Turbine Engine Inlet Systems

Question 1

Describe the purpose, design and principles of operation of the engine inlet duct. (4)

Answer 1

must slow any air to subsonic speed, and convert the kinetic energy (speed) of the airflow into pressure energy

must present the air to the fan or compressor as smoothly and as linearly (in a straight line) as possible, at all stages of flight (take off, descent, and maneuvering)

Cause as little drag as possible

Ram recovery. As an aircraft starts moving forwards on the take off roll, the engine and inlet itself creates drag, which reduces thrust. As the aircraft speed increases, the ram effect of air being gathered by the inlet duct, accompanied by the diffusion (air spreading out) of the air inside the duct, causes an increase in thrust.

Question 2

Describe the purpose of a compressor in a turbine engine

Answer 2

To compressor and deliver to combustion.

Can also provide air for other function such as cooling air, pneumatic starting, cabin pressurisation, inlet anti icing and hydraulic system pressurisation.

Question 3

Describe the basic principles of operation of centrifugal and axial flow compressors.

Answer 3

centrifugal compressors operate by inducting air into the eye of the impeller, and then rotating it radially outward along veins. The diffuser works similarly to increase the pressure further.

Axial flow compressors convert kinetic energy to pressure energy by passing air through consecutive rows of rotating and stationary blades. the rows of rotating blades direct and accelerate the air into the corresponding rows of stationary diffuser blades.

Question 4

Describe and explain the purpose of a subsonic divergent inlet duct.

Answer 4

It increases the static pressure of the air and reduces its velocity known as ram recovery. Higher pressure, higher the ram efficiency of the intake/compressor combo.

Question 5

Describe typical compressor pressure ratios for the various types and configuration of turbine engine and SFC relation

Answer 5

Centrifugal - 7:1 ratio per stage but only across about two stages before inefficiency.

Axial - 1.2 : 1 per stage

SFC is the flow rate of fuel measured by weight (kg/hr) used for each unit of thrust produced by a turbine. As compressor ratio increases SFC decreases meaning its more efficient

Question 6

Describe the compressor arrangements found on the various types of turbine engine.

Answer 6

Turboprop/turbo shaft engines tend to be either, a centrifugal compressor arrangement or an axial flow/centrifugal compressor arrangement, due to the lower performance requirements of these engines when compared to long haul airliners and the rugged conditions that these engines tend to be exposed to.

Turbojet/turbofan engines are only ever axial flow type compressor equipped due to the much higher performance requirements of the aircraft these engines are designed for and the higher operating efficiencies of the axial flow compressor arrangement

Question 7

Describe factors affecting compression ratio

Answer 7

Type of compressor and number of compressor stages.

Question 8

Define bypass ratio

Answer 8

A comparison between the mass flow rate of air drawn into the engine through the fan disk that goes around the engine core with the mass flow rate of the air that goes through the engine core.

Question 9

Describe the design of, and bypass ratios associated with, various bypass fans, from low bypass to ultra-high bypass.

Answer 9

High bypass - high ratio of air bypassing the engine core. E.g 10:1 meaning for every 1kg of air going through the engine core 9.6 kg is routed through the bypass duct.

Low bypass - e.g 2:1 not as efficient.

Ultra high - more then high, typical jet engine use 1-2 propellers mounted either at the rear or front of the engine. If two props r used they’re contra-rotating.

They differ from turboprop where blades are much larger and air similar to a turbofan engine without a cowling.

Question 10

Describe purpose and function of impeller

Answer 10

Rotating disc with vanes that are forged in one piece. The vanes form divergent passes which radiate from the centre.

Impellers are designed to compress the air received by them; rotating at high speed they accelerate the received air from their eye radially outwards, imparting both velocity and pressure. Are found in Centrifugal compressors.

Question 11

Describe purpose,build and function of diffuser.

Answer 11

Diffusers are areas within the engine, aft of the compressor section with an increasing cross sectional area.

designed to further slow the processed air down and create a final increase in pressure within the air before passing the air into the combustion chamber.

Question 12

Describe rotor blades

Answer 12

Rotors are blade which are turned by the turbine, they pick air up and force it rearwards.

The air is accelerated by the rotors and its pressure is increased.

Question 13

What is a stator blade.

Answer 13

The other part of a stage (combo of stator and rotor blade) which is fixed and doesn’t rotate.

It’s job is to keep the flow from spiralling around the axis by bringing the flow back parallel to the axis.

Question 14

What is a variable stator blade.

Answer 14

Stator blades on many turbine engines which can rotate to ensure airflow strikes the next rotor blade at the correct angle to ensure maximum efficiency and blade stalling.

Question 15

What is an inlet guide vane.

Answer 15

Prior to first rotor there’ll be a ring of inlet guide vanes (another set of stators) and they change the direction of the flow of air so that it meets the first rotor blade at the correct AoA.

They cause air to increase in velocity very slightly, decrease in static pressure.

Question 16

What are Variable inlet guide vanes.

Answer 16

Same as inlet guide vanes but they can change their Angle.

They allow air to strike rotor blades at the correct angle.

Question 17

State the reason for the small pressure change per stage in an axial flow compressor.

Answer 17

through processing the air steadily without huge changes the air can be further processed without being limited by the factors that limit the centrifugal compressor.

This gradual rise allowing further processing means overall a much larger pressure rise is possible thereby increasing compressor efficiencies.

Question 18

State the reason for the decrease in size and increase in the number of compressor blades towards the rear of an axial flow compressor.

Answer 18

As air travels through compressor its pressure and temp increase but velocity remains constant.

