Z-type Schlieren Setup and its Application to High-Speed Imaging of Gasoline Sprays

Schlieren and shadowgraph imaging have been used for many years to identify refractive index gradients in various applications. For evaporating fuel sprays, these techniques can differentiate the boundary between spray regions and background ambient gases. Valuable information such as the penetration rate, spreading angle, spray structure, and spray pattern can be obtained using schlieren diagnostics. In this study, we present details of a z-type schlieren system setup and its application to port-fuel-injection gasoline sprays. The schlieren high-speed movies were used to obtain time histories of the spray penetration and spreading angle. Later, these global parameters were compared to specifications provided by the injector manufacturer. Also, diagnostic parameters such as the proportion of light cut-off at the focal point and the orientation of knife-edge (schlieren-stop) used to achieve the cut-off were examined. From the experiment, it was found that a light cut-off of approximately 60% performed the best to image the internal pattern of the gasoline sprays. An interesting finding from the knife-edge orientation study was that a vertically oriented stop increased the contrast of the spray pattern in the vertical direction. Similarly, a horizontal stop showed higher contrast and more turbulent spray structures in the horizontal direction. A combination of a horizontal and a vertical stop therefore unveiled the most of the gasoline spray pattern and structure. The light cut-off proportion and knife-edge orientation, however, did not affect the refractive-index gradient of the spray border and therefore no significant variations in the tip penetration and spreading angle were measured.