Gas Turbine Environment Research Articles

The Effect of Spray Distance on the Hot Corrosion Behavior of HVOF‐Sprayed NiCrAlY Coatings in a Simulated Gas Turbine Environment

ABSTRACTTo utilize the unique properties of coatings produced using the high‐velocity oxy‐fuel (HVOF) method, this study examined the effect of spray distance on the hot corrosion behavior of NiCrAlY coatings deposited on a steel substrate. The coating samples were obtained at spray distances of 20, 25, and 30 cm. The flow rates of oxygen and propane were maintained constant at 300 and 60 lit/min, respectively. Additionally, the powder feeding rate was 32 g/min. The samples were then subjected to the hot corrosion test in a salt mixture of Na2SO4−60% V2O5 at 900°C in an ambient atmosphere. The surface and the cross section of the samples were observed using a scanning electron microscope (SEM) and the chemical composition of different sections was determined by energy‐dispersive spectroscopy (EDS). The phases were also characterized using X‐ray diffraction analysis (XRD). The best corrosion resistance was found in the sample coating formed at a spraying distance of 20 cm, as the results showed that increasing the spraying distance led to a reduction in the thermal and kinetic energy of the flame, resulting in increased porosity in the coating. This behavior can be attributed to the low oxide content of the coating, the higher aluminum content, and the richer NiAl phase, which serves as a reservoir for aluminum.

Surface corrosion behavior of β-(Ni,Pt)Al coating: α-Al2O3 decohesion-induced failure mechanism

In marine and gas turbine environments, impurities like hydrogen (H), sulfur (S) and chlorine (Cl) ions frequently impact continuity and integrity of the oxide scale of Pt-modified aluminide coating. This study systematically investigated the segregation behavior of these impurities using first-principles calculations, to reveal their weakening potency on both α-Al2O3 bulk and grain boundary (GB). The energetic properties, including separation work, surface energy and GB energy, are strongly associated with existing form of impurities. The interstitial S and Cl atoms are particularly intensive in reducing cohesion of α-Al2O3 in comparison with solute state. The single-atom S- and Cl-doping with S and Cl have similar effects on the embrittlement of α-Al2O3 bulk/GB, and the Cl-introduction exhibits extreme brittleness once the atomic concentration increases. Interestingly, single H-addition has no significant effect on the α-Al2O3 decohesion, while its weakening potency on α-Al2O3 is further enhanced upon contacting with S and Cl atoms. Therefore, we decomposed strengthening energy into mechanical contribution and chemical contribution, finding that the incorporation of H increases the chemical contribution to decohesion of α-Al2O3. This work, along with initial corrosion behavior of various (Ni,Pt)Al coatings, aims to deepen understanding of H, S and Cl on α-Al2O3 decohesion from multiple perspectives.

Thermal properties and calcium-magnesium-alumino-silicate (CMAS) interaction of novel γ-phase ytterbium-doped yttrium disilicate (γ-Y1.5Yb0.5Si2O7) environmental barrier coating material

Rare-earth disilicates are promising candidates for thermal and environmental barrier coatings (TEBC) in gas turbines that safeguard SiCf/SiC ceramic matrix composites (CMCs) from thermal degradation and environmental attacks. Here, we report a systematic investigation on novel TEBC material, γ-Y1.5Yb0.5Si2O7. The γ-phase quarter molar ytterbium–doped yttrium disilicate exhibited low thermal conductivity (1.72 W·m−1·K−1 at 1200 °C) and reduced intrinsic thermal expansion (3.17 ± 0.22 × 10−6 K−1 up to 1000 °C), ensuring promisingly effective thermal insulation and minimized thermal stress with CMC substrates. Using density functional theory (DFT), the heat capacity of γ-Y1.5Yb0.5Si2O7 was predicted higher than that of undoped γ-Y2Si2O7. Comparing these predictions to results calculated using the Neumann–Kopp (NK) rule revealed only minor variations. A metastable CMAS interaction byproduct, cyclosilicate phase Ca3RE2(Si3O9)2, was identified based on energy dispersive X-ray spectrometer (EDS) and electron backscatter diffraction (EBSD) techniques, appearing at 1300 °C but disappearing at 1400 °C. The γ-Y1.5Yb0.5Si2O7 exhibited good CMAS resistance on both dense pellets and sprayed coatings, forming a protective apatite (Ca2RE8(SiO4)6O2) interlayer that effectively hindered CMAS infiltration at evaluated temperatures. The relatively higher Y:Yb atomic ratio (> 3) in the apatite grains indicate differential reactivity with molten CMAS and provides crucial insights into the CMAS corrosion mechanism. These findings highlight the potential of γ-Y1.5Yb0.5Si2O7 as a CMC coating material, emphasizing the need for tailored microstructural optimization as a thermal sprayed coating to enhance long-term performance in extreme gas turbine environments.

