Gas Turbine Applications Research Articles

Improvement in thermal shock resistance of gadolinium zirconate coating by addition of nanostructured yttria partially-stabilized zirconia

Gadolinium zirconate (Gd2Zr2O7, GZ) as one of the promising thermal barrier coating materials for high-temperature application in gas turbine was toughened by nanostructured 3mol% yttria partially-stabilized zirconia (YSZ) incorporation. The fracture toughness of the composite of 90mol% GZ-10mol% YSZ (GZ–YSZ) was increased by about 60% relative to the monolithic GZ. Both the GZ and GZ–YSZ composite coatings were deposited by atmospheric plasma spraying on Ni-base superalloys and then thermal-shock tested under the same conditions. The thermal-shock lifetime of GZ–YSZ composite coating was improved, which is believed to be mainly attributed to the enhancement of fracture toughness by the addition of YSZ. In addition, the failure mechanisms of the thermal-shock tested GZ–YSZ composite coatings were discussed.

Numerical simulation of oxy-fuel combustion for gas turbine applications

Relevant reduction on worldwide greenhouse gases emissions shall be based on more efficient power generation systems linked to a carbon capture and storage technology (CCS). Integrated gasification combined cycle and natural gas combined cycle (IGCC) would play an effective role to these objectives. To that, oxy-fuel combustion is an important alternative for the implementation of CCS technology, claiming, however, modifications to the conventional operation of gas turbines combustors.This work presents CFD simulations of oxy-fuel combustion in a can-type model gas turbine combustor. Equilibrium combustion model for non-premixed flames was used to model chemical reactions and turbulence was accounted for by Reynolds Stress Model (RSM). Numerical predictions were carried out after validating the numerical schemes and turbulence model. In addition to the baseline (propane/air combustion) four oxy-fuel cases were studied: propane and syngas oxy-fuel combustion with two different O2/CO2 ratios.The predictions showed that the propane/oxy-fuel flame (O2/CO2 = 1) resembles the baseline case (propane/air) though, with much higher temperature levels in the central core of the combustor, resulting in a poor gas turbine combustor pattern factor. The oxy-syngas flames showed intense reactions in the primary zone, before the first dilution holes, leading to a very good patter factor.The numerical model proposed for this study can be considered a relevant tool in the preliminary design phase of gas turbine combustors applied to CCS technology.

Analysis of Pollutant Emissions in Double-stage Swirl Chamber for Gas Turbine Application

Abstract The present paper presents an experimental investigation about a double-stage swirl combustor for future application in gas turbine, in which the unfavorable conditions for pollutant formation (CO, UHC, NOx) are achieved by reagents and burned gases flow dynamics control into the combustor. The lean global combustion regime takes place in two chambers and is controlled by the parameters: global equivalence ratio (O), fuel jet Reynolds number (Rej) and swirler blades angle (a). The results have showed that when these parameter contribute to recirculation zone intensification formed at secondary chamber, ie, higher swirler angles and smaller Reynolds numbers, CO and UHC are reduced. For NOx this behavior is also observed, the only exception is the swirler angle, i.e., the NOx increases when the swirler angle also increases. In addition, as expected, when the equivalence ratio increase the CO e UHC emissions reduce, on the other hand, the NOx emissions increase.

High Speed Machining of Inconel 718 Focusing on Tool Surface Topography of CBN Tool

Nickel-based superalloys such as Inconel 718 provide several advantages, including a high-temperature strength and high corrosion resistance. Therefore the demand for such materials has rapidly increased, particularly in the aviation and gas turbine applications. On the other hand, they are known as one of the most difficult-to-cut materials due to their mechanical and chemical properties and the tool life is extremely short. Recently, Cubic-Boron-Nitride (CBN) has received a considerable attention as a material for cutting tools and has already established itself in many areas of machining. However, the performance of CBN tools is still insufficient in practical use, especially in the high speed machining of Inconel 718. To overcome this problem, we first conducted orthogonal cutting experiments on Inconel 718 in a wide range of cutting speeds (20 m/min - 300 m/min) in order to clarify the wear mechanisms of the CBN tool in high speed machining. As a result, it was found that the micro tool surface topography, such as grinding marks on the tool rake face, has a significant impact on the progression of the crater wear of the CBN tool when the cutting speed was increased to over 100 m/min. Based on these results, we prepared several cutting tools with different surface topography and evaluated their cutting performances. The experimental results showed that the polished rake face reduced the initial crater wear by approximately 40% compared to the non-polished tool.

