High Temperature Gas Turbine Components Research Articles

Stress evaluation of TBCs with inclined film-cooling hole considering CMAS penetration-induced change of thermo-mechanical properties

CaO-MgO-Al2O3-SiO2 (CMAS) penetration is an essential factor leading to the failure of thermal barrier coatings (TBCs), especially around the film-cooling hole structure of high-temperature gas turbine components. The complex temperature distribution and the free-edge effect around the cooling holes aggravate the influence of CMAS penetration on the failure of the TBCs. In this study, a three-dimensional (3D) model of a TBC-film cooling system considering CMAS attack is presented based on the observation of a retired turbine blade. CMAS penetration-induced change of thermo-mechanical properties of the ceramic top coat (TC) was considered. The thermal gradient around a cooling hole and the uneven growth of the thermally grown oxide (TGO) on the stress distribution in TBCs were investigated. The results showed that the thermal insulation performance of TBCs declined considerably under CMAS penetration, and the induced maximum increase in the temperature at the TC/bond coat (BC)interface reached 60 K. CMAS penetration also led to a considerable stress increase in the TBC-film system. We therefore conclude that CMAS penetration accelerates the crack initiation at the TC/TGO and TGO/BC interfaces.

Review of Film Cooling in Gas Turbines with an Emphasis on Additive Manufacturing-Based Design Evolutions

Film-cooling technology is used in high-temperature components of gas turbines to extend their service lives. Hot-gas path components are susceptible to damage or failure in the absence of film cooling. Much of the optimization research efforts have been focused on film hole shapes, heat/mass transfer measurement techniques, and film cooling performance under various mainstream and coolant side operating conditions. Due to recent rapid advancements in the areas of measurement techniques (e.g., pressure-sensitive paints and fast high-resolution imaging) and metal additive manufacturing (AM), film cooling technology has undergone significant changes and shows potential new development. In this review, a historical perspective is discussed covering over five decades of innovation: the geometrical effects from injection angle and hole shapes; flow effects from density ratio, momentum-flux ratio, blowing ratio, advective capacity ratio, and freestream conditions; and more items related to AM. The impact of AM on film hole design strategies, the challenges posed by state-of-the-art AM technology, and pathways for future research are discussed. A comparative analysis of AM assisted film hole fabrication and conventionally manufactured film holes is elaborated.

Multi-objective materials bayesian optimization with active learning of design constraints: Design of ductile refractory multi-principal-element alloys

Bayesian Optimization (BO) has emerged as a powerful framework to efficiently explore and exploit materials design spaces. To date, most BO approaches to materials design have focused on the materials discovery problem as if it were a single expensive-to-query ‘black box’ in which the target is to optimize a single objective (i.e., material property or performance metric). Also, such approaches tend to be constraint agnostic. Here, we present a novel multi-information BO framework capable of actively learning materials design as a multiple objectives and constraints problem. We demonstrate this framework by optimally exploring a Refractory Multi-Principal-Element Alloy (MPEA) space, here specifically, the system Mo-Nb-Ti-V-W. The MPEAs are explored to optimize two density-functional theory (DFT) derived ductility indicators (Pugh’s Ratio and Cauchy pressure) while learning design constraints relevant to the manufacturing of high-temperature gas-turbine components. Alloys in the BO Pareto-front are analyzed using DFT to gain an insight into fundamental atomic and electronic underpinning for their superior performance, as evaluated within this framework.

Conjugate Heat Transfer and Flow Features of Single-Hole and Combined-Hole Film Cooling With Rib-Roughened Internal Passages

Abstract Internal cooling and film cooling, as two main cooling methods in modern gas turbines, work together to protect the high-temperature components of gas turbines. This paper presents the results of a computational study on cooling performance for a flat plate with both film cooling and internal cooling using a conjugate heat transfer analysis. Three internal delivery channel geometries, smooth channel, channel roughened by square ribs (SR), and channel roughened by crescent ribs (CR), are studied with two film cooling geometries, cylindrical hole, and sister holes (SS). The respective conjugate cooling performances are compared. Detailed flow and heat transfer characteristics are presented and discussed. Results show that both film cooling effectiveness and internal cooling performances are influenced by the delivery channel geometry near the hole inlets. The sink flow effects of film cooling enhance the heat transfer coefficient near the film cooling hole inlet. At the same time, film cooling performance is affected by the internal channel as the flow inside the film cooling hole is influenced by the ribs near the hole inlets. When using sister holes, ribs in the internal channels make the anti-kidney vortex structure created by sister holes more effective by changing the mass flow distribution among the three holes.

