Flow Through a Turbomachine

Abstract

A turbomachine consists of rows of blades, some of which rotate (rotor) while others remain stationary (stator). As explained earlier, work can be extracted or added in a rotating row of blades only, where the stagnation pressure and stagnation temperature change, while in the stationary row of blades, the stagnation temperature and stagnation pressure do not change (except for the small effect of friction on the stagnation pressure). Hence, in a compressor, the air is accelerated in the rotor (the stagnation pressure is increased) and is decelerated in the stator, where the kinetic energy of air is converted into the pressure (same total pressure; the static pressure is increased). On the other hand, air is accelerated in a nozzle or stator blades’ row at constant stagnation temperature and static pressure with increasing air (gas) speed, and subsequently the kinetic energy is converted into mechanical energy in a rotating row of blades, where the stagnation pressure and temperature decrease. For a single stage of turbomachinery blades, we thus have a row of rotating blades and a row of stationary blades; for compressors, the rotating blades come first, but for turbines, the stationary blades come first. We therefore have two rows of blades, and depending on the direction of motion of air (gas), they act as a compressor or turbine stage, as shown in Fig. 4.1. They are numbered 1–3 from the rotor side. If it is a compressor, air is scooped at blade edge 1 in the rotor and is pushed toward edge 2 of the rotor after accelerating it; it is subsequently decelerated in the stator row from 2 to 3. On the other hand, as turbine air (or gas) enters the stator at 3, it is accelerated up to 2 while the static pressure is lowered, but the stagnation temperature and stagnation pressure remain the same (except for a small loss in stagnation pressure due to friction), and then while passing through the rotor and exhausting at 1, it converts the kinetic energy into mechanical energy.