The influence of non‐Newtonian flow on effective viscosity and channel efficiency in screw pumps

A method is given for selecting the “effective viscosity” for isothermal flow of non‐Newtonian liquids in screw pumps or melt extruders. Effective viscosity is the Newtonian viscosity that would give the same screw‐pump performance with non‐Newtonian liquids at the same flow rate and speed. When effective viscosity is known, it can be inserted in performance equations for simple Newtonian flow. The analysis is restricted to shallow screw‐pump channels with large aspect ratios and to shear stress/shear rate curves with modest curvatures when shown in a double logarithmic plot. The shear stress/shear rate curve is replaced by a power‐law tangent to that curve in the domain of prevailing shear rates, but the analysis could be extended to more complex behavior. Curves are also included for calculating the efficiency of the screw‐pump channel, which can be used to estimate the energy dissipated in screw‐pumps. It is shown that efficiency decreases with decreasing power‐law exponent.