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AER345 - Aerospace Propulsion > Axial Turbines and Combustion > Flashcards

Axial Turbines and Combustion Flashcards

1
Q

Describe the basic design features of an axial turbine

A
  • turns flow away from engine axis
  • flow area decreases
  • average velocity increase
  • average pressure decreases
  • extracts energy from hot gases and drives the compressor
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2
Q

Give differences between inpulse and reaction turbine

A

Impulse:
all pressure drops is in the nozzle effect on blades is momentum only

Reaction:
Pressure drop in both rotor and stator

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3
Q

How does Cw vary between turbine and compressor

A

Cw is greater in Turbine

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4
Q

What is stage loading

A

defines how much work can be obtained from a stage

work increases usually means efficiency decrease

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5
Q

What does a high value flow coefficient mean

A

Flow coefficient = Cx / U

high flow coefficient - low pitch-chord ratio

low flow coefficient - high pitch-chord ratio

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6
Q

What does a Smith chart show

A

temperature drop coefficient vs. flow coefficient

  • used as a guide for a new turbine design
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7
Q

List requirements for combustors

A
  • high combustor efficiency (fuel burnt and heat liberated)
  • reliable smooth ignition
  • wide stability limits
  • free from pressure pulsations
  • low pressure loss
  • good outlet temp traverse (blade and nozzle life)
  • min. cost and ease of maintencance
  • durability
  • size constraints
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8
Q

What is the purpose of a baffle

A

used to create a flow reversal and recirculation region in which a flame can be sheltered and sustained

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9
Q

How is pressure loss reduced in comrpessor

A

air flow is decelerated using a diffuser
reduced by a factor of 5

  • optimal diffuser 7-12°
  • frictional losses
  • short diffusers are required
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10
Q

What is an approximate AFR

A

AFR - Air-Fuel Ratio

around 35:1

  • outside the flammability limits so air is introduced in stages
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11
Q

Why is air introduced in stages in a combustor

A

Air-Fuel ratio is outside the flammability limits so introducing in stages allows for ignition

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12
Q

What is the cold pressure loss

A

this is the combined pressure losses due to the diffuser and air-fuel holes

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13
Q

List advantages and disadvantages of tubular combustion system

A

A:

  • mechanically robust
  • fuel and airflow patterns easily matched
  • rig testing easy small fraction of engine flow

D:
- Bulky and heavy

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14
Q

List advantages and disadvantages of annular combustor

A

A:

  • min. length and weight
  • min. engine frontal area
  • min. pressure loss
  • easy light around

D:

  • outer liner buckling loads
  • rig test require high airflows
  • difficult to match fuel and air patterns
  • inlet velocity profiles can effect outlet traverse
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15
Q

List advantages and disadvantages of tubo-annular combustion system

A

A:

  • mechanically robust
  • fuel and air patterns easily matched
  • rig testing easy small fraction of engine flow
  • low pressure loss
  • shorted and lighter than tubular

D:

  • less compact than annular
  • requires interconnectors
  • cooling requirements
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16
Q

List the stages of initial to a developed spray

A
  • dribble
  • distorted pencil
  • onion stage
  • tulip stage
  • fully developed spray
17
Q

How does blade stress loading vary to accommodate the varying temperatures seen

A

tip - temp low
med - temp high
root - temp low

There blade stress loading decreases from hub to tip so can allow temperatures up the span

18
Q

List alternative fuels

A
  • Biofuel
  • Hydrogen
  • Methane
  • Ethanol
19
Q

List reasons why Biofuel not used

A
  • argued that this would reduce global warming
  • risk of food production reduced
  • emission would still be there
20
Q

List reasons for hydrogen fuel

A
  • emissions are just water vapour

- density too small however

21
Q

Why is kerosene used

A
  • easy to handle
  • stable within a wide range of temperatures
  • highly calorific
22
Q

What is the reason for cooling turbine blades

A
  • so blades can cope with increase in combustor outlet

- this is so efficiency increases by having a higher specific work out

23
Q

How much can cooling decrease the temp of blade by

A

200-300K

24
Q

How are blades made

A

cast - single uni-directional crystal
or
forged

25
Q

How to increase heat transfer in blade cooling

A

make airflow turbulent

26
Q

What are NGV

A

Nozzle guide vanes:

  • just after the combustor
  • do not rotate
  • this means less stresses so higher temp possible
  • oxidation often the biggest problem
27
Q

How do high rotational speeds effect stresses and stage loading

A

Higher rotational speed:

  • increase stresses
  • reduce stage loading
  • also cooler temperatures seen as V2 seen
28
Q

List 3 cooling methods

A
  • impingement
  • passage
  • film cooling
29
Q

What is impingement cooling

A
  • internally air is drawn in at high compression and blasted onto the side walls of the turbine blade
30
Q

What is passage cooling

A
  • high turbulent air is passed through tubes within the blade
31
Q

What is film cooling

A
  • cooler air then exhaust is bled over the blade to create a barrier
32
Q

Give equation for cooling efficiency

A

efficiency = Tcool_out - Tcool_in / Tblade - Tcool_in

33
Q

Give equation for cooling effectiveness

A

effectiveness = Tgas - Tblade / Tgas - Tcool_in

34
Q

What is the compromise when trying to reduce pressure loss

A
  • reduce pressure loss by decrease speed
  • decrease speed by increase area
  • increase area means longer blades
  • longer blades mean more cooling and more friction and more likely to break-down aerodynamics
35
Q

What are the penalties for choosing to cool blades

A
  • Cost of manufacturing blades
  • more high pressure air required from compressor (6.5%)
  • film cooling disrupts the boundary layers, potentially increasing the profile losses
36
Q

Describe how varying where airflow is ejected from blades changes the situation

A

ejecting from tip:

  • requires additional work which reduces total power output
  • also a waste but mechanical integrity is not always possible to eject elsewhere

ejecting from trailing edge:

  • coolant flow will have been turned in the rotor cooling passages and will thus do some useful work
  • reduced entropy generation due to lower mixing losses
  • will mean trailing edge design has to be thicker however meaning a poorer wake
  • ejecting coolant here fills in that wake and ideally coolant velocity = mean velocity

AER345 - Aerospace Propulsion (5 decks)

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