Abstract
The objective of this study is to clarify the flow characteristics of drag-reducing flow and to elucidate the mechanism underlying this phenomenon. The surfactant and counter ion we used were Lipothoquad O/12 and sodium salicylate, respectively. The drag reduction rate (DR%) was measured by using a recirculating system with a diameter of 25.6 mm. We also measured the flow characteristics of the surfactant solutions with different concentrations and temperatures by using particle image velocimetry (PIV). From the experimental results, DR% at an average velocity of 2.0 m/s increased from 0 to 68% as the temperature increased from 10 to 40 °C at a constant concentration of surfactant (300 mg/L). From the velocity contour plot obtained from PIV, we found that the thickness of the lower-velocity region of the drag-reducing flow near the pipe wall was thick at 20 °C, whereas vortex motions seemed controlled at 30 °C. On the other hand, the lower-velocity region thickened as the concentration of the surfactant increased at 25°C. Even if the same level of drag-reducing effects occurred, the flow patterns were quite different depending on the concentration and temperature.
Highlights
It is well known that certain cationic surfactant solutions with counter ions significantly reduce drag in turbulent flow
In comparison with the coherent structure of normal turbulence, the vortex motion of drag-reducing flow can be restrained by the rheological properties of surfactant solutions, greatly changing flow patterns as a result
Negative values of DR% occurred sometimes in the lower flow rate range, which caused a higher viscosity of a surfactant solution than that of water
Summary
It is well known that certain cationic surfactant solutions with counter ions significantly reduce drag in turbulent flow. Since the cationic surfactant oleylbishydroxyethyl-methyl-ammonium chloride (brand name: Lipothoquad O/12) shows a sufficient and stable drag-reducing effect, it has been adopted at more than 200 sites in air-conditioning systems throughout Japan, including office buildings, hotels, hospitals, supermarkets, airport facilities, and industrial factories (Saeki and Tokuhara, 2014). In comparison with the coherent structure of normal turbulence, the vortex motion of drag-reducing flow can be restrained by the rheological properties of surfactant solutions, greatly changing flow patterns as a result. The effects of the concentration of the surfactant and the solution temperature on the drag-reducing effect are not fully understood in connection with the coherent structure of the flow. PIVs of surfactant systems were measured with varying concentrations and solution temperatures. The objective of our study was to clarify the flow characteristics of drag-reducing flow and to elucidate the mechanism underlying this phenomenon
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