The thermal load of tubes in radiation heaters

In the tubes of radiation heaters, which are under high thermal loads, apart from stresses caused by the pressure of the working medium also thermal stresses are produced. Owing to the nonuniform irradiation over the tube periphery, these thermal stresses have to be determined taking into account the radial and tangential temperature gradients in the tube wall. It is shown by the example of the pulverized-coal-fired heater of a closed-cycle gas turbine plant to what extent thermal stresses are caused, assuming linearly elastic behaviour of the tube material, and how the method of fixing the tube ends influences the total stress.     

Failure Analysis and Remedy Procedure for Cracking of Fuel Nozzles in a Gas Turbine

A common failure in a certain type of gas turbine, observed during the first periodic inspection, is radial cracks in the tip plate of gas fuel nozzles. Here, each gas turbine has 18 nozzles. In all nozzles and in all similar units, these cracks of lengths ranging from 1 mm to a maximum of 14.5 mm are observed. As prescribed by the manufacturer, the defective part must be removed and replaced by welding and machining of a new one. But this problem is repeated and observed in the next periodic visits, and in all units. Depending on the number of nozzles in each gas turbine unit and the number of units in total, these repairs are very expensive and time-consuming. In this paper, the failure is analyzed and the causes of the cracks in the nozzles are investigated. Studies show that the main causes of nozzle failure are residual stresses caused by welding and thermal stresses caused by the start-up and shutdown processes. According to results, a solution has been proposed to release these residual and thermal stresses. After the implementation of this method in 1998, no more failure has been reported by the repair team, which proves the effectiveness of this solution. Since this paper has been prepared based on technical reports from the years between 1996 and 1998, the cited references of this paper are these technical reports.     

Investigation on crystallographic behaviors of nickel‐base single crystal alloy cooling holes with different spanwise injection angles

Film cooling as an important thermal protection technology is widely used in aviation and ground gas turbine blades. But film cooling holes reduce the strength of blade seriously, which have become a key region of crack nucleation. In this paper, the plastic behaviors of nickel‐base single crystal alloy turbine cooling holes in spanwise injection angles range from 0° to 40° are investigated on basis of crystallographic constitutive theory. The results show that there are both higher stress regions and lower stress regions around multi‐column cooling holes, where suffer stress interference. The maximum Mises stress occurs at the hole in the center column. The places where the maximum resolved shear stresses occurs change with load and spanwise injection angle. The maximum Mises stress around holes with injection angle of 0° is lowest. With the injection angle increases, the maximum Mises stress increases until injection angles up to 30°. In all the slip systems, the resolved shear stress of hexahedral slip system is most sensitive to the changing of spanwise injection angle and load.     

Fatigue resistance of high-temperature alloys under cyclic temperature variation. Report 1. Study technique and results

We examine the determination of the serviceability of the material of gas turbine engine (GTE) turbine rotor blades. These blades are among the most highly loaded engine components and determine the reliability of the turbine as a whole. We describe briefly the NUM-3 setup, intended for study of the fatigue resistance of high-temperature alloys under conditions simulating operation. We present the results of fatigue studies of the EP962 and EI698 alloys in isothermal conditions and under combined thermal and mechanical loading. Analysis of the data makes it possible to identify several relations governing the influence of thermal cycling and the associated variable thermal stresses on the fatigue resistance of the subject materials.Translated from Problemy Prochnosti, No. 3, pp. 13–20, March, 1994.     

Property gradation for modification of response of rotating MMC discs

Abstract

Aerospace and industrial gas turbine engine components such as discs, blades, and propeller shafts for which long fibre reinforced metal matrix composites are being made, operate under complex mechanical and thermal loading conditions. A major aim of functionally graded materials application is to optimise component response through appropriately tailored microstructures. This paper explores the influences of property gradation, centrifugal body force loading, and thermal loading on stresses in rotating discs. The discs were modelled as non-homogeneous orthotropic materials such as those obtained through non-uniform reinforcement of a metal matrix by long fibres. The results show how different temperature change distribution patterns and property gradation types correlate with hoop stresses developed in the disc.     

