycle Efficiency With CMC Gas Turbine Blades
Authors: Martelli Emanuele , Girardi Marco , Chiesa Paolo
Published: October 2022

This work focuses on the optimal cycle design criteria for heavy duty gas turbines employing CMC blades. A combined cycle model with detailed description of the cooled gas turbine is used to determine the optimal Turbine Inlet Temperature and pressure ratio for two values of maximum allowed CMC wall temperatures (1300, 1500 °C). Results indicate that the maximum efficiency of the combined cycle is achieved with pressure ratio above 60, posing the need of adopting two shafts for the gas turbine and high temperature materials for the high-pressure compressor. Net electric efficiency above 70% (LHV basis) can be approached with CMC blade wall temperatures of 1300 °C (TIT = 2000 °C) and efficiency values approaching 72% with CMC blade wall temperatures of 1500 °C and TIT of 2200 °C. Compared to current H-class gas turbines, the simple cycle efficiency increases by about 10 percentage points (> 53% vs. 43% LHV basis) and the net specific work/net power output by 66%–110% depending on the pressure ratio and TIT. The most critical challenge to achieve such performance appears to be the temperature gradient across the blade wall and the temperature capability (maximum surface temperature and temperature gradient) of the blade coatings.
Ignition Chemistry of Syngas Highly Diluted in CO2
Authors: Cooper Sean P. , Mathieu Olivier , Mohr Darryl J. , Petersen Eric L.
Published: October 2022

Syngas is a desirable, high-hydrogen fuel source for combustors utilizing the Allam-Fetvedt cycle involving supercritical-CO2 (sCO2). Minimal data are available with high concentrations of CO2, with much of the available data being few in number and reported over very narrow temperature ranges. Considerable model disagreements have been shown at intermediate pressures (10–100 atm), while good agreement is seen for higher pressures (> 100 atm). Further examination of literature data highlights that the ignition delay time (IDT) characteristics of syngas at these pressures show little dependence on pressure, mixture composition, and equivalence ratio. To verify these observations, literature experiments were replicated using a high-pressure shock tube facility. Ignition delay time data were collected for syngas mixtures for pressures of 20 and 40 atm with 85% CO2 mixtures at stoichiometric conditions and H2:CO fuel ratios of 1:1 and 1:4. Literature results are limited to IDTs less than 500 μs, and data presented herein expand these data to considerably longer IDTs over a wider temperature range. Some disagreement with literature data is seen, and sources of discrepancy from the literature results are discussed. However, similar trends are seen for syngas ignition delay time characteristics and chemical kinetic models do not replicate this behavior. In particular, AramcoMech 2.0 replicates IDT near 40 atm for a H2:CO fuel ratio of 1:1, but is significantly under reactive for the 1:4 fuel ratio. To this end, a detailed sensitivity analysis using the AramcoMech 2.0 chemical kinetics mechanism highlights important chemical reactions. Of these reactions, significant model improvements are shown using the reaction rate suggested by Tsang and Hampson for CO+HO2 ⇌ CO2+OH.
Stress Blended Eddy Simulations for Hydrogen-Air Rotating Detonation Engine With Wall Heat Transfer
Authors: Elasrag Hossam A. , Gallagher Timothy , Rankin Brent A. , Schumaker S. Alexander
Published: October 2022

