Turbofan Aero-engine Research Articles

A performance degradation assessment method for complex electromechanical systems based on adaptive evidential reasoning rule

The evidence reasoning (ER) rule has been widely used in various fields to deal with both quantitative and qualitative information with uncertainty. However, when analyzing dynamic systems, the importance of various indicators frequently changes with time and working conditions, such as performance degradation assessment of complex electromechanical systems, and the weights of the traditional evidence reasoning rules cannot be appropriately adjusted. To solve this problem, this paper proposes an adaptive evidence reasoning (AER) rule that can adjust weights according to different times and working conditions. The AER rule has two unique features: adaptive weight operation under time division and adaptive weight operation under working-condition division, which are used to solve the problem of dynamic weight adjustment under different times and working conditions. The CMA-ES algorithm is used to optimize the model parameters. Two case studies of performance degradation assessment are established to prove the advantage of the AER rule: a computer numerical control experiment and a simulation experiment of turbofan aeroengine. The results verify the effectiveness and practicability of the proposed method.

Rapid aerodynamic characterization of surface heat exchangers for turbofan aeroengines through optical techniques and additive manufacturing

Surface heat exchangers that use the bypass flow as heat sink are becoming a widely used solution to alleviate the high thermal load of modern aeroengines. Experimental characterization of such heat exchangers in full-scale engine tests is extremely expensive and time-consuming, so carrying out experiments in scaled wind tunnels that can replicate their very challenging flow conditions is highly desirable. However, intrusive instrumentation can affect the actual aerodynamics of the component due to the reduced size of the section and the typical high air velocities of turbofan engines. For this reason, a methodology to characterize a surface heat exchanger designed to work in the turbofan bypass using optical techniques is presented in this work. Additionally, the possibility of replicating the experimental conditions using additively manufactured models of exchangers would allow rapid preliminary characterization of these components. In this investigation, different non-intrusive techniques such as PIV, LDA, Schlieren, or LDV have been applied to determine the heat exchanger aerodynamic performance and vibration response, and a detailed characterization of the flow field has been carried out. Data have been cross-validated using different measurement techniques. A 3D-printed model has also been built to compare with the aluminum heat exchanger, showing almost an identical behavior in terms of velocity distribution downstream of the heat exchanger and pressure drop induced by its fins, as well as corrected frequencies, confirming thus the suitability of additive manufacturing for the aerodynamic characterization of these devices in preliminary stages.

Influence of gear modification on dynamic characteristics of star gearing system in geared turbofan (GTF) gearbox based on a closed-loop dynamic analysis method

The star gearing transmission system is a key component of a geared turbofan (GTF) aero-engine. Accurate prediction of the dynamic characteristics of the star gearing transmission system, especially after gear modification, is important for the design of high-performance engines. Firstly, this paper establishes a dynamic time-varying mesh stiffness (TVMS) and a comprehensive transmission error (TE) calculation model for herringbone gear pair considering changes in the meshing state. Then, the dynamic internal excitation calculation model is combined with the lumped parameter dynamic model of the system to form the “excitation–response–feedback” closed-loop coupling dynamic analysis method of the GTF star gearing system. Compared with traditional analysis methods, this method takes into account the effect of contact state changes on internal excitation during the calculation process and corrects internal excitation in real time. Finally, the effect of different topology modification schemes on the system's dynamic characteristics is investigated based on this method. The results show that the closed-loop coupled dynamics analysis method is able to capture contact state changes of the same gear pair at different moments and between different gear pairs at the same moment. Modified #2 is only to modify the planet gear. This scheme can effectively reduce the vibration displacement of the ring gear, and reduce the peak-to-peak value of the ring gear's vibration displacement in all directions by more than 70%. At the same time, the scheme also reduces the load-sharing coefficient of the internal and external meshing pairs of the system by 1.92% and 2.01%, respectively, and the peak-to-peak value of the dynamic factor of the internal and external meshing pairs can be reduced by 57.91% at most.

Conceptual design and optimization of polymer gear system for low-thrust turbofan aeroengine accessory transmission

Abstract The advancement in materials and lubrication has significantly improved the load-carrying capability of polymer gears, making them ideal for replacing metallic gears in power transmission. However, this conversion is not as simple as substituting steel with polymer; it requires a thorough redesign of the structural parameters specific to polymer gears. To enable the metallic-to-polymer conversion of gear in power transmission, a model for optimizing polymer gear systems was developed. An investigation of the accessory transmission system of a low-thrust turbofan aeroengine was conducted. A comprehensive performance index for the accessory transmission was developed using combined weighting coefficients to achieve the optimization goals including total mass, transmission efficiency, maximum transmission error, and so on. The polymer gear system developed using the proposed optimization model demonstrated a 70.4% reduction in total mass compared with the metallic gear system, as well as a transmission error decrease of over 29% when compared with polymer gear systems with standard tooth profiles. The contribution analysis results demonstrated that optimizing the tooth width, pressure angle, and addendum height of polymer gears can significantly enhance the load-carrying capacity of the polymer gear system while maximizing tooth profile flexibility.

