Visualization of secondary atomization in emulsified-fuel spray flow by shadow imaging

Abstract

In this study, emulsified fuels having different dispersed water droplets and n-dodecane spray were injected separately into high-temperature inert gas, and their spray droplet behavior was studied. A measurement system consisting of high-speed video camera (125,000fps) coupled with a shadow imaging technique was used to visualize secondary atomization of spray droplets. In addition, the size distribution of the spray flow was measured using digital processing at different points. Although puffing (i.e. vapor eruption from droplet surface) and partial micro-explosion (i.e. a large part of the droplet bursts) were observed in this study, complete micro-explosion (i.e. the entire droplet bursts) was rarely observed. A certain amount of dispersed water was necessary to cause complete micro-explosion; however, the size of the dispersed water droplets (about 2–3μm) was insufficient to cause micro-explosion until coalescence is well progressed. In addition, the coalescence of dispersed water droplets had not progressed much because of the short waiting time of 20–30ms. Meanwhile, puffing occurred, regardless of coalescence, and it was sufficient to provide fine droplets, although less fine than those created by micro-explosion. As emulsified fuel moved downstream with heating, the droplet size significantly decreased; in contrast, heating had less effect on the droplet size of n-dodecane spray. Clearly, secondary atomization that included puffing improved spray characteristics, although the spray size of the emulsified fuel was larger than that of the original fuel near the injection point because of increased viscosity resulting from emulsification. Moreover, large dispersed water droplets improved the intensity of secondary atomization and provided a finer spray flow. The initial size of the dispersed water droplets played an important role in the spray characteristics of the emulsified fuel spray.