Nucleation and growth of Mg condensate during supersonic gas quenching

Abstract

A one-dimensional model based on classical nucleation and growth has been developed as a diagnostic tool for predicting the impact of different process conditions and nozzle geometries on particle size distributions produced from supersonic quenching of magnesium vapours. The model was validated against experimental data for water and SF 6, showing good qualitative agreement with the data. For the cases in the study—magnesium concentration from 1 to 20 mol% and the inlet temperature varying from 1600 to 1900 K—the model predicts that 99% of the condensation is due to growth of particles nucleated during an initial high nucleation rate stage. The ultimate average particle size is therefore dependent on the magnitude of the nucleation rate during that initial stage of nucleation and to the degree of subsequent growth of those particles which are, in turn, a complex function of the conditions in the nozzle. The distribution of condensate size is somewhat sensitive to the inlet temperature of nozzle, increasing the temperature from 1600 to 1900 K increases the mean size of the condensate by 25%. The molar concentration of magnesium in the gas affects the final particle size but this does not follow a simple trend. The size distribution of particles predicted from the model is very sensitive to changes in surface tension and sticking coefficient, highlighting the need for a more rigorous treatment of these parameters.