Browsing by Author "Eryilmaz, Ibrahim"

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    A design approach for controlled blade-off in overspeeding turbines
    (Elsevier, 2022-04-18) Eryilmaz, Ibrahim; Pachidis, Vassilios
    Following a shaft failure or loss of load in a gas turbine engine, the turbine overspeeds due to the continuing expansion through the stage(s). The overspeed may result in hazardous conditions which have to be prevented. Several mitigation methods include the control system’s response by shutting the fuel flow, mechanical friction to reduce turbine acceleration, and blade release at a predetermined rotational speed. The release of the blades not only terminates the gas torque which accelerates the disk, but also increases the disk burst speed at reduced centrifugal load. In this manuscript, a design space exploration is presented to avoid disk burst by blade-off in a civil large turbofan engine through a parametric design of blade firtree and disk post system. The firtree design parameters used in the study are the contact angle between the blade firtree and the disk post, firtree bottom flank angle, firtree flank length and firtree thickness with respect to the disk post. The LS-DYNA finite element software was used in the simulations to generate possible failure scenarios. These were ‘disk burst’ and ‘blade-off’. Blade-off conditions manifested in two ways as a function of design parameters. The first type was blade release from serrations without disk post failure, and the second type was blade escape with disk post failure. Following the design space exploration, the effect of several design and material parameters on the design space was investigated. These parameters are; the contact friction coefficient between the blade firtree and disk post, firtree serration number, and the strain hardening behavior of the material.
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    Design space exploration of turbine blade shroud interlock for flutter stability
    (American Society of Mechanical Engineers, 2021-01-11) Larrieta, Olatz; Alonso, Roberto; Pérez Escobar, Óscar; Eryilmaz, Ibrahim; Pachidis, Vassilios
    The geared turbofan engines bring the potential to rotate the fan at lower speed and allow an increase in diameter, which in turn leads to an increase in propulsive efficiency through high by-pass ratio. The low-pressure turbine stages driving the fan can also rotate at high speed resulting in fewer stages when compared to traditional turbofans. However, when operating at high speed, pressure fluctuations due to self-excited vibrations increase and may provoke flutter instabilities. In a geared architecture, to deliver the high power required by the fan and the intermediate-pressure compressor, the low-pressure turbine system operates at higher temperatures compared to its predecessors. This phenomenon requires structural materials with higher heat resistance, which carries the inconvenience of poor welding suitability. That is the reason why alternative non-welded blade shroud joint techniques are so important, techniques as the blade interlock mechanism studied in this work. This manuscript examines the effects of different design parameters of a low-pressure turbine blade shroud interlock on flutter stability, to make future recommendations for geared engines. The shrouded turbine rotor blades feature blade interlocks, which enhances the dynamic stability by providing stiffness to the rotor blade row. To assess the stability of the system, a parametric design of a turbine blade-disk assembly was prepared. In the parametric model the design variables that define the blade interlock are the interlock angle, interlock axial position, interlock contact length and height, knife seal position and pre-twist angle. After parametrization, a finite element model of the turbine blade and disk assembly was prepared with cyclic symmetry boundary condition. The stresses caused by rotation were calculated in a static structural analysis and these were used as pre-stress boundary conditions in modal analysis. The modal results were afterwards exchanged with an aerodynamic model to obtain the aerodynamic damping for different blade interlock design configurations. In the present work, the dynamic response of the first three excitation modes was analyzed. It was found that the third mode was stable for all the design points, whereas first and second modes were unstable at least for the reference design point. Among the considered six different parameters that define the blade interlock geometry, the interlock contact position turned to be the most influential parameter for modal response and for flutter stability. Moving the interlock contact position towards the trailing edge gave the most beneficial results. On the other hand, the interlock angle showed the least influence on both, the modal analysis and flutter behavior. The accomplished Design of Experiments and subsequent optimization process also conclude that there exists an interdependency between the studied parameters.
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    Multi-blade shedding in turbines with different casing and blade tip architectures
    (Elsevier, 2019-02-25) Eryilmaz, Ibrahim; Guenchi, Biläl; Pachidis, Vassilios
    A shaft failure in a gas turbine engine results in the decoupling of the turbine and the compressor. The turbine continues extracting work from the air flow causing the acceleration of the free-running turbine which can result in debris release or even disc burst. Post shaft failure the structural integrity of the engine must be guaranteed for product safety and certification purposes. To achieve this, speed limiting systems have to be integrated. One common method is friction between the rotor and stationary structures which occurs during unlocated failures where the bearing arrangement allows axial movement of the rotor under end load. Another possible mechanism is the destruction of the turbine rotor blades such that they cease to extract power from the incoming flow, decelerating progressively. Blade shedding involves rupture of blades which may result in their containment within the casing or rupture of the turbine casing. This research investigates the effects of excessive damage as blades rupture in the high-pressure turbine of a large civil engine. The research investigates different casing inclinations and shrouded/unshrouded blade configurations respectively. The nonlinear finite element software LS-DYNA is used to model two blade release scenarios which are; i) simultaneous release of all blades, and ii) simultaneous sectoral release of blades. The blades are released from firtrees considering the worst case scenario from a containment point of view. It is observed that a sector having a sufficient number of blades can result in the same effect caused by all blades impacting the casing. Containment requirements of shrouded and unshrouded rotors with different casing inclinations are compared as a function of the blade kinetic energy. Provided that the blade mass is kept constant, the effect of the casing inclination is found to be dominant when compared to the effect of blade tip geometry. Together with a rotor overspeed trajectory, the containment requirement of a simultaneous multi-blade shedding application for disk burst prevention is given. The research provides improved understanding of blade tip-to-casing interactions, to be used as an overspeed prevention mechanism, and contributes towards developing design guidelines for the next generation of aero engines in terms of fail-safe engine architectures.
