甲烷-空气混合气体放电等离子体增强点火机理分析

Analysis on Mechanism of Plasma Enhanced Ignition of

关键词:  等离子体  物理模型  数值模拟  约化场强  粒子演化  点火延迟

Key words:  Plasma  Physical model  Numerical simulation  Reduced electric field  Particle evolution  Ignition delay

摘要:

为研究放电过程产生的等离子体对缩短甲烷点火延迟时间的效果，针对脉冲式放电，本文耦合了密度方程、能量传递方程以及Boltzmann方程，对于甲烷-空气混合气体放电粒子浓度变化规律进行了研究分析。将计算得到的放电过程中激发态分子及活性自由基作为初始组份代入CHEMKIN中进行计算，计算了放电条件下等离子体对于甲烷点火延迟时间的影响。相比于附着过程，甲烷粒子弹性碰撞、激励、电离过程的碰撞截面要大2~5个数量级。随着粒子能量的增加，各个过程碰撞截面的变化并不单调，均存在碰撞截面最大的点。混合气体的激发过程导致了80%以上的能量损失。当约化场强逐渐增大时，甲烷的电离效应逐渐增强。混合气体的附着与弹性碰撞效应造成的能量损失比较小，相比激发与电离效应可以忽略。放电过程能够产生大量不同种类的活性粒子与自由基，不同活性粒子随时间变化的规律不相同。其中，随着放电，振动激发态氮分子浓度保持为1015/cm3量级。电子激发态氮分子粒子数密度随着放电的进行，在10-8s～10-7s会产生一个峰值。模型计算的单脉冲放电产生的活性粒子，在大多数点火温度下，可将点火延迟时间缩短10%以上。脉冲式放电对于甲烷-空气混合气体点火有显著的增强效果。

Abstract:

The density equation，energy transfer equation as well as Boltzmann equation were coupled aimed at analyzing particles concentration of pulsed discharge in order to study the effect in reducing ignition delay acted by plasma during the discharge. The excited molecular and active radical calculated were added as initial components in CHEMKIN model. The influence of methane ignition delay at the discharge was calculated. The cross section of elastic，excitation and ionization is 2~5 orders of magnitude larger than that of attachment. The excitation process leads to more than 80% energy loss of the mixed gas. With the increase of particle energy，each cross section of process is not monotonous. There were maximum values for each process. The ionization effect of methane is gradually enhanced with the increasing of the reduced electric field. Energy loss of attachment and elastic is smaller for the mixed gas compared with the excitation and ionization. Therefore it can be ignored. It can produce a large number of different kinds of active particles and radicals during the discharge process，and the changing rule of different active particles over time is different. The concentration of vibration excitation nitrogen molecules keeps 1015/cm3 orders of magnitude with the discharge. The concentration of ionization excitation nitrogen molecules produces a peak in 10-8s～10-7s with the discharge. The active particles produced by single pulsed discharge could shorten the ignition delay by 10% at least in most of the ignition temperature. Pulsed discharge can enhance the ignition of methane-air mixed gas dramatically.