Experimental Investigation Onturbulent Drag-Reducing Flows With Injection Of Surfactant Additives In The Boundary Layer

The skin friction drag can consume large amount of power when fluids flows over a surface. In order to save the energy, therefore the usage of drag-reducing techniques for wall turbulence in industrial application is required and important. In 1948 Toms found the addition of small amount of flexible long-chain polymers to the turbulent flow can significantly reduce the skin friction drag, which is known as Toms effect or turbulent drag-reducing effect. The technique has been applied to the practical applications owing to the drag reduction characteristics. In most cases, the drag-reducing effect is caused by homogenous solution with drag reducing additives mixed uniformly. In consideration of the economy and conditions of the practical applications, some scholars proposed that the introduction of drag-reducing additives can be added by injection, which can reduce the drag with the process of mixing. Therefore, an experimental system for this target is designed, and the paper aims at understanding of the characteristics of the turbulent drag-reducing flow with the surfactant injected from the wall.. The measurements of rheological characteristics, the velocity field and concentration field were measured in the channel flow, respectively. This paper use Cetyltrimethyl chloride(CTAC) as the drag reducer.Firstly, we measured the rheological characteristics of CTAC/NaSal solution by rotary rheometer. Under the condition of small shear rate the viscosity of CTAC/NaSal solution at high concentrations is much higher than that of water. Also the viscosity increases when the concentration of solution becomes large. The shear-thinning phenomenon in the profiles of viscosity of surfactant solution is also observed.Secondly, by planar laser-induced fluorescence(PLIF)we measured the concentration field in x-y plane in a drag-reduced flow with CTAC/NaSal solution injected from the porous slot. The results show that the diffusion of surfactant solution is slower than that of water by both means of qualitative and quantitative analysis. A surfactant concentration boundary layer is formed in the channel, which thickenes downstream along the channel. at Besides, colored water solutions distribute more evenly than colored surfactant solution.Then by particle imaging velocimetry(PIV)we measure the velocity field in x-y plane in a drag-reduced flow with CTAC/NaSal solution injected from the porous slot. It is found that the turbulent statistics depends on the normalization due to outer variables. For example, the Reynolds shear stress and wall-normal turbulent intensities are significantly suppressed in the drag-reduced flow, while the mean velocity profile and streamwise turbulent intensities are similar to that of ordinary turbulent. Also the eddy structures are different from those in the Newtonian fluid flow.Finally, we study the drag-reducing effect at different concentration, different Reynolds number, different dosing rate of surfactant solution and different position away from the slot. It is found that the drag-reducing effect firstly increases and then decreases with the increase of the solution concentration under the same condition; drag-reducing effect decreases with increase of the Reynolds number; drag-reducing effect improves with the same dosing rate of surfactant solution under the same concentration and Reynolds number. Except that, drag-reducing effect is found to be enhanced at downstream position during the process of development.