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MECH217 - Thermodynamics > Gas Turbine Cycle > Flashcards

Gas Turbine Cycle Flashcards

1
Q

What assumptions (3) are made in the Brayton cycle?

A

Working fluid is air, ideal gas (constant cp, cv).
Processes are internally reversible.
Closed loop/cycle.

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2
Q

What are the 4 processes that occur in the Brayton cycle?

A

1 -> 2: isentropic compression
2 -> 3: constant pressure heating
3 -> 4: isentropic expansion
4 -> 1: constant pressure cooling

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2
Q

What are the 4 components in the idealised Brayton cycle?

A

1 -> 2: Compressor (Wc)
2 -> 3: Heat Exchanger (combustor, Qin)
3 -> 4: Turbine (Wt)
4 -> 1: Heat Exchanger (Qout)

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3
Q

At which points of the Brayton cycle are the pressures equal?

A

P1 & P3
P2 & P4

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3
Q

At which stage of the Brayton cycle is the temperature at a maximum?

A

Stage 3 (after the ‘combustion’/constant pressure heating)

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4
Q

At which stage of the Brayton cycle is the temperature at a minimum?

A

Stage 1 (after constant pressure cooling)

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5
Q

What are the two main applications of the Brayton cycle?

A
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6
Q

In the Brayton cycle, from stage 1 -> 2 how does the pressure/temperature/entropy/enthalpy change?

A

Pressure: P2 > P1 (increases)
Temperature: T2 > T1 (increases)
Entropy: delta S = 0 (no change)
Enthalpy: h2 > h1 (increases)

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7
Q

In the Brayton cycle, from stage 2 -> 3 how does the pressure/temperature/entropy/enthalpy change?

A

Pressure: P2 = P3 (no change)
Temperature: T3 > T2 (increases)
Entropy: S3 > S2 (increases)
Enthalpy: h3 > h2 (increases)

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8
Q

In the Brayton cycle, from stage 3 -> 4 how does the pressure/temperature/entropy/enthalpy change?

A

Pressure: P3 > P4 (decreases)
Temperature: T3 > T4 (decreases)
Entropy: delta S = 0 (no change)
Enthalpy: h3 > h4 (decreases)

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9
Q

In the Brayton cycle, from stage 4 -> 1 how does the pressure/temperature/entropy/enthalpy change?

A

Pressure: P4 = P1 (no change)
Temperature: T4 > T1 (decreases)
Entropy: S4 > S1 (decreases)
Enthalpy: h4 > h1 (decreases)

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10
Q

In which processes in the Brayton cycle does the temperature increase?

A

1-2 Isentropic compression
2-3 Constant pressure heating

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11
Q

In which processes in the Brayton cycle does the temperature decrease?

A

3-4 Isentropic expansion
4-1 Constant pressure cooling

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12
Q

In which processes in the Brayton cycle does the entropy change and in which direction?

A

2-3 Constant pressure heating - entropy increases
4-1 Constant pressure cooling - entropy decreases

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13
Q

In which process(es) in the Brayton cycle does the enthalpy decrease?

A

3-4 Isentropic expansion
4-1 Constant pressure cooling

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14
Q

In which process(es) in the Brayton cycle does the enthalpy increase?

A

1-2 Isentropic compression
2-3 Constant pressure heating

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15
Q

In which stage of the Brayton cycle is work (Wc) put into the cycle?

A

1-2 Isentropic compression (work used to compress working fluid)

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16
Q

In which stage of the Brayton cycle is heat (Qin) put into the cycle?

A

2-3 Constant pressure heating

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17
Q

In which stage of the Brayton cycle is work (Wt) taken out of the cycle?

A

3-4 Isentropic expansion (work used to power turbine for external use)

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18
Q

In which stage of the Brayton cycle is heat (Qout) taken out of/lost from the cycle?

A

4-1 Constant pressure cooling

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19
Q

Is the ideal Brayton cycle open or closed?

A

Closed

20
Q

What are the two main applications of the Brayton cycle?

A

Aircraft propulsion system (product = thrust, not power generation, power produced only used to operate compressor and other auxiliary systems in the air craft)

Stand-alone power generation system (product = work, its used to mechanically generate electricity)

21
Q

Draw a PV diagram for the ideal Brayton cycle.

