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Basic Gas Turbine Theory > Compressor > Flashcards

Compressor Flashcards

1
Q

What are the primary and secondary functions of the compressor?

A

1st is to increase air pressure.
2nd is to supply sufficient air to combustion chambers.

Secondary function:
Supply Bleed air for cooling.

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2
Q

What is a pressure ratio?

A

The proportional increase in pressure through the compressor.

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3
Q

What is the preferred pressure ratio for a compressor?

A

High-pressure ratio.

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4
Q

What are the two types of compressors?

A

Centrifugal compressor and axial flow compressor.

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5
Q

What are the three main components of centrifugal compressors?

A

Impeller, Diffuser, and compressor manifold.

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6
Q

What is the principle of operation for a centrifugal-type compressor?

A

Air is ducted to the centre of the compressor, flow is axial initially, then the impeller accelerates air outwards due to centrifugal reactor to the impeller rotational speed. Air expands in the diffuser due to the divergent ducts, and then Bernoulli’s principle acts to reduce velocity and increase static pressure.

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7
Q

How are the vanes set up on the diffuser?

A

They are tangential to the impellor, they are a divergent duct that will convert the kinetic energy to pressure energy.

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8
Q

What are the advantages and disadvantages and characteristics of the centrifugal compressor?

A 
Advantages: More resistant to damage
Easier to manufacture
Cheaper and smaller
Disadvantages:
Significant performance limitations
Characteristics: Usually combined with other compressors, not usually found on transport jet aircraft.
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9
Q

What is the benefit of axial flow compressor efficiency compared to the centrifugal compressor?

A

The axial flow compressor is able to achieve a high degree of efficiency at a range of pressure ratios. Where the centrifugal-type peaks at about 5 then reduce dramatically.

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10
Q

What is the SFC equation?

A

fuel flow (lb/hr)/thrust (lb)

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11
Q

What causes the centrifugal compressor to not be able to produce acceptable efficiencies.

A

Maximum flow capacities of the diffuser and the inherent inertia of the rotating impeller. Since an increase in engine demand requires more fuel flow to the combustion chamber if the impeller is not able to supply the required air over fueling could occur.

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12
Q

What effects does over fuelling have on the combustion chamber?

A

It will lead to a higher combustion chamber pressure, which then induces an adverse pressure gradient followed by flow stagnation.

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13
Q

What is the principle of the axial flow compressor?

A

Converts kinetic energy to pressure energy by passing air through consecutive rows of rotating and stationary blades.

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14
Q

What is the compressor rotating section driven by on axial flow compressor?

A

Driven by one or more turbines at the rear of the engine.

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15
Q

When two compressor assemblies are fitted what are the two called?

A

Multi Spool compressor with two compressor
The first is called the low-pressure compressor and the second is the high-pressure compressor.
They are driven of a Low pressure turbine and high pressure turbine respectively.

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16
Q

What is the purpose of rotor blades?

A

To move air rearward

17
Q

What are the rotor blades generally made out of?

A

Normally made out from titanium or aluminum

18
Q

What is the purpose of the stator blades?

A

To receive high-velocity air and diffuse it to reduce velocity and increase pressure by way of a divergent duct.
Change kinetic energy to potential as pressure
Aerofoil shape (divergent)
Stationary Unit attached to the outer casing of the compressor

19
Q

What is the operating principle of the rotor and stator vanes in the compressor?

A

The stator vanes slow the air by the means of a divergent duct, converting the accelerated velocity from the rotor blades to higher static pressure. The vanes are positioned at an angle such that the air from the stator is directed into the rotor blades of the next stage at the most efficient angle of attack. This process gets repeated multiple times.

20
Q

What are Inlet guide Vanes?

A

Inlet guide vanes may be stationary or variable inlet guide vanes. They are ahead of the compressor rotor which directs airflow into the 1st rotor stage at the optimum angle of attack.
Part of the inlet

21
Q

What are speed operations is stationary Inlet guide Vanes designed for?

A

Relatively constant speed compressor.

22
Q

What are variable inlet guide vanes designed for?

A

Airflow control so the compressor can operate at a wider speed range.

23
Q

What is the purpose of the axial diffuser?

A

It is an integral part of the compressor casing or sometimes a separate assembly. It is a divergent passage to decrease air velocity and increase air pressure. It directs the airflow to the combustor section

24
Q

What is the purpose of the exit guide vane?

A

To straighten the air to axial flow
Assist with compression
Direct the airflow from the last stage of the compression to the combustor.

25
Q

Combination Compressors?

A

Combine both centrifugal and axial designs.

Uses advantages of both being lightweight, ease of starting, and can reduce chances of the stall

Axial produces initial compression and Centrifugal give a final boost

Produces a high mass airflow in its axial section for a small cross-sectional area, due to the high axial velocity present.

Eliminates the disadvantages of a centrifugal engine

Small engine application

Most sutiable when combined with reverse flow combustor.

26
Q

What are the main advantages of an axial-flow compressor?

A

Consumers more air for the same frontal area and can be designed to attain much higher pressure ratios. Therefore will give more thrust to the same frontal area.

27
Q

Advantages of Multispool compressors.

A

Dual and triple spool compressors were developed for operational flexibility as they provide high compression ratios and quick acceleration and better control of stall characteristics.

28
Q

What is a spool?

A

Combination of a compressor and a turbine.

29
Q

What is a compressor stall?

A

Breakdown in airflow across one or more compressor blades and or one or more compressor stages.

30
Q

What is a compressor surge?

A

A complete breakdown in airflow through the compressor section leading to flow reversal

31
Q

How would you be able to identify a compressor stall and surge?

A

Compressor stall has no symptoms and can go unnoticed

Compressor surge you may feel vibrations and temperature and fuel flow and thrust indications may fluctuate.

32
Q

Causes of stalls and surges?

A

Condition of compressor blades depart from the desired states
Positive incidence angle at the low-pressure stage at low speeds
Negative incidence angle at the high-pressure stage at high speeds
Turbulent air or disrupted airflow
Excessively rich or lean fuel mixtures cause abrupt engine acceleration/deceleration
Contaminated or damaged compressors
Damaged Intake ducting
Damaged turbine components
Engine operation above or below designed.

33
Q

What are the symptoms of compressor stall or surge?

A 
Vibrations
High Gas Temperature
Fluctuating engine instruments
Loss of power
Engine surge (flow reversal)
Engine sneeze or loud bang
Flames were seen from the front of the engine
34
Q

How did engine designers fix the problem of the front stages of the compressor were overloaded and the rear stages would operate inefficiently.

A

Arranging a large number of stages is required on two spools, each driven by its own turbine which provides the best speed for compression at those stages.

35
Q

What are the blades like in the LP and HP Compressor?

A

LP blades are longer than the HP

36
Q

What is the LP and HP rotational speed

A

N1 AND N2

37
Q

How can compressors be repaired?

A

Cleaning, blending damage
Individual blade replacement
Deeper level maintenance

38
Q

What could be caused by dirty or damaged compressors?

A

Reduce SFC
Reduction in power.
A stuck bleed air valve will drastically reduce compressor efficiency due to the huge loss of air and drop in pressure.

39
Q

How can it be an indication of a dirty or damaged compressor?

A

High EGT and low thrust/power

Basic Gas Turbine Theory (14 decks)

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