The Ins And outs of Gas Turbine Engine Compressor
The three parameters of a compressor
The theory of gas turbine engine compressor is essentially converting mechanical energy(the turning of compressor blades by its attached turbine) to pressure energy(the amount of energy that air contains). To understand the rest of the content we first need to discuss the three parameters of a compressor.
1. Compressor Efficiency:
The process of taking in outside air and compressing it is an isentropic process, meaning that the total energy before the first stage to the end of the last stage stays constant(assuming no energy is added nor removed). However, this only happens theoretically. The actually operation of an compressor will always incur energy loss during the process of converting mechanical energy into pressure energy. This will be discussed more later in this chapter.
2. Pressure Ratio(Pi or Π)
Pressure ratio is the total air pressure at the exit of the compressor(denoted by Pt3) divided by the total air pressure at the entry of the compressor(Pt2).
So this: Π=Pt3/Pt2
This parameter plays a significant role in engine thrust, fuel consumption, engine efficiency and engine weight. To increase pressure ratio, the easiest way is to increase the number of compressor stages. This leads to an increase in compression pressure and higher temperature, thus making manufacturing much more demanding. Even the redesign of entire engine might be needed.
Note: the number 3 and 2 in the above equation comes from here, it is known as a station number.
3. Air-flow rate, or mass-flow rate
Air-flow rate is the amount of air that the compressor is able to process in a unit of time, normally in second. We can do thermal cycle analysis by controlling the air-flow rate. And engine size can also be classified by this parameter.
All three parameters are closely related.
In modern days, compressor efficiency can be achieved to nearly 90%, pressure ratio to 16:1 and air-flow rate to 200kg/s. For a high bypass ratio engine, the pressure ratio could achieve 30:1 and a air-flow rate of 900kg/s.
The Two Types of Compressor
Centrifugal flow and Axial flow. Before 1950, except for some German aircraft, all other early fighter planes' compressors are of centrifugal type. Today, except for smaller applications like helicopters, M1 tanks and APUs, most engines are of axial flow type. This is due to the demand for high thrust and it can only be achieved by engines with axial flow configuration. Hence, the majority of the discussion in this chapter is about axial flow compressor.
Centrifugal Flow
Figure I
The theory of a centrifugal flow compressor is essentially the same with a car turbocharger. It sucks in air from the middle front and distribute them out through the rotors. The air then enters the diffuser, further increase its pressure and decrease its velocity(Figure I). Note that the figures above are correlated to each other, and the horizontal lines demonstrate just that. About half of the pressure generated is through the rotor blades and the other half is through the diffuser. The diffuser here has another feature(or a limitation), that is changing the direction of the incoming air to a 90 degree angle.
The RPM of the rotor in a centrifugal flow compressor is between 20,000 to 30,000. The size of the rotor directly affects the RPM, the bigger the rotor gets, the slower it spins. The pressure ratio of a centrifugal compressor is roughly 5:1, with an efficiency of around 80%. We can increase the pressure ratio further by increasing the rotor's RPM but the counter effect of that is a significant loss in compressor efficiency. It is because the fluid(air) at the tip of the rotor becomes supersonic, causing shock waves in the diffuser. There is a solution to this: adding another stage of centrifugal flow compressor behind the first stage. This way, pressure ratio can be further increased without causing shock waves. The good old Rolls-Royce Dart engine is a typical example of such design. Its design discontinued due to the weight and the complexity of air flow directions.
Overall, comparing to the axial flow compressor, the centrifugal flow type has a much higher compression rate per stage: 5:1 comparing to 120-130% in a axial flow compressor. However, the most stages a centrifugal flow compressor has ever been produced was only 2. The centrifugal compressor is also easier and cheaper to manufacture.
Axial Flow
Unlike any centrifugal flow compressor, the rotors of an axial flow compressor does not pressurize air by centrifugal force, instead, it utilizes Bernoulli's principle and Newton's law to compress air by many carefully engineered airfoils. Because of the nature of such aerodynamics, this type of design may lead to a dangerous phenomenon known as Surging if not designed carefully. Compressors of axial flow type are also very demanding both in terms of manufacturing process and material strength.
However, the axial flow design has its perks. For one it doesn't turn the air 90 degrees all the time like the centrifugal flow compressor does, minimizing the waste of dynamic pressure. For two, the design generally comes with a smaller front facing surface area, making parasite drag minimal.
Figure II
An axial flow compressor consists of several parts:(Figure II)
1. Compressor Front Frame
2. Compressor Casing with Stator Vanes
3. Rotor with Rotor Blades
4. Compressor Rear Frame
Now before we dive into the theory of its operation, let's first look at the role of each part.
Compressor Front Frame:
Figure III
The compressor front frame is supported by 6 to 8 strut bars. These strut bars are stationary and hollow by design. The hollow space allows the deliveries of engine oil to things like bearings and other machinery in the center. It also provides a place for wiring to go through when needed, but most importantly the transportation of mechanical power like the IDGs(Integrated Drive Generator) which provides AC power to the aircraft. Furthermore, the hollow design of struts also allows the run though of bleed air which can prevent ice formation. It's worth noting that the front vanes here are what is known as the VIGV(Variable Inlet Guide Vanes). They guide the incoming airflow to a desirable direction before entering the first stage of the compressor. They are hydraulically actuated and are controlled by continuous measurement of LPC(Low-Pressure Compressor) inlet temperature and HPC(High-Pressure Compressor) discharge static pressure.
