The turbulent diffusion mechanism of particles in a round air jet are studied both theoretically and experimentally, with particular attention to the relative velocity between particles and fluid, overshooting effect of particles, and the distributions of fluid properties in space. The results indicate that the particle diffusivity decreases with the increase of the particle inertia. In general, the turbulent diffusivity of particles in an air jet is smaller than that of fluid scalar quantities. The particle inertia and the fluid large eddies, which are expressed by the Stokes number and the integral scale, respectively, play an important role in the transport mechanism of particles.

We have numerically simulated the particle and gas flows in the raceway region in an actual blast furnace of which dimension is the same as that of the commercial blast furnace using Distinct Element Method for the computation of the multi-body interaction among coke particles, Hard Sphere Model for two body interaction of powder particles based on Direct Simulation of Monte-Carlo Method and Finite Difference Method for the numerical analysis of Navier-Stokes equations with the interaction terms between gas and particles for the gas flows. In the simulations we have taken the existence of softening melting zones into account. The present calculation results indicate the raceway pattern, its fluctuations with various periods. The results also indicate the velocity distributions of coke, powder and gas, and packing ratio distributions of these particles. The dynamical characteristics fluctuate and are unstable. The highly packed coke and powder particle layers are formed in the lower core and in the lower wall regions under the tuyere due to the air and these particle flows.The high air velocity region appears in the layer between the softening melting zones and the highly packed furnace lower core region, and the unstable high air velocity region is produced near the furnace wall and on the raceway by the existence and the disappearance of softening melting zones. The coke and powder particles and the softening melting zones would yield the unstable state in the furnace.The powder particles circularly spread and make the circularly multi-layered powder particle clusters caused by the particle collisions and the breakage of clusters. Some of the powder particles flow upward and others are packed in the furnace lower core region by the particle and the air flows.Our calculated results present an unusual phenomenon example in the blast furnace, that is, the unusual high air velocity wide region touched to the furnace wall is formed due to the effect of the softening melting zones and the accumulation of small coke particles in the furnace center region.

A theoretical equation is derived for the collision rate of aerosol particles in a homogeneous and isotropic turbulent system. This equation takes into account the relative velocity between fluid and particles. The calculated results indicate that the relative velocity between fluid and particles is the main factor in the turbulent coagulation (agglomeration, coalescence) of unequally sized particles in an air flow. This hold true, even when the particle sizes are less than 1 micron. For particles of equal radii the coagulation coefficient reaches its minimum value, because the effect of motion relative to the fluid now becomes zero and only the spatial variation of turbulent motion remains to cause collisions between the particles. For particles following a fluid motion completely, as in a water stream, the equation for the collision rate reduces to the Saffman and Turner equation. SHlNlCHl YUU SCOPETurbulence causes collisions between neighboring particles by increasing their random motion. These collisions then result in agglomeration of particles or coalescence of droplets (refered to subsequently as "coagulation of particles"). Since most flows in which particles are suspended are turbulent, it is important to understand this process and to determine the effect that it has on particle coagulation and on the change with time of particle concentration and size distribution. An approach to this is to first determine the collision rate. Then the coagulation process can be determined by the numerical calculation of the population balance equation.Saffman and Turner (1956) derived the collision rate equation in an isotropic turbulent flow by considering the spatial variations of turbulent velocity. Their result indicates that the collision rate increases proportionally to the square root of the viscous dissipation rate of the fluid. Levich (1962) and Beal(l972) obtained a similar result of Saffman and Turner, considering the particle collision as the deposition of one particle onto another particle due to the diffusion process. However, these studies did not have completely taken into account the relative velocity between fluid and particle. Even a slight relative velocity between fluid and particles affects the collision rate, and this should not be neglected even for particles under 1 pm in size.The objective of this study is to describe theoretically the mechanism of the coagulation of small particles in a turbulent flow. To do this a theoretical equation for the collision rate of small particles in a homogeneous and isotropic turbulence is derived as follows. First, the variance of particle relative velocities is expressed as in terms of the turbulent intensity of each particle, the variance of the particle velocity gradient, and their correlation. These turbulent characteristics of the particles are then converted into those of the fluid by means of the equation of particle motion and by the empirical formula for the Lagrangian fluid velocity correlation. Finally, the equation of collision rate is obtained a...

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