Tangential impellers

An example of a typical turboexpander is shown in Fig. 29-46. Radial-flow turbines are normally single-stage and have combination impulse-reaction blades, and the rotor resembles a centrifugal-pump impeller. The gas is jetted tangentially into the outer periphery of the rotor and flows radially inward to the eye, from which the gas is jetted backward by the angle of the rotor blades so that it leaves the rotor without spin and flows axially away. 

H = work per lb of fluid U2 = impeller peripheral veloeity Ux = indueer veloeity at the mean radial station Vff2 = absolute tangential fluid veloeity at impeller exit Vgx = absolute tangential air veloeity at indueer inlet... 

Supposing constant rotational speeds, no slip, and an axial inlet, the velocity triangles are as shown in Figure 6-10. For the radial vane, the absolute tangential fluid velocity at the impeller exit is constant—even if the flow rate is increased or decreased. 

For backward-curved vanes, the absolute tangential fluid velocity at the impeller exit increases with the reduction of flow rates and decreases with the increase in flow rate as shown in the following equation ... 

In a stationary impeller, the flow is expeeted to follow the blade shape and exit tangentially to it. A high adverse pressure gradient along the blade passage and subsequent flow separation are not eonsidered to be general possibilities. 

The rotor or impeller of the radial-inflow turbine eonsists of a hub, blades, and in some eases, a shroud. The hub is the solid axisymmetrieal portion of the rotor. It defines the inner boundary of the flow passage and is sometimes ealled the dise. The blades are integral to the hub and exert a normal foree on the flow stream. The exit seetion of the blading is ealled an exdueer and it is eonstrueted separately like an indueer in a eentrifugal eompressor. The exdueer is eurved to remove some of the tangential veloeity foree at the outlet. 

Vu2 = tangential component of the absolute velocity U2 = impeller tip velocity... 

The forces applied by an impeller to the material contained in a vessel produce characteristic flow patterns that depend on the Impeller geometry, properties of the fluid, and the relative sizes and proportions of the tank, baffles and impeller. There are three principal types of flow patterns tangential, radial and axial. Tangential flow is observed when the liquid flows parallel to the path described by the mixer as illustrated in Figure 7. 

When the flow pattern in a mixed tank is primarily tangential, the fluid discharge from the impeller to the surroundings and its entrainment into the impeller are small. Also, fluid transfer in the vertical direction is at a minimum. The mixing effect is lowest when the rotational velocity of the liquid approaches that of the mixer. 

Axial flow, in which the liquid enters the impeller and discharges along a parallel path to the axis, is shown in Figure 9. The radial and longitudinal components are primarily responsible for the derived mixing action. The tangential component is important when the shaft has a vertical orientation and is positioned near the center of the tank. 

Baffles are responsible for restricting the tangential velocity component, u, and augment the vertical component, while simultaneously increasing the radial velocity, U,. The net result is that the liquid discharges from the impeller in a wider flow radius. 

To force the air to flow through the impeller blades of a centrifugal fan, a tangential force is needed. According to the momentum law this force is... 

Here c, is the tangential component of the air velocity at the inlet to the impeller blades, and is the same at the exit. q , is the mass flow of air through the impeller. 

Equation (9.79) shows that the static pressure increases with an increase in the tangential speed and the distance between the incoming and leaving edges of the impeller blade. This will not influence the nature of the flow process in the impeller. 

Up to this stage, only the fan characteristics have been considered, without investigating the influence of different impeller blade shapes. Consider the flow at the edge of the impeller blade. Normally, for cost reasons, leading devices are not installed in front of the fan propeller, resulting in radial gas flow into the propeller, with the tangential velocity component = 0. 

In Eq. (9.90), C2 is the tangential component of the absolute velocity at the exit if the flow is exactly in the blade direction. Since the slip factor is ieSs than 1, the total pressure increase will decrease according to Eq. ( 9.72) for the same impeller and isentropic flow. 

In Eq. (9.113), subscript u indicates the velocity to the tangential velocity component, the first subscript 2 indicates impeller exit, the first subscript 1 indicates impeller inlet, the second subscript 2 stands for 2> and second subscript 1 stands for rzy. The proportionality constant k is... 

Tangential acceleration Acceleration of a fluid tangentially to a vane or impeller due to rotary motion. 

If it is assumed that the fluid leaves the impeller tangentially at the same speed as the impeller (an approximation), then an angular momentum balance on the fluid in contact with the impeller gives ... 

Axial-flow impellers generate currents parallel to the axis of the impeller shaft. Radial-flow impellers generate currents in a direction tangential or radial to the axis of the impeller shaft. Within the two classes of impellers, there are three main types of impeller design. These are propeller, turbine, and paddle. The three main types are utilized in about 95% of most batch liquid agitation systems. Standard propellers have three blades, but two-bladed, four-bladed, or impellers encased by a circular guard can also be used. 

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