High Temperature Gas Turbine Engines Research Articles

Tailoring microstructure of double-layered thermal barrier coatings deposited by suspension plasma spray for enhanced durability

Gadolinium zirconate (GZ)-based TBCs comprising GZ as the top layer and yttria stabilized zirconia (YSZ) as the base layer, are attractive double-layered thermal barrier coatings (TBCs) for high temperature gas turbine engine application. This work attempts to understand the influence of individual layer microstructure on the durability of GZ/YSZ double-layered TBCs processed by suspension plasma spray (SPS). Two different spray parameters were chosen to obtain a combination of three microstructurally distinct GZ/YSZ double-layered TBCs i.e. GZ porous (P)/YSZ porous (P), GZ dense (D)/YSZ porous (P) and GZ dense (D)/YSZ dense (D). Thermal diffusivity of the as-deposited coatings was measured using Laser Flash Analysis (LFA) technique and the thermal conductivity of the TBCs was calculated. The GZ/YSZ double-layered TBC specimens were subjected to two different durability tests, i.e. thermal cyclic fatigue (TCF) and burner rig test (BRT). Sintering behavior of the individual layer TBC microstructures was evaluated by comparing the porosity evolution in as-deposited and TCF tested TBCs. Fracture toughness measurements performed on each layer of the double-layered TBCs were correlated with the durability results. Thermal cycling results amply demonstrate that the individual layer microstructure of GZ/YSZ double-layered TBC influenced its durability. Detailed failure analysis of the TCF and BRT failed specimens revealed similar failure modes for GZ (P)/YSZ (P), GZ (D)/YSZ (P) and GZ (D/YSZ (D) TBCs under identical thermal cyclic test conditions. However, failure modes differed when subjected to different thermal cyclic test conditions (TCF and BRT) and the probable causes are discussed. Findings from this work provide key insights on designing durable GZ/YSZ double-layered TBCs.

Technology for the Deposition of Wear-Resistant Coatings on the Airfoil Shroud Platforms of GTE Turbine Blades

Experimental data are used to estimate the possibility of the deposition of a wear-resistant nickel-based VKNA-2M intermetallic alloy onto the contact surfaces of the airfoil shroud platforms of the gas turbine blades in a high-temperature gas turbine engine (GTE) by facing in an argon atmosphere. A technique is proposed to restore the geometric sizes of the wear-resistant coatings on the airfoil shroud platforms of gas turbine blades to increase the functional properties of the repaired coated parts.

MICROSTRUCTURAL CRITERIA FOR DETERMINING THERMO-MECHANICAL CONDITIONS OF OPERATIONAL VULNERABILITY OF THE WORKING BLADES OF HIGH-TEMPERATURE GAS TURBINE AVIATION ENGINES

The necessity of studying the structure of the blade material and corresponding in-service structural changes is attributed, on the one hand, to the fact that the structural changes may impair the performance of the material thus promoting the blade fracture. On the other hand, the structural changes can serve as diagnostic signs in determination of thermo-mechanical conditions of the blade destruction. Moreover, the structural changes in the blade material often determine the service life of the blades: aside of creep mechanism, in-service depletion of the blade lifetime in high temperature gas turbine engines of maneuvering aircrafts is attributed to complex thermo-mechanical loading of the blades which, in turn activates the mechanisms of fatigue and thermo-mechanical fatigue. Monitoring of the structural state of the blade material with equiaxed crystallization (ZhS6U, VZhL12U), directed and monocrystallization (ZhS26, ZhS32) and studying the features of structural changes attributed to the operational vulnerability of the turbine blade material are the main goals of the study. The features of changes in the morphology of the strengthening γ’-phase and carbide transformation in the blade material that occur in-service are considered. The results of monitoring the structural state of the blade material are compared with the results of testing the same samples under laboratory conditions simulating thermo-mechanical loading. Microstructural criteria are recommended for more accurate evaluation of thermo-mechanical conditions of operating vulnerability of the turbine blades of high-temperature aircraft engines.