Changes in high-temperature thermal properties of modified YSZ with various rare earth doping elements

To protect the high-temperature components of gas turbines, 6–8 wt.% yttria stabilised zirconia (YSZ) has been extensively used as a thermal barrier coating (TBC) material. However, its application is severely limited at high temperatures because of zirconia phase transition and sintering densification above 1200 °C. This study developed modified YSZ with enhanced high-temperature thermal properties owing to the addition of various rare-earth doping elements. Among the various rare earth-doped compositions, all the thermal properties were significantly improved in compositions containing scandium, gadolinium, and dysprosium. Furthermore, in the selected compositions, the high-temperature thermal properties were analysed under heat treatment conditions of 1300 °C, with a target turbine inlet temperature (TIT) of 1500 °C. The high-temperature phase stability of the tetragonal phase was significantly improved in the newly developed compositions, and they exhibited glass-like low thermal conductivity (∼0.984 W/mK) due to the influence of lattice distortion caused by the differences in the substituent-ion mass and size, and the oxygen vacancies. Moreover, there was notable improvement in the thermal expansion coefficient (∼11 × 10−6/K) and resistance to high-temperature densification.

Method for Predicting Thermal Fatigue Life of Thermal Barrier Coatings Using TGO Interface Stress

Thermal barrier coatings (TBCs) applied to high-temperature components of gas turbines consist of a ceramic top coat, a metallic bond coat, and thermally grown oxide (TGO) generated between the top coat and bond coat. Because TBCs are subjected to repeated thermal stress at the coating interface under thermal fatigue conditions and eventually breakage, it is crucial to evaluate the thermal fatigue durability of TBCs according to the stress. In this study, coin-type TBC specimens were prepared by depositing commercial coating powders on Ni-based super alloys via the air plasma spray method, and the thermal fatigue life of the TBCs was experimentally evaluated. According to the test results and references, a finite-element analysis was conducted. The maximum stress of the TGO interface was evaluated according to the thickness and equivalent elastic modulus, and simulating the microstructure including the pores of the top coat. Using these relationships, a thermal fatigue life prediction equation considering the coating thickness (t), equivalent elastic modulus (E), and operating temperature (T) was derived, and subsequently verified.

Flow Characteristics of Microscale Porous Media with Varying Porosity and Pore Size

Sintered porous media are considered to be one of the best candidates to cool high temperature gas turbine components with blades and vanes for next-generation air cooling. The sintered porous media used in this study is made of the stainless steel SUS316L by metal injection molding process. The complex interaction between fluid and the porous medium causes fluid pressure drop when the fluid is forced to flow through the porous surface. In addition, the information on the relation between porosity and pressure drop of the porous media is important to saving the costs of the filter application and cooling process. This research is intended to understand flow characteristics of the porous media with the air fluid movement according to different porosity and pore size for gas turbine application. The experimental data compared with the calculation are examined for different experimental conditions, which indicate flow behavior of erratic and compressible on the microscopic scale porous media.

Isothermal Oxidation Comparison of Three Ni-Based Superalloys

Ni-based superalloys are used for high-temperature components of gas turbines in both industrial and aerospace applications due to their ability to maintain dimensional stability under conditions of high stress and strain. The oxidation resistance of these alloys often dictates their service lifetime. This study focuses on the isothermal oxidation behavior of three Ni-based superalloys, namely, polycrystalline cast IN738LC, single-crystal N5, and a ternary Ni-Fe-Cr (TAS) powder metallurgy alloy. The isothermal oxidation tests were conducted at 900 °C in the static air up to 1000 h, and the specific aspects studied were the oxidation behavior of these chromia-forming and alumina-forming alloys that are used extensively in industry. In particular, the behavior of oxide scale growth and subsurface changes were analyzed in detail using various techniques such as SEM, EDS, and AFM. From the isothermal oxidation kinetics, the oxidation rate constant, k p, was calculated for each alloy and found to be; k p = 2.79 × 10−6 mg2 cm−4 s−1 for IN738LC, k p = 1.42 × 10−7 mg2 cm−4 s−1 for N5 and k p = 1.62 × 10−7 mg2 cm−4 s−1 for TAS. Based on a microstructural analysis, IN738LC exhibited a continuous dense outer scale of Cr2O3 and discontinuous inner scale of Al2O3, whereas N5 and TAS showed a dense outer scale of Al2O3 alone. The results suggested that the N5 and PM-TAS alloys are more oxidation resistant than the IN738LC under these conditions.

