Gas Turbine Applications Research Articles

Surface roughness and topography of EDM machining of Inconel 718

An alloy with a nickel–chromium basis known as Inconel 718 is utilised in gas turbine and jet engine applications. The machining of the Inconel 718 is challenging because of its higher strength and hardness. Electrical Discharge machining is the most common operation used for machining Inconel alloys because of its higher accuracy in machining the alloy. The machining accuracy is evaluated by the surface roughness of the material. In this research, the machining of Inconel 718 alloy using the electric discharge machining is done by varying the input parameters. Taguchi design is used for varying the input parameters of Electrical discharge machining. The efficiency in machining is evaluated by measuring the surface roughness of the Inconel alloy. The signal-to-noise ratio analysis optimises process parameters for excellent surface roughness. The analysis of variance is used to determine the impact of each input variable on the surface roughness. The result of this research is helpful for researchers and industries to select the accurate process parameter for machining Inconel 718 alloy.

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.

Assessment of surface integrity and hole quality in graphene-based NMQL Micro-drilling of ceramic-coated Nimonic 90 for gas turbine applications

Nickel-based superalloys have superior strength properties at higher temperature ranges and thus have become increasingly important in manufacturing gas turbine components for aerospace industry. However, the desire for a larger thrust-weight ratio has raised the typical operating temperature in a gas turbine; thus, thermal barrier coatings are essential. The present work compares the micro-drilling performance of ceramic-coated Nimonic 90 nickel superalloy under dry, flood and 0.5% graphene-based NMQL conditions. The biodegradable acid oil was used as a base oil, and the assessment comprised surface integrity in terms of surface roughness inside the hole and micro-crack formation and hole quality based on the diametrical overcut and taper ratio. Spindle speed (1000, 2000 and 3000 rpm) and feed rate (3, 6, and 9 μm/rev) were changed in three levels, and Taguchi L9 array was applied for the design and analysis of the experiments. Ti-Al-N coated tungsten carbide drill of diameter 700 μm was used, and Analysis of variance (ANOVA) revealed that spindle speed was the utmost important parameter impacting surface roughness, while speed and feed rate both influenced overcut and taper ratio. 0.5% Graphene-based NMQL lubrication condition significantly diminished the surface roughness by 52.67%, overcut by 46.86% and the taper ratio by 48.87% as compared to dry condition. Furthermore, in the NMQL condition, micro-crack development and ceramic layer damage were minimized, resulting in better surface integrity. In addition, burr development was minimized at the hole periphery, and tool wandering was not seen in the NMQL condition. Hence the hole quality was superior in NMQL conditions as compared to the dry and flood lubrication.

Effect of multi-hole trench cooling on an adiabatic flat plate for gas turbine application

Abstract A 3D numerical analysis on an adiabatic flat plate for multi-hole trench cooling with forward, backward and mixed injection holes is performed in the current investigation. The numerical setup is validated before the performances of different cooling configurations are compared. The effect of three different multi-hole trench arrangements, square-diamond, long-diamond, and super-long-diamond with constant perforated percentage (3.27%), on film cooling performance is studied at blowing ratio 1.0. The row-to-row interaction between coolant jets and mainstream is analysed, and lateral film cooling effectiveness is calculated downstream. The dimensionless temperature contour overlaid with streamlines concluded that the SLD trench hole arrangement with forward injection forms a developed effusion layer due to counter-rotating vortex pairs, which helps in proper mixing of coolant jets into the mainstream and improves film cooling effectiveness in lateral as well as in longitudinal direction. It is observed that super-long-diamond arrangement with forward injection provides the highest film cooling effectiveness than square-diamond and long-diamond arrangements and favours early development of the coolant film layer.

