Nucleation flow-induced

Flow induced nucleation A Control volume thermodynamics approach... 

Here, we will formulate a novel model of flow induced nucleation by using control volume thermodynamics. The general idea is very simple and goes through the lines indicated along this chapter, that is, by assuming that nonequilibrium equations of state can be formulated in a way consistent with the laws of thermodynamics when a flow is imposed on the system. 

This intensified flow will cause an increased flow-induced nucleation and so on. 

Equation (18-36) is the general expression for impeller-induced nucleation. In a fixed-geometry system in which only the speed of the circulating pump is changed and in which the flow is roughly proportional to the pump speed, Eq. (18-36) may be satisfactorily replaced with... 

When the flow through the CNC was exhausted outside of the laboratory, we observed particle formation at higher SOp concentrations as expected (Table II). To prove that the radical scavenger effect is reproducible, another radical scavenger (92 ppb nitric oxide) was used in the presence of 110 ppb SOp concentration and 2% humidity, and the supression in particle formation was observed. Another possible mechanism that supressed the particle formation is that more neutralization of polonium ions occurred at the higher humidities and thus ion-induced nucleation would be suppressed. 

Metastability with respect to ice crystal formation was not apparent in this freezer, probably because particulate matter in the salt solution induced nucleation. Nevertheless, more ice may be produced for a given pressure difference and surface when the slurry is recycled. The ice in the flowing slurry presents more surface and precludes the necessity of high driving forces to induce nucleation. Also, the size of the ice crystals is increased by recycle of slurry. 

The objective is to reduce volatiles to below 50-100-ppm levels. In most devolatilization equipment, the solution is exposed to a vacuum, the level of which sets the thermodynamic upper limit of separation. The vacuum is generally high enough to superheat the solution and foam it. Foaming is essentially a boiling mechanism. In this case, the mechanism involves a series of steps creation of a vapor phase by nucleation, bubble growth, bubble coalescence and breakup, and bubble rupture. At a very low concentration of volatiles, foaming may not take place, and removal of volatiles would proceed via a diffusion-controlled mechanism to a liquid-vapor macroscopic interface enhanced by laminar flow-induced repeated surface renewals, which can also cause entrapment of vapor bubbles. 

All faces can be grown dislocation-free, and the current below the critical overvoltage has been found to be zero. A pulse excitation can induce nucleation, preferably on the front (100) face, followed by a current flow. As expected, the current-time curve has a veiy specific form in this case (Fig 5.11b). Tlie four cusps of the curve indicate the moments where the growing monolayer reach consecutively the four edges of the rectangular front face. 

Orientation-induced nucleation, caused by alignment of macromolecules, e.g. in exten-sional flow field. 

Avrami exponent of flow induced crystallization is closely associated with the aspect ratio of nucleating agent. ... 

The increasing number of activated nuclei is the first noticeable effect induced by flow. The effect of flow on growth rate was also observed (Monasse 1995), but it is usually less important and can be neglected (Koscher and Fulchiron 2002), while the effect of flow on nucleation must be considered. 

Other authors, for example, Coppola et al. (2001) and Zheng and Kennedy (2004), considered the flow-induced change in free energy as a driving force. The starting point to construct a formulation is based on theories of Lauritzen and Hoffman (1960) and Ziabicki (1996) for the quiescent nucleation rate, which is then extended to include the flow-induced change of free energy. Coppola et al. 

More recently, Leung et al. (2010) and Wang et al. (2010) demonstrated that the melt flow induced by the expansion of previously nucleated bubbles could generate new bubbles around them, as shown in Figure 3.3. The authors speculated that... 

The previous discussion relates to the initial state of the flow-induced structures, which form the fingerprint for the final structures enhanced point nucleation creates a more fine-grained structure than the equivalent quiescent structure, mild stretch will lead to internal orientation of the spherulites while higher stretch levels will create fibrils (shish) on which lamellae (kebabs) will grow. The initial nucleated structure is, therefore, of utmost importance for the evolving post-shearing structure. 

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