Hybrid stress blended eddy simulations (SBESs) are performed for the AFRL rotating detonation engine burner with detailed finite rate chemistry. The near wall boundary layer flow is modeled using URANS k-ω-SST model and the turbulent internal flow is modeled using Large Eddy Simulation (LES) dynamic Smagorinsky model. Using one-dimensional (1D) detonation tubes, the chemical mechanism and the chemistry integration algorithm are validated against the Chapman Jouguet (CJ) conditions and the Zel’dovich-von Neumann-DÖring (ZND) calculations with reasonably good accuracy. The impact of spatial and temporal resolutions on the wave propagation speed (WPS) and the von Neumann (VN) peak is studied using 1D detonation tube simulations. Using second order (SO) spatial accuracy, the SO temporal scheme over-predicts the VN peak with numerical dispersion near the pressure’s peak, while the first order (FO) temporal scheme is found to add the correct numerical dissipation. The SO and FO profiles show identical wave structure at different resolutions in the expansion region downstream of the compression shock’s peak. With FO temporal scheme, the VN peak was found to vary within 2%–6% band within a spatial resolution range of 0.5–0.001 mm and within 2%–20% band within a temporal resolution range of 1E−7 s – 1E−8 s. The spatial and temporal resolutions, however, are found to have a smaller impact on the WPS (varies within 2%–5%). For three-dimensional (3D) simulations the SO and FO comparisons with data were overall comparable. The results show very good agreement with the measurements mean static pressure data, WPS, specific thrust and specific impulse. The SO temporal scheme predicted a WPS with a mean error of approximately 11%–10% while the FO temporal scheme mean error is 6%. Back-flow due to the detonation wave in the air and fuel plenums are quantified to be 10% and 15%, respectively. The impact of wall heat transfer modeling on the detonation wave is studied. As the wall temperature increases more deflagration burning in the refill zone occurs ahead of the detonation wave. The fraction of heat released by deflagration and detonation is quantified for each case using cut-off values of 5 atm and 10 atm. Although the impact of wall heat transfer on WPS is found to be small, the fraction of heat released by deflagration increased by 10% for 600 K isothermal wall temperature compared to the 300 K isothermal wall temperature. For local static temperatures > 300 K, the fraction of heat released in the deflagration mode was 75%, 53%, and 40%, for the adiabatic walls, 600 K, and 300 K isothermal walls, respectively. The current numerical study shows that accurate wall heat transfer modeling is important for rotating detonation engine (RDE) numerical simulations.
Characterization of an Apparatus to Study Solid Deposit Formation in Lubricating Oils at High Temperatures
Authors: Juárez Raquel , Gutierrez Noble , Petersen Eric L.
Published: October 2022

When exposed to excessively high temperatures, the lubricating oils in turbines undergo extreme oxidation and thermal degradation processes that eventually result in the formation of solid deposits that can, in turn, cause operational problems. This work describes the testing and characterization process of a newly constructed test rig that studies the propensity of lubricants to form such deposits. In the test rig, the lubricant flows through a heated tube, thermally degrades or oxidizes, and forms solid deposits that adhere to the heated surface. An oil flow rate calibration curve as a function of pump speed was developed using two oils, Castrol GTX 20W-50 and Mobil DTE 732. The steady-state axial temperature distribution along the heated tube was measured for temperature settings of up to 475°C and flow rates of 10.4, 12.2, and 13.9 mL/min using Mobil DTE 732. The results showed that the surface temperature is not constant, but that it varies axially as is expected from the heat transfer in the thermal entrance region of a pipe with laminar flow. Examples of the results of tests performed with the apparatus using Mobil DTE 732 and Castrol GTX 20W-50 flowing at 10.4 mL/min with surface temperatures between 328 and 489°C are presented. Based on these tests, it is clear that the induction time for solid-deposit formation can be unambiguously determined using the experimental apparatus. The induction times of the sample tests described were found to be 638 ± 10 min and 86 ± 10 min for Mobil DTE 732 and Castrol GTX 20W-50, respectively.
Axial Compressor Map Generation Leveraging Autonomous Self-Training AI
Authors: Burlaka Maksym , Moroz Leonid
Published: October 2022

The paper describes the study performed by SoftInWay in the scope of the Phase I SBIR project funded by NASA.

The project was dedicated to a study of optimization of the variable geometry reset angle schedules with the use of innovative autonomous AI technology. In the scope of the project, an automated compressor performance data generation workflow was developed. Three highly loaded multistage axial compressors were designed. The developed workflow was used to generate the training, validation, and test data sets for all three compressors. Multiple different architectures of artificial neural networks were studied, and parametric models for the representation of performance speedlines were developed.

