fluid_mechanics

646 Chapter 12 ■ Turbomachines
Exhaust gas
flow
Compressed
air flow
Exhaust gas flow
Air flow
Exhaust gas flow
Shaft rotation
Air flow
Turbine
rotor
Exhaust gas
flow
Compressed
air flow
Compressor
rotor
F I G U R E 12.1 Automotive turbocharger turbine and compressor rotors.
(Photograph courtesy of Concepts NREC.)
shaft power. In this case we have a turbine. On the other hand, we can exert a shaft torque, typically
with a motor, and by using blades, flow channels, or passages force the fluid to move. In this
case we have a pump. In Fig. 12.1 are shown the turbine and compressor 1pump2 rotors of an automobile
turbocharger. Examples of turbomachine-type pumps include simple window fans, propellers
on ships or airplanes, squirrel-cage fans on home furnaces, axial-flow water pumps used in
deep wells, and compressors in automobile turbochargers. Examples of turbines include the turbine
portion of gas turbine engines on aircraft, steam turbines used to drive generators at electrical
generation stations, and the small, high-speed air turbines that power dentist drills.
Turbomachines serve in an enormous array of applications in our daily lives and thus play an
important role in modern society. These machines can have a high power density 1large power transfer
per size2, relatively few moving parts, and reasonable efficiency. The following sections provide
an introduction to the fluid mechanics of these important machines. References 1–3 are a few examples
of the many books that offer much more knowledge about turbomachines.
12.1 Introduction
Turbomachines involve
the related
parameters of force,
torque, work, and
power.
(Photograph courtesy
of Mid American
Energy.)
Turbomachines are mechanical devices that either extract energy from a fluid 1turbine2 or add energy
to a fluid 1pump2 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.
Using a food blender to make a fruit drink is an example of turbo-pump action. The blender
blades are forced to rotate around an axis by a motor. The moving blades pulverize fruit and ice
and mix them with a base liquid to form a “smoothie.”
Conversely, the dynamic effect of the wind blowing past the sail on a boat creates pressure differences
on the sail. The wind force on the moving sail in the direction of the boat’s motion provides
power to propel the boat. The sail and boat act as a machine extracting energy from the air. Turbine
blades are like sails. See, for example, the enormous wind turbine blades in the figure in the margin.
The fluid involved can be either a gas 1as with a window fan or a gas turbine engine2 or a
liquid 1as with the water pump on a car or a turbine at a hydroelectric power plant2. While the basic
operating principles are the same whether the fluid is a liquid or a gas, important differences
in the fluid dynamics involved can occur. For example, cavitation may be an important design consideration
when liquids are involved if the pressure at any point within the flow is reduced to the
vapor pressure. Compressibility effects may be important when gases are involved if the Mach
number becomes large enough.
Many turbomachines contain some type of housing or casing that surrounds the rotating blades
or rotor, thus forming an internal flow passageway through which the fluid flows 1see Fig. 12.22.