Abstract
.The dynamics of inertial particles in Rayleigh-Bénard convection, where both particles and fluid exhibit thermal expansion, is studied using direct numerical simulations (DNS) in the soft-turbulence regime. We consider the effect of particles with a thermal expansion coefficient larger than that of the fluid, causing particles to become lighter than the fluid near the hot bottom plate and heavier than the fluid near the cold top plate. Because of the opposite directions of the net Archimedes’ force on particles and fluid, particles deposited at the plate now experience a relative force towards the bulk. The characteristic time for this motion towards the bulk to happen, quantified as the time particles spend inside the thermal boundary layers (BLs) at the plates, is shown to depend on the thermal response time, tau_{T}, and the thermal expansion coefficient of particles relative to that of the fluid, K = alpha_{p}/alpha_{f}. In particular, the residence time is constant for small thermal response times, tau_{T} lesssim 1, and increasing with tau_{T} for larger thermal response times, tau_{T} gtrsim 1. Also, the thermal BL residence time is increasing with decreasing K. A one-dimensional (1D) model is developed, where particles experience thermal inertia and their motion is purely dependent on the buoyancy force. Although the values do not match one-to-one, this highly simplified 1D model does predict a regime of a constant thermal BL residence time for smaller thermal response times and a regime of increasing residence time with tau_{T} for larger response times, thus explaining the trends in the DNS data well.Graphical abstract
Highlights
Inertial particles in thermally driven flows are abundant in both nature and technological applications
We investigate the dynamics of thermally responsive inertial particles in Rayleigh-Benard convection, where we include thermal expansion of both particles and of fluid
Since in RayleighBenard convection (RBC) the temperature gradients are largest in the thermal boundary layers (BLs) while the temperature in the bulk fluctuates around the average temperature [17, 36], particles are expected to distribute differently in the bulk than in the thermal BLs when thermal expansion is included
Summary
Inertial particles in thermally driven flows are abundant in both nature and technological applications. When considering a dilute suspension (where particles are not expected to influence the fluid flow or temperature) where the size of particles is independent of temperature, thermal inertia will not influence the motion of the inertial particles. Bubbles in boiling convection will grow in the warmer spots of the flow and shrink in the cooler spots [13,14,15], affecting their buoyancy and changing the upward and downward motion of these bubbles.
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