Investigation on Failure in Thermal Barrier Coatings on Gas Turbine First-Stage Rotor Blade

Failure in turbine blades can affect the safety and performance of the gas turbine engine. Results of coating decohesion, erosion and cracking at the first-stage high-pressure (HPT) blade working in gas turbine engine are being reported in this paper. This investigation was carried out for the possibility of various failure mechanisms in the thermal barrier coating exposed to high operating temperature. The blade was made of nickel-based superalloy, having directionally solidified grain structure coated with thermal barrier coatings of yttria-stabilized zirconia with EB-PVD process and platinum-modified aluminum (Pt–Al) bond coat with electro-deposition. The starting point of analysis was apparent coating decohesion close to the leading edge on the suction side of blade. The coating decohesion was found to be widening of interdiffusion zone toward the bond coat at higher operating temperature which could change the composition and induce thermal stresses in the bond coat. The erosion, cracking and decohesion of the coating on the pressure side was also observed during failure investigation. The erosion of the coating was coupled by two factors: one by increase in temperature as demonstrated by change in microstructure of the substrate and second by increase in coating inclination toward the trailing side. As a result of high operating temperature, swelling and thickening of TGO was observed due to outward diffusion of aluminum from the bond coat to form alumina (non-protective oxide) which causes internal stresses that leads to top coat decohesion and cracking. The possibility of hot corrosion was also investigated, and it was found that top coat decohesion did not involve this failure mechanism. Visual inspection, optical microscopy, scanning electron microscopy and energy-dispersive spectroscopy have been used as characterization tools.     

Predicting failure within TBC system: Finite element simulation of stress within TBC system as affected by sintering of APS TBC,geometry of substrate and creep of TGO

Demand for economically efficient and environmental friendly gas turbine engines leads to the usage of a thermal barrier coating (TBC) system, which is usually sprayed on the top of a superalloy substrate. The system includes a ceramic TBC, a bond coat (BC) and a thermally grown oxide (TGO) layer. Thermo-mechanical mismatch stresses created within the coating at the end of a thermal cycle lead to spallation of the ceramic coating and a rapid increase in the temperature of the substrate. The thickness of the oxide layer and the amount of aluminium depleted during high temperature operation also affect the lifetime of the TBC. As a first step to the prediction of the failure mechanisms and the lifetimes of TBCs, a preliminary study of how the stress distribution within the TBC system is affected by different factors is required. This paper investigates the effects of the sintering of the ceramic layer, of the geometry of the substrate and of the creep of the TGO, on the stresses built up in the TBC system. Three different TBC system geometries were modelled using plane strain FE models with three different sets of TGO creep properties. An Arrhenius equation was fitted to the temperature dependent modulus of the sintered TBC using results published in the open literature. The equation was later implemented within the FE model. It is concluded that the TBC on the top of flatter regions of substrate produces smaller tensile residual stresses compared to sharp corners of the substrate. It was also found that the initiation and propagation of cracks within a TBC, during steady state operation depends on the choice of the creep parameters of the TGO. At the cooling stage, increase in the modulus of the TBC, due to sintering, has been shown to produce stresses within the TBC near the TGO interface that are as large as twice the value that is predicted using a model without sintering.     

The Influence of the Blade Airfoil Bend Angle on Its Thermal and Thermal Stress State under Nonuniform Heating

The author studied the influence of the turbine-blade-airfoil bend angle on the blade thermal and thermal stress state under nonsteady-state temperature conditions. It is shown that at nonuniform heating, the influence of the bend angle on the level of temperatures and temperature distribution over the airfoil is insignificant, whereas the level of thermal stresses on the blade edges changes appreciably due to internal constraint. It has been found that high levels of thermal stresses may occur in turbine rotor and nozzle blades irrespective of the magnitude of the bend angle when their thermal bending due to nonuniform heating is restricted by the external constraint.     

Thermophysical measurements during thermal cycling of gas turbine engine blades with ceramic coatings

A design-experiment method is proposed for high-frequency induction heating of turbine blades and models of combustion chamber flame tubes of gas turbine engines with heat-resistant ceramic coatings. The proposed method has been developed taking into account the electrophysical and thermophysical properties exhibited by the materials in thermal cycling tests. The results of thermophysical measurements, design-experiment studies of the nonstationary thermal state of the parts with coatings with the use of a thermal vision system, and thermal cycling tests of rotating blades and models of flame tubes with heat-resistant ceramic coatings are presented.     

