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Gas Turbines > Subjects 4 - 6 (incomplete) > Flashcards

Subjects 4 - 6 (incomplete) Flashcards

1
Q

What is the purpose of a compressor?

A

Supply air to the combustor at the correct temp, pressure and velocity.

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2
Q

What are the 3 mechanical arrangements of compressors?

A
  • Centrifugal
  • Axial flow
  • Axial Centrifugal
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3
Q

What are the 3 basic components on a centrifugal compressor?

A

Impeller (vaned disc), diffuser, compressor manifold

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4
Q

What materials are used to make the impeller?

A

Aluminium or titanium alloys

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5
Q

What is needed when a double entry impeller is used?

A

Requires a plenum chamber on the rearward face due to lack of ram pressure.

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6
Q

What is the advantage of a double-sided impeller vs a single sided impeller?

A

Allows for higher mass airflow and smaller engine diameter

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7
Q

What is the diffuser section for in a centrifugal compressor?

A

Used to decrease the velocity and increase pressure
- Kinetic energy into static pressure

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8
Q

What is the purpose of a compressor manifold?

A

Changes airflow direction from radial flow to axial flow from the diffuser into the combustor.
- Manifold has 1 outlet duct and elbow per combustion chamber

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9
Q

Describe the operation of a centrifugal compressor

A
  1. Centrifugal force slings air outwards. Tip speeds can reach 1.3 Mach but remains subsonic.
  2. The pressure in the compressor casing allows for high energy transfer and prevents airflow separation.
  3. Airflow enters diffuser which reduces velocity and increase pressure
  4. Airflow enters manifold which has turning vanes to duct airflow into the compressor.
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10
Q

What is the single biggest factor affecting centrifugal compressor efficiency?

A

Tip clearance

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11
Q

What happens if the tip clearance is too small or too big on a centrifugal compressor?

A

Too Small:
- Causes aerodynamic buffeting, vibration, and unsteady airflow.
Too big:
- Causes large air leakage between impeller and compressor casing

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12
Q

What is a Dual stage centrifugal compressors used for and why is it rarely used?

A

Used for obtaining a higher-pressure ratio
- Rarely used due to energy losses in airflow (due to multiple turns in ducting) and extra weight.

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13
Q

On a double entry centrifugal compressor, the volume of air moved is determined by what?

A

Diameter of the compressor

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14
Q

What problems are encountered when a high tip diameter is used on a centrifugal compressor

A
  • High tip speeds reduce efficiency
  • Larger volume makes it hard to aerodynamically smooth
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15
Q

What is the best way to increase the volume of air without increasing diameter on a centrifugal compressor?

A

Use a double entry impeller

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16
Q

What is the purpose of a Plenum chamber in a centrifugal compressor

A
  • Holds air at higher pressure
  • Smooths out pulsations in air
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17
Q

What are the advantages to an axial flow compressor over centrifugal compressor?

A

Much higher compression ratio and smaller cross section (easier to streamline)

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18
Q

Describe the operation of an axial flow compressor

A
  1. Air flows through inlet guide vanes to meet the rotors at the right angle (slightly increases velocity)
  2. The air flows over the rotors which are powered by the energy taken from the turbine, increasing velocity.
  3. Airflow enters the stators which act like a diverging duct, increasing pressure and decreasing velocity.
  4. Once airflow passes the compression section, exit guide vanes removes swirls and directs it to the diffuser and combustor.
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18
Q

What are the disadvantages to an axial flow compressor over centrifugal compressor?

A

More expensive, heavier and more prone to FOD.

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19
Q

Where are Inlet Guide Vanes (IGVs) located?

A

Installed on the first rotor section

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20
Q

What is the Compressor Pressure Ratio?

A

The ratio of compressor discharge pressure to the compressor inlet pressure
- Ratio of exit pressure vs inlet

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21
Q

In terms of design, what is the goal when creating a compressor?

A

Highest compressor pressure ratio with low mass airflow
- However, higher pressure means stronger materials and higher cost

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22
Q

What is a typical large jet compression ratio?

A

50:1

23
Q

What is the equation for Total Compression Ratio?

A

Compressor discharge pressure / Compressor inlet pressure

24
Q

What is the cascade effect?

