Meaning
Fluid mechanics calculations must account for complex behaviors when the viscosity of a medium varies with the applied rate of shear. Characterizing this behavior as non-newtonian rheology allows chemical engineers to design piping systems for slurries, polymers and emulsions that do not exhibit a constant flow resistance. This discipline is essential for industries ranging from food processing to oil drilling where complex fluids are pumped under high pressure.
It stops being a useful framework when the shear rates are so low that the fluid acts as a simple linear-elastic solid.
Viscosity Variation
Shear thinning and shear thickening represent the two primary responses of these complex fluids to mechanical agitation. Dilatant fluids become more resistant to flow as force increases, while pseudoplastic materials flow more easily under stress. This behavior is critical for pump selection and pipe sizing.
Process Design
Slurry transport lines require careful velocity control to prevent the settling of suspended solids without causing excessive pressure drop. Viscosity drops at higher flow velocities can lead to turbulent flow transitions that alter heat transfer rates. Engineers must balance these effects to optimize system throughput.
Experimental Analysis
Capillary and rotational rheometers are utilized to measure the shear-stress response across a wide range of shear rates. These laboratory tests generate flow curves that are used to fit mathematical models like the power-law equation. Accurate curves prevent startup failures in industrial mixing vessels.
