In Thermodynamics there are two general categories: power cycles, which produce a net power output, and refrigeration and heat pump cycles, which consume a net power input. Gas cycles and Vapour cycles are the two categories of Power Cycle. In gas cycles, the working fluid remains in the gas phase throughout the entire cycle. In vapor cycles, the working fluid exits as a vapor during one part of the cycle and as a liquid during another part of the cycle.
Assumptions in Ideal Brayton Cycle
The compression process is assumed to be perfect without any losses; no friction, no heat loss, 100% efficient compressor.
The combustion process (Heat Source) in real engines are replaced with an equivalent “Reversible” Heat Addition process at constant pressure (Isobaric).
The Expansion process (through the turbine) is assumed to be perfect without any losses; no friction, no heat loss, 100% efficient turbine.
The “Exhaust” and new “Intake” in real engines is replaced with a “Reversible” Heat Rejection process to regenerate a fresh air intake.
Brayton Cycle Process
The ideal Brayton cycle consists of four operations as follows:
Isentropic (Perfect) Compression
Isobaric (Fixed Pressure) Heat Addition
Isobaric Heat Rejection
Brayton Cycle Efficiency Derivation
We will derive the efficiency of Brayton cycle in the below notes. To understand Efficiency derivation, you should understand PV and TS diagram for Brayton cycle.
Brayton Cycle pv Diagram
In an ideal Brayton cycle, heat add to the cycle at a constant pressure process (process 2-3).
qin = h3 – h2 = cP(T3 – T2)
Heat rejection at a constant pressure process (process 4 -1).
qout = h4 – h1 = cP(T4 – T1)
Brayton Cycle TS Diagram
Process 1-2 and process 3-4 are isentropic processes, thus,
Since P2 = P3 and P4 = P1,
Considering all the relations above, the thermal efficiency becomes,
Airstandard Brayton Cycle
where rP = P2/P1 is the pressure ratio and k is the specific heat ratio. In most designs, the pressure ratio of gas turbines range from about 11 to 16.
Open End Brayton Cycle Thermodynamics
In this process the below actions happen.
A compressor takes in fresh ambient air (state 1), compresses it to a higher temperature and pressure (state 2).
Fuel and the higher pressure air from the compressor are sent to a combustion chamber, where fuel is burned at constant pressure. The resulting high-temperature gases move the turbine (state 3).
The high-temperature gases expand to the ambient pressure (state 4) in the turbine and produce power.
The exhaust gases leave the turbine.
In Turbine Work generates. Compressors consume part of the work. The fraction of the turbine work used to drive the compressor is called the back work ratio.
Since fresh air enters the compressor at the beginning and exhaust are thrown out at the end, this cycle is an open cycle.
In an actual gas-turbine cycle, the compressor consumes more work and the turbine produces less work than that of the ideal Brayton cycle. The irreversibilities in an actual compressor and an actual turbine can be considered by using the adiabatic efficiencies of the compressor and turbine.
Actual Brayton and Ideal Brayton Cycle difference
In real engines many differences should be considered:
Air compressors do have frictional losses, random turbulent fluid mixing losses, and heat losses which make them have nominal efficiency values approximately around 80%.
The heat addition occurs due to fuel combustion, which generates high temperature differences and thus “Irreversible” process, which mean more losses.
The turbine do have frictional losses, heat losses and the process is “Inisentropic”. Typical turbine efficiency values approximately range around 85%.
Instead of regenerating fresh air by the heat rejection in Brayton Cycle, real jet engines exhausts the hot gases and sucks a new air intake to complete the power cycle.
In real engines pressure losses through ducts, combustion chambers, and heat exchangers do present.
At the jet engine compressor inlet, there exists a diffuser to enhance air compression.
At the turbine outlet a nozzle is located to provide the necessary thrust to propel the Plane by the reverse action of propelling hot gases at very high velocities (Conservation of Momentum)
What is the effect of the compressor inlet temperature or turbine inlet temperature on the Brayton cycle?
Lowering the inlet air temperature increases the output of the turbine. Cold air is dense and hence easy to compress. Do the compressor discharge pressure increases and hence the load. Turbine inlet temperature (TIT) is the temperature of the gases just as they leave the turbine. Higher the turbine inlet temperature, higher is the efficiency and output. Metallurgical limitations constraint the increase in efficiency.
What are the difference between rankine cycle and Brayton Cycle
Air enters into Jet Engines. The Compessor Compresses the air, Fuel burns with the Compressed air. Turbine releases the burnt gas to the atmosphere. Thus making it an Open cycle. And there is no phase change in the medium (Inlet-Gaseous phase, Exit- Gaseous phase)
Rankine cycle is for Steam engines, the water is boiled, evaporated, used for work and then condensed for re-use. Thus making it an Closed cycle. And there is phase change in the medium (Liquid to Gaseous phase)
Brayton cycle vs Rankine Cycle
The major difference between the two is that the Rankine is a vaporcycle, where the working fluid is cycled between vaporand liquid states, while the Brayton is a gas cycle, where the working fluid always exists as a gas.