Experimental study of the heat transfer properties of self-sustained biphasic thermally driven turbulence

We experimentally investigate the detailed heat transfer characteristics of a recently identified biphasic, two-component thermal turbulence [1]. The system comprises a cylindrical (water-based) Rayleigh-Benard convection setup, to which we introduce a small volume fraction (phi similar to 0.5%, 1% and 4%) of a low boiling temperature (T-cr), heavy immiscible liquid (HFE-7000). We show evidence of the catalystlike additive biphasic species undergoing self-sustained boiling-condensation cycles, leading to significant heat transfer enhancements. We vary the HFE-7000 volume fraction, superheat and underheat parameters. When the system is maintained at the same superheat, while varying the experimental parameters, the heat transfer enhancement is able to reach levels of up to 800 percent, which exceeds prior observations. This occurs at a volume fraction of 4% of the HFE-7000 species, facilitated by intense biphasic activity within the convection cell. Upon raising the superheat, an abrupt growth in the temperature fluctuations is observed for all three different phi tested, due to intermittent quenching and heating driven by the biphasic species on the top and bottom plates. We show that the thickness of the layer of HFE-7000 on the bottom plate crucially influences the temperature fluctuations and heat transfer properties in the explored parameter regime. Lastly, we discuss the two main contributing mechanisms to the heat transfer enhancement, 1) the boiling-condensation cycles of the biphasic species (latent heat), and 2) their wake-induced liquid agitation, which results in heightened mixing. (C) 2020 Elsevier Ltd. All rights reserved.