Lecture 6 - Joule Brayton Gas Flashcards
What gas is normally used and how is it treated?
Atmospheric air
Ideal gas
Why is there no need for a cooler?
The working fluid isn’t reused- a continuous supply of fresh air is drawn into the compressor
What drives the compressor?
The turbine
as more energy is generated than required to drive compressor
What is represented by lines 1-2, 3-4, 3-5 and 5-4?
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
How do aircraft cycles differ from cycles used for generation?
The turbine expansion is smaller. Expansion is stopped at 5, so remaining enthalpy in fluid us converted to kinetic energy via a nozzle
What may be eliminated from the SFEE in Brayton cycles, and why?
Potential terms
Density is very low
No height changes
How is the enthalpy change calculated?
c(p)DeltaT = Deltah
The cycle is reversible if:
- The isothermal expansion and compression are reversible
2. The expansion and compression are isentropic
How is the efficiency of the cycle calculated?
eff = (W(turbine) - W(compressor))/Q(2-3)
= (-(T2 - T1) - (T4 -T3))/(T3 - T2)
What equation can be used because there are no pressure losses in the system?
(P3/P4) = (P2/P1) = r(p) Where r(p) is the cycle pressure ratio
What increases efficiency (for a simple and ideal cycle)?
A higher pressure ratio
What are the effects of increasing temperature?
Increasing work ratio
Decreasing size of gas cycle plant for a given power output
What is the downside to increased temp?
The higher the temp of combustion products, the shorter the turbine blade life
What could happen if the work ratio is low?
Any decrease in turbine or compressor performance could cause the cycle to collapse
What happens if the isentropic efficiency of the adiabatic processes is less than 100%?
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
Are turbines or compressors generally more efficient?
Turbines, as it goes with the decrease in pressure
What is the effect of a combustor pressure drop?
The expansion begins from a reduced pressure
The outlet temperature from the turbine is raised, so power is reduced from the turbine
What is a gas turbine?
it is an engine designed to convert the energy of fuel into some form of useful power
What does a gas turbine consist of?
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
What ideally happens in a combustion chamber in a gas turbine?
pressure does not decrease but usually drop about 5/10%
What are the advantages of gas turbines?
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
Draw a TS diagram of a brayton cycle?
e
Draw a pressure volume diagram of a brayton cylce?
e
What are the different stages of a brayton cycle?
isentropic compression in compressoor
heat supplied at constant pressure
isentropic expansion through turbine
heat rejected at constant pressure
What are the three processes in a brayton cycle and how is the work done and heat out calculated?
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)
How are inlet and outlet temperatures calculated for turbines and compressors?
Using the ideal gas equation
T2/T1 = (P2/P1)^n-1/n
What is the equation for the ideal cycle efficiency:
eta = 1 - (1/Rp)^n-1/n
What is the work ratio?
Useful turbine work/total turbine work
How can the cycle efficiency be improved through introducing components?
using a recovery heat exchanger - using waste heat from the turbine to heat the gas after the compressor
Whats the issue with a recovery heat exchanger?
They are huge so cannot be used for aircraft engines - only suitable for industrial gas turbine
What happens in an ideal heat exchanger
T6=T2
T3=T5
What is the approach temperature
the closest that temperatures from the cooling and heating process can by brought to delta Ta
What are the final temperature after the heat exchanger if the approach temperature is taken into account
After the condenser can only be raised to T3 =T5 - deltaTa
After the turbine can only be cooled to T6 = T2+ deltaTa
What is heat exchanger effectiveness?
epsilon = actual heat transfer / maximum heat transfer
What is the mathematical equation for heat exchanger effectiveness?
epsilon qactual/qmaximum = mCp(T3-T2)/mCp(T5-T2)
only take into account approach temperature once
How can approach temperature be found from heat exchanger effectiveness?
deltaTa = (1-epsilon) * (T5-T2) where T5 is turbine outlet and T2 is compressor outlet