Phase composition and thermal conductivity of zirconia-based thermal barrier coatings

Atmospheric plasma spraying of powder materials has been used to produce thermal barrier coatings (TBCs) based on ZrO2 stabilized with 7 wt % Y2O3, including coatings doped with neodymium and samarium oxides, for state-of-the-art and next-generation high-temperature gas turbine engines. Doping with neodymium and samarium oxides has been shown to reduce the thermal conductivity of the TBCs by 10–20%. At the same time, changes in the phase composition, crystal structure parameters, and microstructure of the TBCs during heat treatment at the service temperature lead to an increase in the thermal conductivity of all the coatings by 50–70%.

Evaluation of the Influence on the Efficiency Curvature of the Surface Film Cooling

The results of numerical simulation of film cooling of the turbine rotor blade with the flow of coolant to the area of the leading edge through three rows of cylindrical holes. To determine the effect of surface curvature on film cooling effectiveness, and a comparison of results obtained with the results of simulation modeling of flow of a plate with film cooling at the inlet cylindrical region. The studies were conducted at the magnitude of the blowing ration of 0.5 to 2.0. Dimensions calculation model and input data received characteristic of the high pressure turbine blades of high-temperature gas turbine engine. Modelling performed using software package ANSYS CFX. The result obtained by calculations that the presence of the cylindrical profiled surfaces instead of flat surface leads to a significant decrease in the average film cooling effectiveness, both cylindrical and the concave surfaces in the investigated range blowing ration values. For all blowing ration film cooling effectiveness parameter values on the suction side is much higher than the pressure side. At the same time, the average efficiency of the film cooling on the suction side when m < 1.5 is higher than on a plot of flat wall, and when m > 1.5 marginally below. Application dependencies obtained on models with flat walls, for the calculation of the film cooling efficiency of suction side and pressure side of the blades is only possible with such amendments as may be determined by the results of this work.

Influence of thermal protection on the temperature state of the screen and casing

This work is devoted to the experimental study of the method of thermal protection of the environment by shielding from the hot body of the high-temperature gas turbine engine on a model setup under conditions of free convection (bulk screen), and with the injection of air through a porous screen. The generalized dependences of thermal protection are obtained that permit the temperature of the screen and the casing under free convection and injection of air through a porous screen to be predicted.

An anisotropic enhanced thermal conductivity approach for modelling laser melt pools for Ni-base super alloys

Ni-base super alloys are extensively used in high temperature gas turbine engines and energy industries. Due to the high replacement costs of these components, there are huge economic benefits of repairing these components. Laser direct metal deposition processes (LDMD) based on laser cladding, laser fusion welding, and laser surface melting are some of the processes which are used to repair these high value components. Precise control of these processes is important to achieve the desired microstructure, stress distribution, distortions due to thermal stresses and other important output variables. Modelling of these processes is therefore an extremely important activity for achieving any degree of control/optimisation. However, modelling of these processes is not straight-forward due to melt pool flows dominated by Marangoni and buoyancy driven convection. Detailed CFD models are required for accurate prediction of melt pool geometry. But these models are computationally expensive and require greater expertise. To simplify and speed up the modelling process, many researchers have used the isotropic enhanced thermal conductivity approach to account for melt pool convection. A recent study on mild steel has highlighted that isotropic enhanced thermal conductivity approach is not able to accurately predict the melt pool geometry. Based on these findings a new approach namely anisotropic enhanced thermal conductivity approach has been developed. This paper presents an analysis on the effectiveness of the isotropic and anisotropic enhanced thermal conductivity approaches for laser melting of Inconel 718 using numerical technique. Experimental melt pool geometry has been compared with modelling results. It has been found that the isotropic enhanced thermal conductivity approach is not able to accurately predict the melt pool geometry, whereas anisotropic enhanced thermal conductivity approach gives good agreement with the experimental results.

Investigation of the effect of irradiance and pulse duration in industrial percussion laser-drilling of Waspaloy

As the use of pulsed lasers in the manufacture of cooling holes in high-temperature gas-turbine engine components continues to increase, understanding the material behavior during laser-material interaction and the subsequent effects on hole quality becomes critical. In this study, atomic emission spectroscopy is used to monitor the optical characteristics of ejected material during percussion laser-drilling of Waspaloy over irradiance and pulse duration values in the ranges of 10–20 MW/cm2 and 0.3–1.1 ms, respectively. While irradiance is a widely used target parameter, it was experimentally determined that pulse duration had a larger impact on both the calculated electron temperature of the ejected material as well as on the thickness of residual resolidified, or recast, layer of material on the inside walls of drilled holes.