Data-driven nonlinear MIMO modeling for turbofan aeroengine control system of autonomous aircraft

The mathematical modeling problem of a general turbofan aeroengine control system used in autonomous aircraft is investigated in this paper. Unlike the thermodynamics or physical mechanism-based modeling methods in the existing literature, a pure data-driven modeling approach is presented with high accuracy via online and offline data only. This is achieved by using the nonlinear autoregressive neural network with exogenous inputs (NARX) neural network. In comparison with the existing NARX-based MIMO modeling methods, the series–parallel structure and the parallel structure are integrated for the neural network training. Faster convergence and stability as well as higher modeling accuracy are thus achieved. Another feature of this approach is that it is independent of all the components’ dynamics in the turbofan aeroengine. Simulation results are given to verify the effectiveness of the proposed modeling approach. This is further validated by experimental testing with the method applied to mini turbofan aeroengine testbed.

Aerodynamics of a short intake in crosswind

The next generation of turbofan aero-engines are likely to have an increase in fan diameter to reduce the specific thrust and increase the overall propulsive efficiency. More compact nacelles with possibly shorter intakes may be used to reduce weight and drag and achieve a net reduction of fuel consumption. For these compact nacelles a key consideration is the design of the short intake at the off-design conditions such as crosswind and high incidence operations. The close coupled interaction between a short intake and the fan at these off-design conditions is one of the key challenges. Previous work focused on the impact of short intake aerodynamics on the fan but there is a similar requirement to understand the impact of the fan on the viable short intake design space. This paper addresses the influence of the fan on the separation onset of the flow within a short intake under crosswind conditions. The effect of the fan on the separation characteristics of the intake boundary layer was considered both from a steady and an unsteady point of view. A hierarchy of fan computational models was used to separately assess the different aerodynamic contributions and to evaluate a net effect of the fan on the intake critical condition. Steady computational fluid dynamics analyses showed a notable positive effect of the fan on total pressure loss at post-separation conditions relative to a configuration without the fan. However, unsteady analyses revealed that fan unsteadiness has an adverse impact on the intake separation characteristics which reduces the intake critical conditions by about 15%. The main mechanisms behind the unsteady interaction were identified. Overall this work addresses, for the first time, the role of fan unsteadiness on the separation characteristics of the boundary layer within a short intake in crosswind.

Experimental aerothermal characterization of surface air-cooled oil coolers for turbofan engines

Thermal management is a major challenge for new generation turbofan aero-engines. One of the most promising heat exchangers are the so-called surface air-cooled oil coolers (SACOCs). In this study, an experimental methodology is proposed and implemented in order to characterize SACOCs mounted in turbofan bypass ducts. Three different SACOC geometries have been characterized under the same nominal operating point, while the actual velocity profile in the bypass was reproduced by means of a distortion screen upstream the test section. The heat exchangers were mounted in counterflow configuration and feature the same fin geometry in the oil side. The three prototypes varied only in the air side, being the first a baseline flat plate, the second a SACOC with standard trapezoidal fins and the third featuring optimized fins designed to reduce the pressure drop. Aerothermal results demonstrated that the effect of the SACOC on the bypass flow was confined to a region about the same height and width of the finned area, avoiding the need of reproducing the whole bypass duct. However, for this reduced-height experimental approach to be valid, we show that the velocity profile needs to be rearranged to match the specific section of the whole bypass. We also demonstrate how the optimized fin geometry achieved a 10% lower friction factor than the standard one at nominal flow conditions while increasing the overall heat transfer coefficient by 5.2%.

