Drag-reducing solutions exhibit simultaneous friction and heat transfer reductions, yet it has been widely believed that there is a decoupling between the two effects, an unexplained and puzzling concept. In this work, we have conducted a dedicated study to reexamine this issue, using a wide range of flow conditions from onset to asymptotic, pipe diameters, and various fluids ranging from polymer to surfactant solutions. All our data and those of some other investigators were found to be remarkably well correlated by a constant ratio between the heat transfer and drag reductions over the entire range of conditions examined. In addition, this ratio was found to hold even at and near the onset of drag reduction phenomenon, and also upon departure from the asymptotic regime. We have also shown that the onsets for drag and heat transfer reductions are the same, and that degradation or dilution result in simultaneous departure from asymptotic conditions. All these results support the concept that heat transfer and drag are indeed strongly coupled under all conditions investigated. This information should provide new insight into the fundamental mechanisms underlying the relationship between the drag and heat transfer reduction phenomena. In addition, the practical engineering impact of this finding is very significant in that it allows us now to predict directly the heat transfer from friction measurements or vice-versa, for these fluids.

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