A New approach in characterizing secondary-breakup regimes for newtonian liquids

An image processing technique developed earlier using backlit shadowgraph for primary and secondary atomization study has been extended in this work to investigate the breakup evolution of a broad range of biodiesels, ethanol and fossil diesel. The backlit system uses a high speed CMOS camera in conjunction with a long distance microscope objective lens with the aid of a diffused light beam generated from a diode stack high-speed Nd-YAG laser. In the current work, a new approach of using the spatial gradient of area of fragment shapes is developed to improve understanding of the droplet-to-fragment and fragment-to-fragment breakup processes. A definition of breakup length is given here as the length between the initial location of the parent droplet and the location where the spatial gradient of area of ligaments and unbroken objects becomes close to zero. The non-dimensional breakup-time characteristic is then estimated using the measured breakup length and an appropriate velocity scale. This quantitative approach is capable of comparing the breakup time amongst different fuels and could be used to improve current breakup-time models. The trend of breakup-time observed in this study matches quite well with that reported by Pilch and Erdman [1]. The time however, is one order of magnitude shorter than that noticed by Pilch and Erdman. This is attributed to the difference in the breakup time definitions and the experimental techniques used in Pilch and Erdman’s work and those in this study, as has also been observed elsewhere [1].