Multiphase Heat Transfer
The heat transfer mechanism in Rayleigh-B´enard convection in a liquid with a mean temperature close to its boiling point is studied through numerical simulations with point-like vapor bubbles, which are allowed to grow or shrink through evaporation and condensation and which act back on the flow both thermally and mechanically. It is shown that the effect of the bubbles is strongly dependent on the ratio of the sensible heat to the latent heat as embodied in the Jakob number Ja. For very small Ja the bubbles stabilize the flow by absorbing heat in the warmer regions and releasing it in the colder regions. With an increase in Ja, the added buoyancy due to the bubble growth destabilizes the flow with respect to single-phase convection and considerably increases the Nusselt number.
Nanofluids
The figures show the specific heat capacity and the conductivity as function of the particle volume fraction for a suspension of aluminum oxide nanoparticle in water. The experimental data are predicted by a theoretical model named undulatory theory of phonons. The temperature diffusion is governed by the propagation of elastic waves traveling with the sound velocity. The elastic wave can be regarded as the spatial collection of the phonon effects which take place at the nanoscale.
Pollutant Diffusion
Tracer dispersion within a highly convective planetary boundary layer is studied by means of a large-eddy simulation (LES) model for the continuous phases describing the temperature and velocity fields, and with the Lagrangian tracking of particle trajectories. The collective motion of four particles, initially located at the vertices of regular tetrahedra, is studied. The figure shows the distribution of the tetrads as a function of the normalized surface distance and of time. We plot the fraction of tetrads, with respect to the total number, whose centre of mass is located at time (t) at height (z). The evolution of tetrad shape and orientation is contrasted with those obtained in homogeneous and isotropic flows. Results show that an agreement is achieved at small time lags. At larger times, the boundary layer reveals its anisotropic structure and the tetrad shape statistics deviate from results obtained in ideal flows.