Statistical properties of thermally expandable particles in soft-turbulence Rayleigh-Bénard convection

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). 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 1 arXiv:1907.00049v1 [cond-mat.soft] 22 Jun 2019 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, τ T , and the thermal expansion coefficient of particles relative to that of the fluid, K = α p /α f . In particular, the residence time is constant for small thermal response times, τ T 1, and increasing with τ T for larger thermal response times, τ T 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 τ T for larger response times, thus explaining the trends in the DNS data well.

Enhanced heat transport in thermal convection with suspensions of rod-like expandable particles

“…The working fluid is then used for all the subsequent experiments, which are conducted in the range 2 • C T 32 • C and 3 × 10 8 Ra 4 × 10 9 . The Ra range is about 2-3 orders of magnitude larger than the Ra values in Alards et al (2019), and about one order of magnitude smaller than that in Wang et al (2019) and Joshi et al (2016). More importantly, PDMS has a large thermal expansion coefficient that is more than three times that of the working fluid (table 1), making the particle thermally responsive and more active than the background fluid.…”

“…The settling particles may form porous layers that reduce the heat transport (Joshi et al 2016). The distribution of thermally expandable point-like particles in RBC has also been studied numerically in the 'soft turbulent' regime, with the assumption that the presence of the particles does not modify the flow (Alards et al 2019). However, to the best of our knowledge, no enhancement of Nu has been reported in turbulent RBC with solid inertial particles.…”

Bubbly and Buoyant Particle–Laden Turbulent Flows

“…Therefore, the effect of bubbles and light particles on heat transfer has been subject of several experimental and numerical investigations. One approach for enhancing heat transport is to create vapor bubbles (Prosperetti 2017) or biphasic (Wang et al 2019) and thermally expandable particles (Alards et al 2019). Some of these have resulted in impressive heat transport enhancements as compared to the single phase case (Lakkaraju et al 2013;Wang et al 2019;Zhong et al 2009).…”