Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.2 Bubbles dynamics and boiling 371
Figure 7.2.7. Relative decrement of free oscillations of
air bubble versus concentration of POE in water. [By
permission of IOP Publishing Limited from W.D.
McComb, and S. Ayyash, J. Phys. D: Appl. Phys., 13,
773, 1980, the reference 39]
Figure 7.2.9. Collapse of a bubble near a solid wall in
water (curves 1 and 4) and in dilute POE aqueous solution
(curves 2, 3). [By permission of the American Institute
of Physics from G.L. Chahine, and D.H. Fruman,
Phys. Fluids., 22, 1406, 1979, the reference 37]
Figure 7.2.8. Geometrical parameters of the collapsing
bubble. [By permission of the American Institute of
Physics from G.L. Chahine, and D.H. Fruman, Phys.
Fluids., 22, 1406, 1979, the reference 37]
nomenon observed 40
in ultrasonic
insonification of liquid polybutadiene with
molecular massM~10 5 and η p ~10 6 Pas. At
ultrasound frequency f~18 kHz the acoustic
cavitation and well-developed pulsations of
bubbles were detected, in spite of for
low-molecular liquids with so high viscosity
it is completely impossible. 41
For non-spherical bubbles the effect of
polymeric additives becomes essential at
lower concentrations, as compared to the
spherical bubbles. For example, retardation
of the bubble collapse near a solid wall was
observed 37,42 in such concentration interval
where dynamics of spherical bubbles has remained
unchanged. In Figures 7.2.8 and
7.2.9 the data 37 are reproduced, where the Rayleigh time, t 0, was chosen for a scaling time,
t R, and for curves 1- 4 R c,max/L = 0.5, 0.56, 1.39, 1.25, respectively. The bubble collapse is
accompanied by generation of a microjet towards the wall and addition of polymer led to
stabilization of the bubble shape and retardation of the jet formation. This effect is connected
with the increase in the elongational viscosity of a polymeric solution in flow around
collapsing bubble.
The effect of polymeric additives on collective phenomena, associated with the dynamics
of bubbles, can be illustrated by the example of hydrodynamic cavitation, caused by
abrupt decrease in local pressure (in flows around bodies, after stream contraction, in jets).
It has been found that the use of polymers permits to decrease the cavitation noise, lower the
cavitation erosion, and delay the cavitation inception. For example, adding a small amount
of POE to a water jet issuing from the orifice caused the decrease of the critical cavitation
number, κ cr, by 35-40%. 43,44 In experiments with rotating disk 45 the value of κ cr was decreased
by 65% with addition of 500 ppm POE. Note, however, that these features are
linked not only to the changes in individual bubble dynamics, but to the influence of
macromolecules on the total flow regime as well. In particular, phenomena listed above are