Application of ADMM to robust model predictive control problems for the turbofan aero-engine with external disturbances

<abstract><p>In this paper, we investigate a class of optimal control problems for turbofan aero-engines considering external disturbances. The alternating direction method of multipliers (ADMM) is embedded in the framework of robust model predictive control (RMPC), which is not only able to reach a predetermined value of the engine fan speed, but is also developed to maintain the robustness of the engine control system. First, to consider the optimal control strategy for the worst-case scenario, this optimal control problem is formulated as a minimum-maximum convex optimization problem with constraints. Second, through a transformation technique, the problem can be equivalently described by a variational inequality, which is then transformed into a quadratic programming (QP) problem using a proximal point algorithm (PPA). Finally, the ADMM algorithm is used to solve a series of optimization subproblems based on the structural characteristics of the model. Computational examples illustrate the solution efficiency and robustness of the improved algorithm (RMPC-ADMM).</p></abstract>

Thermodynamic Performance and Creep Life Assessment Comparing Hydrogen- and Jet-Fueled Turbofan Aero Engine

There is renewed interest in hydrogen as an alternative fuel for aero engines, due to their perceived environmental and performance benefits compared to jet fuel. This paper presents a cycle, thermal performance, energy and creep life assessment of hydrogen compared with jet fuel, using a turbofan aero engine. The turbofan cycle performance was simulated using a code developed by the authors that allows hydrogen and jet fuel to be selected as fuel input. The exergy assessment uses both conservations of energy and mass and the second law of thermodynamics to understand the impact of the fuels on the exergy destruction, exergy efficiency, waste factor ratio, environmental effect factor and sustainability index for a turbofan aero engine. Finally, the study looks at a top-level creep life assessment on the high-pressure turbine hot section influenced by the fuel heating values. This study shows performance (64% reduced fuel flow rate, better SFC) and more extended blade life (15% increase) benefits using liquefied hydrogen fuel, which corresponds with other literary work on the benefits of LH2 over jet fuel. This paper also highlights some drawbacks of hydrogen fuel based on previous research work, and gives recommendations for future work, aimed at maturing the hydrogen fuel concept in aviation.

Vibration Characteristics of Multi-Stage Blade–Disk–Shaft Integrated Structure with Three-Dimensional Crack

According to the structural parameters of an actual turbofan aeroengine, the dynamics modeling and vibration analysis of multi-stage blade-disk-shaft integrated structure with three-dimensional crack are conducted. Based on the theory of fracture mechanics, the stress intensity factor is considered as a fracture criterion. This paper proposes a three-dimensional (3-d) equivalent notch model which is simpler and more convenient than the traditional crack simulation model. Moreover, its validity is analyzed. Aiming at the harsh working environment of the blades, the penetration crack is considered at the edge and the middle of a blade root, respectively. The influence of the crack on the strength failure of the rotor system is studied. Results show that the appropriate small angle equivalent notch model can be convenient and effective to simulate the fracture cracks, in which 5° is proved to be the best angle. The crack aggravates the vibration of blades obviously. A parametric study is conducted, with a particular focus on the effects of pressure load and centrifugal load on the stress intensity factors and displacement fields.

Investigation of Unmeasured Parameters Estimation for Distributed Control Systems

The problem of unmeasured parameters estimation for distributed control systems is studied in this paper. The Takagi–Sugeno fuzzy model which can appropriate any nonlinear systems is employed, and based on the model, an observer-based fuzzy H∞ filter which has robustness against time-delay, external noise, and system uncertainties is designed. The sufficient condition for the existence of the desired filter is derived in terms of linear matrix inequalities (LMIs) solutions. Moreover, the underdetermined estimation problem in which the number of sensors available is typically less than the number of state variables to be estimated is specifically addressed. A systematic method is proposed to produce a model tuning parameter vector of appropriate dimension for the estimation of the filter, and the optimal transformation matrix is selected via iterative solution to minimize the estimated error. Finally, a simulation example for turbofan aeroengine is given to illustrate the effectiveness of the proposed method, and the estimated error is less than 2.5%.

Turbofan aero-engine efficiency evaluation: An integrated approach using VSBM two-stage network DEA

The application of Data Envelopment Analysis (DEA) has been wide, especially for the purpose of evaluating efficiency among similar production processes within enterprises belonging to particular industries. Although research pertinent to DEA has primarily focused on efficiency of production systems or corporate entities/organizations (e.g., terminals, hospitals, universities/schools, banks), fairly little attention has been given to efficiency evaluation among engineering systems featuring common configurations (e.g., automobiles, power plants). Furthermore, the limited previous literature involving efficiency evaluation of engineering systems has implemented DEA methodologies with limited discriminative power, i.e. there is a quite increased portion of efficient Decision Making Units (DMUs). In the current paper, a methodological framework deploying Variable intermediate measures Slacks-Based Measure (VSBM) Two-Stage Network DEA is implemented, in order to evaluate the efficiency of turbofan aero-engines, currently utilized by active-duty commercial and military aircraft. Apart from exploring the positive correlation of DEA efficiency with engineering efficiency, we also develop a methodology evaluating the features of near-future turbofan designs in terms of DEA efficiency, thus comprising a potential tool for efficiency assessment of any turbofan aero-engine being in the conceptual or preliminary design stage.

