Variable Cycle Engine Research Articles

Aerodynamic Characteristics of Morphing Supersonic Cascade Under Low-Upstream-Mach-Number Condition

For fans of a variable-cycle engine, a good aerodynamic performance over a wide range of rotating speeds is essential. However, when supersonic cascades designed for a high upstream Mach number operate under a low upstream Mach number, a starting problem may occur, which significantly decreases the aerodynamic performance of fan blades. As a result, the operating range of the supersonic cascades is severely limited. To solve the starting problem caused by the mismatch between geometry of supersonic cascades and upstream Mach number, a morphing supersonic cascade is developed. Thus, this study mainly focuses on the effect of the static deformation of supersonic cascades driven by smart materials on aerodynamic characteristics under a low upstream Mach number. To investigate this issue, numerous simulations are conducted on an S-type cascade by finite element method and computational fluid dynamics. As demonstrated by flow structure analysis, the morphing cascades are started under certain morphing configurations while the original cascade operates at nonstarted state. The results show that the deformation driven by smart materials alters the shock wave structures under a low upstream Mach number by adjusting key cascade geometric parameters. Specifically, the morphing cascades achieve 80% reduction of detached shock loss compared with the original cascades.

Optimization of ACE mode transition control schedule considering geometric adjustment speed

Abstract Aiming to optimize the mode transition control schedule of the adaptive cycle engine (ACE), an optimization method based on a gradient algorithm was proposed. During the mode transition, the compressor surge margin, the total turbine inlet temperature and the combustion chamber fuel-air ratio are not exceeded. The integration of thrust by time was selected as the optimization objective. The geometric parameters were updated via the optimization target derivative. The speed limit of the geometric adjustment mechanism was also considered in the optimization process. The control schedules of two mode transition processes, from triple to double bypass mode and from double to triple-bypass mode were optimized. The optimization method presented in this paper can be applied to various mode transitions and the geometric adjustment speed meets all the constraints.

Optimization of ACE mode transition control schedule considering geometric adjustment speed

Abstract Aiming to optimize the mode transition control schedule of the adaptive cycle engine (ACE), an optimization method based on a gradient algorithm was proposed. During the mode transition, the compressor surge margin, the total turbine inlet temperature and the combustion chamber fuel-air ratio are not exceeded. The integration of thrust by time was selected as the optimization objective. The geometric parameters were updated via the optimization target derivative. The speed limit of the geometric adjustment mechanism was also considered in the optimization process. The control schedules of two mode transition processes, from triple to double bypass mode and from double to triple-bypass mode were optimized. The optimization method presented in this paper can be applied to various mode transitions and the geometric adjustment speed meets all the constraints.

Development and Validation of a Novel Control-Volume Model for the Injection Flow in a Variable Cycle Engine

The variable area bypass injector (VABI) plays a crucial role in variable cycle engines by regulating the flow mixing process in complex bypass ducts, and low-dimensional theoretical models are the key to revealing its working mechanism while estimating its aerodynamic performance. An improved VABI model using the control volume method is established, through which the feature parameters that determine the VABI aerodynamic performance are summarized. To acquire an accurate prediction of the injection ratio, a calibration item is introduced to the governing equations to consider the static pressure discrepancy on the mixing plane, and a numerical database is developed to obtain the calibration item. Results show that the aerodynamic parameters that determine the VABI performance include the bypass total pressure ratio, bypass backpressure, and the injection ratio, while the injection angle and the VABI opening area also influence the injection flow characteristics. The injection ratio is increased by reducing the bypass total pressure ratio, decreasing the bypass backpressure, and closing the VABI. Numerical validation shows that the calculation error of the improved model is generally below 3%. The improved VABI model is then validated by a well-arranged experiment, for which the annular flow is simplified into a rectangular duct flow with an error of less than 5%. The experimental validation also proves the accuracy of the model.

Acceleration technique for global optimization of a variable cycle engine

A variable cycle engine (VCE) is a potential power system for future advanced military and civil aircraft. The involvement of numerous adjustable parameters enables a VCE to meet multiple flight requirements simultaneously; however, it also increases the difficulty of engine design. Global algorithms can be adopted to maximally exploit the performance potential of a VCE while satisfying constraints. However, the optimization of a VCE is a highly constrained and nonlinear problem with high computational cost. In this article, a novel acceleration technique for the global optimization of VCEs is presented. The initial guesses for the iterations of the VCE model were optimized along with the optimization variables to reduce the model call number (MCN). The differential evolution (DE) algorithm was chosen to demonstrate and verify the effectiveness of this acceleration technique. The results show that the technique can significantly reduce the MCN under most conditions without affecting the optimization process. The more rapidly the DE converges, the more significantly the MCN is reduced. In some cases, this technique can help improve the optimization results. Moreover, the technique can be applied to various DE improvement strategies. This technique can also be employed in the performance optimization of other aero engines or similar systems.

