Aero-engine Applications Research Articles

A REVIEW OF NEW COMPOSITE MATERIALS FOR THERMAL PROTECTION IN AN AERO-ENGINE APPLICATION

The paper is aimed at exploring new composite materials for thermal protection in an aero-engine application. Aluminium hybrid composites have been widely used to substitute materials in the aerospace industry because of their high thermal resistance, wear resistance and strength-to-weight ratio. This review article covers standard fire test results of different types of burners and burn-through times of the composites and polymeric materials. The paper evaluates the performance of different structural composite materials including synthetic fibres, natural fibres and some type of coating on the materials in a pool-fire scenario. It also deals with the practical use of flame retardancy and performance factors including safety in an aircraft related to polymer composites. It was found that most of the materials used could only be categorized as fire resistance materials except GLARE materials and synthetic/natural fibre composites and aerogel composites which showed the characteristic of fireproof materials.

Micropatterning on stainless steel surface using electrochemical micromachining

Advanced materials propose an outstanding combination of biocompatibility and weight-specific properties that create these materials as ideal components in biomedical and aeroengine applications. However, the important microengineering applications are hindered by the prerequisite to produce high quality ellipse micropattern on stainless steel foils. The advanced and cost-effective method i.e., maskless electrochemical micromachining (EMM) has been established to manufacture micropatterns on SS-304 foils. The dispersal of current flux influences the shape and uniformity of microtextures and spreading of current density depends upon the mask thickness and strength in this process. So, micro ellipse profiles can be generated by controlling mask thickness and process parametric combination. In this paper, the development of investigational setup and utility of appropriate mask are described to fabricate the regular microtextures. The outcome of input factors such as electrolyte concentration, voltage, and machining time on various microtextured characteristics are investigated. A study has been made to achieve the best process variables based on micrographs. From investigational results attained in a sequence of experimentation, the appropriate SU-8 2150 mask is reused for making uniform micropatterns. Eventually, a group of micropatterns is produced effectively with the vital accuracy by maskless EMM.

Study on multi stage forging process with combination of different strain rate and temperature region in IMI685 aero engine compressor disc forging

IMI 685, a near alpha Ti alloy has been used in high pressure compressor discs for aero engine application due to its high strength to weight ratio and high creep resistance. Manufacturing process plays an important role in obtaining the required mechanical properties for the highly complex and critical products. The suitable manufacturing process is not available in open literature. Hence, in this paper non-isothermal approach of manufacturing compressor disc combining hydraulic press and pneumatic hammer at different temperature for different stages of forging has been explored. Mechanical and metallurgical analysis of processing in partial α-β/β temperature range resulted in higher creep resistance with acicular/needle alpha platelets with a basket weave structure. This experiment has proven that with a proper selection of thermo mechanical processing parameters (combination of different forging temperature and strain rates in forging sequence), required creep properties can be met with the non-isothermal processing route.

Fatigue property-performance relationship of additively manufactured Ti-6Al-4V bracket for aero-engine application: An experimental study

In this paper, the Low Cycle Fatigue (LCF) performance of a weight optimized SLM Ti-6Al-4V bracket has been investigated. The bracket, a ‘struts & connectors’ shape for an aero-engine application, was designed to operate within the material elastic limit. The tensile and LCF fatigue data obtained from the coupon tests are discussed first which was then used to establish the loading levels for the LCF tests on the bracket. Cyclic softening was observed in the LCF coupon tests at strain levels higher than the material elastic limit, which was attributed to the pile-up of dislocations and formation of sub-grains in the SLM Ti-6Al-4V material. The SLM Ti-6Al-4V bracket met the LCF target cycles when operated near the material elastic limit and also when cycled at displacement levels causing plastic strain in the bracket elements. The carefully designed, weight optimized bracket has shown promising results in terms of its LCF performance and this provides a good encouragement to practicing engineers to adopt SLM technology for load bearing applications.

