Optimal Control Of Ramjet Engines For Airframe-propulsion Integrated System

With the perspective of airframe-propulsion integrated system(APIS),the optimal design and control of ramjet engines have to attach importance to the strong coupling characteristics between air-breathing engine and airframe,as well as the distributed parameter characteristics of the flow-combustion coupling process of engines.The optimization on these complicated characteristics can broaden the flight envelope of ramjet engines and improve its propulsion performance.Based on optimal control theory and the relevant methods,in this paper,aiming at the optimization problems of the APIS propelled by a ramjet engine,theoretical studies are carried out as follows:To determine the optimal trajectory for the ascent phase of a general supersonic vehicle propelled by a hydrocarbon-fueled ramjet,a strategy is developed based on the coupling characteristics of a flight-propulsion system.With two different cost functions and a series of necessary constraints,minimum-fuel and minimum-time trajectory problems are formulated.Both problems are transformed into typical continuous Bolza problems,which can be solved via the Gauss pseudospectral method.The results show that along the two optimal trajectories,the vehicle accelerates horizontally at the onset of the ascent process until the dynamic pressure can provide sufficient lift.Efficiency analysis is conducted to reveal the coordination between engine performance and atmosphere characteristics.Further,the effect of required system performance,ramjet engine performance and the system constraints on the results of trajectory optimization problems are analyzed,and the trajectory optimization problem based on supersonic combustion is discussed.In scramjet engines,burning in supersonic stream must always cause relatively higher heat addition losses.It has been always a major concern to seek the ideal heat release of supersonic combustion to the smallest entropy increase,which corresponds to the performance limit of supersonic combustion.Many of the efforts to date centered on parameterizing the heat release and then performing p arameter optimizations.Inevitably,the parameterization will introduce high-dimensional problem due to the infinite degree of freedom of heat addition.In the present study,the problem of optimizing heat release is viewed from a perspective of functional and calculus of variations.Different from the previous studies devoted to finding the optimal parameters in the finite-dimensional parameter space,the present study attempts to seek the ideal heat release of supersonic combustion in the infinite-dimensional function space.Mathematically,the optimization problem is a constrained maximization of functional with differential equation constraints and fixed end points.Correspondingly,the method of optimal control and Lagrange multipliers is applied to solving the optimal solution of the functional subject to the physical constraints.Solutions of the ideal heat release of supersonic combustion in both constant-area combustors and diverging combustors are provided.To seek the optimum heat release and inner configuration of a supersonic combustor,a strategy was proposed to address the coupling optimization problem in an infinite dimensional function space.The total pressure recovery coefficient of the combustor was chosen as the objective function to minim ize the entropy increment,which also corresponds to the performance limit of a supersonic combustor.With properly defined boundary conditions and necessary constraints,the problem was formulated and solved using optimal control theory.The results of th e optimum heat release and inner configuration of a supersonic combustor were provided,and the influences of important factors were analyzed.