Turbulent convection in emulsions: the Rayleigh-B\'enard configuration

TL;DR

This paper investigates how multiphase Rayleigh-Bénard convection with emulsions affects heat transfer and turbulence, revealing that dispersed droplets can enhance or reduce heat transfer depending on properties and concentrations.

Contribution

It provides new insights into the effects of dispersed-phase volume fractions, viscosity ratios, and thermal diffusivity on heat transfer and turbulence in multiphase convection.

Findings

Dispersed droplets increase energy transfer to smaller scales.

Heat transfer can increase by up to 25% with certain emulsion properties.

Higher thermal diffusivity in droplets reduces overall heat transfer.

Abstract

This study explores heat and turbulent modulation in three-dimensional multiphase Rayleigh-B\'enard 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.