Heat transfer at various methods of thermal protection in the systems of power-driven gas turbine plants (GTP)

The scheme variants of implementing the thermal protection against heat flows being generated by the body of a high-temperature stationary gas turbine engine (GTE) are presented. The scheme of the experimental bench with a working section is given. Methodical approaches to the heat transfer calculation at different variants of forced and natural convection organization and under various operating conditions are described. The generalized results of the experiments carried out using a heat curtain being generated by porous injection are presented.

Adiabatic and conjugate cooling effectiveness analysis of a new hybrid scheme

The adiabatic film cooling effectiveness and flow characteristics for a new hybrid scheme and a circular hole were investigated numerically. The new film cooling scheme is proposed for high temperature gas turbine engines in aerospace and electric power generation applications. The hybrid scheme included two consecutive film hole configurations with interior bending to direct the secondary flow in stream-wise direction. The film cooling performance with flow-field analysis for the hybrid scheme and the circular hole were investigated at blowing ratio of 0.5, 1.0 and 2.0 for 0.95 density ratios. The results showed that the hybrid scheme provided a superior local and average film cooling effectiveness performance that was enhanced for further increase in blowing ratio. Moreover, the new scheme enhanced the downstream overall area-average film cooling effectiveness compared to the circular film hole. Subsequently, film cooling and conjugate heat transfer were combined to investigate the cooling effectiveness of the hybrid scheme at Br = 0.5 and 1.0 for different flow arrangements, specifically: parallel flow and jet impingement with two different gap heights (0.8 d and 1.2 d). The hybrid scheme presented a high cooling effectiveness by combining film cooling and conjugate heat transfer. The jet impingement configuration enhanced the upstream flow circulation which has a significant effect on convective heat transfer. Furthermore, a large gap height with jet impingement enhanced the downstream cooling effectiveness compared to other conjugate and adiabatic cases studied. The conjugate configuration with large gap height provided the highest overall laterally averaged cooling effectiveness compared to other conjugate and adiabatic cases studied. As a result, the hybrid scheme is able to minimize the cooling flow rate significantly since it provided superior cooling effectiveness at a lesser secondary flow rate, thus increasing overall engine efficiency.

Combined coating for turbine blades of high-temperature gas turbine engines

A combined coating for protecting turbine blades of high-temperature gas turbine engines is studied. Comparative tests of coatings under laboratory conditions and of coated blades in engine operation are performed. The microstructure of the coating is studied and the concentration profiles of alloying elements are determined by the method of x-ray diffraction analysis. Tests for high-temperature strength are performed.

Turbine Blade Cooling Studies at Texas A&M University: 1980-2004

Gas turbines are used extensively for aircraft propulsion, land-based power generation, and industrial applications. Developments in turbine cooling technology play a critical role in increasing the thermal efficiency and power output of advanced high-temperature gas turbine engines. Gas turbine blades are cooled internally by passing the coolant through several rib-enhanced serpentine passages to remove heat conducted from the outside surface. External cooling of turbine blades by film cooling is achieved by injecting relatively cooler air from the internal coolant passages out of the blade surface to form a protective layer between the blade surface and hot gas-path flow. The most important research contributions on turbine blade cooling studies at Texas A&M University's Turbine Heat Transfer Laboratory from 1980 to 2004 are summarized. For turbine blade internal cooling, the focus is on the effect of rotation on rotor blade coolant passage heat transfer with rib turbulators, pin fins, dimples, and impinging jets. For turbine blade external cooling, the focus is on unsteady high freestream turbulence effects on film-cooling performance with a special emphasis on turbine blade edge region heat transfer and cooling problems.