Analysis of micro-recuperators in small-sized gas turbines –manufacturing potential of Iran

Heat recuperation is often used to improve the overall cycle efficiency of gas turbines. However, generally in small- scale gas turbines, it has a negative effect on turbine inlet temperature, pressure ratio and pressure drops, and thus decreases the overall cycle efficiency. In this paper, a thermodynamic model is performed to evaluate recovered heat as a function of heat exchanger effectiveness, pressure drops, and defines the overall cycle energy and exergy efficiency. A high heat exchanger effectiveness, and low pressure drops are favorable to achieve maximal cycle energy efficiency. The main challenge in recuperator designis to find compromise between these conflicting requirements. Hence, a thermodynamic model is developed to determine recuperator design with an aim to maximize overall cycle energy and exergy efficiency. Further, to analyze Iran's manufacturing and its technological capabilities,a pre-requisite is considered for the proposed model. Hence, Iran's capability level could be determined. Finally, a case study of selecting recuperator of a 200kW gas turbine is conducted. Industrial gas turbines show performance characteristics that distinctly depend on ambient and operating conditions. They are influenced by site elevation, ambient temperature, and relative humidity. Proper application of gas turbines requires consideration of these factors. Thus, in this study, the effect of ambient conditions on overall cycle energy and exergy efficiency is performed by the proposed thermodynamic model.

Indentations on Air Plasma Sprayed Thermal Barrier Coatings Prepared by Different Starting Granules

The effect of starting granules on the indentation properties of air plasma sprayed thermal barrier coatings (TBCs) is investigated in this paper. Various kinds of spray‐dried granules are prepared from different processing conditions, especially varying solvent and dispersant, showing a deformed hollow‐typed and a filled spherical‐typed granule. The similar coating thicknesses are prepared by adjusting process parameters during air plasma spray. All XRD peaks in phase analysis are tetragonal and cubic phases without any monoclinic phase after the starting granules were heat‐treated. A relatively porous microstructure of the coating layer could be obtained from the monodisperse granules, while a relatively dense microstructure resulted from the hollow‐typed granules. The morphology and distribution of the granules crucially affect the microstructure of thermal barrier coatings and thus have influences on indentation properties such as indentation stress‐strain curves, contact damage, and hardness. The implication concerning microstructure design of TBCs for gas turbine applications is considered.

Microstructural characterization of a new mechanically alloyed Ni-base ODS superalloy powder

The microstructure of a new Ni-base oxide dispersion strengthened superalloy powder was studied for high temperature gas turbine applications after the mechanical alloying process. In this study, an atomized powder with a composition similar to the CMSX-10 superalloy was mechanically alloyed with yttria and Hf powders. The mechanically alloyed powder included only the supersaturated solid solution γ phase without γ′ and yttria provided by severe plastic deformation, while after the 3-step aging, the γ′ phase was precipitated due to the partitioning of Al and Ta to the γ′ and Co, Cr, Re, W, and Mo to the γ phase. Mechanical alloying modified the morphology of γ′ to the new coherent γ–γ′ nanoscale lamellar structure to minimize the elastic strain energy of the precipitation, which yielded a low lattice misfit of 0.16% at high temperature. The γ′ lamellae aligned preferentially along the elastically soft [100] direction. Also, the precipitated oxide particles were refined in the γ phase by adding Hf from large incoherent YAlO3 to fine semi-coherent Y2Hf2O7 oxide particles with the average size of 7nm and low interparticle spacing of 76nm.

