The dynamics of compound drops rising in a quiescent viscous fluid

Abstract

Compound drops are fluid particles composed by more than one fluid phase. Their complex structure governs their dynamic behavior, whose knowledge is of the utmost importance in several multiphase flow applications. The present work is devoted to the analysis of the gravitational motion of millimeter-sized gas-liquid compound drops in a quiescent viscous fluid at high Reynolds numbers, from 70 to 700 roughly, by means of flow visualization techniques. With this aim, experiments in an isothermal environment employing the high-speed shadowgraph and particle image velocimetry (PIV) techniques were conducted in the Flow Visualization Laboratory within the premises of the Multiphase Flow Research Center (NUEM – UTFPR). The influence of the properties of the fluids and the geometry of the compound drop was investigated by employing different fluids (silicone, corn, and mineral oils) and altering the size of the internal bubble. Two rising regimes were identified based on a critical range of diameter ratios, namely the rectilinear and the oscillatory motions. The governing effects of each motion regime were discussed. The compound drops preserved their spherical shape through all the measurements, suggesting that the internal fluid movement might play an essential role in the motion transition. The oscillatory behavior is described by the orientation angles of the phases of the compound drop that follow a pendular-like motion. The internal bubble reduces the viscous dissipation and the drag coefficient presented intermediate values compared to single-fluid drops and bubbles. The unsteadiness of the wake is at the onset of path instabilities. When the size of the bubble exceeds a critical value, the symmetry of the wake is lost due to the increased vorticity. The Strouhal number showed a different evolution compared to bubbles, drops, and spheres, revealing different mechanisms of oscillations. These observations are corroborated by mechanistic modeling and supported by the experimental data.