Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.2 Bubbles dynamics and boiling 381
Figure 7.2.18. Growth of vapor bubbles on the heating surface at high (a) and low (b) pressures. [Reprinted from
S.P. Levitsky, B.M. Khusid and Z.P. Shulman, Int. J. Heat Mass Transfer, 39, 639, Copyright 1996, the reference
66, with permission from Elsevier Science]
with manifestation of elastic properties of the solution at vapor bubble growth on the heating
surface. 66
The general character of the bubbles evolution at boiling under atmospheric and
subatmospheric pressures, respectively, is clarified schematically in Figure 7.2.18. In the
first case (at high pressures) the base of a bubble does not “spread” 67 but stays at the place of
its nucleation. Under such conditions the decrease in the curvature of the bubble surface
with time, resulting from the increase in bubble radius, R, leads to liquid displacement from
the zone between the lower part of the microbubble and the heating surface. This gives rise
to the local shear in a thin layer of a polymer solution. A similar shear flow is developed also
in the second case (at low pressures), when a microlayer of liquid is formed under a
semi-spherical bubble. As known, at shear of a viscoelastic fluid appear not only tangential
but also normal stresses, reflecting accumulation of elastic energy in the strained layer (the
Weissenberg effect 3 ). The appearance of these stresses and elastic return of the liquid to the
bubble nucleation center is the reason for more early detachment of the bubble from the
heating surface, reduction in its size and growth in the nucleation frequency. All this ultimately
leads to enhancement of the heat transfer.
The above discussion permits to explain the experimental results. 61 Their reasons are
associated with substantial differences in the conditions of boiling on a thin wire and a plate
or a tube. Steam bubbles growing on a wire have a size commensurable with the wire diameter
(the growing bubble enveloped the wire 61 ). This results in sharp reduction of the boundary
layer role, the same as the role of the normal stresses. Besides, the bubble growth rate on
a wire is smaller than on a plane (for a wire R~t n where n < 1/4). 67
The elastic properties of the solution are responsible also for stabilization of the spherical
shape of bubbles observed in experiments on boiling and cavitation. Finally, the observed
reduction in a coalescence tendency and an increase in the bubble sizes uniformity
can also be attributed to the effects of normal stresses and longitudinal viscosity in thin
films separating the drawing together bubbles.
The linkage between the enhancement of heat transfer at boiling of dilute polymer solutions
and the elastic properties of the system is confirmed by the existence of the optimal
concentration corresponding to α max (Figure 7.2.14). Similar optimal concentration was established
in addition of polymers to water to suppress turbulence - the phenomenon that also
owes its origin to elasticity of macromolecules. 1,3,9 Therefore, it is possible to expect that the
factors favoring the chain flexibility and increase in the molecular mass, should lead to
strengthening of the effect.
The data on boiling of concentrated polymeric solutions 20 demonstrate that in such
systems thermodynamic, diffusional, and rheological factors are of primary importance.