Synthesis of a new tungsten-free γ–γ′ cobalt-based superalloy by tuning alloying additions

The paper presents the synthesis of a new class of γ–γ′ cobalt-based superalloy that is free of tungsten as an alloying addition. It has much lower density and higher specific strength than the existing cobalt-based superalloys. The current superalloys have a base composition of Co–10Al and are further tuned by the addition of a binary combination of molybdenum and niobium, with the optimum composition of Co–10Al–5Mo–2Nb. The solvus temperature of the alloy (866°C) can be further enhanced above 950°C by the addition of Ni to give the form Co–xNi–10Al–5Mo–2Nb, where x can be from 0 to 30at.%. After heat treatment, these alloys exhibit a duplex microstructure with coherent cuboidal L12-ordered precipitates (γ′) throughout the face-centred cubic matrix (γ), yielding a microstructure that is very similar to nickel-based superalloys as well as recently developed Co–Al–W-based alloys. We show that the stability of the γ′ phase improves significantly with the nickel addition, which can be attributed to the increase in solvus temperature. A very high specific 0.2% proof stress of 94.3MPag−1cm−3 at room temperature and 63.8MPag−1cm−3 at 870°C were obtained for alloy Co–30Ni–10Al–5Mo–2Nb. The remarkably high specific strength of these alloys makes this class of alloy a promising material for use at high temperature, including gas turbine applications.

Materials for future gas turbine applications

The complexity of adopting new or using existing advanced alloy systems in demanding environments where safety is paramount is highlighted. Of particular importance is the ability to define operative deformation and degradation mechanisms that may limit part life or functionality. With respect to the underlying fundamental material science there is a high degree of commonality between the aero and power generation sectors. The vast array of new experimental and modelling techniques that can be brought to bear on long standing technical challenges offers the potential for a renaissance in materials science. Of particular interest from an industrial context would be the ability to ‘design’ material microstructure and texture optimised for service use manufactured via the most energy and time efficient route. The requirements and potential for progress in this area is summarised with an emphasis on gas turbine and nuclear reactor applications for hcp metals.

Conjugate heat transfer analysis of the effects of impingement channel height for a turbine blade endwall

Advancements in cooling for applications such as gas turbines components require improved understanding of the complex heat transfer mechanisms and the interactions between those mechanisms. Critical cooling applications often rely on multiple thermal protection techniques, including internal cooling and external film cooling in gas turbine airfoils, to efficiently cool components and limit the use of coolant. Most research to quantify the effectiveness of such cooling technologies for gas turbine applications has isolated internal and external cooling in separate experiments. The research presented in this paper uses a conjugate heat transfer approach to account for the combined effects of both internal and external cooling. The geometry used for this study is a turbine blade endwall that includes impingement and film cooling as well as the relevant conduction through the endwall. Appropriate geometric and flow parameters were scaled to ensure engine relevant dimensionless temperatures were obtained. Using the conjugate heat transfer approach, the effect of varying the height of the impingement channel was examined using spatially resolved external wall temperatures obtained from both experiments and simulations. A one-dimensional heat transfer analysis was used to derive the average internal heat transfer coefficients from the experimental results. Both experiments and simulations showed good agreement between area averaged cooling effectiveness and impingement heat transfer coefficients. The cooling effectiveness and heat transfer coefficients peaked for an impingement channel height of around three impingement hole diameters. However, the heat transfer coefficients were more sensitive than the overall effectiveness to the changes in height of the impingement channel.

The Application of Discontinuous Galerkin Methods in Conjugate Heat Transfer Simulations of Gas Turbines