High Temperature Oxidation of Additively and Traditionally Manufactured Inconel 718

Abstract Additively manufactured nickel-based superalloys are now being considered as a replacement for traditionally manufactured components on commercial and military aircraft. While the potential of additive manufacturing for gas turbine engines is promising, the quality and performance of additive manufactured components still need extensive study. Coupon-sized test specimens comprised of as-printed additively and traditionally manufactured Inconel 718 with and without yttria-stabilized zirconia thermal barrier coating were tested under simulated isothermal and thermal cycling combustion conditions that were representative of gas turbine environments. Pre-test scanning electron microscopy indicated that traditionally manufactured coupons had a smooth surface finish with minor imperfections while additive manufactured coupons had a rough surface finish. Post-test scanning electron microscopy exhibited differences in oxide scale between the isothermal and thermal cycling conditions. The thermal cycling condition increased the amount of oxide scale for both additively manufactured and traditionally manufactured Inconel 718. The size of the oxide islands on traditionally manufactured coupons was significantly larger than the additively manufactured coupons. The results indicated that differences in surface roughness may affect the growth of the oxidation scale in a high-temperature combustion environment. The benefits of yttria-stabilized zirconia thermal barrier coating were characterized. The substrate of the coupons experienced little to no formation of oxide scales compared to the uncoated additive and traditionally manufactured coupons. The results further suggested that yttria-stabilized zirconia thermal barrier coating can be utilized to provide both insulation and oxidation protection when desired. Post-test energy-dispersive X-ray spectrometry results corresponded well with known elemental compositions and oxidation mechanisms.

Dry air cyclic oxidation of mixed Y/Yb disilicate environmental barrier coatings and bare silica formers

Mixed Y and Yb disilicate coatings (Y/Yb)DS have been proposed as dual function thermal and environmental barrier coatings (EBCs) for protecting SiC-based ceramic matrix composites in gas-turbine environments. As an initial step, the 1350 °C dry air cyclic oxidation of atmospheric plasma sprayed (Y1.2/Yb0.8)DS and ytterbium disilicate/ytterbium monosilicate (YbDS/YbMS) EBCs deposited onto Si bond coatings was compared. As a baseline for evaluating EBC oxidant permeability, the dry air cyclic oxidation scale growth rates for bare silica formers (SiC, Si) were also measured and were consistently higher than rates previously measured after isothermal oxidation. Regarding Si bond coat oxidation rates underlying (Y/Yb)DS and YbDS/YbMS EBCs, the thinner silica scale formed under the thinner and denser (Y/Yb)DS coatings suggested a lower oxidant permeability than YbDS/YbMS. After 500 1-h cycles, the (Y/Yb)DS coating was comprised of only the β-polymorph disilicate and minor amounts of the X-2 phase monosilicate phase. Negligible differences in oxidation kinetics for (Y/Yb)DS coatings over the 90 – 240 µm thickness range were observed.

Effects of wet oxidation on residual stress variation of SiCf/SiC composites

Continuous silicon carbide fiber-reinforced silicon carbide matrix (SiCf/SiC) composites, used as thermal structures for gas turbines, are prone to degrade under the gas stream (mainly composed of O2 and H2O). Here, the degradation of the composites in wet oxygen environment was investigated by Raman spectroscopy. Results show that the composites exhibit significant residual stress as the oxidation proceeds. SiC matrix in the as-received condition exhibits average hydrostatic tensile stresses of approximately 1350 MPa, while SiC fiber exhibits compression stress of approximately 1450 MPa. Both the residual stresses in matrix and fiber decrease with the increasing oxidation temperature. Moreover, the residual stress evolution is found to be related to the formation of cristobalite and the volatilization of Si(OH)4, CO, H2. This work can open new alternatives for understanding the degradation mechanism of SiCf/SiC composites in gas turbine environment, and for developing mathematical model to predict their service life.

