Tomographic Spray Shadowgraphy - A Feasibility Study

Abstract

This contribution introduces tomographic shadowgraphy, a volume resolving imaging technique which is capable of recovering the liquid phase of a spray both spatially and temporally. The method is based on a multiple view camera setup with inline illumination provided by current pulsed LEDs which results in droplet shadows being projected onto the sensor plane. Each camera records image pairs with short interframing times which allows the trajectories of the individual droplets to be estimated using conventional three-dimensional particle tracking approaches. The observed volume is calibrated with a traversed micro target. A comparison is made between several photogrammetric and polynomial least square camera calibration techniques regarding their accuracy in deep volume calibration at magnifications close to unity. A calibration method based on volume calibration from multiple planar homographies at equally spaced z-planes was found to produce the most reliable calibration. Sequential combination of back-projected images at each voxel plane efficiently reproduces the droplet positions in three-dimensional space. The quality of spray reconstruction is investigated using experimental data from multiple view shadowgraphs of a hollow cone spray.