Research On Characteristics Of Unsteady Pipe Flow And Relevant Deviation Rules Of Pipe Flow Measurement

The rheological parameters of gelled crude oil are important parameters for the calculation of pipeline restart process.The looping device in laboratory is one of the main methods to measure rheological characteristics of fluids and its theoretical basis is Poiseuille flow.During the initial unsteady process of Poiseuille flow,the radial velocity distribution in pipe changes with time and thus causing the differences bwtween rhelogical parameters obtained from measurement principle and actual ones.To realize the flow characteristics of unsteady process and deviation rules of measurement caused by it,this paper carried out related research and main content includes building numerical models and analyzing influences of different boundary conditions and physical parameters on unsteady process.Researches on unsteady Poiseuille flow process of Newtonian fluid under different boundary conditions were carried out.Considering the unsteady velocity distribution within pipe,the radial space within pipe was divided into cells.Momentum equation and constitutive equation of fluid are used to build numerical model.Dimensionless viscosity and dimensionless time were used to represent unsteady process and actual measured viscosity and dimensionless viscosity were obtained based on viscosity measurement principle.Result shows that unsteady process will cause measured viscosity larger than actual viscosity of fluid.Dimensionless viscosity will decrease with the increasement of time and finally reaches 1.Based on analysis of dimensionless viscosity curves,dimensional time when dimensionless viscosity reaches 1 was certain under different boundary conditions.When types of boundary conditions were decided,value of dimensionless viscosity was only relevant with dimensionless time.For different types of boundary conditions,dimensionless time for fluid to reach steady under constant average velocity conditions were always shorter than that under velocity increasing linearly from 0 conditions and constant pressure conditions.Unsteady Poiseuille flow model of Cross thixotropic fuild was built and influence of fluid physical parameters and boundary conditions on viscosity measurement deviation was analyzed.It was found out that during the initial unsteady Poiseuille flow process of Cross thixotropic fluid,there is little difference between structure parameters everywhere and unsteady flow process is similar with that of Newtonian fuild.Based on calculation results,influence of fluid physical parameters on unsteady flow process were obtained through comparison and analysis.Boundary conditions have little influence on relative deviation,structure cracking and recovering parameters of fluid will influence viscosity relative deviation in rapid cracking process while pipe diameter and initial viscosity have large influence during the whole process.Besides,results under different boundary conditions showed that time for initial process to reach homogeneous flow under constant pressure is longer and relative deviation is larger.During obvious cracking process of fluid,deviation of measured viscosity is also larger than that of constant average velocity boundary.Thus,as for viscocity measurement,constant average velocity boundary condition is preferred.Researches on unsteady Poiseuille flow process of thixotropic fuild whose shear stress is power with shear rate were carried out and influence of different shear rate equivalent methods on the relative deviation of consistence measurement.It was found out that for the thixotropic fluids power with shear rate,measured consistence obtained when shear rate was equivalent as 8V/D 3n+1/4n is more close to actual consistence under ideal homogeneous flow than equivalent as 8V/D.For different rheological behavior index(0<n<1),as n gets larger,relative deviation of measured consistence obtained from these two ways is smaller and vice versa.So,for those fuilds with small rheological behavior index n,influence of n must be considered during the calculation of equivalent shear rate to reduce measure deviation.