Study On Unsteady Shear Flow Characteristics Of Gelled Crude Oil

Rheological testing is the main method to obtain the unsteady shear mechanical response characteristics of gelled crude oil.When rheometer is started or stopped instantaneously,there is an unsteady flow in the test system gap,which affects the rheological test results.Based on transient theory analysis of viscoelastic fluids and creep recovery oscillation test of gelled crude oil,this paper systematically studies the transient shear flow characteristics of viscoelastic fluids.The main research contents and results are as follows:Considering the torque loss during transient rotation of the test system,combining the real torque balance relationship at the boundary of the rotor and the viscoelastic fluid constitutive equation,a viscoelastic fluid transient flow model is established.Compared with the analytical solution and numerical solution of the unsteady flow of viscoelastic fluids,the result indicates that the calculation results of the two methods are exactly the same when the measurement gap is small.Comparing the difference between the steady flow and the unsteady flow of viscoelastic fluids,it is found that viscoelastic fluids exhibit periodic oscillations with gradually decreasing amplitude during the unsteady shear flow.The coupling of the rotational inertia of the test system with the viscoelasticity of the fluid is the essential cause of oscillation.Analyze the effects of fluid viscoelasticity and moment of inertia on transient testing.For Jeffery fluid,elasticity modulus G mainly affects oscillation period and amplitude,parallel dashpot ?₁ mainly affects amplitude and decay speed,and series dashpot ?₂ mainly influences slope of the strain curve.The greater the moment of inertia,the more pronounced the oscillations that occur during the unsteady state process.The transient viscoelastic response characteristics of gelled crude oil were studied.The creep recovery oscillation tests of gelled crude oil under different test conditions were performed using four different test systems to explore the damping oscillation law of gelled crude oil.At the beginning of creep,the strain response changes as a sine function with gradually decreasing amplitude.At the beginning of recover,the strain response changes as a cosine function with gradually decreasing amplitude.The viscoelasticity of gelled crude oil is different under various test conditions.Viscoelasticity has an effect on the oscillation curve.The stronger the viscoelasticity,the weaker the oscillations exhibited.Transient strain response of gelled crude oil under different test systems is related to the structural parameter.The larger the value of a,the more pronounced the oscillation.Taking into account the mechanical response characteristics of rheometer,the Jeffery start-up flow model considering the short-term correction was obtained.The fitting results show that the model can accurately represent the strain-time curve of gelled crude oil during the initial unsteady flow stage.