Heat Transfer in Flows with Drag Reduction

Abstract

Publisher Summary The chapter reviews the models proposed to describe both momentum and heat transfer in flows with drag reduction and examine their limitations. A phenomenological model for calculating heat transfer in flows with drag reduction is presented. The model is based on a mean velocity closure that uses Van Driest's mixing length expression with a variable damping parameter and the classical Reynolds–Prandtl analogy between momentum and heat transfer. The model has been used to analyze the heat transfer in pipe flows both in the thermally established region and in the entrance region. The effect of the temperature-dependent fluid properties is also analyzed. Algorithms providing implicit relations between the heat transfer, the friction, the properties of the fluid and the flow, which do not include any empirical coefficients, are derived for each case.Drag reduction is due to the reduction of the turbulent exchange of momentum near the wall. This reduction is accompanied by a comparable reduction in the turbulent transfer of heat and mass. When the relative transfer by the vertical turbulent fluctuations in the wall region is reduced, features that characterize heat transfer in laminar flows are observed. It is well known that in laminar flows it is necessary to distinguish between the two modes of heat transfer, that the effect of the temperature dependent fluid properties is larger than in turbulent flows, and that the entrance region is longer, and that its length is Reynolds number dependent. Although the proposed model does not explain the physical mechanisms of the phenomenon of drag reduction, it provides a better understanding of the observed features, and a valuable tool for analyzing and predicting transport processes in flows with drag reduction.