Effects of polymer additives on heat transport and flow structure in Rayleigh-Bénard convection

We experimentally investigate the effects of polymer additives in turbulent Rayleigh-Bénard convection (RBC) in a cubic cell with an aspect ratio $\Gamma =1$. The heat transport efficiency is measured with the polymer concentration $c$ varied in the range of $0\le c\le 200$ ppm and the $Wi$ number in the range of $0.15\le Wi\le 1.45$. In addition, the structure of the large-scale flow, the dynamics of the velocity boundary layer, and the velocity and temperature statistics in the bulk flow are investigated at various polymer concentrations $c$. It is found that polymer additives reduce the global heat transport. The degree of the reduction increases with $c$ for fixed $Wi$ and decreases with $Wi$ for fixed $c$. Measurements of the flow structure show that when $c$ exceeds a threshold of approximately 60 ppm, the large-scale circulation typically observed in turbulent RBC is replaced by vertically moving thermal plumes. This change in flow structure is also reflected in the statistics of the temperature and velocity in the bulk flow, i.e., the bulk flow becomes more coherent and the vertical velocity fluctuations are enhanced. It is also found that polymer additives stabilize the boundary layer by reducing both the velocity magnitude and velocity fluctuation. This stabilization leads to the thickening of the velocity boundary layer. These findings shed new light on how polymer additives affect the flow structure and heat transport in turbulent thermal convection.