THEORY OF PROPULSION 7' Centrifugal Compressors

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THEORY OF PROPULSION7. Centrifugal Compressors

• P. M. SFORZA
• University of Florida

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Circular Cascade
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The circular cascade
Array of identical blades spaced at an angular
pitch of wp
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The energy equation in the cascade
h e p/r
For liquids, as well as for gases undergoing a
small pressure rise, e constant (small T
change) and therefore r constant and the
equation becomes
For pumps or small pressure rise, e.g., fans
Then
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The power transfer in the cascade
Velocity component relation
In these equations no assumption about
compressibility has been made so far
For constant density flow, the stagnation
pressure change across the cascade is
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Compressibility effect
In a gas compressor the ideal process is an
isentropic compression where p rk and thus
We may now calculate the density and temperature
changes for an ideal compression
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Isentropic compression of gases
k1.4
Axial flow compressor range
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Isentropic compression of gases
k1.4
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Mach number effect on pressure
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Centrifugal compressors
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Velocity diagrams
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Configuration of a typical compressor
Axial entry (no pre-whirl)c2u0
const
Radial exit c3uu3
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Entry blade twist with no pre-whirl
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Mass flow considerations
The mass flow is constant
compressible
incompressible
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Radial exit with no pre-whirl
No pre-whirl
Radial exit
Power required
Total head
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General exit with no pre-whirl
Slip coefficient t
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Head rise for zero pre-whirl
b3 inc
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Work required for compression
Isentropic relation
Compressor work