The performance of modern heavy-duty gas turbines is greatly determined by the accurate numerical predictions of thermal loading on the hot-end components. The purpose of this paper is: (1) to present an approach applying a novel numerical technique—the discontinuous Galerkin (DG) method—to conjugate heat transfer (CHT) simulations, develop the engineering-oriented numerical platform, and validate the feasibility of the methodology and tool preliminarily; and (2) to utilize the constructed platform to investigate the aerothermodynamic features of a typical transonic turbine vane with convection cooling. Fluid dynamic and solid heat conductive equations are discretized into explicit DG formulations. A centroid-expanded Taylor basis is adopted for various types of elements. The Bassi-Rebay method is used in the computation of gradients. A coupled strategy based on a data exchange process via numerical flux on interface quadrature points is simply devised. Additionally, various turbulence Reynolds-Averaged-Navier-Stokes (RANS) models and the local-variable-based transition model γ-Reθ are assimilated into the integral framework, combining sophisticated modelling with the innovative algorithm. Numerical tests exhibit good consistency between computational and analytical or experimental results, demonstrating that the presented approach and tool can handle well general CHT simulations. Application and analysis in the turbine vane, focusing on features around where there in cluster exist shock, separation and transition, illustrate the effects of Bradshaw’s shear stress limitation and separation-induced-transition modelling. The general overestimation of heat transfer intensity behind shock is conjectured to be associated with compressibility effects on transition modeling. This work presents an unconventional formulation in CHT problems and achieves its engineering applications in gas turbines.

A New Polycrystalline Co-Ni Superalloy

In 2006, a new-ordered L12 phase, Co3(Al,W), was discovered that can form coherently in a face-centered cubic (fcc) A1 Co matrix. Since then, a community has developed that is attempting to take these alloys forward into practical applications in gas turbines. A new candidate polycrystalline Co-Ni γ/γ′ superalloy, V208C, is presented that has the nominal composition 36Co-35Ni-15Cr-10Al-3W-1Ta (at.%). The alloy was produced by conventional powder metallurgy superalloy methods. After forging, a γ′ fraction of ~56% and a secondary γ′ size of 88 nm were obtained, with a grain size of 2.5 μm. The solvus temperature was 1000°C. The density was found to be 8.52 g cm−3, which is similar to existing Ni alloys with this level of γ′. The alloy showed the flow stress anomaly and a yield strength of 920 MPa at room temperature and 820 MPa at 800°C, similar to that of Mar-M247. These values are significantly higher than those found for either conventional solution and carbide-strengthened Co alloys or the γ/γ′ Co superalloys presented in the literature thus far. The oxidation resistance, with a mass gain of 0.08 mg cm−2 in 100 h at 800°C, is also comparable with that of existing high-temperature Ni superalloys. These results suggest that Co-based and Co-Ni superalloys may hold some promise for the future in gas turbine applications.

CFD analysis of solar tower Hybrid Pressurized Air Receiver (HPAR) using a dual-banded radiation model

Solar receivers used for central tower Concentrated Solar Power (CSP) plants use either a surface-based or volumetric heat transfer region. Volumetric receivers are more efficient but require more sophisticated and expensive materials that can withstand the elevated temperatures (in excess of 1000°C). For a solarized gas-turbine application, pressurized air from the compressor is used as heat transfer fluid (HTF) in order to increase both the density and heat capacity of the HTF. If a volumetric receiver is used, it needs to be located in a pressure vessel with a pressurized quartz window located at the aperture of the concentrator. An alternative approach to utilize pressurized air in a pseudo-volumetric fashion is to populate a volumetric region with piped pressurized air. A tubular-type volumetric receiver (named a Hybrid Pressurized Air Receiver (HPAR)) is studied here. The HPAR provides the challenge of enabling maximum heat transfer without causing hot spots on the side of the solar irradiation source. Heat transfer in the HPAR would not be as effective as for a true volumetric receiver because the heat transfer area is limited by the tube wall. The heat transfer to the HTF is however enhanced through mixing generated by external forced convection caused by suction due to a downstream fan in the receiver cavity. In addition, the aperture of the cavity contains a glass windowed louver system, to limit re-radiation losses. A Computational Fluid Dynamics (CFD) model is generated of the solar receiver cavity. The commercial CFD code ANSYS Fluent v14.5 is used to evaluate the heat transfer between the incoming solar flux and the HTF. A numerical validation is performed to illustrate mesh dependency and the choice of turbulence model. The incoming solar irradiation and its absorption, reflection and transmission are modeled using the Discrete Ordinates (DO) radiation model in ANSYS Fluent. An idealized and a solar flux map based on the PS10 field are used as source. For comparison, both a gray (without a glass louver) and a semi-gray two-banded DO approach are used when modeling the absorption of high-wavelength re-radiation by the glass in front of the aperture. This two-banded approach is also applied to investigate the influence of a band-selective absorber tube and cavity surfaces. The DO model also predicts the emission of thermal re-radiation from all surfaces. The geometry is parameterized in order to allow for various cavity layouts to be automatically generated. Results include typical CFD results of a candidate geometry to illustrate the solar irradiation input, the effect of tubular layout as well as temperature and heat flux distributions for single and dual-band radiation.

