Coalescence collisions

Keywords Binary drop colUsions Bouncing Coalescence Collision model Crossing separation Gaseous environment Immiscible liquids Lattice-Boltzmann simulation Miscible liquids Navier-Stokes simulation Reflexive separation Satellite droplets Spray flow simulation SPH simulation Stretching separation... 

F. Mashayek, N. Ashgriz, W. J. Minkowycz, B. Shotorban Coalescence collision of liquid drops, Int. J. Heat Mass Transfer 46, 77-89 (2003). 

Keywords Coalescence Collisions Combustion Entrainment Included angle Interactions Nozzles Interacting sprays... 

We have thus far considered only those factors which can minimise any tendency of antifoam drops to cream, collide and coalesce. Collision may however result in the formation of only loosely bound flocculated drops. This does not of course represent a desirable product attribute even though such drops may be readily redispo sed by simply shaking the containing vessel. Moreover if those drops should subsequently coalesce, then redispersal by such means may be extremely difficult The concomitant loss of antifoam action may be permanent This situation is obviously to be avoided if possible by selection of appropriate stabilizing surfactants or polymers. However, little is apparently known at all about the coalescence of polyorganosiloxane-hydrophobed silica drops in the types of environment provided by detergent liquids. 

Passandideh-Fard, M., Roohi, E. (2006). Coalescence collision of two droplets Bubble entrapment and the effects of important parameters. In 14th Annual (International) Mechanical Engineering Conference, Isfahan. 

The first form of aerosol modifier is a spray chamber. It is designed to produce turbulent flow in the argon carrier gas and to give time for the larger droplets to coalesce by collision. The result of coalescence, gravity, and turbulence is to deposit the larger droplets onto the walls of the spray chamber, from where the deposited liquid drains away. Since this liquid is all analyte solution, clearly some sample is wasted. Thus when sensitivity of analysis is an issue, it may be necessary to recycle this drained-off liquid back through the nebulizer. 

Modeling and Simulation subsection.) It is necessary to determine both the mechanism and kernels which describe growth. For fine powders within the noninertial regime of growth, all collisions result in successful coalescence provided binder is present. Coalescence occurs via a random, size-independent kernel which is only a func tion of liquid loading, or... 

The coalescence-redispersion (CRD) model was originally proposed by Curl (1963). It is based on imagining a chemical reactor as a number population of droplets that behave as individual batch reactors. These droplets coalesce (mix) in pairs at random, homogenize their concentration and redisperse. The mixing parameter in this model is the average number of collisions that a droplet undergoes. 

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. 

Following growth by condensation, droplets grow further by collision coalescence (colliding mainly due to different fall speeds). Some small... 

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). 

This response time should be compared to the turbulent eddy lifetime to estimate whether the drops will follow the turbulent flow. The timescale for the large turbulent eddies can be estimated from the turbulent kinetic energy k and the rate of dissipation e, Xc = 30-50 ms, for most chemical reactors. The Stokes number is an estimation of the effect of external flow on the particle movement, St = r /tc. If the Stokes number is above 1, the particles will have some random movement that increases the probability for coalescence. If St 1, the drops move with the turbulent eddies, and the rates of collisions and coalescence are very small. Coalescence will mainly be seen in shear layers at a high volume fraction of the dispersed phase. 

As dispersion proceeds drops come into close contact with each other and may coalesce. Coalescence is commonly divided into three sequential steps (Chesters, 1991) collision or close approach of two droplets, drainage of the liquid between the two drops, and rupture of the film (see Fig. 26). 

Smaller drops are more likely to coalesce after collision than larger... 

Kapur and Fuerstenau (K6) have presented a discrete size model for the growth of the agglomerates by the random coalescence mechanism, which invariably predominates in the nuclei and transition growth regions. The basic postulates of their model are that the granules are well mixed and the collision frequency and the probability of coalescence are independent of size. The concentration of the pellets is more or less fixed by the packing... 

It is instructive to simplify the above picture somewhat and consider the coalescence or sticking of two particles schematically shown in Fig. 13. One can assume that due to shear forces in the mixer, a fluidized bed in the present case, the two particles posses a relative velocity U0 which ensures collision at some point on their trajectory and possible sticking under appropriate conditions. It is essential that some binder be present at the point of contact, as depicted in the figure. From this simplified picture, allmechanisms... 

Conditions of Coalescence. The outcome of the collision of two binder-covered particles is determined by the ratio of the initial kinetic energy of the system and the energy dissipated in the liquid bridge and in the particles. This can be expressed analytically by the definition of a so called Stokes number, St... 

Vaux (1978), Ulerich et al. (1980) and Vaux and Schruben (1983) proposed a mechanical model of bubble-induced attrition based on the kinetic energy of particles agitated by the bubble motion. Since the bubble velocity increases with bed height due to bubble coalescence, the collision force between particles increases with bed height as well. The authors conclude that the rate of bubble-induced attrition, Rbub, is then proportional to the product of excess gas velocity and bed mass or bed height, respectively,... 

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