Utilizing the developed approaches, the artificial neural networks were trained for all three compressors to predict their performance with a relative error below 3 %. The trained neural networks were successfully used in the optimization of the variable inlet guide vanes and variable stator vanes reset angle schedules with a relative error of total-to-total pressure ratio prediction below 2 % for most of the points and relative error of total-to-total efficiency prediction below 1 % for all the points of the operational line.

The capability of the developed AI models to accurately predict the optimal combination of reset angles and efficiency of the axial compressor with multiple vanes controlled independently allowed doing quick evaluations of efficiency and stability margins. The availability of such information enables the opportunity to make technical-economical decisions about the reasonability of implementation of independent variable vanes and their number during engine system analysis.
Particle Image Velocimetry in a High-Speed Short-Duration Turbine Rig
Authors: Okada Mizuki , Pinho Jorge , Cernat Bogdan , Lavagnoli Sergio
Published: October 2022

This article presents the implementation of a Particle Image Velocimetry (PIV) into the high-speed short-duration rotating turbine facility of the von Karman Institute. The advantage of PIV as a whole field measurement is emphasized in such circumstances for which the use of optical technique can drastically reduce the number of tests and the need for multiple intrusive expensive probes, ultimately lowering the testing cost.

Practical solutions were demonstrated that address various challenges for the effective application of PIV. An endoscope delivered the laser sheet to the region of interest and a planoconcave window provided optical access for the measurement in the annular test section. Sub-micron scale oil droplets were seeded into main flow through custom-made probes located upstream the nozzle guide vane and into a pipe line supplying rim seal purge flow. A high-speed laser system and a high-speed camera were synchronized at 1 kHz sampling rate. Complementary measurements and dedicated image processing were performed to ensure the synchronization of the PIV images with the rotor position that was monitored through an encoder.

The region of interest was the blade-to-blade plane at the 58% span turbine exit on a rectangular field of view covering approximately one rotor pitch and 0.15 rotor axial chord from the rotor trailing edge. Phase-locked-average velocity fields are obtained from PIV and compared against steady-state Reynolds Averaged Navier Stokes (RANS) simulations along with four-hole probe measurement results. Together with an uncertainty analysis, the results demonstrate the promising robustness and accuracy of PIV. A practical guideline for PIV application in such kind of turbine test rigs is provided as a conclusion of the paper.
The Effect of Diffuser Shape for Film Cooling Holes With Constant Expansion Angles and Area Ratio
Authors: Lundburg Evan , Lynch Stephen , Liu Kevin , Xu Hongzhou , Fox Michael
Published: October 2022

Shaped film cooling holes are used in gas turbine components to deliver coolant to the high temperature surfaces of turbine blades and vanes to improve their durability. In general, shaped holes are created by expanding the outlet of the hole, resulting in a large area at the outlet of the hole that diffuses the flow. It has been shown in past studies that increasing the diffuser outlet to meter inlet area ratio causes a lower average momentum of the coolant jet at the hole exit, thereby producing better cooling performance. Instead of increasing the size of the diffuser section by increasing the area ratio, the present study focuses on changing the cross-section shape of the diffuser. This is done to mimic changes observed in the diffuser shape of conventionally manufactured film cooling holes. The present study utilizes 10-10-10 diffuser expansion angles and maintains a constant diffuser to meter area ratio. However, the diffuser shape is varied by changing the diffuser edge angle, κ, located between the diffuser sidewall and the diffuser downstream wall. Three film cooling hole shapes were tested using three different diffuser edge angles, resulting in a narrow outlet, a wide outlet, and a standard outlet film cooling hole. Each hole shape was tested in a large wind tunnel with coolant supplied to the film cooling holes at three different blowing ratios by a co-flow and counterflow delivery channel, similar to the delivery method in a turbine vane with an internal baffle. In addition, the film cooling holes were tested with simulated diffuser roughness. Adiabatic effectiveness measurements indicate that film cooling hole performance is most impacted by diffuser roughness. The film cooling hole shape arising from the diffuser edge angle directly impacts the sensitivity to blowing ratio and coolant feed direction. Therefore, it is recommended that manufacturing of film cooling holes focus on reducing roughness in the diffuser for the highest performance. It is also recommended that the tolerance of the film cooling hole shape be biased towards wider film cooling holes to minimize sensitivity to the blowing ratio and coolant feed direction.
Novel UAS Propeller Design Part 1: Using an Unloaded Tip to Reduce Power Requirements and Lower Generated Sound Levels for Propellers Designed for Minimum Induced Drag
Authors: Wisniewski Charles , Van Treuren Kenneth
Published: October 2022

