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Lecture 15 & 16
BRAYTON CYCLE: The Ideal Cycle For
Gas Turbines (Ch-9)
Thermodynamics - II
Zia Ud Din
2
BRAYTON CYCLE: THE IDEAL CYCLE FOR
GAS-TURBINE ENGINES
An open-cycle gas-turbine engine. A closed-cycle gas-turbine engine.
1) Air as an ideal gas is the working fluid.
2) The combustion process is replaced by a constant-pressure heat-addition
process from an external source, and the exhaust process is replaced by a
constant-pressure heat-rejection process to the ambient air.
1-2 Isentropic compression (in a compressor)
2-3 Constant-pressure heat addition
3-4 Isentropic expansion (in a turbine)
4-1 Constant-pressure heat rejection
Areas on T-s Diagram
2-3-a-b-2 = Heat Addition
1-4-a-b-1= Heat Rejection
Areas on P-v Diagram
1-2-a-b-1 = Work Input
3-4-b-a-3 = Work Done
1-2 Isentropic compression (in a compressor)
2-3 Constant-pressure heat addition
3-4 Isentropic expansion (in a turbine)
4-1 Constant-pressure heat rejection
Air-Standard BRAYTON Cycle:(Constant Pressure Cycle)
Back Work Ratio for Cycle: Thermal Efficiency:
Using Control Volume mass & energy rate balance:
12
1 2
Wh h
m
233 2
Qh h
m
411 4
Qh h
m
34
3 4
Wh h
m
Air-Standard BRAYTON Cycle Analysis:(Constant Pressure Cycle)
η is the ratio of network of cycle to Qin:
When air table data are used to conduct an
analysis involving ideal Brayton Cycle the
following relationship apply for isentropic
processes 1-2 and 3-4
(for variable specific heats)
Where, p4/p3 = p1/p2
Air-Standard BRAYTON Cycle Analysis:(Considering variable specific heats)
On constant specific heats basis, following expressions used for
isentropic processes
On a cold air-standard basis, Thermal
efficiency is:
Eq. 9.23
Eq. 9.24
Air-Standard BRAYTON Cycle Analysis:(Considering constant specific heats)
Thermal efficiency of the ideal Brayton
cycle is a function of pressure ratio
across the compressor
Air-Standard BRAYTON Cycle Analysis:(Considering constant specific heats)
Thermal
efficiency of the
ideal Brayton
cycle as a
function of the
pressure ratio.
8
For fixed values of Tmin and Tmax, the net
work of the Brayton cycle first increases
with the pressure ratio, then reaches a
maximum at rp = (Tmax/Tmin)k/[2(k - 1)], and
finally decreases.
The fraction of the turbine work
used to drive the compressor is
called the back work ratio.
The two major application areas of gas-
turbine engines are aircraft propulsion
and electric power generation.
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.
9
Deviation of Actual Gas-Turbine Cycles from
Idealized Ones
The deviation of an actual gas-
turbine cycle from the ideal
Brayton cycle as a result of
irreversibilities.
Reasons: Irreversibilities in turbine and compressors, pressure
drops, heat losses
Isentropic efficiencies of the
compressor and turbine
10
Example 9-5
11
Example 9-5
12
Example 9-5
13
Example 9-5
14
Example 9-6
15
Example 9-6
16
Example 9-6
17
Example 9-6
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