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We present experimental evidence that a minute amount of polymer additivescan significantly enhance heat transport in the bulk region of turbulentthermal convection. The effects of polymer additives are found to be the\textit{suppression} of turbulent background fluctuations that give rise toincoherent heat fluxes that make no net contribution to heat transport, and atthe same time to \textit{increase} the coherency of temperature and velocityfields. The suppression of small-scale turbulent fluctuations leads to morecoherent thermal plumes that result in the heat transport enhancement. The factthat polymer additives can increase the coherency of thermal plumes issupported by the measurements of a number of local quantities, such as theextracted plume amplitude and width, the velocity autocorrelation functions andthe velocity-temperature cross-correlation coefficient. The results from localmeasurements also suggest the existence of a threshold value for the polymerconcentration, only above which can significant modification of the plumecoherent properties and enhancement of the local heat flux be observed.Estimation of the plume emission rate suggests that the second effect ofpolymer additives is to stabilize the thermal boundary layers.

Effects of polymer additives in the bulk of turbulent thermal convection