Handbook of Turbomachinery Second Edition Revised - Ventech!

Projected Shape in Meridional Plane
Let us first consider the shape projected by a rotor on the meridional plane,
which is a plane through the axis of rotation. This shape is illustrated by the
contours shown for the centrifugal impellers in Figs. 2 and 4. The
boundaries of the indicated flow passage turn the flow radially outward
and thereby produce additional velocity gradients normal to the stream
surfaces. The velocity of a gas is higher at the casing than at the hub.
Blade Shapes
Refer now to Eq. (16) and note that oa must have finite values in centrifugal
and mixed-flow impellers. The velocity gradients described by Eq. (17)
augment those noted in the previous subsection when the magnitude of oa is
less than one half of the angular velocity of the impeller. Both the vorticities
and the curvatures of the hub and shroud induce high gas velocities at the
outer casing and low velocities at the hub. The effect of vorticity is reversed
at greater values of oa. Vorticity and the outward curvature of the flow have
been responsible for stagnated flow at the hub and separated flow at the
casing. Outward curvature of the hub and casing should thus be deferred
until the value of oa is about 50% of the angular velocity of the rotor. For
this preliminary design, one may estimate the gradients by assuming that the
path of the flow follows the shape of the blades.
One should strive to design the blades so that as much free vortex
rotation as possible is imparted to the gas in the regions where the flow has a
strong axial component. Recall, however, that mechanical stresses can limit
this objective.
As a tentative diffusion limit, the relative velocity at the impeller outlet
should be no less than 60% of the maximum relative velocity at the inlet. If
the resulting relative velocity component at the impeller outlet is
uncomfortably high, the use of relative tangential velocities opposite to
those of the impeller blades should be considered. The blades of such
impellers are said to have backward curvature. The absolute value of the
outlet tangential component of gas velocity is reduced, which lessens the
task of the following stator or diffuser. Impeller cost, size, stresses, and
possibly efficiency are the principal limitations to the amount of backward
curvature that may be selected.
Slip Factor
Unfortunately, there is no good general rule for estimating the deviation
angle or an equivalent angle. A quantity known as slip factor is an
Copyright © 2003 Marcel Dekker, Inc.