Visualization and Measurement of Rarefied Gas Flow using O_2-PLIF

氧分子的吸收波段范围很广，从紫外线的114纳米到红外线的1580纳米。主导的Schumann-Runge吸收带系统对应于基态CHI^3SIGMA_8^到激发态BETACHI^3SIGMA_u^-的跃迁，具有175 ~ 250 nm的离散吸收带和130 ~ 175 nm的连续吸收带。已知β状态被许多排斥电位曲线交叉，并且被预解离。由于强预解离率使荧光的寿命短，约为10^<-12>秒，因此可以假设荧光在分子间碰撞之前产生。在这项研究中，我们研究了O_2-LIPF（激光诱导预解离荧光）对稀薄气体流动的成像能力。由于氧是空气的主要成分之一，O_2-LIPF可以在没有示踪剂的情况下可视化高速风洞中的气流。然而，由于使用O_2-LIPF在室温以下流场的可视化非常困难，因此它只应用于高焓和燃烧流^<(1)>，而没有应用于低温高速流。在本研究中，使用ArF准分子激光器（193 nm）和高灵敏度CCD相机，用O_2-LIPF可视化了稀薄的氧气流动。如果设置CCD相机的长曝光时间（1200秒），并充分抑制散射激光束，O_2-LIPF可以成功成像室温以下的氧气流。

Oxygen molecules have the absorption bands in the broad range from 114 inthe ultra violet to 1580 nm in the infrared. The dominant Schumann-Runge absorption band system corresponds to the transition of the ground state CHI^3SIGMA_8^-to the excited state BETACHI^3SIGMA_u^-, and has the discrete absorption bands from 175to 250 nm and the continuum ones from 130 to 175 nm. The BETA state has been known to be crossed by many repulsive potential curves and to be predissociated. Sincethe strong predissociation rate makes the lifetime of the fluorescence short in the order of 10^<-12> seconds, it is possible to assume that the fluorescence yields prior to the intermolecular collision.In this study, we investigate the capability of imaging of rarefied gas flow using O_2-LIPF(Laser-Induced Predissociative Fluorescence). Since oxygen isone of main component of air, O_2-LIPF may make it possible to visualize the air flow in the high speed wind tunnel without the tracer. However, because the visualization of the flow field below the room temperature using O_2-LIPF is verydifficult, it has been applied only to the high entalpy and combustion flow^<(1)>, not to high speed flow with low temperature. In this study, the rarefied oxygen flow is visualized by O_2-LIPF, using an ArF excimer laser (193 nm) and a high-sensitive CCD camera. If long exposure time (1200 seconds) of the CCD camera is set and the scattered laser beam is suppressed sufficiently, the oxygen flow below the room temperature using O_2-LIPF can be imaged successfully.