Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.2 Bubbles dynamics and boiling 379
In view of the discussed results, the work 61 attracts special attention since it contains
data on boiling of dilute solutions, opposite to those reported earlier. 56-60 The addition of
PAA, PEO and HEC to water in concentrations, corresponding to the viscosity increase up
toηp= 1.32×10 -3 Pas, has brought about reduction in heat transfer. The boiling curve in coordinates
q (heat flux) vs. ΔT displaced almost congruently to the region of larger ΔT values
with c (Figure 7.2.17a). It was demonstrated 61 that the observed decrease in α with addition
of polymer to water can be both qualitatively and quantitatively (with the Rohsenow pool
boiling correlation for the heat transfer coefficient 62 ) associated with the increase in the solution
viscosity (Figure 7.2.17, b). The experiments 61 were performed using a thin platinum
wire with diameter 0.3 mm.
Explanation of experimental data needs more detailed discussion of physical factors
that can reveal themselves in boiling of polymeric solutions. They include possible changes
in capillary forces on interfaces in the presence of polymeric additives; absorption of
macromolecules on the heating surface; increase in the number of weak points in the solution,
which facilitates increase in the number of nuclei; thermodynamic peculiarities of the
polymer-solvent system; the effect of macromolecules on the diffusion mass transfer in
evaporation of solvent; hydrodynamics of
convective flows in a boiling layer and the
motion of bubbles; manifestation of rheological
properties of solution.
The capillary effects were indicated as
one of the reasons for the intensification of
heat transfer, since many polymers (in particular,
HEC, PEO, etc.), 63
similar to
Figure 7.2.16. Heat transfer coefficient for nucleate pool
boiling of PEO aqueous solutions. (p f0 = 9.8×10 3 Pa).
Curve 1 corresponds to pure water, for curves2-7, c=
0.01, 0.02, 0.04, 0.08, 0.16 and 1.28%, respectively.
[Adapted, from S.P. Levitsky, and Z.P. Shulman, Bubbles
in polymeric liquids, Technomic Publish. Co.,
Lancaster, 1995, with permission from Technomic Publishing
Co., Inc., copyright 1995]
low-molecular surfactants, 64 are capable of
decreasing the surface tension. As a result,
they decrease both the work of the nucleus
formation, W cr, and the critical size of bubble,
R cr:
Figure 7.2.17. Boiling curves for aqueous solutions of PAA Separan AP-30. (a) experimental data; for curves1-6
η r = 1.00, 1.01, 1.04, 1.08, 1.16 and 1.32, correspondingly; (b) calculations made with the use of the Rohsenow
pool boiling correlation; for curves1-5,η r= 1.00, 1.01, 1.04, 1.16 and 1.32, respectively (η r = η p/η s). [By permission
from D.D. Paul, and S.I. Abdel-Khalik, J. Rheol., 27, 59, 1983, reference 61]