Numerical study of atmospheric pressure Ar plasma jets under different electrode structures
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摘要: 采用二维轴对称流体模型对比研究了3种不同电极结构下大气压Ar等离子体射流的基本特性。第一种是带绝缘介质的针电极结构（电场方向和气体流方向平行），第二种是在第一种电极结构的介质管外增加一个垂直气流方向的接地环电极，第三种是不带绝缘介质的裸针电极结构。研究结果表明，接地环电极的引入对介质管内外的射流传播影响不同。在介质管内，接地环电极使管内表面附近的径向电场增加，电子密度升高，射流传播速度加快，但对中心轴附近的电场和电子密度影响很小；然而在介质管外，接地环电极的引入导致轴向和径向电场均减小，从而引起射流的传播长度减小，射流通道径向收缩。通过带绝缘介质的针电极和裸针电极结构的对比研究发现，保持其他条件不变，去掉包裹在针电极上的介质后，由于等离子体电势升高，电场增加，射流的传播长度几乎增加一倍，峰值电子密度增加近一个数量级，而且在整个射流通道内电子密度都保持相对高的值。此外，对3种电极结构下的主要活性粒子的产生和输运进行了比较研究。
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关键词:
- 大气压Ar等离子体射流 /
- 电极结构 /
- 二维模拟 /
- 活性粒子
Abstract: In this paper, the basic properties of the atmospheric pressure Ar plasma jet with three different electrode structures are comparatively studied using a two-dimensional axisymmetric fluid model. The results show that the introduction of the grounded ring electrode affects the jet propagation both inside and outside the dielectric tube. Inside the dielectric tube, the grounded ring electrode increases the radial electric field near the inner surface of the tube, leading to the increase of the electron density and jet propagation length. However, it has a slight effect on the electric field and electron density near the central axis. Outside the dielectric tube, the introduction of the grounded ring electrode results in the reduction of the electric field both in axial and radial directions. This inevitably causes the decrease of the jet propagation length and a contraction of the jet channel radius. For the bare needle electrode structure, the removal of the medium wrapped around the needle electrode increases the electric field due to the elevated plasma potential. This makes the increase of the jet propagation length. The peak electron density in the jet channel increases about one order of magnitude. Meanwhile, the electron density in the whole channel is relatively higher in the bare needle electrode structure. In addition, the production and transport of the main active particles under the three electrode structures are comparatively studied. -
图 1 计算中采用的装置示意图和模拟域
Figure 1. Discharge device and simulation domain used in our calculation
图 3 第一种和第二种电极装置下电子密度的时空演化
Figure 3. Temporal and spatial evolution of electron density for the first and the second electrode device
图 4 不同电极结构下径向和轴向电子密度时间演化
Figure 4. Radial and axial electron density evolutions for different electrode device
Note: dashed line: the first electrode device; solid line: the second electrode device.
图 5 电离波在介质管中传播时不同电极结构下电离头处轴向电场和径向电场演化
Figure 5. Evolution of the axial and radial electric field in the ionization head when the ionization wave propagates inside the tube for different electrode device
图 6 电离波在介质管外传播时不同电极结构下电离头处轴向电场和径向电场演化
Figure 6. Evolution of the axial and radial electric field in the ionization head when the ionization wave propagates outside the tube for different electrode device
图 7 不同电极结构下50 ns时活性粒子的空间分布
Figure 7. Spatial distribution of reactive species density at 50 ns for two electrode devices
图 8 两种电极装置（分别用虚线和实线表示）下活性粒子轴线数密度演化
Figure 8. Evolution reactive species density on the axis for two electrode devices
图 9 裸针电极装置下电子密度的时空演化
Figure 9. Temporal-spatial evolution of electron density for bare needle electrode device
图 10 裸针电极装置下轴向电子密度演化
Figure 10. Axial electron density evolution for the bare needle electrode device
图 11 裸针电极装置下电离波传播过程中电离头处轴向电场和径向电场的演化
Figure 11. Evolution of the electric field during the propagation of the ionization wave under the bare needle electrode device
图 12 裸针电极（实线）和带绝缘介质针电极（虚线）结构下轴向等离子体电势分布
Figure 12. Axial plasma potential distribution at different moments for the bare needle electrode (solid line) and needle electrode with insulation dielectric (dashed line)
表 1 中性气体流计算边界条件
Table 1. Boundary conditions for the neutral gas flow model
boundary velocity condition background species condition BC ${u_{ {\textit{z}} } } = {u_0},{u_{{r} } } = 0$ ${w_{\rm{Ar} } } = 0.999, {w_{\rm{air} } } = 0.001$ BJ,CI,IH,HD,GF ${u}_{ {\textit{z}} }=0,{u}_{{r} }=0$ ${{n} } \cdot { {{J} }_{{i} } } = 0$ DE ${u}_{ {\textit{z}} }=0.03{u}_{0},{u}_{{r} }=0$ ${w_{\rm{air}}} = 1$ KG ${u_{{r} } } = 0$ $\dfrac{ {\partial {w_{{i} } } }}{ {\partial r} } = 0$ EF 0.1 MPa ${{n} } \cdot { {{J} }_{{i} } } = \rho {w_{{i} } }{u_{{r} } }$ Note: ${u_{{r}}}$ and ${u_{ {\textit{z}} } }$ are the velocity in the axial and radial directions, respectively. ${{n}}$ is the unit vector pointing toward the boundary. 
