Changes in temperature and pressure:

The changes in temperature and pressure of the gases through a gas turbine engine are illustrated in Figure 1.5 The efficiency with which these changes are made will determine to what extent the desired relations between pressure, temperature and velocity are obtained.  The more efficient the compressor, the higher is the pressure generated for a given work input - i.e. for a given temperature rise of the gas.  Conversely, the more efficiently the turbine uses the expanding gas, the greater is the output of work for a given temperature drop in gas.

VELOCITY AND PRESSURE

During the passage of the air (gas) through the engine, aerodynamic and energy requirements demand changes in its velocity and pressure. For example, during compression a rise in the pressure of the air is required with no increase in its velocity. After the air has been heated and its internal energy increased by combustion, an increase in the velocity of the gases is necessary to cause the turbine to rotate. Also at the propelling nozzle, a high velocity is required, for it is the change in momentum of the air that provides the thrust on the aircraft. Local decelerations of gas flow are also required - for example, in the combustion chambers to provide a low velocity zone for the flame.