Next-generation Gas Turbine Research Articles

High-throughput composition screening of high-entropy rare-earth monosilicates for superior CMAS corrosion resistance up to 1873 K

Superior calcium-magnesium-aluminum-silicate (CMAS) corrosion resistance is the key to the applications of high-entropy rare-earth monosilicates (HEREMs) as environmental barrier coatings (EBCs) for next-generation gas turbine engines. Here, we report a high-throughput composition screening strategy to achieve superior CMAS corrosion resistance in HEREMs for the first time. Specifically, we first fabricate ten kinds of (Ho0.25Yb0.25Lu0.25X0.25)2SiO5 (X = Sc, Tm, Er, Y, Dy, Gd, Eu, Sm, Nd, and La) HEREM samples using a high-throughput pressure-less sintering technique, and subsequently screen their CMAS corrosion resistance at 1673 K via a high-throughput testing apparatus. Among all the as-fabricated samples, the HEREM-Er samples are found to have the best CMAS corrosion resistance, even up to 1873 K, which outperforms the EBC materials that have been reported. Such superior CMAS corrosion resistance of the HEREM-Er samples is primarily attributed to the formation of a dense apatite barrier layer, which results from its decreased melting point, as confirmed by first-principles calculations and molecular dynamic simulations. Our work provides a guide for designing exceptional anti-CMAS corrosion high-entropy rare-earth silicates.

A Bayesian optimization framework for the control of combustion instability of a bluff-body stabilized combustor

Control of combustion instability for a realistic gas-turbine combustor is challenging. This work aims to establish an efficient numerical framework for optimization to improve the combustion stability of a bluff-body combustor. Large eddy simulations of the spray combustion process are conducted, and the experimental measurements are used to evaluate the numerical accuracy of the baseline case. The air preheating temperature, the Sauter mean diameter of fuel droplets, and the location of liquid fuel injection are regarded as input variables. The root mean square of pressure amplitude is regarded as an optimization objective. The Bayesian optimization framework is proposed that includes the sampling process, surrogate model, acquisition function, and genetic algorithm optimizer processes. It is found that PRMS can be reduced by 64% for the optimized case compared to the baseline case using only 17 sample evaluations. This work is promising as it provides an effective optimization framework for the development of next-generation gas-turbine combustors.

Porous spheres enabling excellent high-temperature abradability and long lifetime for abradable seal coating

Low hardness and high abradability are often required for abradable seal coating (ASC) to protect the blade tip of gas turbines and aeroengines. However, the low bonding strength often lead to spallation failure. In this paper, a novel abradable seal coating with thin-wall porous spheres was designed and prepared. Results show that this coating has both a low hardness of 48.6 HR15Y and a high bonding strength of 25.4 MPa. The abradability and erosion resistance of this coating are 4 times and 63 times higher than reported metal-based coatings. In addition, no surface densification occurs after incursion due to the novel abradable mechanism of crushing of thin-wall sphere. This demonstrates the coating can retain excellent abradability during service and have a long lifetime. The combination of durable excellent abradability and adhesive strength contribute to a long lifetime required by next-generation gas turbines and aeroengines towards higher efficiency and reliability.

Overall cooling characteristics of double-wall effusion system considering material selections and hole arrangements

Enhancing thermal protection of hot components in gas turbines can be accomplished using advanced materials with various thermal conductivities and optimized cooling structures that exhibit high internal-external cooling efficiency. The double-wall effusion system (DWES) stands out as a promising solution for effectively cooling gas turbine hot components. However, its thermal response to new materials remains inadequately explored, and the combined effect of hole arrangement and thermal conductivity within the DWES system has not been thoroughly investigated. In this study, the impacts of material selection and hole arrangement on the overall cooling characteristics of the impingement-pinfin-effusion based DWES were thoroughly investigated. Three different materials with varying thermal conductivities were selected to control Biot number, and various hole arrangements, including effusion-only or impingement-effusion, as well as forward or backward film injection, were compared to quantify the contributions of internal and film cooling on the overall cooling performance. The results showed that the stainless-steel scheme achieves the highest overall cooling effectiveness due to its superior thermal conductivity, while the polycarbonate scheme obtains the lowest overall cooling effectiveness. Compared to the corresponding effusion-only schemes, the area-averaged overall cooling effectiveness of the stainless-steel impingement-effusion scheme demonstrates a remarkable increase of up to 29%. Notably, employing backward film injection yields remarkable improvements in overall cooling effectiveness, with the stainless-steel impingement-effusion scheme exhibiting an increment of up to 16% compared to the forward film injection configuration. Furthermore, the correlations that establish the relationship between the overall cooling effectiveness, Biot number, and blowing ratio were developed for various configurations. These correlations can serve as valuable design tools for the development of next-generation gas turbines that incorporate the DWES technology.

