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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.
An example of a velocity triangle drawn for the inlet of a turbomachine. The "1" subscript denotes the high pressure side (inlet in case of turbines and outlet in case of pumps/compressors). A general velocity triangle consists of the following vectors: [1] [2] V = absolute velocity of the fluid. U = blade linear velocity.
The velocity triangle [2] (Figure 2.) for the flow process within the stage represents the change in fluid velocity as it flows first in the stator or the fixed blades and then through the rotor or the moving blades. Due to the change in velocities there is a corresponding pressure change. Figure 2. Velocity Triangle for fluid flow in turbine
The color triangles formed by velocity vectors u,c and w are called velocity triangles and are helpful in explaining how pumps work. c 1 {\displaystyle c_{1}\,} and c 2 {\displaystyle c_{2}\,} are the absolute velocities of the fluid at the inlet and outlet respectively.
A Kaplan turbine is an example of an axial flow turbine. In the figure: U = Blade velocity, V f = Flow velocity, V = Absolute velocity, V r = Relative velocity, V w = Tangential or Whirl component of velocity. Radial Turbomachine's Velocity Diagram [1]
Usually the flow velocity (velocity perpendicular to the tangential direction) remains constant throughout, i.e. V f1 =V f2 and is equal to that at the inlet to the draft tube. Using the Euler turbine equation, E/m=e=V w1 U 1, where e is the energy transfer to the rotor per unit mass of the fluid. From the inlet velocity triangle,
From an energy exchange point of view axial compressors are reversed turbines. Steam-turbine designer Charles Algernon Parsons, for example, recognized that a turbine which produced work by virtue of a fluid's static pressure (i.e. a reaction turbine) could have its action reversed to act as an air compressor, calling it a turbo compressor or pump.
Being quite small in axial impellers (inlet and outlet flow in the same direction), slip is a very important phenomenon in radial impellers and is useful in determining the accurate estimation of work input or the energy transfer between the impeller and the fluid, rise in pressure and the velocity triangles at the impeller exit.