|Alternative Title||Effect of Acceleration and Plasma on Supersonic Combustion Structure of Scramjet|
|Thesis Advisor||张新宇 ; 顾洪斌|
|Place of Conferral||北京|
The scramjet is promising power component for hypersonic vehicles, The stable combustion of the combustor is the core of the performance of the scramjet. However combustion instability and hysteresis occur during disturbances such as acceleration, deceleration of the aircraft, and changes in the equivalence ratio. Regardless of the type of flame stabilization, the cavity or the strut, will undergo flame structure transion during acceleration and deceleration. Taking the cavity flame as an example, the jet flame and shear layer flame are two typical flame structure. During the transition process, the two flame structures are constantly switching, resulting in unstable combustion and ultimately affecting engine performance. Whether unstable combustion or hysteresis, it has a great impact on the overall performance of the engine. These instability can be eliminated by various means. Traditional flame-stabilizing methods such as cavities or strut cannot achieve the elimination of instability during dynamic processes. Active interfere to combustion is required. Therefore, by studying the characteristics of flame instability in the process of scramjet acceleration and adding plasma active interference to the flame structure, a new method of scramjet flame stabilization is presented, and the plasma assisted combustion mechanism is given by the Bunsen burner experiment. This study is divided into three parts, the study of engine acceleration performance, the influence of microwave and arc plasma on supersonic flame structure, and the influence mechanism of microwave and arc on Bunsen flame. The first part is based on the direct connection test facility, the continuous acceleration process of high-altitude flight was simulated, and dynamic changes of the flame structure of the combustor of the super-combustion ramjet and the engine performance was studied. The second part introduces microwave and gliding arc plasma-assisted cavity supersonic combustion to study the effects of microwave and plasma on supersonic flame structure. In the third part, the effects of plasma on the combustion were studied by Bunsen burner combustion experiments.
The dynamic continuous acceleration of the scramjet engine was realized in the the scramjet on the ground, and the fixed equivalent ratio and continuous decreasing equivalent ratio experiment were carried out. The high-altitude acceleration process of the hypersonic vehicle was simulated, and the Mach number of the combusor accelerated from 2.4 to 2.9. The RP3 room temperature liquid kerosene was used in the experiment, which was successfully achieve ignition and stable combustion. A discontinuous point of the combustion chamber pressure occurred and the instability of the combustion was found during the acceleration process. In the acceleration process, whether it is a fixed or variable equivalent ratio, pressure instability occurs. Unstable pressure does not necessarily result in instability of the overall performance of the engine. Because if the pressure fluctuates upstream of the engine, it may be absorbed by the downstream flow field. And if the pressure instability occurs downstream of the engine, it will lead to instability of the overall engine performance such as abrupt of thrust. The direct cause of pressure instability is the instability of combustion heat release. CH* characterizes the combustion zone of hydrocarbon fuel combustion. In this study, the flame in the acceleration process is studied by shooting the flame CH* luminous image. During the acceleration process, the equivalence ratio remains fixed, and the actual fuel flow rate is adjusted according to the incoming flow, and the fuel jet pressure. At low speeds, the flame can be stabilized in the jet zone. At high speeds, the flame cannot be stabilized in the jet zone and transited into a pure cavity shear layer flame. The process of flame transition from the jet flame to the shear layer is an unstable process. During this period, two flames in the flame zone will oscillate back and forth. After this oscillation process, a stable pure shear layer flame will be formed. In the high-speed flow field, when only the single-cavity burned, due to the combustion back pressure is insufficient and the ignition delay, and a stable flame cannot be formed.
In order to study the transformation of the flame structure and the flame zone, and propose new flame stabilization method, new single-cavity, single-sided expansion engine model was designed and microwave and gliding arc plasma were added to the combustion chamber. The effects of microwave and plasma on engine performance and supersonic flame were investigated by measurement of pressure and flame CH*. After the plasma is added, the combustion chamber flame structure was transition. This is manifested by changes in the pressure structure and changes in the average CH* luminescence region. The addition of microwaves makes combustion easier for the flame to stabilize in the jet flame zone. The transition process of the combustion flame structure is a sudden change process, and the sudden change of the combustion affects the sudden change of the pressure at the cavity position. In the jet flame mode, the fuel in the cavity position tends to burn inside the cavity, which increases the back pressure of the combustor, thereby forming a jet flame. After the microwave is added, the electromagnetic field is concentrated in the cavity, and the flame is more likely to stabilize in the electromagnetic field position, and is transitied downward by the shear layer combustion. Under the condition that the jet pressure is constant, the flame stable position is changed, and a new stable mode is formed.
The self-similar characteristics of the supersonic combustion flame boundary are obtained, by analyzing the fractal geometric characteristics of the flame boundary. With the addition of microwave power, the fractal dimension of the flame boundary is positively correlated with the microwave power. Since the fractal geometric dimension of the flame boundary is positively correlated with the turbulent flame velocity, it can be said that the addition of microwaves increases the speed of the turbulent flame of the supersonic combustion. At the same time, the spectral characteristics of the pressure are obtained by monitoring the high frequency pressure characteristics of the cavity position. Microwave and gliding arc plasma were found to have an effect on the pressure spectrum. The main frequency oscillation phenomenon was found after the addition of microwave and slip arc. This is related to the self-excited waves of the cavity and the vortex shedding of the flame boundary.Combined with the fractal geometry, the influence of microwave on the combustion of small-scale vortex is explained.
In order to further explain the influence of microwave and plasma on the flame, this paper studies the chemical reaction intermediates of the flame after microwave and arc addition by using a simple Bunsen burner experiment to obtain a single variable. It was found that the OH radicals increased after microwave addition, and the number of Cx elemental free radicals decreased. The effect of the arc on the flame is more pronounced, and the addition of the arc causes the OH radical to rise significantly. When the power supply is large enough, the flame acts as an arc path, at which point the chemical reaction intermediates leave almost only OH radicals. This shows that the action of the arc greatly reduces the chemical reaction path and promotes the combustion reaction.
The characteristics of flame structure transformation during scramjet acceleration are studied, the plasma was successfully used to control the flame structure when the flow and fuel state are constant, and the mechanism of plasma effect on combustion was explained by further experiments with Bunsen burner, complete research from phenomenon to preliminary mechanism was completed.
|孟宇. 超燃冲压发动机加速过程及等离子体对超声速火焰结构的影响[D]. 北京. 中国科学院大学,2019.|
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