The gas turbine is used in a wide range of applications. Common uses include power generation plants and military and commercial aircraft. In Jet Engine applications, the power output of the turbine is used to provide thrust for the aircraft.
In a simple gas turbine cycle, low pressure air is drawn into a compressor (state 1) where it is compressed to a higher pressure (state 2). Fuel is added to the compressed air and the mixture is burnt in a combustion chamber. The resulting hot products enter the turbine (state 3) and expand to state 4. Most of the work produced in the turbine is used to run the compressor and the rest is used to run auxiliary equipment and produce power.
Air standard models provide useful quantitative results for gas turbine cycles. In these models the following assumptions hold true.
- The working substance is air and treated as an ideal gas throughout the cycle
- The combustion process is modeled as a constant pressure heat addition
- The exhaust is modeled as a constant pressure heat rejection process
In cold air standard (CAS) models, the specific heat of air is assumed constant at the lowest temperature in the cycle.
Brayton Cycle
The Brayton cycle depicts the air-standard model of a gas turbine power cycle.
The four steps of the cycle are:
Brayton Cycle
The Brayton cycle depicts the air-standard model of a gas turbine power cycle.
The four steps of the cycle are:
- (1-2) Isentropic Compression
- (2-3) Reversible Constant Pressure Heat Addition
- (3-4) Isentropic Expansion
- (4-1) Reversible Constant Pressure Heat Rejection
The pv and Ts diagrams are shown below.
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