Research on the Analytical Redundancy Method for the Control System of Variable Cycle Engine

The safety and reliability of the measuring elements of an aero-engine are important preconditions of the stable operation of the engine control system. The number of control parameters of a variable cycle engine increases by 20%–40% compared to traditional engines. Therefore, it is important to conduct study on the analytical redundancy, design fault diagnosis and isolation of the sensors, as well as the signal reconstruction system, so as to increase the ratability and fault-tolerant capability of the variable cycle engine control system. The analytical redundancy method relies on the accuracy of the mathematical model of the engine. During the service cycle of the engine, it is inevitable that the engine performance will deteriorate, resulting in a mismatch with the model. In this paper, the adaptive model of the variable cycle engine is built with a Kalman filter. Based on this, the strategy of analytical redundancy logic is built and the dynamic adaptive calculation of the threshold is introduced. Simulation results reflect that this method can effectively increase the reliability of sensor fault diagnosis and the accuracy of the analytical redundancy when there is performance degradation of the variable cycle engine.

An efficient multi-fidelity simulation method for adaptive cycle engine ejector nozzle performance evaluation

The three-bypass duct structure of Adaptive Cycle Engine (ACE) provides unique performance advantages while also challenging the design of its exhaust system. The separated exhaust nozzles widely adopted in current research may not meet the performance requirement of ACE. The feature of combining two exhaust streams to discharge makes the ejector nozzle suitable as the exhaust system of ACE. However, due to the connection between the third bypass duct and the ejector nozzle, the ejector nozzle and ACE form a highly coupled system, thus it is necessary to perform an integrated simulation of the ejector nozzle and ACE. The multi-fidelity simulation could offer accurate performance simulation as well as consider the complicated component matching working relations. In this paper, a multi-fidelity simulation method for ACE ejector nozzle simulation is developed for fast and accurate performance evaluation, and the design requirements of ACE ejector nozzle are initially explored. A multi-fidelity joint simulation method for the (0-D) ACE performance model and the (2-D) ejector nozzle Computational Fluid Dynamics (CFD) model is constructed through a dynamically optimized and updated surrogate model. The result shows, when ensuring the ejector nozzle does not affect the ACE working point, the peak gross thrust coefficient occurs in subsonic cruise condition as 0.9895. The gross thrust coefficients of acceleration conditions decrease with the increase of flight Mach number due to the increase of shroud divergent angle. Only 350 times CFD simulations are required to complete multi-fidelity simulations of 29 ACE operating points (As a comparison, the de-coupled method requires about 105 times CFD simulations for the whole characteristics, and the fully integrated method requires about 100 times CFD simulations for one ACE operating point). The diameter of the ejector shroud shoulder must be variable, otherwise it will cause a maximum thrust loss of 22.6% at dry acceleration condition and fan surge at cruise and reheat acceleration conditions. This method can further realize the evaluation of the installed performance and the control law optimal design of the ACE ejector nozzle.

A method for numerical simulation of mode transition process of compression system of variable cycle engine based on throttle valve model

In order to study the aerodynamic characteristics of the compression system of a variable-cycle engine during its mode transition, a low-order model was established based on the throttle valve theory to describe the throttle characteristics of the mode selection valve at its different opening degrees. The model was applied to the outlet of a double-bypass variable-cycle fan in the form of characteristic boundary. A 3D low-order hybrid computational model was established for the numerical simulation of flow fields in the mode transition process of the compression system. The calculation accuracy and effectiveness of the throttle valve model were verified by comparing its calculation results with those of the 3D valve model. The simulation method was further applied to the prediction and analysis of the fan performance variation during the single and double bypass mode transitions of the variable-cycle fan. The simulation results show that: the throttle coefficient, which represents the throttle intensity, has something to do only with the flow area ratio of the valve but has nothing to do with its angle. Therefore, the throttle valve model is universal for valves with different motion angles; the throttle valve model can accurately predict the variation trend of the aerodynamic characteristics of the fan in all stages of the mode transition process. Compared with the 3D valve model, the prediction error of the pressure ratio of the fan in all stages is less than 1.93%, and the efficiency error is less than 1.05%. In the valve closing process, the second-stage rotor performance changes more dramatically, and the first-stage rotor performance changes with lag.

