Condensation of Water Vapour on Heterogeneous Nuclei in a Shock Tube.

摘要

A macroscopic model of heterogeneous nucleation is used for a theoretical study of condensation of water-vapour/carrier-gas mixtures in a nonequilibrium nonstationary rarefaction wave generated in a shock tube. The results are compared with those from homogeneous nucleation. Nucleation is assumed to take place heterogeneously on idealized smooth, spherical solid particles of Aitken nuclei, which are chemically and electrically inert, In the processes of heterogeneous condensation, the controlling factors are the size-distribution of nuclei, the concentration of monomers on the surface of the substrate and the contact angle of embryos. Of these factors the most dominant is the contact angle, which can reduce greatly the activation energy of nucleation. Heterogeneous condensation results in less supercooling of the mixture but a faster approach to the equilibrium state. By choosing a suitable value for the contact angle, the numerical results can be made to fit the experimental data. Although this is not entirely satisfactory, it is probably preferable to changing the value of surface tension as in the homogeneous nucleation case in order to obtain agreement with experimental results. (Author)