Boiling of macromolecular liquids

Experimental investigations of heat transfer at boiling of polymeric hquids 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 fiequency, etc.). Let s discuss the existing experimental data in more detail. 

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 smface 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 

The capillary effects were indicated as one of the reasons for the intensification of heat transfer, since many polymers (in particular, HEC, PEO, etc.), similar to low-molecular siufactants, are capable of decreasing the siuface tension. As a result, they decrease both the woik of the nucleus formation, W and the critical size of bubble, 

In highly diluted solutions the change in Newtonian viscosity due to polymer is insignificant, and though the correlation between heat transfer enhancement and increase in viscosity has been noticed, it cannot be the reason for observed changes of a. In hydrodynamics, the effect of turbulence suppression by small polymeric additives is known, but it also cannot be considered for such a reason because laminarization of the boimdary layer leads to reduction of the intensity of convective heat transfer. Nevertheless, the phenomenon of the decrease of hydrodynamic resistance and enhancement of heat transfer in boiling dilute solutions have a common nature. The latter effect was connected 

The above discussion permits to explain the experimental results. 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 ). 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 where n 1/4).  

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