Baffled cup and end-effects of a vane-in-a-large cup rheometer for Newtonian fluids

Abstract

A four-blade vane in a large cup was tested for Newtonian fluids using a controlled stress rheometer. The end-effects of a hypothetical cylinder were evaluated as a function of vane distance from the base of the container. A hypothetical cylinder radius and an effective height of the vane that includes the contribution of the “end-effect” were estimated. Also, the effect of a non-baffled and a baffled cup was investigated. Average values of shear stress and shear rate were calculated assuming that the shear conditions are similar to those existing in an analogous concentric cylinder. Furthermore, experimental angular velocity as a function of torque was correlated when using the power and Reynolds dimensionless numbers. Flow characterization with a vane-in-a-large cup is possible if laminar flow is verified. End-effects are minimized when vane is placed at the surface of fluid and the distance between the bottom of the vane and the base of the container is higher than a half of the vane height (for vane heights equal or higher than the vane diameter). Baffled did not show an effect on the laminar flow contrary to the large effect observed in the turbulent flow. For shear factors calculations of a vane-in-a-large cup, Couette analogy must only be applied using the finite cup radius equations, and corrected for non-Newtonian flow when necessary.