Experimental And Calculated Study On Drag-reducing Mechanism Heat Transfer Characteristics Of Surfactant Solution

A small amount of surfactant of Cetyltrimethyl Ammonium Chloride (CTAC) and NaSal dramatically depresses the turbulent friction, which can apply in circle system to reduce pumping power application. The research on the addition of drag-reducing additives to the circulating water of distinct heating and cooling system is becoming promising work in recent years, however, the mechanism of surfactant drag-reducing flow is not clear and the research on its heat transfer characteristics is not much. In order to study the drag-reducing mechanism and heat transfer characteristics of surfactant solution flows, the detailed experimental and calculated projects are implemented of CTAC drag-reducing flow.Present research mainly consists of following sections: the experiments of drag-reducing performances for CTAC fluid flow; the experiments of the turbulent structure and the turbulent kinetic energy structure for CTAC drag-reducing flow and water; the experiments of rheological characteristics of distilled surfactant solutions; the calculated study by using shear-thinning model (Carreau-Bird model) for surfactant solution flows; the direct numerical simulation (DNS) using Giesekus viscoelastic model for surfactant solution flows; the experiments of heat transfer characteristics of surfactant fluid flow.The experimental study of surfactant solution performances show that the first critical Reynolds number is increased as the concentration. There is optimal concentration of surfactant solution in completely drag-reducing region.The changes in turbulence structure and turbulent kinetic structure of surfactants have been obtained, such as a thickening of the buffer layer, corresponding to an offset of the logarithmic region. The peak of the streamwise turbulence intensity (or rms of the streamwise velocity fluctuations) is substantially larger for the drag-reducing flow than for water, while the position is shifted wall-outward. The turbulent shear stress profiles for the drag-reducing flow are decreased with respect to water, and show a Reynolds stress deficit. This means that the surfactants have a significant contribution to the shear stress. The production of turbulent kinetic energy of the drag-reducing flow is depressed totally. The peak position of the production of turbulent kinetic energy is shifted wall-outward.The shear viscosities of turbulent drag-reducing surfactant solution have been measured as a function of concentration, shear rate and temperature by using AR-G2 rheometer. The conclusions of the appearance critical concentration with SIS in surfactant solutions and the SIS is not the necessary condition for drag-reducing fluid have been proposed for the first time. There is the optimal concentration of surfactant solution in the application of heating and cooling system.One of the key properties for drag reduction by surfactant additives is shear-thinning. The simulation with the viscous shear-thinning Carreau-Bird model has been shown, that the shear-thinning has effects on the drag reduction except for lower drag reduction than that of the experiments. This is clear up the question that turbulent intensities of surfactant solution with higher concentration are all depressed, however, the turbulent intensities of surfactant solution with lowerer concentration are changed.Another key property for drag reduction by surfactant additives is viscoelasticity. A direct numerical simulation using viscoelastic Giesekus model shows that the closely drag reduction to that experimental study, which further conclude that viscoelasticity has a mainly effects on the surfactant solution flow. The instantaneous fields of velocity fluctuation of surfactant solution flow show that the vortex structure becomes elongated in the streamwise direction,especially in the region near the wall. The low speed streaks become more elongated and the average spacing of the streaks becomes wider. The turbulent kinetic energy of CTAC drag-reducing fluid flow has been changed completely compared to that of water.Based on the experimental and calculated analysis, the model of drag-reducing mechanism has been established.The heat transfer performances experiments show that the surfactant solution need more heat flux compared to water for heating the same fluid. The experiments for measuring temperature field show that the larger mean temperature gradient and smaller thermal diffusivity of the surfactant solution flows in the buffer layer while the main heat resistance of the water exits in the viscous sublayer. The frequency of temperature fluctuation is small and the characteristics of the temperature vary with the variation of distance from the wall for surfactant solution flows. The mean temperature fluctuations and velocity fluctuations have the similar tendency and the peak values are out of the wall. Temperature fluctuation governs the streamwise turbulent heat flux more than the velocity fluctuation does and the peak value is located close to the peak of the temperature fluctuation. The decorrelation between streamwise velocity fluctuation and wall-normal velocity fluctuation shows the depression of Reynolds shear stress and directly results in DR. Similarly, the decorrelation between temperature fluctuation and wall-normal velocity fluctuation shows the depression of wall-normal turbulent heat flux and directly results in HTR.