Stress-Rupture Characterization in Nickel-Based Superalloy Gas Turbine Engine Components

Today’s gas turbine engines utilize high volume fraction gamma prime (γ′) strengthened alloys for turbine airfoils, which typically operate at temperatures greater than ∼0.5T _m of the alloy. At these temperatures and at stresses below yield, time-dependent deformation (creep) of the airfoil can occur and, if left unabated, can result in complete separation of the airfoil. This process is commonly referred to as stress rupture. Insufficient cooling air, unintentional interruptions of cooling air as well as abnormal engine operating conditions are typical causes of stress-rupture failures in gas turbine blade components. Stress-rupture fractures are generally heavily oxidized, tend to be rough in texture, and are primarily intergranular and/or interdendritic in appearance compared to smoother, transgranular fatigue type fractures. Often, gross plastic yielding is visible on a macroscopic scale. Commonly observed microstructural characteristics include creep voiding along grain boundaries and/or interdendritic regions. Internal voids can also nucleate at carbides and other microconstituents, especially in single crystal castings that do not possess grain boundaries. Other signs of overtemperature include partial resolutioning of the γ′ strengthening precipitates, with the remaining volume fraction of γ′ commonly used to estimate blade metal temperatures. This article highlights the visual, fractographic, and metallographic characteristics typically encountered when analyzing stress rupture of turbine airfoils. An erratum to this article can be found at     

Failure analysis of gas turbine buckets

The failure of a gas turbine first stage bucket was investigated by visual inspection and finite element analysis. The failure of a major bucket cooling passage was a critical cause of the separation of a bucket segment and caused microstructural deterioration of the neighboring regions by serious thermal load. Changes of microstructural morphologies of the damaged buckets under the thermal and mechanical stress were observed. After coating stripping, the bucket surface condition was evaluated through visual inspection and finite element analysis. The TMF (thermal-mechanical fatigue) cracking of surface coatings on the suction and pressure sides of the bucket was described.     

Thermo-mechanical damage modeling of a glass–phenolic composite material

This study is on the development of a thermo-mechanical damage model (TMDM) for glass–phenolic composite materials subject to high temperature and thermal radiative environments. The damaged composite is expressed as two regions of non-charred and charred materials. Homogenization methods are used to formulate the damaged material in terms of the volume fractions associated with composite fiber, resin and char. Equations are derived that employ Darcy’s law to account for the gas transport within the structure. Mechanical response of the composite is taken into account by solving a homogenized system of linear elasticity equations which introduces the gas-phase pressure in a self-consistent manner. A finite element method is developed to solve the thermal and mechanical equations for a two-dimensional clamped composite beam subject to thermal radiative heating. Overall, good agreement is obtained between the numerical predictions and experimental data for temperature and gas pressure. Results show the decomposition of the resin and char formation create local stress concentrations across the pyrolysis front. The origin of these stresses are from thermal expansion and contraction across the front and the generation of locally high pore gas pressures from resin decomposition.     

Failure analysis of gas turbine transition pieces,leading to a solution for prevention

In this study the failure analysis of transition pieces of a gas turbine is investigated. Transition piece connects combustion chamber to the turbine and acts as a nozzle which leads hot gases to stationary blades of turbine. The problem of this transition piece in this gas turbine, which was common in other similar units, was cracks developing on the lower wall near the connection to the turbine. Study of gas turbine operation history, cracks apparent form microstructure analysis and fracture surface, revealed that the thermal fatigue was the main reason for the failure and also oxidation facilitated the crack propagation. In order to prevent the failure of transition pieces, it was proposed to create a row of holes, at 1 cm above the impaired region for local cooling and stopping probable cracks. Due to the implemented solutions, the failure of transition pieces and their annual repair are prevented.     

燃气-蒸汽联合循环热电厂噪声分析及控制

根据国内以天然气为燃料的燃气-蒸汽联合循环热电厂特点,工程实践表明,蒸汽轮机房、燃气轮机、余热锅炉、机力通风冷却塔、变压器、天然气调压站等区域,以及循环水泵房、化水车间等辅助设备房是主要的噪声辐射区,并对这些区域的声源设备及声源特性进行分析。同时,对各重点声源区域噪声提出具体控制技术措施,可为同类电厂实施噪声控制提供参考。     

Comparative analysis of steady state heat transfer in a TBC and functionally graded air cooled gas turbine blade

Internal cooling passages and thermal barrier coatings (TBCs) are presently used to control metal temperatures in gas turbine blades. Functionally graded materials (FGMs), which are typically mixtures of ceramic and metal, have been proposed for use in turbine blades because they possess smooth property gradients thereby rendering them more durable under thermal loads. In the present work, a functionally graded model of an air-cooled turbine blade with airfoil geometry conforming to the NACA0012 is developed which is then used in a finite element algorithm to obtain a non-linear steady state solution to the heat equation for the blade under convection and radiation boundary conditions. The effects of external gas temperature, coolant temperature, surface emissivity changes and different average ceramic/metal content of the blade on the temperature distributions are examined. Simulations are also carried out to compare cooling effectiveness of functionally graded blades with that of blades having TBC. The results highlight the effect of including radiation in the simulation and also indicate that external gas temperature influences the blade heat transfer more strongly. It is also seen that graded blades with about 70% ceramic content can deliver better cooling effectiveness than conventional blades with TBC.     

