Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.2 Bubbles dynamics and boiling 377
temperature and concentration D(T,k) was neglected (α = 0), whereas for the curve 5� it was
accounted for according to the experimental data. 55 Other curves were evaluated with the
following parameter values: curves 1 - 5 correspond to α =0,k 0= 0.99, 0.95, 0.7, 0.5, 0.3; 1
-4:χ= 0.1. Thermophysical parameters of the liquid and vapor are the same as in the Figure
7.2.10.
Thus, the rate of expansion of vapor bubbles in superheated solution of polymer is
lower than in pure solvent due to diffusion resistance. But in diluted solution at rather small
superheats the mechanism of diffusional retardation can be suppressed due to a weak dependence
of T s on k 0 in this concentration range. Another important conclusion is that in
concentrated solutions it is practically impossible to attain values Ja >> 1 by increasing the
superheat because of low values of the corresponding Sn numbers.
7.2.3 BOILING OF MACROMOLECULAR LIQUIDS
Experimental investigations of heat transfer at boiling of polymeric liquids cover highly diluted
(c = 15 to 500 ppm), low-concentrated (c ~ 1%), and concentrated solutions (c>10%).
The data represent diversity of physical mechanisms that reveal themselves in boiling processes.
The relative contribution of different physical factors can vary significantly with
changes in concentration, temperature, external conditions, etc., even for polymers of the
same type and approximately equal molecular mass. For dilute solutions this is clearly demonstrated
by the experimentally detected both intensification of heat transfer at nucleate
boiling and the opposite effect, viz. a decrease in the heat removal rate in comparison with a
pure solvent.
Macroscopic effects at boiling are associated with changes in the intrinsic characteristics
of the process (e.g., bubble shape and sizes, nucleation frequency, etc.). Let’s discuss
the existing experimental data in more detail.
One of the first studies on the effect of
water-soluble polymeric additives on boiling
was reported elsewhere. 56 For a plane
heating element a significant increase in
heat flux at fixed superheat, ΔT = 10-35K,
was found in aqueous solutions of PAA
Separan NP10 (M = 10 6 ), NP20 (M =
2×10 6 ), and HEC (M ~ 7×10 4 to about 10 5 )at
concentrations of 65 to 500 ppm (Figure
7.2.13). The experiments were performed at
atmospheric pressure; the viscosity of the
solutions did not exceed 3.57×10 -3 Pas. The
following specific features of boiling of
polymer solution were revealed by visual
observations: (i) reduction in the departure
Figure 7.2.13. Effect of the HEC additives on the boiling
curve. 1 - pure water; 2, 3 and 4 - HEC solution with
c = 62.5, 125 and 250 ppm, correspondingly. [Reprinted
from P. Kotchaphakdee, and M.C. Williams, Int. J. Heat
Mass Transfer, 13, 835, Copyright 1970, the reference
52, with permission from Elsevier Science]
diameter of bubbles, (ii) more uniform bubble-size
distribution, (iii) decrease in the
tendency to coalescence between bubbles.
The addition of HEC led to faster covering
of the heating surface by bubbles during the
initial period of boiling and bubbles were