Vibration fault signal analysis and diagnosis of flue gas turbine

Flue gas turbine is a kind of typical rotating machinery. It uses the flue gas expansion which contains a lot of heat energy and pressure energy generated in the catalytic cracking process to drive the axial-flow compressor or generator to rotate. Due to the complex structure, bad operating environment, poor stability, and large vibration of the flue gas turbine, its failure rate is high, which causes immeasurable economic losses and safety hazards. Therefore, how to effectively obtain, extract and identify the fault information of flue gas turbine is a difficult problem in the field of fault diagnosis technology. In this paper, aiming at two kinds of faults in the tp9-90 flue gas turbine system, i.e., rotor misalignment and unbalance causing excessive vibration, the corresponding geometric model and mathematical model are established, and the fault mechanism is revealed. Through on-line monitoring of flue gas turbine rotor vibration signal, the running state of flue gas turbine is comprehensively analyzed, and the technological transformation is carried out according to the fault causes, the improved flue gas machine runs smoothly, and the vibration and failure rate are obviously reduced. This study provides an important reference for condition monitoring and fault diagnosis of flue gas turbines.

Evaluating steam oxidation kinetics of environmental barrier coatings

AbstractEnvironmental barrier coatings (EBCs) are a commercially proven means of protecting SiC‐based materials in gas turbine environments. However, there are little specific data in the literature on the impact of coatings like Yb2Si2O7 on preventing accelerated SiO2 growth in the presence of H2O. Quantification of reduced rates are necessary for evaluating and comparing EBC effectiveness and incorporation of silica growth rates into future EBC lifetime models. In this study, baseline kinetics of silica formation on bare Si and chemically vapor deposited (CVD) SiC in the 1250–1425℃ range were obtained via 100 h isothermal exposures in dry air and steam environments utilizing a SiC reaction tube to mitigate specimen volatility. An Arrhenius plot of the resulting rates was constructed, representing baseline minimum and maximum rates for Si and SiC oxidation at ambient pressure. Various EBC systems on CVD SiC substrates including air plasma sprayed (APS) EBCs with and without a Si bond coating and with surface roughening to enhance Yb2Si2O7 adhesion were subjected to 1‐h furnace cycle testing in air with 90vol%H2O at 1250–1350℃ for up to 500 cycles. After exposure, silica formation rates were measured and compared to the baseline rates to assess EBC effectiveness, where EBC effectiveness is gauged as the propensity to reduce underlying rates of silica formation. With a Si bond coating, a ~180 µm Yb2Si2O7 (YbDS) top coating reduced rates over the entire 1250°‐1350℃ range. Without a Si bond coating, ~60 µm (YbDS) coatings deposited directly onto CVD SiC exhibited poor adhesion, and had to be deposited onto substrates with enhanced roughness at 1350℃. While exhibiting good adhesion at 1350℃, overall the single layer YbDS coating exhibited a decreasing effectiveness from 1250° to 1350℃.

Hot Corrosion of Shipboard Gas Turbine Blades

Turbine blades removed from the first stage of a shipboard gas turbine engine for excessive degradation were characterized. Scanning electron microscopy coupled with energy-dispersive spectroscopy, X-ray diffraction, and inductively coupled optical emission spectroscopy was used to characterize corrosion deposits and features of field hardware that are not typically obtained in controlled laboratory settings. Corrosion was associated with deposits of varying compositions on the airfoil, beneath the platform, and within cooling passages. Deposits on the airfoil were primarily sodium sulfate presumably derived from seawater. Deposits below the platform and within cooling channels were crystalline aggregates of Ca, Mg, Al, and Si compounds presumably derived from dust and sand. FactSage thermochemical calculations were performed for gas turbine environments, and results are used to explain variations in deposit chemistry. The results show that solid sodium sulfate may not be retained in some gas turbine conditions, leaving the deposits rich in Ca and Mg compounds.

