A centrifugal pump impeller is cast in a duplex stainless steel (

PHYSICS · Multi-part · Fermi University Physics · Fluids & Continuum Physics · Solid continuum mechanics and complex fluids · Solid continuum mechanics and complex fluids

Bending, Ringing, Breaking and Flowing, Ch Bubbles, drops and cavitation

Problem

A centrifugal pump impeller is cast in a duplex stainless steel (ρ=7900 kg m−3\rho = 7900\,\mathrm{kg\,m^{-3}}, longitudinal wave speed c=5800 m s−1c = 5800\,\mathrm{m\,s^{-1}}, Poisson's ratio ν=0.30\nu = 0.30, 0.20.2 per cent proof stress 500 MPa500\,\mathrm{MPa}). Collapsing cavitation bubbles near a blade send re-entrant jets of speed 120 m s−1120\,\mathrm{m\,s^{-1}} into the surface. The pumped water has ρ=1000 kg m−3\rho = 1000\,\mathrm{kg\,m^{-3}} and speed of sound c=1480 m s−1c = 1480\,\mathrm{m\,s^{-1}}. ((a)) Find the acoustic impedance of the water and of the impeller alloy. ((b)) Estimate the impact pressure of a jet striking the impeller. ((c)) Assuming the metal directly under the impact spot is in a state of uniaxial strain, estimate the von Mises stress there and decide whether a single impact yields the alloy. ((d)) Even though a single impact does not yield the metal, impellers of this kind pit badly after long service under cavitating conditions. Explain in a few sentences why.

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