A turbine is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. A turbine is a turbo-machine with at least one moving part (the rotor assembly), which is a shaft or drum with blades attached. Moving fluid acts on the blades so that they move and impart rotational energy to the rotor. Early turbine examples are windmills and water wheels.
THEORY OF TURBINE OPERATIONS
Working fluid contains potential energy (pressure head) and kinetic energy (velocity head). The fluid may be compressible or incompressible. Several physical principles are employed by turbines to collect this energy. Impulse turbines change the direction of flow of a high velocity fluid or gas jet. The resulting impulse spins the turbine and leaves the fluid flow with diminished kinetic energy. There is no pressure change of the fluid or gas in the turbine blades, as in the case of a steam or gas turbine; all the pressure drop takes place in the stationary blades. Before reaching the turbine, the fluid’s pressure head is changed to velocity head by accelerating the fluid with a nozzle. Pelton wheels and de Laval turbines use this process exclusively.
Impulse turbines do not require a pressure casement around the rotor since the fluid jet is created by the nozzle prior to reaching the blading on the rotor. Newton’s second law describes the transfer of energy for impulse turbines. Reaction turbines develop torque by reacting to the gas or fluid’s pressure or mass. The pressure of the gas or fluid changes as it passes through the turbine rotor blades.
A pressure casement is needed to contain the working fluid as it acts on the turbine stage(s) or the turbine must be fully immersed in the fluid flow. The casing contains and directs the working fluid and, for water turbines, maintains the suction imparted by the draft tube. Francis turbines and most steam turbines use this concept. For compressible working fluids, multiple turbine stages are usually used to efficiently harness the expanding gas. Newton’s third law describes the transfer of energy for reaction turbines.
In the case of steam turbines, such as would be used for marine applications or for land-based electricity generation, a Parsons type reaction turbine would require approximately double the number of blade rows as a de Laval type impulse turbine for the same degree of thermal energy conversion. While this makes the Parsons turbine much longer and heavier, the overall efficiency of a reaction turbine is slightly higher than the equivalent impulse turbine for the same thermal energy conversion.
In practice, modern turbine designs use reaction and impulse concepts to varying degrees whenever possible. Wind turbines use an airfoil to generate a reaction lift from the moving fluid, imparting it to the rotor. Wind turbines also gain some energy from the impulse of the wind, by deflecting it at an angle. Crossflow turbines are designed as impulse machines, with nozzles, but in low head applications they maintain some efficiency through reaction, like traditional water wheels.
Turbines with multiple stages may use either reaction or impulse blading at high pressure. Steam turbines were traditionally more impulse based but continue to move toward reaction designs similar to those used in gas turbines. At low pressure, the operating fluid medium expands in volume for small reductions in pressure. Under these conditions, blading becomes strictly a reaction type design with the base of the blade solely impulse. This is due to the effect of the rotation speed for each blade.
As the volume increases, the blade height increases, and the base of the blade spins at a slower speed relative to the tip. This change in speed forces a designer to change from impulse at the base, to a high reaction style tip. Classic turbine design methods were developed in the mid 19th century. Vector analysis related the fluid flow to turbine shape and rotation. Graphical calculation methods were used at first. Formulas for the basic dimensions of turbine parts are well documented, and a highly efficient machine can be reliably designed for any fluid flow condition. Some of the calculations are empirical or rule of thumb formula, and others are based on classic mechanics.
As with most engineering calculations, simplifying assumptions were made. Velocity triangles can be used to calculate the basic performance of a turbine stage. Gas exits the stationary turbine nozzle guide vanes at absolute velocity Va1. The rotor rotates at velocity U. Relative to the rotor; the velocity of the gas as it impinges on the rotor entrance is Vr1. The gas is turned by the rotor and exits, relative to the rotor, at velocity Vr2. However, in absolute terms, the rotor exit velocity is Va2. The velocity triangles are constructed using these various velocity vectors. Velocity triangles can be constructed at any section through the blading, but are usually shown at the mean stage radius.