The effect of Prandtl number on the dynamics of a convective turbulent
studied by numerical experiments. In particular, three series of experiments
performed; in two of them the Rayleigh number spanned about two decades
Prandtl number was set equal to 0.022 (mercury) and 0.7 (air). In the third
contrast, we fixed the Rayleigh number at 6×105 and the
Prandtl number was varied
from 0.0022 up to 15. The results have shown that, depending on the Prandtl
there are two distinct flow regimes; in the first (Pr[lsim ]0.35)
the flow is dominated by
the large-scale recirculation cell that is the most important ‘engine’
for heat transfer.
In the second regime, on the other hand, the large-scale flow plays a negligible
in the heat transfer which is mainly transported by the thermal plumes.
For the low-Pr regime a model for the heat transfer is derived
and the predictions
are in qualitative and quantitative agreement with the results of the numerical
simulations and of the experiments. All the hypotheses and the consequences
of the model
are directly checked and all the findings are consistent with the predictions
experimental observations performed under similar conditions. Finally,
in order to
stress the effects of the large-scale flow some counter examples are shown
the large-scale motion is artificially suppressed.