Page 206 - Fluid Mechanics and Thermodynamics of Turbomachinery

Three-dimensional Flows in Axial Turbomachines 187

FIG. 6.9. Variation of the distribution in axial velocity through a row of guide vanes
(adapted from Hawthorne and Horlock 1962).

taken from a review paper by Hawthorne and Horlock (1962), shows the distribution
of the axial velocity component at various axial distances upstream and downstream
of an isolated row of stationary inlet guide vanes. This figure clearly illustrates the
appreciable redistribution of flow in regions outside of the blade row and that radial
velocities must exist in these regions. For the flow through a single row of rotor
blades, the variation in pressure (near the hub and tip) and variation in axial velocity
(near the hub) both as functions of axial position, are shown in Figure 6.10, also
taken from Hawthome and Horlock. Clearly, radial equilibrium is not established
entirely within the blade row.
A more accurate form of three-dimensional flow analysis than radial equilibrium
theory is obtained with the actuator disc concept. The idea of an actuator disc is
quite old and appears to have been first used in the theory of propellers; it has
since evolved into a fairly sophisticated method of analysing flow problems in
turbomachinery. To appreciate the idea of an actuator disc, imagine that the axial
width of each blade row is shrunk while, at the same time, the space chord ratio,
the blade angles and overall length of machine are maintained constant. As the
deflection through each blade row for a given incidence is, apart from Reynolds
number and Mach number effects (cf. Chapter 3 on cascades), fixed by the cascade