Flow control of double bypass variable cycle engine in modal transition

To study the change mechanism and the control of the variable cycle engine in the process of modal transition, a variable cycle engine model based on component level characteristics is established. The two-dimensional CFD technology is used to simulate the influence of mode selection valve rotation on the engine flow field, which improves the accuracy of the model. Furthermore, the constant flow control plan is proposed in the modal transition process to reduce the engine installed drag. The constant flow control plan adopts the augmentation linear quadratic regulator control method. Simulation results indicate that the control method is able to effectively control the bypass ratio and demand flow of the variable cycle engine, and make the engine transform smoothly, which ensures the stable operation of the engine in modal transition and the constant demand flow of the engine.

Fault-Tolerant Control Scheme for the Sensor Fault in the Acceleration Process of Variable Cycle Engine

This paper presents a fault-tolerant control scheme for the sensor fault in the acceleration process of the variable cycle engine. Firstly, an adaptive equilibrium manifold model with multiple inputs and multiple outputs is established. Combined with the Kalman filter bank, sensor fault diagnosis is carried out to realize the diagnosis and signal reconstruction of the engine in the case of a single sensor and double sensor faults. On this basis, isolation and group isolation are used to diagnose sensor faults and reconstruct signal in speed closed-loop control. Then, the control plan of the acceleration process is optimized based on the target shooting method, aiming to simulate the variation of various variables in the engine acceleration process more accurately, so as to verify the feasibility of the sensor fault-tolerant control scheme. Finally, a hardware-in-loop simulation platform is built based on the idea of distributed control, and the fault-tolerant control scheme of the sensor proposed previously is verified based on this platform. The results show that the proposed scheme can accurately diagnose the sensor faults and reconstruct the signal within 0.2 s, and the actual speed can rise from 67.87% to 99.9% in 4 s, ensuring the safe and rapid completion of the acceleration process.

Analytical redundancy of variable cycle engine based on variable-weights neural network

In this paper, variable-weights neural network is proposed to construct variable cycle engine’s analytical redundancy, when all control variables and environmental variables are changing simultaneously, also accompanied with the whole engine’s degradation. In another word, variable-weights neural network is proposed to solve a multi-variable, strongly nonlinear, dynamic and time-varying problem. By making weights a function of input, variable-weights neural network’s nonlinear expressive capability is increased dramatically at the same time of decreasing the number of parameters. Results demonstrate that although variable-weights neural network and other algorithms excel in different analytical redundancy tasks, due to the fact that variable-weights neural network’s calculation time is less than one fifth of other algorithms, the calculation efficiency of variable-weights neural network is five times more than other algorithms. Variable-weights neural network not only provides critical variable-weights thought that could be applied in almost all machine learning methods, but also blazes a new way to apply deep learning methods to aeroengines.

The exploration of bypass matching limitation and mechanisms in a double bypass engine compression system

In order to clarify the influence of bypass configuration on the variation range of the bypass ratio in a variable cycle engine, as well as revealing the effect of bypass ratio adjustment on the engine aerodynamic performance, an integrated zero-dimensional bypass model is established based on a double bypass engine compression system. The bypass model employs the control volume method and is validated by the bypass flow database which is established using a novel high-accuracy simplified bypass system computational method. By relating the model parameters to the engine component operation points, evaluation of the bypass aerodynamic performance in the real engine environment is realized, and the feature parameters decisive for the bypass matching characteristic are proposed. The operating range of the bypass system is then disclosed using the bypass model, which is instructive for the aerodynamic design of the whole compression system. Results show that to increase the total bypass ratio, one can decrease bypass backpressure, reduce the core driven fan stage (CDFS) total pressure ratio, increase the mode selector valve (MSV) opening angle or open up the forward variable area bypass injector (FVBAI). Different methods tend to influence the bypass matching characteristics differently. The non-monotonicity characteristic of the dividing flow loss around the MSV will induce a multi-solution zone for the double bypass configuration and make it difficult to diagnose the system operating state. Moreover, the inner bypass will reach the critical condition before the outer bypass, yet the supercritical condition is not recommended as it requires unique bypass geometry and will bring about high flow loss.

