Fast Expansion Chamber

FIGURE 11.6 Fast expansion chamber. Initially undersaiurated (5< 1) at po, 7b for a time interval of typically 1 ms supercritically supersaturated (S > S, ), where Sm, is the saturation ratio corresponding to a threshold nucleation rate, at pmm, Tmin finally still supersaturated, but subcritical (1 < S < Scri<) to permii droplet growth only. Sketch at right indicates the distribution of clusters with size at the three stages of operation. 

Experimentally nucleation rates can be determined in expansion chambers [35], The vapor is expanded in a fast and practically adiabatic process. Then it cools down. Since at low temperatures, the equilibrium vapor pressure is much lower, supersaturation is reached. Partially, this is compensated for by the pressure reduction during the expansion, but the temperature effect dominates. The density of nuclei can be measured by light scattering. 

Smaller size water droplets can be obtained by condensation, using sudden expansion of saturated steam, for example, in a shock tube high-pressure chamber [17-23]. The study [24—27] revealed that the fast forced or accidental expansion of a gas mixture containing steam, generates a two-phase gas + micro-droplets mixture (fog). 

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