Significant heat transfer enhancement via polymer additives in two-dimensional sheared convection

TL;DR

This study demonstrates that polymer additives can significantly enhance heat transfer in two-dimensional sheared convection, especially through wall-attached polymer structures that reorganize flow patterns.

Contribution

The paper reveals how polymer-induced elastic effects can dramatically increase heat transfer efficiency in sheared convection flows, identifying specific flow states and structures responsible.

Findings

Polymer additives can boost heat flux by up to 1100%.

Wall-attached polymer hooks reorganize flow into strong counter-rotating rolls.

Unattached hooks offer a thermally efficient heat transfer regime.

Abstract

Heat dissipation is critical in modern engineering systems. Polymer additives offer a potential route to improve fluid-based cooling. Here, we study elasticity-enhanced heat transfer in two-dimensional, thermally-stratified Poiseuille flow. At Reynolds numbers, $Re$, $\lesssim 1000$, we observe two types of linearly unstable modes: the recently identified elasticity-induced centre mode (Khalid et al., J. Fluid Mech. 915, 2021) and the classical buoyancy-driven convective mode (Kelly, Adv. Appl. Mech. 31, 35-112, 1994). Direct numerical simulations show that the centre mode develops into a nonlinear `arrowhead' state but yields negligible heat transfer enhancement (typically $\approx 0.03\%$ increase compared to the conductive state). By contrast, polymers can enhance the heat flux associated with the convective mode by up to $1100\%$. The nonlinear convective-mode states take the form of either periodic orbits or travelling waves, and are dominated by hook-like polymer-stress structures that can attach to the walls. The unattached hooks act as `speed bumps' that reduced streamwise velocity and promote wall-normal motion, whereas wall-attached hooks form effective `polymer walls', reorganising the flow into strong counter-rotating rolls and triggering the extreme-enhancement regime. The elasto-buoyant nature of these states is confirmed by perturbation kinetic energy budgets, which show that polymer and buoyancy sustain the states synergistically. The wall-attached hooks enable rapid thermal equilibration but impose a large hydraulic penalty, making them suitable for process streams requiring fast temperature adjustment. Unattached hooks provide a more thermally efficient regime for heat-transport applications. These results highlight the potential of elastic fluids for future heat transfer enhancement technologies.