Study On The Stability Of Constitutive Relation For Waxy Oils

Waxy oils show complex non-Newtonian fluid characteristics,owning to the structure formed by precipitated wax crystals at low temperature.Most of current studies are based on traditional rheology experiments and many models are proposed to characterize the apparent rheological behavior.Few studies involve possible constitutive instability of waxy crude oil.In order to ascertain whether waxy oil has constitutive instability,experiments with loading of linear increasing shear rate,constant stress and step shear rate are performed,using the stress-controlled rheometer.The experimental results are that waxy oils show constitutive instability behavior at low temperature,such as hysteresis loop,viscosity bifurcation and non-monotonic equilibrium flow curve,respectively.The effect of shear history on the equilibrium state is studied,making equilibrium flow curve capable to be applied to the description and establishment of model.The results show that the equilibrium flow curve is determined by the maximum shear rate,which is directive for steady state experiment.Then,the non-monotonic equilibrium flow curve of waxy oil is obtained which is a typical constitutive instability behavior and the effect of temperature and maximum shear rate on it is analyzed.In order to explore the method for quantitative description of constitutive instability,the fitting effect of Houska model and that of Coussot model are compared based on the data of equilibrium flow curves.According to the average relative deviation,the Houska model is better.Then it is used for the numerical simulation of the velocity distribution of non-Newtonian fluid under steady laminar flow and the effect of constitutive instability on the velocity distribution in pipeline is analyzed.The results show that the influence of the instability on the velocity distribution is related to the characteristics of the fluid itself,the initial structure state,the pressure and diameter of the pipeline.The stronger the instability of the fluid,the stronger the initial structure,the smaller pressure of the pipeline and the larger diameter of the pipe,the greater the influence of the constitutive instability have on the velocity distribution.In conclusion,the constitutive instability behavior of waxy oil at low temperature is explored in this paper.The influence of shear history on equilibrium state is illustrated and the factors of constitutive instability affecting steady flow are analyzed.