Selection of forging process for compressor disc for aero engine using finite element analysis

Compressor discs for aero-engine applications are very critical in nature with respect to application and near alpha Ti alloys are used for this application due to its high specific strength combined with good corrosion resistance, fracture toughness, superior high temperature properties. Its processing technique is also highly complex and in open literatures is hardly available. In this paper, forging process sequence for manufacturing the compressor disc has been optimized using finite element analysis. Simulation results are analyzed for load, temperature, strain and material flow behavior. Based on the simulation studies, upsetting in two stages from cylindrical billet using hydraulic press and die forging in pneumatic hammer is recommended..

Picosecond laser machining of ceramic matrix composite

In recent years, there has been increased interest in using silicon carbide (SiC) based ceramic matrix composites (CMC) materials for aero-engine applications; however, both conventional and non-conventional machining of SiC CMC is challenging. Ultrashort pulse laser machining is evolving as an attractive option for materials like SiC CMC; however, the current understanding is based only on low-power lasers, whose removal rate is not in-line with the industrial requirements. This paper presents the results of 300 W picosecond laser machining of SiC CMC. The laser machining characteristics of SiC CMC were investigated at high average power and discussed in detail. A 300 W picosecond laser can be used to achieve a material removal rate of ∼230 mm3/min (0.76 mm3/minW) for a machining depth of 2 mm in SiC CMC, however, irrespective of the laser parameters a wall taper of ∼7° was observed over the sides of the slots that need to be addressed.

Maximizing strength and corrosion resistance of InterPulsed TIG welded Superalloy 718 joints by RSM for aerospace applications

Inconel 718 (IN-718) is a nickel-based alloy, mostly used in the aeroengine applications due to its good mechanical strength and resistance to corrosion. However, it is amenable to alloying segregation and Laves phase growth in fusion zone (FZ) which severely deteriorates the strength and corrosion resistance of IN-718 joints. To overcome this problem, InterPulsed TIG (IP-TIG) welding process, characterized by high frequency current pulsation and arc constriction, was attempted to join thin sheets of IN-718 alloy. The 3D graphs of response surfaces were built to establish the optimized welding parameters for maximizing the joint strength. The parametric mathematical equation was constructed to predict the strength of IN-718 joints. The mathematical equation was also built to predict the tensile strength from Laves phase. The potentiodynamic corrosion test was done in a 3.56 wt.% NaCl solution for the selected joints to determine the corrosion behavior of IN-718 joints. The IP-TIG welded IN-718 joints exhibited 99.20% of base metal strength and superior corrosion resistance at optimized condition of welding parameters. This refers to the refining of FZ dendritic structure which aids in minimizing the Laves phase. The IN-718 joints developed using optimized IP-TIG parameters exhibited 22–32% higher strength and superior corrosion resistance than TIG welded IN-718 joints and comparable to EBW and LBW IN-718 joints.

The dependency of compressive response of epoxy syntactic foam on the strain rate and temperature under rigid confinement

Syntactic foam is being increasingly conceived for aero engine applications to enhance overall structural reliability under impact loading. Systematic compressive tests without and with lateral confinement are carried out to investigate the deformation and failure of an epoxy syntactic foam (ESF) at quasi-static 0.01/s, medium rate 10–100/s and high strain rates 500–1100/s at gradually increasing temperatures from room temperature 25 °C to 150 °C. The stress–strain response and the corresponding deformation process, complemented by Digital Image Correlation (DIC) technique, were monitored to reveal the dynamic deformation and strain localization of the foam. The elastic-brittle behavior without confinement at high strain rates can be suppressed by the lateral confinement and by elevated temperatures above 100 °C. The lateral confinement slightly increases yield stress and significantly improves the energy absorption of ESF. Both confined yield stress and energy absorption evolve nonlinearly with strain rate and temperature. The strain hardening in the elastic–plastic behavior of ESF under confinement is strain rate independent, compared to its modest temperature dependency. A nonlinear phenomenological model is found to be able to describe the unconfined and confined responses of ESF and its temperature-strain rate equivalence, and reveal the competition between strain hardening and strain softening which is influenced by temperature.

