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.