Particle Counters and the COVID-19 Pandemic

Author: Geoff Sheard, President, AGS Consulting, LLC

The COVID-19 pandemic has brought into sharp focus the need to understand the transmission mechanisms of respiratory viruses. Understanding these mechanisms requires studying three broad fronts: identifying virus transmission paths, learning how the virus circulates, and experimentally validating these transmission and circulation models. Before the spread of COVID-19, in preparation for an anticipated influenza pandemic, the scientific community had already shown that the short-range aerosol route is an important, though often neglected, virus transmission path.

Particle counters are typically used to measure the number of airborne particles in cleanrooms, research labs, and operating rooms, and they are emerging as a technology that can help study aerosol behavior. Accurately measuring the number and size of aerosol particles carrying the COVID-19 virus is critical to validating any transmission or circulation model. One of the challenges with this type of virus is that it travels in water particles expelled by an infected individual sneezing, coughing, or breathing. Water particles evaporate over time, and their particle size changes.

To tackle this challenge, Particles Plus, an engineering and manufacturing company located in Massachusetts, partnered with AGS Consulting, LLC, to apply computational fluid dynamics (CFD) and single-particle tracking simulations to analyze the flow field within a particle counter. They chose Cadence (formerly Numeca) to perform the CFD analysis because of their reputation for simulation tools that accurately predict real-world product performance.

Methodology

The numerical process adopted comprised two steps: first, modeling the particle counter flow field that constitutes the particles' support media; second, calculating the trajectory of particles as they pass through the particle counter.

Due to the complexity of the geometry, engineers at Cadence chose to work with Fidelity Flow, using the unstructured meshing capabilities of Fidelity Automesh. First, for comparison with experimental results, flow field simulations were run in Fidelity Flow to predict pressure drop through the particle counter. Here a pressure-based solver was used, which is faster and more accurate for incompressible flow field simulations.

Figure 1: Particle trajectories presented over two planes through the center of the particle counter - 3D streamlines showing particle trajectories

Single particle tracking was used to predict the trajectory of different-sized particles through the particle counter. Particles were launched from the inlet, and trajectories were calculated as they passed through the particle counter chamber.

Two assumptions were made when calculating trajectory:

• Particle trajectory is driven by the flow field within the particle counter, but particles do not have a significant impact on the flow field. The particle-to-air ratio is considered low enough for this one-way coupling approach to be assumed valid.
• Particle-to-particle interaction is neglected. Again, the particle-to-air ratio is considered low enough for this approach to be assumed valid.

Figure 2: Predicted trajectories for each class of particle over Cut 1 (left) and Cut 2 (right).

Simulation Results

Horizontal and vertical cutting planes through the particle counter were defined, and particle trajectories were mapped onto each plane. The trajectories of Class 1, 2, and 3 particles were studied over the vertical plane, with each class of particle behaving differently:

• The Class 1 (5.0 micrometers) particles pass through the chamber.
• The Class 3 (0.3 micrometers) particles recirculated in the vicinity of the chamber exit. However, they do not migrate back into the main body of the chamber.
• The Class 2 (1.0 micrometer) particles both recirculated in the vicinity of the chamber exit and, critically, do migrate back into the main body of the chamber.

When particle trajectory was studied over the horizontal cutting plane, particle trajectories were concluded to be similar, with the exception of Class 2 particles. The Class 2 particles exhibited a more intense recirculation at the exit of the chamber. The intensity of this recirculation was tentatively concluded to be driving the migration of Class 2 particles back into the main body of the chamber.

Table 1: Definition of Class 1, Class 2, and Class 3 particles (respectively 5.0, 1.0, and 0.3 micrometers diameter) used in the particle tracking simulation. As particle diameter reduces, the number of particles increases logarithmically.

Conclusions

This experimentally validated CFD simulation of the flow field within a particle counter identified that large, medium, and small particles behave differently as they pass through the particle counter. The two-step transfer of 1.0 micrometers particles from inlet-jet to exit-recirculation and then exit-recirculation to main-chamber recirculation was not anticipated.

This insight into particle counterflow field physics and the associated physical mechanisms at play within the particle counter provided Particles Plus with the basis for an ongoing project aimed at identifying critical particle sizes prone to recirculation and the optimization of particle counter geometry to minimize that recirculation.

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