Quasi-two-dimensional description of rapid-type nucleation in a free turbulent jet

Abstract

A method of computation of vapour-to-particle conversion during the quenching of a turbulent jet is described. The mixing zone is divided into concentric regions by conical surfaces having their apices at the jet pole. The dilution of the hot material in each region is defined by solving the equation for the mixing kinetics which are treated as being due to mass exchange between two neighbouring regions rather than to dilution with clean material from the stagnant medium into which the jet is propelled. Computation has been performed for “rapid” type nucleation in which the formation of particulate matter is controlled by the collision efficiency of vapour molecules with the smallest cluster rather than by their condensation rate which is the rate-determining factor in slow (classical) nucleation theory. Results are compared with those obtained by the unidimensional approximation in which the kinetics of mixing and condensation are averaged over the apical angle at the jet pole. In the initial part of the mixing zone condensation proceeds in quite a different way from that predicted by the unidimensional approximation; proceeding downstream the difference vanishes gradually. It is shown that predictions of the unidimensional model can be completely wrong for thick jets and for the condensation of very dilute vapours.