Evaporating Diesel Spray Visualization using a Double-pass Shadowgraphy/Schlieren imaging

Evaporating diesel sprays from multi-hole nozzles have been visualized by means of a double-pass Shadowgraphy/Schlieren technique using a metal mirror attached on the injector-mount plane. Injection has been carried out in an inert environment of nitrogen gas in order to study the mixture process, avoiding mirror and glass window fouling by soot deposition from combustion. Studies have been carried out for four different gas densities during spray injection, changing pressure and temperature.

The effect of optical system alignment on image characteristics has been analysed for different beam angles relative to the optical axis. This parameter was varied in a range of 2 degrees to define the optimal optical configuration. If the angle equals 0, the Shadowgraphy/Schlieren system using a light reception back from the metallic mirror captures the spatial gradients of ambient gas density, and turbulent flow structures are clearly registered. However, the spray is observed as a mere shadow on the image. By modifying the angle, image patterns change so that ambient gas flow structure disappears and sprays are presented as a high-contrast visible image. In this imaging, bright areas reveal the location of vapour phase fuel while dark areas represent the dense region of the spray, although it does not only correspond to the liquid phase.