Damage Model for Degradation Induced by Volcanic Ash Deposition on Thermal Barrier Coatings

The importance of a land-based gas turbine has been increased in order to maintain energy security in Japan. Since our country has been always exposed to a natural disaster, it is necessary to consider about influence of air intake of volcanic ash spouted from a volcanic explosion on high-temperature components in gas turbine as well as vibration by an earthquake event. In this study, a high-temperature exposure tests were conducted for 8YZ/CoNiCrAlY/IN-738LC sample on which volcanic ash was deposited. Here, we selected the natural ash picked up from Shin-moe mountain where had been exploded in 2011. As a result, it was found that the top coating was peeled completely off at the exposure condition 1273 K for 100 hours, which means that volcanic ash deposition brings about some acceleration for the delamination life of TBC. The microstructure observation after the exposure tests revealed that Si and Ca contents involved in the volcanic ash was detected inside open-pores. Thus, it was identified that those ashes infiltrated into open pores along splat boundaries under a high-temperature exposure environment. The damage model for explaining degradation induced by volcanic ash deposition was developed based upon micromechanics. The formulae for constitutive equation of TBC was done by considering about influence of both crystal distortion by tetragonal to monoclinic phase transformation and thermal shrinking by temperature drop in the heat exposure treatment. Residual stress and interfacial energy were evaluated using the proposed constitutive equation in order to clarify degradation mechanisms for ash attack to TBC.

Biaxial Thermomechanical-Fatigue Life Property of a Directionally Solidified Ni-Base Superalloy

High-temperature components in thermal power plants are subjected to creep-fatigue loading where creep cavities initiate and grow on grain boundaries. Development of life assessment methods of high-temperature components in gas turbine for maintenance and operating cost reduction is strongly demanded by Japanese utilities. Especially, first row blades are subjected to complicated thermomechanical-fatigue (TMF) loading during start, steady state, stop cycles. Therefore it is important to clarify the TMF life property of blade materials to develop a life assessment procedure. In this study, tension-torsion biaxial TMF tests have been performed between 450°C and 870°C on a Ni-base directional solidified superalloy. Strain ratio ϕ was defined as shear strain range, Δγ, to normal strain range, Δε, and ϕ varied from 0 to infinity. The “Blade wave form,” which simulated temperature and strain condition of the blade surface, was employed. The biaxial TMF tests were also carried out on coated specimens with CoCrAlY. Fatigue life under the biaxial TMF loading showed strain ratio dependency giving shorter life with increasing ϕ. Considering biaxial stress effect on the failure life, an equivalent shear strain range was derived based on the Γ-plane theory, and the biaxial TMF life was well correlated with the equivalent shear strain range. The biaxial TMF life was reduced by introducing strain hold duration at the maximum temperature. The maximum stress increased by introducing the hold time due to increasing mean stress level in the Blade wave form. It was concluded that creep damage gradually accumulated during cycles resulting in reduction in the TMF life. The nonlinear creep-fatigue damage accumulation model was applied to predict failure life of the hold time tests. As a result, the failure lives were predicted within a factor of 1.5 on the observed life. It was found that the fatigue life of CoCrAlY coated material reduced 1∕2 to 1∕3 from that of the substrate. From observation of the longitudinal section of the coated specimens, many cracks started from the coating surface and penetrated into the substrate. It was concluded that the CoCrAlY coating reduced the biaxial TMF life due to acceleration of crack initiation period in the substrate.

MGC部材の極限環境下における熱安定性と高温ガスタービン部材への適用

MGC部材の極限環境下における熱安定性と高温ガスタービン部材への適用

Experimental Investigations of Radially Rotating Miniature High-Temperature Heat Pipes

A radially rotating miniature high-temperature heat pipe employs centrifugal force to return the condensate in the condenser section to the evaporator section. The heat pipe has a simple structure, very high effective thermal conductance and heat transfer capacity, and can work in hostile high-temperature environments. In this research, a high-speed rotating test apparatus and data acquisition system for radially rotating miniature high-temperature heat pipes are established. Extensive experimental tests on two heat pipes with different dimensions are performed, and various effects of influential parameters on the performance characteristics of the heat pipes are investigated. The ranges of the important parameters covered in the current experiments are: 470⩽ω2Za¯/g⩽1881; 47 W⩽Q⩽325W; di=1.5 and 2 mm; and 1.05×10−3m3/s⩽W⩽13.4×10−3m3/s. The experimental data prove that the radially rotating miniature high-temperature heat pipe has a high effective thermal conductance, which is 60–100 times higher than the thermal conductivity of copper, and a large heat transfer capacity that is more than 300 W. Therefore, the heat pipe appears to be feasible for cooling high-temperature gas turbine components.

