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Hydrodynamics of Pumps
Christopher E. BrennenCalifornia Institute of Technology,
Pasadena, California
With many thanks to Allan Acosta, Dave Japikse, innumerable colleagues, a special group of students
at Caltech, anda special debt to NASA Marshall, to Loren Gross,
Otto Goetz and Henry Stinson.
Prediction of problems:
Turbomachine Power proportional to L53 = L2(L)3
Therefore, same power, same fluid, if L decreases then L must increase
and since is prop. to (L)-2
cavitation must increase
Also…
Since fluid pressures prop. to (L)2
Then blade stresses prop. to
(L)2 (L/T)2
And therefore for the same power,same fluid, same geometry,
blade stress is prop. toL-4/3
Lecture One:
Introduction Specific Speed and Pump Design Non-cavitating performance Secondary flows incl. Prerotation
Geometric Notation:
Streamtube:
VelocityTriangle:
Incidence Angle Deviation Angle
Reynolds Number effects:
Non-cavitating pump performance analysis
Using Bernoulli’s equation in rotating coordinates, a simple expression for the viscous losses (f),
assuming simple geometry,zero deviation, and no preswirl,
leads to a simple pump performance analysis:
And with only slightly more complex lossmechanisms (mD):
Deviation from inviscid calculation:
Viscous losses in blade wakes (axial cascade):
Axial cascade losses:
Centrifugalcascadeanalysis:
Displacement component of inviscid flow:
Busemann slip factor for inviscid flow:
Viscous wakes in centrifugal pumps:
Three-dimensional analysis:A radial equilibrium
calculation
Secondary Flows
Some secondary flows:
Within the blade passage At inlet – tip clearance flow and backflow for an unshrouded impeller Shrouded centrifugal pump Cutwater separation in volute
Prerotation
Widespread misunderstanding Prerotation may be caused only by
Backflow
or
Upstream Asymmetry
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