Design and Performance of Experimental Axial-Discharge Mixed-Flow Compressor 1: Impeller Design Theory Page: 4 of 36
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NACA RM No. E8F04
Basic Assumptions
The following assumptions are used in establishing flow rela-
tions and velocity distribution:
1. At the entrance and exit (stations 1 and 3 in fig. 1), the
entropy is constant on an axial plane, which is the equivalent of
assuming no boundary layer on the shrouds and equal entropy increases
for all streamlines in the impeller.
2. The gas inside the impeller flows on surfaces of revolution.
3. Tangential velocity at the casing in the discharge annulus
is 0.95 of the tip speed.
4. The velocity reduction at the blade tips in the inducer
section is one-third the inccning relative velocity.
5. Pressure rise allowed on the case is slightly less than the
centrifugal pressure rise on the assumption that the boundary layer
limits the pressure rise.
6. At the exit, the maximum allowed absolute Mach number
is 1.4.
7. At the entrance, the maximum allowed relative Mach number
is 1.0.
For computing the shape of the impeller there are the follow-
ing assumptions:
1. The blade tips are shaped according to two-dimensional
stream-filament theory, which is applied to compute the velocity
on the blade surface. Suitability of the velocity distribution is
judged by two-dimensional boundary-layer theory. The rest of the
blade, extending down to the root, is shaped to maintain radial
blade elements.
2. Inside the impeller the entropy is constant on surfaces of
revolution normal to the meridional projection of the streamlines.
3. The frictional forces on only the blades and impeller are
taken into account; those on the case are neglected.
4. An infinite number of blades is assumed for computing the
hub shape.3
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Goldstein, Arthur W. Design and Performance of Experimental Axial-Discharge Mixed-Flow Compressor 1: Impeller Design Theory, report, August 12, 1948; (https://digital.library.unt.edu/ark:/67531/metadc57686/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.