Abstract
We present results on the global and local characterisation of heat transport in homogeneous bubbly flow. Experimental measurements were performed with and without the injection of ${\sim}2.5~\text{mm}$ diameter bubbles (corresponding to bubble Reynolds number $Re_{b}\approx 600$) in a rectangular water column heated from one side and cooled from the other. The gas volume fraction $\unicode[STIX]{x1D6FC}$ was varied in the range 0 %–5 %, and the Rayleigh number $Ra_{H}$ in the range $4.0\times 10^{9}{-}1.2\times 10^{11}$. We find that the global heat transfer is enhanced up to 20 times due to bubble injection. Interestingly, for bubbly flow, for our lowest concentration $\unicode[STIX]{x1D6FC}=0.5\,\%$ onwards, the Nusselt number $\overline{Nu}$ is nearly independent of $Ra_{H}$, and depends solely on the gas volume fraction $\unicode[STIX]{x1D6FC}$. We observe the scaling $\overline{Nu}\,\propto \,\unicode[STIX]{x1D6FC}^{0.45}$, which is suggestive of a diffusive transport mechanism, as found by Alméras et al. (J. Fluid Mech., vol. 776, 2015, pp. 458–474). Through local temperature measurements, we show that the bubbles induce a huge increase in the strength of liquid temperature fluctuations, e.g. by a factor of 200 for $\unicode[STIX]{x1D6FC}=0.9\,\%$. Further, we compare the power spectra of the temperature fluctuations for the single- and two-phase cases. In the single-phase cases, most of the spectral power of the temperature fluctuations is concentrated in the large-scale rolls/motions. However, with the injection of bubbles, we observe intense fluctuations over a wide range of scales, extending up to very high frequencies. Thus, while in the single-phase flow the thermal boundary layers control the heat transport, once the bubbles are injected, the bubble-induced liquid agitation governs the process from a very small bubble concentration onwards. Our findings demonstrate that the mixing induced by high Reynolds number bubbles ($Re_{b}\approx 600$) offers a powerful mechanism for heat transport enhancement in natural convection systems.
Highlights
Enhancing the heat transport in flows is desirable in many practical applications
Our findings demonstrate that the mixing induced by high Reynolds number bubbles (Reb ≈ 600) offers a powerful mechanism for heat transport enhancement in natural convection systems
It is known that the motion of injected bubbles induces mixing of warm and cold parcels of the liquid phase, which in industrial applications where heat transport is coupled with the bubbly flow can lead to a 100 times greater heat transfer coefficient when compared to the single-phase case (Deckwer 1980)
Summary
Enhancing the heat transport in flows is desirable in many practical applications. To achieve this in systems with natural convection, several approaches have been adopted. In a more recent numerical study on forced convective heat transfer in turbulent bubbly flow in vertical channels, Dabiri & Tryggvason (2015) showed that both nearly spherical and deformable bubbles, improve the heat transfer rate They found that a 3 % volume fraction of bubbles increases the Nusselt number by 60 %. Previous studies (performed in systems with natural convection) mostly focused either on heat transfer in inhomogeneous bubbly flow, where bubbles of different sizes were introduced close to the hot wall, and where the thermal stability of the used set-ups remained unclear, or the studies were performed in a well-defined Rayleigh–Bénard system, but in which the bubbles mainly consisted of vapour and where bubble volume fraction and the bubble size varied due to evaporation and condensation. In this study we choose the height to be the characteristic length scale
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