(441j) Assessing the Impact of Internal Flow Dynamics on Low-Cost Optical Particle Counters

Environmental pollution represents a pressing global concern, prompting international efforts to mitigate the emission of harmful pollutants into the atmosphere. These pollutants emanate from a diverse array of sources, including construction activities, agriculture, power generation, transportation (particularly from automobiles), and the utilization of cookstoves for cooking. Among the primary pollutants of concern are Oxides of Nitrogen (NOx), Oxides of Sulfur (SOx), Particulate Matter (PM), and Ammonia. Notably, PM has garnered significant attention due to its adverse health impacts on the human populations upon inhalation [1].

PM is classified based on their aerodynamic diameters into three categories: fine particles (PM_2.5) with diameters less than or equal to 2.5μm, ultra-fine particles (PM_0.1) smaller than 0.1μm, and coarse particles (PM₁₀) with diameters less than 10μm. The health implications of PMs are size-dependent, with PM_2.5 posing a greater risk as they can penetrate deeper into the respiratory system, leading to conditions such as asthma, bronchitis, and mortality [2].

In the United States, monitoring instruments for PM include the Federal Reference Method (FRM) and the Federal Equivalent Method (FEM), which, however, report PM concentrations over extensive geographic areas [4]. To facilitate localized, real-time measurements, mobile PM sensors have been developed [5]. Various methodologies, including gravimetric, optical, and microbalance techniques, are employed for measuring PMs. Among these, the optical method, particularly Optical Particle Counters (OPC), is most prevalent. OPCs operate by drawing particle-laden air through a system where particles scatter light in a scattering region, with the intensity of scattered light measured by a detector to indicate particle concentration [6]. The scattering region is the region in an OPC where a particle interacts with light to cause scattering. There are expensive (has mirror or lens) and low-cost (no mirror or lens) OPCs. Low-cost OPCs are readily available to purchase and use.

Unfortunately, the design and operational efficiency of OPCs raise several critical questions concerning their ability to accurately measure ambient particle concentrations and sizes [7]. Factors such as the concentration of particles drawn into the OPC compared to ambient levels, the geometry of the OPC, the impact of fan speed, and variations in concentration within the OPC are crucial considerations. Furthermore, determining the appropriate sampling time is essential for accurate measurements.

This work investigates the performance of OPCs through two approaches. Initially, a comprehensive Computational Fluid Dynamics (CFD) analysis of simple defined geometries, which mimic the tortuous path flow characteristic of commercially available OPCs, was conducted. This foundational analysis elucidated the impact of device geometry on variation in concentrations and subsequently on measurement accuracy. Leveraging on insights gained from the initial CFD study, the research was expanded to explore the performance of commercial low-cost OPCs. This examination focused on addressing critical design and operational questions previously highlighted. Through this inquiry, the research provided substantive evaluations and potential enhancements for the accuracy and reliability of low-cost OPCs in real-world environmental monitoring applications. By employing two CFD software packages for comparative analysis, the research also enhanced the understanding of OPCs' performance and identified potential improvements for their accuracy and reliability in environmental monitoring.

In the initial CFD study of the simply designed geometries, SolidWorks, a computer-aided design (CAD) software, was utilized to create two geometric models: a singular box chamber and a dual-box chamber. The dual-box chamber was employed to investigate the effects of geometric variations on OPC accuracy. The method described in this section focuses more on the use of Converge studio for the initial simple geometric studies.

The simple designed geometries were imported into Converge Studio CFD for simulation setup. The simulations employed a transient turbulence model (RANS-k-epsilon), ensuring detailed flow dynamics analysis. Injector specifications and orientations were defined for each geometry's inlet, alongside comprehensive boundary condition settings for inlets, outlets, and walls, adhering to a no-slip condition for the stationary walls.

The discrete phase model (DPM) in Converge Studio was used. Air served as the continuous phase, while carbon particles represented the discrete phase, and a Rosin-Rammler particle size distribution was used to inject particles with varying sizes. The particles injected were given a Sauter mean diameter of 2.5μm, targeting a concentration of 35μg/m³. Following case setup, input files were generated, and simulations were run. Tecplot360, a post processing software, was used to analyze the results from the simulations of simple geometries. To facilitate a detailed investigation of regional concentrations at various time intervals, a custom Python script was developed. This script strategically segmented each geometry into distinct regions, enabling precise calculations of regional concentrations over different simulation times. Plots of various regional concentrations versus time were generated for different time averaged concentrations. Particle size distributions plots were also generated, enabling a better understanding of different particles sizes in various regions at different times. The simulations in Ansys Fluent CFD also used the DPM model, similar parameters, and the same assumptions as used in Converge studio CFD. Two CFD packages were used to provide additional validation of the simulation results.

The findings from the simulations of the simple geometric models revealed notable variances in particle concentrations across the chamber's regions. While some regions exhibited significant deviations from the target concentration of 35μg/m³, other regions closely aligned with or distributed around this target level. Furthermore, the size distribution plot underscored disparities in particle size distribution within the regions.

Additionally, two commercial OPC designs were also modeled using CAD to capture all relevant geometrical components influencing measurement outcomes. The analysis between two commercially available OPCs delved into the disparities in concentration and particle size distributions across various regions in the geometry.

The insights derived from this study are expected to inform strategic considerations in selecting specific region(s) within an OPC for the scattering region and detector position, leading to improved measurement accuracy. Through these endeavors, the research aims to advance the development of more accurate and reliable OPCs, ultimately contributing to enhanced environmental monitoring capabilities and a deeper understanding of the role of PM in air quality and health.

References

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2. Cormier, S.A., et al., Origin and health impacts of emissions of toxic by-products and fine particles from combustion and thermal treatment of hazardous wastes and materials. Environ Health Perspect, 2006. 114(6): p. 810-7.
3. U.S Environmental Protection Agency(EPA). 2024 PM Nation Ambient Air Quality Standards Final overview presentation. 2024; Available from: https://www.epa.gov/pm-pollution/national-ambient-air-quality-standards-naaqs-pm.
4. Christopher, S.A. and P. Gupta, Satellite remote sensing of particulate matter air quality: the cloud-cover problem. J Air Waste Manag Assoc, 2010. 60(5): p. 596-602.
5. Steinle, S., et al., Personal exposure monitoring of PM2.5 in indoor and outdoor microenvironments. Sci Total Environ, 2015. 508: p. 383-94.
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7. Trejo, R.E.G., et al., A Study on the Behavior of Different Low-Cost Particle Counter Sensors for PM-10 and PM-2.5 Suspended Air Particles, in Telematics and Computing. 2022. p. 33-50.