Axial turbine

An axial turbine is a turbine in which the flow of the working fluid is parallel to the shaft, as opposed to radial turbines, where the fluid runs around a shaft, as in a watermill. An axial turbine has similar construction as an axial compressor, but it operates in the reverse, converting flow of the fluid into rotating mechanical energy.

A set of static guide vanes or nozzle vanes accelerates and adds swirl to the fluid and directs it to the next row of turbine blades mounted on a turbine rotor.

The angles in the absolute system are noted by alpha (α) and the angles in the relative system are noted by beta (β). Axial and tangential components of both absolute and relative velocities are shown in the figure. Static and stagnation values of pressure and enthalpy in the absolute and relative systems are also shown.

It is often assumed that the axial velocity component remains constant through the stage. From this condition we get,
c_x = c₁ cos α₁ = c₂ cos α₂:= w₂ cos β₂ = c₃ cos α₃ = w₃ cos α₃ Also, for constant axial velocity yields a useful relation:

tan α₂ + tan α₃ = tan β₂ + tan β₃

A single-stage impulse turbine is shown in Figure

There is no change in the static pressure through the rotor of an impulse machine. The variation of pressure and velocity of the fluid through the stage is also shown in Figure.

The absolute velocity of the fluid increases corresponding to the pressure drop through the nozzle blade row in which only transformation of energy occurs. The transfer of energy occurs only across the rotor blade row. Therefore, the absolute fluid velocity decreases through this as shown in the figure. In the absence of any pressure drop through the rotor blades the relative velocities at their entry and exit are the same for fricitionless flow. To obtain this condition the rotor blade angles must be equal. Therefore, the utilization factor is given by

When the pressure drop available is large, it cannot all be used in one turbine stage. A single stage utilizing a large pressure drop will have an impractically high peripheral speed of its rotor. This would lead to either a larger diameter or a very high rotational speed. Therefore, machines with large pressure drops employ more than one stage.

One of the methods to employ multi-stage expansion in impulse turbines is to generate high velocity of the fluid by causing it to expand through a large pressure drop in the nozzle blade row. This high velocity fluid then transfers its energy in a number of stages by employing many rotor blade rows separated by rows of fixed guide blades. A two-stage velocity compounded impulse turbine is shown in Figure

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