Reaction turbines&action=edit&redlink=1 "Reaction (physics) (not yet written)") develop torque by reacting to the pressure or mass of the gas or fluid. The pressure of the gas or fluid changes as it passes through the turbine rotor blades.[3]​ A pressure frame is needed to contain the working fluid as it acts on the turbine stage(s) or the turbine must be completely submerged in the fluid flow (as in wind turbines). 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 often used to harness the expanding gas efficiently. Newton's third law describes energy transfer for reaction turbines. Reaction turbines are better suited to higher flow rates or applications where the fluid head (upstream pressure) is low.[3]​.

In the case of steam turbines, such as those that would be used for marine applications or for generating electricity on land, a Parsons-type reaction turbine would require approximately twice as many rows of blades as a 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 that of an equivalent impulse turbine for the same thermal energy conversion.

In practice, modern turbine designs use reaction and impulse concepts to varying degrees wherever possible. "Wind turbines" use an airfoil to generate reaction lift from the moving fluid and impart it to the rotor. Wind turbines also derive some energy from the momentum of the wind, deflecting it at an angle. Turbines with multiple stages may use reaction or impulse blades at high pressure. Steam turbines were traditionally more impulse-driven, 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 pressure reductions. Under these conditions, the blade becomes strictly a reaction type design with the base of the blade only impulse. The reason is due to the effect of the rotation speed of each blade. As the volume increases, the height of the blade increases and the base of the blade rotates at a slower speed relative to the tip.

Classical turbine design methods were developed in the mid-century. Vector analysis related fluid flow to the shape and rotation of the turbine. At first graphical calculation methods were used. The formulas for the basic dimensions of turbine parts are well documented and a highly efficient machine can be designed for any fluid and flow condition. Some of the calculations are empirical or "rule of thumb" formulas, and others are based on classical mechanics. As with most engineering calculations, simplifying assumptions were made.

Velocity triangles") can be used to calculate the basic efficiency of a turbine stage. The gas leaves the nozzle guide vanes of the stationary turbine at absolute speed V. The rotor rotates at speed U. Relative to the rotor, the speed of the gas as it impinges on the rotor inlet is V. The gas is rotated by the rotor and leaves, relative to the rotor, at a speed V. However, in absolute terms, the rotor exit speed is V. Velocity triangles are constructed using these various velocity vectors. Velocity triangles can be constructed at any section across the blade (e.g. hub, tip, mid-section, etc.), but are usually shown at the mean stage radius. The mean stage performance can be calculated from the velocity triangles, at this radius, using Euler's equation:

That's why:.

where:.

The turbine pressure ratio is a function of and the turbine efficiency.

Modern turbine design takes calculations further. Computational fluid dynamics dispenses with many of the simplifying assumptions used to derive classical formulas, and computer software facilitates optimization. These tools have led to constant improvements in turbine design over the past forty years.

The primary numerical rating of a turbine is its specific speed. This number describes the speed of the turbine at its maximum efficiency with respect to power and flow. The specific speed is derived to be independent of the size of the turbine. Given the fluid flow conditions and the desired shaft exit speed, the specific speed can be calculated and an appropriate turbine design selected.

Specific speed, along with some fundamental formulas, can be used to reliably scale an existing design of known performance to a new size with corresponding performance.

Off-design performance is normally shown as a turbine map or characteristic.

The number of blades on the rotor and the number of blades on the stator are usually two different prime numbers to reduce harmonics and maximize the passing frequency of the blades.[4]​.

