| The unstart phenomenon caused by flow chocking in the combustion is one of the major threats to the safe flight of a hypersonic aircraft powered by a scramjet.At present,domestic and foreign scientific researchers mainly focus on unstart caused on high back pressure in the combustion and how to predict and control it,but pays little attention to the establishment and development of the pressure in the combustion before the flow chocking is triggered.Chocking caused by combustion heat release has been considered as the main cause of Chocking in supersonic combustion.The Rayleigh flow model has become one of the main models for predicting the limiting equivalence ratio of a ramjet.However,in recent years,a series of flight and ground experiments such as Hy Shot II have gradually found that the theoretical limit equivalent ratio based on Rayleigh flow is much higher than the experimental observation value,and the combustion heat release is formed when it is far lower than the theoretical expectation.The combustion back pressure is sufficiently high and further causes the engine to reach the unstart boundary.In fact,in actual scramjet combustion,many physical factors such as fuel injection,wall friction,combustion heat release,flow channel area throttling,and irreversible entropy increase such as shock waves will cause the pressure in the combustion chamber to increase.Therefore,the back pressure formed by the combustion process is not only derived from the single factor of combustion heat release,but involves a complex hydrodynamic process.The establishment and development mechanism of the pressure in the combustion needs further study.Considering that the combustion heat release factors have been extensively researched,this article is mainly based on the cold flow perspective,combined with wind tunnel test observation and theoretical analysis,to study and obtain fuel injection quality and wall friction under different Mach numbers.The influence of physical factors such as flow channel area throttling and their coupling effects on the establishment and development of pressure in the combustion chamber.The main research contents and conclusions include:First of all,this paper studies the process and law of the establishment and development of the pressure in the combustion chamber induced by a single physical factor,such as fuel injection and mass,wall friction.It was found that when a single variable such as the injection pressure and friction coefficient of the fuel working fluid is sufficiently large,a sufficiently high back pressure can be established downstream of the combustion chamber to trigger the unstart shock wave;the injection of the fuel working fluid will be in the injection area.The local formation of aerodynamic compression shock waves significantly increases the pressure of the nearby airflow and the incident wall surface area,but has a small impact on the overall pressurization process along the combustion;wall friction mainly affects the airflow through the growth of the wall surface layer in the combustion and consume energy of gas.The effective circulation area will have an important impact on the pressurization process of the air flow in the combustion.Based on the study of the above basic laws,this article gives preliminary theoretical criteria for the establishment of pressure by a variety of physical factors in the scramjet combustion.Secondly,this paper studies the process and law of the establishment and development of pressure in the combustion under the coupling of multiple physical factors.The results show that the inlet Mach number of the combustion chamber has a significant effect on the pressure build-up and development process in the combustion.Under the condition of high Mach number inflow,it shows a strong coupling effect.When the inlet Mach number is low,the main source is the heat release from the combustion reaction.The wall friction effect has a certain degree of influence on the built-up pressure,while the injection effect of the fuel working fluid is basically negligible,wall friction and fuel injection pressure brought by the gap between vary within a certain distance in 3 to5 times.With the increase of the Mach number at the inlet of the combustion,the compressibility of the incoming air Increasing gradually,while the wall friction will increase the surface layer and significantly affect the effective flow area of the airflow,it will also induce a complex compression shock wave(train)structure,which will significantly increase the pressure rise of the combustion induced by wall friction,the pressure increase caused by wall friction reachs to about 10 times compared with pressure increased by injection,gradually becoming One of the main sources of pressurization in the combustion.Finally,based on the study of the above-mentioned basic phenomena and laws,this paper analyzes and refines the main pathways for the establishment and development of pressure in the combustion induced by different physical factors,and conducts experimental verification through the design of the wall expansion of the combustion.The results show that fuel injection boosting and its induced local aerodynamic compression,wall friction effects,and other physical factors and their coupling effects induced by pressurization in the combustion chamber can be quantified and adjusted through the design of the wall expansion of the combustion.A3°expand combustor can make jam to pressure drawdown.Based on the above research,this paper has initially obtained the pressure build-up process and its influence law induced by various physical factors and their coupling effects in the combustion chamber of scramjet combustion.The innovation lies in: through theoretical analysis and experimental verification,the multiple physical coupling phenomenon of supersonic combustion pressure build-up is revealed,and the main pressurization mechanism under different incoming Mach numbers is established.The research work in this paper is of great significance to further study and reveal the pressure field formation and evolution mechanism dominated by the complex combustion flow coupling in the supersonic combustion,and to develop and propose a critical state prediction model suitable for near-real conditions. |
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