High-Speed Imaging of a Vaporizing GDI Spray: A Comparison Between Schlieren, Shadowgraph, DBI and Scattering 2020-01-0326
The evolution of the liquid and vapor phases of an iso-octane GDI spray was investigated in a constant volume vessel, under inert environment, using high-speed imaging techniques. The tests were performed in nitrogen, at temperatures and density varying between the operating conditions representative of late injection, flash boiling conditions and early injection in a GDI engine. Large scale parameters of the spray were obtained by processing Schlieren, shadowgraph, DBI and scattering images. The segmentation of spray images, for both the liquid and vapor phase, was carried out through an in-house image processing method. The method essentially involves three basic steps: an optimal filtering of the spray images by means of their regularization through variational methods; an original thresholding procedure based on the iterative application of the Otsu's method; the magnification of the Schlieren/Shadowgraph intensity pattern by means of the principal curvatures of the spray image surface. The boundaries of the vapor phase were obtained from the Schlieren and shadowgraph images of the spray. The processing method allows to accurately distinguish the contours of the vapor phase, whether it is a Schlieren or a shadowgraph image. The contours of the liquid phase obtained from DBI and scattering images were compared with each other and with the contours obtained by segmenting the shadowgraph images of the spray. Our findings demonstrate that the liquid phase signature in a shadowgraph image can be distinguished with some precision from that of the vaporized fuel. This would allow getting simultaneously the parameters of the two phases just from shadowgraph images of a spray.