Today, over 500 eVTOL vehicles are being developed for everything from Urban Air Mobility to package delivery. In addition, Unmanned Aircraft Systems (UASs) are used in a variety of applications to include intelligence, surveillance, and reconnaissance (ISR) as well as resupply for the military. Most of these vehicles utilize electric propulsion systems with propellers. Efficient propellers with reduced noise generation are necessary for these vehicles to coexist in urban environments or be used for ISR missions. Traditional propeller design uses Blade Element Momentum Theory (BEMT) to minimize induced losses. The current investigation describes a novel propeller design approach that minimizes induced drag by reducing the thrust loading near tip. This reduces the tip vortex strength lowering the Sound Pressure Level (SPL) and power required. A parametric study of 18 propellers examined performance using traditional BEMT design near the hub and progressing to zero thrust loading at the tip. A microphone on a traverse mounted behind the propeller was used to measure the peak SPL and its location in a wind tunnel. The best performing unloaded tip propeller showed 5.2% reduction in power required and 12 dB SPL reduction compared to a commercial propeller at the same test conditions.
Combined Heat and Power Supply Demonstration of Micro-Mix Hydrogen Combustion Applied to M1A-17 Gas Turbine
Authors: Horikawa Atsushi , Ashikaga Mitsugu , Yamaguchi Masato , Ogino Tomoyuki , Aoki Shigeki , Wirsum Manfred , Funke Harald H.-W. , Kusterer Karsten
Published: October 2022

Kawasaki Heavy Industries, Ltd. (KHI), Aachen University of Applied Sciences, and B&B-AGEMA GmbH have investigated the potential of low NOx micro-mix (MMX) hydrogen combustion and its application to an industrial gas turbine combustor. Engine demonstration tests of a MMX combustor for the M1A-17 gas turbine with a co-generation system were conducted in the hydrogen-fueled power generation plant in Kobe City, Japan.

This paper presents the results of the commissioning test and the combined heat and power (CHP) supply demonstration. In the commissioning test, grid interconnection, loading tests and load cut-off tests were successfully conducted. All measurement results satisfied the Japanese environmental regulation values. Dust and soot as well as SOx were not detected. The NOx emissions were below 84 ppmv at 15 % O2. The noise level at the site boundary was below 60 dB. The vibration at the site boundary was below 45 dB.

During the combined heat and power supply demonstration, heat and power were supplied to neighboring public facilities with the MMX combustion technology and 100 % hydrogen fuel. The electric power output reached 1800 kW at which the NOx emissions were 72 ppmv at 15 % O2, and 60 %RH. Combustion instabilities were not observed. The gas turbine efficiency was improved by about 1 % compared to a non-premixed type combustor with water injection as NOx reduction method. During a total equivalent operation time of 1040 hours, all combustor parts, the M1A-17 gas turbine as such, and the co-generation system were without any issues.
Evaluation of the Performance of a 200kW Recuperated Gas Turbine Engine Operated on Up to 30% Hydrogen Added to Natural Gas
Authors: Villatoro Walther , McDonell Vincent , Hu Ray , Ayers Don
Published: October 2022