Thermal conductivity and mechanical properties of YSZ/LaPO4 composites

High temperature gas turbine sealing is an important issue for increasing the thermal efficiency of gas turbine. In this purpose, layered structured LaPO₄ has been selected as the soft phase to add into the commercialized thermal barrier coating material 7 wt% yttria stabilized zirconia (7YSZ). The consequent thermal conductivities and mechanical properties versus the content of LaPO₄ have been researched systemically in this paper. Phase composition and microstructure of the high-temperature sintered LaPO₄/7YSZ composites were characterized. The thermal conductivity decreases significantly due to the second phase effects and the interface thermal resistance was also strongly involved according to the composite model. The hardness decreased by composed LaPO₄ phase so that to reduce attrition of the vanes at high temperature. The slight increase of fracture toughness and bending strength in the results were also favored in operation. The experimental results demonstrate that the LaPO₄/7YSZ composite will be an excellent candidate abradable sealing material for high temperature gas turbine.     

Der Einfluß von thermozyklischer Beanspruchung auf den Eigenspannungszustand in elektronenstrahlgedampften Wärmedämmschichten aus Zirkonoxid

Influence of Thermal Cycling on Residual Stresses in Zirconia Thermal Barrier Coatings Produced by Electron Beam – Physical Vapor Deposition. Flat specimens and aircraft turbine engine blades were coated with partially stabilized zirconia by electron beam - physical vapor deposition. Residual stresses of these thermal barrier coatings were measured before and after thermal cycling by x-ray stress analysis. All flat specimens and blades showed compressive residual stresses. Stress values in the longitudinal direction were significantly greater than those measured in the transverse direction. No changes of stress states and line widths were observed after thermal cycling. The specimens showed considerable differences in texture which made evaluation of the stress measurements difficult and which may have influenced degradation behaviour during thermal cycling.     

Finite Element Simulation on Thermal Fatigue of a Turbine Blade with Thermal Barrier Coatings

In this paper, a finite element model was developed for a turbine blade with thermal barrier coatings to investigate its failure behavior under cyclic thermal loading. Based on temperature and stress fields obtained from finite element simulations, dangerous regions in ceramic coating were determined in terms of the maximum principal stress criterion. The results show that damage preferentially occurs in the chamfer and rabbet of a turbine blade with thermal barrier coatings and its thermal fatigue life decreases with the increase of thermal stress induced by high service temperature.     

工业燃气轮机涡轮叶片用铸造高温合金研究及应用进展

工业燃气轮机具有热效率高、污染低等突出优点,成为未来发电机组与大型水面舰船动力的首选设备。铸造高温合金是工业燃气轮机涡轮叶片等热端部件的关键材料,其性能和制备水平在一定程度上决定了先进燃气轮机的功率、效率、寿命等性能。本文重点综述了工业燃气轮机及其涡轮叶片用铸造高温合金材料的研究及应用现状,并对工业燃气轮机涡轮叶片用铸造高温合金及涡轮叶片制造技术的发展趋势进行了展望。未来,先进定向凝固,“材料基因工程”等技术将逐渐应用到工业燃气轮机涡轮叶片用铸造高温合金的研制中;此外,先进工业燃气轮机上定向/单晶高温合金的应用将越来越广泛。     

Stability analysis of a gas turbine exhaust stack

Stability analysis, based on finite element method, was carried out on a gas turbine exhaust stack following obvious appearances of significant gaps between its silencer casings and embracing rings. The finite element model was developed by referring to the corresponding technical drawing of the exhaust stack and the information obtained during the site inspection. In the case of axial constraint at both ends of the exhaust stack, the original design was found incapable of withstanding the thermal loading experienced during operation of the gas turbine, leading to instability of the structure. A design modification of the exhaust stack was proposed to rectify this problem. The outcome of the finite element analysis indicated that the modified design of the exhaust stack had improved stability as compared to the original design when subjected to typical thermal loading of the gas turbine operation.