126 AxialFlow and MixedFlow Pumps
As noted previously, centrifugal pumps are radial-flow machines that operate most efficiently for applications requiring high heads at relatively low flowrates. This head-flowrate combination typically yields specific speeds Nsd that are less than approximately 4000. For many applications, such as those associated with drainage and irrigation, high flowrates at low heads are required and centrifugal pumps are not suitable. In this case, axial-flow pumps are commonly used. This type of pump...
Pelton Wheel Failure Modes
where hT is the turbine head. This simplifies to since p1 p2 and z1 z2. Note that the impulse turbine obtains its energy from a reduction in the velocity head. The largest turbine head possible and therefore the largest power occurs when all of the kinetic energy available is extracted by the turbine, giving This is consistent with the maximum power condition represented by Fig. E12.7 amp . As indicated by Eq. 1, if the exit absolute velocity is not in the plane of the rotor i.e., b lt 180 ,...
WShaft mU m2u2v02
Again, the value of Vs is positive when Vs and U are in the same direction and negative otherwise. Also, Wshaft is positive when the shaft torque and v are in the same direction and negative otherwise. Thus, Wshaft is positive when power is supplied to the contents of the control volume pumps and negative otherwise turbines . This outcome is consistent with the sign convention involving the work term in the energy equation considered in Chapter 5 see Eq. 5.67 . Finally, in terms of work per...
124 The Centrifugal Pump
One of the most common radial-flow turbomachines is the centrifugal pump. This type of pump has two main components an impeller attached to a rotating shaft, and a stationary casing, housing, or volute enclosing the impeller. The impeller consists of a number of blades usually curved , also sometimes called vanes, arranged in a regular pattern around the shaft. A sketch showing the essential features of a centrifugal pump is shown in Fig. 12.6. As the FIGURE 12.6 Schematic diagram of basic...
129 Compressible Flow Turbomachines
Compressible flow turbomachines are in many ways similar to the incompressible flow pumps and turbines described in previous portions of this chapter. The main difference is that the density of the fluid a gas or vapor changes significantly from the inlet to the outlet of the compressible flow machines. This added feature has interesting consequences, benefits, and complications. Compressors are pumps that add energy to the fluid, causing a significant pressure rise and a corresponding...
v2 vi 2g
with sections 1 and 2 at the pump inlet and exit, respectively. The head, ha, is the same as hp used with the energy equation, Eq. 5.84, where hp is interpreted to be the net head rise actually gained by the fluid flowing through the pump, i.e., ha hp hs hL. Typically, the differences in elevations and velocities are small so that The power, Pf, gained by the fluid is given by the equation and this quantity, expressed in terms of horsepower is traditionally called the water horsepower. Thus,...
128 Turbines
The two basic types of hydraulic turbines are impulse and reaction. As discussed in Section 12.2, turbines are devices that extract energy from a flowing fluid. The geometry of turbines is such that the fluid exerts a torque on the rotor in the direction of its rotation. The shaft power generated is available to drive generators or other devices. In the following sections we discuss mainly the operation of hydraulic turbines those for which the working fluid is water and to a lesser extent gas...
123 Basic Angular Momentum Considerations
When shaft torque and rotation are in the same direction, we have a pump otherwise we have a turbine. In the previous section we indicated how work transfer to or from a fluid flowing through a pump or a turbine occurs by interaction between moving rotor blades and the fluid. Since all of these turbomachines involve the rotation of an impeller or a rotor about a central axis, it is appropriate to discuss their performance in terms of torque and angular momentum. Recall that work can be written...
Turbomachines
The complete material for Chapter 12 is contained in the E-book only. Turbomachines are dynamic fluid machines that add for pumps or extract for turbines flow energy. In previous chapters we often used generic black boxes to represent fluid machines such as pumps or turbines. The purpose of this chapter is to understand from a fluid mechanics standpoint how these devices work. Pumps and turbines sometimes called fluid machines occur in a wide variety of configurations. In general, pumps add...
121 Introduction
Turbomachines involve the related parameters of force, work, and power. Turbomachines are mechanical devices that either extract energy from a fluid turbine or add energy to a fluid pump as a result of dynamic interactions between the device and the fluid. While the actual design and construction of these devices often require considerable insight and effort, their basic operating principles are quite simple. The dynamic interaction between a fluid and a solid is often based on flow and fluid...