Gas Turbine Engine Compressor Research Articles

Artificial seeding of fatigue cracks in NDI reference coupons

One of the most challenging aspects of quantifying the performance of non-destructive inspection (NDI) techniques for their calibration, detection capability and reliability is the development of representative coupons containing crack-like discontinuities. Electrical discharge machining (EDM) using shim or wire electrodes offers opportunities to develop rudimentary level discontinuities; however, these have several characteristics that differ from actual fatigue cracks. EDM slots are often triangular in an attempt to represent corner cracks or rectangular to represent surface cracks. Unfortunately, these are crude approximations of the quarter-elliptical or semi-elliptical cracks found in service. Deep EDM slots also have a significantly larger width, whereas shallow EDM slots can behave more like a machining mark rather than a sharp discontinuity, thereby affecting the responses of NDI techniques such as eddy current and ultrasonics. Five case studies for which reference NDI coupons were developed with carefully seeded fatigue cracks are presented. The principles used were similar, with a few nuances. Typically, the specialised geometry was machined undersize, which allowed the machining of starter EDM slots. The parts were then subjected to fatigue cycling to generate sharp cracks from the EDM slots. Once the required crack sizes were achieved, the starter EDM slots were machined away, removing the evidence of where and how the cracks were introduced. If required, there was post-machining of the appropriate geometry and addition of assembly details to create more representative reference coupons and structural design details for assessing the ability of NDI techniques to find simulated in-service cracks. These case studies cover approaches for creating fatigue cracks: at critical radii; inside fastener holes at corners, countersinks and mid-bores ; on free surfaces to create surface cracks (for example for monitoring crack development and growth under bonded repairs); inside simulated gas turbine engine compressor bores; and underneath bushings in simulated helicopter lugs. Examples of the differences that EDM slots and fatigue cracks have on detectability and reliability are provided.

Mitigation of Fretting Fatigue Damage in Blade and Disk Pressure Faces With Low Plasticity Burnishing

Low plasticity burnishing (LPB) is now established as a surface enhancement technology capable of introducing through-thickness compressive residual stresses in the edges of gas turbine engine blades and vanes to mitigate foreign object damage (FOD). The “fatigue design diagram” (FDD) method has been described and demonstrated to determine the depth and magnitude of compression required to achieve the optimum high cycle fatigue strength, and to mitigate a given depth of damage characterized by the fatigue stress concentration factor, kf. LPB surface treatment technology and the FDD method have been combined to successfully mitigate a wide variety of surface damage ranging from FOD to corrosion pits in titanium and steel gas turbine engine compressor and fan components. LPB mitigation of fretting-induced damage in Ti–6Al–4V in laboratory samples has now been extended to fan and compressor components. LPB tooling technology recently developed to allow the processing of the pressure faces of fan and compressor blade dovetails and mating disk slots is described. Fretting-induced microcracks that form at the pressure face edge of bedding on both the blade dovetail and the dovetail disk slots in Ti-6-4 compressor components can now be arrested by the introduction of deep stable compression in conventional computer numerical control (CNC) machine tools during manufacture or overhaul. The compressive residual stress field design method employing the FDD approach developed at Lambda Technologies is described in application to mitigate fretting damage. The depth and magnitude of compression and the fatigue and damage tolerance achieved are presented. It was found that microcracks as deep as 0.030in.(0.75mm) large enough to be readily detected by current nondestructive inspection (NDI) technology can be fully arrested by LPB. The depth of compression achieved could allow NDI screening followed by LPB processing of critical components to reliably restore fatigue performance and extend component life.

Flow Characteristics of Tip Injection on Compressor Rotating Spike via Time-Accurate Simulation

The performance of gas turbine engines is often limited by compressor stall. Steady tip injection stall control technologies have demonstrated their effectiveness to increase the stable operating range of gas turbine engine compressors. To help understand the fluid mechanic processes of stall with and without stall control and how stall is mitigated by stall control technology, the establishment of a capability to simulate the flow details leading to stall would be extremely helpful. This paper presents results of the simulations of a high- speed single-stage axial compressor. These simulations show the evolution of rotating instabilities and range extension as a result of tip injection. The simulations are time-accurate simulations of the three-dimensional Navier-Stokes equations encompassing the full-annulus grid and are executed using high-performance parallel computing. Computed compressor characteristics are compared to experimental data. The general flow features of the compressor with and without tip injection at the onset of stall are presented using time- accurate pressure traces and visualized by a novel disturbance cell concept.

