氩气/甲烷/空气介质阻挡放电等离子体的发射光谱诊断

Emission Spectrum Diagnosis of Argon/Methane/Air Dielectric Barrier Discharge Plasma

为了获得可燃气体的放电及等离子体发射光谱特性,进一步揭示等离子体助燃作用下燃料在稀燃状态的点火与燃烧特性,在常压下以氩气作为载气对预混的甲烷和空气进行放电研究。实验基于平行板电极射频(13.56 MHz)介质阻挡放电的等离子体发生装置,首先在常压下对体积分数为90%氩气/10%空气的混合气体开展放电研究;再在90%氩气含量不变的情况下,调节空气含量并加入与之能形成燃烧化学当量比Φ=1的甲烷,氩气/甲烷/空气的混合气体同样能实现稳定而均匀的放电;最后分别在90%氩气含量不变,甲烷和空气在当量比为Φ=0.4~1.9六种情况下进行放电实验。由光谱仪记录不同放电工况下的发射光谱信息,诊断反应产物类型,利用观测到的氮分子第二正带系(0-2)380.4nm和(1-3)375.4nm处的发射谱线,与自编程序计算的模拟谱线拟合,得出分子转动温度(即气体温度)。研究结果表明:通过拟合模拟光谱与实验所测发射光谱的方法推测分子转动温度,进而获得气体的平动温度,氩气/空气放电的气体温度可达到1 150K,氩气/甲烷/空气Φ=1时放电气体温度升高到1 390K;甲烷与空气形成不同当量比时,所测等离子体气体温度相对于90%氩气/10%空气混合气体温度的温升在70~240K范围变化;由光谱信息观测到CH,H,OH和CH_2O等活性粒子的存在以及气体温度的升高,表明可燃成分混合气在射频电场放电作用下发生等离子体燃烧化学反应并释放出化学热。

The discharge of methane/air mixture gas had been investigated by using argon as carrier gas under atmospheric pressure for the first time. The characteristics of the discharge and the emission spectra of the plasma were gained to reveal the properties of ignition and combustion of the mixture gas assisted by plasma under lean-burn condition. Based on the parallel-plate electrode radio frequency （13.56 MHz） dielectric barrier discharge plasma generator, the discharge behavior of the gas mixture, with volume fraction of 90% argon and 10% air, was studied by experiments under atmospheric pressure. A stable and uniform discharge was achieved when methane had been added into the mixture gas at stoichiometric ratio （qb= 1）, while the content of argon maintained at 90%. At last, six discharge experiments were made at different level of the equivalent ratio （0. 4 to 1.9）. To identify the radicals generated during the process, the data of the emission spectrum were recorded by the spectrometer under different discharge conditions. By using a program compiled by the authors for the nitrogen second positive band system simula- tion, comparison between the experimental and simulated spectra of band （0--2） 380. 4 nm, （1--3） 375.4 nm was used to de- termine the rotational temperature of nitrogen molecules. The rotational temperature is considered as the temperature of dis- charge gas. Research results show that the discharging gas temperature would reach 1 150 K in the case of 90%argon/10%air; the temperature would rise to 1 390 K when the mixture was added stoichiometrie air and methane; in the case of different equiv- alence ratios, the plasma gas temperature would go up at the range of 70～240 K, against the mixture of 90%Argon/10%air; The existence of some active radicals, such as CH/H/OH/CH2 O, and the raise of temperature indicated that plasma chemical reaction occurred and the release of chemical heat.