Comparative studies on the hot corrosion behavior of air plasma spray and high velocity oxygen fuel coated Co-based L605 superalloys in a gas turbine environment

An improvement in the corrosion resistance of alloys at elevated temperature is a factor for their potential use in gas turbines. In this study, CoNiCrAlY has been coated on the L605 alloy using air plasma spray (APS) and high-velocity oxygen fuel (HVOF) coating techniques to enhance its corrosion resistance. Hot corrosion studies were conducted on uncoated and coated samples in a molten salt environment at 850°C under cyclic conditions. Thermogravimetric analysis was used to determine the corrosion kinetics. The samples were subjected to scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction for further investigation. In coated samples, the formation of Al2O3 and Cr2O3 in the coating acts as a diffusion barrier that could resists the inward movement of the corrosive species present in the molten salt. Coated samples showed very less spallation, lower weight gain, less porosity, and internal oxidation as compared to uncoated sample. HVOF-coated sample showed greater corrosion resistance and inferred that this is the best technique under these conditions.

Tízéves a teljes hatáskörű digitális elektronikus hajtómű-szabályozó rendszerek fejlesztése a BME VRHT-n

A gázturbinás hajtóművek napjainkban már szinte kizárólagosan digitális elektronikus szabályozó rendszerrel (Full Authority Digital Electronic Control, FADEC) rendelkeznek, jobb pontosságuk, kisebb méretük, tömegük és karbantartásigényük miatt. A BME VRHT jogelődjén 2009-ben indult meg a TKT-1 gázturbinás sugárhajtómű megalkotását és üzembe állítását követően a szabályozórendszer tervezése. A cikk bemutatja a fejlesztés főbb állomásait a kiinduló, valamint további típusokon (Deutz T216, PD-60R), a különféle alkalmazott szabályozási törvényszerűségeket (fordulatszám, hajtómű nyomás- és teljesítményviszony). Továbbá bemutatjuk azokat az algoritmusokat is (PID, lineáris kvadratikus és csúszómód-szabályozások), amelyeket FADEC-rendszereinkben kidolgoztunk.Munkánk egyik legnagyobb eredménye, hogy a gázturbinás sugárhajtómű próbapadján ezeketaz algoritmusokat valós körülmények között is le tudjuk ellenőrizni.

High temperature behavior of superalloy under simulating gas turbine environment

On prolonged exposure of superalloys at elevated temperature, the surface degradation is prone to involve. In this study two nickel – based superalloys, Inconel617 and Hastelloy are the materials used to perform surface studies. The behaviors of superalloys at elevated temperatures are studied with and without salt mixture. Samples are exposed at 900° and 1000°C for a period of fifty hours. From the investigation, the cumulative mass changes per unit area are calculated with reference to the proportion of surface scale formation. The substantial role of the alloying elements is analyzed through SEM and XRD analysis. The Hastelloy has better resistance towards corrosion and oxidation resistance than Inconel617. The predominant oxides formed during the exposure at high temperature have a major contribution towards the protection of the samples.

Environmental Barrier Coatings Made by Different Thermal Spray Technologies

Environmental barrier coatings (EBCs) are essential to protect ceramic matrix composites against water vapor recession in typical gas turbine environments. Both oxide and non-oxide-based ceramic matrix composites (CMCs) need such coatings as they show only a limited stability. As the thermal expansion coefficients are quite different between the two CMCs, the suitable EBC materials for both applications are different. In the paper examples of EBCs for both types of CMCs are presented. In case of EBCs for oxide-based CMCs, the limited strength of the CMC leads to damage of the surface if standard grit-blasting techniques are used. Only in the case of oxide-based CMCs different processes as laser ablation have been used to optimize the surface topography. Another result for many EBCs for oxide-based CMC is the possibility to deposit them by standard atmospheric plasma spraying (APS) as crystalline coatings. Hence, in case of these coatings only the APS process will be described. For the EBCs for non-oxide CMCs the state-of-the-art materials are rare earth or yttrium silicates. Here the major challenge is to obtain dense and crystalline coatings. While for the Y2SiO5 a promising microstructure could be obtained by a heat-treatment of an APS coating, this was not the case for Yb2Si2O7. Here also other thermal spray processes as high velocity oxygen fuel (HVOF), suspension plasma spraying (SPS), and very low-pressure plasma spraying (VLPPS) are used and the results described mainly with respect to crystallinity and porosity.

