Explain with neat diagram:Axial flow turbine
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Answer:
Explanation:
Axial turbine
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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.
Contents
1 Stage velocity triangle
2 Single impulse stage
3 Multi stage velocity compounded impulse
4 Multi stage pressure compounded impulse
5 Reaction stages
6 Blade-to-gas speed ratio
7 Losses and efficiencies
8 See also
9 References
Stage velocity triangle
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.
Velocity triangle for a turbine stage
It is often assumed that the axial velocity component remains constant through the stage. From this condition we get,
cx = 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
Single impulse stage
A single-stage impulse turbine is shown in Figure
Single-stage impulse turbine
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
{\displaystyle E=U{\frac {(c_{y2}+c_{y3})}{{\frac {1}{2}}c_{2}}}}E=U{\frac {(c_{{y2}}+c_{{y3}})}{{\frac {1}{2}}c_{{2}}}}
Multi stage velocity compounded impulse
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
Two-stage velocity-compounded impulse turbine
The decrease in the absolute velocity of the fluid across the two rotor blade rows (R1 and R2) is due to the energy transfer; the slight decrease in the fluid velocity through the fixed guide blades (F) is due to losses. Since the turbine is of the impulse type, the pressure of the fluid remains constant after its expansion in the nozzle blade row. Such stages are referred to as velocity or Curtis stages.
Multi stage pressure compounded impulse
There are two major problems in velocity-compounded stages:
The nozzles have to be of the convergent-divergent type for generating high (supersonic) steam velocity. This results in a more expensive and difficult design of the nozzle blade rows.
High velocity at the nozzle exit leads to higher cascade losses. Shock waves are generated if the flow is supersonic which further increase the losses.
To avoid these problems, another method of utilizing a high pressure ratio is employed in which the total pressure drop is divided into a number of impulse stages. These are known as pressure-compounded or Rateau stages. On account of the comparatively lower pressure drop, the nozzle blade rows are subsonic (M < 1). Therefore, such a stage does not suffer from the disabilities of the velocity stages.
Two-stage pressure-compounded impulse turbine
Figure shows the variation of pressure and velocity of steam through the two pressure stages of an impulse turbine. The nozzle blades in each stage receive flow in the axial direction.
Some designers employ pressure stages up to the last stage. This gives a turbine of shorter length as compared to the reaction type, with a penalty on efficiency.
Reaction stages