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• December 16, 2016 Create Date
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National Advisory Committee for Aeronautics, Technical Notes - Effect of Changing Passage Configuration on Internal Flow Characteristics of a 48 Inch Centrifugal Compressor - I - Change in Blade Shape

The passage contour of a 48—inch centrifugal impeller was modified
by changing the shape of the blades with the objective of reducing the
deceleration rates along the blade faces and thereby improving the
internal efficiency of the impeller. A comparison of the internal—flow
characteristics was made of the original and modified.passage contours
at the design flow rate. In addition, the internal-flow characteristics
of the modified passage are presented and analyzed over the entire flow
range from.maximum flow to surge at a corrected impeller tip speed of
700 feet per second.

At design flow, the modified impeller, which had lesser decelera-
tion rates along the blade surfaces, showed a general improvement in
efficiency throughout the passage over that of the original impeller.
It is concluded that impellers designed for prescribed velocity distri—
butions with low rates of deceleration should have higher internal
efficiencies than impellers with high rates of deceleration. At high
weight flows (negative angles of attadk), there were large separation
losses at the inlet because of the shape of the leading edge; at low
flow rates (positive angles of attack), the separation losses were not
as great. The low-efficiency regions in the passage generally occurred
near the flow surfaces with decelerating flow, except along the driving
face where most boundary—layer build—up by adverse velocity gradients
was probably eliminated.by secondary flows or bleeding through the
clearance space.

The flow characteristics within the rotating passages of a large
radial—inlet centrifugal impeller have been determined.experimentally
at the NACA Lewis laboratory (references 1 and 2). Along the trailing
face, a low-efficiency region generally occurred as a result of a shift
of low-energy air toward the trailing face and losses caused by the
deceleration of air. By changing the passage area and the contour of
the blades, the velocity distribution was adjusted to give smaller
decelerations along the trailing face; better flow characteristics
throughout the passage resulted.