A
22
Q

Draw a TS diagram for the ideal Brayton cycle.

A
23
Q

What is the equation for thermal efficiency of the Brayton cycle based on rp & y?

A

n = 1 - 1/(rp^((y-1)/y))

24
Q

What is the equation for thermal efficiency of the Brayton cycle based on heat?

A

n = 1 - Qout / Qin

25
Q

What is the equation for thermal efficiency of the Brayton cycle based on temperatures?

A

n = 1 - (T4 - T1)/(T3 - T2)

26
Q

What is the equation for an isentropic process?

A

PV^y = constant (before and after process occurs)

27
Q

What is the equation to find pressures from temperatures at different points in the cycle (or vice versa)?

A

T2/T1 = (P2/P1)^((y-1)/y)

28
Q

What is the equation for the pressure ratio?

A

rp = P2/P1 = P3/P4

29
Q

What is the relationship between the pressure ratio and thermal efficiency?

A

As pressure ratio increases, thermal efficiency increases logarithmically, levels off at n = 0.6-0.7.

30
Q

What is a typical pressure ratio for gas turbine engines?

A

Between 5-20

31
Q

Why can the thermal efficiency of the Brayton cycle not increase after a while if the pressure ratio is still increasing?

A

Due to practical limitations and material properties at high temperatures.

32
Q

What are the two roles of the air in the Brayton cycle?

A

O2 = source for combustion
Coolant to keep temperature within safe limits.

33
Q

What are the main differences between the ideal and actual gas turbine cycles?

A

There is not isentropic compression or expansion, as there are irreversibilities.

In the compressor: Wactual > Wisentropic
(Wa > Ws)
In the turbine: Wactual < Wisentropic
(Wa < Ws)

34
Q

What are the three ways that the actual efficiency of the compressor can be found in the Brayton cycle?

A

nc = Ws/Wa
nc = (h2s - h1)/(h2a - h1)
nc = (T2s - T1)/(T2a - T1)

35
Q

Draw and annotate the differences between the actual and ideal gas turbine cycles.

A

See Week 9 slide 16

36
Q

Where are the irreversibilities in the actual gas turbine cycle?

A

1-2 Compression is not isentropic
3-4 Expansion is not isentropic

37
Q

What are the three equations for efficiency of the turbine of the actual gas turbine cycle?

A

nt = Wa/Ws
nt = (h3 - h4a) / (h3 - h4s)
nt = (T3 - T4a) / (T3 - T4s)

38
Q

What are the improvements made to the gas turbine cycle to increase efficiency?

A
  1. Increasing the turbine inlet temperatures: e.g. use ceramic coatings on blades and design better cooling techniques.
  2. Increasing the efficiencies of machinery: computer aided-design enabled improvements on designs aerodynamics, minimising loss of energy.
  3. Adding modifications to the cycle: regeneration, intercooling, reheating.
39
Q

What are three modifications that can be made to the gas turbine cycle?

A

Regeneration
Intercooling
Reheating

39
Q

What is the back work ratio?

A

Power consumed by compressor : Power produced by turbine

40
Q

Describe how the gas turbine cycle is modified by adding regeneration? How does this make it more efficient?

A

Counter-flow heat exchanger (regenerator) transfers heat from exhaust gases to the high pressure air leaving the compressor.
This reduces the amount of fuel needed in the combustor.
Improves thermal efficiency.
Ideally the exhaust gases would heat the air up to T5 but actually it heats to T3 (below T5).

41
Q

Draw a gas turbine system containing a regenerator.

A

Week 9 slide 18

42
Q

Draw the components of a gas turbine.

A
43
Q

Draw a gas turbine cycle with intercooling and reheating.

A

Week 9 slide 19

44
Q

Why are intercooling and reheating always combined with regeneration for gas turbine cycle modification?

A
45
Q

How does intercooling and reheating improve the gas turbine cycle?

A

Improves back work ratio, not necessarily thermal efficiency.

46
Q

What is the function of an intercooler?

A

It reduces the temperature at which heat is supplied????????

47
Q
A

MECH217 - Thermodynamics (1 decks)

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