VALIDATION OF A STOCHASTIC BREAKUP MODEL FOR TURBULENT JETS IN HIGH-SPEED CROSSFLOW: ASSESSMENT OF TURBULENT INTERACTIONS AND SENSITIVITY TO BOUNDARY CONDITIONS

Improving the mixing of fuel and air by injecting a turbulent liquid fuel jet into a high-speed cross-flowing gas can reduce the emissions of gas turbine applications. To facilitate and hasten the development of such low-emissions technologies, accurate predictions of the spray characteristics are needed. The objective of the present study is to validate the predictive capabilities of a stochastic breakup model for turbulent transverse jets over a wide range of representative pressures and atomization characteristics. The effect of turbulence modeling is also assessed to provide accurate and computationally less expensive Eulerian-Lagrangian transient approaches. To do so, the predictions made with the large eddy simulation (LES) approach for different subgrid-scale (SGS) models and with the synthetic eddy method (SEM) are compared to the ones made using the unsteady Reynolds-averaged Navier-Stokes (RANS) approach with and without a turbulent dispersion model. The sensitivity of the numerical methodology to the upstream velocity profile, pressure, and momentum flux ratio were also assessed. Properly accounting for the upstream gas velocity profile was found to be critical to ensure accurate predictions of the spray characteristics. The unsteady RANS (URANS) turbulent approach coupled with the turbulent dispersion model showed good agreement with experimental data, but the LES approach tends to overpredict the spray penetration and underpredict the Sauter mean diameter (SMD). This could be due to the lower turbulent interactions it predicts, which may lead to lower momentum transfer between the phases.

Duration-affected creep behaviors of Ni-based single crystal superalloys with/without rhenium addition designed for IGT application

In this study, the creep behaviors of two Ni-based single crystal superalloys with/without Re addition designed for industrial gas turbine application were investigated at 900 °C under a range of applied stresses (200 MPa–330 MPa), and the examinations of the microstructures and dislocation configurations were performed at the minimum creep rate and creep rupture under each condition of both alloys. The results indicated that although the creep lives spanned over thousands of hours under different stresses, dislocations underwent the similar mechanisms including the proliferation and movements by gliding-climbing process in decelerating stage, followed by the progressive increase in dislocations shearing into the γ′ phases until creep rupture in each alloy. This led to a special rafting regime which was characterized by the sluggish formation of the rafting structure in the present superalloy, considering the relatively lower γ′ volume fraction, lower lattice misfit, and lower elemental diffusion of the experimental superalloys than those of traditional Ni-based superalloys. Thus, the “duration effect” was proposed to describe the strong correlation between the creep duration and microstructural evolution, which showed insensitivity to the applied stress and accumulation strain, resulting in the consistently exponential relationships between the time to the minimum creep rate and the applied stress or creep life of each alloy within a wide range of stresses. The addition of Re gave rise to an inversion of the rafting tendency during high-temperature creep without changing the deformation mechanism under different stresses. Nevertheless, the addition of Re could still enhance the creep property under each stress, which contributes to provide insights for optimizing the alloying design and enhancing long-term reliable service of industrial gas turbine blades.

Attributes of Hall current and radiation absorption in unsteady tangent hyperbolic nanofluid flow over a stretching surface with activation energy: A numerical study

This work describes the effects of Hall current and radiation absorption on magnetohydrodynamic tangent hyperbolic nanofluid flow over a stretching sheet considering binary chemical reaction with activation energy and heat source. Convective heat is applied to the sheet from the bottom, whereas a zero mass flux condition is imposed on the surface. Furthermore, the radiative heat transfer is considered in view of Rosseland’s approximation. A nonlinear set of ordinary differential equations, in non-dimensional form, is obtained by employing appropriate transformations to the governing equations. The built-in bvp4c function in MATLAB is used to find the numerical solution of the system of ordinary differential equations. A data comparison for a limiting case with previously reported results is presented to validate the numerical findings. Further, the flow, as well as heat and mass transfer characteristics, are shown graphically as a result of the impacts of various flow factors. It is noticed that the fluid flow is accelerated with the rise of the Hall current parameter. The thermophoresis parameter and activation energy have an ascending influence on the concentration profile. Moreover, Biot number and radiation absorption parameter can uplift heat transfer rate. This study has applications in nuclear power reactors, gas turbines, oil reservoirs, reactor safety, and heat insulation.

