Rheological Model of the Full Flow Curve of Highly Viscous and Solidifying Oil at Starting and Steady-State Mode Conditions

The article is devoted to the need of taking into account the thixotropic properties of oils when oil pipeline starts after a long stop, as well as to calculation of the time of its safe stop. To evaluate the parameters of thixotropy, it is proposed to use the results of laboratory experiments of oil samples on a rotary viscometer. Typical properties of the rheological flow curves of thixotropic oils for the starting flow mode are determined. The use of the wave equation of damped vibrations is proposed to describe the rheological flow curve of the forward course of measurements of a rotational viscometer, characterized by the presence of ascents and descents of shear stresses. This equation, supplemented by a model of nonlinear-visco-plastic liquids, allows accurately finding the areas of appearance of thixotropic properties of oils. The article presents the results of experimental studies of the manifestation of thixotropic properties of two oil samples at a lower temperature. The wave equation of damped vibrations allows describing mathematically the ascents and descents of shear stresses observed at low oil temperatures. Experiments have shown that the wave properties of thixotropic oil do not change with a decrease in temperature except for the amplitude of maximum vibrations and are invariant properties of the studied liquid. The presented explanation of the physical mechanism for the manifestation of wave properties based on the hardening of the supramolecular paraffin crystal lattice characteristic of thixotropic oils explains the invariance of the frequency change function and the attenuation coefficient during starting processes at the beginning of the movement of the movable cylinder of a rotary viscometer. The result of laboratory studies allows determining the nature of changes in the contours of the zones of manifestation of thixotropic oil properties as a dependence on the temperature and shear rate.