Similar to the diesel jet penetration measurement (Naber, 1996), schlieren or shadowgraph imaging is also recommended to identify the vapor boundary of gasoline sprays. Collimated light is passed along a line of site through the combustion vessel. A concave mirror or large collection lens is normally used to direct the light onto a high-speed camera sensor. Schlieren imaging techniques are sensitive to gradients in refractive index, formed by either density or composition differences along a line-of-sight, and can therefore be used to mark the boundary of the vapor or liquid-phase of a penetrating jet.
Simultaneous to schlieren or shadowgraph imaging, Mie-scatter imaging (front-view or side-view) may be performed to gain a general sense of liquid plume penetration and direction, but it should not be considered quantitative as an ECN standard. The standard method for liquid penetration is extinction imaging.
Examples of simultaneous Mie-scattering/schlieren imaging of an iso-octane spray are shown in the high-speed movies below.
Fig. Movies of side-view Mie scatter (top left), front-view Mie scatter (top right), and schlieren (bottom). The vapor boundary of the spray is determined through image analysis described below.
Recommendations for vapor penetration measurements
- Apply focused shadowgraph or schlieren technique with high sensitivity
- The available Sandia-developed processing algorithm may be used to identify the boundary of spray region, while the definition of penetration in this algorithm is not for a gasoline spray but rather a single-plume diesel spray. For multi-plume gasoline injectors, define penetration as the maximum axial penetration of any plume in the z direction (the axis of the injector, not the plume).
- Evaluate the spatial and temporal distribution of standard deviation of the high-speed images to determine the vapor border.
- At this time, no standardized definitions for spray angle are suggested.
- Using the vapor boundary in the primary orientation, process the width of the entire vaporized spray at z = 15 mm.
Vapor penetration measurements are available to download for the gasoline Spray G condition using schlieren imaging, as documented by Manin et al., SAE 2015-01-1894. The vapor penetration is defined as the maximum axial penetration of any plume.