Creep life prediction for udimet 720 material using the initial strain method (ISM)

Despite of considerable research results of uniaxial tension creep available for superalloys, few studies have been made on high temperature creep using the Initial Strain Method (ISM). In this paper, the real-time prediction of high temperature creep strength and creep life for the nickel-based superalloy Udimet 720 (high-temperature and high-pressure gas turbine engine materials) was performed on round-bar type specimens under pure static load at the temperatures of 538°C, 649°C, and 704°C. The predictive equation derived from the ISM in creep tests showed better reliability than those from LMP(Larson-Miller Parameter) and LMPISM (Larson Miller Parameter-Initial Strain Method) specially for long time creep prediction (103-105h)

Heat-resistant eutectic alloys with carbide-intermetallic strengthening

Some important characteristics of gas turbine engines (in the first place the thrust and the efficiency) depend on the gas temperature and hence are determined by the service life of the blades. Heat-resistant nickel alloys are used for casting blades that can work at maximum temperatures of 1050–1100°C. However, more efficient engines need materials with a higher operating temperature and high-temperature strength. The work concerns the principles of alloying and structure formation of eutectic heat-resistant alloys on a nickel base with carbide and intermetallic strengthening. Their main mechanical properties at high temperatures are analyzed in comparison with the characteristics of high-temperature single-crystal nickel alloys, and the possibility of using them for casting turbine blades of high-temperature gas-turbine engines is considered.

A program of equivalent tests for turbine blades under laboratory conditions

A method of conducting equivalent tests of turbine blades under laboratory conditions, which ensures reproduction of the operational pattern of the loading in its most heavily loaded components, is proposed. A method for construction of the equivalent-test program is analyzed on a stage-I turbine blade of one of the high-temperature aviation gas-turbine engines.

Ceramic to Metal Attachment Using Low Modulus BRUNSBOND® Pad

BRUNSBOND® Pad is a low modulus sintered fiber metal material developed for attaching high temperature ceramic coatings to metal substrates. The fiber metal pad acts as a compliant strain absorbing interlayer to accomodate the differences in thermal expansivity and operating temperature between the ceramic and metal substrate. Ceramic coatings, 0.060 – 0.125 in. thick, remain intact on metal substrates through severe multiple cycle thermal excursions. The fiber metal pad is comparable in insulating value to the ceramic coating. Applications include ceramic thermal barrier coatings for high temperature gas turbine engine seals and combustors, MHD electrodes, and internal combustion engine insulators.

A feasibility study of a diffusion barrier between Ni-Cr-Al coatings and nickel-based eutectic alloys

Coating systems have been proposed for potential use on eutectic alloy components in high temperature gas turbine engines. In a study to prevent the deterioration of such systems by diffusion, a tungsten sheet 25 μm thick was placed between eutectic alloys and an Ni-Cr-Al layer. Layered test specimens were aged at 1100°C for as long as 500 h. Without the tungsten barrier the δ phase of the eutectic deteriorated by diffusion of niobium into the Ni-Cr-Al. Insertion of the tungsten barrier stopped the diffusion of niobium from the δ phase. Chromium diffusion from the Ni-Cr-Al into the γ/γ' phase of the eutectic was greatly reduced by the barrier. However, the barrier thickness decreased with time, and tungsten diffused into both the Ni-Cr-Al and the eutectic. When the δ platelets were aligned parallel rather than perpendicular to the Ni-Cr-Al layer, diffusion into the eutectic was reduced.

Paper 11: Regenerators for High Temperature Gas Turbine Engines

In various companies throughout the world a first generation of production vehicle gas turbine engines are being engineered. A vital component involved is the regenerative heat exchanger. The relative merits of the rotary regenerative and static recuperative heat exchanger are compared. Thermal efficiency and competitive initial cost are the two vital issues involved in the design of small gas turbines for the commercial establishment of gas turbine vehicles. The selection of a material for the rotary regenerator is essentially related to resolving the two vital issues of future small gas turbines and is, therefore, analysed. The account of the pioneering work involved in engineering the glass ceramic and other non-metal rotary regenerators includes a complete failure analysis based on running experience with over 200 ceramic regenerators. The problems of sealing, supporting and manufacturing the glass ceramic rotary generator are discussed and future practical regenerative designs are outlined. Heat exchange theory applied to small gas turbines is also reviewed.