| Submicron size monodisperse particles are of interest in many industrial and scientific applications. These include the manufacture of ceramic parts using fine ceramic particles, the production of thin films by deposition of ionized clusters, monodisperse seed particles for laser anemometry, and the study of size dependence of cluster chemical and physical properties. An inexpensive and relatively easy way to generate such particles is by utilizing the phenomenon of spontaneous condensation. The phenomenon occurs when the vapor or a mixture of a vapor and a noncondensing gas is expanded at a high expansion rate. The saturation line is crossed with the supercooled vapor behaving like a gas, until all of a sudden at the so called Wilson point, condensation occurs, resulting in a large number of relatively monodisperse droplets. The droplet size is a function of the expansion rate, inlet conditions, mass fraction of vapor, gas properties, etc.; In this dissertation, spontaneous condensation of (a) steam and (b) water vapor and air mixture in one dimensional nozzle has been modeled and the resulting equations solved numerically. The classical (Frenkel and Volmer) and Deich's nucleation theories were used. Unlike classical theory, Deich's theory also accounts for the critical droplet size dependence upon the expansion rate in the nozzle. The droplet size distribution at the exit of various one dimensional nozzles and the flow characteristics such as pressure ratio, mean droplet radius, vapor and droplet temperatures, nucleation flux, supercooling, wetness etc., along the axial distance were obtained. The numerical results compared very well with the available experimental data. Both nucleation theories predicted the pressure ratio properly but the classical theory tended to overestimate the droplet sizes. Additionally, since the classical theory cannot be used for obtaining a correlation for surface tension ratio, Deich's theory is preferred for analyzing and designing new nozzles.; The effect of inlet conditions, nozzle expansion rates and vapor mass fractions on droplet mean radius, droplet size distribution and pressure ratio were examined. By manipulating the various parameters, different mean radius and pressure ratio can be obtained. However, this only has minor impact on the relatively monodisperse droplet size distribution. |
