Drop Breakage Mechanisms

In an agitated suspension, the dispersed phase can be broken into small drops when its surface is disrupted. That disruption can be caused either by frictional forces (via viscous shear) or by inertial forces (via turbulence) [26]. The ratio of the external disrupting force to the interfacial tension force is often expressed as the Weber number, We. Drop deformation increases as We increases. When We exceeds a critical value, a drop will break into smaller drops. 

The fluids in agitated vessels are often turbulent. If the turbulence in local regions can be regarded as isotropic, a criterion for the drop breakage mechanism can be developed [26, 27]. In turbulent flow, random eddies are superimposed on the main flow. Eddy sizes are influenced by the location of the vessel walls and are 

In theories of local isotropy, it is assumed that the small eddies are statistically independent of each other. Velocity fluctuations are determined by the local rate of energy dissipation per unit mass of fluid (e) and by the kinematic viscosity (v). 

Kolmogorov [29], by dimensional reasoning, derived an expression [Eq. (1)] for the length of the smallest eddy (tj). 

Rushton et al. [30] suggested that local isotropy could exist when the Reynolds number is 10 . The macroscale of turbulence, L (approximately equal to the impeller diameter), is then much larger than the microscale of turbulence. Here, the Reynolds number Re) is defined by Eq. (2), where N is the impeller speed, D is the impeller diameter, and v is the kinematic viscosity of the dispersion. 

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Drop sizes depend on many factors that are discussed throughout this chapter. For any given system, drop sizes are never uniform rather, they exist in a continuous size spectrum. The large end of the drop size spectrum is controlled by agitation intensity, and the small end by the physics of drop breakage events. The DSD is sometimes bimodal or trimodal. Multimodal distributions are usually a result of multiple breakage mechanisms and unusual breakage patterns, such as those that result when viscous and/or viscoelastic drops are dispersed. Certain coalescence events can also lead to bimodal drop size distributions. 

Simple laminar shear or extension flow produces orderly dispersion since the flow field surrounding the drop is constant and continuous. In contrasL simple turbulent flows produce more random breakup events, due to the time-dependent nature of fluid-drop interactions. The effect of breakage mechanism on the resulting DSD is sometimes counterintuitive. 

In a more recent study, Marks (1998) observed the deformation and breakup of a drop in SSF that was exposed to a steady shear rate greater than Gmax-His results show that the breakage mechanism and breadth of the daughter DSD... 

Both authors argued that for dynamically similar breakage mechanisms, the equilibrium DSD should only depend on the ratio of disruptive (Xc) to cohesive (Xs and/or xj) forces acting on the drops. Thus, the individual DSDs could be collapsed to a single correlation by normalization with d 2- Defining X = d/d32, the volume probability density function becomes... 

Dispersion behavior becomes more complex for (id > 500 cP. The DSD broadens considerably and transitions to a lognormal distribution in volume, due to a shift in the breakage mechanism, resulting in the production of numerous small satellite drops. The reader is referred to the original work of Calabrese et al. (1986a). 

Blount, J. M. (1995). Mechanisms of drop breakage in dilute, agitated liquid-liquid systems, M.S. thesis. University of Maryland, College Park, MD. 

The No.7 blast furnace of WISCO has been used for about 5 years and it was mid-repaired in March, 2012. The top phenomena of worn hot air pipelines are brick drop, breakage, severely local wear and deformation, existing hidden danger, so the worn hot air pipelines were renewed. Investigations have been done on the stove damage and design [1,2]. In order to elarify the wear mechanism, the used refractories for hot air pipelines were sampled and analyzed, the properties and structures of worn materials and reused materials are relatively studied. 

Poor choice of FR additives can lead to excessive loss of PBT molecular weight upon processing, hence leading to impaired mechanical properties (usually seen as part melt-viscosity (MV) drop and, if severe, part breakage). In some cases, generation of acidic halide species can cause mold or electrical contact corrosion. 

In practice, this model is oversimplified since the exciting wake shedding is by no means harmonic and is itself coupled with the shape oscillations and since Eq. (7-30) is strictly valid only for small oscillations and stationary fluid particles. However, this simple model provides a conceptual basis to explain certain features of the oscillatory motion. For example, the period of oscillation, after an initial transient (El), becomes quite regular while the amplitude is highly irregular (E3, S4, S5). Beats have also been observed in drop oscillations (D4). If /w and are of equal magnitude, one would expect resonance to occur, and this is one proposed mechanism for breakage of drops and bubbles (Chapter 12). 

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