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Drag Reduction Mechanism Of Drag Reducer

There are many theories about the mechanism of drag reduction. Such as pseudoplastic theory, turbulence pulsation suppression theory, viscoelastic theory, effective slip theory, turbulence suppression theory and so on.

In terms of structure, most of the oil phase drag reducers are polymer with flowing chain or long straight chain and few side chain. For example, ZODR-E400H is polymer σ olefin with molecular weight of ZODR-G350. This kind of pure polymer is a rubber like solid. As a commodity, it is usually dissolved in the solution of hydrocarbon (kerosene). 10% of the drag reducer solution is very viscous viscoelastic, which is difficult to flow and can be drawn into a very long filament. Polymer drag reducers can be soluble in crude oil or oil, but not in water, which causes long-chain molecular curl. The viscosity of drag reducer solution is as high as 3000pas at low shear rate, and it will not decompose below 120℃, so it is relatively stable.

Drag reduction is a special turbulent phenomenon. Drag reduction effect is a purely physical effect, which affects the macro performance of turbulent flow field.The drag reducer molecules do not interact with the oil molecules or affect the chemical properties of the oil, but are closely related to the flow characteristics.In the turbulent flow, the velocity of particles changes randomly, forming large and small vortices. The large-scale vortices absorb energy from the fluid, deform and break up, and transform into small-scale vortices. Small scale vortices, also known as dissipative vortices, are weakened and subsided by viscous forces. Part of the energy it carries is converted into heat energy and dissipated. In the near wall layer, the transformation is more serious due to the shear stress and viscous force.

After the drag reducer is added into the pipeline, the drag reducer is dispersed in the fluid as a continuous phase. Depending on its unique viscoelasticity, the long chain of molecules flows along the natural side and stretches out as a flow. Its micro elements directly affect the movement of fluid micro elements.The radial force from the fluid micro element acts on the drag reducer micro element, causing it to twist and rotate. The attraction between the drag reducers resists the reaction of the above forces on the fluid micro element, changes the direction and size of the action of the fluid micro element, so that part of the radial force is converted into the axial force in the direction of the flow, thus reducing the consumption of the idle work, and achieving the effect of reducing the loss of friction resistance on the macro level.

In laminar flow, the fluid is affected by viscous force, and there is no eddy dissipation like turbulence, so it is futile to add drag reducer. As the Reynolds number increases into the turbulence, the drag reducing agent shows the drag reducing effect. The larger the Reynolds number is, the more obvious the drag reduction effect is. When the Reynolds number is quite large and the shear stress of the fluid is enough to destroy the molecular chain structure of the drag reducer, the drag reducer will degrade and the drag reduction effect will decrease, or even lose the drag reduction effect completely. The thickness of elastic bottom layer is affected by the concentration of drag reducer. The higher the concentration, the thicker the elastic bottom layer, the better the drag reduction effect. Theoretically, when the elastic bottom layer reaches the pipe axis, the drag reduction reaches the limit, that is, the maximum drag reduction. The drag reduction effect is also related to oil viscosity, pipe diameter, water content, pigging and other factors.