Axial, flat

A basic stirred tank design is shown in Fig. 23-30. Height to diameter ratio is H/D = 2 to 3. Heat transfer may be provided through a jacket or internal coils. Baffles prevent movement of the mass as a whole. A draft tube enhances vertical circulation. The vapor space is about 20 percent of the total volume. A hollow shaft and impeller increase gas circulation (as in Fig. 23-31). A splasher can be attached to the shaft at the hquid surface to improve entrainment of gas. A variety of impellers is in use. The pitched propeller moves the liquid axially, the flat blade moves it radially, and inclined blades move it both axially and radially. The anchor and some other designs are suited to viscous hquids. 

Another approach is to recognise that initially for a flat sheet, the axial stiffness is high but the transverse stiffness is relatively low. As the corrugation depth increases then the transverse stiffness increases but at the expense of the axial stiffness. It is readily shown that the axial deflection per unit load for the corrugations for the new geometry compared with the flat sheet is given by... 

Figures 5-3 and 5-5 illustrate a few of the types of impellers used for mixing. They may be basically classified as axial, radial and mixed. In general the most generally applicable are the. 3-bladed propeller, the flat-blade turbine, the curved blade turbine, and the paddle. The many other designs are either modifications of these or specially designed for a very special purpose with respect to a fluid system and/or its performance.

Rakes, Gates Spirals, Anchors, Paddles Propellers Axial Flow Turbines Flat Blade Turbine Bar Turbine Bladeless Impeller Close Clearance Impeller and Stator... 

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. 

If a propeller is located quite close to the bottom of a tank, the flow becomes radial like that of the flat blade turbine. In a properly baffled system the propeller flow is axial. WTien dynamic similarity is accomplished, the systems are similar [21]. For a first approximation, placing the impeller at E of liquid height off the bottom is good. 

If the constant speed characteristic of a pump is superimposed on a system curve, there is usually one intersection point, shown in Figure 32.39. If a flat system curve is being matched with a mixed or axial flow machine there can be flow instability, as illustrated in Figure 32.40, which is only corrected by changing pump speed or the static lift, or selecting a different pump. 

Solution The numerical integration techniques require some care. The inlet to the reactor is usually assumed to have a flat viscosity profile and a parabolic velocity distribution. We would like the numerical integration to reproduce the paraboUc distribution exactly when q, is constant. Otherwise, there will be an initial, fictitious change in at the first axial increment. Define... 

At lower Reynolds numbers, the axial velocity profile will not be flat and it might seem that another correction must be added to Equation (9.14). It turns out, however, that Equation (9.14) remains a good model for real turbulent reactors (and even some laminar ones) given suitable values for D. The model lumps the combined effects of fluctuating velocity components, nonflat velocity profiles, and molecular diffusion into the single parameter D. 

Adiabatic Reactors. Like isothermal reactors, adiabatic reactors with a flat velocity profile will have no radial gradients in temperature or composition. There are axial gradients, and the axial dispersion model, including its extension to temperature in Section 9.4, can account for axial mixing. As a practical matter, it is difficult to build a small adiabatic reactor. Wall temperatures must be controlled to simulate the adiabatic temperature profile in the reactor, and guard heaters may be needed at the inlet and outlet to avoid losses by radiation. Even so, it is hkely that uncertainties in the temperature profile will mask the relatively small effects of axial dispersion. 

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