Breakup collision

Chiappetta, L. M. 2001. Tests of a comprehensive model for ciirblast atomizers including the effects of spray initialization (primary atomization), secondary breakup, collision and coalescence. UTRC Report. 

Equation (17) indicates that the entire distribution may be determined if one parameter, av, is known as a function of the physical properties of the system and the operating variables. It is constant for a particular system under constant operating conditions. This equation has been checked in a batch system of hydrosols coagulating in Brownian motion, where a changes with time due to coalescence and breakup of particles, and in a liquid-liquid dispersion, in which av is not a function of time (B4, G5). The agreement in both cases is good. The deviation in Fig. 2 probably results from the distortion of the bubbles from spherical shape and a departure from random collisions, coalescence, and breakup of bubbles. 

The cameras are usually used in two different modes, front light and backlight. Standard images using the front light technique are very useful in tracking particle movements, collisions, breakup, and coalescence (Figure 15.2). 

Formulations for SMD of secondary droplets have also been derived by other researchers, for example, O Rourke and Amsden)3101 and Reitz.[316] O Rourke and Amsden[310] used the % -square distri-bution[317] for determining size distribution of the secondary droplets. They speculated that a breakup process may result in a distribution of droplet sizes because many modes are excited by aerodynamic interactions with the surrounding gas. Each mode may produce droplets of different sizes. In addition, during the breakup process, there might be collisions and coalescences of the secondary droplets, giving rise to collisional broadening of the size distribution. 

Detailed modeling study of practical sprays has a fairly short history due to the complexity of the physical processes involved. As reviewed by O Rourke and Amsden, 3l() two primary approaches have been developed and applied to modeling of physical phenomena in sprays (a) spray equation approach and (b) stochastic particle approach. The first step toward modeling sprays was taken when a statistical formulation was proposed for spray analysis. 541 Even with this simplification, however, the mathematical problem was formidable and could be analyzed only when very restrictive assumptions were made. This is because the statistical formulation required the solution of the spray equation determining the evolution of the probability distribution function of droplet locations, sizes, velocities, and temperatures. The spray equation resembles the Boltzmann equation of gas dynamics[542] but has more independent variables and more complex terms on its right-hand side representing the effects of nucleations, collisions, and breakups of droplets. 

An attempt has been made by Tsouris and Tavlarides[5611 to improve previous models for breakup and coalescence of droplets in turbulent dispersions based on existing frameworks and recent advances. In both the breakup and coalescence models, two-step mecha-nisms were considered. A droplet breakup function was introduced as a product of droplet-eddy collision frequency and breakup efficiency that reflect the energetics of turbulent liquid-liquid dispersions. Similarly, a coalescencefunction was defined as a product of droplet-droplet collision frequency and coalescence efficiency. The existing coalescence efficiency model was modified to account for the effects of film drainage on droplets with partially mobile interfaces. A probability density function for secondary droplets was also proposed on the basis of the energy requirements for the formation of secondary droplets. These models eliminated several inconsistencies in previous studies, and are applicable to dense dispersions. 

Raindrop breakup also occurs when drops collide, and this has been studied by a number of workers [see (M8)]. It is probable that the collision mode of... 

Droplet Dispersion. The primary feature of the dispersed flow regime is that the spray contains generally spherical droplets. In most practical sprays, the volume fraction of the liquid droplets in the dispersed region is relatively small compared with the continuous gas phase. Depending on the gas-phase conditions, liquid droplets can encounter acceleration, deceleration, collision, coalescence, evaporation, and secondary breakup during their evolution. Through droplet and gas-phase interaction, turbulence plays a significant role in the redistribution of droplets and spray characteristics. 

When a gas stream is introduced into a turbulent liquid flow in a motionless mixer, the gas is broken up into bubbles. The breakup is due mainly to the turbulent shear force of the liquid but also partly to the collision between gas and the leading edge of an element. 

Note that the preceding equation is for ideal cases, in which the particles are monodis-persed, spherical, and totally elastic, and the contact surface is clean. In practice, the particles are usually nonspherical and polydispersed the collision could have involved some heat loss, plastic deformation, or even breakup and the contact surface may have impurities or contaminants. In these cases, a correction factor tj is introduced to account for the effects of these nonideal factors. The applicable form of the electric current through the ball probe is, thus, given by... 

The mechanical erosion of a solid surface such as a pipe wall in a gas—solid flow is characterized by the loss of solid material from the solid surface due to particle impacts. The collisions of the particles either with other particles or with a solid wall may lead to particle breakup, known as particle attrition. Pipe erosion and particle attrition are major concerns in the design of a gas-solid system and during the operation of such a system. The wear of turbine blades or pipe elbows due to the directional impact of dust or granular materials, the wear of mechanical sieves by the random impact of solids, and the wear of immersed pipes in a fluidized bed by both directional and random impacts are examples of the erosion phenomenon in industrial systems. The surface wear associated with the erosion phenomenon of a gas-solid flow has been exploited to provide beneficial industrial applications such as abrasive guns, as well. 

Figure 21 Schematic illustration of (a) countercurrent flows in conventional macroscale devices, (b) droplet generation because of breakup due to high shear stress in an ordinary microchannel, and (c) collision of two phases in an ordinary microchannel. Abbreviation Aq., aqueous phase, Org., organic phase (Aota et al, 2007c).

---