  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 $\sim 2.5$ mm diameter bubbles (corresponding to $Re_b
\approx 600$) in a rectangular water column heated from one side and cooled
from the other. The gas volume fraction $\alpha$ 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 $\alpha = 0.5\% $ onwards, the Nusselt number $\overline{Nu}$ is
nearly independent of $Ra_H$, and depends solely on the gas volume
fraction~$\alpha$. We observe the scaling $\overline{Nu} \propto
\alpha^{0.45}$, which is suggestive of a diffusive transport mechanism. 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
$\alpha = 0.9\%$. 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. 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.
