The American Society of Mechanical Engineers

GONGWER—A THEORY OF CAVITATION FLOW IN CENTRIFUGAL-PUM P IM PELLERS 33
off and the 0.5 per cent point, here labeled th e line of cavitation
inception, for a num ber of cavitation runs taken, as described,
a t different values of C2 over th e pum p range of capacities. From
measurements of the vane setting, point A has been found to
correspond to zero angle of attack of the vane leading edges in
accordance w ith Equation [9], The close spacing of th e tw o
lines, of cavitation inception and breakoff, a t point A corresponds
to a very sharp breakoff in the head as the suction head
is lowered. The discontinuities a t B and C have been found to
occur for the pumps tested wherever the data have been taken
over a sufficient range of capacities and will be discussed later.
In Figs. 7, 8, and 9, actual cavitation data are plotted in this
form. The first two plots are for several impellers w ith the same
eye design and widely differing outside diam eters. Since, however,
the same foundry p attern w as not used for all th e impellers
and slight differences in vane thickness, setting, and spacing
unavoidably occurred, there is a slight scatter in the points of
the breakoff curve. The scatter is seen to become progressively
greater as the degree of cavitation decreases and the reason for
this is apparent from Fig. 1 in th e low slope of th e head curve
at high suction heads. However, careful inspection of these
points has shown th a t the differences in results among the impellers
of different outside diam eters is of small order w ith respect
to the scatter for any one impeller and, therefore, th e plots have
been considered to represent eye performance only. The importance
of this fact is emphasized, particularly in th e light of the
interpretation which is put on point B, Figs. 5 and 6, in the
discussions which follow.
From th e nature of th e three cavitation indexes, it is possible
to represent in the general cavitation plots the complete characteristics
of the particular eye design, and regardless of eye
diam eter (scale), speed, or capacity, Ca is the same measure of
flow sim ilarity and Ci and C3 are th e same m easures of the cavitation
regime. The general utility of this type of chart is therefore
great, particularly for th e designer.
Evaluation of Eye Coefficients From Experimental Data. In
E quation [9] th e expression for the angle of attack /S was derived
F i g . 9
r--&,
Cz ND’
I m p e l l e r C a v it a t io n C h a r a c t e r is t ic s
0 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.022 0.024
r - Q
2 ND3
F i a . 8
D i m e n s i o n l e s s C a v i t a t i o n P l o t