A similarity between the turbulent transport of heat and momentum in drag-reduced flows by polymer additives

Heat transfer reduction (HTR) in drag-reduced turbulent flows of dilute polymer solutions has been examined using DNS of fully developed viscoelastic turbulent channel flows (Re_τ = 125 and Pr = 5) with a constant heat flux boundary condition. The polymer stress is modeled using the finitely extensible nonlinear elastic-Peterlin constitutive model, and low (15%), intermediate (34%), and high drag reduction (DR) (52%) cases are examined. The present results show that the HTR is larger than DR in the viscoelastic flows; the HTRs are 19%, 41%, and 62% for DR = 15%, 34%, and 52% flow, respectively. As DR increases, the wall-normal turbulent heat flux decreases, which leads to an increase in the slope of the mean temperature profile and thus results in the heat transfer reduction. The conditionally averaged fields for the events with a large contribution to the wall-normal turbulent heat flux are very similar to those for the Q2 events in both Newtonian and viscoelastic flows. This suggests that the turbulent transport of momentum and heat are associated with almost the same flow structure in drag-reduced flows as well as Newtonian flow, which is consistent with our recent study (Kim and Sureshkumar, 2018, Phys. Fluids 30).