Statistical Analysis of Accuracy for Electrochemical Working of a Compressor Blade

The article is about statistical investigation of accuracy achieved with electrochemical machining of gas turbine engine compressor blades. On the base of received data the authors propose improvement the output technical characteristics of ECM operation. It is shown that dimensional accuracy approximately 0.1 mm and surface efficiency nearby Ra = 1,6 mcm are reliably realized during ECM.

Расчет суммарных газодинамических характеристик компрессоров по идентифицированной модели с использованием CFD-технологий

Issues related identification of 1-D gas dynamic characteristics based on stage mapping using CFD technologies are addressed in this paper. Compressor of GT25000 gas turbine engine developed and manufactured at «Zorya»-«Mashproekt» SC GTRPC is taken as a test object. Methods developed and described in this paper enable to obtain full scale total gas dynamic compressor characteristics at low processing time

Erosion resistance of refractory alloys modified by ion beams

The objective of the present research is the critical analysis of test results, dedicated to the effect of ion-beam irradiation conditions upon the sand particle erosion resistance of refractory alloys. Gas turbine engine compressor blades, produced with titanium alloys (VT9 and VT18U) as well as refractory steels (EP866sh and EP718ID) were used as the study objects. The ion implantation and irradiation of targets for the high-power pulsed ion beam were accomplished by means of Delta, Raduga-2 and Temp-M accelerators. The irradiation conditions were varied within the following ranges: B, N, C, La, Pd, Sm and Hf ion implantation — E = 30–80 keV (ion energy), j = 40–5·10 3 μA/cm 2 (ion current density), f = 30 Hz (pulse frequency), D = 10 16–2 10 19 ion/cm 2 (irradiation dose); high-power pulsed ion-beam irradiation — ions of carbon (60–70%) and protons, E = 250–300 keV, j = 40–200 A/cm 2, τ = 50 ns (pulse duration), n = 3–10 pulses (number of pulses). After irradiation some targets were subjected to vacuum annealing for 2 h at their service temperature. Erosion tests of initial and irradiated blades were performed in vacuum on the gas-dynamical branches with a sand load to 200 mg/mm 2 at the particle velocity of 200 m/s. The average size of the sand particles was equal to 80–120 μm. The target surface state prior to and after erosion tests was studied by Auger electron spectroscopy, scanning electron microscopy, transmission electron microscopy, optical metallography and X-ray structural analysis. The fracture surface was studied by optical and electron fractography. The test results showed that the erosion resistance of blades, subjected to the ion-beam irradiation with post-process vacuum annealing at the optimum conditions, could be increased by 20–200% depending on the type of implanted ion. This positive effect of ion-beam treatment on the erosion resistance of refractory alloy blades is evidently connected with strengthening the material in the surface layer with thickness of 1 μm (ion implantation) up to 10 μm (high-power pulsed ion-beam irradiation). Using electron fractography it was found, that the erosion resistance increase was associated both with the type change of craters created during beginning stages of the erosion tests and with the decrease of erosion rate due to the surface microrelief formed during the fracture incubation period (“start–inertial” mechanism of fracture).

An oxidation model for predicting the life of titanium alloy components in gas turbine engines

The excellent combination of lightweight and good mechanical properties makes the titanium alloys more attractive for fabrication of gas turbine engine components. However, titanium alloys readily absorb oxygen when exposed in air at elevated temperatures, leading to α-case formation and oxidation. It severely limits the high temperature capability of titanium alloys with respect to mechanical properties and causes failures during service. In order to extend the use of titanium alloys for gas turbine engine compressor components, it is essential to understand the properties of titanium alloys before introducing into service. In the present paper, an oxidation model has been developed based on the experimental observations, to predict the life of the components fabricated from the titanium alloys. The results show that there is a very good agreement between the measured and predicted values and no assumptions are required. The developed model is very simple, easy to apply, promising, and is found to be extremely useful.

