Large Gas Turbine Engines Research Articles

Fatigue dimples? – Fractured bulkhead ring bolts in a large gas turbine engine

Abstract Bulkhead ring fasteners, four in total, were found fractured in a large gas turbine engine after 1700 operating hours and a high number of engine starts and shut-downs for such a relatively short time in service. A metallurgical failure investigation was ordered by the client for determining the metallurgical cause of the cracking. The subject bolts failed due to high-stress low cycle fatigue fracture.

Increased Productivity in the Superfinishing of Large Gas-Turbine Engine Blades

Increased Productivity in the Superfinishing of Large Gas-Turbine Engine Blades

What if fractography does not tell us anything? – Failure of a side seal retainer

Abstract A fractured transition side seal retainer, together with an intact piece for reference, was received from the client on December of 2023. The subject part fractured next to the fastening hole, liberating a long ligament that was never found but reportedly caused some damage to hot gas path components downstream of a large gas turbine engine. It is speculated that the subject side seal retainer fractured due to high-stress, not fully reversed bending high cycle fatigue failure. It is recommended to make the design sturdier by considering dampening the long free length of the part, and by specifying a larger fillet radius in the initiation area. Also, over-torquing upon assembly of the subject part should be avoided.

Intergranular Corrosion in X39CrMo17-1 – A Review

Abstract The aim of this contribution is to review cases of repeated snap ring failures in large gas turbine engine service due to intergranular attack, or intergranular corrosion, IGC, and to shed some light on metallurgical aspects of the wet corrosion resistance of the subject high-alloy martensitic stainless chromium steel X39CrMo17-1, DIN 1.4122. It is important to note that only snap rings, a.k.a. retaining rings for bores or Seeger rings, were affected by those failures, namely fractures resulting from intergranular attack (IGA, Kornzerfall). It was determined that the metallurgical root cause of failure was sensitization, resulting from secondary chromium carbide precipitation at grain boundaries (GB), and the ensuing chromium depletion in narrow bands adjacent to the GB, which rendered the matrix there sensitive to selective corrosion (the chromium content in these narrow seams, measuring only a few 10 nm in width, drops below the corrosion resistance threshold of ca. 10.5 %). Snap rings, as the name suggests, require a certain “snappiness” to serve their purpose. It is because of this requirement that all snap rings that experienced failures in the field were heat treated per DIN EN 10088-2, providing for a low tempering temperature of max. 250 °C, leading to a relatively high hardness of ca. 580 HV 10, providing said “snappiness”. It turned out that the microstructure resulting from this heat treatment (H/T) is sensitized, rendering the material sensitive to IGA. To the knowledge of the authors, no other components made of this martensitic stainless chromium steel are given this H/T, but rather a high-temperature tempering of 650 °C–750 °C per DIN EN 10088-3, producing a lower hardness in the order of ca. 300 HV 10 and largely a lack of “snappiness”, the latter being not required by those non-snap ring applications. Metallurgically, it became clear that high-temperature tempering per DIN EN 10088-3 eliminates sensitization by “backfill” diffusion of chromium from the non-depleted matrix into formerly depleted regions next to GB. It was therefore decided in the authors’ organization to eliminate X39CrMo17-1 per DIN EN 10088-2 as a possible material selection. The metallurgical investigations described in this contribution corroborate and support this decision.

Hot Tearing in an Investment Cast Burner Swirler

Abstract The damage case dealt with in the present work relates to a generally less common manufacturing defect that was discovered during its installation. The part was sourced from a supplier. An installation of the defective part in a large gas turbine engine used for energy generation could thus be prevented. The defect, a solidification crack, could have been repaired by welding and the component could have been used for operation. Instead, it was subjected to destructive examinations aiming at identifying what caused the crack. As the component could not possibly perform its function prior to being repaired by welding we are, in any event, talking about a damage case. The solidification crack initiated at a site of oxide infiltration. It was thus clearly a casting defect. The respective supplier was advised to reduce the content of non-metallic inclusions by the use of ceramic filters when the material is poured into the shell mold and a better control of the vacuum in the casting furnace.

