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.

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.

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.

Closed cycle gas

turbine engine

Open cycle gas

turbine engine

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

Isentropic compression (in a compressor)

Constant pressure heat addition

Isentropic expansion (in a turbine)

Constant pressure heat rejection

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 compression

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

Turbine

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.

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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).

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

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

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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.

Brayton cycle

Working fluid – air

Ideal gas

Specific heats steady or variable

High temperature reservoir

Open or closed model

Steady pressure heat exchange

Relation between efficiency and pressure ratio

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.

Brayton cycle

Ideal•ηth =45.6%With irreversibilities•ηth =24.9%With regenerator* ηth =56.8%

Ideal•ηth =45.6%With regenerator•ηth =57%

Brayton Cycle With Intercooling, Reheating, &

RegenerationThe 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.

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