Compressor Casing and Stator Blades:
For ease of maintenance, compressor casings are usually assembled from two pieces, the top half and the bottom half, connected using flanges. Compressor casings are normally forged using lightweight titanium alloy. Such alloy allows for structural expansion and provides rotor blades tip clearance with the casing under high heat conditions.
Note: the number 3 and 2 in the above equation comes from here, it is known as a station number.
3. Air-flow rate, or mass-flow rate
Air-flow rate is the amount of air that the compressor is able to process in a unit of time, normally in second. We can do thermal cycle analysis by controlling the air-flow rate. And engine size can also be classified by this parameter.
All three parameters are closely related.
In modern days, compressor efficiency can be achieved to nearly 90%, pressure ratio to 16:1 and air-flow rate to 200kg/s. For a high bypass ratio engine, the pressure ratio could achieve 30:1 and a air-flow rate of 900kg/s.
The Two Types of Compressor
Centrifugal flow and Axial flow. Before 1950, except for some German aircraft, all other early fighter planes' compressors are of centrifugal type. Today, except for smaller applications like helicopters, M1 tanks and APUs, most engines are of axial flow type. This is due to the demand for high thrust and it can only be achieved by engines with axial flow configuration. Hence, the majority of the discussion in this chapter is about axial flow compressor.
Centrifugal Flow
Figure I
The theory of a centrifugal flow compressor is essentially the same with a car turbocharger. It sucks in air from the middle front and distribute them out through the rotors. The air then enters the diffuser, further increase its pressure and decrease its velocity(Figure I). Note that the figures above are correlated to each other, and the horizontal lines demonstrate just that. About half of the pressure generated is through the rotor blades and the other half is through the diffuser. The diffuser here has another feature(or a limitation), that is changing the direction of the incoming air to a 90 degree angle.
The RPM of the rotor in a centrifugal flow compressor is between 20,000 to 30,000. The size of the rotor directly affects the RPM, the bigger the rotor gets, the slower it spins. The pressure ratio of a centrifugal compressor is roughly 5:1, with an efficiency of around 80%. We can increase the pressure ratio further by increasing the rotor's RPM but the counter effect of that is a significant loss in compressor efficiency. It is because the fluid(air) at the tip of the rotor becomes supersonic, causing shock waves in the diffuser. There is a solution to this: adding another stage of centrifugal flow compressor behind the first stage. This way, pressure ratio can be further increased without causing shock waves. The good old Rolls-Royce Dart engine is a typical example of such design. Its design discontinued due to the weight and the complexity of air flow directions.
Overall, comparing to the axial flow compressor, the centrifugal flow type has a much higher compression rate per stage: 5:1 comparing to 120-130% in a axial flow compressor. However, the most stages a centrifugal flow compressor has ever been produced was only 2. The centrifugal compressor is also easier and cheaper to manufacture.
Axial Flow
Unlike any centrifugal flow compressor, the rotors of an axial flow compressor does not pressurize air by centrifugal force, instead, it utilizes Bernoulli's principle and Newton's law to compress air by many carefully engineered airfoils. Because of the nature of such aerodynamics, this type of design may lead to a dangerous phenomenon known as Surging if not designed carefully. Compressors of axial flow type are also very demanding both in terms of manufacturing process and material strength.
However, the axial flow design has its perks. For one it doesn't turn the air 90 degrees all the time like the centrifugal flow compressor does, minimizing the waste of dynamic pressure. For two, the design generally comes with a smaller front facing surface area, making parasite drag minimal.
Figure II
An axial flow compressor consists of several parts:(Figure II)
1. Compressor Front Frame
2. Compressor Casing with Stator Vanes
3. Rotor with Rotor Blades
4. Compressor Rear Frame
Now before we dive into the theory of its operation, let's first look at the role of each part.
Compressor Front Frame:
Figure III
The compressor front frame is supported by 6 to 8 strut bars. These strut bars are stationary and hollow by design. The hollow space allows the deliveries of engine oil to things like bearings and other machinery in the center. It also provides a place for wiring to go through when needed, but most importantly the transportation of mechanical power like the IDGs(Integrated Drive Generator) which provides AC power to the aircraft. Furthermore, the hollow design of struts also allows the run though of bleed air which can prevent ice formation. It's worth noting that the front vanes here are what is known as the VIGV(Variable Inlet Guide Vanes). They guide the incoming airflow to a desirable direction before entering the first stage of the compressor. They are hydraulically actuated and are controlled by continuous measurement of LPC(Low-Pressure Compressor) inlet temperature and HPC(High-Pressure Compressor) discharge static pressure.
Compressor Casing and Stator Blades:
For ease of maintenance, compressor casings are usually assembled from two pieces, the top half and the bottom half, connected using flanges. Compressor casings are normally forged using lightweight titanium alloy. Such alloy allows for structural expansion and provides rotor blades tip clearance with the casing under high heat conditions.
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