Effect of flow velocity and temperature on ignition characteristics in laser ignition of natural gas and air mixtures

Laser induced spark ignition offers the potential for greater reliability and consistency in ignition of lean air/fuel mixtures. This increased reliability is essential for the application of gas turbines as primary or secondary reserve energy sources in smart grid systems, enabling the integration of renewable energy sources whose output is prone to fluctuation over time. This work details a study into the effect of flow velocity and temperature on minimum ignition energies in laser-induced spark ignition in an atmospheric combustion test rig, representative of a sub 15MW industrial gas turbine (Siemens Industrial Turbomachinery Ltd., Lincoln, UK). Determination of minimum ignition energies required for a range of temperatures and flow velocities is essential for establishing an operating window in which laser-induced spark ignition can operate under realistic, engine-like start conditions. Ignition of a natural gas and air mixture at atmospheric pressure was conducted using a laser ignition system utilizing a Q-switched Nd:YAG laser source operating at 532nm wavelength and 4ns pulse length. Analysis of the influence of flow velocity and temperature on ignition characteristics is presented in terms of required photon flux density, a useful parameter to consider during the development laser ignition systems.

Thermal Cycling Life of Porous Ceramic Coatings Subjected to Thermal Gradient and Coolant Pressure

A transpiration cooling system for gas turbine applications has significant benefit to reduce the amount of cooling air and to increase cooling efficiency. We had developed a porous ceramic coating deposited by plasma spraying process, which can infiltrate cooling gas, and examined about those mechanical properties. In this study, thermal cycling life of this porous ceramic coating is revealed in order to apply this technology to advanced gas turbine blade in practical use. The thermal cycling test is conducted by using the hand-made device which can heat cyclically up and down around the surface of the porous coating while infiltrating cooling gas. The number of thermal cycles up to reach the coating delamination is related with maximum exposed temperature and pressure of the cooling gas as the test condition, consequently.

Influence of plasma coatings on Inconel 617 for gas turbine applications

The Inconel 617 with plasma sprayed Al2O3–40%TiO2 (A40T) and NiCr–Cr2O3 coatings were investigated for the hot corrosion behaviour at 1000°C for gas turbine applications. The behaviour of the alloy under the mixed salt environment with and without V2O5 in Na2SO4 and NaCl was analysed by means of mass change per unit area, optical and SEM images along with X-ray diffraction and EDAX. Microstructural and EDAX analyses revealed presence of oxides, rich in Ni and Mo on the surface of the samples. The coated sample exhibited better corrosion resistance in A40T and NiCr – Cr2O3 coatings than bare alloy.

Flow Control Over a Circular Cylinder Using Pulsed Dielectric Barrier Discharge Actuators

