Dust, small particles, fumes, mist and/or fibers can be harmful for many production processes. Their path and destination are often unpredictable and difficult to determine with experiments. Luckily, we have simulation, and with a smart set-up using the right assumptions you can get a grip on it!
Defining the physics
We must ensure the correct physics is included in the simulations, so some hand calculations are required.
The Stokes number helps us to find how much the particle moves with the flow, or if its path is mostly determined by its own inertial forces. The figure below shows two simulations of a saw on a plate, by which particles of different sizes are created. The small particles (left) have low Stokes numbers and therefore move with the flow to the extraction tube. A 'simpler' tracer simulation, ignoring the particle inertia, might have been sufficient. The larger particles have a totally different experience; their inertial forces are too large and make them bump into the extractor walls. The simulation shows in what range the current extractor design is effective.
The Péclet number tells us if we should include diffusive transport of the particles in the simulation. Or maybe the whole problem is diffusion dominant, and we don’t even need to consider the flow field. This can save you a lot of computational effort!
Even electromagnetic forces, concentration gradients (molecular contamination) or temperature gradients (via thermophoretic forces) can be important for your contamination problem and need to be considered before setting up your simulation.
Defining the time domain
Sometimes the flow-field is changed by moving hardware or varying input conditions, which imposes another challenge: transient contamination modelling. If the timescale of the flow field and of the particle path are of the same order, they have to be considered at the same time! In the example below the movement of the arm creates small dust-like particles with a low Stokes number. During the particle path, the flow field is constantly changed by the arm movement. A transient simulation with tracer contamination modelling is then the perfect choice.