Achieving carbon neutrality and sustainability has prompted research efforts into reducing carbon dioxide and carbon monoxide emissions in combustion systems without significantly increasing nitrogen oxides (NOx). Because of its high specific energy density, hydrogen is a prime candidate to eventually replace natural gas. Presently, using renewable hydrogen mixtures in natural gas turbines can reduce carbon dioxide and carbon monoxide emission as a step towards carbon neutrality. In the present effort, a commercial natural gas 200kW microturbine generator produced by Capstone Green Energy Corporation is used to study the impact of hydrogen addition to natural gas. Exhaust emissions, injector temperatures, and power output are evaluated for increasing levels of hydrogen in the fuel mixture. Relative to emissions on 100% natural gas, the experimental results show that, for full load, carbon monoxide and carbon dioxide emissions decrease as the amount of hydrogen in the fuel mixture increases while the NOx emissions increase. This is attributed to the increased reactivity of hydrogen resulting in a jet flame stabilized closer to the injector exit as the amount of hydrogen is increased. While the injector tip temperatures increased modestly with additional hydrogen, visual inspection of the injectors following hydrogen addition tests indicate that flashback or combustion inside the injectors was not occurring. This is consistent with the modest changes in NOx with hydrogen addition. If flashback were occurring, the emissions levels would be expected to be significantly higher. The results also demonstrate that added hydrogen to the fuel does not have a statistically significant impact on overall system efficiency. In general, the results show that the injectors can withstand the temperature increase from hydrogen addition while maintaining low emissions at full load, indicating that addition of hydrogen is possible without any modification to this gas turbine engine which has been optimized for performance on natural gas. Parametric studies were carried out to demonstrate how modest changes in engine operating points and fuel injectors can be incorporated to attain single digit NOx levels with up to 30% hydrogen.
Two Stage Radial Compressor for a Kilowatt Scale Supercritical Carbon Dioxide Power Block: Design Considerations
Authors: Seshadri Lakshminarayanan , Patel Ashutosh , Biradar Vijay , Kumar Pramod , Gopi Pramod Chandra
Published: October 2022

Potential applications of kilowatt (kW) scale supercritical Carbon dioxide (sCO2) Brayton power systems include exhaust waste heat recovery in diesel engines and bottoming cycles for biomass gasifier-driven gas turbines. However, the practical realization of the kW scale single-stage sCO2 turbomachinery is associated with several challenges. While a hermetically sealed arrangement is most beneficial, it is fraught with large windage losses in the motor rotor cavity. The high windage losses in the single-stage setup are primarily due to high shaft speeds. These issues limit the commercial utility of kW scale sCO2 Brayton power cycles. In order to mitigate these issues, the design of a two-stage unshrouded radial compressor unit for a kW scale sCO2 Brayton cycle is described in this study. The working pressures are 103 bar/ 170 bar, and the compressor mass flow rate is 2 kg/s. These optimal operating pressures are obtained taking into consideration both thermodynamic power and motor/disc windage losses. The two rotors are mounted on a single shaft in a double-ended configuration where the axial thrusts act in opposite direction. The compressor blade profiling is carried using a standard design tool. The performance of the compressor is assessed using a 1D mean-line calculation to estimate the overall efficiency. Using this approach, an overall compressor efficiency of ∼ 71.8 % (including motor and disc windage losses) is attained. A 3D CFD simulation is carried out at the design point to validate 1D data. Rotodynamic analysis is carried out to determine the first and second shaft critical speeds of the rotor-shaft configuration. Structural analysis is carried out to ensure that the maximum Von-Mises stresses are well below the material limits.
The Role of Turbine Operating Conditions on Combustor-Turbine Interaction – Part 1: Change in Expansion Ratio
Authors: Notaristefano Andrea , Gaetani Paolo
Published: October 2022

Aeroengine lean-burn combustors release vorticity and temperature perturbations that, interacting with the first turbine stage, impact the stage aerodynamics, the blade cooling and noise production. The first of these issues is addressed in this paper that is part 1 of a two-fold contribution. A detailed experimental analysis is carried out to study the impact on the combustor-turbine interaction of the off-design conditions experienced by aero-engines in their duty.

