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Gas Turbine Systems Theory 55
blade. Manufacturers have overcome this, in part, by decreasing the
length of the airfoil and increasing the width (or chord).
For single stage operation, the angle of attack depends on the
relation of airflow to speed. It can be shown that the velocity relative
to the blade is composed of two components: the axial component
depends on the flow velocity of the air through the compressor, and
the tangential component depends on the speed of rotation of the
compressor (Figure 4-5). Therefore, if the flow for a given speed of
rotation (rpm) is reduced, the direction of the air approaching each
blade is changed so as to increase the angle of attack. This results in
more lift and pressure rise until the stall angle of attack is reached.
This effect can be seen on the compressor characteristic curve.
The characteristic curve plots pressure against airflow (Figure 4-6).
The points on the curve mark the intersection of system resistance,
pressure, and airflow. (Note that opening the bleed valve reduces
system resistance and moves the compressor operating point away
from surge.) The top of each constant speed curve forms the loci for
the compressor stall line.
Therefore, the overall performance of the compressor is depicted
on the compressor performance map, which includes a family
of constant speed (rpm) lines (Figure 4-7). The efficiency islands are
included to show the effects of operating on and off the design point.
At the design speed and airflow, the angle of attack relative to the
blades is optimum and the compressor operates at peak efficiency. If
flow is reduced at a constant speed, the angle of attack increases until
the compressor airfoil goes into stall.
As flow is increased at a constant speed the compressor characteristic
curve approaches an area referred to as “ stone wall.” Stone
wall does not have the dynamic impact that is prevalent with stall,
but it is a very inefficient region. Furthermore, operation at or near
stone wall will result in over-temperature conditions in the turbine
section.
From the mechanical point of view, blade stresses and blade
vibration are limiting factors. The airfoil must be designed to handle
the varying loads due to centrifugal forces, and the load of compressing
air to higher and higher pressure ratios. These are conflicting requirements.
Thin, light blade designs result in low centrifugal forces,
but are limited in their compression-load carrying ability, while thick,
heavy designs have high compression-load carrying capability, but