MEE218
Chapter 9 (Part 2)
Brayton Cycle
Thermodynamics II Asst.Prof.Dr. Abdulrazzak AKROOT
➢ The highest temperature in the cycle is limited by the maximum temperature that
the turbine blades can withstand. This also limits the pressure ratios that can be
used in the cycle.
➢ The air in gas turbines supplies the necessary oxidant for the combustion of the
fuel, and it serves as a coolant to keep the temperature of various components
within safe limits. An air–fuel ratio of 50 or above is not uncommon.
The fraction of the turbine work used to drive
the compressor is called the back work ratio.
Development of Gas Turbines
1. Increasing the turbine inlet (or firing) temperatures
2. Increasing the efficiencies of turbomachinery components (turbines, compressors):
3. Adding modifications to the basic cycle (intercooling, regeneration or recuperation, and
reheating).
THE BRAYTON CYCLE WITH REGENERATION
➢ In gas-turbine engines, the temperature of the exhaust gas leaving the turbine is often
considerably higher than the temperature of the air leaving the compressor.
➢ Therefore, the high-pressure air leaving the compressor can be heated by the hot
exhaust gases in a counter-flow heat exchanger (a regenerator or a recuperator).
➢ The thermal efficiency of the Brayton cycle increases as a result of regeneration since
less fuel is used for the same work output.
➢ The effectiveness of most regenerators used in practice is below 0.85.
A gas-turbine engine with regenerator.
Effectiveness of
regenerator
When the cold-air-standard assumptions are utilized,
it reduces to T-s diagram of a Brayton
cycle with regeneration.
The thermal efficiency depends on the ratio of the minimum to maximum temperatures
as well as the pressure ratio.
Regeneration is most effective at lower pressure ratios and low minimum-to-maximum
temperature ratios.
Under the cold-air-standard assumptions, the thermal efficiency of an ideal Brayton
cycle with regeneration is
Can regeneration be used at high
pressure ratios?
➢ This figure shows that regeneration is most
effective at lower pressure ratios and low
minimum to-maximum temperature ratios.
Thermal efficiency of the ideal Brayton cycle with and
without regeneration.
Example 3
A Brayton cycle with regeneration using air as the working fluid has a pressure ratio of 7.
The minimum and maximum temperatures in the cycle are 310 and 1150 K. Assuming an
isentropic efficiency of 75 percent for the compressor and 82 percent for the turbine and
an effectiveness of 65 percent for the regenerator, determine (a) the air temperature at
the turbine exit, (b) the net work output, and (c) the thermal efficiency.
Also, determine the total exergy destruction associated with the cycle, assuming a
source temperature of 1500 K and a sink temperature of 290 K. Also, determine the
exergy of the exhaust gases at the exit of the regenerator. Use variable specific heats for
air.
solution
Analysis (a) The properties of air at various states are given in Table A-17
State 1
State 2
State 3
State 4
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Thus,
(a) the air temperature at the turbine exit
(b) the net work output
(c) the thermal efficiency.
Then,
Quiz
A gas-turbine power plant operates on the regenerative Brayton cycle between the pressure
limits of 100 and 700 kPa. Air enters the compressor at 25 °C at a rate of 12.6 kg/s and
leaves at 260 °C. It is then heated in a regenerator to 400 °C by the hot combustion gases
leaving the turbine. A diesel fuel with a heating value of 42,000 kJ/kg is burned in the
combustion chamber with a combustion efficiency of 97 percent. The combustion gases
leave the combustion chamber at 871 °C and enter the turbine whose isentropic efficiency is
85 percent. Using variable specific heats to determine (a) the isentropic efficiency of the
compressor, (b) the effectiveness of the regenerator, (c) the air–fuel ratio in the combustion
chamber, (d) the net power output and the back work ratio, (e) the thermal efficiency, and ( f )
the second-law efficiency of the plant. Also determine (g) the second-law efficiencies of the
compressor, the turbine, and the regenerator, (h) the rate of the energy flow with the
combustion gases at the regenerator exit, and (e) the entropy generation and exergy
destruction through the regenerator and combustion chamber .
