Abstract :
A procedure for estimating the size and duration of a nucleation pulse is described. The assumed forms of temperature and concentration profiles, ignoring condensation, are used to calculate the rate of increase of nucleation rate, J, prior to the time of peak nucleation, tp. The rate of fall of J due to condensation alone after tp is also calculated. The procedure involves stepping forward in time, calculating these two rates and identifying tp as the time when they are equal. The reciprocal of the rate of fall at tp gives a measure of the duration of the pulse and the number nucleated can be estimated from J divided by this rate. Similar considerations are applied to estimating the time scale for condensation on seed aerosol and, in particular, to estimate when such condensation will prevent homogeneous nucleation taking place. Values from this method are compared with a more accurate numerical solution for the case of vapour flowing into a well-mixed chamber and found to agree to within a factor two over a wide range of flow rates. The method is also used to examine the effects of condensation on nucleation in turbulent water jets. For a typical laboratory jet, it is found that at temperatures below about 90°C, condensation has negligible effect so the number of droplets flowing axially is independent of flow velocity. For temperatures above about 95°C, condensation quenches nucleation and the number flowing increases with increasing flow velocity. The approach is intended to aid in the design of systems where simultaneous nucleation and growth occur and to indicate if more detailed modelling is required.