Aerothermal characteristics of thin double-wall effusion cooling systems with novel slot holes and cellular architectures for gas turbines

Double-wall effusion cooling system is a promising cooling solution for next-generation gas turbines. Thinning the blade wall can bring the coolant closer to the hot stream, thereby enhancing overall cooling effectiveness. However, a thin blade wall creates film cooling holes with a short length-to-diameter ratio, which increases jet-crossflow mixing and reduces coolant attachment, inevitably degrading film cooling performance. Herein, we constructed a new thin double-wall effusion cooling system by integrating novel short slot holes for better coolant attachment and triply periodic minimal surface (TPMS) based cellular architectures for improved internal heat transfer. The effects of wall thickness, film hole shape, and internal turbulators configurations were numerically studied. Adiabatic and overall film cooling effectiveness, vortex structures, and aerodynamics loss were comprehensively assessed. The results demonstrate that the novel slot hole significantly improves the overall film cooling effectiveness under thin wall conditions by enhancing the attachment of kidney vortices to walls. Notably, compared to the short fan-shaped hole (scheme B), the novel slot hole (scheme C) shows an improvement in the averaged overall cooling effectiveness of over 20% and a reduction in the total pressure loss coefficient by 60%. Furthermore, three TPMS-based cellular architectures produce similar overall film cooling effectiveness and reduction of total pressure loss coefficient in the range of 0.57-0.81 and 23%-53%, respectively. This work provides guidance for designing thin double-wall effusion cooling systems for next-generation gas turbines.

Bayesian optimization with active learning of design constraints using an entropy-based approach

The design of alloys for use in gas turbine engine blades is a complex task that involves balancing multiple objectives and constraints. Candidate alloys must be ductile at room temperature and retain their yield strength at high temperatures, as well as possess low density, high thermal conductivity, narrow solidification range, high solidus temperature, and a small linear thermal expansion coefficient. Traditional Integrated Computational Materials Engineering (ICME) methods are not sufficient for exploring combinatorially-vast alloy design spaces, optimizing for multiple objectives, nor ensuring that multiple constraints are met. In this work, we propose an approach for solving a constrained multi-objective materials design problem over a large composition space, specifically focusing on the Mo-Nb-Ti-V-W system as a representative Multi-Principal Element Alloy (MPEA) for potential use in next-generation gas turbine blades. Our approach is able to learn and adapt to unknown constraints in the design space, making decisions about the best course of action at each stage of the process. As a result, we identify 21 Pareto-optimal alloys that satisfy all constraints. Our proposed framework is significantly more efficient and faster than a brute force approach.

High-throughput exploration of the WMoVTaNbAl refractory multi-principal-element alloys under multiple-property constraints

