Recirculation patterns

Fig. 11. Computer-simulated recirculating patterns in a mixing tank with full baffles (a) elevation view shows circulation patterns generated by turbine blades (b) plane view shows the effect of the baffle on the radial velocity vectors above the turbine blades.

Large tanks tend to develop a recirculation pattern from the impeller through the tank back to the impeller. This results in a behavior similar to that for a number of tanks in a series. The net result is that the mean circulation time is increased over what woiild be pre-dic ted from the impeller pumping capacity. This also increases the standard deviation of the circulation times around the mean. 

There are three types of mixing flow patterns that are markedly different. The so-called axial-flow turbines (Fig. 18-3) actually give a flow coming off the impeller of approximately 45°, and therefore have a recirculation pattern coming back into the impeller at the hub region of the blades. This flow pattern exists to an approximate Reynolds number of 200 to 600 and then becomes radial as the Reynolds number decreases. Both the RlOO and A200 impellers normally require four baffles for an effective flow pattern. These baffles typically are V12 of the tank diameter and width. 

After combustion, the rich burning mixture leaves the combustion zone and flows between the rows of air jets entering the liner. Each jet entrains air and burning fuel and carries it toward the combustor axis, forming torroidal recirculation patterns around each jet that result in intensive turbulence and mixing throughout the combustor. 

The velocity profile is uniform across the entire width of the channel if the channel is open at the electrodes, as is most often the case. However, if the electric field is applied across a closed channel (or a backpressure exists that just counters that produced by the pump), a recirculation pattern forms in which fluid along the center of the channel moves in a direction opposite to that at the walls further, the velocity along the centerline of the channel is 50% of that at the walls (Fig. 11.32a, see Plate 12 for color version). Figure 11.32b (see Plate 12 for color version) illustrates an electric field generating a net force on the fluid near the interface of the fluid/solid boundary, where a small separation of charge occurs due to the equilibrium between adsorption and desorption of ions. The charge region from excess cations localized near the interface by coulombic... 

Recirculation with Throughflow. For relatively rapid recirculation compared to throughflow, the system as a whole acts as one large stirred tank hence, the observed tracer signal is simply the superposition of the recirculation pattern and the exponential decay of an ideal stirred tank. This is shown in Fig. 14.6 where Cq is the concentration of tracer if it is evenly distributed in the system. 

Thus far, we have considered the reaction engineering of molecules in containers that conserve mass. The same equations describe the populations of living systems in their environment, a container operating in batch or flow mode with inputs, outputs, disturbances, catalysts, internal recirculation patterns, and even diffusion (svvirtirtiirig, walking, flying) (Figure 8-19). 

Figure 11.19 presents the pressure distribution along the x-axis for a Newtonian solution using several bank-to-nip ratios. The solutions are presented with the analytical predictions using McKelvey s lubrication approximation model presented in Chapter 6. The graph shows that the two solutions are in good agreement. Fig. 11.20 presents a sample velocity field for the Newtonian case with a bank-to-nip ratio of 10. As can be seen, the velocities look plausible and present the recirculation pattern predicted by McKelvey s lubrication approximation model and seen in experimental work done in the past [18]. 

The separation-layer technique benefits from the unique feature of micro mixers, such as to operate in a laminar flow regime [135], By the absence of convective recirculation patterns, at least close to the inlet, the separation layer remains as a barrier between the solution to be mixed, as long as it is not passed by molecules owing to diffusive transport. 

The continuous change of flow direction in zig-zag channels can induce secondary flow patterns at sufficiently high Re, which besides diffusion can act as a mixing mechanism. By means of recirculation patterns, material is transported transverse to the flow direction and improves the mixing. 

The plot shown in Fig. 45 is useful in understanding the flow properties of bubble-flow equipment. Also, it is quite probable that the Davidson and Harrison relation (D4, PI 1) with (3 = 1 is for a fluidized bed with bulk recirculation, although the recirculation pattern may be different. 