Engine-representative disturbances are generated by a combustor simulator able to produce swirling entropy waves. Two injection positions and four injection cases are studied. Experimental measurements are carried out at three traverses: upstream of the stator, at the interstage, and downstream of the rotor.

This paper analyses the effect of the stage expansion ratio: two values are studied, namely 1.4 and 1.76, representative of subsonic and transonic flow conditions. They are chosen imposing similar velocity triangles at the rotor inlet. Results show that the swirl profile considerably impacts the stage aerodynamics. The aerothermal flow field downstream of the stator is modified significantly by the combustor disturbances. Conversely, downstream of the rotor, the differences in aerodynamics lessen. However, the entropy wave persists at the stage outlet and its transport depends on both the operating point and the injection position.
A Novel Engine Architecture for Low NOx Emissions
Authors: Blondeel Tim , Yin Feijia , Gangoli Rao Arvind
Published: October 2022

The fuel efficiency of turbofan engines has improved significantly, hence reducing aviation’s CO2 emissions. However, the increased operating pressure and temperature for fuel efficiency cause adverse effects on NOx emissions. Therefore, a novel engine concept, which can reduce NOx emissions without affecting the cycle efficiency, is of high interest to the aviation community. This paper investigates the potential of an intercooler and inter-turbine burner (ITB) for the future low NOx aircraft propulsion system.

The study evaluates performance and NOx emissions of four engine architectures: a very high bypass ratio (VHBR) turbofan engine (baseline), a VHBR engine with intercooler, a VHBR engine with ITB, and a VHBR engine with both intercooler and ITB. The cycles are optimized for minimum cruise Thrust Specific Fuel Consumption (TSFC), considering the same design space, thrust requirements, and operational constraints. The ITB is only used during take-off to minimize cruise fuel consumption. The analysis shows that using an ITB solely, with the energy split of 75% (the first burner) / 25% (ITB), reduces the cruise NOx emission by 26%, and the cruise TSFC slightly by 0.5%. The intercooler alone reduces the NOx emissions by 16% and the cruise TSFC by 0.8%. The combination of intercooler and ITB reduces the NOx emissions further by 38%. The analysis confirms that introducing an intercooler and ITB can potentially resolve the contradicting effects of fuel efficiency and NOx emissions for the future advanced turbofan engine.
Improved Design Space Exploration by Machine Learning Estimation for a Parametric Turbofan Model
Authors: Foster Chad , Moore Jack
Published: October 2022

In the preliminary design phase, selecting an architecture requires understanding the influence of noise parameters while exploring the design range. Given that architecture simulations are slow, the normal approach is to generate meta-models or fast running models that are sufficiently accurate. These meta-models are used to efficiently select robust designs by quickly estimating the impact of noise distributions for different design independents.

The creation of these meta-models is efficiently accomplished by designed experiments. Predetermined runs are decided by factorial experiments, Latin hypercube designs, or sequential experimentation. These simulation runs have combinations of independents and noise parameters that are outside of previous experience. Without accurate initial estimates the simulations may not converge or have long simulation times. A method is presented to improve these simulations through more effective initial estimates.

The estimates are created by a data-based, machine learning, model that is created from previous successful runs. This additional model is used to make accurate initial condition guesses.

