IMECH-IR  > 高温气体动力学国家重点实验室
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
Alternative TitleEffect of Acceleration and Plasma on Supersonic Combustion Structure of Scramjet
孟宇
Thesis Advisor张新宇 ; 顾洪斌
2019-11
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline流体力学
Keyword超燃冲压发动机,燃烧模态,火焰结构,加速过程,微波,滑移电弧,等离子体
Abstract

超声速燃烧冲压发动机是临近空间高超声速飞行器的理想动力部件,燃烧室火焰稳定是超燃冲压发动机性能的核心。然而在飞行器的加速、减速以及当量比变化等扰动过程中均会出现燃烧的不稳定和迟滞现象,无论何种稳焰方式(如凹腔或支板)在加速和减速过程中均会发生火焰结构转换,例如凹腔稳焰,由射流和剪切层稳焰转换到纯凹腔剪切层稳焰的转换过程中,两种火焰结构不断切换导致了燃烧的不稳定,最终影响发动机性能。无论是燃烧的不稳定或者迟滞均对发动机的整体性能产生较大影响,因此需要通过各种手段消除这些不稳定现象。传统的稳焰方法改变燃烧室几何结构从而改变流场,属于被动稳焰,也是燃烧不稳定或燃烧振荡的诱因之一,均不能实现消除动态过程中产生的不稳定现象,因此需要主动的稳焰方法对燃烧进行干扰。本文通过研究双模态超燃冲压发动机加速过程中火焰不稳定现象的特征,并进一步加入等离子体主动干扰火焰结构,给出一种新的超燃冲压发动机稳焰方法,并通过本生灯实验给出等离子辅助燃烧机理。本文分为三个部分,包括发动机加速性能研究,等离子体对超声速火焰结构的影响研究,以及微波和滑移电弧对本生灯火焰影响的机理研究。第一部分通过直连实验台模拟高空飞行连续加速过程,研究超燃冲压发动机燃烧室火焰结构的动态变化和发动机性能;第二部分引入等离子体辅助凹腔超声速燃烧,研究等离子体对超声速火焰结构的影响;第三部分通过本生灯助燃实验研究微波和滑移电弧等离子体对火焰的影响。

首次在地面实现了超燃冲压发动机动态连续加速实验,模拟了高超声速飞行器高空加速过程,燃烧室马赫数2.4加速至2.9,采用RP3液态常温煤油,成功实现了点火和稳定燃烧,并进行了等当量比和连续递减当量比实验。通过研究发现无论是定当量比还是变当量比,在加速过程中均有压力的不稳定出现,并发现燃烧不稳定现象。而导致压力不稳定的直接因素之一是燃烧释热的不稳定。因此本文通过拍摄火焰CH*自发光图像,研究了加速过程中火焰结构的变化过程。实验表明,在低速来流条件下火焰能够在射流区稳定,但高速来流条件下火焰不能在射流区稳定,而是转化为纯凹腔剪切层稳焰。射流稳焰向剪切层稳焰转化是一个不稳定过程,期间会出现两个稳焰区火焰的来回震荡,经过这一震荡过程之后才会形成稳定的纯剪切层火焰。在高速流场中由于点火延迟,单级凹腔常温煤油燃烧反压不足,不能形成稳定火焰。

为了进一步研究火焰结构和稳焰区域的转化,并提出新的火焰稳定手段,本研究设计了新的单凹腔、单面扩张的发动机模型,并在燃烧室加入了微波和滑移电弧等离子体。通过测量发动机沿程压力及拍摄火焰CH*发光研究等离子体对发动机性能和超声速火焰的影响。实验发现加入等离子体后燃烧室火焰结构发生转换,具体表现为压力结构的改变和平均CH*发光释热区域的变化,由此证明等离子体的加入使火焰更容易稳定在射流稳焰区域。同时发现燃烧火焰结构的转换过程是一个突变的过程,火焰结构的突变影响了凹腔位置压力的突变。其原因是微波加入后电磁场在凹腔集中,火焰更容易稳定在电磁场位置,在射流压力不变的条件下,燃烧由剪切层向下转移,改变了火焰稳定位置并形成了新的稳定模式。

通过分析火焰边界的分形几何特征分析,得到超声速燃烧火焰边界的自相似性,并且发现随着微波功率的加入,火焰边界的分形维数与微波功率成正相关。由于火焰边界的分形几何维数与湍流火焰速度为正相关关系,由此可推断微波的加入使超声速燃烧的湍流火焰速度增大。同时,通过监测凹腔位置的高频压力特征得到压力的频谱特性,发现微波和滑移电弧等离子体对压力频谱均有影响,加入微波和滑移电弧后发现了主频震荡现象,这与凹腔的自激振荡和火焰边界燃烧涡团脱落均有关系。同时结合分形几何特征,解释了微波对燃烧小尺度涡团的影响。

为了更进一步的解释微波和等离子体对火焰的影响,本文通过简单的本生灯实验力求单一变量,研究了微波和电弧加入之后火焰的化学反应中间产物。实验发现加入微波后OH自由基有上升趋势,同时Cx单质自由基数量下降。电弧对火焰的影响更加明显,电弧的加入使OH自由基显著上升。当电源功率足够大时,火焰充当电弧通路,此时化学反应中间产物几乎只剩下OH自由基,这也说明电弧的作用使化学反应路径大大缩减,促进燃烧反应。

本论文从双模态超燃冲压发动机加速过程出发,研究了加速过程中的火焰结构转换特性,并利用等离子体成功实现来流和燃料状态恒定时主动控制火焰结构完成转换,通过进一步的本生灯实验解释了等离子体对火焰影响的机理,完成了从现象到初步机理的完整研究。

Other Abstract

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.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/80037
Collection高温气体动力学国家重点实验室
Recommended Citation
GB/T 7714
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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