AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
D2O and Nonane Non-equilibrium Droplet Growth in the Free Molecular Regime
HARSHAD PATHAK, Kelley Mullick, Barbara Wyslouzil, Shinobu Tanimura, The Ohio State University
Abstract Number: 219 Working Group: Aerosol Physics
Abstract Nanodroplet growth of water and n-alkanes is observed in industrial processes including, for example, the removal of condensibles from natural gas or during the expansion of steam in low pressure turbines. The process commences with homogenous nucleation of the supersaturated vapor to form droplets. Droplet growth then quenches nucleation by depleting the vapor and releasing heat to the flow. The competition between nucleation and the initial stages of droplet growth determines the number of droplets formed and, thus, strongly influences the aerosol size distribution. We study the growth of D$_2O or nonane droplets in the free molecular regime under the highly non-equilibrium conditions found in supersonic nozzles and compare the measured growth rates to the predictions of the isothermal Hertz-Knudsen (HK) and non-isothermal Hertz-Knudsen-Smolders (HKS) growth laws. Pressure trace measurements (PTM) combined with small angle X-ray scattering (SAXS) characterize the droplet size and number densities as a function of the flow time in a supersonic nozzles with 5-10 microsecond time resolution. For the D$_2O aerosols coagulation clearly plays a role, while for the nonane aerosols number densities are low enough that coagulation is not important. Fourier transform infrared spectroscopy experiments also detect freezing in the coldest D$_2O droplets. For the nonane droplets, there is essentially no difference between the predictions of the isothermal and non-isothermal growth laws since the equilibrium vapor pressures are so low that the ratio of the evaporation to condensation rates is effectively zero. The experimental D$_2O growth rates are more closely predicted by the non-isothermal growth law rather than isothermal growth law, but there are still significant differences that cannot be explained by coagulation.