Nonisothermal homogeneous nucleation

Classical homogeneous nucleation theory is extended to nonisothermal conditions through simultaneous cluster mass and energy balances. The transient nucleation of water vapor following a sudden increase in saturation ratio is studied by numerically solving the coupled mass and energy balance equations. The ultimate steady state nucleation rate, considering nonisothermal effects, is found to be lower than the corresponding isothermal rate, with the discrepancy increasing as the pressure of the background gas decreases. After the decay of the initial temperature transients, subcritical clusters in the vicinity of the critical cluster are found to have temperatures elevated with respect to that of the background gas

Binary nucleation in acid–water systems. II. Sulfuric acid–water and a comparison with methanesulfonic acid–water

This work presents a systematic investigation of binary nucleation rates for sulfuric acid and water and the effect of temperature on these rates at isothermal, subsaturated conditions. The results from nucleation rate measurements for the sulfuric acid (H2SO4)–water system are discussed and compared to those previously presented for methanesulfonic acid (MSA)–water [B. E. Wyslouzil, J. H. Seinfeld, R. C. Flagan, and K. Okuyama, J. Chem. Phys. (submitted)]. Experiments were conducted at relative humidities (Rh) ranging from 0.006<Rh<0.65, relative acidities (Ra) in the range of 0.04<Ra<0.46, and at three temperatures, T=20, 25, and 30 °C, in the continuous flow mixing-type apparatus described in Paper I. Particles were formed by binary nucleation and grew by condensation as the mixed stream flowed through an isothermal glass tube. Number concentrations observed at the exit of the nucleation and growth tube as a function of Rh and Ra are extremely sensitive to the binary nucleation rate, and from these data the nucleation rate was estimated as a function of saturation level and temperature. Particle size distributions were also measured using a specially constructed differential mobility analyzer. As anticipated, the H2SO4 particles formed by nucleation and growth are much smaller than those formed in the MSA–water experiments, but particle size distribution measurements confirm that most of the particles formed are being observed. The ratio of experimental to theoretical nucleation rates, Jexpt/Jtheor, was found to be a strong function of the predicted number of acid molecules in the critical nucleus for both the H2SO4–water and MSA–water systems

Experimental and theoretical studies of multicomponent vapor condensation. Final report, 15 May 1992--15 May 1994

This report describes the results of experimental and theoretical studies of nucleation and condensation in multicomponent gas mixtures conducted over the past ten years. The program goals were to: (1) improve basic understanding of nucleation and droplet growth, (2) stringently test theories of nucleation at high nucleation rates and under nonisothermal conditions, (3) develop improved theories where needed, (4) enlarge the data base for systems of both fundamental and practical interest, and (5) provide reliable means for predicting the behavior of mixtures in practical devices and in the atmosphere. Condensible vapors, mixed with a carrier gas, were cooled in a supersonic Laval nozzle to obtain high nucleation rates under steady state conditions. Interferometry and laser light scattering were used to detect the `onset` of condensation and to monitor subsequent droplet growth. Theoretical calculations of the droplet size distribution along the flow axis were performed to assess competing theories of nucleation and droplet growth. This report briefly summarizes the accomplishments of the first eight years. There follows a more detailed summary of the program`s final two years, and a series of appendices describing each of the studies of the final two years in considerably more detail

The influence of nucleation and droplet growth on the onset of condensation in supersonic nozzle expansions

Unlike most other techniques used to study nucleation, supersonic nozzles do not yield nucleation rates directly because the length of time over which nucleation contributes significantly to particle formation is not easy to determine or control. Nevertheless, experiments in nozzles are extremely important because they provide higher rates of cooling, higher supersaturations and higher nucleation rates than any of the other techniques. Their operating conditions are more typical of important industrial conditions found in aerodynamic and turbomechanical flows where homogeneous condensation can have serious consequences for the gas flow behavior. Because the fluid mechanics of nozzles is well-understand, condensation experiments in the nozzle are amenable to sophisticated modeling efforts, and much useful insight can be gained regarding the nucleation and droplet growth processes under these severe cooling conditions. This paper summarizes our recent experimental work using a gently diverging supersonic Laval nozzle to investigate the variation of onset temperature and pressure for varying amounts of condensible vapor in an excess of carrier gas. Many similar studies have been carried out previously, but the results of these studies are usually not sufficiently well documented to enable us to do modeling studies that permit assessment of the condensate characteristics at onset. By carrying out modeling of the particle size distributions for our own experiments, we can avoid this difficulty. In modeling our experimental results, we have found that the mechanism for producing observable condensate varies considerably with conditions. Nucleation of small droplets can dominate at one extreme, but droplet growth can also be found to play a dominant role at other conditions