Mathematical model of turbofan engine weight estimation taking into account the engine configuration and size

The paper presents a new correlation-regression model of estimating the turbofan engine weight considering the effect of the engines design schemes and dimensions. The purpose of this study was to improve the efficiency of the conceptual design process for aircraft gas turbine engines. Information on 183 modern turbofan engines was gathered using the available sources: publications, official websites, reference books etc. The statistic information included the values of the total engine air flow, the total turbine inlet gas temperature, the overall pressure ratio and the bypass ratio, as well as information on the structural layout of each engine. The engines and the related statistics were classified according to their structural layout and size. Size classification was based on the value of the compressor outlet air flow through the gas generator given by the parameters behind the compressor. Depending on the value of this criterion, the engines were divided into three groups: small-sized, medium-sized gas turbine engines, and large gas turbine engines. In terms of the structural layout, all engines were divided into three groups: turbofan engines without a mixing chamber, engines with a mixing chamber and afterburning turbofan engines. Statistical factors of the improved weight model were found for the respective groups of engines, considering their design and size. The coefficients of the developed model were determined by minimizing the standard deviations. Regression analysis was carried out to assess the quality of the developed model. The relative average error of approximation of the developed model was 8%, the correlation coefficient was 0,99, and the standard deviation was 10,2%. The model was found to be relevant and reliable according to Fisher's test. The obtained model can be used to assess the engine weight at the stage of conceptual design and for its optimization as part of an aircraft.

Combustion chamber development features for converted small gas turbine

Conversion and modernization of a gas turbine engine involves solving a number of tasks, one of which is the development of a combustion chamber for operation on various gaseous fuels. The complexity of solving this problem is associated with a variety of interrelated processes occurring in the CC, which are complex and insufficiently studied. The work on the small-size gas turbine engine CC is even more complicated by the lack of a generally accepted and well-developed finishing technique typical of large gas turbine engines, which is emphasized both in domestic and foreign works. The report considers the main approaches to classifying the dimensions of combustion chambers of gas turbine engines [1] according to various criteria, namely: to the power type size (according to CIAM)Classification based on the reduced air flow through the compressor. The criteria included in the classification are analysed, in relation to the implemented designs, the most objective classification is selected for further consideration of the design of the combustion chamber of a small-sized gas turbine engine when converting it to gaseous fuel. The features and technical limitations of converting the combustion chamber from liquid to gaseous fuel are considered, which is an urgent task against the background of gasification programs of the fuel and energy complex in the Russian Federation. Attention is paid to the factors that have the greatest impact on the environmental characteristics of the converted combustion chambers.

EGT prediction of a micro gas turbine using statistical and artificial intelligence approach

Small size gas turbine application has increased with the advancement of technology. These engines have simple structures, fewer number of operating components, and less design and construction costs in comparison with large gas turbines. Small size gas turbines can be used in different fields such as remotely piloted vehicles, unmanned aerial vehicles, helicopters, and special purpose aircraft such as vertical takeoff and landing aircrafts [1]. The advantages of these engines compared to the large gas turbine engines leads to high usage of small-sized engines and growing demand for research activities and experimental test results on the operating parameters of these engines, such as exhaust gas temperature (EGT). In general, research on gas turbine engines, related with the present work, can be divided into two parts. The first part includes studies about past experiments and modeling of gas turbine engines, and the second part focuses on data mining methods applied on gas turbine engines. These two areas are discussed in the following sections.

Development of Compressor Blade Prognostics and Health Management for Large Gas Turbine Engine

DOOSAN has been developed large gas turbine since 2014 as a national project in Korea. Recent researches showed that one of major risk item in gas turbine development is compressor rotor blade failure. There are several events of compressor rotor blade failure which have occurred over the last few years, in some cases causing catastrophic damage to the entire compressor. In response to these issues, DOOSAN started to develop compressor monitoring system to prevent failures and diagnose blades to assess the structural integrity of each blades. This paper presents the ideas and plans how DOOSAN will develop the system with latest sensors. Additionally, validation methods will be presented using sub-scaled test rig in 2018 and real scale engine test in 2019, respectively.