Excellently abradable seal coating with spherical closed-pore for next-generation aero-engine application

To improve the aero-engine compressor efficiency and reliability, abradable seal coatings need to have good abradability resulting from reduced hardness. However, low hardness is often coupled with low bonding strength, resulting in coating spalling off failure during service. To defeat the conflicts between low hardness and high bonding strength, a new abradable seal coating with a spherical closed-pore structure was prepared in this study. Compared to conventional coatings, the new coating has both lower hardness and higher bonding strength. In the high-speed scraping test, the coating presented excellent abradability without any coating spalling. The new abradable seal coating is promising to improve the efficiency and reliability of the next-generation aero-engine.

Bearing Skidding Detection for High Speed and Aero Engine Applications

Abstract Rolling-element bearings (REB) can develop severe damage due to skidding (slipping) between the rolling elements and bearing races. Skidding can be described as gross sliding between the bearing surfaces in relative motion and can result in significant surface distress such as smearing, especially at light loads and high rotational speeds. Under these conditions, skidding occurs between the rolling elements and the bearing races, leading to increased wear (higher friction coefficient), elevated bearing temperature, significant power losses and reduced service life of the bearing. The main objective of this study is to investigate the significance of various sensing technologies (induction, vibration, ultrasound, acoustic and optical) in detecting skidding in standard series roller bearings as well as custom-made roller bearings for aero engine applications. The bearings have a bore diameter of 60 mm and 90 mm, respectively. Jet and under race lubrication techniques have been used to supply oil to the bearings under test. The custom-made aero engine test bearing features special channels to allow under race lubrication of the rollers/races contacts as well as the cage land. The effect of radial load, rotational speed and oil flow on roller skidding have also been investigated and analyzed. Tests have been performed on a dedicated high speed experimental bearing facility and data was recorded using a commercially-available data acquisition system.

In-situ multi-axial testing of three-dimensional (3D) woven organic matrix composites for aeroengine applications

The present paper focuses on in-situ multi-axial testing of three-dimensional (3D) woven organic matrix composites for aeroengine applications. Damage onset is characterised by X-ray µ-Computed Tomography (µCT) based in-situ tests, supported by Acoustic Emission (AE) analysis and by Finite Element (FE) simulations. The synergy between these techniques allows having knowledge of the solicitation at which damage onsets and defines the first points necessary for the construction of a damage onset surface at the sample scale.

The Impact of Various Modeling Decisions on Flow Field Predictions in a Centrifugal Compressor

Abstract Computational tools have become increasingly important in design and research applications in recent years due to increasing computational resources. In most cases, model geometry and flow physics are simplified to reduce the complexity of the computational model. While this was necessary historically, modern computational tools are capable of including realistic features such as fillets, surface roughness, and heat transfer. This work presents extensive and systematic numerical results from a simulation of a centrifugal compressor stage for an aero-engine application. Numerical results are compared to detailed experimental data to investigate the effect of various modeling decisions, including turbulence models, on the predicted aerodynamics developing through the diffuser passage. Roughness and the inclusion of fillets significantly impact the flow development, especially with the shear-stress transport (SST) turbulence model. This approach leads to the conclusion that the BSL-EARSM model is best able to predict the experimentally determined diffuser flow profiles and overall performance trends with the inclusion of the previously mentioned model features. Additionally, misleading conclusions can be reached if modeling decisions are based on merely matching overall performance values. Finally, frozen rotor simulations are used to roughly approximate the impact of unsteadiness on the flow field. The results show a significant impact and also that the inclusion of approximate unsteady effects tends to further improve the predictive capability of the computational models that were considered.

Heat Treatment and Properties of High-Temperature Steels for Rolling Bearings

Abstract It is demonstrated that high speed steels are suitable for manufacturing of heat resistant rolling element bearings especially for applications in aero engines. A detailed analysis of heat treating processes and metallurgical evaluation shows how the properties of these steels grades can be adjusted to meet the specific requirements for engine bearings. Based on results from rig testing it is shown how variations in heat treating parameters can influence the rolling contact fatigue performance. The role of carbides, a typical constituent of high speed steels, on bearing applications is discussed in detail. The high level of heat resistance of high speed steels allows a nitriding in addition to the conventional heat treatment. By nitriding an increase in hardness close to the surface can be achieved which can make bearings more robust against indentation of hard particles. ◼

On the estimation of the windage power losses of spiral bevel gears: An analytical model and CFD investigation

