Nucleation vapor-liquid

Clinch, J. M., andH. B. Karplus, 1964, An Analytical Study of the Propagation of Pressure Waves in Liquid Hydrogen-Vapor Mixtures, Report IITRI-N-6054-6, IIT Research Inst., NASA-CR-54015. (3) Cobb, C. B., and E. L. Park, Jr., 1969, Nucleate Boiling—A Maximum Heat Flux Correlation for Corresponding States Liquids, Chem. Eng. Prog. Symp. Ser. 65(92) 188—193. (4)... 

The experiment began by charging the equilibrium cell with about 30 cm3 of either phenoPp-cresol or phenol-water solution mixture. The cell was then pressurized with either methane or carbon dioxide until the phenol clathrate formed under sufficient pressure. The systems were cooled to about 5 K below the anticipated clathrate-forming temperature. Clathrate nucleation was then induced by agitating the magnetic spin bar. After the clathrates formed, the cell temperature was slowly increased until the clathrate phase coexisted with the liquid and vapor phases. The nucleation and dissociation steps were repeated at least twice in order to diminish hysteresis phenomenon. The clathrates, however, exhibited minimal hysteresis and the excellent reproducibility of dissociation pressures was attained for all the temperatures and found to be within 0.1 K and 1.0 bar at each time. When a minute amount of phenol or p-cresol clathrate crystals remains and the system temperature was kept constant for at least 8 hours after attaining pressure stabilization, the pressure was considered as an equilibrium dissociation pressure at that specified temperature. 

Thus, if a liquid contains suspended particles with complex microrelief, a vapor-gas nucleus often remains in small cracks in such particles. With poor wettability by the liquid, nucleation of a cavity under action of tensile acoustic stresses should always begin from the vapor-gas state in the entrance of the crack. 

Figure 4. Boundary of limiting superheats of acetone and water solutions in acetone 1 - acetone, 2 - acetone + 10 % water, 3 - acetone + 30 % water. Solid line - line of liquid-acetone vapor phase equilibrium, C - critical point, dashed line - calculation by homogeneous nucleation theory for J = 10 s m (acetone). ...

The heat transfer mechanisms that are active in boiling in micro-channels can be summarized as follows (i) in bubbly flow, nucleate boiling and liquid convection would appear to be dominant, (ii) in slug flow, the thin film evaporation of the liquid film trapped between the bubble and the wall and convection to the liquid and vapor slugs between two successive bubbles are the most important heat transfer mechanisms, also in terms of their relative residence times, (iii) in annular flow, laminar or turbulent convective evaporation across the liquid film should be dominant, and (iv) in mist flow, vapor phase heat transfer with droplet impingement will be the primary mode of heat transfer. For those interested, a large number of two-phase videos for micro-channel flows from numerous laboratories can be seen in the e-book of Thome [22]. 

In addition to showing that solvent-free melt and sublimation crystallization conditions offer an attractive route to new polymorphs, a CSD survey of these methods of crystallization and the frequency of Z was performed (Table 3-5). There is a dramatic increase in the occurrence of Z > 3 crystal structures when melt or sublimation crystallization conditions are used [20]. The occurrence of high Z in melt crystallization and sublimation methods is ascribed to the rapid cooling of the hot liquid or vapor (100-300° C) in the open flask or on the cold finger (kinetic phase), conditions under which hydrogen-bonded clusters are likely to condense in a pseudo-symmetric crystalline arrangement. On the other hand, the slower nucleation process of solution crystallization gives the frequent situation of Z < 1 (88% hits). 

For multicomponent mixtures, the mechanisms are even more complex, involving mass transfer in both liquid and vapor phases. Detailed measurements of bubble frequency (/), bubble departure diameter (dd), and number of active nucleation sites ((V ) are reported by Bier and Schmidt [127] for the propane/n-butane mixtures studied, the bubble departure diameter initially increases (relative to its value for n-butane) with increasing mole fraction of propane and then decreases to a value less than that for pure propane before increasing rapidly with concentration and propane mole fractions greater than about 0.9. Ilie bubble frequency shows the opposite trends. The number of active sites (Na) passes through a minimum as the mole fraction of propane is increased, the maximum reduction being around a factor of 3. It is clear, therefore, that the effects of having a multicom-... 

Another effective technique to study vapor-to-liquid nucleation is an expansion cloud chamber (Schmitt 1981, 1992 Wagner and Strey 1981 Strey and Wagner 1982 ... 

For the case of single-component vapor-to-liquid nucleation the above equation, in conjunction with (23), yields... 

For droplet condensation in supercooled vapors or bubble formation in superheated liquids, density functional theory predicts that the free energy barrier to nucleation vanishes at the spinodal curve. This is an important improvement on classical nucleation theory, which predicts finite barriers irrespective of the depth of penetration into the two-phase region. Density functional theory is an extremely powerful technique for the rigorous calculation of free energies barriers to nucleation. Examples of calculations in non-ideal systems include bubble nucleation in the superheated Yukawa and Lennard-Jones liquids [55, 57] liquid nucleation in dipolar vapors [61] binary nucleation of liquids from vapors [58] and of bubbles from liquids [62] and crystal nucleation [59]. 

McGraw, R., and Laaksonen, A. (1996) Scaling properties of the critical nucleus in classical and molecular-based theories of vapor-liquid nucleation. Physical Review Letters 76, 2754-2757. 

Take for example a vessel containing butane at room temperature (20°C), in which liquid and vapor are at equilibrium at an absolute pressure of 2 atm (point M in Fig. 22.2). If, due to the thermal radiation from a fire, the temperature increases to 70°C, the pressure inside the vessel will be 8 atm (point N). If, at these conditions, the vessel bursts (due to the failure of the material or an impact, for example), there will be an instantaneous depressurization from 8 atm to the atmospheric pressure. At the atmospheric pressure, the temperature of the liquid-vapor mixture will be -0.5°C (point O in Fig. 22.2) and the depressurization process corresponds to the vertical line between N and O. As this line does not reach the tangent to the saturation curve at the critical point, the conventional theory states that there will be no BLEVE strictly speaking although there will be a strong instantaneous vaporization and even an explosion, nucleation in all the liquid mass will not occur. 

Homogeneous nucleation of solid particles in solution is generally analyzed in terms of the classical theories developed for vapor-to-liquid and vapor-to-solid transformations which are described in detail by Christian (47). We shall briefly outline the main features of the classical theories for vapor-to-liquid transformation and then examine how they are applied to nucleation of solid particles from solution. In a supersaturated vapor consisting of atoms (or molecules), random thermal fluctuations give rise to local fluctuations in density and free energy of the system. Density fluctuations produce clusters of atoms referred to as embryos, which can grow by addition of atoms from the vapor phase. A range of embryo sizes will be present in the vapor with vapor pressures assumed to obey the Kelvin equation ... 

Figure 4 shows some calculations of Pcav(T)- The simplest approach is called classical nucleation theory (CNT) [38,39]. CNT considers the nucleation of a spherical bubble with an infinitely sharp interface between liquid and vapor Pcav is then directly related to the bulk surface tension of the liquid. At low... 

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