Oxidation and hot corrosion studies on Fe-based superalloy A-286 pulsed current GTA weldments in gas turbine environment

Cyclic air oxidation and hot corrosion performance of pulsed current gas tungsten arc weldment (PCGTAW) of Fe-based superalloy A-286 are evaluated in the air and Na2SO4–7.5%NaVO3–5%NaCl molten salt at 700 °C. Thermogravimetric analysis is carried out to evaluate the corrosion kinetics of the weldments. Scanning electron microscope, energy dispersive spectroscopy, and x-ray diffraction methods are used to investigate the surface morphology, composition and phases of the air oxidized and hot corroded weldments. The depth of corrosion attack and distribution of alloying elements in the oxidized and hot corroded weldments are investigated at the cross sections using SEM microstructure and elemental mapping analysis. Some oxide scales spalling and sputtering are observed in PCGTA weldment in the molten salt environment during hot corrosion study which is not observed in air oxidized weldment at the end of 50 cycles. The hot corroded weldment shows a higher weight gain than oxidized weldment. Presence of molten salt increases the rate of corrosion. The cross-sectional analysis clearly explains, in both the cases higher oxidation rate is observed in the base metal zone.

Investigations of hot corrosion resistance of HVOF coated Fe based superalloy A-286 in simulated gas turbine environment

The main aim of the present work is to investigate the high-temperature corrosion behavior of bare, high-velocity oxy-fuel coated on iron-based superalloy A-286. Ni-20%Cr and Cr3C2-25%NiCr powders are deposited on the Fe based superalloy A-286 material by high-velocity oxy-fuel (HVOF) process. Surface roughness and coating thickness of the coated specimen is evaluated using surface roughness tester and optical microscope (OM). The hot corrosion study was performed on bare, and HVOF coated Fe-based superalloy A-286 in Na2SO4-50%V2O5-10%NaCl environment at 700 °C under cyclic conditions. Kinetics of corrosion has been determined by thermo-gravimetric analysis using mass change measurement; Cr3C2-25%NiCr coating shows the better corrosion resistance due to the formation of stable Cr2O3 layer. The uncoated superalloy is severely suffered by spallation and sputtering of oxide scales during hot corrosion study. The surface microstructure, chemical composition and phase identification of coatings before and after hot corrosion are analyzed using scanning electron microscope/ energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analysis. Depth of corrosion attack and oxide scales are characterized using EDS compositional analysis and elemental mapping technique. The result of the cross-sectional image shows that uncoated material was severely suffered by hot corrosion to a depth of 200 µm.

Investigation of Applicability of Transporting Water Mist for Cooling Turbine Blades

Abstract This paper presents a numerical study to investigate the feasibility of transporting water mist to the rotating blades of a high-pressure turbine. The idea of using mist film cooling to enhance conventional air cooling has been proven to be a feasible technique under laboratory conditions. However, there are challenges in implementing this scheme for real gas turbine systems. The first challenge is how to transport the mist to the rotating blades and the second challenge is delivering the mist to the injection holes and getting the particles to survive within the harsh gas turbine environment. Both a zero-dimensional mist evaporation analytical model and a 3D computational fluid dynamics (CFD) scheme are employed for analysis. In the CFD simulation, the Lagrangian–Eulerian method is used along with the discrete phase model (DPM) to track the evaporation process of each individual water droplet. For transporting the mist to the blades, the high-pressure water mist is injected into the stream of cooling air extracted from the compressor through two different passages. The first passage passes through the rotor cover-plate cavity before entering the blade base. The second passage passes through a diaphragm box on the base of the second vane, then tangentially through a cooling passage in the rotating shaft, and eventually to the blade base. The results show that it is feasible to transport the mist from the turbine casing to the blade through both passages, provided that droplets with sufficient particle diameter and mist loading are used. The shorter passage, through the nozzle diaphragm, alleviates a lot of challenges facing the passage through the blade cavity and seems to be more practical. A side benefit of transporting mist through the internal passages is the additional cooling of the preswirler and rotor cover plates. The results are encouraging for implementing the mist cooling technique under real gas turbine conditions.