By using a chained linear regression model, the initial conditions for half the parameters are improved compared with using the previous run. The resulting meta-model can be used for additional design studies. It is also shown that improvement may not translate into significant run time differences as single parameter estimates may drive convergence.
Experimental and Numerical Assessment of Liquid Film Thickness Under High-Speed Gas Flow
Authors: Okui Takayuki , Mizumi Shunsuke , Tabata Soichiro , Mizushima Yuki , Sanada Toshiyuki
Published: October 2022

For improving steam turbines, we need to solve pending issues derived from the gas-liquid two-phase flow phenomena, i.e., turbine blade erosion and wetness loss. These troubles result from the droplets atomized from the liquid-film flow on the blade. Although the most conventional study has successfully analyzed the liquid film that spreads all over bottom floor of the channel, the blade surface’s liquid film is supposed to be a rivulet flow. Therefore, analysis considering the rivulet-like condition has significance for accurate estimation of the liquid-film flow on the blade contributing to the atomization of the droplets.

In this study, we demonstrated thickness measurement for the rivulet-like film flow using an Optical-fiber-based Reflective Probe (ORP). Airflow (jG = 40∼75 m/s) was introduced into the rectangle test section, then a small amount of tap water (Q = 5 ∼ 15 × 10−7 m3/s) was injected on the channel plate. We measured the thickness of the liquid film simultaneously by using ORP and high-speed visualization. Further, we also estimated the thickness of the liquid-film flow by CFD using ANSYS Fluent. We will discuss their qualitative agreement and tendency, which describe some differences from similar previous experiments. At last, whether the ORP measurement applies to the rivulet-like film flow or not is validated, including its uncertainty, experimentally and numerically.
Numerical Analysis of an Axial High Pressure Partial Admission Turbine
Authors: Hu Heng , Askebjer Per , Genrup Magnus , Hushmandi Narmin
Published: October 2022

The transformation of the energy systems will call for new turbomachinery designs for harvesting low-grade heat. The word “low-grade” has many definitions and ranges from levels where steam is the preferred selection to very low grades where specially adopted ORC technology is used. In this work, turbomachinery for cascading the steam cycle with a very low-temperature ORC technology is studied. The steam turbine being investigated here is, in principle, a highly-loaded small backpressure turbine. One indeed important feature is the trade-off between losses related to partial arc admission and losses due to having very short blades. This sets the overall architecture of the turbine and is one of the most prominent selections during the design. Due to the special working/boundary conditions, a supersonic axial turbine model and partial-arc admission were necessary. Previous open literature had shown better performance with one admitted arc. A full three-dimensional analysis of such a turbine model using CFD techniques is rather challenging. Due to the special character of the supersonic flow and the interaction of possible shocks with boundary layer flow, a fine grid is required for this application. On the other hand, due to the unsymmetrical feature of the geometry imposed by the existence of only one blocked arc, modeling of the full annulus of the turbine is unavoidable. Computations are done to assess the characteristics of the flow inside threedimensional supersonic turbine using commercial CFD codes. The unsteady loads of the rotor blades are analyzed both in time and frequency domains.
Analysis of Ultra-High Bypass Ratio Turbofan Nacelle Geometries With Conventional and Short Intakes at Take-Off and Cruise
Authors: Magrini Andrea , Bousi Denis , Benini Ernesto
Published: October 2022

Turbofan engines with ultra-high bypass ratio require more aggressive nacelle designs to limit the drag and weight associated to the increased fan diameter. Reducing the inlet length-to-diameter ratio appears a possibility for further weight saving and mitigation of the total pressure losses. However, these benefits are counteracted by the higher fan exposure to nonuniform in-flow conditions. In this paper, we compare three nacelle geometries for an ultra-high bypass ratio turbofan with a progressively shorter intake having a length-to-diameter ratio of 0.475, 0.35, and 0.22 and two different compact cowl designs. The complete isolated nacelle model, including the exhaust system and a pylon, is simulated numerically with the fan rotor and stator cascades represented through a body force method, allowing to capture the engine/nacelle interaction. The three geometries are examined at take-off and cruise, measuring the variation of integral metrics related to the inlet performance and the propulsive forces. Although with shorter intakes the total pressure losses and the separation onset are positively impacted at high incidence, a careful shaping of the lip is needed to control the diffusion in the limited axial extension available. The effect on the propulsive forces is found to be limited at take-off for a fully attached flow, whereas at cruise the gross thrust improvement of the more compact nacelle is counterbalanced by a larger drag.
Laser Additive Manufacturing of Iron-Aluminum for Hybrid Steam Turbine Blades
Authors: Rittinghaus Silja-Katharina , Hama-Saleh Rebar , Brunn Oliver , Salit Victor , Mokulys Thomas
Published: October 2022

