Engineering context

Non-Newtonian fluids have a viscosity that changes with shear rate rather than staying constant, so their pressure drop and pump performance can differ markedly from a Newtonian liquid. They are modelled with the fluid defined in the database as a non-Newtonian liquid and its viscosity described by an appropriate rheology model.

A non-Newtonian fluid has no single viscosity — it depends on shear rate — so pressure drop and pump duty cannot be predicted from one Newtonian viscosity value. Choosing the rheology model that matches the fluid’s measured behaviour is what makes the results meaningful.

Engineering workflow

1. Confirm the fluid is non-Newtonian. Rheology test data showing shear-dependent behaviour confirms a non-Newtonian fluid. For a slurry without test data, solids made up mostly of particles 75 microns and smaller indicate a non-settling slurry with non-Newtonian characteristics.
2. Define the fluid in the database as a non-Newtonian liquid — define its properties in FluidFlow’s fluid database.
3. Define the viscosity using the most appropriate rheology model — Power law, Bingham plastic, Herschel-Bulkley, or Casson.
4. Define the boundary conditions — add the boundaries and select the non-Newtonian fluid from the fluid database, which carries its properties. Choose the pressure model correctly — stagnation for vessels, static for pipes.
5. Build and connect the network — add pipes, fittings, elevations, and equipment, including any centrifugal pumps.
6. Model the system as a liquid system with centrifugal pump non-Newtonian derating — enable the option to derate centrifugal pump performance for non-Newtonian viscosity effects.
7. Solve, then analyse as a typical liquid system — review pipe velocity, capacity, and NPSH margin.
8. Analyse pump performance — check pumps for the drop in performance due to non-Newtonian viscosity effects.

Why the full system matters

Because viscosity changes with shear rate, pressure drop in a non-Newtonian system changes with flow velocity in a way that a Newtonian model cannot reproduce. The apparent viscosity may be much higher or lower than any single measured value, depending on the shear rate in the pipe. Pump performance is also derated by non-Newtonian viscosity effects, so both the pressure drop and the pump duty point must be evaluated in the connected model.

How FluidFlow helps

FluidFlow models non-Newtonian fluids within its steady-state pipe network solver: the fluid is defined in the fluid database as a non-Newtonian liquid with a rheology model (Power law, Bingham plastic, Herschel-Bulkley, or Casson), and centrifugal pumps are derated for non-Newtonian viscosity effects. It reports velocity, capacity, pressure drop, and NPSH across the connected network.

Go deeper