Lecture 6 - Joule Brayton Gas

Question 1

What gas is normally used and how is it treated?

Answer 1

Atmospheric air

Ideal gas

Question 2

Why is there no need for a cooler?

Answer 2

The working fluid isn’t reused- a continuous supply of fresh air is drawn into the compressor

Question 3

What drives the compressor?

Answer 3

The turbine

as more energy is generated than required to drive compressor

Question 4

What is represented by lines 1-2, 3-4, 3-5 and 5-4?

Answer 4

1-2: work done by compressor
3-4: work done by turbine
3-5: work needed to drive compressor
5-4: ‘extra’ energy which can be used to drive a generator

Question 5

How do aircraft cycles differ from cycles used for generation?

Answer 5

The turbine expansion is smaller. Expansion is stopped at 5, so remaining enthalpy in fluid us converted to kinetic energy via a nozzle

Question 6

What may be eliminated from the SFEE in Brayton cycles, and why?

Answer 6

Potential terms
Density is very low
No height changes

Question 7

How is the enthalpy change calculated?

Answer 7

c(p)DeltaT = Deltah

Question 8

The cycle is reversible if:

Answer 8

1. The isothermal expansion and compression are reversible

2. The expansion and compression are isentropic

Question 9

How is the efficiency of the cycle calculated?

Answer 9

eff = (W(turbine) - W(compressor))/Q(2-3)

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

Question 10

What equation can be used because there are no pressure losses in the system?

Answer 10

(P3/P4) = (P2/P1) = r(p) 
Where r(p) is the cycle pressure ratio

Question 11

What increases efficiency (for a simple and ideal cycle)?

Answer 11

A higher pressure ratio

Question 12

What are the effects of increasing temperature?

Answer 12

Increasing work ratio

Decreasing size of gas cycle plant for a given power output

Question 13

What is the downside to increased temp?

Answer 13

The higher the temp of combustion products, the shorter the turbine blade life

Question 14

What could happen if the work ratio is low?

Answer 14

Any decrease in turbine or compressor performance could cause the cycle to collapse

Question 15

What happens if the isentropic efficiency of the adiabatic processes is less than 100%?

Answer 15

The compression process will require more work
The turbine process will produce less work
The output of the cycle will be decreased
The heat input required will decrease

Question 16

Are turbines or compressors generally more efficient?

Answer 16

Turbines, as it goes with the decrease in pressure

Question 17

What is the effect of a combustor pressure drop?

Answer 17

The expansion begins from a reduced pressure

The outlet temperature from the turbine is raised, so power is reduced from the turbine

Question 18

What is a gas turbine?

Answer 18

it is an engine designed to convert the energy of fuel into some form of useful power

Question 19

What does a gas turbine consist of?

Answer 19

a gas generator section and a power conversion section which can either be a nozzle to produce thrust or extra turbine stages to produce electricity

Question 20

What ideally happens in a combustion chamber in a gas turbine?

Answer 20

pressure does not decrease but usually drop about 5/10%

Question 21

What are the advantages of gas turbines?

Answer 21

large power to weight ratio
long mechanical life and low cost of maintenance
quick to start
wide variety of fuels can be utilised
Atmospheric air is used and no coolant is required

Question 22

Draw a TS diagram of a brayton cycle?

Answer 22

e

Question 23

Draw a pressure volume diagram of a brayton cylce?

Answer 23

e

Question 24

What are the different stages of a brayton cycle?

Answer 24

isentropic compression in compressoor
heat supplied at constant pressure
isentropic expansion through turbine
heat rejected at constant pressure

Question 25

What are the three processes in a brayton cycle and how is the work done and heat out calculated?

Answer 25

1-2 isentropic compression -Wc = mCp(T2-T1)
2-3 Isobaric heat addition Qb=mCp(T3-T2)
3-4 Isentropic expansion -WT = mCp(T4-T3)

Question 26

How are inlet and outlet temperatures calculated for turbines and compressors?

Answer 26

Using the ideal gas equation

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

Question 27

What is the equation for the ideal cycle efficiency:

Answer 27

eta = 1 - (1/Rp)^n-1/n

Question 28

What is the work ratio?

Answer 28

Useful turbine work/total turbine work

Question 29

How can the cycle efficiency be improved through introducing components?

Answer 29

using a recovery heat exchanger - using waste heat from the turbine to heat the gas after the compressor

Question 30

Whats the issue with a recovery heat exchanger?

Answer 30

They are huge so cannot be used for aircraft engines - only suitable for industrial gas turbine

Question 31

What happens in an ideal heat exchanger

Answer 31

T6=T2

T3=T5

Question 32

What is the approach temperature

Answer 32

the closest that temperatures from the cooling and heating process can by brought to delta Ta

Question 33

What are the final temperature after the heat exchanger if the approach temperature is taken into account

Answer 33

After the condenser can only be raised to T3 =T5 - deltaTa

After the turbine can only be cooled to T6 = T2+ deltaTa

Question 34

What is heat exchanger effectiveness?

Answer 34

epsilon = actual heat transfer / maximum heat transfer

Question 35

What is the mathematical equation for heat exchanger effectiveness?

Answer 35

epsilon qactual/qmaximum = mCp(T3-T2)/mCp(T5-T2)

only take into account approach temperature once

Question 36

How can approach temperature be found from heat exchanger effectiveness?

Answer 36

deltaTa = (1-epsilon) * (T5-T2) where T5 is turbine outlet and T2 is compressor outlet