Agitation 191 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 show what happens when the angle and the shape is such that there is a separation of the fluid from the airfoil body The A310 impeller( Fig. 9a)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 material 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 A315 (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 A410(Fig. 9f)which has a very marked increase in twist angle of the 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 elocity as a function of time on a strip chart. As can be seen in Fig. 12 the fluidfoil impeller type(A310) has a very low velocity fluctuation and usesAgitation 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