Design optimization of a compressor transition S-shaped duct using a teaching–learning-based optimization algorithm

A high bypass turbofan aero-engine delivers compressed air from the low-pressure to the high-pressure compressor through a compressor transition duct. Weight and design space limitations impel to its S-shaped design. Despite that, the compressor transition duct has to guide the flow very carefully to the high-pressure compressor without disturbances and flow separations. Hence, the present paper is devoted to elaborate on the application of the proposed heuristic optimization technique known as multi-objective teaching–learning-based optimization (MO-TLBO) algorithm in the area of S-shaped transition compressor duct. The present algorithm is applied for obtaining the optimum design of the S-shaped duct while minimizing the total pressure loss coefficient and non-uniformity index at the S-shaped duct outlet. These objectives are also optimized independently and as a multi-objective consideration as well. Because of the conflicting nature of the objective functions, a Pareto-optimal curve is also presented for a successful trade-off between the objectives. Pareto-optimal curve gives flexibility to the designer for selecting the best set of the parameters based on the objectives. Successful execution of the proposed MO-TLBO algorithm is examined by comparing the optimum results acquired by the multi-objective genetic algorithm and comparisons divulge that the TLBO algorithm can be successfully practiced to obtain the optimized design of compressor transition S-shaped duct. Optimized duct shows the reduction in total pressure loss and non-uniformity by 28.80% and 36.67%, respectively, despite the reduction of 14.74% in overall length.

Conceptual Multidisciplinary Design Optimization of Commercial Aero-engine System

Commercial aero-engine design is a complicated task, which involves many disciplines, such as thermodynamics, aerodynamics, solid mechanics, heat transfer and materials science. Besides, for the sake of market requirements, designing a commercial aero-engine should not only consider the performance, fuel consumption, thrust and weight, but also pay special attention to noise, emissions and other environmental indices. In order to balance conflicts among various discipline indicators, and design an aero-engine with high comprehensive performance, we apply the multidisciplinary design optimization (MDO) technology to the conceptual design of an aero-engine system in this paper. Particularly, the system conceptual design of a high-bypass-ratio turbofan aero-engine is selected as a case study. A total of five disciplines, including thermal cycle, aerodynamics, structural mechanics, emission and noise, are considered. Meanwhile, six modules are integrated and analysed carefully. These modules include performance evaluation at the design point, performance evaluation at fifteen different off-design points, engine size assessment, engine weight assessment, engine emission assessment as well as engine noise assessment. On the basis of these modules, a MDO platform is set up. In addition, one optimization study with a single target of fuel burn is carried out by utilizing the adaptive simulated annealing (ASA) algorithm. Moreover, another optimization with multiple targets of fuel burn as well as nitrogen oxide emission is also studied by employing the non-dominated sorting genetic (NSGA-II) algorithm. The results shows that the MDO method for the commercial aero-engine can obtain better solutions for the system by fully taking into account the mutual effects among different disciplines.

Unsteady Simulations of a Fan/Outlet-Guide-Vane System: Broadband-Noise Computation

The fan broadband noise of a rotor–stator system representing a turbofan aeroengine is investigated using the improved delayed detached-eddy-simulation approach. The baseline configuration of the NASA source diagnostic test with the entire hard-wall nacelle is simulated at approach and takeoff conditions. In preceding studies, the simulation methodology for the aerodynamics and the treatment of turbulence was established; subsequently, the blade-passing-frequency tones were analyzed. In this study, the forward- and aft-radiating broadband noise associated with the rotor–stator interaction is investigated. The radiated sound power levels are calculated using two methods, and the results are compared with the experimental data: 1) the Ffowcs Williams and Hawkings method with a permeable surface enclosing the nacelle, which is determined through careful studies of the integration-surface design; and 2) the mode-extraction method applied in the fan duct, in which the cost function is tailored for the broadband noise. The power-level spectra from these two numerical methods generally agree, except for the forward-radiating noise in the takeoff condition. Compared with the experiment, both methods predict the spectra typically within 3 dB for both the inlet and the exhaust in the approach condition, but they underpredict the broadband levels in the takeoff condition. The azimuthal-mode decompositions in the inlet and exhaust ducts are also generally consistent with the experiment.

A case against the trickle-down effect in technology ecosystems.