Windage power loss, a form of load-independent power loss, is always one of the hotspots for reducing fuel consumption and overall weight in aero-engine applications. This paper represents a considerable improvement in the analytical model for estimating the losses of spiral bevel gears and proposes a novel overset mesh method for the prediction of the windage losses. The improved analytical model details the total windage losses as the sum of air dragging effects on the periphery and toe/heel. Once each loss is calculated, the total losses could be then determined. Furthermore, by leveraging the computational fluid dynamics (CFD) software and exploiting the overset mesh strategy, the windage behavior of the gear is evaluated. Moreover, direct comparisons between theoretical /numerical results and experimental findings demonstrate that both methods are capable of estimating the windage power loss values. The results clearly show that pressure effect is the chief cause of the overall windage loss for the spiral bevel gear and the major windage losses are located nearby the gear teeth, much greater than the toe/heel.

A binary beta titanium superalloy containing ordered-beta TiFe, alpha and omega

Alpha-beta titanium alloys excel for aeroengine applications but are typically limited to ~550°C. An alternative strategy is reinforcement with the ordered-beta TiFe intermetallic, toward ‘β-Ti superalloys’, however, there has been minimal study of TiFe precipitation in the binary system.Here, a Ti-20Fe (at.%) alloy was homogenised at 1050°C in the β-Ti phase field and aged at 600°C where the Fe supersaturation promoted TiFe precipitation. Curiously, as the TiFe volume fraction increased, the alloy hardness decreased, due to an interplay of mechanisms: (1) Fe solid solution strengthening, which reduces as the β-Ti Fe content falls to 16.2% on ageing; (2) ω precipitation strengthening, as ω-like incommensurate modulated domains were identified by transmission electron microscopy in the homogenised β-Ti parent phase and are suggested to change in size and structure after ageing, resulting in reduced ω-strengthening; (3) softening as softer TiFe and α-Ti phases precipitate from the harder ω‑strengthened β-Ti parent phase.

A Joint Experimental and Large Eddy Simulation Characterization of the Liquid Fuel Spray in a Swirl Injector

Abstract The quality of liquid fuel spray injection determines to a large extent the performance of aero-engine combustors. This investigation focuses on the detailed characterization of the liquid fuel spray in a test rig targeted at aero-engine applications. The liquid fuel is injected as a hollow cone by a simplex atomizer and the injector comprises a radial swirler. Two features of the droplet distribution are less commonly found. First, the distributions of droplet diameters exhibit nonaxisymmetric patterns, which are investigated for three types of swirlers. Second, it is found that the size-conditioned velocity distributions feature a single wide peak for small droplets and become bimodal for the largest droplets, with a first peak at low velocities, and a second one at higher velocities. Experiments are complemented with large eddy simulations and Lagrangian particle tracking. The spray interacts with the lateral injector surface and requires a droplet–wall interaction model for the liquid film. Simulations do not retrieve the lack of rotational symmetry that is found experimentally indicating that this feature is not linked to the nature of the flow. This is also consistent with further experiments with a different atomizer confirming that this is due to imperfections in the atomizer geometry. Another result is that certain swirler designs are more robust to atomizer imperfections. Simulations accounting for the liquid film yield a bimodal distribution for the droplets' axial velocity distribution which is not obtained without this model, indicating that it is important to represent the droplet–wall interaction.

Monitoring Damage in Nonoxide Composites at High Temperatures Using Carbon-Containing CVD SiC Monofilament Fibers as Embedded Electrical Resistance Sensors

