الفصل الثاني

2-1.Introduction:
The flow through an axial flow turbomachine is primarily in the axial direction. Axial flow turbomachine have airfoil-shaped surfaces, called blades, attached to the periphery of a rotating disk like spokes on a hub. The unit is known as rotor and is usually enclosed by casing to minimize leakage over the tips of the blades. The fluid flows axially through the annular space between the hub and casing. An example of the axial flow turbine is the Kaplan turbine which is usually chosen for heads of 15 m or less. Figure (2-1) shows the principle of an axial flow rotor.
Many axial flow machines have sets of alternating moving blades and stationary surfaces called vanes. The vanes are also airfoil-shaped and attached to the inside of the casing. A circumferential set of vanes is called stator. The stator does not change the mechanical energy of the flow but simply alters the proportion between static and dynamic pressure. Since the fluid is confined, substantial changes in static pressure can occur. One rotor and its adjacent downstream stator are called a stage. Some axial flow compressor may have as many as 15 stages; axial flow turbines usually have no more than three stages.
Axial flow compressor often have a set of fixed surfaces called inlet guide vane, at the entrance to the compressor to bring the flow at the proper angle on to the first stage rotor. Note that the annular area decreases in the direction of the flow because the air is being compressed and its density increased.