Agitation I91
3.1 Fluidfoil Impellers
The introduction of fluidfoil impellers, as shown in Fig. 9a through 9f,
give a wide variety of mixing conditions suitable for high flow and low fluid
shear rates. Fluidfoil impellers use the principles developed in airfoil work
in wind tunnels for aircraft. Figure 10a shows what is desirable, which is no
form separation of the fluid, and maximum lift and drag coefficients, which
is what one is trying to achieve with the fluidfoil impellers. Figure 10b shows
what happens when the angle and the shape is such that there is a separation
ofthe fluid from the airfoil body. The A3 10 impeller (Fig. Sa) was introduced
for primarily low viscosity fluids and, as can be seen, has a very low ratio of
total blade surface area compared to an inscribing circle which is shown in
Fig. 11. When the fluid viscosities are higher, the A3 12 impeller is used
(shown in Fig. 9b) which is particularly useful in fibrous materials.
To give a more responsive action in higher viscosities, the A320 is
available which works well in the transition area of Reynolds numbers. When
gas-liquid processes are used, the A3 15 (Figure 9d) has a still higher solidity
ratio. It is particularly useful in aerobic fermentation processes. Impellers in
Figs. 9(a-d) are formed from flat metal stock.
To complete the current picture, when composite materials are used,
the airfoil can be shaped in any way that is desirable. The A6000 (Fig. 9e)
illustrates that particular impeller type. The use of proplets on the end of the
blades increases flow about 10% over not having them. An impeller which
is able to operate effectively in both the turbulent and transitional Reynolds
numbers is the A4 10 (Fig. 90 which has a very marked increase in twist angle
ofthe blade. This gives it a more effective performance in the higher viscosity
fluids encountered in mixers up to about 3 kW.
One characteristic of these fluidfoil impellers is that they discharge a
stream that is almost completely axial flow and they have a very uniform
velocity across the discharge plane of the impeller. However, there is a
tendency for these impellers to short-circuit the fluid to a relatively low
distance above the impeller. Very careful consideration of the coverage over
the impeller is important. If the impeller can be placed one to two impeller
diameters off bottom, which means that mixing is not provided at low levels
during draw off, these impellers offer an excellent flow pattern as well as
considerable economies in shaft design.
To look at these impellers in a different way, three impellers have been
compared at equal total-pumping capacity. Figure 12 gives the output
velocity as a function of time on a strip chart. As can be seen in Fig. 12 the
fluidfoil impeller type (A3 10) has a very low velocity fluctuation and uses
Fermentation and Biochemical Engineering Handbook192
much less power than the other two impellers. For the same flow, the A200
impeller has a higher turbulent fluctuation value. The Rl 00 impeller has still
higher power consumption at the same diameter than the other two impellers,
and has a much more intense level of microscale turbulence.
The fluidfoil impellers are often called "high efficiency impellers", but
that is true only in terms offlow, and makes the assumption that flow is the
main measure of mixing results. Flow is one measure, and in at least half of
the mixing applications is a good measure of the performance that could be
expected in a process. These impellers are low in efficiency in providing shear
rates-either of the macro scale or the micro scale.
The use of computer generated solutions to problems and computa-
tional fluid dynamics is also another approach of comparing impellers and
process results. There are software packages available. It is very helpful to
have data obtained from a laser velocity meter on the fluid mechanics of the
impeller flow and other characteristics to put in the boundary conditions for
these computer programs.
Figure 9a. A310 fluidfoil impeller.
Agitation
193
Figure 9b. A312 Fluidfoil impeller.
Figure 9c. A320 fluidfoil impeller.
Fermentation and Biochemical Engineering Handbook194
Figure 9d. A315 fluidfoil impeller
Figure ge. A6000 fluidfoil impeller made of composite materials
Agitation
195
Figure 9(. A410 fluidfoil impeller made from composite materials with high twist angle
ratio between tip and hub.
Figure lOa. Typical flow around airfoil positioned for maximwn lift; minimwn drag.
Fermentation and Biochemical Engineering Handbook196
Figure lOb. Typical profile of airfoil at an angle of attack that gives fluids separation from
the airfoil surface.
Figure 11. Solidity ratio defmed as a ratio of blade area to circle area. The solidity ratio
for four different fluidfoil impellers is shown.
Agitation I9 7
4-
3-
OUTLET VELOCITY vs TIME
1 4. 4
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Figure 12. Velocity trace with time for three different impeller types, A3 10 fluidfoil, A200
axial flow turbine and RlOO radial flow turbine. The impellers are compared at equal
discharge pumping capacity, equal diameter and at whatever speed is required to achieve
this flow. The power required increases from left to right.
As an example of other types of programs that can be worked on, Fig.
13 shows a velocity profile from an A410 impeller, Fig. 14, a map of the
kinetic energy dissipation in the fluid stream and in the third one (Fig. 15)
model of heavier than the liquid particles in a random tracking pattern.
Additional models can be made up using mass transfer, heat transfer
and some reaction kinetics to simulate aprocess that can be defined in one or
more of those types of relationships.
The laser velocity meter has made it possible to obtain much data from
experiments conducted in transparent fluids. Figure 16 shows a typical
output of such a measurement, giving lines that are proportioned to the fluid
at that point and also relate to that angle of discharge. Studies on the blending
and process performance of these various fluidfoil impellers will be covered
in later sections of this chapter.
198
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Agitation 201
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Figure 16. Typical laser output from the measurement of velocities by means of a Doppler
velocity meter.
4.0 BAFFLES
Figure 17 illustrates the flow pattern in an unbaffled tank. The swirl
and vortex are normally undesirable. Putting four baffles in the tank, as
shown in Fig. 18, allows the application of any amount of horsepower to the
system without the tendency to swirl and vortex. Either 3,6 or 8 baffles can
be used if preferred. The general principle is to use the same total projected
area as exists with four baffles, each 1/12 the tank diameter in width. For
square or rectangular tanks the baffles shown in Fig. 19 are typical. At power
levels below 1 hp/1000 gal., the corners in square or rectangular tanks often
give sufficient baffling that additional wall baffles are not needed.
202 Fermentation and Biochemical Engineering Handbook
BOTTOM
SIDE
Figure 17. Vortexing flow pattern obtained with any type impeller which is unbafled.
SIDE VIEW
-BAFFLES
BOTTOM VIEW
Figure 18. Flow pattern obtained with any type of impeller.
Agitation 203
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Figure 19. Suggested bafles for square and rectangular tanks.
5.0 FLUID SHEAR RATES
Figure 20 illustrates flow pattern in the laminar flow region from a
radial flat blade turbine. By using a velocity probe, the parabolic velocity
distribution coming off the blades of the impeller is shown in Fig. 2 1. By
taking the slope of the curve at any point, the shear rate may be calculated at
that point. The maximum shear rate around the impeller periphery as well as
the average shear rate around the impeller may also be calculated.
An important concept is that one must multiply the fluid shear rate from
the impeller by the viscosity of the fluid to get the fluid shear stress that
actually carries out the process of mixing and dispersion.
Fluid shear stress = p(fluid shear rate)
Even in low viscosity fluids, by going from 1 cp to 10 cp there will be 10 times
the shear stress of the process operating from the fluid shear rate of the
impeller.