Characteristics of Fuel Droplets Discharged From a Compensated Fuel/Ballast Tank

Fuel droplets, formed by the interaction of fuel plumes with a water/fuel interface, can be discharged during the refueling of water-filled compensated fuel/ballast tanks. Motivated by increasingly stringent environmental regulations, a study was initiated to understand the physical mechanisms involved in the formation and transport of fuel droplets by complex immiscible flows inside a model tank. In particular, optical measurements were made of the size distribution of fuel droplets in water discharged from a three-bay model of a compensated fuel/ballast tank. The volumetric fuel concentration of discharge from the tank was inferred from measurements of droplet size and number. Flow visualizations inside the model were coupled to optical measurements of fuel droplets at the tank outlet to show that the presence of fuel in the discharged water was correlated to the formation of fuel plumes within the water-filled tank. The size distribution of fuel droplets at the tank exit is found to differ from the size distribution reported for the generation zone (near the fuel plumes) inside the tank. Thus, the advection of fuel droplets from the generation zone to the tank outlet is shown to affect the characteristics of discharged fuel droplets. The transport process specifically prevents large-diameter droplets from reaching the tank exit. Buoyancy tends to cause larger fuel droplets generated within the tank to rise and separate out of the flow before they can be discharged. The buoyancy time, $τb(D)$⁠, relative to the characteristic advection time, $τa$⁠, of fuel droplets is a key parameter in predicting the fate of fuel droplets. The influence of buoyancy on the size distribution of discharged droplets was found to be modeled reasonably well by a Butterworth filter that depends on the ratio of timescales $τa∕τb(D)$⁠. This model, which relates the size distribution of discharged droplets to generated droplets, is found to produce the correct qualitative behavior that larger fuel droplets are discharged when the fuel plumes move closer to the tank exit, i.e., for decreasing advection time $τa$⁠.