Research On Thermodynamic Cycle And Control Law Of Precooled Airbreathing Propulsion System

Precooled cycle engines that uses onboard fuel as heat sink to cool the intake air provides another propulsion solution for hypersonic flight.Before practical application,there are still some critical issues to be solved for the engine family.First of all,the numerous cycle schemes of precooled engines provide abundant options for high-speed propulsion;nevertheless,up to date there is a lack of systematic research on the entire engine family,and the fragmented analysis of individual cycle will inevitably bring about some inconsistent or even contradictory understandings to the whole precooled engine family.Secondly,as the heat sink for air precooling,the cooling capacity that the fuel can provide is very limited,and consequently,for efficient utilization of fuel heat sink,not only the related heat exchanging process should be optimized,it is more crucial to optimize the thermodynamic cycle such that the utilizing efficiency of fuel heat sink can be fundamentally ensured.Thirdly,as new type of cycles with extraordinary working principles,the releasing of the full performance potentials of precooled engines depends not only on engine design,but also it depends on engine control,especially for indirect precooled engines.Considering of the aforementioned problems,the following studies are carried out in this work:Firstly,the cycles of precooled engines can be constructed flexibly,and the choice of cycle schemes is abundant,therefore it is crucial to develop a unified analysis model for the different cycles such that the common influential factors and principles relating to performance of the engine family can be analyzed integrally.On account of this,overall cycle analysis of heat exchanger based precooled engines is carried out.The cycle schemes of the direct and indirect precooled engines are abstracted,and based on a general framework of precooling-compression system(PCS)that describes the core working principle of precooled engines,an overall thermodynamic model for precooled family is developed,with which the performance improving of the engine family,the influence of fuel properties and figure of merit of the precooling-compression system etc.are studied to establish model and theoretical basis for subsequent in-depth research.Secondly,based on the overall thermodynamic cycle model developed for precooled engines,Performance study of heat exchanger based precooled engines is conducted.With the defined performance index for PCS system,numerical performance analyzing model is constructed through component-based modeling framework for the over cycle model and PCS system.Improving of PCS system performance is discussed from the aspects of system operation parameters,fuel properties and PCS schemes.Particularly,the impact of chemical endothermic effect of pyrolyzable fuels on PCS performance improving is evaluated.The design tradeoff between intake ram compression and PCS compression is analyzed on account of preference boundaries of the precooled family.The engine and vehicle mission level performances for representative candidate fuels are studied,with the performance characteristics of the precooled family is further revealed.Thirdly,for efficient utilization of the limited fuel heat sink and reducing fuel consumption in precooling,research on optimum configuration of the multi-branch indirect precooled PCS system is performed.Considering the performance advantages of indirect PCS system,the scheme with single branch design is analyzed first.Through theoretical optimization analysis,optimum design method for the tandem regeneration-compression system of the multi-branch PCS is developed.The optimum air-cooling scheme for a multi-segment cascade precooling system is studied to maximize the utilization of cooling capacity of the working fluid.Correctness of the design method is verified through numerical optimization or comparing with literature design method.Performance of the single branch design and the multi-branch scheme is analyzed and compared on both the PCS system and engine levels,which shows that the multi-branch scheme is more advantageous as the total heat capacity flow rates of the PCS system cannot be matched.The potential of using fuel recooling scheme to further improve performance of PCS system is evaluated.Finally,to ensure the safety and high-performance operation of precooled engines under non-design conditions,off-design performance character and control law for a single branch indirect precooled engine are studied.Off-design performance analyzing model for the engine is developed,and based on cycle-performance/heatexchanger-design conjugated optimization method,design point parameters of the engine are matched.Control variables of the engine are determined,with the regulation mechanism of which to state of the core engine is revealed.The control characteristics of the engine is studied to make clear the variation of engine performance with the key state parameters,though which the controlled variables are determined,and the control law for maximum thrust sate of engine is established.Based on the intake performance and flight trajectory of similar engine design,the correctness of the developed maximum thrust control law is finally validated under real operation conditions.