To ensure this constant velocity rotors at rear must be shorter to create a convergent shape.

Secondly as air is compressed it requires less volume to occupy done by reducing the area of the blades over stages.

Question 19

State the reasons for and advantages of multiple spool compressors.

Answer 19

Having separate spools on turbofans allows the low pressure and high pressure sections to operate at different RPMs, as the fan blade tips would go supersonic if they tried to spin at the same RPM as the much smaller high pressure blades.

Question 20

Advantages of centrifugal compressor (3)

Answer 20

• Simpler,cheaper and easier to manufacture
• more sturdy so more resistance to damage
• HP rise per stage ( up to two stage) which can reach between 10:1 and 15:1 over the two stages combined

Question 21

Disadvantages of centrifugal compressor (3)

Answer 21

• Large frontal area for a given air flow rate compared to the axial flow compressor, therefore more drag
• Cannot have more than two stages due to the energy loss between stages
• Poor acceleration leading to surging, stalling and choking.

These are due to the diffuser only being able to cope with a certain amount of air flow, and the inertia of the impeller (it takes a while to spool up to the speed commanded)

Question 22

Axial flow compressor advantages (4)

Answer 22

• Smaller frontal area for a given flow rate, therefore less drag than a centrifugal compressor
• In manufacturing, more stages can be added to obtain a higher compression ratio -not possible with a centrifugal compressor due to losses experienced by the diffuser and impeller
• Can provide a high compression ratio without large losses in efficiency
• quick acceleration

Question 23

Axial flow disadvantages (2)

Answer 23

• More complex design, engineering, maintenance, and therefore more costly
• Vulnerable to foreign object damage, and birds

Question 24

Describe the merits of combined centrifugal and axial flow compressor combinations in turbine engines.

Answer 24

The advantages of both can be leveraged without the disadvantages.

Question 25

Describe the common source of bleed air

Answer 25

It’s air extracted from the compressor of the engine and is used for a variety of reasons like to heat the engine inlets to prevent ice from forming, detaching and being ingested into the engine.

Question 26

Explain what’s meant by compressor stall

Answer 26

The compressor rotor blades are aerofoil shaped. If the airflow strikes these rotors at too high an angle of attack then, just like the wing of an aircraft, the blades will stall.

Since the rotors have a fixed pitch, a change in the relative velocity of air entering the engine (the axial velocity) can cause a stall.

Question 27

What’s a rotating stall

Answer 27

Usually, a group of blades will be affected. Airflow breaks away from the upper surfaces of the blades and will reduce the available air space between the blades. Air will be deflected to adjacent blades causing an increase in angle of attack for those on one side and a decrease on the other.

This causes the stall ‘cell’ to move around the blade row. This is known as a rotating stall.

Question 28

Describe what its meant by compressor surge

Answer 28

The complete breakdown in compression resulting in a reversal of flow and the violent expulsion of previously compressed air out through the engine intake. (Like a balloon being blown, if its full, air comes out)

caused by the compressor’s inability to continue working against the already-compressed air behind it.

The compressor either experiences conditions which exceed the limit of its pressure ratio capabilities, or is highly loaded such that it does not have the capacity to absorb a momentary disturbance, creating a rotational stall which can form in less than a second and can include the entire compressor.

Question 29

What is the surge point

Answer 29

Airflow and compressor pressure ratio at which surge occurs is called the surge point.

Anything beyond the surge line results in compressor stall or surge.

Question 30

Describe the symptoms of compressor stall/surge (8)

Answer 30

• Flashes or flame from the inlet or exhaust of an engine (can be confused from the ground as an engine fire)
• Vibration
• Banging noises
• Engine RPM fluctuating
• Fuel flow rate fluctuating
• Rising engine temperature, visible on the ITT, TIT or EGT gauges
• Loss of thrust
• Yawing of the aircraft towards the engine experiencing the surge or stall

(ITT = Interstage Turbine Temperature, TIT = Turbine Inlet Temperature, EGT = Exhaust Gas Temperature)

Question 31

State the conditions that are commonly known to produce compressor stall/surge with particular regard to—
(a) compressor maintenance

Answer 31

If components aren’t maintained properly excessive contamination build up on the blades cause disrupted air flow and will cause blades to be more sensitive to sudden RPM changes causing stall.

Question 32

State the conditions that are commonly known to produce compressor stall/surge with particular regard to—
Blade damage

Answer 32

Occur due to fatigue or due to engine ingesting foreign stuff.

As they spin quick, even a small amount of damage causes a large amount of centrifugal forces and vibration which combined cause a lot of damage.

Question 33

State the conditions that are commonly known to produce compressor stall/surge with particular regard to—
Inlet damage

Answer 33

Job of the inlet is to direct inflow smoothly into engine. It can fail if its damaged or the aircraft is is operating in a way where inlet cannot compensate.

Question 34

State the conditions that are commonly known to produce compressor stall/surge with particular regard to—
Fuel scheduling

Answer 34

If excessive fuel is added to the combustion chamber too quickly then the combustion chamber pressure will quickly rise to a point that is higher than the compressor outlet pressure.

results in a backflow through the compressor and hot gases exiting the engine through the engine intake.

Question 35

Describe the use of bleed valve and bands

Answer 35

They are used to control the AoA when it meets the compressor and to ensure that theres not too much pressure in the rear stages of the compressor. Its done by bleeding off some of the air pressure in the compressor.

They are open at low RPM and close as RPM increase.

Question 36

Describe use of variable inlet guide vanes

Answer 36

First row of stator blades and they’re used to change the direction of the flow of air so it meets the first rotor blades at the correct AoA.