Development of next-generation gas turbines requires the design and fabrication of novel high-temperature structural materials capable of operating beyond 1300°C. We propose a high-throughput alloy design framework under multiple-property constraints to discover new refractory multi-principal element alloys (MPEAs) for high-temperature applications. The framework treats the development of MPEAs as a composition-agnostic constraint satisfaction problem, i.e., no prescriptions are made concerning the design space before performing investigatory calculations. We target alloys in the WMoVTaNbAl chemistry space that are predicted to meet constraints on the following properties simultaneously: single-phase stability, density, solidus temperature, yield strength at 1300°C, and ductile-to-brittle-transition temperature. These properties are relevant to both applications in gas turbines and manufacturability. A set of 214 MoNbV-rich alloys meet these relevant constraints. These feasible alloys are investigated with density functional theory (DFT) to provide a fundamental electronic basis for their superior properties. Three compositionally representative alloys from the feasible design space (Mo45Nb35Ta5V15, Mo25Nb50V20W5, and Mo30Nb35Ta5V25W5) are selected with a k-medoids-based design scheme for detailed DFT analysis and experimental characterization. The DFT analysis predicted a single-phase BCC at high temperatures with a high yield strength for all three MPEAs, in agreement with CALPHAD (CALculation of PHAse Diagrams) and experiments, respectively. These three alloys are benchmarked against a public database of 1546 MPEAs. Concerning the aforementioned constraints, the Mo30Nb35Ta5V25W5 alloy outperforms these 1546 MPEAs. The present work demonstrates the ability of the proposed design methodology to identify candidate alloys for a given application under multiple property constraints in a combinatorically vast design space.

The Isothermal Oxidation of a New Polycrystalline Turbine Disk Ni-Based Superalloy at 800 °C and Its Modification with Pre-oxidation

Ni-based superalloys with enhanced oxidation resistance at high temperatures are crucial for next-generation gas turbine engines. A new polycrystalline Ni-based superalloy (C19) that combines improved microstructural stability with environmental resistance has been developed. Its oxidation resistance has been determined through measurements of the specific mass change and morphological evolution of the formed oxides following furnace exposures at 800 °C in air for up to 1000 hours; the results of which were benchmarked against Nimonic 105. C19 showed hybrid Type II/Type III behavior as a marginal Al2O3 former and performed similarly to established superalloys at 750 °C. The Wagner model for the transition from internal to external oxide formation predicted that C19 should form a continuous Al2O3 scale at higher temperatures. A pre-oxidation treatment at 1100 °C for 1 hour was, therefore, selected and shown to dramatically improve the oxidation resistance during subsequent exposure at 800 °C. Oxide cross-sectional analysis showed that C19 formed a continuous and protective Al2O3 scale after the pre-oxidation treatment, whereas Nimonic 105 retained discontinuous Al2O3 finger-like intrusions beneath a Cr2O3 overscale.

Energy and exergy analysis of supercritical/transcritical CO2 cycles for water injected hydrogen gas turbine

This paper investigated the supercritical/transcritical carbon dioxide cycles for water injected hydrogen gas turbine with the advantages of zero carbon emission, low pollution, high efficiency, low cost and so on. This paper selected several typical combined cycles from more than a dozen layouts studied by our team. We constructed a thermodynamic energy and exergy analysis model and verified it with the experimental models from GE. The maximum energy efficiency and its corresponding exergy efficiency that each layout can achieve are obtained after parameter sensitivity analysis, water mixing research and exergy analysis. It can be found that the combined cycle energy efficiency decreases as the water-hydrogen ratio increases. The component with the largest exergy loss is the combustor, accounting for 23.58%. And the transcritical CO2 dual recuperated combined cycle is the best layout with the combined cycle energy efficiency of 64.39% and the combined cycle exergy efficiency of 62.96%. The research done in this paper can provide a basis for the design of next-generation gas turbine.

High-Temperature Solid Particle Erosion in a Melt-Infiltrated SiC/SiC Ceramic Matrix Composite

Abstract Ceramic matrix composites are an enabling propulsion material system that offer weight benefits over current Ni-based superalloys, and have higher temperature capabilities that can reduce cooling requirements. Incorporating ceramic matrix composites into the hot section of gas-turbine engines therefore leads to an increase in engine efficiency. While significant advancements have been made, challenges still remain for current and next-generation gas turbines; particularly when operating in dust laden or erosive environments. Solid particles entrained in the gas flow can impact engine hardware resulting in localized damage and material removal due to repeated, cumulative impacts. In this study, the erosion behavior of a melt-infiltrated (MI) silicon carbide fiber-reinforced silicon carbide (SiC/SiC) ceramic matrix composite is investigated at high temperature (1200 °C) in a simulated combustion environment using 150 μm alumina particles as erodent. Particle impact velocities ranged from 100 to 200 m/s and the angle of impingement varied from 30 to 90 deg. Erosion testing was also performed on α-SiC to elucidate similarities and differences in the erosion response of the composite compared to that of a monolithic ceramic. Scanning electron microscopy was used to study the posterosion damage morphology and the governing mechanisms of material removal.

