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 2:5 mm diameter bubbles (corresponding to bubble Reynolds number Reb 600) in a rectangular water column heated from one side and cooled from the other. The gas volume fraction was varied in the range 0 %–5 %, and the Rayleigh number RaH in the range 4:0109–1:21011. 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 D0:5% onwards, the Nusselt number Nu is nearly independent of RaH, and depends solely on the gas volume fraction . We observe the scaling Nu / 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 D 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 (Reb 600) offers a powerful mechanism for heat transport enhancement in natural convection systems.
- Multiphase and particle-laden flows
- Gas/liquid flows
- multiphase and particle-laden flows
- gas/liquid flows