激光诱导荧光聚焦纹影系统及超声速燃烧流场应用

纹影是一种常用的流动显示技术，广泛应用于可压缩流动显示及超声速燃烧流场实验.然而，在变Mach数超声速燃烧实验中，燃烧室总温随来流Mach数变化.受准稳态/非定常温度变化影响，光学玻璃窗口的折射率发生显著改变，影响基于密度梯度的纹影成像质量.同时，普通纹影为光程体积沿程积分，难以同二维燃烧场成像信息进行直接比较以开展燃烧与流动耦合研究.聚焦纹影技术可抑制燃烧室内高温引起的玻璃窗口折射率变化，并实现毫米级的急剧聚焦深度，获得二维流场结构，同时配合纳秒级脉宽Nd:YAG激光光源可冻结高超声速流场.在传统聚焦纹影系统基础上发展了激光诱导荧光聚焦纹影系统并应用于变Mach数超声速燃烧实验，创新点在于使用激光诱导荧光染料，以荧光作为光源消除原本激光光源中的相干噪声，同时发展了边缘增强图像处理方法.实验结果表明激光诱导荧光聚焦纹影系统及边缘增强图像处理方法能够有效消除激光光源相干噪声，捕捉二维超声速燃烧流场结构. 

Laser-Induced Fluorescence Focusing Schlieren Systemand Its Application in Scramjet Combustor

Schlieren imaging visualizes flow structures with variations in density gradient and has been widely applied in the study of non-reacting and reacting compressible flow experiments.The supersonic combustion experiments were accompanied by high temperature and pressure,which caused distortions of the optical window and changes of its refractive index.Schlieren technique captures density gradient variation along the entire light path.Thus,it is sensitive to the change of temperature induced by window distortion,especially during the long test run of transient experiments.Focusing schlieren is a two-dimensional flow visualization technique with short depth of focus(DOF).The narrow DOF of focusing schlieren reduces the effect of the changes in the refractive index.The Nd:YAG laser has been widely used to illuminate the highly transient supersonic flow field for sufficient flow freezing.However,coherent noise is not evitable.A measurement technique combining the laser-induced fluorescence and focusing schlieren(LIF-FS)system was proposed.The novel contribution is to use fluorescence induced by laser as the light source.The results show that LIF-FS can clearly capture flow structures in supersonic combustion experiments and reduce coherent noise caused by laser illumination.