Abstract Electrical resistance (ER) has become a technique of interest for monitoring SiC-based ceramic composites. The typical constituents of SiC fiber-reinforced SiC matrix composites, SiC, Si, and/or C, are semiconducive to some degree resulting in the fact that when damage occurs in the form of matrix cracking or fiber breakage, the resistance increases. For aero engine applications, SiC fiber reinforced SiC, sometimes Si-containing, matrix with a boron nitride (BN) interphase are often the main constituents. The resistivity of Si and SiC is highly temperature dependent. For high temperature tests, electrical lead attachment must be in a cold region which results in strong temperature effects on baseline measurements of resistance. This can be instructive as to test conditions; however, there is interest in focusing the resistance measurement in the hot section where damage monitoring is desired. The resistivity of C has a milder temperature dependence than that of Si or SiC. In addition, if the C is penetrated by damage, it would result in rapid oxidation of the C, presumably resulting in a change in resistance. One approach considered here is to insert carbon “rods” in the form of CVD SiC monofilaments with a C core to try and better sense change in resistance as it pertains to matrix crack growth in an elevated temperature test condition. The monofilaments were strategically placed in two nonoxide composite systems to understand the sensitivity of ER in damage detection at room temperature as well as elevated temperatures. Two material systems were considered for this study. The first composite system consisted of a Hi-Nicalon woven fibers, a BN interphase and a matrix processed via polymer infiltration and pyrolysis (PIP) which had SCS-6 monofilaments providing the C core. The second composite system was a melt-infiltrated (MI) prepreg laminate which contained Hi-Nicalon Type S fibers with BN interphases with SCS-ultramonofilaments providing the C core. The two composite matrix systems represent two extremes in resistance, the PIP matrix being orders of magnitude higher in resistance than the Si-containing prepreg MI matrix. Single notch tension–tension fatigue tests were performed at 815 °C to stimulate crack growth. Acoustic emission (AE) was used along with ER to monitor the damage initiation and progression during the test. Post-test microscopy was performed on the fracture specimen to understand the oxidation kinetics and carbon recession length in the monofilaments.

Review of a Custom-Designed Optical Sensing System for Aero-Engine Applications

Fibre bundle-based reflective optical sensors are good candidates for parameter monitorisation in aero engines. Tip clearance is one of those parameters of great concern that is necessary to monitor. Within this optical technology, the evolution experienced by a custom-designed optical sensor is presented from its first configuration up to the fifth one. The performance of the last configuration is compared with those of other two optical sensors that are also based on a fibre bundle design. The comparison has been carried out in an experimental program in a transonic wind tunnel for aero engines. The proven high resolution and sensitivity of the last configuration of the optical sensor opens up the possibility to detect blade defects, cracks, etc. that could otherwise be hard to track.

Modeling of Partially Wetting Liquid Film Using an Enhanced Thin Film Model for Aero-Engine Bearing Chamber Applications

Abstract In the case of aero-engine, thin lubricating film servers dual purpose of lubrication and cooling. Prediction of dry patches or lubricant starved region in bearing or bearing chambers are required for safe operation of these components. In this work, thin liquid film flow is numerically investigated using the framework of the Eulerian thin film model (ETFM) for conditions, which exhibit partial wetting phenomenon. This model includes a parameter that requires adjustment to account for the dynamic contact angle. Two different experimental data sets have been used for comparisons against simulations, which cover a wide range of operating conditions including varying the flowrate, inclination angle, contact angle, and liquid–gas surface tension coefficient. A new expression for the model parameter has been proposed and calibrated based on the simulated cases. This is employed to predict film thickness on a bearing chamber which is subjected to a complex multiphase flow. From this study, it is observed that the proposed approach shows good quantitative comparisons of the film thickness of flow down an inclined plate and for the representative bearing chamber. A comparison of model predictions with and without wetting and drying capabilities is also presented on the bearing chamber for shaft speed in the range of 2500 RPM to 10,000 RPM and flowrate in the range of 0.5 liter per minute (LPM) to 2.5 LPM.

Porosity analysis in SiC/SiC ceramic matrix composites

AbstractCeramic matrix composites made of silicon carbide fibers embedded in silicon carbide matrix (SiC/SiC CMCs) are widely recognized as excellent replacements for denser superalloys such as Nickel based super alloys for high‐temperature applications in aero engines. During the manufacturing process with chemical vapour infiltration, large amount of pores are introduced in the CMC and their presence can have considerable impact on the structural performance of a component. In this work, we analyze the pore size and spatial distribution in SiC/SiC CMCs for different geometrically‐identical specimens. Porosity within specimens are quantified through computed tomography (CT) scans. Different statistical approaches are utilized for describing the amount of pores, their size and spatial distribution within each specimen.