Molecular flow effects in evaporation and condensation phenomena
It is well known that the continuum description of fluid flow in general becomes inadequate under conditions at which the macroscopic gradient scales in the flow become comparable to the molecular mean free path in the fluid. This is high-lighted by the classical Chapman-Enskog approach that contracts the Boltzmann kinetic theory into the Navier-Stokes description of continuum fluid mechanics. The general result is that the continuum description is valid for small values of the so called Knudsen number, which is the ratio between the mean free path and the macroscopic flow scales. However, there are exceptions (singular cases) for small Knudsen numbers; one being the structure of shock waves, and another group being evaporation and condensation at interphase surfaces in single component systems. This latter group has been the subject of considerable interest from the physics of fluids community during the last few decades, and some of the initially fundamental results are by now implemented in various multi scale approaches for problems of current technological interest. The talk will focus on this latter group and review some of the basic kinetic theory results for evaporation/condensation problems and their links to macroscopic heat and mass transfer concepts. Specific problems to be considered include; (i) Evaporation into a shear flow, (ii) Explosive boiling of a liquid droplet, (iii) Laser ablation of carbon, and (iv); Flow in heat pipes. In addition, some mention will be made as to the next molecular scale; the Ångstrøm scale and its importance to further details on phase transition.
Published March 10, 2011