Axial-Flow Compressors
The axial-flow compressor compresses its working fluid by first acceler-ating the fluid and then diffusing it to obtain a pressure increase. The fluid isaccelerated by a row of rotating airfoils (blades) called therotor， 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. One rotor and one stator make-up a stage in a compressor. One additional row of fixed blades (inlet guide vanes) is frequently used at the compressor inlet to ensure that air enters the first-stage rotors at the desired angle. In addition to thestators， another diffuser atthe exit ofthe compressor furtherdiffuses the fluid andcontrols its velocity entering the combustors. Although the working fluid can be any compres-siblefluid， only air will be considered here.
In an axial compressor air passes from one stage to the next， each stage raising the pressure slightly. By producing low-pressure increases on the orderof 1.1:1 to 1.4:1， very high efficiencies can be obtained. The use of multiple stages permits overall pressure increases ofup to 40:1. Figure 7-1 shows a multistage high-pressure axial compressor. The low-pressure increase per stage also simplifies calculations in the design ofthe compressor byjustifying the air as incompressible in its flow through a stage.
As with other types of rotating machinery， an axial compressor can be described by a cylindrical coordinate system. The Z axis is taken as runningthe length of the compressor shaft， the radius r is measured outward fromtheshaft， and the angle of rotation e is the angle turned by the blades in Figure 7-2. This coordinate system will be used throughout this discussion of axial-flow compressors.
Figure 7-3 shows the pressure， velocity， and total enthalpy variation forflow through several stages ofan axial compressor. As indicated in Figure 7-3，
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Figure 7-1. A multistage high-pressure axial compressor (Courtesy of Westing-house Electric Corporation.)
the length of theblades， and the annulusarea， which is the area betweenthe shaft and shroud， decreases through the length of the compressor. This reduction in flow area compensates for the increase in fluid density as it iscompressed， permitting a constant axial velocity. In most preliminary calcu-lations used in the design of a compressor， the average blade height is used as the blade height for the stage.
Blade and Cascade Nomenclature
Since airfoils are employed in accelerating and diffusing the air in acompressor， much of the theory and research concerning the flow in axial compressors are based on studies of isolated airfoils. The nomenclature and methods of describing compressor blade shapes are almost identical to that of aircraft wings. Research in axial compressors involves several blades in a row to simulate a compressor rotor or stator. Such a row is called a cascade.When discussingblades， all angles， which describe the blade and its orienta-tion， are measured with respect to the shaft ( Z axis) of the compressor.
Figure 7-2. Coordinate system for axial-flow compressor.
CL
P0’ T0’ Total Pressure and Temperature PS’ TS’ Static Pressure and Temperature
V Absolute Velocity
Figure 7-..Variation ofenthalpy，velocity， and pressure through an axial-flow compressor.
The airfoils arecurved， convex on one side and concave on theother， with the rotor rotating toward the concave side. The concave side is called thepressure side of theblade， and the convex side is called the suction side of the blade. The chordline of an airfoil is a straight line drawn from the leadingedge to the trailing edge of the airfoil， and the chord is the length of the chordline. (See Figure 7-4.) The camberline is a line drawn halfway betweenthe twosurfaces， and the distance between the camberline and the chordline is the camber of the blade. The camber angle e is the turning angle of the camber line. The blade shape is described by specifying the ratio of the chordto the camber at some particular length on the chordline， measured from the leading edge. The aspect ratio AR is the ratio of the blade length to the chord length. The term ""hub-to-tip ratio"" is frequently used instead of aspect ratio. The aspect ratio becomes important when three-dimensional flow character-istics are discussed. The aspect ratio is established when the mass flow and axial velocity have been determined.
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