Gas Turbine Handbook : Principles and Practices

Gas Turbine Systems Theory 61
Angle of Deviation ≡ β 2
– β 2
'
Stagger Angle ≡ γ
Pitch ≡ s
Turning Angle ≡ ε = β 2
' – β 1
'
Chord Length
Leading Edge
Trailing Edge
Maximum Thickness ≡ t
Width ≡ w
Height ≡ h
Aspect Ratio ≡ ratio of blade height to blade chord
Angle of Attack or angle of incidence ≡ i = β 1
– β 1
'
Considering the Centrifugal Compressor
The centrifugal compressor, like the axial compressor, is a dynamic
machine that achieves compression by applying inertial forces
to the air (acceleration, deceleration, turning) by means of rotating
impellers.
The centrifugal compressor is made up of one or more stages,
each stage consisting of an impeller and a diffuser. The impeller is
the rotating element and the diffuser is the stationary element. The
impeller consists of a backing plate or disc with radial vanes attached
to the disc from the hub to the outer rim. Impellers may be
either open, semi-enclosed, or enclosed design. In the open impeller
the radial vanes attach directly to the hub. In this type of design the
vanes and hub may be machined from one solid forging, or the vanes
can be machined separately and welded to the hub. In the enclosed
design, the vanes are sandwiched between two discs. Obviously, the
open design has to deal with air leakage between the moving vanes
and the non-moving diaphragm, whereas the enclosed design does not
have this problem. However, the enclosed design is more difficult and
costly to manufacture.
Air enters the compressor axially and turns in the impeller
inlet (eye) to flow through the impeller. The flow through the impeller
takes place in a plane perpendicular to the axis or shaft of the
machine. This is easier to understand when viewing the velocity
diagrams for a centrifugal compressor stage. Although the information
presented is the same, Figure 4-11 demonstrates two methods of
preparing velocity diagrams.