Flame stabilization and thermoacoustic instability during operating condition modulations: Roles of pilot and main flames

Operating condition modulation plays a key role in the startup process of practical gas turbine applications. The present article investigates the operating condition modulations of a centrally staged swirl burner regarding the flame stabilization and thermoacoustic instability, in which the roles of pilot and main flames have been distinguished. The global characteristic of thermoacoustic instabilities during operating condition modulations has been explored in a wide operating range by incrementally increasing equivalence ratios in the pilot stage (ϕp) and main stage (ϕm). Transitions of stability regimes under self-excited conditions have been observed with increasing ϕm and show difference with ϕp= 1.1–1.3. Two modes of stable flame stabilizations under un-excited conditions have been identified with ϕm variation, including pilot and staged flames. Their main difference is the varied attachment of flame roots along pilot shear layers and main shear layer. At self-excited conditions, three different instability modes have been revealed with increasing ϕm, i.e., pilot mode, hybrid mode, and staged mode. The periodic interaction between the pilot flame and the main stage stream is shown to be necessary for sustaining a large-scale convective motion of pilot flame to trigger strong thermoacoustic instabilities in pilot and staged modes. For the hybrid mode, the periodic attachment of the main flame root has suppressed the convective motion and, thus, caused weakened thermoacoustic couplings. The present investigation reveals the unique and various interactions between pilot and main flames during operating condition modulations, which can assist startup design in practical centrally staged combustion systems.

Transitions of thermoacoustic modes and flame dynamics in a centrally-staged swirl combustor

In the start-up process of practical gas turbine applications, the modulation of operating condition, especially the variation of global equivalence ratios (ϕglo), could cause the variations of stability regimes. The above process is accompanied by the increase of total air flow rates (ṁair) when approaching the stable operation condition, and thus the effect of this parameter should also be considered in the operating condition modulation process. This work presents an experimental investigation into the transitions of thermoacoustic modes and flame dynamics in a centrally-staged swirl combustor over a range of ϕglo and ṁair. The present thermoacoustic system exhibits two transitions of thermoacoustic modes with ϕglo variation, showing distinct characteristics of pressure oscillation (p′) and flame structures. To further explore the detailed variation of p′ characteristics and its link to flame dynamics during two transitions, a nonlinear dynamic analysis of p′ including its phase and recurrence plots as well as a proper orthogonal decomposition (POD) study have been conducted. The thermoacoustic network analysis and local Rayleigh index analysis have clarified the mode transition mechanism with varying ϕglo and ṁair. The present results can provide a guideline for quantifying the stability regimes of fuel-flexible combustors during the operating condition modulation processes.

The temperature dependence of strengthening mechanisms in Ni-based superalloys: A newly re-defined cuboidal model and its implications for strength design

The development of advanced Ni-based superalloys for gas turbine applications is strongly reliant on alloy strength design and optimisation through microstructure control. Herein, a new model for precipitation strengthening in Ni-based superalloys with large amounts of cuboidal γ’ particles has been further developed from a previous work with re-defined physical parameters, allowing for a smooth integration of weak and strong pair-coupling models. The predicted results were directly validated using commercial alloy (Alloy720Li)-based single-crystal tie-line model alloys with specially designed γ’ particle sizes and volume fractions, which made it possible to extract the temperature dependence of all factors responsible for the overall strength, such as the γ/γ’ mixture, particle strengthening, and solid-solution strengthening. The physical parameters controlling the strength increment in particle strengthening were thoroughly examined as well. The proposed pair-coupling model with the assumption of cuboidal γ’ particles accurately predicts alloy strength over a wide temperature range (up to 760 °C) for volume fractions > 45%, whereas the classical model that assumes the presence of spherical γ’ particles is valid only for volume fractions < 20%. More importantly, our finding suggests that the cuboidal γ’, with its stronger dependence of strengthening on volume fractions than on size and higher strength increment with increasing APB energy, can allow easier strength optimization of turbine discs over spherical γ’.

