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Module 15 - Gas Turbine Engines > 15.4 - Compressors > Flashcards

15.4 - Compressors Flashcards

1
Q

What are the two types of compressor?

A

Centrifugal flow compressor

Axial flow compressor

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2
Q

What are the main parts of a centrifugal flow compressor?

A

Impeller (accelerating the air flow)

Diffuser (produces the pressure rise)

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3
Q

What are the main parts of a axial flow compressor?

A

Rotors and stators

Accelerating the air and diffusing the air until required pressure rise is obtained

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4
Q

How is the efficiency of a centrifugal flow compressor maintained?

A

By keeping tip clearances between the impeller and the casing as small as possible.

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5
Q

What is the design point of an engine?

A

The point at which the engine will operate during most of its service life based on cruise speed and altitude (air density)

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6
Q

What is the purpose of variable stator vanes in an axial flow compressor?

A

They ensure the airflow is directed onto the following stage of rotor blades at an acceptable angle.

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7
Q

Why are high tip speeds unwanted in a axial flow compressor?

A

Shock waves will be produced if travelling over their optimum (Mach 0.8) which reduces the efficiency of the compressor.

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8
Q

What are the three rotor blade fixing methods?

A

The dovetail root
The fir tree rood
The solid root

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9
Q

What are most fan blades made out of?

A

Titanium alloy

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10
Q

What is the main purpose of the last row of stator vanes in a compressor?

A

They act as air straighteners, removing swirl prior to entering the combustion chamber at uniform axial velocity

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11
Q

What happens to the outlet pressure of a compressor, if the inlet pressure increases?

A

It increases

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12
Q

What happens to the compressors capacity and ability to compress air if the inlet temperature decreases?

A

It increases

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13
Q

Air pressure increases with square of…

A

RPM

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14
Q

What is a stall?

A

A partial breakdown of the airflow through the compressor. It is a progressive condition which can cause a surge if not checked.

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15
Q

What is a surge?

A

A total breakdown of the airflow through the engine compressor which can cause the airflow in the engine in the worst case scenario, flow in the opposite direction!

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16
Q

What systems are in place to ensure stalls and surges do not happen?

A

Multispool compressors
Variable inlet guide vanes (VIGVs)
Variable stator vanes (VSVs)
Variable bleed valves (VBVs)

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17
Q

What does FADEC stand for?

A

Full Authority Digital Engine Control

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18
Q

When do compressor bleed valves open?

A

At low engine RPM so that excessive airflow can escape and prevents airflow blockage in the aft stages.

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19
Q

What are the disadvantages of using compressor bleed valves to prevent stalls and surges at low RPM?

A
  • decreased mass airflow through engine
  • decreased thrust for a given setting
  • raises engines’s SFC
  • raises EGT due to less cooling air flowing through combustion chamber
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20
Q

What is the ‘stagger angle’ on compressor blades?

A

They maintain exit velocity airflow at the same value along the blade length

21
Q

What are the inlet guide vanes’ purpose?

A

To control the angle of airflow into the compressor.

22
Q

What is a centrifugal compressor’s maximum tip speed?

A

1.2. Mach

23
Q

What is the cascade effect?

A

Axial flow compressors use this effect; the series of rotors and stators causing a cumulative effect when the output of one serves as the input of the other

24
Q

How are variable bleed valves (VBVs) operated?

A

By a fuel hydro-mechanical controlling system

25
Q

When are the Variable Bleed Valves (VBVs) open?

A

Fully open at idle speed and slowly close as the throttle increases.

26
Q

What is the compression ratio of a jet engine?

A

The ratio between the compressor output pressure and the compressor inlet pressure

27
Q

What are shroud fillet areas?

A

Critical areas on the blades where no damage is permissible

28
Q

When is a compressor most likely to stall?

A

During acceleration or deceleration.

29
Q

What do the variable stator vanes do?

A

Allow the correct angle of attack for smooth and rapid engine acceleration.
At low compressor speeds, VSVs are partially closed.

They control correct compressor pressure ratio (CPR) for a given mass airflow.

30
Q

When will the VSVs start to open in ISA conditions?

A

65% N2

31
Q

What is the advantage of using VSVs?

A

They allow the engine to operate a higher compression ratio for a given RPM and not have tendencies to stall.

32
Q

What happens to VSVs at low CIT?

A

At low compressor inlet temperature, VSVs will open at lower compressor speeds to increase airflow

33
Q

What is the advantage of using a FADEC system?

A

Better and more precise control of the airflow through the engine due to extended amount of input parameters.

Tests can also be done, reducing time on the ground and saving money

34
Q

What are the VBVs controlled by?

A

Hydraulically actuated by fuel pressure

35
Q

What is the EVMS?

A

Engine vibration monitoring system

36
Q

What is the ACMS?

A

Aircraft condition monitoring system

37
Q

What is the ECMS?

A

Engine condition monitoring system

38
Q

What is the ‘one shot trim balance’?

A

A less inexpensive way to check the fan imbalance. Can be done inflight or just a single ground run compared to previous years.

Fan blades and balances must be known prior to the test for a computer to calculate the corrective weights required to balance the fan and reduce vibration. Stops weights needing to be added manually during the tests and multiple engine runs.

39
Q

What is the maximum tip speed of a compressor blade?

A

1.2 Mach

40
Q

What position are the inlet guide vanes at when starting the engine?

A

Fully closed

41
Q

At low RPM, what position are the VIGV and bleed valves?

A

VIGV’s at MAX swirl position and bleed valves OPEN

42
Q

What is the average pressure rise across a single spool axial flow compressor?

A

up to 15:1 pressure rise

43
Q

What will severe rubbing of compressor blades usually cause?

A

Galling - cold fusing/sticking due to metallic contact

44
Q

What components in a gas turbine engine aid in guiding the airflow during load thrust operations?

A

Inlet guide vanes

45
Q

What are the signs of a compressor stall?

A

Banging in the jet pipe and a rapid rise in engine gas temperature

46
Q

What do the diffuser vanes do to convert energy in compression?

A

They convert kinetic energy into potential energy through divergence

47
Q

What is the purpose of the cross sectional area decreasing in the compressor at each stage?

A

It maintains the velocity of the air under rising pressures

48
Q

What is the purpose of the rotating guide vanes on a centrifugal compressor?

A

They direct the air smoothly into the impeller

Module 15 - Gas Turbine Engines (22 decks)

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