Tomiyama et al, 1998). The fully contaminated bubble behaves as a solid particle in fact, the accumulation of impurities at the gas-liquid interface makes the interface rigid. In contrast, in pure systems the absence of impurities allows the formation of a recirculation pattern inside the bubble, which decreases the viscous drag. For pure systems and for an isolated bubble, Hadamard (1911) proposed the following relationship ... 

Evidently, mixing efficiency improves strongly with miniaturization of the mixing elements for the caterpillar micromixer [27]. For all three mixers tested, mixing efficiency improves with increasing flow rate, which is due to more intense recirculation patterns and thus interfacial stretching. The slope is steeper for the smaller caterpillar micromixers, i.e., the 800-pm device shows a more pronounced increase of mixing efficiency with flow rate. 

Fig. 15 Mixing in microdroplets flowing in a microcharmel (a) Recirculation flow generated in a straight channel, (b, c) Mixing patterns generated in winding channels that causes (b) stretch, fold, and reorientation of the fluids interface and (c) asymmetrical recirculation patterns in the droplet halves, (d) Experimental results showing the chaotic advection thus generated in microdroplets (Adapted from [151] with permission)...

FIGRUE 9.9 Solids mean dynamic behavior (circumferentially averaged) in a cylindrical fluidized bed for 500-pm glass particles at ujumf = 2 (a) recirculation pattern, (b) mean velocity vector field, and (c) density distribution... 

A very complex solids flow pattern will result when solid obstacles exist in the fluidized bed. The solids recirculation pattern in a cylindrical bed with a single sphere was presented by Lin, Chen, and Chao (1985), and in a 2-D bed with a single and multiple cylinders by Ai (1991). It was demonstrated that large obstacles would not only affect the local solids velocity, but also the global solids circulation patterns. 

Figures 7.71 and 7.72 are plots of NO, (NO -1- NO2) af locafions before and in fhe middle of the convection section at various depths. The data clearly show that NO, varied by a maximum of 11% over a disfance of 46 cm (18 in.). Nofice fhaf fhe NO appeared to be lower af a locafion before fhe convecfion section than at a location in the middle of fhe convection section, for all depfhs tested. If is unclear why this occurs because the total NO, (NO -f NO2) should be conserved between these two locations. Perhaps the recirculation pattern in the upper corner of fhe firebox creates this anomaly. At a probe depth of 61 cm (24 in.), the average NO, in the stack appears to be in the center of the data scatter for NO, collected af various locations in the convection section.

The process burner test facility will have a limited number of types of furnaces available. It will never have an identical furnace into which the burners will be installed. The flue gas recirculation patterns and heat sinks will be significantly different. It is often the case that the burner will still behave slightly differently in the field than in the test furnace due to these reasons and others. Due to these reasons it can be a challenge to extrapolate the test results to the actual furnace conditions and predict how the burner performance may change as a function of these differences. 

Usually, a uniform heat flux is desired to produce better yields. Most industrial processes take advantage of flue gas to provide heat flux distribution inside the furnace. Note that in oxy-combustion, the flue gas volume is significantly reduced (by more than three times that of air-combustion). Hence, considerable efforts are needed to properly manage the flue gas recirculation pattern within the furnace and heat flux. Leroux et al. 

As expected, the highest average glass surface temperatures measured were at locations 4 and 5 in the region of the spring zone. These peak temperatures were due to the combined effects of the recirculation pattern in... 

The thrombus that formed on the rotating rod was usually quite uniform in thickness over the apical portion, as seen in Figure 3. Measurements of the platelet and fibrinogen content per unit surface area of the rod confirmed this pattern (Figure 4) (5). The increase in deposit on the last fourth of the rod is attributed to recirculation patterns in flow around the exit port of the chamber. Because of this effect, only data from the apical three-fourths of the rods are reported and discussed. 

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