Study of Water Vapor Condensation on Shock-Tube Walls

Abstract

Optical techniques have been used to study the condensation of water vapor on the inside surface of a glass section of a shock tube. Because of the thermal boundary layer, the gas (pure water vapor) at the walls experienced essentially an isothermal compression. By adjusting the initial conditions of the experiments, supersaturation of the water vapor was obtainable. The use of a shock tube resulted in very rapid compressions, permitting measurements to be made on a time scale the order of microseconds. This is a much faster time scale than has been reported by other investigators of condensation processes. The results of these measurements showed that the condensation process was very sensitive to the nature of the surface. For clean hydrophilic surfaces it was found that condensation occurred in the form of discrete droplets having an average radius of the order of about 0.2 μ, until at least about 100 μsec after compression. Hydrophobic surfaces produced condensation in the form of a continuous film. After reaching a thickness of about 100 Å in about 10 μsec after compression, the film appeared to begin to break up into many small droplets. The thin films of condensate were studied by measuring their optical transmissivity. The droplets were studied by making use of their light‐scattering properties. The results of the measurements are shown to be in agreement with mechanisms of the condensation process suggested by other authors.