Experimental investigations on the influence of polymerization degree in polymer-assisted pool boiling enhancement

The addition of polymer has been widely demonstrated effective for heat transfer enhancement. However, the influence of polymerization degree is rarely studied independently. In this paper, polyvinylpyrrolidones (PVP) with different polymerization degrees were attempted to enhance nucleate pool boiling performance at the mass fraction from 100 ppm to 3200 ppm. For PVP K15, K60 and K90, their highest heat transfer coefficients were 141.88 kW/(m2 center dot K) at 3200 ppm, 150.64 kW/(m2 center dot K) at 3200 ppm and 141.50 kW/(m2 center dot K) at 1600 ppm respec-tively, all significantly higher than 61.37 kW/(m2 center dot K) of distilled water. The mechanism was explored by solution property measurements and bubble behavior observations. The variations of nucleate sites, bubble size and departure frequency were observed, with the detection of special behaviors including bubble repulsion, bubble leaving retardancy, bubble-bubble penetration and liquid entrainment. The mechanism on the influence of polymerization degree is comprehensively proposed. Higher polymerization degree corresponds to lower surface tension, higher local viscosity, stronger normal stress between bubble and surface, and greater repulsion force between bubbles. The findings in this paper is of great significance, not only for predicting results of present polymers, but also for providing guidance for exploration of next-generation boiling-enhanced substance in field of heat conversion and transfer.