Modeling Drop-Drop Collision Regimes for Variable Pressures and Viscosities

Abstract

The conditions associated with droplets colliding in gasses are considered in terms of the primary outcomes for low to moderate impact velocities. For these conditions, predictive models were developed for the boundaries of the five primary outcome regimes: slow coalescence (SC), bounce (B), fast coalescence (FC), head-on separation (HS), and grazing separation (GS). For the slow coalescence/bouncing (SC/B) boundary which can be important for viscous hydrocarbon drops in gasses, a boundary prediction was herein developed to incorporate effects of the gas characteristics (including temperature, pressure and vapor concentration). For the B/FC boundary, a B/FC models is herein developed for the effects gas pressure and gas viscosity (which can supply a cushioning effect for bounce but were not incorporated into previous models). Additionally, drop viscosity modifications were incorporated into the Brazier-Smith model for the FC/GS boundary and into a simplified Ashgriz-Poo model for FC/HS boundary. To evaluate the performance of these new predictions, an extensive review with available dropdrop collision data was conducted. The present models were found to be reasonable, though approximate, for a wide variety of test conditions, including variation in drop diameter ratio, drop fluid properties (density and viscosity), gas pressure, and gas properties (density and viscosity). However, additional analysis and data is necessary to improve these predictions and to generalize for effects of droplet shape and charge.