Simultaneous measurements of velocity and temperature fluctuations in thermal boundary layer in a drag-reducing surfactant solution flow

Abstract

The mechanism of turbulent heat transfer in the thermal boundary layer developing in the channel flow of a drag-reducing surfactant solution was studied experimentally. A two-component laser Doppler velocimetry and a fine-wire thermocouple probe were used to measure the velocity and temperature fluctuations simultaneously. Two layers of thermal field were found: a high heat resistance layer with a high temperature gradient, and a layer with a small or even zero temperature gradient. The peak value of \( \overline{{u^{ + } \theta ^{ + } }} \) was larger for the flow with the drag-reducing additives than for the Newtonian flow, and the peak location was away from the wall. The profile of \( - \overline{{v^{ + } \theta ^{ + } }} \) was depressed in a similar manner to the depression of the profile of \( - \overline{{u^{ + } v^{ + } }} \) in the flow of the surfactant solution, i.e., decorrelation between v and θ compared with decorrelation between u and v. The depression of the Reynolds shear stress resulted in drag reduction; similarly, it was conjectured that the heat transfer reduction is due to the decrease in the turbulent heat flux in the wall-normal direction for a flow with drag-reducing surfactant additives.