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Brayton Cycle: Mr.S.Vijayakumar M.E-Thermal Engineering (Hons)

The Brayton cycle is a constant pressure cycle used for gas turbine power plants. In gas turbines, over half the turbine work output is typically used to drive the compressor, with back work ratios ranging from 40-80%. The Brayton cycle involves four processes: 1) adiabatic compression in the compressor, 2) constant pressure heat addition in the combustion chamber, 3) adiabatic expansion in the turbine, and 4) constant pressure heat rejection to the atmosphere. Net work output is maximized at the optimum pressure ratio where the compressor and turbine outlet temperatures are equal.

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185 views6 pages

Brayton Cycle: Mr.S.Vijayakumar M.E-Thermal Engineering (Hons)

The Brayton cycle is a constant pressure cycle used for gas turbine power plants. In gas turbines, over half the turbine work output is typically used to drive the compressor, with back work ratios ranging from 40-80%. The Brayton cycle involves four processes: 1) adiabatic compression in the compressor, 2) constant pressure heat addition in the combustion chamber, 3) adiabatic expansion in the turbine, and 4) constant pressure heat rejection to the atmosphere. Net work output is maximized at the optimum pressure ratio where the compressor and turbine outlet temperatures are equal.

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BRAYTON CYCLE

Brayton cycle is a constant pressure cycle for a perfect gas. It is also called Joule
cycle.
Brayton cycle, popularly used for gas turbine power plants.

In gas-turbine power plants, the ratio of the compressor work to the turbine work,
called the back work ratio, is very high. Usually more than one-half of the turbine
work output is used to drive the compressor.

Mr.S.Vijayakumar M.E-Thermal Engineering (Hons)


riyasvijaythermal@gmail.com
9751023140

Typical back work ratios of gas turbines range from 40 to 80%. In comparison, the
back work ratios of vapor power plants are normally only 1 or 2%.

Brayton cycle shows following processes:


1-2: Adiabatic compression, involving (ve) work, WC in compressor.
2-3: Constant pressure heat addition, involving heat Q add in combustion chamber
3-4: Adiabatic expansion, involving (+ve) work, WT in turbine.
4-1: Constant pressure heat rejection, involving heat, Q rejected in atmosphere.
Air standard analysis of Brayton cycle gives work for compression and expansion
as;
WC = m1 (h2 h1)
WT = m3 (h3 h4)
for air standard analysis, m1 = m3, where as in actual cycle
m3 = m1 + mf , in open type gas turbine
m3 = m1, in closed type gas turbine
For the fuel having calorific value CV the heat added in air standard cycle;
Qadd = m1 (h3 h2), whereas Qadd = mf CV for actual cycle.
Net work = WT WC
Wnet = {m3 (h3 h4) m1 (h2 h1)}

Air standard cycle efficiency =

Mr.S.Vijayakumar M.E-Thermal Engineering (Hons)


riyasvijaythermal@gmail.com
9751023140

Mr.S.Vijayakumar M.E-Thermal Engineering (Hons)


riyasvijaythermal@gmail.com
9751023140

For any given value of t, there must be an optimum pressure ratio to given a
maximum specific output

Mr.S.Vijayakumar M.E-Thermal Engineering (Hons)


riyasvijaythermal@gmail.com
9751023140

This means that the specific work output is maximum when the pressure ratio is
such that the compressor and turbine outlet temperatures are equal.

NOTE:
1. Work ratio=Wnet / WT
i. 0.45 WR 0.55 (for gas turbine plants)
ii. 0.95 WR 0.98 (for steam turbine plants)
2. W=0 at pressure ratio (r)=1
3. Wmax at optimum pressure ratio
4. Wmax [roptimum] at T2=T4 compressor and turbine outlet temperatures are equal.
5. Wmax=Cp[Tma x -Tmin]
Mr.S.Vijayakumar M.E-Thermal Engineering (Hons)
riyasvijaythermal@gmail.com
9751023140

6.
7.
8.
9.

max at r = rmax
max=1-(Tmax / Tmin)

roptimum= rmax
Temperature ratio (t)= Tma x / Tmin
i. t = 3.5 to 4 (for Industrial plants)
ii. t = 5 to 5.5 (for air craft engine with cooled turbine blades)
For all values of pressure ratio r lying between 1 and t /2(1) temperature T4 is
greater than T2. In order to reduce heat transfer from external source for improving
efficiency a heat exchanger can be incorporated.
This concept of using heat exchanger gives birth to modified form of simple gas
turbine cycle with heat exchange.
This modified form of cycle is also called regenerative gas turbine cycle.

Mr.S.Vijayakumar M.E-Thermal Engineering (Hons)


riyasvijaythermal@gmail.com
9751023140

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