Page 324 - Fluid Mechanics and Thermodynamics of Turbomachinery

Hydraulic Turbines 305
problems with regard to construction and maintenance. Equation (9.24) shows that
the greater the available head H E at which a turbine operates, the lower it must be
located relative to the surface of the tailrace.

Sonoluminescence
The collapse of vapour cavities generates both noise and flashes of light (called
sonoluminescence). Young (1989) has given an extended and interesting review of
experiments on sonoluminescence from hydrodynamic cavitation and its causes.
The phenomenon has also been reported by Pearsall (1974) who considered that the
collapse of the cavity was so rapid that very high pressures and temperatures were
created. Temperatures as high as 10 000 K have been suggested. Shock waves with
pressure differences of 4000 atm have been demonstrated in the liquid following the
collapse of a cavity. The effect of the thermal and pressure shocks on any material
in close proximity causes mechanical failure, i.e. erosion damage.
Light has been reported in large energy distributions in field installations. An
example again quoted by Young is that of the easily visible light observed at night
in the tailrace at Boulder Dam, USA. This occurs when sudden changes of load
necessitate the release of large quantities of high-pressure water into an energy-
dissipating structure. Under these conditions the water cavitates severely. In a further
example, Young mentions the light (observed at night) from the tailrace of the
hydroelectric power station at Erochty, Scotland. The luminescence appeared for
up to ten seconds shortly after the relief valve was opened and was seen as a blue
shimmering light stretching over an area of the water surface for several square
metres.

References

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Danel, P. (1959). The hydraulic turbine in evolution. Proc. Instn. Mech. Engrs., 173,36 44.
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