Axial Flow Compressor is a type of compressor in which the working fluid flows parallel to the axis of rotation of the rotor or axially.
In this compressor, the working fluid is compressed by first accelerating the fluid and then diffusion it to increase its pressure.
Compressors are mechanical devices used to compress fluids. In other words, they increase the pressure of gases using various methods. The gases entering the compressor will be at low pressure. However, it will exit at very high pressure. This pressurizing property of the compressors is used in filling gases in cylinders and tires.
Compressors are normally classified into two categories – Positive Displacement Compressors and Dynamic Compressors. The former increases the pressure of a system by reducing the volume, whereas the later achieves its objectives through a series of rotor-stator arrangements.
Axial Flow Compressors are a type of dynamic compressors. These compressors allow the fluid to attain high velocity initially but creates a restriction in the airflow later. It will cause the gas pressure to rise. In addition to axial flow type, centrifugal compressors and mixed-flow compressors form other categories of dynamic compressors.
In an axial flow compressor, the gas enters and exits the compressor in the axial direction. And the compression process takes place in two steps.
- The flow is accelerated while moving through the blades or rotor.
- The accelerated flow is then restricted by the stator blades. It causes the kinetic energy to transform into static pressure energy.
Axial Flow compressors consist of rotor blades placed on the rotor and the stator on the casing. The axial flow of air through these alternately placed rotors and stators results in an increase in their final pressure.
Components of an Axial Flow Compressor
A typical Axial Flow Compressor consists of the following components.
- Rotor Blades
- Stator Blades
Axial-Flow Compressor has a rotating part called the rotor. It consists of a rotor drum. The rotor drum is connected to the shaft. The shaft is the actual rotating element. It transfers the rotary motion to the drum.
The mounting for the rotor blades is on the rotor drum.
Rotor Blades are placed on the rotor drum. The rotor blades rotate and are hence responsible for increasing the kinetic energy of gases.
Materials for the construction of Rotor Blades are as follows.
- Nickel alloy
The casing forms an outer protective membrane for the axial flow compressor. It also performs several other functions.
- The stator blades are mounted on the casing. They are the fixed blades in an axial flow compressor.
The casing of an axial flow compressor is made from one of the following components.
- Iron or high-quality steel
Unlike the rotor blades, which rotate along with the rotor drum, the stator blades remain stationary throughout the compression process. The stator blades reside in the casing of an axial flow compressor.
The stator blades are from the following materials:-
- Nickel alloy
The functions of Stator Blades include the following:
- They act as guide vanes. They guide the gases to flow in a particular direction inside the compressor. It allows the gases to hit on the rotor blades with a proper angle of contact.
- They are also responsible for increasing the pressure of the gases.
Working of an Axial Flow Compressor:
The basic working principle behind an axial flow compressor is that the rotor imparts kinetic energy to the gas. This kinetic energy is later converted to static pressure when it is diffused through passages or when it strikes on the rotor.
For an axial flow compressor, the flow of gases is along the axis.
At the start, inlet guide vanes or stator blades are present. It is to ensure that the gases strike on the rotor with the proper angle of attack. And the rotor is placed next to the stator blade. The aerofoil shape of the blade is a crucial feature for this action.
Axial Flow Compressor works in many stages. A rotor and a stator together constitute one. Generally, there can be five to fifteen stages in an axial flow compressor. The number of stages is determined by the pressure ratio that needs to attain as well as the amount of gas.
The stator and rotor blades are alternately packed. Gases first collide on the stator or the first guide vane. It guides the gases to the rotating rotor blade that follows next.
The rotor blades then increase the kinetic energy of the gases and supply it to the adjacent stator blade.
The gases then fall on the stator blades. When it hits on the stator, its pressure energy increases, but at the cost of its kinetic energy. The gases with a decreased velocity will now move to the next rotor.
The rotor will now increase the kinetic energy of the gases. But the kinetic energy will be far less than the kinetic energy supplied by the rotor before.
These processes repeat until the gases reach the exit. At the exit of the compressor, the gases have very high pressure.
Advantages of Axial Flow Compressors
- Very high peak efficiency as compared to other types of compressors.
- Very high mass flow rate.
- The pressure ratio is high, leading to very high efficiency. Increased number of stages also help them attain a handsome pressure ratio. A pressure ratio of even 40:1 is achievable in an axial flow compressor.
- Because of the small frontal area for a given flow, it reduces aerodynamic drag.
Disadvantages of Axial Flow Compressors
- Manufacturing an axial flow compressor is difficult.
- This machine is expensive.
- This machine is very heavy, making its installation difficult.
- The axial flow compressors need very high power to start or initiate the compression process.
- The pressure rise in individual stages is low. It is the reason why we employ multiple stages for axial flow compressors.
Applications of Axial Flow Compressors
Aircraft turbojet engines use axial-flow compressors that give a high mass flow rate. The compressor pressurizes the gas that enters through the inlet. The compressed air then flows to the combustion chamber. The fuel ignites in the presence of highly compressed air or oxygen-producing the energy required for propulsion.
Losses in an axial flow compressor:
Axial Flow compressors, like any other mechanical device, are not devoid of losses. Commonly, these compressors experience the following type of losses.
- Profile loss
- Skin Friction Loss or Annulus Loss
- Secondary Flow Loss
- Tip Leakage Loss
Profile loss – This loss arises due to the growth of the boundary and its subsequent separation on the blade profile. In other words, this loss arises due to the peculiarity of their aerofoil structure.
Skin Friction or Annulus loss –This is a type of viscous loss that arises due to the axial growth of the boundary layer. It accounts for approximately 50% of the total losses.
Secondary Flow Loss -When the flow takes place through curved blade passages, secondary flows occur. Losses arising from this secondary flow are called secondary loss.
Tip Leakage Flows – Due to the gap between the casing and the blade, there occurs a flow from the pressure surface to the suction surface, creating losses for the machine.
Comparison between Axial Flow and Centrifugal Compressor
The difference between axial flow compressor and centrifugal compressors are given below.
|AXIAL FLOW COMPRESSOR||CENTRIFUGAL COMPRESSOR|
|The design and manufacture of axial flow compressors are difficult.||The design and manufacturing are comparatively easier for Centrifugal Compressors.|
|It can handle a very high flow rate and offer a higher pressure ratio.||It tolerates a comparatively less flow rate and pressure ratio.|
|The pressure gradient for a single stage is very less.||It can create more differential pressure in a single stage.|
|It is very expensive and the machine is very heavy.||This machine is comparatively cost-effective.|
Velocity Diagram of Axial Flow Compressors
The absolute velocity of the gas when it strikes the rotor blades at the inlet – C1
The angle of incidence of gas with respect to the axis – α1
The absolute velocity of gas when it leaves the rotor blades – C2
The angle with the axis, when the gas leaves the rotor – α2
The tangential velocity of the rotor – U
The relative velocity at the inlet – V1
The relative velocity at outlet – V2
The angle made by relative velocity vector in the axial direction – β1
[Ca and Cwforms the components of the velocity vector C]
According to Euler’s equation, work done on the air is given by;
Wc = U (Cw2 + Cw1)
From the velocity triangles given above;
U/ Ca = tanα1 +tanβ1
U/ Ca = tanα2+tanβ2
Ca = C2 = Ca1 is the axial velocity and is assumed constant throughout the stage.
Substituting these values in the above equation, we get the work done equation in terms of pressure angle as follows.
Wc = (tanα2-tanα1) UCa
Wc = (tanβ1-tanβ2) UCa
Velocity triangles are useful to determine the characteristics of the blade including its design features. It also helps understand the nature of the flow.