Impulse turbines "Impulse (physics)") change the direction of flow of a high-velocity fluid or gas jet. The resulting impulse spins the turbine and leaves the fluid flowing with decreased kinetic energy. There is no pressure change of the fluid or gas on the turbine blades (the moving blades), as in the case of a steam or gas turbine, all the pressure drop takes place on the stationary blades (the nozzles). Before reaching the turbine, the "pressure head" of the fluid is changed to "velocity head" by accelerating the fluid with a nozzle. Pelton wheels and Laval turbines use this process exclusively. Impulse turbines do not require a pressure frame around the rotor, as the fluid jet is created by the nozzle before reaching the rotor blades. Newton's second law describes the energy transfer for impulse turbines. Impulse turbines are more efficient to use in cases where the flow is low and the inlet pressure is high.[3]​.

Reaction turbines&action=edit&redlink=1 "Reaction (physics) (not yet written)") develop torque by reacting to the pressure or mass of the gas or fluid. The pressure of the gas or fluid changes as it passes through the turbine rotor blades.[3]​ A pressure frame is needed to contain the working fluid as it acts on the turbine stage(s) or the turbine must be completely submerged in the fluid flow (as in wind turbines). 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 often used to harness the expanding gas efficiently. Newton's third law describes energy transfer for reaction turbines. Reaction turbines are better suited to higher flow rates or applications where the fluid head (upstream pressure) is low.[3]​.

In the case of steam turbines, such as those that would be used for marine applications or for generating electricity on land, a Parsons-type reaction turbine would require approximately twice as many rows of blades as a 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 that of an equivalent impulse turbine for the same thermal energy conversion.

In practice, modern turbine designs use reaction and impulse concepts to varying degrees wherever possible. "Wind turbines" use an airfoil to generate reaction lift from the moving fluid and impart it to the rotor. Wind turbines also derive some energy from the momentum of the wind, deflecting it at an angle. Turbines with multiple stages may use reaction or impulse blades at high pressure. Steam turbines were traditionally more impulse-driven, 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 pressure reductions. Under these conditions, the blade becomes strictly a reaction type design with the base of the blade only impulse. The reason is due to the effect of the rotation speed of each blade. As the volume increases, the height of the blade increases and the base of the blade rotates at a slower speed relative to the tip.

Classical turbine design methods were developed in the mid-century. Vector analysis related fluid flow to the shape and rotation of the turbine. At first graphical calculation methods were used. The formulas for the basic dimensions of turbine parts are well documented and a highly efficient machine can be designed for any fluid and flow condition. Some of the calculations are empirical or "rule of thumb" formulas, and others are based on classical mechanics. As with most engineering calculations, simplifying assumptions were made.

Velocity triangles") can be used to calculate the basic efficiency of a turbine stage. The gas leaves the nozzle guide vanes of the stationary turbine at absolute speed V. The rotor rotates at speed U. Relative to the rotor, the speed of the gas as it impinges on the rotor inlet is V. The gas is rotated by the rotor and leaves, relative to the rotor, at a speed V. However, in absolute terms, the rotor exit speed is V. Velocity triangles are constructed using these various velocity vectors. Velocity triangles can be constructed at any section across the blade (e.g. hub, tip, mid-section, etc.), but are usually shown at the mean stage radius. The mean stage performance can be calculated from the velocity triangles, at this radius, using Euler's equation:

That's why:.

where:.

The turbine pressure ratio is a function of and the turbine efficiency.

Modern turbine design takes calculations further. Computational fluid dynamics dispenses with many of the simplifying assumptions used to derive classical formulas, and computer software facilitates optimization. These tools have led to constant improvements in turbine design over the past forty years.

The primary numerical rating of a turbine is its specific speed. This number describes the speed of the turbine at its maximum efficiency with respect to power and flow. The specific speed is derived to be independent of the size of the turbine. Given the fluid flow conditions and the desired shaft exit speed, the specific speed can be calculated and an appropriate turbine design selected.

Specific speed, along with some fundamental formulas, can be used to reliably scale an existing design of known performance to a new size with corresponding performance.

Off-design performance is normally shown as a turbine map or characteristic.

The number of blades on the rotor and the number of blades on the stator are usually two different prime numbers to reduce harmonics and maximize the passing frequency of the blades.[4]​.