This Presentation was prepared for Brayton Cycle. This is very important in the study of THermodynamics.

1. Brayton Cycle: Ideal Cycle for Gas-Turbine Engines
Gas turbines usually operate on an open cycle .
Air at ambient conditions is drawn into the compressor,
where its temperature and pressure are raised. The high
pressure air proceeds into the combustion chamber,
where the fuel is burned at constant pressure.
The high-temperature gases then enter the turbine
where they expand to atmospheric pressure while
producing power output.
Some of the output power is used to drive the
compressor.
The exhaust gases leaving the turbine are thrown out
(not re-circulated), causing the cycle to be classified as
an open cycle.

2. The basic gas turbine cycle is named for
the Boston engineer, George Brayton, who
first proposed the Brayton cycle around
1870.
The Brayton cycle is used for gas turbines
only where both the compression and
expansion processes take place in rotating
machinery.

3.  Gas turbines usually operate on an open cycle.
 Fresh air at ambient conditions is drawn into the
compressor, where its temperature and pressure are
raised.
 The high-pressure air proceeds into the combustion
chamber, where the fuel is burned at constant
pressure.
 The resulting high-temperature gases then enter the
turbine, where they expand to the atmospheric
pressure through a row of nozzle vanes.

4. Closed cycle gas
turbine engine
Open cycle gas
turbine engine

5.  The open gas-turbine cycle can be modeled as a closed
cycle by using the air-standard assumptions.
 Here the compression and expansion process remain
the same, but a constant-pressure heat-rejection process
to the ambient air replaces the combustion process.
 The ideal cycle that the working fluid undergoes in this
closed loop is the Brayton cycle, which is made up of
four internally reversible processes

6.  Isentropic compression (in a compressor)
 Constant pressure heat addition
 Isentropic expansion (in a turbine)
 Constant pressure heat rejection

7. Efficient compression of large volumes of air is essential for a
successful gas turbine engine.
This has been achieved in two types of compressors.
The axial-flow compressor and the centrifugal – or radial-flow
compressor .
Most power plant compressors are axial-flow compressors.
An axial-
flow
compressio

8. The chemical combination of a substance with certain elements.
The function of the combustion chamber is to accept the air from
the compressor and to deliver it to the turbine at the required
temperature.
For the common open-cycle gas turbine, this requires the Internal
combustion of fuel.
A Combustion
Chamber Can

9. Turbine

10.  Gas turbines move relatively large quantities
of air through the cycle at very high velocities.
 The gas turbine in its most common form is a
heat engine operating through a series of
processes.
 It is similar to the gasoline and Diesel engines
in its working medium and internal
combustion, but is like the steam turbine in the
steady flow of the working medium.

11. 11

12.  All four processes of the Brayton cycle are executed in
steady flow devices so they should be analyzed as
steady-flow processes.
 When the changes in kinetic and potential energies are
neglected, the energy balance for a steady-flow process
can be express, on a unit-mass basis.
 The thermal efficiency of an ideal Brayton cycle
depends on the pressure ratio of the gas turbine and the
specific heat ratio of the working fluid (if different
from air).


13.  Specific heat ratio: A physical property of a
material. The specific heat is defined as the amount
of heat required to raise a unit of mass of a substance
one degree.
 In brayton cycle, it is given as;
 rp=p2/p1

14. In theory, as the pressure ratio goes up, the efficiency rises.
The limiting factor is frequently the turbine inlet
temperature.
The turbine inlet temp is restricted to about 1,700 K or 2,600
F.
  k1k
pr
1
1 


15. Actual Gas-Turbine Cycles
Some pressure drop occurs during the heat-addition
and heat rejection processes.
The actual work input to the compressor is more, and
the actual work output from the turbine is less,
because of irreversibilities.
Deviation of actual compressor and turbine behavior
from the idealized isentropic behavior can be accounted
for by utilizing isentropic efficiencies of the turbine and
compressor.

16. Brayton cycle
Working fluid – air
Ideal gas
Specific heats steady or variable
High temperature reservoir
Open or closed model
Steady pressure heat exchange

17. Relation between
efficiency and
pressure ratio

18. Brayton Cycle With Regeneration
Temperature of the exhaust gas leaving the turbine is
higher than the temperature of the air leaving the
compressor.
The air leaving the compressor can be heated by the
hot exhaust gases in a counter-flow heat exchanger
(a regenerator or recuperator) – a process called
regeneration.
The thermal efficiency of the Brayton cycle increases
due to regeneration since less fuel is used for the
same work output.
Note:
The use of a regenerator is
recommended only when the
turbine exhaust temperature is
higher than the compressor exit
temperature.

19. Brayton cycle
Ideal
•ηth =45.6%
With irreversibilities
•ηth =24.9%
With regenerator
* ηth =56.8%
Ideal
•ηth =45.6%
With regenerator
•ηth =57%

20. Brayton Cycle With Intercooling, Reheating, &
Regeneration
The net work output of a gas-turbine cycle can be increased by either:
a) decreasing the compressor work, or
b) increasing the turbine work, or
c) both.
The compressor work input can be
decreased by carrying out the compression
process in stages and cooling the gas in
between using multistage compression
with intercooling.
The work output of a turbine can be increased by expanding the gas
in stages and reheating it in between, utilizing a multistage
expansion with reheating.

21. Gas Turbine
Advantages
oHigh power:weight ratio
oCompact
oOne-direction motion –
vibration
oFewer moving parts
oBetter reliability
oVariety of fuels
oLow emissions
Disadvantages
oHigher cost