3. Action Principle of Crude Oil Drag Reducer

The task of drag reduction mechanism research is to explain many characteristics of drag reduction from mechanism, such as pipe diameter effect, concentration effect, turbulence velocity distribution change, polymer characteristics, so as to grasp the law of drag reduction essentially. At present, there are many theories about drag reduction mechanism, such as Toms pseudoplastic hypothesis, Virk's effective slip hypothesis, viscoelastic hypothesis, turbulence suppression theory and so on.

(1) Toms' pseudoplastic hypothesis

After Toms found the drag reduction phenomenon in 1948, he put forward the hypothesis of drag reduction mechanism. He thinks that the polymer drag reducer solution has pseudoplasticity, that is, the shear rate is inversely proportional to the apparent viscosity, and the increase of shear rate leads to the decrease of the apparent viscosity, which leads to the decrease of the resistance. However, with the development of non-Newtonian fluid mechanics, the Toms hypothesis has been gradually denied. As long as simple experiments are carried out, it can be found that the measured value of friction resistance of drag reducer solution in turbulent flow has a great error with the calculation of pseudoplastic fluid, and the pseudoplasticity of dilute drag reducer solution is very weak, even no pseudoplasticity at all. Its rheology is almost the same as Newtonian fluid, but the drag reduction rate is large. Walsh's test shows that the expanded plastic fluid also has a strong drag reduction effect.

(2) Virk's effective slip hypothesis

According to Virk, when the fluid is turbulent in the pipe, the layer close to the wall is viscous bottom layer, followed by elastic layer, and the center is turbulent core. He measured the velocity distribution through experiments, and found that the velocity in the turbulent core area of the drag reducer solution was a certain value larger than that of the pure solvent, but the velocity distribution was the same, and the velocity gradient of the elastic layer increased, resulting in the decrease of the resistance.

According to Virk's hypothesis, when the concentration of drag reducer increases, the thickness of elastic layer also increases. When the elastic layer extends to the tube axis, the drag reduction reaches the limit. This hypothesis can explain the maximum drag reduction and the pipe diameter effect. The chemical agent that can reduce the resistance of crude oil pipeline is called crude oil drag reducer.

(3) Viscoelasticity hypothesis

The viscoelasticity hypothesis suggests that the drag reduction of polymer solution is the result of the interaction between viscoelasticity and turbulent vortex. Many researchers have carried out time experiments on specific polymer drag reducer dilute solution, and found that the relaxation time of polymer molecules is longer than the duration of turbulent vortex, indicating that the elasticity of polymer molecules does play a role. Therefore, part of the kinetic energy of the turbulent vortex is absorbed by polymer molecules and stored in the form of elastic properties, which reduces the kinetic energy of the vortex and achieves the effect of drag reduction.

(4) Turbulence suppression hypothesis

According to the turbulence retention inhibition hypothesis, after the drag reducer is added to the pipeline, the drag reducer will stretch naturally along the long chain of the molecule due to its viscoelasticity, and its micro elements will directly affect the movement of the fluid micro elements. The radial force from the fluid micro element on the drag reducer micro element causes it to twist and rotate. The molecular gravity of drag reducer resists the reaction of the above forces on the fluid micro element, changes the magnitude and direction of the force of the fluid micro element, changes part of the radial force into the axial force along the flow direction, thus reducing the consumption of idle work, and plays the role of reducing the friction loss on the macro level. That is to say, polymer molecules inhibit the generation of turbulent vortices, thus reducing the intensity of pulsation and ultimately the energy loss.

Based on the above hypotheses, it can be concluded that the drag reducer added to the oil flow depends on its unique viscoelasticity, and the macromolecular chain naturally stretches along the flow direction, which will affect the movement of the fluid particle. The radial force of the fluid particle causes the drag reducer molecules to twist and rotate. The drag reducer molecules rely on the interaction between molecules to resist the force of the fluid particles, change the direction and size of the action of the fluid particles, so that a part of the radial force for reactive work is converted into the axial force in the direction of the flow, thus reducing the consumption of reactive work, and the macroscopic performance reduces the friction loss of the fluid, that is to say, it plays the role of drag reduction.