High-Speed Centrifugal Compressor Instabilities during Speed Transients

Quasi-steady instability initiation studies do not model the actual operation of a compressor in an installed engine. Namely, gas turbine engine compressors are subject to transient operation. To provide needed high-speed centrifugal compressor instability initiation information addressing transient operation, an experimental study is performed to characterize the instability behavior of a high-speed centrifugal compressor during accelerations and decelerations at various throttle settings. Data are acquired from high response dynamic pressure transducers at the inlet and exit of the compressor. Analysis of these data shows that three distinct instabilities occur: Deep surge, mild surge, and rotating stall. The rotating stall is consistent with a nine-lobed stall pattern occurring in or near the diffuser. During accelerations, rotating stall occurs prior to surge initiation. During decelerations, all three instabilities are observed. Rotating stall is observed prior to and following both mild and deep surge.

Workbench for the first stages of gas turbine engine compressors

The description of the structural features of the working shoulder blade with the inflow in the root portion and the results of the comparative numerical simulation of the three-dimensional flow of ordinary and proposed work blades designed for the transonic level of the compressor

The Air Force Manufacturing Technology Laser Peening Initiative

Laser peening has been demonstrated to be a unique and valuable method to increase the resistance of aircraft gas turbine engine compressor and fan blades to foreign object damage (FOD) and improve high cycle fatigue (HCF) life. Laser peening is also known as the LaserPeen process (LPP) or laser shock processing (LSP). LaserPeen processing’s high value as a surface enhancement process to mitigate high cycle fatigue failures is driving e fforts to expand its application from gas turbine engines to aircraft structures, land vehicles, weapon systems, and for general industrial use. One of the major impediments to the broad acceptance and use of the LaserPeen process has been high processing costs with relatively low throughput. Great strides are being made to significantly decrease laser peening costs and increase throughput, thanks to three important Air Force ManTech (manufacturing technology) programmes being conducted at LSP Technologies, Inc. (LSPT). Several key issues are being addressed to meet ManTech goals to lower costs and increase throughput, which include the development of production quality laser peening services and equipment and the expansion of the end user base for this technology. Additionally, a fourth ManTech programme is being conducted by Universal Technology Corporation (UTC) in conjunction with LSPT, Pratt & Whitney, and Howmet. The objective of this programme is to demonstrate that expensive wrought turbine engine blades can be replaced with low cost cast blades, which have been LaserPeen processed. SE / S214

Probabilistic Analysis of Aircraft Gas Turbine Disk Life and Reliability

Two series of low cycle fatigue (LCF) test data for two groups of different aircraft gas turbine engine compressor disk geometries were reanalyzed and compared using Weibull statistics. Both groups of disks were manufactured from titanium (Ti-6Al-4V) alloy. A NASA Glenn Research Center developed probabilistic computer code Probable Cause was used to predict disk life and reliability. A material-life factor A was determined for titanium (Ti-6Al-4V) alloy based upon fatigue disk data and successfully applied to predict the life of the disks as a function of speed. A comparison was made with the currently used life prediction method based upon crack growth rate. Applying an endurance limit to the computer code did not significantly affect the predicted lives under engine operating conditions. Failure location prediction correlates with those experimentally observed in the LCF tests. A reasonable correlation was obtained between the predicted disk lives using the Probable Cause code and a modified crack growth method for life prediction. Both methods slightly overpredict life for one disk group and significantly under predict it for the other.

High-power ion beam treatment application for properties modification of refractory alloys

This paper reviews the basic principles and applications of high-power pulsed ion beam (HPPIB) treatment for modification of turbine engine components service properties. This process of high-energy and high-intensity pulsed ion treatment has been evaluated for a number of titanium and nickel-iron base alloys. After a discussion of the basic phenomena occurring at the sample surface during irradiation, results are presented on the surface structure, chemical and phase contaminations, and service properties modifications. HPPIB treatment is shown to increase the following characteristics of gas turbine engine compressor blades from refractory alloys: (1) the fatigue strength (30–200%); (2) the oxidation resistance (2–3 times); and (3) the salt corrosion resistance (6–8 times). Tests performed on Russian engines have confirmed the improvement of the material behaviour.

