Mixing impellers Turbulence

Entrainment is an important element in the mixing operation and involves incorporation of low velocity fluid into the mass of the fluid stream or jet issuing from a source such as a mixing impeller. The axial flow from a propeller under proper physical conditions serves as a circular cross-section jet to produce mixing by turbulence and entrainment. The flat-blade turbine issues a jet for entrainment at the top and bottom, areas of the ring [2]. It is significant to estimate the relative amount of liquid involved due to entrainment, as this helps to describe the effectiveness of the operation. 

Dispersions may be classified into two types, based upon size range of the droplets formed. Turbulence creators (mixing impellers, mixing valves, eductors, orifice plates) will produce fine emulsions of micron-size droplets. Nozzles, perforated plates, bubble caps, tower packings, etc., can form discrete drops of relatively large size which will quickly settle through the continuous phase. 

The principal feature of turbulent flows is the presence of eddies which are large compared with the molecular scale and which aid the mixing process (turbulent dispersion). This mixing mechanism in liquids is often rapid compared with the other processes of bulk flow and molecular diffusion. The eddies vary in size, having a maximum scale, L, which is of the order of the scale of the equipment (i.e. impeller or baffles) down to a minimum value, /, which, according to Kolmogoroff, for isotropic turbulence depends only upon the power input per unit mass to the system and the kinematic viscosity of the liquid ... 

The above relationship is based on the assumptions that the ratio of local energy dissipation rate and its average value for the entire vessel is the same for different impellers having the same DIT ratio. Supporting evidence for Equation 7B.10 has come from the results of Kramers et al. (1953), Norwood and Metzner (1960), Prochazka and Landau (1961), and Holmes et al. (1964). Norwood and Metzner s study clearly showed that for Re > lO , mixing by turbulent diffusion can be assumed to be predominant such that Nx0 = constant. 

The power number depends on impeller type and mixing Reynolds number. Figure 5 shows this relationship for six commonly used impellers. Similar plots for other impellers can be found in the Hterature. The functionality between and Re can be described as cc Re in laminar regime and depends on p. N in turbulent regime is constant and independent of ]1. 

As in the case of turbulent flow, then, smaU-diameter impellers (D < Df/3) are useful for (1) rapid mixing of diyparticles into hquids,... 

Impeller Reynolds Number a dimensionless number used to characterize the flow regime of a mixing system and which is given by the relation Re = pNDV/r where p = fluid density, N = impeller rotational speed, D = impeller diameter, and /r = fluid viscosity. The flow is normally laminar for Re <10, and turbulent for Re >3000. 

Mixing is accomplished by the rotating action of an impeller in the continuous fluid. This action shears the fluid, setting up eddies w hich move through the body of the system. In general the fluid motion involves (a) the mass of the fluid over large distances and (b) the small scale eddy motion or turbulence which moves the fluid over short distances [21, 15]. 

In general, below a Reynolds number of 50, all impellers give viscous flow mixing between 50 and 1,000 the pattern is in the transition range and from Nrp above 1000 the action is turbulent. 

Because the most common impeller type is the turbine, most scale-up published studies have been devoted to that unit. Almost all scale-up situations require duplication of process results from the initial scale to the second scaled unit. Therefore, this is the objective of the outline to follow, from Reference [32]. The dynamic response is used as a reference for agitation/mixer behavior for a defined set of process results. For turbulent mixing, kinematic similarity occurs with geometric similarity, meaning fixed ratios exist between corresponding velocities. 

Generally as system size increases the impeller flow per fixed power input will increase faster than will the turbulence of the system. Even the same degree of turbulence is no insurance that the rates of mixing, mass transfer. 

High turbulence is required for efficient mixing this is created by the vortex field which forms behind the blades. For all the gas to flow through this region it must enter the vessel close to and preferably underneath the disk hence it is recommended that spargers should always be nearer, about a distance of DJ2 below the agitator, where D( is the impeller diameter. 

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