Turbines may be defined as turbomachines that extract energy from the fluid and convert it into mechanical/electrical energy. A classification of turbines is shown in Figure 14.1. Turbines may be classified based on whether the surrounding fluid is extended or enclosed. An example of an extended fluid turbine is a wind turbine, which may be a horizontal axis wind turbine or a vertical axis wind turbine. Enclosed turbines may be classified based on whether the working fluid is either incompressible or compressible. Hydraulic turbines (mostly water turbines) deal with incompressible fluids. Compressible turbines may deal with either steam or gas. Gas turbines may operate as subsonic or supersonic turbines. Turbines may also be classified based on whether the gas flow direction within its passage is axial, radial, or mixed. In axial turbines, the flow moves parallel to the axis of rotation. In radial turbines, the gas moves perpendicular to the axis of rotation. In mixed-flow turbines, the gases have a combined radial and axial motion. Another classification is related to the role of the turbine rotor in extracting power from the gas flowing through its passages; thus, there may be either impulse or reaction turbines. In reaction turbines, both stator and rotor share power extraction from the gas, while in impulse types, stator only do the job.