Evaluation of vibration-induced stresses in teeth of rotor disks of compressors of gas-turbine engines

We show that loads induced in turbine blades by vibration make a significant contribution to the dynamic stressed state of the teeth of compressor disks. The dynamic stresses formed in the teeth of the investigated blade wheels can be as high as 42% of the stresses induced in the blades. Together with fretting corrosion, static stresses, and other factors, this may lead to initiation and propagation of cracks in teeth and, as a final result, their fracture.

Modification of the properties of aircraft engine compressor blades by uninterrupted and pulsed-ion beams

The influence of uninterrupted and arc-pulsed ion implantation, high-power ion beam treatment and final annealing regimes on the chemical composition and phase-structural state of surface layers and on the surface properties of gas turbine engine compressor blades manufactured from refractory alloys was investigated. Long-term full-scale tests of the production and irradiated compressor blades were carried out under gas turbine engine operating conditions. After completion of the tests the physical-chemical state of the blades was studied.

Procedure and results of the investigation of full-scale lock joints of blades of compressors of gas-turbine engines for fretting fatigue

We analyze the experimental procedures aimed at testing specimens which simulate contact pairs and full-scale lock joints of blades of compressors of gas-turbine engines with grooved disks for fretting fatigue. The experimental results corroborate the efficiency of these procedures and enable one to choose coatings and procedures of treatment of contacting surfaces which guarantee the attainment of higher values of the endurance limit than those exhibited by blade roots under the conditions of fretting. The indicated modes of treatment are characterized by high technological and economical efficiency.

Possibility of increasing the service life of damaged blades

Results are given of an investigation of the effectiveness of a method of strengthening the working titanium alloy blades of a gas turbine engine compressor. It is established that the proposed strengthening method can lengthen to more than double the service life of damaged blades. In addition it is shown that in these blades, in comparison with nonstrengthened blades, cracks develop in a narrower zone and predominantly on the side of the inlet edge.

U.S. NAVY GAS TURBINE INLET DEVELOPMENT PROGRAM

ABSTRACTThe marine salt aerosol has been the primary factor in gas turbine engine compressor fouling and hot section corrosion. The need to minimize the effects on engine performance through high performance air filtration systems is recognized by the U.S. Navy and resulted in the current Gas Turbine Inlet Development Program. The purpose of this paper is to present the status of the Program and to summarize the results of testing to date.

Rub energetics of compressor blade tip seals

The rub mechanics of the abradable blade tip seals of aircraft gas turbine engine compressors were studied under simulated engine conditions. In twelve statistically planned instrumented rub tests using single titanium blades and fibermetal rubstrips, five basic rub parameters were varied to determine their effects on the rub energy, the heat split between the blade, rubstrip surface and rub debris and the blade and seal wear. The rub energies were found to be most significantly affected by the incursion rate while rub velocity and blade thickness were of secondary importance. In five additional rub tests using single nickel alloy blades and multiple titanium alloy blades, rub energy and wear effects were found to be similar for titanium and nickel alloy blades while rub energies increased for multiple blades relative to single-blade test results.

A study of the influence of service operating time and a corrosive medium on the life of gas-turbine engine compressor blades

A study of the influence of service operating time and a corrosive medium on the life of gas-turbine engine compressor blades

Modeling of gas turbine engine compressor blades for vibration analysis

stall flutter design system. Such an analysis must properly account for all the parameters important to aeroelastic stability before durable rotors can be confidently designed with minimum weight and performance penalties. Efforts at Pratt & Whitney Aircraft and the United Aircraft Research Laboratories to provide an accurate and dependable design system have recently been intensified. Major emphasis is being concentrated on developing an energy method of stability analysis. Attempts to generate the unsteady stalled cascade aerodynamics for the analysis include both theoretical investigations and experimental measurement of unsteady lift and moment in an oscillating cascade rig. The blade mode shape prediction is also being improved. NASTRAN is being developed for determining dynamic chordwise deflection, and shroud models are being improved to account more accurately for stiffness and boundary conditions (slippage). The ultimate goal of this work is a system that extends the state-ofthe-art to make stall flutter a problem that can be successfully handled during preliminary fan and compressor design.