TESTS OF CERTAIN APPROXIMATIONS IN COMPUTATIONS MODELING THE FORMATION OF CLUSTERS IN SUPERSONIC-FLOW

Two approximations made in previous treatments modeling the nucleation and growth of molecular clusters generated in nonequilibrium nozzle flow are examined along with the sensitivity of results to uncertainties in input parameters. The first approximation, namely, the assignment of identical temperatures to all clusters at a given point in the flow, was found to be not seriously in error. The kinetics of mass and heat transfer must be suitably taken into account, however, to obtain the correct difference between the cluster temperature and that of the surrounding carrier gas. The second approximation, according to which the Kelvin equation is applied to critical nuclei but not subsequently to growing clusters, has an appreciable effect. When small clusters are given their enhanced volatilities at all times during growth, the slower growth and heat evolution into the medium delay termination of nucleation and result in somewhat cooler clusters that are smaller when fully grown. Although the thermal accommodation coefficient is crucial in the kinetics of heat transfer between clusters and gas, the calculated thermal history of clusters does not depend strongly upon the exact value of this poorly known coefficient. Likewise, final results are relatively insensitive to asssumptions about other uncertain parameters. Overall, results are robust enough to make computer modeling a valuable aid in the interpretation of the types of cluster phases encountered in experimental studies. © 1990 American Chemical Society.