text: Usual Rankine cycles are thermodynamic cycles that employ water as a working fluid to produce electric power from thermal sources. In Organic Rankine cycles, by contrast, water is substituted by molecularly complex organic compounds. Since the vaporization temperature of these kinds of fluids is lower than that of water at atmospheric pressure, low-to-medium temperature sources can be exploited allowing for heat recovery, for example, from biomass combustion, industrial waste heat, or geothermal heat. For these reasons, ORC technology belongs to the class of renewable energies.
  For the design of mechanical components, such as turbines, working in ORC plants, it is fundamental to take into account typical non-ideal gas-dynamic phenomena. In fact, the single-phase vapor at the inlet of an ORC turbine stator usually evolves in the non-ideal thermodynamic region close to the liquid-vapor saturation curve and critical point. Moreover, due to the high molecular mass of the complex organic compounds employed, the speed of sound in these fluids is low compared to that of air and other simple gases. Therefore, turbine stators are very likely to involve supersonic flows even if rather low flow velocities are reached. High supersonic flows can produce large losses and mechanical stresses in the turbine blades due to the occurrence of shock waves, which cause a strong pressure raise. However, when working fluids of the BZT class are employed, expander performances could be improved by exploiting some non-classical phenomena.