Gas Turbine Engine Impellers

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