ɤ-TiAl alloy: revisiting tensile creep deformation behaviour and creep life at 832 °C

ABSTRACT Creep deformation in single-phase ɤ-TiAl alloys manufactured using different processing techniques has been an extensively studied topic owing to the high specific strength and excellent creep properties of these alloys at temperatures between 760 and 1000°C. In addition, these lightweight and creep-resistant alloys are being presently considered as replacements to the comparatively heavier Ni-based superalloys for application in the low-pressure turbine blades of the next-generation gas turbine engines. However, there is limited information on the tensile creep deformation behaviour and creep life of ɤ-TiAl alloys at 832°C where these alloys have been reported not to exhibit steady-state creep. To this end, the present work revisits the work on understanding the tensile creep deformation behaviour of wrought single-phase ɤ-TiAl alloy by Saha [1] and is aimed to develop an understanding of the tensile creep deformation behaviour at 832°C and the influence of creep activation energy on the creep life of wrought single-phase ɤ-TiAl alloy for stress levels of 69.4 and 103.4 MPa at 832°C using Monkman–Grant [2] approach.

Perspectives on selected alloys in contact with eutectic melts for thermal storage: Nitrates, carbonates and chlorides

Increasing the operating temperature of molten salt-based concentrated solar power plants is of paramount importance to enable next-generation gas turbines and an overall increase in power conversion efficiency. The issue is how to mitigate the degradation of necessary metallic components in highly corrosive salt environments. In this study, three eutectic salt mixture candidates, nitrates, carbonates, and chlorides, are brought into contact with stainless steel (316H or 304L) and the FeCrAl alloy Kanthal® APMT. The post-exposure analysis is discussed in terms of the overall performance of each alloy as concerns mass change, scale growth, internal attack, and leaching. Significant reduction of corrosion is realised through the ability of Kanthal® APMT to form aluminium oxide species at the surface in contact with alkali nitrates and carbonates. On the other hand, aluminium is leached most efficiently in contact with chlorides, which causes a deeper attack on Kanthal® APMT than alloy 304L.The overall conclusion is that only by employing a holistic perspective on all individual measurements can a long-term performance estimation be formulated.

Anisotropic Thermal Conductivity of Nickel-Based Superalloy CM247LC Fabricated via Selective Laser Melting

Efforts to enhance thermal efficiency of turbines by increasing the turbine inlet temperature have been further accelerated by the introduction of 3D printing to turbine components as complex cooling geometry can be implemented using this technique. However, as opposed to the properties of materials fabricated by conventional methods, the properties of materials manufactured by 3D printing are not isotropic. In this study, we analyzed the anisotropic thermal conductivity of nickel-based superalloy CM247LC manufactured by selective laser melting (SLM). We found that as the density decreases, so does the thermal conductivity. In addition, the anisotropy in thermal conductivity is more pronounced at lower densities. It was confirmed that the samples manufactured with low energy density have the same electron thermal conductivity with respect to the orientation, but the lattice thermal conductivity was about 16.5% higher in the in-plane direction than in the cross-plane direction. This difference in anisotropic lattice thermal conductivity is proportional to the difference in square root of elastic modulus. We found that ellipsoidal pores contributed to a direction-dependent elastic modulus, resulting in anisotropy in thermal conductivity. The results of this study should be beneficial not only for designing next-generation gas turbines, but also for any system produced by 3D printing.