Experimental and mathematical modelling of corrosion behaviour of CMAS coated oxide/oxide CMCs

CMAS corrosion of turbine blades is a crucial failure in turbine engines and their components. In this study, oxide/oxide CMCs (AS-N610), which are candidates for gas turbine (GT) applications, are investigated for its corrosion behaviour at different temperatures and time in presence of CMAS. The corrosion studies using CMAS coating of the CMCs reveal that CMCs had a weight gain of ∼6% owing to formation of α-Al2O3 at 1000 °C. The SE image indicated the penetration of CMAS into the porous CMC. At 1000 °C, CMAS degraded to form a black glassy substance (Calcium alumino silicate) with traces of Mg which led to corrosion of the matrix. Indentation fracture toughness of the oxide/oxide CMCs was 7.78 ± 0.5 MPa m0.5 which degraded by ∼12% at 1000 °C after 10 h in the presence of CMAS. A mathematical model derived through diffusion equation indicated weight gain of ∼0.3 g which was closer to experimental data.

Experimental investigation of the film cooling performance by triangular-shaped thin plates on inclined cylindrical holes

Experimental investigations have been conducted to improve the cooling effectiveness of film-cooling technology. A multiple row of cylindrical holes at the flat surface was considered by inclining the holes at 35° along the streamwise direction. A low-speed wind tunnel with the main flow speed of 10 m/s was devised to create a cross flow at a Reynolds number of 196,167 and five blowing ratios of 0.3, 0.5, 0.7, 1, and 2. The proposed triangular-shaped thin plate and flop at the cylindrical film cooling hole were investigated using flow visualization and thermographic techniques. The resulting cooling performances were also experimentally evaluated and compared with respect to the different film-cooling hole configurations. The typical film cooling hole configuration of cylindrical hole shape shows the cooling effectiveness of 0.1 at blowing ration of 1.0. It should be denoted that this cooling effectiveness is exemplary value of wide range of gas turbine applications. We also experimentally evaluates the cooling effectiveness in order to valid our experimental method. Meanwhile, with triangular-shaped thin plate-flop, the flow detachment is successfully prevented with requiring 8% increase of discharge coefficient. In addition, the film flow is stable even at the distant downstream, resulting in enhanced cooling performance compared to the cylindrical and triangular-shaped thin plate. Consequently, a 242.7% improvement in film cooling effectiveness can be achieved with a triangular-shaped thin plate-flop than with a cylindrical hole owing to its enhanced film coverage.

Turbulent flame speed and morphology of pure ammonia flames and blends with methane or hydrogen

Ammonia appears a promising hydrogen-energy carrier as well as a carbon-free fuel. However, there remain limited studies for ammonia combustion especially under turbulent conditions. To that end, using the spherically expanding flame configuration, the turbulent flame speeds of stoichiometric ammonia/air, ammonia/methane and ammonia/hydrogen were examined. The composition of blends studied are currently being investigated for gas turbine application and are evaluated at various turbulent intensities, covering different kinds of turbulent combustion regimes. Mie-scattering tomography was employed facilitating flame structure analysis. Results show that the flame propagation speed of ammonia/air increases exponentially with increasing hydrogen amount. It is less pronounced with increasing methane addition, analogous to the behavior displayed in the laminar regime. The turbulent to laminar flame speed ratio increases with turbulence intensity. However, smallest gains were observed at highest hydrogen content, presumably due to differences in the combustion regime, with the mixture located within the corrugated flamelet zone, with all other mixtures positioned within the thin reaction zone. A good correlation of the turbulent velocity based on the Karlovitz and Damköhler numbers is observable with the present dataset, as well as previous experimental measurements available in literature, suggesting that ammonia-based fuels may potentially be described following the usual turbulent combustion models. Flame morphology and stretch sensitivity analysis were conducted, revealing that flame curvature remains relatively similar for pure ammonia and ammonia-based mixtures. The wrinkling ratio is found to increase with both increasing ammonia fraction and turbulent intensity, in good agreement with measured increases in turbulent flame speed. On the other hand, in most cases, the flame stretch effect does not change significantly with increasing turbulence, whilst following a similar trend to that of the laminar Markstein length.

