Turbulence particle interaction with

Pain, Y, and Banerjee, S., Numerical simulation of particle interactions with wall turbulence. Phys. Fluids S(10). 2733 (1996a). 

The various collision mechanisms are compared in Fig. 7.7 which shows the collision frequency function for l- m particles interacting with particles of other sizes. Under conditions corresponding to turbulence in the open atmosphere ( j ss 5cm /sec- ), either Brownian motion or differential sedimentation plays a dominant role. Brownian motion controls for particles smaller than 1 jam. At lower altitudes in the atmosphere and in turbulent pipe flows, shear becomes important. 

After a complete structure disintegration, the disperse system behaves as a Newtonian fluid under the laminar flow conditions (Fig. IX-24, region IV). The viscosity r mm of such relatively concentrated system is higher than that of dispersion medium by the amount exceeding the one predicted by Einstein s equation. In this case the concentration of dispersed phase is sufficiently high, and the particles interact with each other. Further increase in the applied shear stress results in deviations from Newton s equation due to the appearance of turbulence. Early transition of the flow to turbulent behavior sometimes results in the absence of region IV (Fig. IX-24). 

In the same way that relative length scales of eddies and blobs affect the breakup of blobs, in multiphase flows the relative response times of particles and eddies determine how particles interact with eddies (Tang et al., 1992). Although we do not discuss this issue in detail, it is important to recognize two things (1) the relevant length scales for multiphase flows can be much more difficult to scale accurately because of the complicated interactions between turbulence and particles and (2) where tracer particles are used in experiments, the scales of motion that can be observed are a function of the particle size and characteristic response time. 

It is the author s conviction that in many (turbulent) dispersed multiphase flows—except probably in very dense multiphase flow systems—the origin of mesoscale structures is in the fluid—particle interaction, with a secondary role for particle-particle interaction (coUisions, coalescence, breakup). Clustering of particles is believed to be intimately connected with the chaotic dynamics of fluid accelerations, as particles converge toward each other where and when the divergence of the acceleration field is positive (Goto... 

This result makes it clear that particle stress is strongly dependent on the interaction between the particles and the interface, so that electrostatic and also hydrophobic and hydrophilic interactions with the phase boundary are particularly important. This means that the stress caused by gas sparging and also by boundary-layer flows, as opposed to reactors with free turbulent flow (reactors with impellers and baffles), may depend on the particle system and therefore applicability to other material systems is limited. 

In addition to bulk liquid turbulence effects, suspended particles maybe involved in collisions with one another or with solid surfaces within the vessel. This phenomenon has been extensively studied in micro-carrier cultures [60] and appears to be significant at high concentrations [61]. Rosenberg [69] and Meijer [72] applied the approach of Cherry and Papoutsakis [60] to the study of collision phenomena involving spherical plant cell aggregates of 190 and 100 pm, respectively. In both cases it was concluded that for typical biomass concentrations, particle-particle interactions were of less significance than particle-impeller collisions. 

In addition, DNS of turbulent flow in a periodic box offer interesting opportunities for studying in a fully resolved mode the intimate details of the flow field, its interaction with particles and the mutual interaction between particles (including particle-particle collisions and coalescence). Such simulations may yield new insights see, e.g., Ten Cate et al. (2004) and Derksen (2006b). The same can be said about our understanding of particle-turbulence interactions in wall-bounded flows this has increased due to Portela and Oliemans (2003) exploiting both DNS and LES and due to Ten Cate et al. (2004). 

Authors efforts in this part of the work have been concentrated on developing turbulence closures for the statistical description of two-phase turbulent flows. The primary emphasis is on development of models which are more rigorous, but can be more easily employed. The main subjects of the modeling are the Reynolds stresses (in both phases), the cross-correlation between the velocities of the two phases, and the turbulent fluxes of the void fraction. Transport of an incompressible fluid (the carrier gas) laden with monosize particles (the dispersed phase) is considered. The Stokes drag relation is used for phase interactions and there is no mass transfer between the two phases. The particle-particle interactions are neglected the dispersed phase viscosity and pressure do not appear in the particle momentum equation. 

Hydrodynamic interactions with particles may certainly play a role in clustering. Horio and Clift [30] noted that particle clusters, a group of loosely held together particles, are the result of hydrodynamic effects. Squires and Eaton [31] proposed that clustering resulted from turbulence modification from an isotropic turbulent... 

Dirty Flames. At this point one could well ask so what happens in real combustors which are turbulent, soot and particle laden and are highly luminous By the end of this morning s session you should be convinced that CARS can be applied to these systems. I don t want to steal all of Alan Eckbreth s slides so I will show only two more. Figure 13 shows the BOXCARS spectrum of N- with a computer fit to a temperature of 2000°K in a laminar sooting propane diffusion flame (12). Figure 14 shows the vertical temperature profile for this same flame system. It should be pointed out that care must be taken under these conditions to account for the laser interaction with carbon in the flame which can generate laser induced Swan Band emission from C2-... 

In case 3 the relative size of the particles (with respect to the computational cells) is large enough that they contain many hundreds or even thousands of computational cells. It should be noted that the geometry of the particles is not exactly represented by the computational mesh and special, approximate techniques (i.e., body force methods) have to be used to satisfy the appropriate boundary conditions for the continuous phase at the particle surface (see Pan and Banerjee, 1996b). Despite this approximate method, the empirically known dependence of the drag coefficient versus Reynolds number for an isolated sphere could be correctly reproduced using the body force method. Although these computations are at present limited to a relatively low number of particles they clearly have their utility because they can provide detailed information on fluid-particle interaction phenomena (i.e., wake interactions) in turbulent flows. 

Bolio, E.J., Yasuna, J.A. and Sinclair, J.L. (1995), Dilute turbulent gas-solid flow in riser with particle-particle interactions, AIChE J., 41, 1375. 

Ocone R, Sundaresan S, Jackson R (1993) Gas-Particle Flow in a Duct of Arbitrary Inclination with Particle-Particle Interactions. AIChE J 39(8) 1261-1271 Patil DJ, Smit J, van Sint Annaland M, Kuipers JAM (2006) Wall-to bed heat transfer in gas-sohd bubbling fluidized beds. AIChE J 52(l) 58-74 Peirano E, Leckner B (1998) Fundamentals of Turbulent Gas-Solid Flows Applied to Circulating Fluidized Bed Combustion. Proc Energy Combust Sci 24 259-296... 

Microscale fluid turbulence is, by deflnition, present only when the continuous fluid phase is present. The coefficients Bpv describe the interaction of the particle phase with the continuous phase. In contrast, Bpvf models rapid fluctuations in the fluid velocity seen by the particle that are not included in the mesoscale drag term Ap. In the mesoscale particle momentum balance, the term that generates Bpv will depend on the fluid-phase mass density and, hence, will be null when the fluid material density (pf) is null. In any case, Bpv models momentum transfer to/from the particle phase in fluid-particle systems for which the total momentum is conserved (see discussion leading to Eq. (5.17)). 

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