A
  • Air flows from low pressure to high pressure due to the close proximity of the rotors and stators and opposite cambers
  • High pressure of the stators gets pulled into the low pressure of rotors.
25
Q

Why are compressor sections tapered?

A

Bernoulli’s theorem: If area is constant, increase in velocity decreases pressure.
- To maintain constant airflow velocity, the compressor area is reduced.

26
Q

What are the two ways a compressor can reduce the area?

A

Either the compressor case gets smaller, or the rotor blades get smaller.

27
Q

In a Single spool Axial flow compressor, how many rotating elements are there?

A

1 rotating element, can only handle a few stages of compression.

28
Q

In a Single spool Axial flow compressor what happens if too many stages of stators/rotors are added?

A

The fwd section of the compressor becomes overloaded and inefficient, causing compressor surge.

29
Q

What is a compressor surge?

A

A severe compression stall and complete stoppage of airflow.
- Results in severe damage.

30
Q

How are compressor surges prevented?

A
  • Interstage compressor bleed
  • Adding another spool (dual spool)
31
Q

What are dual spool compressors?

A

Two rotating elements on their own separate turbines

32
Q

Are dual spool turbines mechanically linked or aerodynamically linked?

A

Aerodynamically linked

33
Q

What is the name given to the rearmost stage on a dual spool compressor?

A

N2

34
Q

On a Dual Spool Axial flow engine, what is N2 used for on the ground and what governs the speed?

A

Used for engine start due to lighter weight
- Speed governed by fuel control

35
Q

On a Dual Spool Axial flow engine, for a given power setting does N2 change speeds with flight conditions?

A

N2 speed remains constant
N1 speed changes with flight conditions

36
Q

How is a constant mass airflow given to N2 from N1?

A

By varying N1 speed

37
Q

How many rotating elements does a Triple spool have?

A

3 rotating elements

38
Q

What is the name given to N1 in a Triple spool axial compressor?

A

Fan

39
Q

What is the name given to N2 in a Triple spool axial compressor?

A

Intermediate compressor

40
Q

What is the name given to N3 in a Triple spool axial compressor?

A

HP compressor

41
Q

What is the stagger angle of a compressor blade and what does it do?

A

Twists in the blade that maintain constant airflow velocity

42
Q

Why is the trailing edge knife thin on a compressor blade?

A

Reduce turbulence and increase efficiency

43
Q

What are the two categories or compressor blade in terms of manufacturing?

A

Flat-machined top - Blades cut square at top

Profile (Squealer tip) - Reduced thickness at top

44
Q

What are Profile tips? (Squealer tips)

A
  • Reduced thickness at the top to increase resonant frequency (less vibration)
  • Designed as vortex tips which increase velocity and ensure minimal leakage.
45
Q

Why are Profile tips called Squealer tips?

A
  • Due to close tip clearance, the compressor shroud is lined with abradable material
  • Engine shutdown = High pitch squeal
46
Q

How does IGVs affect airflow and velocity?

A
  • Minimal effect
47
Q

What are the two categories or IGVs?

A
  • Fixed angle
  • Variable angle (controlled hydraulically using fuel, prevents compressor stalls)
48
Q

How is a Stator blade designed?

A
  • Aerofoil shape, similar to rotors
  • Mounted to make a divergent duct
49
Q

The first few stator stages can be controlled, what does it use to control the orientation?

A
  • Uses fuel from the engines fuel control to hydraulically move stators
50
Q

What materials are used to create the stator blades?

A

Steel or nickel alloys
- Must have high fatigue strength when ‘notched’ by ingestion damage

51
Q

Where is the diffuser located in an axial flow engine?

A

Located just after the exit guide vanes and prior to the combustor

52
Q

What are the two purposes of a diffuser in an axial flow engine?

A
  • Increase pressure and decrease velocity to combustion
  • High pressure, High temperature bleed air
53
Q

What applications are Axial-Centrifugal (Compound) compressors used in?

A

Small gas turbines
- e.g Helicopters

54
Q

What are some advantages to a Axial-Centrifugal compressor?

A
  • Small cross-sectional area and high mass airflow -AXIAL SECTION
  • Good compression ratio over wider range -CENTRIFUGAL SECTION

Gas Turbines (5 decks)

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