Research On The Effects Of Transient Shear Flow On Rheological Measurements In The Start-up Stage

On the basis of transient tests and theoretical analyses of the initial unsteady stage in the coaxial cylinder rheometer,the systematic research of the effects of transient shear flow on rheological measurements during the start-up stage has been conducted carefully.It aims to provide a certain basis for more accurate understandings of material rheology during this transient start-up stage.The paper mainly includes: the experimental studies of the transient start-up stage,the theoretical studies of the transient start-up stage in the coaxial cylinder rheometer,and the correction studies based on the “Experimental Correction Method” during transient start-up stage.The initial transient rheological processes of Newtonian fluids were studied under different loading conditions in cylinder rheometer.The distortion phenomena of test results during the initial start-up process were verified among a relatively wide range of viscosity,shear rate and shear stress.The smaller the viscosity of Newtonian fluid is,the longer the transient stage lasts;with same loading condition and sample,the measuring system with larger rotating inertia has shorter transient stage;with same measuring system and sample,the transient stage under Controlled Shear Rate(CR)mode is shorter than that under Controlled Stress(CS)mode.In view of the initial experimental laws of Newtonian fluids,the analytical and numerical studies of the transient stage of Newtonian fluid within the simplified coaxial cylinder rheometer model were studied under different loading conditions.By comparing the numerical and experimental results under same sample and testing condition,the actual boundary condition at the rotating fixture bounder can be determined as satisfying the torque balance relationship.The total torque of rotational rheometer is applied to accelerate the rotating system,overcome air friction loss of motor,and shearing the sample.Current rheometer processing system can only determine the output torque of motor by detecting the output current,and regards the corresponding “shear stress” of “the output torque” as “shear stress applied on sample”,which fails to take the torque loss caused by system inertia into consideration.Thus,a kind of experimental correction method based on current rheological measurement is put forward to correct the fuzzy “shear stress”.Using the proposed “Correction Method of Rheological Parameter”,the transient correction studies were conducted on the experiment results of Newtonian fluids,Power law fluid and viscoelastic thixotropic fluid under Anton Paar MCR 102.The corrected rheological curves of Newtonian fluids and power law fluid all represent no relationship with loading conditions and testing time,which meets the laws under steady shearing.As to the crude oil,it all has been yield during the transient process under CR or CS loading conditions.It is the cumulative effects of shearing that finally leads to material yield not a particular shearing stress/rate.The corrected initial rheological curve shifts toward “down and left”,which has smaller yield stress and yield time.When the measuring temperature is nearing the pour point temperature,the corrected rheological curve can represent more characteristics of Newtonian fluid than without correction.