Design and Characteristics of a New Pre-Evaporation Chamber Geometry for Low-Grade Palm Oil Fuel Evaporation for Micro Gas Turbine Application

Recent increase in fuel prices, greenhouse emissions, global warming issue, as well as future predictions regarding fossil fuel depletion concerns have prompted widespread research on carbon-neutral renewable liquid biofuels as an alternative of petroleum fuels. In the present study, a numerical approach was used to design and optimize a novel design of pre-evaporation chamber, to pre-evaporate crude palm oil fuel using exhaust gas recycling to allow partially premixed fuel vapors to be introduced to the main combustion chamber. Discrete Phase Model (DPM) and species non-premixed model used in ANSYS-FLUENT software to simulate fuel evaporation. The pre-chamber optimization included the addition of variable number of revolve geometry to the walls to induce turbulence and to enhance the cyclonic motion caused by the tangential hot exhaust gas inlets with different fuel injection configurations. The number of revolves was varied in the range of 5-8 and diameter range of 40-90 mm with three injector configurations. The results revealed that the optimum chamber geometry consist of 7 revolves with 70 mm diameter with fuel evaporation rate about 0.0023kg/s and fuel evaporation percentage about 89%. Increasing the exhaust gas temperature from 950 °C up to 1050°C did not show a significant fuel evaporation enhancement. Results also showed that the evaporation enhancement is proportional to the number of fuel injectors where evaporation increased gradually from 73% to 89% when increasing number of injectors from 1 to 3.

Study of turbulent flame characteristics of water vapor diluted hydrogen-air micro-mixing combustion

Hydrogen has been regarded as an important alternative fuel, especially in gas turbine applications. However, the premixed combustion of pure hydrogen has problems such as easily flashback and high NOx emissions. A more reasonable method of combustion is to use micro-mixing technology combined with water vapor dilution. This work experimentally investigated the influences of dilution rate (D) and equivalence ratio (φ) on the flame characteristics of hydrogen-air micro-mixing combustion diluted with water vapor. Results show that increasing D reduces the flame area by reducing the downstream OH radical signal, which is contrary to the effect of CO2 dilution. The flame local curvature radius shows that the micro-mixing combustion is a high turbulence intensity combustion, and the convex structure is more frequent. Moreover, increasing φ or decreasing D leads to a decrease in the instantaneous average curvature radius (Rave), indicating more small-scale wrinkle structures are generated. D and φ have a similar influence on the fluctuations of Rave and flame area in the time domain, while the fluctuation of the flame area is more evident near the lean-burn limit. When D = 25%, the frequency characteristics of the flame structure and chemical reactions are synchronized.

Reaction between environmental barrier coatings material Er2Si2O7 and a calcia-magnesia-alumina-silica melt

The suitability of sintered erbium disilicate (Er2Si2O7) as an environmental barrier coatings (EBCs) for gas turbine applications was assessed by characterizing its high-temperature corrosion behavior in contact with a synthetic calcia-magnesia-alumina-silica (CMAS) melt. Er2Si2O7 was fabricated using spark plasma sintering at 1400 °C for 20 min. Corrosion tests were performed by coating sintered Er2Si2O7 pellets with CMAS and heating them to 1400 °C for 2, 12, and 48 h. High-temperature X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis were used to identify and analyze the reaction products. The two materials were found to react chemically to form an apatite phase, Ca2Er8(SiO4)6O2, at their interface. The Ca2Er8(SiO4)6O2 grains were observed to have shard-like morphologies oriented perpendicular to the Er2Si2O7 surface; the reaction layer thickened with increasing heat-treatment time, with the thickness after exposure for 48 h approximately three times the thickness after 2 h.