Tailoring the initial characterization of fully stabilized HfO2 with Y2O3/Ta2O5

New ceramic materials for thermal barrier coating (TBC) are a key to more powerful and efficient next-generation gas turbines. Here, we investigate the effect of the two novel dopants, Ta2O5 and Y2O3, on the phase stability, thermophysical and mechanical properties of the HfO2-based ceramic systems (i.e., Ta2O5-HfO2; Y2O3-HfO2). For the Ta2O5-HfO2 system, we observe the formation of a new compound, Hf6Ta2O17. In the Y2O3-HfO2 system, stabilized HfO2 with up to 18 mol% YO1.5 (YSH) is generated, whereby monoclinic HfO2 transforms to cubic HfO2. The cubic YSH ceramics demonstrate enhanced phase stability, compared with Hf6Ta2O17 at temperatures up to 1300 °C. The mechanical properties, thermal expansion and fracture toughness of 18–24 mol% YSH (YO1.5 = 18–24 mol%) indicate that cubic YSH exhibits enhanced suitability for higher-temperature.

Sensitivity analysis of internal layout and coating thickness to overall cooling performances of laminated cooling configurations with surface thermal barrier coatings

Laminated cooling coupled by surface thermal barrier coating (TBC) has a large potential to provide the effective protection to components in next-generation gas turbines. However, the conjugate heat transfer characteristics of coupling system is reported rarely. In this work, the combined effects of internal element layout, coating thickness and cooling air amount on the metal and TBC surface temperatures were discussed. The conjugate heat transfer experiments were conducted under engine-similar hot-side Biot (Bi) numbers and mainstream-to-coolant temperature ratio, and thus, a low conductivity material was applied to simulate the actual TBC. Four TBC thicknesses could cause BiTBC/BiMetal = 1.5, 3.0, 4.5 and 6.0 in the actual operating range. The results revealed the TBCs with BiTBC/BiMetal ≥ 4.5 can generate a large thermal resistance to reduce the metal temperature and hence a 50% increment in metal overall cooling effectiveness. However, the dominated internal cooling was also isolated to cause an increase of exterior surface temperature of entire cooling scheme, and a significant reduction of cooling air effect. The surface temperature variations were affected by the internal layout, when adopting the TBCs with BiTBC/BiMetal ≤ 3.0. Relative to the coolant amount and TBC thickness, the sensitivities of cooling effectiveness with internal layout were non-significant.

Microstructure, thermal characteristics, and thermal cycling behavior of the ternary rare earth oxides (La2O3, Gd2O3, and Yb2O3) co-doped YSZ coatings

In this paper, La2O3, Gd2O3, and Yb2O3 co-doped yttria stabilized zirconia (LGYYSZ) thermal barrier coatings were prepared on a nickel-based superalloy by atmospheric plasma spraying. The phase stability, coefficient of thermal expansion (CTE), and thermal conductivity of the LGYYSZ and YSZ coatings were measured. Thermal cycling at 1400 °C was implemented to evaluate the feasibility of LGYYSZ as an optimal ceramic material for thermal barrier coatings (TBCs) of next-generation gas turbines. The results show that the LGYYSZ coating exhibits a stable cubic phase, even after annealing at 1500 °C for 100 h. The CTE of LGYYSZ is slightly higher than that of YSZ. The thermal conductivity of the LGYYSZ coating at 1400 °C is 1.08 W∙m−1∙K−1, which is ~26.5% lower than that of YSZ. The thermal cycling lifetime of the LGYYSZ coating at 1400 °C is 468 cycles, which is ~2.7 times that of the YSZ coating. The longer thermal cycling life of the LGYYSZ TBC can be attributed to the excellent phase stability and lower thermal conductivity.

Structure and Properties of Nonstoichiometric Y1-xNbxO1.5+x for Thermal Barrier Coatings

Understanding the effect of nonstoichiometry on material properties is of critical importance for the Thermal Barrier Coatings (TBCs) application. In this work, the effect of nonstoichiometry was systematically investigated in Y3NbO7, a recently identified promising TBCs material for next-generation gas turbine engines. The results show that the nonstoichiometry effect mediates the microstructure, mechanical properties, thermophysical properties, oxygen ionic conductivity and optical transmittance. The results suggest that the oxygen ionic conductivity is correlated to the mass diffusion in the oxygen-deficient fluorite oxides Y1-xNbxO1.5+x. The stoichiometric composition Y3NbO7, with the lowest thermal conductivity and slowest mass diffusion in Y1-xNbxO1.5+x series, is optimal for the TBCs application, which can be of relevance for TBCs material design and coating fabrication.

