Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.2 Bubbles dynamics and boiling 383
Figure 7.2.20. Stability diagrams for film boiling. 73 Quenching in water (a) and in aqueous polymer solution with η s
= 3.27×10 -3 Pas at 40 o C; (b): 1, 2 - stable and unstable film boiling, 3 - nucleate boiling, 4 - convection. T ms,T m1 and
T m2 are the surface temperatures of the specimen, corresponding to the destabilization of the regimes1-3,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]
rates during the initial stage of quenching that permits to obtain fine-grained supercooled
austenite, and relatively low intensity of heat removal at moderate temperatures, when
martensite transformations take place.
The main feature of quenching in polymeric solutions is the prolongation of the cooling
period as a whole in comparison with water that is explained by extended range of a stable
film boiling (Figure 7.2.20). The increase in polymer concentration leads to reduction of
α on the stage of nucleate boiling and growth of the temperature, corresponding to the onset
of free convection regime.
The effect of polymeric additives on the initial stage of the process was the subject of a
special investigation. 74 Experiments were performed with aqueous polymer solutions of
Breox, PEO and some other polymers withM=6×10 3 to 6×10 5 at pressure of 0.1 MPa. The
platinum heater with short time lag, submerged in solution, was heated in a pulsed regime
with � T~10 5 to 10 6 K/s. The experimental results revealed the existence of a period with enhanced
heat transfer (as compared to water) in solutions with c ~ 1%, which lasted for 10 to
100μs after the onset of ebullition. The sensitivity of heat transfer to the polymer concentration
was sufficiently high. After formation of the vapor film the secondary ebullition was
observed, which resulted from superheating of the liquid outside the region of concentration
gradients near the interface. The mechanism of this phenomena was described. 75 It is associated
with the fact that heat transfer has a shorter time lag than mass transfer, and thus the
thermal boundary layer in a liquid grows faster than the diffusion one.
The experimental data and theoretical results on the growth of vapor bubbles and films
in polymeric solutions explain the efficiency of quenchants, based on water-soluble polymers.
The main reason is stabilization of the film-boiling regime at initial stage of quenching.
Such stabilization is connected with elastic properties of the liquid skin layer, adjacent
to the interface that is enriched by polymer due to solvent evaporation. Appearance of this
layer leads to fast growth of longitudinal viscosity and normal stresses, when perturbations
of the interface arise, thus increasing the vapor film stability. A similar mechanism is responsible
for stabilization of jets of polymeric solutions 9 as well as for retardation of bubble