等离子体火焰介电系数分析及电容层析成像测量研究

发布时间：2018-10-13 13:39

【摘要】：等离子体助燃是近年发展起来的强化燃烧的手段,其可以加快化学反应速率、增加活性基团的种类和数量、减小点火延迟时间、增大燃烧的可燃极限和吹熄极限等从而使其在航空发动机领域有较好的发展前景。等离子体火焰的检测手段目前主要有探针法、质谱法和光谱法,但是,这三种方法均无法获得等离子体火焰的二维分布图像。上个世纪80年代出现了一种新的过程层析成像技术——电容层析成像技术(Electrical Capacitance Tomography,ECT),其原理是基于被测物质相对介电系数不同进而获得其二维物质分布图像。本文利用ECT技术对甲烷-空气等离子体火焰进行成像测量,并结合实验和模拟的方法对等离子体火焰的介电系数进行研究。等离子体火焰中的极化方式主要有极性分子的偶极转向极化、束缚电子的位移极化、热转向极化和自由电子位移极化。本文对等离子体火焰的极化方式进行分析,忽略对相对介电系数贡献较小的极化方式,例如极性分子的偶极转向极化、分子和原子的束缚电子位移极化等。最终确定等离子体火焰中对相对介电系数贡献最大极化方式为自由电子的位移极化,并借此获得了相对介电系数的表达式。等离子体火焰相对介电系数表达式中的未知参数主要是火焰中自由电子的电子密度和电子温度,郎缪尔探针是检测这两个参数最常用的手段之一,即利用等离子体火焰的伏安特性曲线来获得电子密度和电子温度的值。将两者代入到相对介电系数公式中即可获得相对介电系数的探针实验值。利用ECT对等离子体火焰进行测量就可获得其二维图像,图像中不同像素点的灰度值不同,需要对不同灰度值代表的相对介电系数的大小进行标定。本文利用MAXWELL软件进行模拟分析,模拟时所采用的几何模型与实验时一致,实验时空标定与满标定物质的相对介电系数同样成为模拟时的最小与最大相对介电系数,并利用相对介电系数介于两者之间工质进行模拟计算,借此获得相对介电系数与灰度值的对应关系。利用此关系对ECT图像进行标定,获得等离子体火焰相对介电系数ECT实验值。对等离子体火焰相对介电系数的探针实验值与ECT实验值进行分析,获得了相对介电系数的探针实验值与ECT实验值的误差来源。同时,利用相对介电系数公式对火焰温度场进行了标定。
[Abstract]:Plasma combustion is a recently developed means of intensified combustion, which can accelerate the chemical reaction rate, increase the type and number of active groups, and reduce the ignition delay time. By increasing the combustible limit and blowing limit, it has a good prospect in the field of aero-engine. At present, the methods of plasma flame detection mainly include probe method, mass spectrometry method and spectral method. However, none of these three methods can obtain the two-dimensional distribution image of plasma flame. In the 1980s, a new process tomography technique, electrical capacitance tomography (Electrical Capacitance Tomography,ECT), emerged. Its principle is to obtain two-dimensional material distribution images based on the difference of relative dielectric coefficient of measured matter. In this paper, ECT technique is used to measure the methane air plasma flame, and the dielectric coefficient of the plasma flame is studied by means of experiment and simulation. The polarization modes in plasma flame mainly include dipole shift polarization of polar molecule, displacement polarization of bound electron, thermal turn polarization and free electron displacement polarization. In this paper, the polarization modes of plasma flame are analyzed, ignoring the polarization modes which contribute little to the relative dielectric coefficient, such as the dipole shift polarization of polar molecules, the bound electron displacement polarization of molecules and atoms, etc. The maximum contribution to the relative dielectric coefficient in the plasma flame is determined to be the displacement polarization of the free electron and the expression of the relative dielectric coefficient is obtained. The unknown parameters in the expression of relative dielectric coefficient of plasma flame are mainly electron density and electron temperature of free electron in flame. Langmuir probe is one of the most commonly used methods to detect these two parameters. The values of electron density and electron temperature are obtained by using the volt-ampere characteristic curve of plasma flame. The probe experimental data of the relative dielectric coefficient can be obtained by inserting them into the formula of relative dielectric coefficient. The two-dimensional image of plasma flame can be obtained by using ECT to measure the plasma flame. The gray values of different pixels in the image are different, so the relative dielectric coefficient represented by different gray values should be calibrated. The geometric model used in the simulation is the same as that in the experiment. The relative dielectric coefficient of the spatio-temporal calibration and the full calibration is the minimum and the maximum relative dielectric coefficient in the simulation. The corresponding relation between relative dielectric coefficient and gray value is obtained by simulating the relative dielectric coefficient between them. The relative dielectric coefficient of plasma flame is obtained by calibrating the ECT image with this relation. The experimental values of the relative dielectric coefficient of plasma flame were analyzed by using probe and ECT, and the error sources between the probe and ECT were obtained. At the same time, the relative dielectric coefficient formula is used to calibrate the flame temperature field.
【学位授予单位】：北京交通大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TK16

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