Flow and Heat Transfer Intensification by Elastic Turbulence

Elastic instability is proposed as a promising method to intensify heat transfer of a polymer solution at a very low Reynold number. However, the onset of elastic instability highly relies on the rheological properties of the polymer solution, whose concomitant effects on the heat transfer side have not been revealed. Besides, most of the studies adopted passive techniques to induce the elastic turbulence, the active control of polymer motion and thereby induce the elastic turbulence has not been investigated. To address these limitations, this work aims to conduct a systematic study to reveal the effects of polymer rheological behaviours on the flow and heat transfer performance and to probe the potential for the active control of elastic turbulence, which could improve the understanding of mechanism of elastic turbulence and its potential application in heat transfer intensification. At the bulk level, the effects of polymer sensitivities such as polymer concentration and salinity on the rheological properties of hydrolysed polyacrylamide (HPAM) solutions were first investigated experimentally. It showed that with the increase of polymer concentration or the reduction of the salinity, the viscosity of polymer solutions could be increased significantly, showing stronger shear-thinning behaviours. The different mechanisms were interpreted by microscopic HPAM morphology by molecular dynamics simulation (MD). The salt cations shielding the polymer electronic repulsion leaded to the collapse of the polymer molecules, reducing the viscosity consequently. The effects of polymer solution on the onset of elastic turbulence and heat transfer were conducted in two experimental setups, i.e., a swirling flow between two parallel plates and a curvilinear microchannel. The polymer solution either with a high concentration or a low salinity was capable of inducing elastic instability easily, which was consistent with the mixing performance in curvilinear microchannel. However, this salinity-based rheology effects could not be described by the interpretation for the effects of polymer concentration, which is mainly ascribed to the additional shielding effects. A similar trend was observed in heat transfer experiment. For a given flow velocity, polymer solutions with a higher concentration and a low salinity exhibited much significant convective heat transfer enhancement. The salinity effects became weak as the swirling flow velocity continually increased. The maximum enhancement seemed to be independent on salinity, which could be also demonstrated by the fact that a salinity independent power-law exponent at a value of -4.3 was observed in fully developed elastic regime for all of the polymer solutions. When the polymer solutions were normalised by the polymer relaxation, the polymer concentration was no longer influenced the heat transfer performance. Whilst high salinity captured better heat transfer capability than low salinity due to the existence of the dramatic shear-thinning phenomenon. Microscopically molecular dynamics (MD) simulations were conducted to exam the HPAM morphology variation under external electric field to investigate the possibility of active control /induction of elastic turbulence. It showed that with the external electric force field applied on the flow passages or channels, the salinity effects on the polymer rheology became insufficient due to the attraction between charged wall and cations, which would modify the onset of elastic instability. It seems to be possible to control the motion of ions to change the polymer conformation, thereby inducing the elastic instability at even lower Re and Wi, The work advances our understanding of the flow and heat transfer of polymer solutions from both bulk and microscale, and could open a new window of opportunity for elastic turbulence applications.