30 Questions
What is the primary function of the diffuser in a gas turbine engine?
To convert velocity energy into pressure energy
What is the purpose of the turning vanes or cascade vanes in the manifold?
To reduce air pressure losses
What is the function of the compressor outlet elbow?
To act as an air duct
What is another term for a single-entry impeller?
Single-sided
What is the purpose of the manifold in a gas turbine engine?
To deliver air to the combustion chambers
What is a disadvantage of the single-stage dual-sided impeller?
Turbulent side air entry
What material is the diffuser typically made of?
Steel alloy
What is the function of the outlet ducts in the compressor manifold?
To act as an air duct
What is the purpose of inlet guide vanes?
To impart a whirl motion in the direction of impeller rotation
What replaces the single-stage dual-sided impeller?
Single-entry two-stage centrifugal compressors
What is the purpose of stationary pre-swirl vanes?
To ease the change of airflow from axial to radial direction
Where are inlet guide vanes sometimes formed?
At the front of the impeller
What is the direction of airflow and compression in an axial compressor?
Parallel to the rotational axis
What is a common drawback of single-spool compressors?
The forward stages are typically overloaded while the rear stages operate at a fraction of their capacity
What is the main difference between a single-spool and a dual-spool engine?
The mechanical connection between the rotors
What type of axial flow compressor is typically used in small turboshaft and turboprop engines?
Single-spool
What is a characteristic of a triple-spool engine?
It is used in some large and medium-sized turbofan engines
What is an advantage of dual-spool engines over single-spool engines?
They respond more quickly to control input changes
What happens to the kinetic energy of the air as it passes through the stator vanes?
It is converted to potential energy (pressure)
What determines the velocity of the airflow in each stage of the compressor?
The speed of the rotor
What is the secondary function of the stator vanes?
To direct airflow to the next stage of compression at the desired angle
What are the blades typically made of?
Stainless steel alloy or titanium alloy
What is a blisk?
A one-piece blade and rotor unit
What is the material used to make the discs?
Nickel steel alloy or titanium alloy
What is the operating zone between the ‘normal operating line’ and the ‘surge stall line’?
Surge-stall margin
At what point does the engine operate during most of its service life?
During cruise speed, at altitude
What may occur when a stall or surge causes a momentary stagnation of mass airflow followed by an over-rich condition in the combustor?
Exhaust flames
What is the purpose of the compressor anti-stall bleed system?
To unload the compressor during starting and low power operation
Why is the compressor anti-stall bleed system necessary?
To maintain AOA and air velocity within desired limits
What is dumped directly back into the atmosphere by the compressor surge/stall bleed air valve system?
Air not used for aircraft systems
Study Notes
Impellers
- Impellers can be referred to as single-entry (single-sided) or dual- or double- entry (dual-sided)
- Single-stage dual-sided impeller provides the same compressor ratio and mass air flow with a smaller impeller diameter
- Single-entry two-stage centrifugal turbo-propeller replaced the single-stage dual-sided impeller due to its disadvantages
Impeller Components
- Impellers incorporate inlet guide vanes called rotating guide vanes or inducers
- Inlet guide vanes induce rotation of the air into the eye of the impeller
- Inlet guide vanes may be part of, or attached to, the impeller
- Inlet guide vanes can be formed by inclining the front sections of the impeller vanes to impart a whirl motion in the direction of impeller rotation
Diffuser
- High-velocity air from the impeller is slung into divergent ducts within the diffuser
- The purpose of the diffuser is to convert velocity energy into pressure energy
- Diffusers are typically made from aluminum alloy or steel alloy
Manifold
- The purpose of the manifold is to change the airflow direction and deliver air to the combustion chambers
- Turning vanes, or cascade vanes, within the manifold straighten the airflow
- Manifold distributes the air in a smooth flow to the combustion section
Axial Flow Compressors
- Axial flow compressors have airflow and compression occurring parallel to the rotational axis
- There are three types of axial flow compressors: single-spool, dual-spool, and triple-spool
- Single-spool axial flow compressors are commonly used in small turboshaft and turboprop engines
Rotor Blades
- Each rotor consists of a set of blades fitted into a disc, which move air rearwards through each stage
- The speed of the rotor determines the velocity present in each stage
- Kinetic energy is transferred to the air with increased velocity
- Stator vanes receive the air at high velocity and act as a diffuser, changing kinetic energy to potential energy (pressure)
Blisks
- Blisks are one-piece blade and rotor units
- Blisks are commonly used in small turboprop and turboshaft engines
- Blisks are being applied to many smaller fans, compressor rotors, and stators, and to some turbine components
Surge-Stall Margin
- The surge-stall margin is the operating zone between the 'normal operating line' and the 'surge stall line'
- The margin decreases as compressor efficiency deteriorates
- Design point is the point on this line at which the engine operates during most of its service life
Test your knowledge of gas turbine engine components and their functions, including diffusers, turning vanes, compressor outlet elbows, and manifolds.
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