Experimental and numerical study of combustion characteristics of ammonia-hydrogen-air swirling flame with/without secondary air injection

Ammonia (NH_(3)) is currently considered to be a potential carbon-free alternative fuel,and its large-scale use as such would certainly decrease greenhouse gas emissions and meet increasingly stringent emission requirements.Although the low flame propagation speed and high NO production of NH_(3) hinder its direct application as a renewable fuel,co-combustion of NH_(3)–H_(2)is an effective way to overcome these challenges.In this study,the combustion characteristics of NH_(3)–H_(2)swirling flames under different equivalence ratios and H_2blending ratios conditions are both numerically and experimentally investigated.Numerically,the One-Dimensional (1D) laminar flame computation presents a comparison base and the Three-Dimensional (3D) numerical simulation yields detailed flame property distributions.Experimentally,the high-speed camera takes instantaneous swirl flame images and the gas analyzer measures the NO emission at the exit plane of the flame chamber.Qualitative and quantitative analysis is performed on the flame structure and NO emission for a series of NH_(3)–H_(2)swirl flames.The variation trends of the NO emission calculated using different techniques agree very well.The quantitative results show that the NO emissions are much higher at lean equivalence ratios than those at rich equivalence ratios,and such difference is closely related to the combustion flame structure.Moreover,it is shown that the utilization of secondary air injection can achieve a significant reduction in NO emissions at the exit of the combustion chamber at equivalence ratios less than or equal to 0.9.

Nonlinear adaptive flight control system:Performance enhancement and validation

The problem of decreasing stability margins in L1 adaptive control systems is discussed and an out-of-loop L1 adaptive control scheme based on Lyapunov’s stability theorem is proposed.This scheme enhances the effectiveness of the adaptation,which ensures that the system has suffi-cient stability margins to achieve the desired performance under parametric uncertainty,additional delays,and actuator faults.The stability of the developed control system is demonstrated through a series of simulations.Compared with an existing control scheme,the constant adjustment of the sta-bility margins by the proposed adaptive scheme allows their range to be extended by a factor of 4–5,bringing the stability margin close to that of variable gain PD control with adaptively scheduled gains.The engineered practicability of adaptive technology is verified.A series of flight tests verify the practicability of the designed adaptive technology.The results of these tests demonstrate the enhanced performance of the proposed control scheme with nonlinear parameter estimations under insufficient stability margins and validate its robustness in the event of actuator failures.

Robust gain-scheduled missile autopilot design based on multifrequency extended state observers

This paper describes the design and implementation of a three-axis acceleration control autopilot for an asymmetric tail-controlled,skid-to-turn tactical missile.In an earlier flight test,degraded autopilot performance was attributed to multiple disturbances and uncertainties and the presence of hidden coupling terms,giving rise to a miss distance of greater than 20 m.To address these issues,the missile dynamics are decomposed into the angular rate dynamics as fast and the acceleration dynamics as slow subsystem using the singular perturbation theory to analyze a multi-time-scale property.Multifrequency extended state observers are then incorporated into the gain scheduling technique to attenuate disturbances,thus enhancing the control performance significantly.In the proposed engineering/practical design framework for missile autopilot,simple,conventional,and explicit tuning rules are provided.And the proposed control scheme can achieve input-to-state stability across the entire flight envelope under unknown but bounded disturbances.The advantages of the method over existing benchmark approaches are shown through nonlinear numerical simulations.This is supported by evidence from a new flight test result with a miss distance of only 2 m.

新一代绿色超声速民机的发展现状与挑战

民航客机早已成为人们生活和交通运输中不可或缺的一部分,而未来民机发展的一个重要动向就是进一步提高飞行速度来缩减飞行时间,使空中旅行更为高效、便捷和舒适.目前,各航空大国纷纷投身于新一代绿色超声速民机的研发浪潮中.2019年6月30日,中国科学技术协会正式发布了2019年度20个对科学发展具有导向作用,对技术和产业创新具有关键作用的前沿科学问题和工程技术难题,“绿色超声速民机设计技术”成为航空领域入选的唯一代表,为加大社会各界对未来国产绿色超声速民机研制的重视力度、顺应世界民机发展趋势、推动我国超声速民机技术的发展提供了重要机遇.