Beware of Hot Tearing in Lifting Lugs – a Case Study in Mechanical Engineering

Abstract One out of eight lifting lugs of a large gas turbine engine exhibited indications upon magnetic particle surface crack inspection (MT), that were later verified by ultrasonic testing (UT). The subject part was made of a low-alloy steel casting. It had already been used in service and was supposed to be re-used, together with the other lifting lugs of the set. In order to determine the cause of the indications, non-destructive metallographic examinations by means of the replica technique were performed. In that investigation, the indications were determined to have been caused by crack-like decohesions of material. In one of the examined areas, these imperfections were associated with small casting defects, namely micro shrinkage. This might have contributed to crack initiation. The described small surface cracks at the cylindrical part of the lifting lug itself were probably caused by bending stresses in service. Indications were found only on one out of the entire set of eight parts. There, only the upper side of the short spigot, were axial tension on the surface is to be expected in service when the bollards are in lifting action, was affected. In a second investigation, that same part was sectioned in the area with the largest UT indications. The microstructure of this steel casting is unremarkable. However, several rather large macrocracks were found in the sectioning plane. Scanning electron micrographs confirmed this. There is a moderate carbon segregation in the subject area that is unremarkable. The longest crack in the metallographic section was forced open for microfractography in the SEM. A small pre-existing crack that could not have been caused by service loads was found. Within that area, microfractographic features of transgranular hot tearing were detected, as evidenced by a dendritic appearance of the fracture surface. This is a hot cracking mechanism.

Failure of Large High-strength Fasteners by Hydrogen Embrittlement

Abstract Five out of several tens of large high-strength fasteners, used for attaching compressor-end supports of a large gas turbine engine in a test rig to its foundation, failed by deferred fracture after tensioning. The hexagonal bolt heads fractured off at the fillet of the shaft end. This happened in a delayed manner, in some cases six hours or so after tensioning. Because of this peculiar delayed fracture, hydrogen embrittlement (HE) was suspected as the metallurgical cause of failure. Delayed cracking is typical of and consistent with HE. On the fracture surfaces of the liberated hexagonal heads, some microfractographic fracture features, consistent with hydrogen embrittlement, were found.

Finite Element Modeling and Dynamic Analysis of Rotating Disc and Drum Connected by Bolted Joints

The disc-drum type rotors are commonly used in large gas turbine engines. In this paper, the dynamic characteristics of single stage disc-drum structure are studied based on the finite element model, where the connecting bolts are modeled using solid elements and the frictional contact at the joint interface is accommodated. In order to evaluate the nonlinearity resulting from the bolted joints, a finite element (FE) model fixing the disc and drum together is also established, and the simulation results based on the two models are compared. The effects of the bolt preload and the rotating speed on the dynamic characteristics of the disc-drum structure are also discussed. The works proposed in this paper help to enhance the understanding of the dynamics of the disc-drum type rotor.

In-engine turbine heat transfer measurement

The heat transfer coefficient distributions on the rotor blades of a low-pressure turbine in a heavy duty (288 MW) gas turbine engine have been measured. This is the first successful in situ measurement of the heat transfer coefficient in the harsh environment (high temperature, combusted gases) of an operating engine. In the measurement technique, blade surface temperatures and an upstream reference gas temperature are measured, while the engine is subjected to a thermal transient. The temperatures on the surfaces of the rotating blades are measured by means of an infrared pyrometer. The distribution of heat transfer coefficients along the midspan of the blades is then determined from these temperatures and the measured change in the upstream reference gas temperature. The associated measurement errors are assessed using a Monte Carlo analysis. Empirical correlations for stagnation point flow show excellent agreement with the measured heat transfer coefficients in the leading edge region of the blades. The accuracy of the measurements and the low relative cost of the hardware mean that the present measurement approach is well suited for in situ heat transfer measurements of large gas turbine engines.

Vibratory Forcing Functions Produced by Nonuniform Cascades

A method is developed to predict the steady, two-dimensional incompressible flowfield through tandem cascades or multibody cascade geometries. The technique is adapted from the classical Douglas-Neumann method, in which a superposition of singularities is used to simulate the cascade geometries. The method is used to analyze the self-induced potential field interaction of a stator cascade with a finite thickness strut cascade behind it or integral within it, such as occurs in fan ducts and intermediate cases of large gas-turbine engines. The calculation predicts the magnitude and location of the nonuniform pressure distortion, which would be imposed on an upstream engine component, for a variety of cascade designs. Such a distortion is known to produce resonant stresses in an upstream rotor. Experimental corroboration of the analysis is presented for a cascade operating at M∞ = 0.49. Sensitivity of the distortion pattern to small strut restaggering is calculated and compared to test data.