A multi-fidelity simulation method research on front variable area bypass injector of an adaptive cycle engine

Front Variable Area Bypass Injector (Front-VABI) is a component of the Adaptive Cycle Engine (ACE) with important variable-cycle features. The performance of Front-VABI has a direct impact on the performance and stability of ACE, but the current ACE performance model uses approximate models for Front-VABI performance calculation. In this work, a multi-fidelity simulation based on a de-coupled method is developed which delivers a more accurate calculation of the Front-VABI performance based on Computational Fluid Dynamics (CFD) simulation. This simulation method proposes a form of Front-VABI characteristic and its matching calculation method between it and the ACE performance model, constructs a coupling method between the (2-D) Front-VABI model and the (0-D) ACE performance model. The result shows, when ACE works in triple bypass mode, the approximate model cannot account for the effect of Front-VABI pressure loss on Core Driven Fan Stage (CDFS) design pressure ratio, and the calculated error of high-pressure turbine inlet total temperature is more than 40 K in mode transition condition (the transition operating condition between triple bypass mode and double bypass mode). In double bypass mode, the approximate model can better simulate the performance of Front-VABI by considering the local loss of area expansion. This method can be applied to the performance-optimized design of Front-VABI and the ACE control law design during mode transition.

Comprehensive exergo- sustainability analysis for a next generation aero engine

In the present study, the exergetic performance behaviours of a variable cycle engine (VCE) which is considered as an alternative engine for the fighter aircraft were analysed for eight flight phases excluding critical fighter phases. In this context, performance analysis of the variable cycle engine model was parametrically performed by on-design relations of an afterburner turbofan. These results were compared with those of conventional military turbofan (CMT) engine. Next, exergetic parameters pertaining to the VCE were measured on component-basis for eight flight phases. Moreover, five exergetic sustainability indexes of the VCE model at all flight phases were investigated in order to observe effects of flight phases on these metrics and these outcomes were compared with the conventional one. According to comparative performance analysis, SFC value of the VCE for cruise phase was estimated as 23.89 g/kN.s whereas that of the CMT engine was computed as 27.25 g/kN.s. On the other hand, the component-based results illustrate that exergy efficiency of the combustor of the VCE has relatively low values ranging from 83.65% to 89.06%, whereas those of high pressure and low pressure turbines have relatively high ratio at concerned flight points. Furthermore, the whole engine-based results show that exergy efficiency of VCE is approximately varies between 7.45% and 31.39% whereas that of the CMT engine changes between 6.31% and 21.5% throughout flight phases. Finally, exergetic sustainability index was calculated the highest at subsonic cruise phases. Namely, it is estimated between 0.45 and 0.6 for the VCE and between 0.31 and 0.34 at these phases for the CMT engine. It is thought that studying exergetic parameters for a military aircraft during the important flight phases could help in understanding which flight segment is useful in terms of sustainability.

Design method of optimal control schedule for the adaptive cycle engine steady-state performance

The alternative working modes and flexible working states are the outstanding features of an adaptive cycle engine, with a proper control schedule design being the only way to exploit the performance of such an engine. However, unreasonable design in the control schedule causes not only performance deterioration but also serious aerodynamic stability problems. Thus, in this work, a hybrid optimization method that automatically chooses the working modes and identifies the optimal and smooth control schedules is proposed, by combining the differential evolution algorithm and the Latin hypercube sampling method. The control schedule architecture does not only optimize the engine steady-state performance under different working modes but also solves the control-schedule discontinuity problem, especially during mode transition. The optimal control schedules are continuous and almost monotonic, and hence are strongly suitable for a control system, and are designed for two different working conditions, i.e., supersonic and subsonic throttling, which proves that the proposed hybrid method applies to various working conditions. The evaluation demonstrates that the proposed control method optimizes the engine performance, the surge margin of the compression components, and the range of the thrust during throttling.

General Design Method of Control Law for Adaptive Cycle Engine Mode Transition

Mode transition (MT) is critical for adaptive cycle engine (ACE) to achieve mission adaptability. There are many constraints for the MT, making the control law difficult to design. At present, most design methods require manual analysis by researchers. When the design parameters of an engine are changed, it takes a lot of time to redesign the control law. What is more, most control laws need to adjust not only the mode selection valve (MSV), but also many other variable geometry components (VGCs) simultaneously, which makes the control system design complex and difficult. In this study, a general design method based on the particle swarm optimization method and the sensitivity calculation method is presented. This method can derive the control law for the MT without manual analysis, improving the efficiency of the design process. At most, one VGC needs to be adjusted during the change of state of the MSV, simplifying the control system. The fluctuation of thrust and airflow is less than 2%, and the surge margin of the compressors is higher than 10%, leading to a safe and stable MT. This method can adapt to the adjustment of the cycle parameters of the engine and the deviation of component performance, making it suitable for the engine performance design stage. It can also be applied to the design for other types of ACEs.

