Shadowgraph or schlieren imaging were used to identify the vapor boundary of a penetrating jet (Naber, 1996). Collimated light is passed along a line of site through the combustion vessel. A concave mirror or large collection lense is used to direct the light onto a high-speed camera sensor. Shadowgraph or schlieren imaging techniqes 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. Examples of schlieren imaging of vaporizing and non-vaporizing sprays are shown in the high-speed movie below.
Fig. 4.1.1. Fuel is #2 diesel fuel. Injector tip to wall is 109 mm.
The boundary of the spray is determined through image analysis. The penetration distance is defined as the distance along the spray axis to the boundary of the spray (Naber, 1996). Jet penetration from CFD is defined as a mixture fraction threshold of 0.1%.
Recommendations for penetration measurements
• Focused shadowgraph technique with high sensitivity
• Use the available Sandia-developed processing algorithm of the texture of the temporal derivative as the spray indicator
• Define penetration as the maximum penetration of spray region
• Avoid bright, saturated flame regions (if possible) with chromatic or temporal filtering
• Conduct future experiments with quantitative mixture fraction results to determine how this definition corresponds to mixture fraction
• Development of standardized experiments and processing for additional spray shape information that is available from data