Saturation of enhanced heat transport in thermal convection in the presence of bi-phasic particles

We investigate the enhancement of turbulent heat transport by phase-change bi-phasic particles in a cylindrical Rayleigh–Bénard convection cell. The system comprises deionised water and a small volume fraction (

) of HFE-7000, a secondary fluid with a low boiling point (

upper T Subscript c r Baseline equals 42 Superscript ring Baseline normal upper C T c r = 42 ∘ C $T_{cr} = 42\,^{\circ }\mathrm{C}$

). By fixing the top-plate temperature at

upper T Subscript t Baseline equals 10 Superscript ring Baseline normal upper C T t = 10 ∘ C $T_t = 10\,^{\circ }\mathrm{C}$

and varying the bottom-plate temperature

upper T Subscript b T b $T_b$

, we characterise the global Nusselt number

italic Nu Nu $\textit{Nu}$

as a function of the superheat

normal upper Delta upper T Subscript italic sup Baseline identical to upper T Subscript b Baseline minus upper T Subscript c r Δ T sup ≡ T b − T c r $\Delta T_{\textit{sup}}\equiv T_b - T_{cr}$

. While the formation of bi-phasic particles initially catalyses heat transport, we report the discovery of a saturation plateau in the enhancement of heat transport for superheats exceeding

2 Superscript ring Baseline normal upper C 2 ∘ C $2\,^{\circ }\mathrm{C}$

, a phenomenon not observed in previous higher-Rayleigh-number studies. Flow visualisation demonstrates that the bi-phasic particles are restricted to the upper subcooled region of the cell, effectively partitioning the flow into a two-layer system. An interface is identified separating a lower single-phase liquid layer from an upper layer containing vigorously moving bi-phasic particles. We find that as the superheat increases, the vertical extent of the upper layer (the effective particle travelling distance) decreases. This geometric constriction leads to a reduction in the local heat transport efficiency of the upper region, which is offset by an increase in the efficiency of the lower layer.