Bubble dynamics and atomization mechanisms in burning multi-component droplets

Abstract

We examine the complete sequence of events associated with the transition in the topology of a single droplet into multiple fragments of secondary droplets in the context of burning multi-component miscible mixtures. The multi-component blends consist of tetradecane as a lower volatile component, while butanol and acetone-butanol-ethanol (A-B-E) are used as the higher volatile constituents. In addition to the widely recognized theory of bubble growth via micro-bubble coalescence, we reveal that the vapor bubble growth also occurs through the merging of large bubbles during the combustion of droplets. The initial bubble growth (Regime I) and collapse cycles were found to increase the rate of bubble nucleation in the droplet, which in turn leads to the growth and merging of two or more vapor bubbles into a single larger bubble (Regime II). The final stage of bubble growth (Regime III) is associated with the Rayleigh-Taylor (RT) instability at the vapor-liquid interface. After the inception of the RT instability, capillary wave propagation is also witnessed on the droplet surface. The breakup of a vapor bubble results in the creation of a ligament that subsequently undergoes pinch-off into one or more secondary droplets. The ligament pinch-off mechanisms are categorized into two types, i.e., tip breakup and tip-base breakup, which govern the diameter and velocity of secondary droplets along with succeeding volumetric shape oscillations in the parent droplet. In particular, the ligament tip-base pinch-off mechanism results in a bimodal distribution of secondary droplets. After the initial breakup event, a vapor bubble may grow either in the secondary droplet or inside the developing ligament, leading to a sequential cascade of breakup events.