In this article we will discuss about:- 1. Introduction to Axial Flow Compressor 2. Construction and Principle of Operation 3. Velocity Triangle 4. Degree of Reaction 5. Efficiency 6. Losses 7. Performance.
- Introduction to Axial Flow Compressor
- Construction and Principle of Operation of Axial Flow Compressor
- Velocity Triangle of Axial Flow Compressors
- Degree of Reaction of Axial Flow Compressor
- Efficiency of Axial Flow Compressor
- Losses in Axial Flow Compressor Stage
- Performance of Axial Flow Compressors
1. Introduction to Axial Flow Compressor:
In these compressors, flow of air is in the axial direction. It consists of alternately placed rows of stator blades and rotor blades. The rotor blades are mounted on the rotor while the stator blades are attached to the casing.
The conversion of kinetic energy into pressure head i.e., the diffusion action takes place partly over the rotor blades and partly over the stator blades. The stator blades also direct air over the successive rotor blades for a shockless entry. The flow passages between rotor blades as well as stator blades provide diverging area.
One row of rotor blades preceded by a row of stator blades is known as a stage of axial flow compressor. A single stage compressor can achieve a pressure ratio of 1.12 to 1.2 only. Therefore, for higher pressure ratio, often several stages of axial flow compressor are necessary.
2. Construction and Principle of Operation of Axial Flow Compressor:
Rotors are of two type viz. drum type and disc type (shown in Fig. 16.17). The disc type rotors being light weight are preferred for aircraft applications. The drum type rotors are used for static industrial applications. (Volume of casing converges from low pressure suction end to high pressure discharge end).
It helps to maintain more or less a constant velocity throughout the length of compressor, though the density of fluid increases gradually. Constant axial velocity throughout the stage is an important design consideration of axial flow compressor. Contraction of the flow annulus is achieved by contraction of casing and diverging the rotor.
The materials used for various components of the compressor are listed below:
(i) Rotor Blades:
Fibrous composites, Aluminium, Titanium, Steel, Nickel alloy.
Steel for shaft, disc. In aircrafts, first stage uses Titanium while the later stages use Nickel steel.
(iii) Stator Blades:
These are made of same materials as those for rotor blades.
It is either a casting of magnesium, aluminium, steel, iron or fabricated from Titanium or steel. The blades of compressor are of aerofoil section. The axial flow compressors can be of impulse type or reaction type. The blade profile and variation of pressure and velocity in the stage are shown in the Fig. 16.17 (a). While in Fig. 16.17 (b), the difference in the shape of Axial flow turbines and Axial flow compressor is illustrated.
In the axial flow compressor, fluid is imparted with kinetic energy from the rotor. This kinetic energy of the fluid is converted into pressure energy by diffuser action. The process occurs over several stages.
The energy analysis is considered for one stage of the compressor. The flow is assumed to occur tangential to the mean blade speed of u.
Figure 16.18 shows velocity triangles for one stage of the compressor. Air approaches rotor blade with absolute velocity V1 at an angle α1 to the axial direction. Combining V1 with blade velocity u1 gives relative velocity Vr1 at inlet, inclined at angle β1 with the direction of motion.
While air flows through the diverging passage between two successive rotor blades, its absolute velocity increases and air leaves the rotor with relative humidity Vr2 at an angle β2. The reduction in the relative velocity from Vr1 at inlet to Vr2 at exit causes some pressure rise over the rotor.
The absolute velocity at exit V2 can be found by vector difference between V2 and u2. The absolute velocity at exit V2 makes an angle α2 with the direction of motion. The air then enters the next stage via stator blades. Over the stator blades, the pressure of air further increases due to diffusion action. Air leaves the stator blades with absolute velocity V3 (less than V2) making an angle α3 with the axial direction. The stator blades are so designed that V3 = V1 and α3 = α1 for a shockless entry over the next stage.
It may particularly be noted that unlike in centrifugal compressors, in this case all angles are measured with axial direction and not from tangential direction.
Referring to the velocity triangles shown in Fig. 16.18 we have,
The degree of reaction is defined as the ratio of pressure rise in the rotor to the pressure rise in the stage.
5. Efficiency of Axial Flow Compressor:
It is defined as the ratio of actual pressure rise to the isentropic pressure rise. Thus,
Friction in rotor and stator blade passages, shock losses, etc, reduce the efficiency of the cascade (stage).
Consider the compression of air in a multistage axial flow rotary compressor shown on T-S diagram in Fig. 16.20.
Polytropic Efficiency η Terms of n:
Consider an actual compression process in a stage of an axial flow compressor, as shown in Fig. 16.21.
Expanding the above equation binomially and ignoring higher order terms, we may write,
It is rather difficult to draw the actual velocity triangles at the root, mean and tip of the blades as accurate estimation of contraction of main stream cannot be done.
The axial velocity at the mean section is somewhat greater than the average axial velocity. The actual temperature rise is somewhat lower than that estimated from the velocity triangles. An empirical relation suggested by Hotwell, for stagnation enthalpy rise across a stage is
CpΔTo = λu (Vf2 tan α2 – Vf1 tan α1)
where λ is work done factor which is approximately 0.85. Hotwell further suggested that the work done factor depends upon the number of stages. Greater the number of stages, the work done factor reduces.
During the flow of fluid through a stage of a compressor, various losses occur.
The pressure loss is due to three factors, namely:
(a) Profile losses on the blade surface;
(b) Skin friction on the annulus walls and
(c) Secondary flow losses.
Figure 16.22 illustrates various losses influencing the stage efficiency.
(a) Profile Losses:
This is a pressure loss of two dimensional cascade arising from the skin friction on the blade surface and due to mixing of fluid particles after the blade.
(b) Skin Friction Losses on the Annulus Walls:
The total pressure loss arise from the skin friction on the annulus walls and some secondary loss.
(c) Secondary Flow Losses:
These losses occur due to combined effects of curvature and boundary layer.
The range of stable operation of axial flow compressor is narrow. Due care has to be taken to match the components for avoiding instability during operation when these compressors are used in gas turbine.
The variation of power, pressure ratio and efficiency of an axial flow compressor against flow rate is depicted in the Fig. 16.24.
Figure 16.24 shows variation of pressure ratio versus volume flow rate for axial flow compressor.