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

2019 ◽  
Vol 42 (9) ◽  
Author(s):  
Kim M. J. Alards ◽  
Rudie P. J. Kunnen ◽  
Herman J. H. Clercx ◽  
Federico Toschi
Keyword(s):  
Thermal Expansion ◽  
Thermal Expansion Coefficient ◽  
Residence Time ◽  
Expansion Coefficient ◽  
Response Times ◽  
Thermal Response ◽  
Benard Convection ◽  
1D Model ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

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}$τT, and the thermal expansion coefficient of particles relative to that of the fluid, $ K = \alpha_{p}/\alpha_{f}$K=αp/αf. In particular, the residence time is constant for small thermal response times, $ \tau_{T} \lesssim 1$τT≲1, and increasing with $ \tau_{T}$τT for larger thermal response times, $ \tau_{T} \gtrsim 1$τT≳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}$τT for larger response times, thus explaining the trends in the DNS data well. Graphical abstract

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

2021 ◽  
Vol 928 ◽  
Author(s):  
Shi-Yuan Hu ◽  
Kai-Zhe Wang ◽  
Lai-Bing Jia ◽  
Jin-Qiang Zhong ◽  
Jun Zhang
Keyword(s):  
Thermal Expansion ◽  
Heat Transport ◽  
Thermal Expansion Coefficient ◽  
Expansion Coefficient ◽  
Thermal Convection ◽  
Temperature Fields ◽  
Glycerol Solution ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard ◽  
Large Rayleigh Number

Thermal convection of fluid is a more efficient way than diffusion to carry heat from hot sources to cold places. Here, we experimentally study the Rayleigh–Bénard convection of aqueous glycerol solution in a cubic cell with suspensions of rod-like particles made of polydimethylsiloxane. The particles are inertial due to their large thermal expansion coefficient and finite sizes. The thermal expansion coefficient of the particles is three times larger than that of the background fluid. This contrast makes the suspended particles lighter than the local fluid in hot regions and heavier in cold regions. The heat transport is enhanced at relatively large Rayleigh number ( $\textit {Ra}$ ) but reduced at small $\textit {Ra}$ . We demonstrate that the increase of Nusselt number arises from the particle–boundary layer interactions: the particles act as ‘active’ mixers of the flow and temperature fields across the boundary layers.

Effect of Temperature Dependent Fluid Properties on Heat Transfer in Turbulent Mixed Convection

2013 ◽  
Vol 136 (2) ◽  
Author(s):  
Francesco Zonta ◽  
Alfredo Soldati
Keyword(s):  
Heat Transfer ◽  
Thermal Expansion ◽  
Mixed Convection ◽  
Thermal Expansion Coefficient ◽  
Expansion Coefficient ◽  
Large Scale ◽  
Effect Of Temperature ◽  
Complex Flow ◽  
Fluid Properties ◽  
Rayleigh Benard

The effect of the uniform fluid properties approximation (Oberbeck-Boussinesq (OB)) in turbulent mixed convection is investigated via direct numerical simulation (DNS) of water flows with viscosity (μ) and thermal expansion coefficient (β) both independently and simultaneously varying with temperature (non-Oberbeck-Boussinesq conditions (NOB)). Mixed convection is analyzed for the prototypical case of Poiseuille-Rayleigh-Bénard (PRB) turbulent channel flow. In PRB flows, the combination of buoyancy driven (Rayleigh-Bénard) with pressure driven (Poiseuille) effects produce a complex flow structure, which depends on the relative intensity of the flow parameters (i.e., the Grashof number, Gr, and the shear Reynolds number, Reτ). In liquids, however, temperature variations induce local changes of fluid properties which influence the macroscopic flow field. We present results for different absolute values of the shear Richardson numbers (Riτ=|Gr/Reτ2|) under constant temperature boundary conditions. As Riτ is increased buoyant thermal plumes are generated, which induce large scale thermal convection that increases momentum and heat transport efficiency. Analysis of friction factor (Cf) and Nusselt number (Nu) for NOB conditions shows that the effect of viscosity is negligible, whereas the effect of thermal expansion coefficient is significant. Statistics of mixing show that (i) mixing increases for increasing Riτ (and decreases for increasing Reτ) and (ii) the effect of thermal expansion coefficient on mixing increases for increasing Riτ (and decreases for increasing Reτ). A simplified phenomenological model to predict heat transfer rates in PRB flows has also been developed.

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