Turbulent convection in emulsions: the Rayleigh-Bénard configuration

Abstract

This study explores heat and turbulent modulation in three-dimensional multiphase Rayleigh-Bénard convection using direct numerical simulations. Two immiscible fluids with identical reference density undergo systematic variations in dispersed-phase volume fractions, $0.0 \leq \Upphi \leq 0.5$, and ratios of dynamic viscosity, ${\lambda }_{\mu }$, and thermal diffusivity, ${\lambda }_{\alpha }$, within the range $[0.1-10]$. The Rayleigh, Prandtl, Weber, and Froude numbers are held constant at $10^8$, $4$, $6000$, and $1$, respectively. Initially, when both fluids share the same properties, a 10\% Nusselt number increase is observed at the highest volume fractions. In this case, despite a reduction in turbulent kinetic energy, droplets enhance energy transfer to smaller scales, smaller than those of single-phase flow, promoting local mixing. By varying viscosity ratios, while maintaining a constant Rayleigh number based on the average mixture properties, the global heat transfer rises by approximately 25\% at $\Upphi=0.2$ and ${\lambda }_{\mu }=10$. This is attributed to increased small-scale mixing and turbulence in the less viscous carrier phase. In addition, a dispersed phase with higher thermal diffusivity results in a 50\% reduction in the Nusselt number compared to the single-phase counterpart, owing to faster heat conduction and reduced droplet presence near walls. The study also addresses droplet-size distributions, confirming two distinct ranges dominated by coalescence and breakup with different scaling laws.

Accepted on the Journal of Fluid Mechanics