Page 27 - Fluid Mechanics and Thermodynamics of Turbomachinery

8 Fluid Mechanics, Thermodynamics of Turbomachinery
might be associated with a Reynolds number effect, for the range of speeds 2500 5
3
N 5 5000 rev/min. For smaller flows, Q/.ND /< 0.025, the flow became unsteady
and the manometer readings of uncertain accuracy but, nevertheless, dynamically
similar conditions still appear to hold true. Examining the results at high flow rates
one is struck by a marked systematic deviation away from the “single-curve” law
at increasing speed. This effect is due to cavitation, a high speed phenomenon of
hydraulic machines caused by the release of vapour bubbles at low pressures, which
is discussed later in this chapter. It will be clear at this stage that under cavitating
flow conditions, dynamical similarity is not possible.

FIG. 1.3. Dimensionless head-volume characteristic of a centrifugal pump.

FIG. 1.4. Extrapolation of characteristic curves for dynamically similar conditions at
N D 3500 rev/min.