Abstract Immersed bodies such as struts, vanes, and instrumentation probes in gas turbine flow systems will, except at the lowest of flow velocities, shed separated wakes. These wakes can have both upstream and downstream effects on the surrounding flow. In most applications, surrounding components are designed to be in the presence of a quasi-steady or at least nonvariant flow field. The presence of unsteady wakes has both aerodynamic and structural consequences. Active flow control of wake generation can therefore be very valuable. One means to implement active flow control is by the use of plasma actuation. Plasma actuation is the use of strong electric fields to generate ionized gas that can be actuated and controlled using the electric fields. The controlling device can be based on AC, DC, or pulsed-DC actuation. The present research was conducted using pulsed-DC from a capacitive discharge power supply. The study demonstrates the applicability of, specifically, pulsed-DC plasma flow control of the flow on a circular cylinder at high Reynolds numbers. The circular cylinder was selected because its flow characteristics are related to gas turbine flowpath phenomena, and are well characterized. Further, the associated pressure gradients are some of the most severe encountered in fluid applications. The development of effective plasma actuators at high Reynolds numbers under the influence of severe pressure gradients is a necessary step toward developing useful actuators for gas turbine applications beyond laboratory use. The reported experiments were run at Reynolds numbers varying from 50,000 to 97,000, and utilizing various pulse frequencies. Further the observed performance differences with varying electric field strengths are discussed for these Reynolds numbers. The results show that flow behaviors at high Reynolds numbers can be influenced by these types of actuators. The actuators were able to demonstrate a reduction in both wake width and momentum deficit.

Oxidation and Hot Corrosion Behavior of Nickel-Based Superalloy for Gas Turbine Applications

This paper investigates the cyclic oxidation and hot corrosion behavior of Inconel 617 at elevated temperatures. The substantial role of alloying elements with respect to oxidation and hot corrosion were analyzed with respect to corrosion kinetics, surface morphology and X-ray analysis of the oxide scales formed. The cumulative mass change shows that the presence of V2O5 in salt mixture plays predominant role in corrosion attack. Consistency of oxide scale, rich in Cr2O3, NiO, Al2O3 and NiCr2O4 was confirmed through X-ray analysis. Inconel 617 has good oxidation resistance and prone to corrosion attack at elevated temperatures due to the presence of vanadium in salt mixture which accelerates hot corrosion.

Influence of overloads on dwell time fatigue crack growth in Inconel 718

Inconel 718 is one of the most commonly used superalloys for high temperature applications in gasturbines and aeroengines and is for example used for components such as turbine discs. Turbine discs can be subjected to temperatures up to ~700°C towards the outer radius of the disc. During service, the discs might start to develop cracks due to fatigue and long dwell times. Additionally, temperature variations during use can lead to large thermal transients during start-up and shutdown which can lead to overload peaks in the normal dwell time cycle. In this study, tests at 550°C with an overload prior to the start of each dwell time, have been performed. The aim of the investigation was to get a better understanding of the effects of overloads on the microstructure and crack mechanisms. The microstructure was studied using electron channelling contrast imaging (ECCI). The image analysis toolbox in Matlab was used on cross sections of the cracks to quantify: crack length, branch length, and the number of branches in each crack. It was found that the amount of crack branching increases with an increasing overload and that the branch length decreases with an increasing overload. When the higher overloads were applied, the dwell time effect was almost cancelled out. There is a strong tendency for an increased roughness of the crack path with an increasing crack growth rate.

Simulations of Autoignition and Laminar Premixed Flames in Methane/Air Mixtures Diluted with Hot Products

This article considers constant-pressure autoignition and freely propagating premixed flames of cold methane/air mixtures mixed with equilibrium hot products at high enough dilution levels to burn within the moderate to intense low oxygen dilution (MILD) combustion regime. The analysis is meant to provide further insight on MILD regime boundaries and to identify the effect of hot products speciation. As the mass fraction of hot products in the reactants mixture increases, autoignition occurs earlier. Species profiles show that the products/reactants mixture approximately equilibrates to a new state over a quick transient well before the main autoignition event, but as dilution becomes very high, this equilibration transient becomes more prominent and eventually merges with the primary ignition event. The dilution level at which these two reactive zones merge corresponds well with that marking the transition into the MILD regime, as defined according to conventional criteria. Similarly, premixed flame simulations at high dilutions show evidence of significant reactions involving intermediate species prior to the flame front. Since the premixed flame governing equations system demands that the species and temperature gradients be zero at the “cold” boundary, flame speed cannot be calculated above a certain dilution level. Up to this point, which again agrees reasonably well with the transition into the MILD regime according to convention, the laminar burning velocity was found to increase with hot product dilution while flame thickness remained largely unchanged. Some comments on the MILD combustion regime boundary definition for gas turbine applications are included.