Handbook of Turbomachinery Second Edition Revised - Ventech!

inherent flow range of the impeller is disclosed by this test. The inlet angles
of the impeller can be deliberately set for flow rates that are lower than
desired. After the initial test, the leading edges of the blades are machined
back in appropriate steps to realize maximum impeller efficiency at the
design flow rate. Similarly, the diameter of the impeller tips can be made
extra large and then machined back so that the desired useful energy is
imparted to the gas.
Vane Island Diffusers
These are the diffusers used for the centrifugal compressors of Figs. 1–4. In
other applications, scroll collectors may be used with these diffusers also.
Vane island diffusers usually have thin leading edges and can be used in
both subsonic and supersonic flow. Note that the vane heights are usually
too limited to alleviate the choking effects noted in the discussion of
supersonic diffusers. Many designers leave sufficient radial distance between
the impeller and the vanes to allow the flow to become subsonic, that is, to
reduce the Mach number to a value of 0.8 or lower.
The diffusion process is mainly controlled by the flow area between the
vanes, although the curvature of the vanes has a noticeable effect. Reneau et
al. [20] and Sagi et al. [21] offer relevent data on these subjects. These
references also show that the efficiency of diffusers is strongly dependent on
the thickness of the boundary layer at the diffuser inlet. The maximum
thickness used in the references should be assumed for the design of these
diffusers.
The leading edges of the diffuser vanes must be properly aligned to the
incoming flow, because this usually controls both the maximum flow rate
and the surge point at rated speed. Measurements of flow angles during tests
with vaneless diffusers are a useful guide. The vanes usually have no twist;
however, determining the flow angle as a function of passage width provides
data for specifying the angle of the leading edge along the span of the blade.
The ability to adjust the vanes during development testing to precisely define
the usable range of flow rates is often cost-effective.
Cascade Diffusers
These diffusers resemble vane island diffusers. Blades similar to those of
axial-flow compressors are placed in one or more rings around an impeller.
The height-to-spacing ratio of the blades for these diffusers is of the order of
unity. Experience with them is limited to subsonic flows. The diffusion
factor developed for axial-flow blades is used to determine how much each
ring may reduce the angular momentum of the gas flow. Unlike axial
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