脆性材料の高温における理想強度

Ideal Strength, aid as a function of temperature was found to be approximated by the following formula:σid≥σid=∫TxT3αE/(1-2μ)dT=3αE(Tx-T)/(1-2μ)where Tx stands for the minimum value of transformation points (melting, sublimation, solid to solid, and brittle to ductile), and α, E, μ respectively for the coefficient of linear thermal expansion, Young's modulus and Poisson's ratio. The formula's error is less than 20%, expressed in the term of error compared with whisker's strength at RT. It was also compared with literature data on high temperature strength of sapphire monofilament, which proved the compatibility of the formula.σid is estimated as 9 GPa for SiC and 3.5 GPa for Si3N4 at 1, 400°C, 1 GPa at 2, 200°C for SiC, and 1 GPa at 1, 700°C for Si3N4. These values prove the usefulness of the materials as high temperature gas turbine components. The formula tells also us that the ideal strength can be well approximated by a straight line regardless of material.The Young's modulus plotted against temperature can be converted to Young's modulus plotted against strain, e, by using e=3∫α/(1-2μ)dT, which can not be given directly from experimental measurement due to the occurrence of fracture in the material. The tangent of the curve for MgO was proved to agree well with the value given by a classical solid state theory. The range of binding force acting across material surface was estimated by using the ideal strength, giving 0.5-1 nm for SiC and Si3N4 and 0.1-0.5 nm for MgO and Al2O3. The allowable atomic distance difference between neighboring layerd material in coherently multilayered films, which has no defect was also estimated to be less than 3% from the ideal fracture strain.

Laser surface treatment of ceramic coatings

Zirconia coatings are widely used for the thermal protection of high-temperature components in gas turbine and diesel engines. In advanced engines, however, the ceramic coatings have to serve as both thermal and chemical barriers. A 3 kW CO 2 laser was successfully used to achieve surface sealing of plasma-sprayed coatings without serious damage. Nevertheless, excessive and random microcracks restricted the mechanical strength. Moreover, zirconia is not protective against oxygen at high temperatures. Reactive laser treatments were therefore conducted, including alumina powder projection in the laser beam. Addition of alumina during the laser surface melting process led to controlled microcracking and produced a surface alumina-zirconia eutectic which greatly strengthened the treated coating.

The mechanical properties of coatings and coated systems

The increasing mechanical demands being placed on high-temperature gas turbine components has generated interest in establishing a sound understanding of the mechanical behaviour of coatings and coated components. This review covers their bulk and in situ properties and behaviour, primarily under cyclic conditions. The close inter-relationship between bulk coating properties, the presence and nature of structural defects within the coating and the interaction between coating and substrate are emphasised. The role of deformation modelling and fracture mechanics in advancing our understanding and design capabilities is highlighted.

Mechanical interactions between coatings and superalloys under conditions of fatigue

The increasing mechanical demands being placed on high-temperature gas turbine components has generated interest in establishing a sound understanding of the mechanical behaviour of coatings and coated components. This review covers their bulk and in-situ properties and behaviour, primarily under cyclic conditions. The close interrelationship between bulk coating properties, the presence and nature of structural defects within the coating and the interaction between coating and substrate are emphasized. The role of deformation modelling and fracture mechanics in advancing our understanding and design capabilities is highlighted.

Plasma spray coating processes

Plasma spray deposition is one of the most important technologies available for producing the high–performance surfaces required by modern industry. Over the past 25 years, there have been significant advances in the understanding of plasma physics and in the development of spraying equipment and techniques. This has enabled a range of materials including metals, alloys, ceramics, and cermets to be plasma sprayed on to a great variety of substrate types and geometries. During this period, the uniquely aggressive environment within the gas turbine engine has provided not only some of the greatest challenges to plasma spraying technology, but also some of its most successful applications. In this paper, the nature of plasma and plasma spray devices are discussed and factors affecting coating quality are considered. Practical aspects of plasma spraying are considered and finally the application of plasma spray coating processes to the protection of high–temperature gas turbine components is discussed using as examples turbine blade overlay coating, coatings for hot gas path seals, and ceramic thermal barrier systems.MST/282

Future developments in plasma spray coating

AbstractFuture coating developments for high–temperature gas turbine components is a topic which encompasses both material and process developments. The materials for use at high temperatures can be classified as wear (abradables and abrasives), corrosion, oxidation, and thermal barrier coatings. In spite of many interesting material modifications, the most important developments are associated with the application process, coating quality control procedures, and efficiency of operation regarding the source of material used to produce the coating.MST/295

Protective coating systems for high temperature gas turbine components

Protective coating systems for high temperature gas turbine components