Brayton cycle is explained along with efficiency, T-s, and P-v, diagrams, etc. This article consists of many easy diagrams, explanations, formulas, and examples to clear the basic idea about the Brayton cycle.
Let’s get into the basic definition of the Brayton cycle, its definition, history.
The Brayton cycle is one of the most widely used and famous thermodynamic cycles in gas turbine engines, internal combustion engines and heat engines.
The Brayton cycles are mainly used in jet planes, usefulness of this Cycle is tremendous due to the fact it is the backbone in driving even helicopters, and submarines.
The name of this cycle is named after George Brayton that described the working of a constant pressure heat engine in 1872.
The original Brayton engine used in the piston compressor, piston expander. But now the modern gas turbine engines are used in air-breathing jet engines.
The cycle used in those is generally the open-type Brayton cycle. For the analysis, this cycle is considered a closed cycle.
This cycle is sometimes called the Joules cycle also.
It is defined as a cycle, where thermal or heat energy is converted into mechanical energy at a constant pressure.
Basically, the Brayton cycle describes the main working process or philosophy of gas turbines or heat engines.
The losses in the compressor and turbines and pressure drops in the combustion chambers are neglected in the ideal Brayton cycle for easy calculation.
So, try to see types, parts, diagrams, actual cycles, etc.
Normally, two processes can be used,
Brayton cycle is the most common cycle known and describes the working of the constant pressure heat engines. The ideal Brayton cycle consists of two isentropic processes and two isobaric processes alternatively.
Let’s check out the components and processes.
Let’s see the Brayton cycle parts along with a basic diagram.
Brayton cycle diagram
There are four main components,
So, try to see we will see a typical ideal Brayton cycle its processes, and some of its variations. In addition, where are the differences between the actual cycle and the ideal cycle?
In the ideal Brayton cycle, there are four processes running, we will check each of them and what will happen in each process.
Let’s delve into the T-S diagram for a better explanation.
Process 1-2 Isentropic compression process
In this process, the working gas is compressed adiabatically from state 1 to state 2 with the help of a compressor.
Process 2-3 Isobaric heat addition process
In this process, the heat is added at constant pressure i.e. isobaric process.
Process 3-4 Isentropic expansion process
In the isentropic expansion process, the heated gas will be expanded adiabatically from the state 3 to 4 in a turbine.
Process 4-1 Isobaric heat rejection
In this process, the expanded gas will have its heat rejection from state 4 to state 1. This process will be completed at constant pressure.
During a Brayton cycle, work is done on the gas by the compressor between states 1 and 2 (isentropic compression).
Work is done by the gas in the turbine between stages 3 and 4 (isentropic expansion).
The difference between the work done by the gas and the work done on the gas is the network produced by the cycle and it corresponds to the area enclosed by the cycle curve (in Pv diagram below).
So, these are the four processes in the ideal Brayton cycle.
Let’s see the P-v diagram,
Let’s find out the Brayton cycle efficiency,
Brayton cycle efficiency = WNet / QInput
Let’s consider the following,
Then we can write,
Wnet = WTurbine – Wcompressor = QOutput =m Cp (T4-T1)
Brayton cycle efficiency = WNet / QInput
= WNet / QInput
= 1 − QExchanger / QInput
= 1 – [ m x Cp x (T4 – T1)] / [ m x Cp x (T3 – T2)]
= 1 – [ T1 x (T4/T1 – 1)] / [ T2 x (T3/T2 – 1)]
From the equation of isentropic process,
we can write,
T2/T1 = T3/T4
or T4/T1 = T3/T2
From isentropic efficiency, we can write,
η = 1 – T1/T2 = T4/T3
If the pressure ratio is rp, then,
Hence, we can conclude,
So, as with any other ideal process, the actual process differs from the ideal process. The ideal process is assumed to make the calculations and setting a specific standard.
In the actual cycle,
We have seen the compression, as well as expansion process, are isentropic in the ideal Brayton cycle.
Actual efficiency shall be as follows,
There are some ways to increase the net output as well as the efficiency in the Brayton cycle.
Like, to increase the power reheating of the working fluid and overspray methods are employed.
In case of improving the efficiency, increasing pressure ratio, recuperator, and co-generation system are used.
So, there are two basic processes to overcome the losses as well as to increase the power output in the Brayton cycle. We will diagrams as well for the below process,
In case of the Brayton reheat cycle, two turbines will be kept in a series.
The first turbine receives the hot gas from the combustor and a reheat combustor is provided between these two turbines.
The reheating method helps to reduce the condensation during the expansion and it helps to reduce the probability of damaging turbine blades.
The name Regeneration means the extract heats will be reused to heat the air after the compressor.
In the gas turbine, the Brayton cycle is used to produce work.
Here, ambient air is introduced to the compressor, and pressure and temperature are increased by the compression process.
After compression, hot air is passed to a mixing chamber and fuel is injected. In this chamber, combustion happens and later on, hot gas is passed through the turbine and expansion happens. This expansion helps to produce energy or work which drives the gas turbine.
The details of the gas turbine are captured in our separate article.
Hence, we have learned the basic idea of the Brayton cycle along with various diagrams, etc.
T-s and P-v diagram also explained to simplify the Brayton cycle. Any questions, please write to us or comment in the below comment box. Happy learning!
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