Numerical Simulation of Unsteady Flows and Shape Oscillations in Liquid Droplets Induced by Deployment Needle Retraction

Abstract

Retractable opposed needles are often used in reduced-gravity droplet combustion experiments to deploy droplets prior to ignition. Needle retraction induces droplet shape oscillations and internal flows that can have important effects on subsequent droplet behaviors. In the present paper, the unsteady flows and droplet shape oscillations associated with deployment needle retraction are investigated using the commercial CFD software package Fluent. A volume-of-fluid method with a second-order upwind scheme and a dual time stepping solver is employed to solve the conservation equations in 2-d and 3-d simulations of droplets prior to ignition. A moving-mesh method is employed to model needle movements. Calculations indicate that rapid needle retraction causes ligament formation between a droplet and a needle, with ligament breakage sometimes resulting in the formation of satellite droplets. Needle retraction also induces droplet shape oscillations that rapidly decay, though significant internal flows are predicted to remain within droplets even after droplet shape oscillations have damped to low levels. The calculations indicate that the initial needle spacing can have important effects on droplet shape oscillations and internal flow characteristics. Comparison of model predictions and experimental data is favorable.