Nano-Fluid: Frictional Force of a Fluid Flow Mixed with Particles

Particle-Laden Flow in a Micro-Bearing (DPM) — ANSYS Fluent CFD Simulation Training

Description

In certain industries — such as Microelectromechanical Systems (MEMS) and microfiltration — micro-channels and micro-bearings with flow thicknesses on the micrometer scale can carry particles on the nanometer scale. Under these conditions, the effect of the carried particles on the frictional force acting on the channel or bearing wall becomes very important. This project uses ANSYS Fluent and the Discrete Phase Model (DPM) to investigate how this wall frictional force differs with and without the presence of particles. The particles are spherical anthracite grains 400 nm in diameter, and the two-way interaction between the particles and the fluid is taken into account.

The geometry consists of two concentric cylinders with diameters of 100 and 70 micrometers, drawn in ANSYS SpaceClaim. The outer cylinder rotates while the inner cylinder remains stationary. The mesh was generated in ANSYS Meshing and comprises 51,840 hexahedral elements — a density that resolves the flow dynamics, turbulence effects, and particle distribution within the domain. The element size is kept larger than the particle size to ensure proper particle tracking.

Methodology

A pressure-based, transient solver is used to track the particle injection over time and capture the interaction between the particles and the fluid. The fluid filling the gap between the cylinders is Polyalphaolefin (PAO) 68. The SST k-omega turbulence model is adopted for its effectiveness in capturing the complex flow that develops around the rotating cylinder, and a suitable wall function resolves the near-wall region — essential for accurately representing the fluid–particle interactions near the walls and the resulting frictional force. Appropriate particle-dynamics forces and models are also included to improve the accuracy of the DPM settings.

Results & Conclusion

The simulation yields the following results:

• The frictional force on the inner cylinder wall increases over time, while the frictional force on the outer cylinder wall decreases over time.

• In both the with-particle and without-particle cases, the frictional force on the outer cylinder wall exceeds that on the inner cylinder wall. This is likely because the outer cylinder is the rotating one, and the frictional force is proportional to the velocity gradient.

• The wall frictional force is higher when particles are present, because the particles raise the effective viscosity of the medium and thereby increase the friction.

• The plots show that, for the inner cylinder, the problem reaches steady state after 27 ms without particles and after 34 ms with particles. For the outer cylinder, steady state is reached after 11 ms without particles and after 20 ms with particles. Overall, then, the case without particles reaches steady state after 27 ms, while the case with particles reaches steady state after 34 ms.