The optimization of steam turbine rotor blades is strongly restricted by centrifugal stresses. To reach higher rotational speed or to obtain larger airfoils it is desirable to realize blade designs with very light, but robust blade tips. Hence, the aim for a composite material design of a turbine blade is to investigate a new method of providing raw material for turbine blades which consists out of standard turbine steel around the root section and much lighter material on the outer diameter of the blade.

Iron aluminide alloys are of increasing interest as a structural material for lightweight construction in hot or corrosive environments due in part to the good and economical availability of the main alloying elements. Currently, the feasibility is being tested of replacing component areas with FeAl through hybrid construction, depending on local load requirements, and thus achieving effective weight reduction. The additive process laser-based direct energy deposition (L-DED) of Fe-28Al is investigated to produce hybrid material consisting of 12% Cr turbine steel and Fe-Al.

Parameters and build-up strategies are varied in order to produce a crack-free and low-stress connection of the material partners while complying with given thermal boundary conditions. Thermography is used to achieve homogeneous process conditions when scaling to component size. Microstructure, hardness and chemical composition of the hybrid material are investigated as well as mechanical strength. It is shown that crack-free machining of test specimens and a component blank is possible after heat treatment.
Towards Fast Prediction of Flame Stability and Emissions of mGT Combustion Chambers: a Chemical Reactor Network Approach
Authors: Savarese Matteo , Bompas Jérémy , De Paepe Ward , Parente Alessandro
Published: October 2022

Hydrogen as energy carrier, in combination with dry, low-NOx micro gas turbines (mGTs), is receiving increasing attention, since it can represent an attractive solution for a low-carbon, highly efficient and decentralized energy restitution system. However, accommodating hydrogen-enriched fuel blends in already existing combustion technologies is not an easy task, due to the increased reactivity of this fuel and the higher associated NOx emission. In this framework, reliable numerical models are needed to assist the industry towards the re-design of existing combustion assets. In particular, predicting thermal efficiency and pollutant emissions is of crucial importance for assessing the performances of a given system, since energy production facilities will have to cope with more stringent regulations. Alternative, physics-based modeling tools, such as Chemical Reactor Networks (CRNs), can represent an appealing solution for fast and reliable predictions of overall combustion efficiency and pollutants emissions. This tool allows to represent complex reactive flow fields as a series of idealized reactor models, thus drastically reducing the computational cost of the simulations. For this reason, detailed kinetic schemes can be employed. Detailed chemistry is crucial for reliable pollutants predictions, especially for NOx and CO, since the formation of those chemical species follows complex chemical pathways. The use of CRNs is quite frequent in literature, especially for performing parametric studies for gas turbines applications. The design of an equivalent CRN for a given combustor is based on the manual observation of the main flow-field features, which can be obtained from experiments or CFD data. The aim of this paper is to construct a highly simplified CRN of a typical mGT, Turbec T100 combustor, in order to perform design exploration studies. The effect of the main operating parameters, such as equivalence ratio, air inlet temperature and fuel composition was studied. Moreover, part load conditions were also simulated. Results highlighted that hydrogen addition does not systematically leads to increased pollutants emissions, since due to its higher reactivity, it offers the possibility to operate in leaner conditions, with respect to natural gas. The obtained results aim at providing useful guidelines for further experimental or detailed numerical design explorations for identified interesting conditions.