Active Valve Resonant Pulse Combustor for Pressure Gain Combustion Applications

This work is an experimental demonstration of a novel resonant pulse combustor technology for which sustained pressure gain can be achieved at thermal equilibrium. The resonant pulse combustor employs an actively driven air inlet valve with passively modulated fuel injection such that the device operating frequency is specified by the valve set-point frequency. This configuration enables more control over the combustor operation compared to passive valve and valveless resonant pulse combustors and enables the durability necessary for consideration in practical applications. Combustor performance is characterized with measured stagnation pressure gain as a function of stagnation temperature and fuel mass flux. Steady-state stagnation pressure ratios between 1.005 and 1.025 are demonstrated for gaseous (ethylene) and liquid (gasoline) fuels at temperature ratios from 2.8 to 4.5. Baseline performance measurements for a well-studied passive valve resonant pulse combustor are presented for comparison. The proposed active valve technology provides a direct path for translation from research-type devices to practical gas turbine applications by addressing key challenges inherent of existing resonant pulse combustor technologies.

NiAl – material for the application in gas turbines

Abstract NiAl alloys are very promising candidates for high-temperature applications such as turbine blades of gas turbines. For such turbine blades, investment casting is the common manufacturing process. Up to now, the processing of NiAl alloys in general and in particular via investment casting is hardly possible due to the very high melting temperature, hardness, reactivity of the melt, and a characteristic ductile-to-brittle transition temperature. In this publication, the development of an investment casting technology will be summarised including the shell mould and core development, casting process development, the advancement of numeric simulation tools, and the formulation of a complete data set.

Nonlinear analysis of a dual-disk rotor system considering elastoplastic contact

The contact effect of disk interfaces has an important influence on the dynamic response of the gas turbine rotor system. In order to improve the accuracy of the contact model, this paper proposes a modified contact model for the full deformation stage based on verified experimental results by considering the influence of elastoplastic deformation, the experimental results and the theoretical results of contact model are compared. In order to simplify the modeling process of the contact interface, the equivalent material layer was introduced to establish the finite element model of the dual-disk rotor-bearing system, and the dynamic response of the rotor system was solved by the Newmark-β method. The validation of the proposed contact model is verified by comparing the time-domain waveform diagram. The dynamic characteristics of the rotor system under different preloads and speeds are analyzed. Results demonstrate that the contact model proposed in this paper can reflect the actual contact state accurately, a certain amount of preload will cause the appearance of multiple combined frequency components, such as fr-2fn, 3fr-4fn (fr is rotating frequency, fn is oil film oscillation frequency). The work provides a theoretical reference for contact effect modeling of rotor system, and has prospective application in gas turbines.

Microstructural evolution and nano-mechanical properties of Ti-Al-based alloys synthesized by laser in-situ alloying

Developing titanium aluminides (Ti-Al) based alloys by means of Laser Engineered Net Shaping (LENS) in-situ manufacturing yields attractive properties as compared to other fabrication methods. Ti-Al alloys have attracted much attention for high-temperature performance in gas turbine and automobile applications because of their attractive properties such as low density, high strength, high stiffness, and good oxidation resistance. In this work, laser in-situ fabricated Ti-Al-2Cr (Sample C3), Ti-Al-3Cr (Sample C1) and Ti-Al-4Cr (Sample C2) alloys were developed and the compositional effect on morphological evolution and nano-mechanical properties were investigated. The developed alloys were heat treated to 1350°C and air-cooled. The microstructural evolution was characterized using the scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Anton-Paar equipment was used to analyze the nanoindentation properties (hardness, elastic modulus and stiffness) and MatLab software was utilized to analyze the stress-strain behaviour of the alloys from the nanoindentation load-displacement curve. The results showed that normalizing heat-treatment played an important role in homogenizing the alloys and there was a significant decrease in nanoindentation hardness, stiffness and modulus of elasticity after heat-treatment.