An Analysis of Droplets and Ambient Air Interaction in a D.I. Gasoline Spray Using LIF-PIV Technique

Measurements of the droplet and ambient air velocities in and around a D.I. gasoline spray were made by combining the laser induced fluorescence (LIF) and the particle image velocimetry (PIV) techniques. Before the fuel spray was injected into a constant volume vessel, rhodamine B-water solution was injected into the ambient air by a swirl-type injector for dispersing the fine fluorescent liquid particles as tracers for the ambient air motion. The fuel spray was injected into the fluorescent tracer clouds by a D.I. gasoline injector and was illuminated by an Nd:YAG laser light sheet (wave length: 532 nm). The light scattered by the droplets in the fuel spray was the same as the Nd:YAG laser wavelength, whereas the light emitted by the fluorescent tracer clouds was at a longer wavelength. The light emitted by the fluorescent tracer clouds was discriminated from the light scattered by the droplets in the fuel spray by an optical high pass filter (>560 nm), and the succeeding two images were captured by using a high-resolution CCD camera. The images acquired were analyzed by the double frame cross-correlation PIV technique, and the droplets and ambient air velocity distributions were obtained. The ambient air velocity distributions by the fluorescent tracer clouds were compared with those by the microballoon tracers to evaluate the tracking ability of the tracers to the ambient air motion. In comparisons of two tracers, similar velocity distributions were observed around the spray. A clear counter-rotating vortex was formed at the periphery of the spray under atmospheric pressure, whereas, at high ambient pressure, a vortex-like structure built up at the periphery of the spray was not vortex motion but the spray flowing up due to increased resistance to the still air and the decreased ambient air motion. The ambient air motion was the large-scale bulk motion symmetric to the spray axis, and it induced the entrainment of ambient air into the spray. The relative velocity between the spray and ambient air was high near the leading edge and at the axis of the spray.