Mechanism of the oxide scale formation in thermally-sprayed NiCoCrAlY coatings modified by CeO2 nanoparticles

The increased operating temperatures of next-generation gas turbines require high-performance coatings with improved oxidation resistance. NiCoCrAlY coatings have significant potential for achieving this goal however improved insights into their oxidation performance are required. In this study, a high-velocity oxy-fuel (HVOF) process was used to compare NiCoCrAlY/nano-CeO2 with NiCoCrAlY conventional coatings. High-temperature oxidation behaviour of the modified coatings was investigated and compared with the conventional coating. Free-standing coatings were then subjected to short- and long-term isothermal oxidation test at 1000 °C. The microstructural features of the modified powders and coatings before and after oxidation were characterized using a Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive Spectroscopy (EDS), Transmission Electron Microscope (TEM), Raman spectroscopy and X-ray Diffraction (XRD). The oxide growth rate of the coatings was also examined and modeled using various diffusion-based calculations. Results showed that, the modified NiCoCrAlY-1.0 wt% nano-CeO2 coating had the highest oxidation resistance (26 % higher than the conventional NiCoCrAlY coatings). The higher oxidation resistance of the modified NiCoCrAlY-1.0 wt% nano-CeO2 coating was attributed to its higher density as well as its lower structural porosity and oxide growth rate after the short- and long-term oxidation tests. Moreover, acceptable conformity of parabolic rate behaviour was obtained for all types of the conventional and modified coatings. The dominant mechanism of oxidation improvement for NiCoCrAlY-1.0 wt.% nano-CeO2 coating was found to the controlling the oxide scale growth rate. As a consequence, the findings indicated that the modified NiCoCrAlY-1.0 wt.% nano-CeO2 coating can be a candidate for the protection of hot sections of gas turbines in the future.

Effect of cooling hole arrangements of micro-effusion cooling on the development of thermal boundary layer on a flat plate

As the next-generation gas turbine efficiency increases, it is required to develop an improved turbine cooling technology to gradually increase the inlet temperature of turbine from 1400 °C to 1600 °C. For this purpose, various studies have been conducted to enhance the external film cooling and the internal cooling performance. This paper proposes the numerical conjugate heat transfer model that is strictly validated with experiment data in order to investigate the heat transfer characteristics of micro-effusion cooling on a flat plate. First, the development of thermal and velocity boundary layer on the flat plate was simulated and analyzed in three kinds of cooling hole sizes. The smaller cooling hole diameter of 0.5 mm maintains a more stable boundary layer along to flow direction than the conventional larger hole size. The second is to compare and evaluate the cooling effectiveness of micro-effusion cooling in two types of cooling hole arrangements, such as aligned and staggered. The staggered array of cooling hole shows the more improved cooling effectiveness by about 350 % from 0.18 to 0.63 than aligned one by 270 % from 0.18 to 0.50 in micro-effusion cooling.

Evaluation of the atomization characteristics of aviation fuels with different viscosities using a pressure swirl atomizer

With advances in the fuel spray field, much attention has been paid to the atomization characteristics of aviation fuels. The physical properties of fuels play a vital role in their resulting spray structure and spatial distribution of droplets. These characteristics are becoming more important for choosing alternative fuels for optimal performance in next-generation gas turbines. The current study experimentally investigated the atomization quality and spray structure of aviation fuels with different viscosities sprayed through a pressure swirl-type nozzle by employing laser diagnostics. A set of CCD cameras synchronized to the Nd:YAG laser characterized the spray structure, and the size and velocity of the drops were quantified by the phase Doppler technique. The fuels were tested under various temperature and injection pressure conditions. An alteration in both the injection pressure and the physical properties of the fuel were found to significantly dominate the spray structure and atomization quality. The transitional development of hollow-conical sprays was shown to require a sufficient amount of swirl intensity, characterized by their physical properties and injection pressure. Fuels with a lower viscosity were experimentally found to generate well-developed hollow-conical sprays with finer drops, wide distribution in size, and higher velocity components, which are recognized as ideal spray properties in the combustion field.