Bubble size distribution dissipation rate

FIGURE I IA.2 Predicted equilibrium bubble size distribution at three values of turbulent energy dissipation rates. Effect of energy dissipation rate on (a) group mass fraction and (b) total surface area. 

Various models for bubble breakage and coalescence rates are presented in the literature. These rates usually depend on physical properties, such as densities, viscosities and surface tension, and on turbulence properties, most commonly the turbulent kinetic energy dissipation rate. To calculate local bubble size distributions, also local physical properties and turbulence level should be used. This can be done via CFD (Alopaeus et al. 1999,2002 Keskinen and Majander 2000). 

In the case of droplets and bubbles, particle size and number density may respond to variations in shear or energy dissipation rate. Such variations are abundantly present in turbulent-stirred vessels. In fact, the explicit role of the revolving impeller is to produce small bubbles or drops, while in substantial parts of the vessel bubble or drop size may increase again due to locally lower turbulence levels. Particle size distributions and their spatial variations are therefore commonplace and unavoidable in industrial mixing equipment. This seriously limits the applicability of common Euler-Euler models exploiting just a single value for particle size. A way out is to adopt a multifluid or multiphase approach in which various particle size classes are distinguished, with mutual transition paths due to particle break-up and coalescence. Such models will be discussed further on. 

The primary task of modelling two-phase reaction systems is the estimation of the average diameter of droplets (bubbles and so on) of a dispersed phase and their size distribution in fast interface processes in diffuser-confusor devices. According to Kholmogorov s theory of isotropic turbulence, the specific kinetic energy of turbulence dissipation rates e are limiting in this case. 

In addition, Chandavimol et al. (1991a,b) have estimated the kinetic rate at which the bubbles go from initial size to the maximum equilibrium size as a function of energy dissipation. The rate of dispersion was found to be approximately proportional to energy dissipation rate. [See Figure 7-24 for a comparison of bubble breakup rate between vortex (HEV) and spiral (KMS type) static mixers.] In general, the equilibrium drop size is reached in a few pipe diameters. However, the drop size distribution is narrowed as the simultaneous processes of drop breakup and coalescence are continued, depending on the mixer design and fluid properties. See also Hesketh et al. (1987, 1991). 

---