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    Multidisciplinary methodology for turbine overspeed analysis
    (Cambridge University Press, 2018-11-15) Eryilmaz, Ibrahim; Pawsey, Lucas; Pachidis, Vassilios
    In this paper, an integrated approach to turbine overspeed analysis is presented, taking into account the secondary air system dynamics and mechanical friction in a turbine assembly following an unlocated high-pressure shaft failure. The axial load acting on the rotating turbine assembly is a governing parameter in terms of overspeed protection since it governs the level of mechanical friction which acts against the turbine acceleration due to gas torque. The axial load is dependent on both the force coming from secondary air system cavities surrounding the disc and the force on the rotor blades. It is highly affected by secondary air system dynamics because rotor movement modifies the geometry of seals and flow paths within the network. As a result, the primary parameters of interest in this study are the axial load on the turbine rotor, the friction torque between rotating and static structures and the axial position of the rotor. Following an initial review of potential damage scenarios, several cases are run to establish the effect of each damage scenario and variable parameter within the model, with comparisons being made to a baseline case in which no interactions are modelled. This allows important aspects of the secondary air system to be identified in terms of overspeed prevention, as well as guidelines on design changes in current and future networks that will be beneficial for overspeed prevention.
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    Performance and operability of an electrically driven propulsor
    (Sage, 2021-12-29) Eryilmaz, Ibrahim; Li, Huayang; Pachidis, Vassilios; Laskaridis, Panagiotis; Zhu, Zi-Qiang; Jewell, Geraint Wyn
    This manuscript discusses the operation of an electrically driven fan for a hybrid-electric propulsion system for BAe-146 aircraft. The thrust requirement is fed into an integrated cycle and aerodynamic design tool for the sizing of a ducted fan as one of the main propulsors, podded under the wing as a replacement for a turbofan engine. The electric motor design is initiated with the torque and speed requirements and with the dimensional constraints arising from the driven fan geometry. The fan operation and aerodynamic design are derived by changing the fan pressure ratio and hub-to-tip ratio to obtain a 2-D design space. Surface-mounted permanent magnet electric motor designs are mapped on the 2-D fan design space. The design and operational flexibility of the system is investigated through three scenarios. In the first scenario, the aircraft rate of climb is changed to downsize the electric motor. In the second scenario, the electric motor rated frequency is changed to increase the power density and in the third scenario the electric motor current density is changed for the same purpose. The investigated three scenarios provide design and operational guidelines for reducing the weight of the electric motor for a direct drive application.
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    Turbine thermomechanical modelling during excessive axial movement and overspeed
    (Cambridge University Press, 2019-03-14) Eryilmaz, Ibrahim; Pachidis, Vassilios
    This manuscript discusses the numerical (finite element) and analytical modelling of structural interactions between gas turbine components in case of excessive axial movement and overspeed. Excessive axial movement, which may occur after a shaft failure, results in contact between rotating and static turbine components under high forces. These forces create friction which can act as a counter torque, potentially retarding the ‘free-rotating’ components. The study is based on a shaft failure scenario of a ‘three-shaft’, high ‘by-pass’ ratio, civil ‘large-fan’ engine. Coupled analytical performance and friction methods are used as standalone tools to investigate the effect of rubbing between rotating and stationary components. The method is supported by ‘high-fidelity’, ‘three-dimensional’, thermomechanical finite element simulations using LS-DYNA software. The novelty of the work reported herein lies in the development of a generalized modelling approach that can produce useful engine design guidelines to minimize the terminal speed of a free running turbine after an unlocated shaft failure. The study demonstrates the advantage of using a fast analytical formulation in a design space exploration, after verifying the analytical model against finite element simulation results. The radius and the area of a stationary seal platform in the turbine assembly are changed systematically and the design space is explored in terms of turbine acceleration, turbine dislocation rate and stationary component mass. The radius of the friction interface increases due to the increasing radius of the nozzle guide vane flow path and stationary seal platform. This increases the frictional torque generated at the interface. It was found that if the axial dislocation rate of the free running turbine wheel is high, the resulting friction torque becomes more effective as an overspeed prevention mechanism. Reduced contact area results in a higher axial dislocation rate and this condition leads to a design compromise between available friction capacity, during shaft failure contact, and seal platform structural integrity.