  Heat transfer by large deformable drops in a turbulent flow is a complex and
rich in physics system, in which drops deformation, breakage and coalescence
influence the transport of heat. We study this problem coupling direct
numerical simulations (DNS) of turbulence, with a phase-field method for the
interface description. Simulations are run at fixed shear Reynolds and Weber
numbers. To evaluate the influence of microscopic flow properties, like
momentum/thermal diffusivity, on macroscopic flow properties, like mean
temperature or heat transfer rates, we consider four different values of the
Prandtl number, which is the momentum to thermal diffusivity ratio: Pr=1, Pr=2,
Pr=4 and Pr=8. The drops volume fraction is Phi=5.4% for all cases. Drops are
initially warmer than the turbulent carrier fluid, and release heat at
different rates, depending on the value of Pr, but also on their size and on
their own dynamics (topology, breakage, drop-drop interaction). Computing the
time behavior of the drops and carrier fluid average temperatures, we clearly
show that an increase of Pr slows down the heat transfer process. We explain
our results by a simplified phenomenological model: we show that the time
behavior of the drops average temperature is self similar, and a universal
behavior can be found upon rescaling by t/Pr^2/3.
