Compressible fluid turbines - Design and operation

Turbines are machines that convert the energy of a fluid into mechanical rotational energy available on a shaft to drive another machine, such as an alternator for electricity generation or a compressor.

• The fluid may be incompressible, as in the case of hydraulic turbines, or compressible, as in the two main families of compressible fluid turbines: gas turbines and steam turbines. Compressible fluid turbines differ from hydraulic turbines in two main respects:

  • by the origin of the energy of the driving fluid; in the hydraulic turbine, this is gravity; for gases, the energy linked to gravity is negligible; the origin of the energy is the pressure and temperature of the fluid; the enthalpy drop replaces the height of the waterfall;

  • the variation in density, which, when the velocity reaches the speed of sound, can lead to significant changes in the flow, in particular the appearance of shock waves.

• This article, although general, is more oriented towards steam turbines, with aspects more specifically related to gas turbines covered in the article "Aeronautical and land-based gas turbines".

After explaining how these turbines work, and in particular the simpler axial turbines, we'll focus on losses and how to reduce them. Although current calculation codes, which are not described here, enable us to predict the flow more and more accurately, the determination of losses is still rather imprecise due to the need for extremely fine meshes, but above all to the still imperfect modeling of turbulence.

If only to "understand" the calculations, and to avoid certain errors, it is necessary to know the physical phenomena that govern the operation of these machines.