The air flow is drawn into the axial compressor and compressed through
multiple stages of stator and rotor blades. The compressed air in the axial
compressor is then mixed with fuel in the combustion chamber, where the
combustion process takes place. The resulting hot gas is expanded through a multi
stage turbine to drive the generator and the compressor.
The fuel flow determines the power output of a gas turbine. The fuel and
air flow together determine the firing temperature, which is the gas temperature at
the exit of the combustion chamber. The fuel flow and air flow are adjusted based
on measurement of the exhaust temperature and the compressor pressure ratio in
order to keep the firing temperature below a design limit. The compressor
pressure ratio is determined from measurements of the inlet and discharge air
pressure of the compressor (for the entire axial compressor this ratio is typical 15
to 20). The air flow can be adjusted by changing the angular position of the
variable inlet guide vanes (VIGVs).
These vanes are essentially the first few stages of the stator blades inside
the axial compressor assembly. When the gas turbine is loaded close to base load,
the VIGVs are wide open. The air flow is a function of VIGV angle, ambient
temperature at compressor inlet, atmospheric pressure and the shaft speed.
Summarizing the gas turbine outputs / inputs dependencies, a black-box
system representation is given in figure 2.