Impact of density ratio on droplet dynamics in pulsating flow

Secondary atomization is extensively studied by investigating a droplet subjected to a steady air/gas stream. However, droplets are often subjected to unsteady or pulsating flows, such as in aero-engines or rockets, because of thermo-acoustic instabilities in the combustion chambers. The investigation focuses on the droplet dynamics and breakup in a pulsating flow for a range of density ratios (ρr), 1000 to 10, under sinusoidal airflow of different amplitudes and frequencies as compared to the dynamics in a steady flow. The volume of fluid multiphase model tracks the liquid–gas interface, and the governing equations are solved using the finite volume method. The two-dimensional axisymmetric pulsating simulations demonstrate accuracy comparable to the corresponding three-dimensional simulations at a much lower computational cost and are used for parametric studies. The droplets under the pulsating flow show a wavy surface, and larger vortex structures are observed during the deceleration period. At a high-density ratio (1000), pulsating flow enhances droplet deformation for a faster breakup, with the flow amplitude having more impact than its frequency. For a medium-density ratio (100), where breakup occurs under steady flow, droplet breakup is inhibited in the pulsating flow at low amplitude and high frequency. In the case of a low-density ratio (10), there is no breakup under steady flow, but pulsating flow promotes breakup, except at low amplitude and high frequency. The droplet breakup is always achieved for the highest amplitude, while lower frequencies push the liquid mass from the center of the droplet to the rim.