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表 2 等离子体模块边界条件
Table 2. Boundary conditions for the plasma model
boundary electrostatic condition species condition BC $\dfrac{{\partial \varPhi }}{{\partial {\textit{z}}}} = 0$ $\dfrac{ {\partial {n_{\rm{e} } } }}{ {\partial {\textit{z} } } } = \dfrac{ {\partial {n_{\rm{\varepsilon} } } } }{ {\partial {\textit{z} } } } = 0,\;\dfrac{ {\partial {n_{\rm{m} } } }}{ {\partial {\textit{z} } } } = 0,\;\dfrac{ {\partial {n_{\rm{i} } } }}{ {\partial {\textit{z} } } } = 0$ BJ， CIHD Eq.(13) Eq.(8)， (9)， (10)， (11) KG $\dfrac{{\partial \varPhi }}{{\partial r}} = 0$ $\dfrac{ {\partial {n_{\rm{e} } } }}{ {\partial r} } = \dfrac{ {\partial {n_{\rm{\varepsilon} } } } }{ {\partial r} } = 0,\;\dfrac{ {\partial {n_{\rm{m} } } }}{ {\partial r} } = 0,\;\dfrac{ {\partial {n_{\rm{i} } } }}{ {\partial r} } = 0$ AK V … GF 0 —— EF 0 $\dfrac{ {\partial {n_{\rm{e} } } }}{ {\partial r} } = \dfrac{ {\partial {n_{\rm{\varepsilon} } } } }{ {\partial r} } = 0,\;\dfrac{ {\partial {n_{\rm{m} } } }}{ {\partial r} } = 0,\;\dfrac{ {\partial {n_{\rm{i} } } }}{ {\partial r} } = 0$ ring 0 —— Note: “ring” is the high voltage electrode.
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表 3 模拟中采用的化学反应
Table 3. Chemistry reactions used in the simulation
index reaction rate coefficients threshold energy/eV reference 1 e＋Ar→e＋Ar BOLSIG＋ / [17] 2 e＋Ar→e＋Ar* BOLSIG＋ 11.5 [17] 3 e＋Ar→2e＋Ar+ BOLSIG＋ 15.8 [17] 4 e＋N2→2e＋N2+ BOLSIG＋ 15.58 [17] 5 e＋N2→e＋2N BOLSIG＋ 13 [17] 6 e＋N2→e＋N2(C3π) BOLSIG＋ 11.03 [17] 7 e＋O2→2e＋O2+ BOLSIG＋ 12.06 [17] 8 e＋O2→e＋2O BOLSIG＋ 5.58 [17] 9 e＋O2→O2− BOLSIG＋ / [17] 10 Ar*＋Ar*→e＋Ar＋Ar+ 6.4×10−16 (m−3/s) / [17] 11 Ar*＋Ar→Ar＋Ar 2.09×10−21 (m−3/s) / [17] 12 Ar*＋N2→Ar＋2N 3.6×10−17 (m−3/s) / [17] 13 Ar*＋O2→Ar＋2O 2.1×10−16 (m−3/s) / [17]
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