Nucleation, Condensation and Evaporation in Waves and Jets

Abstract

In waves and jets of real fluids at states below the critical point phase transitions often occur. These processes are called condensation and evaporation depending on a growth or diminishing of the liquid phase (phase transition of the solid phase is not considered here). In a pressure- volume- temperature-diagram of states of such a fluid (see Fig I) the considered changes take place in the so-called two phase regime. When crossing the border of this regime, the phase boundary, nucleation occurs either on the liquid or the gas side. In the first case the bubble formation is called cavitation, in the second case the droplet formation is referred to as condensation. In both cases the initial process of nucleation up to a critical cluster of atoms or molecules is somewhat different from the later growth beyond the critical size to larger bubbles or droplets. For substances with high molar specific heat some general differences exist in the adiabatic flow process with rapid and large state changes. Because of the different shape of the phase boundary (see Fig 2). surprising phenomena become possible. An expansion flow in a nozzle starting from a superheated liquid state can lead to pure gas flow with supersonic velocities. Otherwise a shock wave in a gas like octane can liquefy this gas completely and the shock wave becomes the surface of a liquid. The instantaneous formation of a liquid surface by circumvention of the normal nucleation process is not yet understood. Thus, it is especially interesting to investigate waves and jets in fluids of high specific heat.