An improved estimation of shear rate for yield stress fluids using rotating concentric cylinder Fann viscometer

•Improved shear rate estimation for yield stress fluids using Fann viscometer.•Rheology of drilling fluid additives.•Consistency plots.•Parametric sensitivity.

This paper deals with the estimation of wall shear rates for yield stress fluids using rotating coaxial-cylinder viscometer readings. The estimation is based on the generalized difference equation for rotating narrow gap coaxial cylinder Fann Viscometer under purely steady, laminar and isothermal tangential fluid flow condition. Rotor rotations and bob deflections are the two important viscometer readings and these are used to predict wall shear rates. The proposed equations are used to calculate shear rates for several Herschel–Bulkley type fluids involving polymer (for example, xanthan gum, poly-anionic cellulose and carbomethoxy cellulose) solutions with varying concentration and bentonite suspension, which are frequently used as drilling fluid additives. Parametric sensitivity is made for stability analysis of the proposed shear rate equations. Finally, the predicted shear rates are compared with the conventional method of estimating the rate of shear for drilling fluids.