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2.0
Axial-Flow Compressors
Meherwan P. Boyce
2121 Kirby Drive, Number 28N
Houston, TX 77019
713-807--0888
713-807-0088 Fax
boycepower.com
mpboyce@boycepower.com
163 163
2.0-1 Introduction
The compressors in most gas turbine applications, especially
units over 5MW, use axial fl ow compressors. An axial fl ow compressor is
one in which the fl ow enters the compressor in an axial direction (parallel
with the axis of rotation), and exits from the gas turbine, also in an axial
direction. The axial-fl ow compressor compresses its working fl uid by fi rst
accelerating the fl uid and then diffusing it to obtain a pressure increase. The
fl uid is accelerated by a row of rotating airfoils (blades) called the rotor,
and then diffused in a row of stationary blades (the stator). The diffusion
in the stator converts the velocity increase gained in the rotor to a pressure
increase. A compressor consists of several stages: 1) A combination of a
rotor followed by a stator make-up a stage in a compressor; 2) An additional
row of stationary blades are frequently used at the compressor inlet and are
known as Inlet Guide Vanes (IGV) to ensue that air enters the fi rst-stage
rotors at the desired fl ow angle, these vanes are also pitch variable thus can
be adjusted to the varying fl ow requirements of the engine; and 3) In addition
to the stators, another diffuser at the exit of the compressor consisting
of another set of vanes further diffuses the fl uid and controls its velocity
entering the combustors and is often known as the Exit Guide Vanes (EGV).
In an axial fl ow compressor, air passes from one stage to
the next, each stage raising the pressure slightly. By producing low-
pressure increases on the order of 1.1:1 to 1.4:1, very high effi ciencies
can be obtained as seen in table 1. The use of multiple stages
permits overall pressure increases of up to 40:1 in some aerospace
applications and a pressure ratio of 30:1 in some Industrial applications.
The last twenty years has seen a large growth in gas turbine
technology. The growth is spear headed by the increase in compressor
pressure ratio, advanced combustion techniques, the growth of materials
technology, new coatings and new cooling schemes. The increase in gas
turbine effi ciency is dependent on two basic parameters:
1. Increase in Pressure Ratio
2. Increase in Firing Temperature
It also should be remembered that the Gas Turbine Axial Flow
Compressor consumes between 55%-65% of the power produced by the
Turbine section of the gas turbine.
Table 1 Axial Flow Compressor Characteristics
Type of
Application
Type of
Flow
Inlet Relative
Velocity Mach
Number
Pressure
Ratio per
Stage
Effi ciency
per Stage
Industrial Subsonic 0.4-0.8 1.05-1.2 88%-92%
Aerospace Transonic 0.7-1.1 1.15-1.6 80%-85%
Research Supersonic 1.05-2.5 1.8-2.2 75%-85%
The aerospace engines have been the leaders in most of the
technology in the gas turbine. The design criteria for these engines was
high reliability, high performance, with many starts and fl exible operation
throughout the fl ight envelope. The engine life of about 3500 hours between
major overhauls was considered good. The aerospace engine performance
has always been rated primarily on its Thrust/Weight ratio. Increase in
engine Thrust / Weight Ratio is achieved by the development of high aspect
ratio blades in the compressor as well as optimizing the pressure ratio and
fi ring temperature of the turbine for maximum work output per unit fl ow.
The Industrial Gas Turbine has always emphasized long life and this
conservative approach has resulted in the Industrial Gas Turbine in many