Research on a component characteristic adaptive correction method for variable cycle engines

Abstract There is inevitably a performance deviation between an engine model and an actual engine that is influenced by unpredictable factors such as the unsuspected environmental conditions and the natural performance degradation in the process of use. Because the engine model precision largely depends on the accuracies of the component maps, it is possible to revise the engine model to determine a better trend for the engine performance from recorded measurements by adjusting the maps. This paper presents a new method for updating the variable geometry component maps of a variable cycle engine (VCE) by using a set of scaling factors estimated with the cubature Kalman filter (CKF). A mapping function is created between the scaling factors and the component characteristic scaling coefficients for the adjustments of the maps. The proposed method is applied to a VCE model according to the VCE benchmark steady-state performance data. The results show that the maximum simulation error of the engine steady-state model decreases from 5.33 to 0.93%, and the CKF-based adaptation method provides a much faster computing rate than the particle swarm optimization (PSO) based adaptation method, which verifies the effectiveness and engineering applicability of the variable geometry characteristic adaptive correction method.

Research on a component characteristic adaptive correction method for variable cycle engines

Abstract There is inevitably a performance deviation between an engine model and an actual engine that is influenced by unpredictable factors such as the unsuspected environmental conditions and the natural performance degradation in the process of use. Because the engine model precision largely depends on the accuracies of the component maps, it is possible to revise the engine model to determine a better trend for the engine performance from recorded measurements by adjusting the maps. This paper presents a new method for updating the variable geometry component maps of a variable cycle engine (VCE) by using a set of scaling factors estimated with the cubature Kalman filter (CKF). A mapping function is created between the scaling factors and the component characteristic scaling coefficients for the adjustments of the maps. The proposed method is applied to a VCE model according to the VCE benchmark steady-state performance data. The results show that the maximum simulation error of the engine steady-state model decreases from 5.33 to 0.93%, and the CKF-based adaptation method provides a much faster computing rate than the particle swarm optimization (PSO) based adaptation method, which verifies the effectiveness and engineering applicability of the variable geometry characteristic adaptive correction method.

Analytical Redundancy of Variable Cycle Engine Based on Proper Net considering Multiple Input Variables and the Whole Engine’s Degradation

In this paper, Proper net is proposed to construct variable cycle engine’s analytical redundancy, when all control variables and environmental variables change simultaneously, also accompanied with the whole engine’s degradation. In another word, Proper net is proposed to solve a multivariable, strongly nonlinear, dynamic, and time-varying problem. In order to make the topological structure of Proper net physically explainable, Proper net’s topological structure is designed according to physical relationship between variables, by which means analytical redundancy based on Proper net achieves higher accuracy with less calculation time. Experiments were compared with performance of analytical redundancy based on Proper net, seven convolutional neural network topological structures, and five shallow learning methods. Results demonstrate that under condition of average relative error less than 1.5%, Proper net is the most accurate and the least time-consuming one, which proves not only the effectiveness of Proper net but also the feasibility of topological structures’ design method based on physical relationship.

Robust design of an adaptive cycle engine performance under component performance uncertainty

The adaptive cycle engine can vary the cycle in wider range and meet the requirement of mixed missions. It is expected to be the propulsion system for the next generation of aircraft and has therefore attracted a lot of attention. The uncertainty in component performance can offset the performance benefits of adaptive cycle engine. Therefore, the robust design should be applied at the engine performance design stage to reduce the effect of component performance uncertainty. Compared with conventional engines, the adaptive cycle engine has more variable geometry components. It causes two major difficulties for the robust design: one is the high computation cost and the other is the complex control law design. In this paper, a robust design method based on a two-level surrogate model and a two-stage design process is presented to overcome these difficulties. In this method, the probabilistic method is applied to quantify uncertainty and the selection principle of optimization target is studied. The cycle parameters and control law are optimized through a two-stage design process. The optimization is developed based on a two-level surrogate model to reduce the computational cost. This robust design method is validated by the Monte Carlo method. According to this study, performance under confidence probability is a better optimization target than the signal-to-noise ratio of performance. Using this method, the probability of engine performance meeting demand can be improved from about 50% to over 96%. The computational cost of uncertainty quantification is only 0.5% of the Monte Carlo method and the computational cost of whole optimization design process is less than the cost of once uncertainty quantification through Monte Carlo method. This method can improve the robustness of adaptive cycle engine performance and reduce the computational cost significantly. It can also be applied to the robust design of other complex gas turbine engines performance.