Emmanuel Belut

The free fall of a drop through gas loaded with solid particles gives rise to multiple physical interactions, which remain poorly documented, especially when the drop is no longer spherical. In particular, no model predicts the particle collection efficiency for drops undergoing deformations or oscillations. This study aims to contribute to this effort by investigating numerically the dynamics of water drops freely falling in air laden with dispersed solid particles for drop Reynolds and Weber numbers such that the drops do (or do not) deform or oscillate (e.g., Re=30, 70, 500, and 876). A Eulerian–Lagrangian framework is adopted. The drop internal and external flows are simulated with direct numerical simulation (DNS), and the dynamics of the liquid–gas interface are tracked using a combination of the volume of fluid (VOF) and level set methods; this approach predicts the interface dynamics in line with experimental data. The trajectories of solid particles are simulated using Lagrangian tracking and taking into account drag, gravity, and Brownian motion. For spherical drops with Reynolds numbers below 200, our methodology replicates previous results. In the presence of oscillations and/or deformations, the flow parameters of the two continuous phases are correctly predicted. The particle collection efficiency also follows the experimental trend, but the values differ significantly from measurements found in the literature. We therefore propose certain areas of improvement with the goal of obtaining better fits to the available experimental data.

The free fall of a drop through gas loaded with solid particles gives rise to multiple physical interactions, which remain poorly documented, esp. when the drop is no longer spherical. In particular, no model predicts the particle collection efficiency for drops undergoing deformations or oscillations. This study aims to contribute to this effort by investigating numerically the dynamics of water drops freely falling in air laden with dispersed solid particles, for drop Reynolds and Weber number such that drops present deformations/oscillations or not (e.g., Re = 30, 70, 500 and 876). An Eulerian-Lagrangian framework is adopted. The drop internal and external flows are simulated with Direct Numerical Simulation (DNS), and the dynamics of the liquid/gas interface are tracked using a combination of the Volume of Fluid (VOF) and Level Set methods, this approach predicts the interface dynamics in line with experimental data. The trajectories of solid particles are simulated using Lagrangian tracking and taking into account drag, gravity, and Brownian motion. For spherical drops with Reynolds numbers below 200, our methodology replicates previous results. In the presence of oscillations/deformations, the flow parameters of the two continuous phases are correctly predicted. The particle collection efficiency also follows the experimental trend, but the values differ significantly from measurements found in the literature. We therefore propose certain areas of improvement with the goal of obtaining better fits to the available experimental data.

Aerosols of semivolatile organic compounds (SVOCs) pose significant health risks to workers in various occupational settings. Measuring human exposure to these aerosols requires a separate assessment of the contribution of particles and gases, which is not resolved by existing sampling techniques. Here, we investigate experimentally the performance of a semivolatile aerosol dichotomous sampler (SADS), proposed in previous studies, for sampling monodisperse liquid particles with aerodynamic diameters between 0.15 and 4.5 µm, corresponding to workplace aerosols. The measured sampling performances are compared to their theoretical counterparts computed by computational fluid dynamics. The effects of leakage rate, repeatability of the assembly, imprecision of the actually machined nozzle diameters, and SADS part misalignment are examined. The SADS assembly is found to be easily leaky, but consequences on sampling can be overcome when a prior leak test with a leakage rate below 4 Pa s⁻¹ is passed. Variation of nozzle diameters in the range (−4.5 %, +3.7 %) with respect to nominal values affects marginally (< 3 %) aerosol transmission efficiency, but sampling performance is little reproducible during successive SADS assemblies (CV = 22.1 % for wall losses). Theoretically unpredicted large (40 %–46 %) wall losses are measured for particles larger than 2 µm, located mostly (80 %) on the external walls of the collection nozzle. Assembly repeatability issues and simulations of SADS parts misalignment effect by computational fluid dynamics (CFD) suggest that these undesirable particle deposits are due to the mechanical backlashes of the assembly. Thus, the current design does not guarantee a nozzle misalignment of less than 5 % of the acceleration nozzle diameter, and other important geometric parameters are not further constrained. The promising theoretical sampling performance of the SADS for SVOC aerosols larger than 1 µm thus falls short of expectations due to mechanical design issues that can be improved before possible field use.

A particle exposure experiment inside a large climate‐controlled chamber was conducted. Data on spatial and temporal distribution of nanoscale and fine aerosols in the range of mobility diameters 8 – 600 nm were collected with high resolution, for sodium chloride, fluorescein sodium and silica particles. Exposure scenarios studied, included constant and intermittent source emissions, different aggregation conditions, high (10 h⁻¹) and low (3.5 h⁻¹) air exchange rates (AERs) corresponding to chamber Reynolds number respectively equal to 1.10⁵ and 3.10⁴. Results are presented and analysed to highlight the main determinants of exposure and to determine if the assumptions underlying two‐box models hold under various scenarios. The main determinants of exposure found were the source generation rate and the ventilation rate. The effect of particles nature was indiscernible and the decrease of airborne total number concentrations attributable to surface deposition was estimated lower than 2% when the source was active. A near‐field/far‐field structure of aerosol concentration was always observed for the AER=10 h⁻¹ but for AER=3.5 h⁻¹ a single field structure was found. The particle size distribution was always homogeneous in space but a general shift of particles diameter (‐8% to +16%) was observed between scenarios in correlation with the AER and with the source position, presumably largely attributable to aggregation.

Models are increasingly used to estimate and pre-emptively calculate the occupational exposure of airborne released particulate matter. Typical two-box models assume instant and fully mixed air volumes, which can potentially cause issues in cases with fast processes, slow air mixing, and/or large volumes. In this study, we present an aerosol dispersion model and validate it by comparing the modelled concentrations with concentrations measured during chamber experiments. We investigated whether a better estimation of concentrations was possible by using different geometrical layouts rather than a typical two-box layout. A one-box, two-box, and two three-box layouts were used. The one box model was found to underestimate the concentrations close to the source, while overestimating the concentrations in the far field. The two-box model layout performed well based on comparisons from the chamber study in systems with a steady source concentration for both slow and fast mixing. The three-box layout was found to better estimate the concentrations and the timing of the peaks for fluctuating concentrations than the one-box or two-box layouts under relatively slow mixing conditions. This finding suggests that industry-relevant scaled volumes should be tested in practice to gain more knowledge about when to use the two-box or the three-box layout schemes for multi-box models.

Deploying mitigation strategies to prevent exposure to hazardous airborne particles relies firstly on identifying and characterizing the sources of pollution. As the location of aerosol sources may be unknown or variable, and their generation rates fugitive, there is a need of developing tractable methods allowing the distant characterization of such sources in the field. The present paper proposes and evaluates an inverse experimental method which enables the reconstruction of the time-varying generation rate of a single aerosol source. The method is suitable for cases where the air flow is statistically steady (forced ventilation) and requires only a reference aerosol source and a real-time concentration sensor. The underlying theory of the method is exposed and situated among existing works. The method is then applied experimentally to hand-held woodworking machines placed in a ventilated cabin, and its performances are directly evaluated by considering the mass balance of the process. Results show that the method is particularly efficient in estimating the total mass of particles released in air. This allows for instance classifying polluting processes with respect to their emission potential and building release libraries, which has important impacts on environmental and occupational hygiene.