Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 22 Single-stage impulse turbine.
is due to unrecoverable losses across the blade row, since the flow is
nonreversible. Analytical methods exist for quantifying the difference
between V2 and V1 and are mentioned in the turbine section of this chapter.
With the flow conditions specified, we can set about discussion of the
turbine axial force components. The resultant turbine axial force is made up
of a summation of two main components. These are a pressure force on the
turbine due to a static pressure difference across the rotor, and a momentum
force due to a change in fluid axial momentum across the turbine. The
turbine pressure force can be thought of (for convenience) as itself consisting
of two components. These are a static pressure difference across the bladerow
annulus, and the difference of the pressure forces acting across the top
and bottom of the turbine disk. For a true impulse-type turbine stage, the
blade-row annulus pressure force is obviously zero, since no static pressure
difference exists across the blade row. The pressure forces acting on the top
and bottom (rotor inlet and rotor outlet) sides of the turbine disk are not so
straightforward. If we refer again to Fig. 22, we can see that both turbine
disk faces are subjected to vortex-type flow of the turbine working fluid. In
the case of the rotor inlet side, the disk (which is moving at a velocity U) is
subjected to flow that moves radially inward, beginning at the disk periphery
where flow is moving at a velocity V1 cos a relative to the disk. Due to
interactions between the disk and the layers of fluid adjacent to it, the
tangential component of the disk-relative fluid velocity is affected in a
Copyright © 2003 Marcel Dekker, Inc.