Self-consistent cluster approach to the homogeneous kinetic nucleation theory

\u3cp\u3eAn alternative, self-consistent formulation of the homogeneous nucleation theory has been proposed. This approach differs from the classical Becker-Döring-Zeldovich theory in two respects: (i) evaporation rates are evaluated by referring to the stable equilibrium of a saturated vapor rather than to the constrained metastable equilibrium of a supersaturated vapor; and (ii) for the reference stable equilibrium state the Fisher theory of condensation is used in order to obtain a self-consistent definition of the free-energy barrier for l-cluster formation, where l is the number of molecules in the cluster. A comparison of the expressions for the nucleation rate and critical cluster size with the corresponding classical expressions has been made for the different parts of the phase diagram (temperature-supersaturation) and the domain where both theories are close has been found. Predictions of the present theory have been compared with the experimental results on nucleation of n-nonane for the three sets of experiments (diffusion cloud chamber, fast-expansion cloud chamber, and two-piston cloud chamber). It has been shown that the present theory has a much better agreement with experimental results for n-nonane than the classical theory.\u3c/p\u3

A pulse-expansion wave tube for nucleation studies at high pressures

The design and performance of a new pulse-expansion wave tube for nucleation studies at high pressures are described. The pulse-expansion wave tube is a special shock tube in which a nucleation pulse is formed at the endwall of the high pressure section. The nucleation pulse is due to reflections of the initial shock wave at a local widening situated in the low pressure section at a short distance from the diaphragm. The nucleation pulse has a duration of the order of 200 µs, while nucleation pressures that can be achieved range from 1 to 50 bar total pressure. Droplet size and droplet number density can accurately be determined by a 90°-Mie light scattering method and a light extinction method. The range of nucleation rates that can be measured is 108 cm-3¿s-

Condensing nozzle flows : Ludwieg tube experiments and numerical/theoretical modelling

The present paper deals with homogeneously condensing flows of humid nitrogen in a Laval nozzle. The modelling of nonequilibrium condensation phenomena can be separated in two distinct processes: homogeneous nucleation and droplet growth. Our objective is to investigate the quality of a condensation model characterised by the following combination: the (corrected) Internally Consistent Classical Theory for the nucleation process and a generalised transitional growth model, with the droplet temperature calculated explicitly via the wet-bulb equation. Our theoretical predictions have been then compared with our experimental results on droplet sizing showing a good agreement

Multicomponent nucleation and droplet growth in natural gas

The first quantitative experimental results are presented on homogeneous nucleation and droplet growth in a multicomponent gas-vapor mixture. Using the pulse-expansion wave tube technique, we investigated the condensation behavior of natural gas consisting of over 30 components. Data were obtained in the pressure range between 6 and 24 bar and at temperatures ranging from 221 to 237 K. The observed droplet growth rates are quantitatively explained using a multicomponent model for diffusion controlled growth. The nucleation rate data are for the moment mainly presented as a challenge to theoreticians, although some qualitative arguments are presented that could be helpful in the interpretation. The data appear to agree at least qualitatively with theoretical values (according to the revised binary classical nucleation theory) for a mixture of n-octane and methane, a model mixture which also shows the same macroscopic phase behavior as natural gas. ©1998 American Institute of Physics

On phase transition in compressible flows: modelling and validation

A phys. model for compressible flows with phase transition is described, in which all the processes of phase transition, i.e. nucleation, droplet growth, droplet evapn. and de-nucleation, are incorporated. The model is focused on dil. mixts. of vapor and droplets in a carrier gas with typical max. liq. mass fraction smaller than 0.02. The new model is based on a reinterpretation of P. G. Hill (1966) method of moments of the droplet size distribution function. Starting from the general dynamic equation, it is emphasized that nucleation or de-nucleation correspond to the rates at which droplets enter or leave droplet size space, resp. Nucleation and de-nucleation have to be treated differently in agreement with their differences in phys. nature. Attention is given to the droplet growth model that takes into account Knudsen effects and temp. differences between droplets and gas. The new phase transition model is then combined with the Euler equations and results in a new numerical method: ASCE2D. The numerical method is first applied to the problem of shock/expansion wave formation in a closed shock tube with humid nitrogen as a driver gas. Nucleation and droplet growth are induced by the expansion wave, and in turn affect the structure of the expansion wave. When the main shock, reflected from the end wall of the low-pressure section, passes the condensation zone, evapn. and de-nucleation occur. As a second example, the problem of the flow of humid nitrogen in a pulse-expansion wave tube, designed to study nucleation and droplet growth in monodisperse clouds, is investigated exptl. and numerically. [on SciFinder (R)

Marangoni flow on an inkjet nozzle plate

\u3cp\u3eIn piezo inkjet printing, nozzle failures are often caused by an ink layer on the nozzle plate. It is experimentally shown that the ink layer at the nozzle is formed through streamers of ink, emanating from a central ink band on the nozzle plate. The streamers propagate over a wetting nanofilm of 13 nm thickness, directed toward the actuated nozzles. The motion of the front end of the streamers follows a power law in time with an exponent 1 2. The observations are consistent with a surface tension gradient driven flow. The origin of the Marangoni flow is an effective lower surfactant concentration of the ink around the nozzle.\u3c/p\u3