Page 32 - Fluid Mechanics and Thermodynamics of Turbomachinery

Introduction: Dimensional Analysis: Similitude 13
when streams of small vapour bubbles appear within the liquid and close to solid
surfaces. This is called cavitation inception and commences in the regions of lowest
pressure. These bubbles are swept into regions of higher pressure where they
collapse. This condensation occurs suddenly, the liquid surrounding the bubbles
either hitting the walls or adjacent liquid. The pressure wave produced by bubble
collapse (with a magnitude of the order 400 MPa) momentarily raises the pres-
sure level in the vicinity and the action ceases. The cycle then repeats itself and
the frequency may be as high as 25 kHz (Shepherd 1956). The repeated action of
bubbles collapsing near solid surfaces leads to the well-known cavitation erosion.
The collapse of vapour cavities generates noise over a wide range of
frequencies up to 1 MHz has been measured (Pearsall 1972) i.e. so-called
“white noise”. Apparently it is the collapsing smaller bubbles which cause the
higher frequency noise and the larger cavities the lower frequency noise. Noise
measurement can be used as a means of detecting cavitation (Pearsall 1966/7).
Pearsall and McNulty (1968) have shown experimentally that there is a relationship
between cavitation noise levels and erosion damage on cylinders and concludes that
a technique could be developed for predicting the occurrence of erosion.
Up to this point no detectable deterioration in performance has occurred. However,
with further reduction in inlet pressure, the bubbles increase both in size and number,
coalescing into pockets of vapour which affects the whole field of flow. This growth
of vapour cavities is usually accompanied by a sharp drop in pump performance
as shown conclusively in Figure 1.3 (for the 5000 rev/min test data). It may seem
surprising to learn that with this large change in bubble size, the solid surfaces
are much less likely to be damaged than at inception of cavitation. The avoidance
of cavitation inception in conventionally designed machines can be regarded as
one of the essential tasks of both pump and turbine designers. However, in certain
recent specialised applications pumps have been designed to operate under super-
cavitating conditions. Under these conditions large size vapour bubbles are formed
but, bubble collapse takes place downstream of the impeller blades. An example of
the specialised application of a supercavitating pump is the fuel pumps of rocket
engines for space vehicles where size and mass must be kept low at all costs. Pearsall
(1966) has shown that the supercavitating principle is most suitable for axial flow
pumps of high specific speed and has suggested a design technique using methods
similar to those employed for conventional pumps.
Pearsall (1966) was one of the first to show that operating in the supercavitating
regime was practicable for axial flow pumps and he proposed a design technique to
enable this mode of operation to be used. A detailed description was later published
(Pearsall 1973), and the cavitation performance was claimed to be much better than
that of conventional pumps. Some further details are given in Chapter 7 of this book.

Cavitation limits
In theory cavitation commences in a liquid when the static pressure is reduced to
the vapour pressure corresponding to the liquid’s temperature. However, in practice,
the physical state of the liquid will determine the pressure at which cavitation starts
(Pearsall 1972). Dissolved gases come out of solution as the pressure is reduced
forming gas cavities at pressures in excess of the vapour pressure. Vapour cavitation
requires the presence of nuclei submicroscopic gas bubbles or solid non-wetted