Flow structures, mixing and droplet production in a fuel-water stratified shear layer

Entrainment and mixing phenomena in a fuel-water stratified shear layer are studied experimentally. Flow visualization leads to identification of four types of interfacial flow structures that include interfacial waves, fingers, vortices and blobs. PIV measurements indicate that concentrated vorticity cores forming close to the interface prior to mixing in the fast-moving water layer initiate the entrainment process. The entrainment can be classified into three regimes characterized by the Richardson number. Once entrainment starts, a ‘mixture’ layer with dense concentration of droplets forms. The development of the mixture layer differs from that of the turbulent shear layer. The mixture layer can grow or shrink depending on the Richardson number, whereas the shear layer keeps on expanding.; Detailed data on the vertical and axial profiles of the mean flow and turbulence statistics in the water shear layer below the interface, along with the characteristic time and length scales are obtained. The turbulence level decreases with increasing distance from the interface. The turbulence decreases with increasing axial distance in the limited/moderate entrainment regimes but maintains an almost constant level when the entrainment is strong. These trends are consistent with the magnitudes of the overturning length scale compared to the mixture layer thickness.; Characteristics of the droplet production are strongly related to the flow structures and turbulence. The droplet size distributions are lognormal and their modes increase with axial distance and decrease with increasing velocity. The variance increases with axial distance. Within the dense mixture layer the droplet concentration is almost an order of magnitude higher than the concentrations outside of the mixture layer. Meandering of the mixture layer has a big effect on droplet spatial distributions. Differences between the turbulent water shear layer and the almost quiescent fuel layer cause an asymmetry in the spatial distributions with respect to the mixture layer.