An inducer is the axial inlet portion of a centrifugal pump rotor, the function of which is to raise the inlet head by an amount sufficient to prevent significant cavitation in the following pump stage. It is used in applications in which the inlet pressure of a pump is close to the vapor pressure of the pumped liquid. Inducers are virtually always included in the turbopumps for liquid propellant rocket engines, although they are also used in other applications which require high suction performance.
Design parameters Inducers are meant to increase the suction performance of a pump to the point that little to no cavitation occurs in its impeller. The key performance metrics of an inducer is its suction specific speed Nss and its flow coefficient Φ (analogous to, but not the same as, the discharge coefficient in pipe flow).
ω s s = ω ⋅ Q ( g ⋅ N P S H R ) 0.75 = π ⋅ 2 0.75 ϕ ( 1 − ν 2 ) τ 0.75 {\displaystyle \omega _{ss}={\frac {\omega \cdot {\sqrt {Q}}}{(g\cdot NPSH_{R})^{0.75}}}={\sqrt {\pi }}\cdot 2^{0.75}{\frac {\sqrt {\phi (1-\nu ^{2})}}{\tau ^{0.75}}}}
N s s = N R P M ⋅ Q g p m N P S H R , f t 0.75 = 2733.00 ⋅ ω s s = 8146.8 ϕ ( 1 − ν 2 ) τ 0.75 {\displaystyle N_{ss}={\frac {N_{RPM}\cdot {\sqrt {Q_{gpm}}}}{NPSH_{R,ft}^{0.75}}}={2733.00}\cdot \omega _{ss}=8146.8{\frac {\sqrt {\phi (1-\nu ^{2})}}{\tau ^{0.75}}}}
ϕ = V a x i a l U t i p = Q A U t i p = Q π ( r t i p 2 − r h u b 2 ) ω r t i p = Q π ω r t i p 3 ( 1 − ν 2 ) {\displaystyle \phi ={\frac {V_{axial}}{U_{tip}}}={\frac {Q}{AU_{tip}}}={\frac {Q}{\pi (r_{tip}^{2}-r_{hub}^{2})\omega r_{tip}}}={\frac {Q}{\pi \omega r_{tip}^{3}(1-\nu ^{2})}}}
N P S H R {\displaystyle NPSH_{R}} = net positive suction head, required (at inducer inlet)
Q {\displaystyle Q} = volumetric flowrate
N {\displaystyle N} = shaft speed
ν {\displaystyle \nu } = hub to tip ratio. Typical values range around 0.2 - 0.4 for a cantilevered inducer, or 0.5-0.6 otherwise.
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