Target-enclosing affine formation control of two-layer networked spacecraft with collision avoidance

This paper addresses a target-enclosing problem for multiple spacecraft systems by proposing a two-layer affine formation control strategy. Compared with the existing methods,the adopted two-layer network structure in this paper is generally directed, which is suitable for practical space missions. Firstly, distributed finite-time sliding-mode estimators and formation controllers in both layers are designed separately to improve the flexibility of the formation control system. By introducing the properties of affine transformation into formation control protocol design,the controllers can be used to track different time-varying target formation patterns. Besides, multilayer time-varying encirclements can be achieved with particular shapes to surround the moving target. In the sequel, by integrating adaptive neural networks and specialized artificial potential functions into backstepping controllers, the problems of uncertain Euler-Lagrange models, collision avoidance as well as formation reconfiguration are solved simultaneously. The stability of the proposed controllers is verified by the Lyapunov direct method. Finally, two simulation examples of triangle formation and more complex hexagon formation are presented to illustrate the feasibility of the theoretical results.

Numerical investigation of dynamic stall suppression of rotor airfoil via improved co-flow jet

The decrease in aerodynamic performance caused by the shock-induced dynamic stall of an advancing blade and the dynamic stall of a retreating blade at low speed and high angles of attack limits the flight speed of a helicopter.However,little research has been carried on the flow control methods employed to suppress both the dynamic stall induced by a shock wave and the dynamic stall occurring at high angles of attack.The dynamic stall suppression of a rotor airfoil by Co-Flow Jet(CFJ)is numerically investigated in this work.The flowfield of the airfoil is simulated by solving Reynolds Averaged Navier-Stokes equations based on the sliding mesh technique.Firstly,to improve the effect of a traditional CFJ on suppressing rotor airfoil shock-induced dynamic stall,an improved CFJ—a CFJ-sloping slot is proposed.Research shows that the CFJsloping slot suppresses the shock-induced dynamic stall more effectively than a traditional CFJ.Moreover,the improved CFJ can also suppress the dynamic stall of rotor airfoil at low speed and high angles of attack.The improved CFJ proposed in this paper is an effective flow control method that simultaneously suppresses the dynamic stall of the advancing and retreating blades.The mechanism of the improved CFJ in suppressing the dynamic stall of the rotor airfoil is studied,and a comparison is made between the improved CFJ and the traditional CFJ in terms of dynamic stall suppression at high and low speed.Finally,the effect of improved CFJ parameters(the jet momentum coefficient,the position of the injection/suction slot,and the size of the injection/suction slot)on shock-induced dynamic stall suppression is analyzed.

Quasi-linear analysis of dispersion relation preservation for nonlinear schemes

In numerical simulations of complex flows with discontinuities,it is necessary to use nonlinear schemes.The spectrum of the scheme used has a significant impact on the resolution and stability of the computation.Based on the approximate dispersion relation method,we combine the corresponding spectral property with the dispersion relation preservation proposed by De and Eswaran(J Comput Phys 218:398-416,2006)and propose a quasi-linear dispersion relation preservation(QL-GRP)analysis method,through which the group velocity of the nonlinear scheme can be determined.In particular,we derive the group velocity property when a high-order Runge-Kutta scheme is used and compare the performance of different time schemes with QL-GRP.The rationality of the QL-GRP method is verified by a numerical simulation and the discrete Fourier transform method.To further evaluate the performance of a nonlinear scheme in finding the group velocity,new hyperbolic equations are designed.The validity of QL-GRP and the group velocity preservation of several schemes are investigated using two examples of the equation for one-dimensional wave propagation and the new hyperbolic equations.The results show that the QL-GRP method integrated with high-order time schemes can determine the group velocity for nonlinear schemes and evaluate their performance reasonably and efficiently.

Numerical study of steady flow characteristics of a rear variable-area bypass injector with alternating area regulator

The mixing effectiveness of the airflow between the inner and outer bypass inlets of a Rear Variable-Area Bypass Injector(RVABI)is the key to the afterburner performance of variable cycle engines.This paper describes an optimized RVABI design based on an alternating area regulator to improve the velocity/temperature uniformity of the incoming flow at the afterburner.Compared with a classical RVABI,numerical simulations show that the proposed alternating RVABI performs better in terms of thermal mixing efficiency and total pressure loss in different variable cycle engine modes.Both the increasing air contact area between the inner and outer bypass of alternating structure RVABI,and a larger streamwise vortex in the inner bypass inlet due to the proposed alternating lobe structure in the RVABI contribute to the significantly increase of mixing effectiveness.Besides,the alternating regulator induces strong streamwise vortex,which helps to improve the airflow mixing with its vortex-induced velocity.The interaction between the streamwise vortex and azimuthal vortex further promises the velocity/temperature uniformity after the RVABI.With the increase of alternating lobe’s height ratio,the covering area of the streamwise vortex and the azimuthal vortex is enlarged,which further enhances the thermal mixing efficiency of the RVABI.This design gives an insight into the future design and optimization of RVABI.