Planar measurements of CO concentrations in flames at atmospheric and elevated pressure by laser-induced fluorescence

The article describes the development and application of a laser-optical technique for the quantitative measurement of carbon monoxide (CO) in combustion environments of elevated pressures relevant of gas turbine environments. Planar CO measurements can provide important information for the understanding of several aspects of the combustion process, such as localized lean blowout conditions or quenching effects of effusion cooling on the combustor flow. While laser-induced fluorescence is a sensitive non-intrusive measuring technique suitable for planar detection, the CO molecule cannot be excited directly in the visible or near UV. For the present implementation, a two-photon excitation scheme with resulting fluorescence in the visible range is utilized. To qualify the technique, planar measurements in a laminar premixed atmospheric flame are performed in a first step; this flame is later on used for calibration of CO concentrations in a laminar premixed flame at elevated pressure up to 5 bar and in a generic turbulent combustor with a swirl-stabilized flame operated at atmospheric pressure. Pressure effects on spectral structures and the calibration procedure are discussed, as well as potential interference by cross-sensitivity with other species. Based on this optimization strategies are defined for the respective experimental arrangements.

Premixed ammonia/hydrogen swirl combustion under rich fuel conditions for gas turbines operation

Energy storage is one of the highest priority challenges in transitioning to a low-carbon economy. Fluctuating, intermittent primary renewable sources such as wind and solar require low-carbon storage options to enable effective load matching, ensuring security of supply. Chemical storage is one such option, with low or zero carbon fuels such as hydrogen, alcohols and ammonia having been proposed. Ammonia provides zero-carbon hydrogen storage whilst offering liquefaction at relatively low pressures and atmospheric temperatures, enabling ease of transportation in a pre-existing infrastructure. Ammonia can also be used directly as a fuel in power plants such as gas turbines to avoid complete conversion back to hydrogen. It is a relatively unreactive fuel, and so it is of interest to explore the potential utilisation of ammonia/hydrogen mixtures. Hence, the goal of this paper is to provide a first assessment of the suitability of a chosen 70%NH330%H2 (%vol) blend for utilisation within a gas turbine environment, based on primary combustion diagnostics including combustion stability – via OH chemiluminescence - and emissions (NOx and NH3). An established optical generic swirl-burner enabled studies of the influence of equivalence ratio (φ > 1), ambient temperature (<484 ± 10 K) and bypass air, with a focus on NOx reduction, one of the main challenges for ammonia combustion. A numerical GT cycle model is developed alongside the experimental investigation. The results demonstrate that the blend has considerable potential as a fuel substitute with reasonable combustion stability and significant reduction of emissions for the cases without bypass air, due to increased chemical reactivity of unburned ammonia. However, emissions are still above those recommended for gas turbine cycles, with a theoretical cycle that still produces low efficiencies compared to DLN methane, highlighting the requirement for new injection techniques to reduce NOx/unburned NH3 in the flue gases whilst ensuring increased power outputs.

Fibre optical sensors for harsh gas turbine environments

Four selected applications of fibre optical sensors in industrial gas turbines will be reviewed. The measurement challenges, the operating principle of the corresponding fibre optical sensing solution and associated test results will be described.

Hot corrosion behaviour of nickel–iron-based superalloy in gas turbine application

ABSTRACT Nickel-based superalloys are mostly used as gas turbine components. During combustion, the sodium sulphate and sodium chlorides with vanadium pentoxide are dominating corrosive species in gas turbine environment. The eutectic point of the salt deposits varies with respect to combustion/operating temperature and corrosion kinetics are initiated. In this research, hastelloy X was chosen as a candidate material in a simulated oxidation and gas turbine hot corrosion at 1100°C. The mass change per unit area is gradual in mixed salt environment and catastrophic under atmospheric air for oxidation studies. This observation may be due to the oxide scale formation and spalling of the same on prolonged exposure. The exposed samples were subject to SEM, EDS and XRD analysis. The samples exposed to molten salt have undergone different surface morphology during the incubation period. The features of the candidate material are discussed in detail.