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AER345 - Aerospace Propulsion > Turbo Jet > Flashcards

Turbo Jet Flashcards

1
Q

List stages in a jet engine

A 
Inlet
Compressor
Combustion Chamber
Turbine
Propelling Nozzle
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2
Q

Giver equation for thrust of a jet engine

A

T = m(dot)C2 + A2(P2 - Pa)

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3
Q

Give reason for inlet

A

Critical for good performance - it guides the flow into the engine and aims to provide the fan with a uniform flow

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4
Q

Give stages in axial compressor and how flow moves over them

A

1 - Rotor (accelerates)

2 - Stator (decelerates)

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5
Q

When is centrifugal compressor used

A

Used where only single stage is required to produce a large pressure rise
- has larger frontal areas than axial (higher casing drag)

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6
Q

How does a centrifugal compressor work

A

Flow enters the rotor ‘eye’ and is accelerated as it flows radially outward to the tip
Flow then decelerates and increases pressure

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7
Q

What happens in a combustor

A

raises enthalpy of the working fluid by burning fuel in the compressed air delivered from the compressor
arranged in an annulus (doughnut) around the drive shaft

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8
Q

What happens in a turbine

A

Extracts energy from high enthalpy gasses after the combustion chamber and drives the compressor
turbine stages - stator then rotor

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9
Q

What is turbo fan used for

A

gets some thrust from the core and most from the fan

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10
Q

What is the BPR

A

Bypass ratio = air flows around core / flow enters core

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11
Q

What does twin-spool mean

A

two compressors driven by two turbines through concentric drive shafts at different speeds

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12
Q

What does triple-spool mean

A

two compressor driven by two turbine. Third turbine drives the fan

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13
Q

Explain using a T-S diagram where energy comes from to provide thrust

A

Distance between 2-1 is smaller than distance 4-3. Therefore due to combustion stage, enthalpy of the gasses is higher and so has more energy. Part is used to power the compressor, the rest is exited out a nozzle to accelerate air and convert enthalpy to kinetic energy

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14
Q

Give equation for work in compressor

A

-Wc = m(dot)Cp(T2-T1)

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15
Q

Give equation for work in combustor

A

Q = m(dot)Cp(T3-T2)

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16
Q

Give equation for work in turbine

A

-Wt = m(dot)Cp(T4-T3)

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17
Q

Give equation for ideal cycle efficiency, η

A

η = Wt - Wc / Q

η = ( (T3 - T4) - (T2 - T1) ) / (T3-T2)

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18
Q

Give equation for Carnot efficiency

A

η = Th - Tc / Th

η = T3 - T1 / T3

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19
Q

Give equation for turbine work ratio

A

ηt = Wt - Wc / Wt

ηt = useful turbine work / total turbine work

20
Q

Which is less efficient - Compressor or Turbine - Why

A

Compressor less efficient than turbine. Flow decelerates across stator …
boundary layer
separation
more losses

21
Q

List efficiency losses in jet engine components

A 
Fluid friction
- intakes
-combustion chambers
- exhaust ducts
- connecting ducts
Friction losses in shaft
- transfer energy from turbine to compressor
Auxiliary power requirement
- oil and fuel pumps
Air cooling blade roots and turbine disks
22
Q

Give equation for compressor efficiency

A

ηc = T02’ - T01 / T02 - T01

T02’ is isentropic stagnation temp
T02 is stagnation temp

23
Q

Give equation for turbine efficiency

A

ηt = T03 - T04 / T03 - T04’

T04’ is isentropic stagnation temp
T04 is stagnation temp

24
Q

Define polytropic efficiency

A

the isentropic efficiency for an elemental stage in a process such that it is constant through the whole process

reason: the increase in temp due to friction in one stage, results in more work being required in the next stage

25
Q

Describe how isentropic efficiency varies with pressure ratio for a compressor and for a turbine

A

Compressor:
medium, converges lower
Turbine:
medium, converges higher

Both start at same point at polytropic efficiency

26
Q

Equation for propulsive efficiency

A

ηp = Thrust power / Thrust power + Jet kinetic power

ηp = 2Ca / (Ca + Cj)

if Cj is almost Ca this means almost no thrust … where exit speed is almost equal to inlet speed

27
Q

Describe graph that shows relationship between ηp and Cj/Ca

A

1/x graph

high ηp to start, decays rapidly and the converges almost at 0 for increasing Cj/Ca

28
Q

What is specific thrust

A

Specific thrust -
the thrust per unit mass of air flow

Fs = F / m(dot)

29
Q

What is SFC

A

Specific Fuel Consumption -

SFC = mf(dot) / F

30
Q

What are the mechanical losses

A

Turbine power transmitted directly to the compressor, however losses in bearing and windage
assume = 0.99 if not given

31
Q

Give Cp for air and exhaust gasses

A

Cp air - 1.4

Cp gasses - 1.333

32
Q

How do inlets effect air flow for different conditions

A

Stationary - inlet acts to accelerate airflow into the engine

cruise speed - inlet acts as a diffuser and slows down airflow

33
Q

What could distortion of inlet air lead to

A

flame out or mechanical damage due to surge

34
Q

How to calculate stagnation temperature

A

T02 = T1 + C1^2/2Cp

35
Q

What is RAM pressure

A

pressure from moving through a fluid
is taken into account in inlet efficiency
ηi = P01 - Pa / P0a - Pa

36
Q

What is used for thrust reversing

A

Thrust is reversed by using clamshell doors to direct airflow in opposite direction

37
Q

How does thrust vary with altitude

A

Thrust decreases initially with increased altitude due to increased momentum drag
then will increase due to beneficial effects of the RAM pressure rise

38
Q

EPR is

A

Engine Pressure Ratio = P04 / P01

39
Q

RPR is

A

Ram pressure ratio = P01 / Pa

40
Q

OPR is

A

Operational pressure ratio

OPR = EPR x RPR

OPR = P04 / Pa

41
Q

Describes possible methods to increase thrust

A
  • re-design for increase in mass flow rate or increase in T3
  • temporary thrust increase:
    • liquid injection
    • afterburning
  • methanol injection:
    spray water methanol mixture at compressor inlet during take-off and climb, equivalent drop in compressor temp therefore less work required to run, so turbine can use more energy on thrust
    this also increase mass flow rate too
42
Q

How do afterburners work

A
  • burn additional fuel in the jet pipe
  • no rotating parts so max temperature increases (2000K)
  • accept penalty in SFC
  • efficiency drops a lot!
43
Q

Difference in noise between turbojet and turbofan

A

noise reduction in turbofan is massive

44
Q

What is the problem with having the fan aft of the engine

A
  • turbine blades for hot stream and fan for cold stream

- sealing

45
Q

If mhot and mcold mix in the nozzle of a turbofan, how should this be treated when calculating

A

Assume:

  • full mixing of the streams
  • inlet static pressure equal

Conduct:

  • energy balance to get exit temperature
  • momentum balance to get exit pressure
46
Q

Give equation for thrust in a turboprop

A

T(tp) = (SP)ηpr + FCa

Sp - shaft power

Designs usually have an unchoked nozzle with Mcruise = 0.6

AER345 - Aerospace Propulsion (5 decks)

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