An empirical model of human aspiration in low-velocity air using CFD investigations

Development of an Empirical Formula for Describing Human Inhalability of Airborne Particles at Low Wind Speeds and Calm Air

Based on experiments conducted in low wind speed and calm air environments, the current International Organization for Standardization (ISO) and European Committee for Standardization (CEN) convention modeling human aerosol inhalability (i.e. aspiration efficiency) may not be valid when wind speeds are less than 0.5 ms-1. Additionally, the convention is based primarily on mouth breathing data and aerosols with aerodynamic diameters smaller than 100 µm. Since the convention's development, experimental inhalation data at wind speeds lower than 0.5 ms-1 for nose, mouth, and oronasal breathing have been generated for aerosols in a wider range of sizes (1.5-135 µm). These data were gathered and modeled with the intention of providing a simple convention recommendation for inhalability in low wind speed (>0 to <0.5 ms-1) and calm air (~0 ms-1) conditions to the ISO Technical Committee (TC) 146, Subcommittee 2, Working Group (WG) 1 ('Particle Size-Selective Sampling and Analysis'), as it relates to standard ISO 7708, and to CEN TC 137/WG 3, as it relates to standard EN 481. This paper presents several equations as possibilities, all relating aspiration efficiency to aerodynamic diameter. The equation AE=1+0.000019dae2-0.009788dae stands out as a possible new convention. This polynomial model balances simplicity and fit while addressing the weakness of the current convention.

An Improved FFR Design with a Ventilation Fan: CFD Simulation and Validation

This article presents an improved Filtering Facepiece Respirator (FFR) designed to increase the comfort of wearers during low-moderate work. The improved FFR aims to lower the deadspace temperature and CO2 level by an active ventilation fan. The reversing modeling is used to build the 3D geometric model of this FFR; the Computational Fluid Dynamics (CFD) simulation is then introduced to investigate the flow field. Based on the simulation result, the ventilation fan of the improved FFR can fit the flow field well when placed in the proper blowing orientation; streamlines from this fan show a cup-shape distribution and are perfectly matched to the shape of the FFR and human face when the fan blowing inward. In the deadspace of the improved FFR, the CO2 volume fraction is controlled by the optimized flow field. In addition, an experimental prototype of the improved FFR has been tested to validate the simulation. A wireless temperature sensor is used to detect the temperature variation inside the prototype FFR, deadspace temperature is lowered by 2 K compared to the normal FFR without a fan. An infrared camera (IRC) method is used to elucidate the temperature distribution on the prototype FFR's outside surface and the wearer's face, surface temperature is lowered notably. Both inside and outside temperature results from the simulation are in agreement with experimental results. Therefore, adding an inward-blowing fan on the outer surface of an N95 FFR is a feasible approach to reducing the deadspace CO2 concentration and improve temperature comfort.

Investigation of the flow-field in the upper respiratory system when wearing N95 filtering facepiece respirator

This article presents a reverse modeling of the headform when wearing a filtering facepiece respirator (FFR) and a computational fluid dynamics (CFD) simulation based on the modeling. The whole model containing the upper respiratory airway, headform, and FFR was directly recorded by computed tomography (CT) scanning, and a medical contrast medium was used to make the FFR "visible." The FFR was normally worn by the subject during CT scanning so that the actual deformation of both the FFR and the face muscles during contact can be objectively conserved. The reverse modeling approach was introduced to rebuild the geometric model and convert it into a CFD solvable model. In this model, we conducted a transient numerical simulation of air flow containing carbon dioxide, thermal dynamics, and pressure and wall shear stress distribution in the respiratory system taking into consideration an individual wearing a FFR. The breathing cycle was described as a time-dependent profile of the air velocity through the respiratory airway. The result shows that wearing the N95 FFR results in CO2 accumulation, an increase in temperature and pressure elevation inside the FFR cavity. The volume fraction of CO2 reaches 1.2% after 7 breathing cycles and then is maintained at 3.04% on average. The wearers re-inhale excessive CO2 in every breathing cycle from the FFR cavity. The air temperature in the FFR cavity increases rapidly at first and then stays close to the exhaled temperature. Compared to not wearing an FFR, wearers have to increase approximately 90 Pa more pressure to keep the same breathing flow rate of 30.54 L/min after wearing an FFR. The nasal vestibule bears more wall shear stress than any other area in the airway.