Technology evolution describes a change in a technology performance over time. The modeling of technology evolution is crucial for designers, entrepreneurs, and government officials to set reasonable R&D targets, invest in promising technology, and develop effective incentive policies. Scientists and engineers have developed several mathematical functions such as logistic function and exponential function (Moore’s Law) to model technology evolution. However, these models focus on how a technology evolves in isolation and do not consider how the technology interacts with other technologies. Here, we extend the Lotka-Volterra equations from community ecology to model a technology ecosystem with system, component, and fundamental layers. We model the technology ecosystem of passenger aircraft using the Lotka-Volterra equations. The results show limited trickle-down effect in the technology ecosystem, where we refer to the impact from an upper layer technology to a lower layer technology as a trickle-down effect. The limited trickle-down effect suggests that the advance of the system technology (passenger aircraft) is not able to automatically promote the performance of the component technology (turbofan aero-engine) and the fundamental technology (engine blade superalloy) that constitute the system. Our research warns that it may not be effective to maintain the prosperity of a technology ecosystem through government incentives on system technologies only. Decision makers should consider supporting the innovations of key component or fundamental technologies.

Study of Bypass Ratio Increasing Possibility for Turbofan Engine and Turbofan with Inter Turbine Burner

Abstract Current trends in the high bypass ratio turbofan engines development are discussed in the beginning of the paper. Based on this, the state of the art in the contemporary turbofan engines is presented and their change in the last decade is briefly summarized. The main scope of the work is the bypass ratio growth analysis. It is discussed for classical turbofan engine scheme. The next step is presentation of reach this goal by application of an additional combustor located between high and low pressure turbines. The numerical model for fast analysis of bypass ratio grows for both engine kinds are presented. Based on it, the numerical simulation of bypass engine increasing is studied. The assumption to carry out this study is a common core engine. For classical turbofan engine bypass ratio grow is compensated by fan pressure ratio reduction. For inter turbine burner turbofan, bypass grown is compensated by additional energy input into the additional combustor. Presented results are plotted and discussed. The main conclusion is drawing that energy input in to the turbofan aero engine should grow when bypass ratio is growing otherwise the energy should be saved by other engine elements (here fan pressure ratio is decreasing). Presented solution of additional energy input in inter turbine burner allow to eliminate this problem. In studied aspect, this solution not allows to improve engine performance. Specific thrust of such engine grows with bypass ratio rise – this is positive, but specific fuel consumption rise too. Classical turbofan reaches lower specific thrust for higher bypass ratio but its specific fuel consumption is lower too. Specific fuel consumption decreasing is one of the goal set for future aero-engines improvements.

Basic singular fields in the theory of impulsive supersonic leading-edge noise

This paper determines the impulsive sound fields produced by sharp-edged gusts striking the leading edge of a supersonic blade or aerofoil, for example in a turbofan aeroengine or a counter-rotating propeller system. A full three-dimensional theory is provided, so that the gust edges can be at any orientation relative to the blade. Complete details are given of the sound fields produced by gust edges in the spanwise and streamwise directions, and by many combinations of such edges, including corners. The mathematical theory depends on singular sound fields produced by gusts with a delta-function upwash; these are used to derive exact analytical formulae for impulsive sound fields of different three-dimensional shapes, and also a Green’s function representation of the field which is especially adapted to numerical evaluation. Gusts with top-hat profiles are given particular attention, and also the effect of Gaussian-function smoothing of both delta-function and top-hat profiles. The investigation is complementary to that in a companion paper (Powles and Chapman, 2019), which determines the smooth sound fields produced by single-frequency gusts. Fourier integration provides the relation between the two types of field.

Acoustic liner test on small turbofan aero-engine

Acoustic liner test on small turbofan aero-engine

Optimum acoustic impedance in circular ducts with inviscid sheared flow: Application to turbofan engine intake

Acoustic liners are widely employed in turbofan aero-engines as acoustic treatment. Their performance is determined by its acoustic impedance in the presence of flow. In recent years, much effort has been devoted to find the “optimum impedance” i.e. the impedance that results in the maximum attenuation. Although such analysis can be carried out by means of numerical simulations, analytical expressions can also be derived in order to predict the optimum impedance based on exceptional points i.e. when two modes coalesce, resulting in a maximum modal decay rate. Previous works focus on the optimum impedance of higher order modes in rectangular ducts with uniform flow. In this work, the analysis is expanded to circular ducts for both uniform and sheared inviscid flows. Typical turbofan engine intake geometry and flight conditions are considered, corresponding to certification points: sideline, cut-back and approach. It is shown that, for sideline and cut-back conditions, the optimum impedance and the maximum attenuation are affected even by the presence of a small boundary layer. For approach condition, uniform and shear flows lead to essentially the same results. Limitations of the analytical expression in the presence of multi-modal propagation is also discussed.