Gas Turbine Engine Impellers

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Questions and Answers

What is the advantage of the single-stage dual-sided impeller?

Increased compressor ratio and mass air flow

Reduced turbulent side air entry

Reduced impeller diameter (correct)

Efficient ram recovery

What is the purpose of inlet guide vanes?

To ease the change of airflow from radial to axial direction

To reduce turbulent side air entry

To increase compressor ratio

To induce rotation of the air into the eye of the impeller (correct)

What is a disadvantage of the single-stage dual-sided impeller?

Inefficient ram recovery (correct)

Increased compressor ratio

Reduced impeller diameter

Efficient airflow direction change

What is a major limitation of centrifugal compressors in larger engines?

Mass airflow Signup and view all the answers

What replaced the single-stage dual-sided impeller?

Single-sided two-stage centrifugal compressors Signup and view all the answers

What is an advantage of a centrifugal compressor over an axial compressor?

Shorter in length Signup and view all the answers

What is formed by inclining the front sections of the impeller vanes?

Inlet guide vanes Signup and view all the answers

What is a characteristic of a single-stage centrifugal compressor?

Can achieve compression ratios of up to 10:1 Signup and view all the answers

What is situated immediately prior to compressor entry when inlet guide vanes are not utilized?

Stationary pre-swirl vanes Signup and view all the answers

Why are more than two stages of single entry centrifugal compressors impractical?

Due to all of the above Signup and view all the answers

What is a characteristic of some centrifugal flow turboprop engines?

They have two separate rotating assemblies Signup and view all the answers

What can be achieved by adding a second compressor stage?

Compression ratio can be increased to 15:1 Signup and view all the answers

What is the primary purpose of the diffuser in a gas turbine engine?

To reduce the air velocity before it enters the combustion chamber Signup and view all the answers

What is the point of highest pressure in a gas turbine engine?

The compressor diffuser section Signup and view all the answers

What are the two forces that combine to form the angle of attack (AOA) of the compressor blade?

Inlet air velocity and compressor rpm Signup and view all the answers

What occurs when there is an imbalance between the inlet air velocity and compressor rpm?

Compressor stall Signup and view all the answers

What is the term used to describe when the entire compressor stalls?

Compressor surge Signup and view all the answers

What can be done to minimize the chance of an intake-induced stall?

Careful inlet design Signup and view all the answers

What is the operating zone between the 'normal operating line' and the 'surge stall line'?

Surge-stall margin Signup and view all the answers

What is the point on the operating line where the engine operates during most of its service life?

Design point Signup and view all the answers

What can occur when a stall or surge causes a momentary stagnation of mass airflow followed by an over-rich condition in the combustor?

Exhaust flames Signup and view all the answers

What is the purpose of a compressor anti-stall bleed system?

To unload the compressor during starting and low power operation Signup and view all the answers

What is the result of compressor efficiency deterioration?

Decrease in surge-stall margin Signup and view all the answers

What is the purpose of the compressor surge/stall bleed air valve system?

To unload the compressor during starting and low power operation Signup and view all the answers

What is the primary purpose of the variable stator vane actuating system in a gas turbine engine?

To maintain the correct AOA relationship between inlet airflow and compressor speed Signup and view all the answers

At low compressor speeds, the variable stator vanes are:

Partially closed Signup and view all the answers

What is the result of varying the vane angle in a gas turbine engine?

Scheduled AOA relationship between airflow and rotor blades Signup and view all the answers

Why do variable stator vanes open as compressor rotor speed increases?

To allow more air to flow through the compressor Signup and view all the answers

What is the effect of varying vane angles on the airstream in a gas turbine engine?

Slows the airstream's axial velocity Signup and view all the answers

What is the benefit of the correct AOA relationship between airflow and rotor blades in a gas turbine engine?

Smooth and rapid engine acceleration Signup and view all the answers

Study Notes

Impeller Types

Impellers can be single-entry (single-sided) or dual-entry (dual-sided)
Single-stage dual-sided impeller provides the same compressor ratio and mass air flow with a smaller impeller diameter
Disadvantages of single-stage dual-sided impeller: turbulent side air entry and inefficient ram recovery

Inlet Guide Vanes

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 can be formed by inclining the front sections of the impeller vanes to impart a whirl motion in the direction of impeller rotation

Centrifugal Compressors

Centrifugal compressors are commonly used in conjunction with axial flow compressors
Centrifugal compressors are used in smaller flight engines, including turboshaft, turboprop, and turbofan
Larger engines use axial flow type compressors due to the limitation on mass airflow
Compressor ratios are about the same for single-sided and dual-sided single-stage impellers
Ratios as high as 10:1 can be obtained from a single-stage centrifugal compressor
Compression can be boosted to about 15:1 by a second compressor stage

Multistage Centrifugal Compressors

More than two stages of single entry are impractical due to:
- Airflow energy loss when making the turns from one impeller to the next
- High weight per stage
- High drive power extraction

Centrifugal Compressor Advantages

Centrifugal compressor is shorter in length than an axial compressor
Centrifugal compressor has advantages due to its shorter length

Compressor Diffuser

The diffuser reduces the velocity of the air to prevent the flame in the combustion chamber from being blown out
The diffuser increases the static pressure of the air, preparing it to enter the combustion area
The diffuser is known as the point of highest pressure in the gas turbine engine

Compressor Stall and Surge

Compressor aerofoils experience an infinite variety of AOAs and air densities
Controlling the AOA is a design function of the inlet duct, the compressor, and the fuel control sensors
AOA of the compressor blade is the result of inlet air velocity and compressor rpm
Compressor stall occurs when there is an imbalance between the two vector quantities
Compressor stall can cause engine damage and a loud audible noise

Anti-Stall Bleed System

The compressor anti-stall bleed system unloads the compressor by dumping unwanted air during starting and low power operation
The system restricts the inlet airflow during starting and low power operation
The system is installed on some gas turbine engines to minimize compressor acceleration and deceleration stall problems at low and intermediate speeds

Variable Vane System

The variable stator vane actuating system is incorporated on many gas turbine engines
The system varies the geometry of the compressor gas path to exclude unwanted air and maintain the proper relationship between compressor speed and airflow
The system is used to minimize compressor acceleration and deceleration stall problems at low and intermediate speeds

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Description

Learn about the types of impellers used in gas turbine engines, including single-entry and dual-entry impellers.