Numerical Investigation of Aerosolization in the Venturi Dustiness Tester: Aerodynamics of a Particle on a Hill.

Understanding particle detachment from surfaces is necessary to better characterize dust generation and entrainment. Previous work has studied the detachment of particles from flat surfaces. The present work generalizes this to investigate the aerodynamics of a particle attached to various locations on a model hill. The present work serves as a model for dust aerosolization in a tube, as powder is injected into the Venturi Dustiness Tester. The particle is represented as a sphere in a parallel plate channel, or, in two dimensions, as a cylinder oriented perpendicular to the flow. The substrate is modified to include a conical hill (3D) or wedge (2D), and the test particle is located at various positions on this hill. The governing incompressible Navier-Stokes equations are solved using the finite-volume FLUENT code. The coefficients of lift and drag are compared with the results on the flat substrate. Enhanced drag and significantly enhanced lift are observed as the test particle is situated near the summit of the hill.

Computational Fluid Dynamics Analysis of the Venturi Dustiness Tester.

Dustiness quantifies the propensity of a finely divided solid to be aerosolized by a prescribed mechanical stimulus. Dustiness is relevant wherever powders are mixed, transferred or handled, and is important in the control of hazardous exposures and the prevention of dust explosions and product loss. Limited quantities of active pharmaceutical powders available for testing led to the development (at University of North Carolina) of a Venturi-driven dustiness tester. The powder is turbulently injected at high speed (Re ~ 2 × 104) into a glass chamber; the aerosol is then gently sampled (Re ~ 2 × 103) through two filters located at the top of the chamber; the dustiness index is the ratio of sampled to injected mass of powder. Injection is activated by suction at an Extraction Port at the top of the chamber; loss of powder during injection compromises the sampled dustiness. The present work analyzes the flow inside the Venturi Dustiness Tester, using an Unsteady Reynolds-Averaged Navier-Stokes formulation with the k-ω Shear Stress Transport turbulence model. The simulation considers single-phase flow, valid for small particles (Stokes number Stk <1). Results show that ~ 24% of fluid-tracers escape the tester before the Sampling Phase begins. Dispersion of the powder during the Injection Phase results in a uniform aerosol inside the tester, even for inhomogeneous injections, satisfying a necessary condition for the accurate evaluation of dustiness. Simulations are also performed under the conditions of reduced Extraction-Port flow; results confirm the importance of high Extraction-Port flow rate (standard operation) for uniform distribution of fluid tracers. Simulations are also performed under the conditions of delayed powder injection; results show that a uniform aerosol is still achieved provided 0.5 s elapses between powder injection and sampling.

Assessing Effectiveness of Ceiling-Ventilated Mock Airborne Infection Isolation Room in Preventing Hospital-Acquired Influenza Transmission to Health Care Workers.

Exposure to airborne influenza (or flu) from a patient's cough and exhaled air causes potential flu virus transmission to the persons located nearby. Hospital-acquired influenza is a major airborne disease that occurs to health care workers (HCW). This paper examines the airflow patterns and influenza-infected cough aerosol transport behavior in a ceiling-ventilated mock airborne infection isolation room (AIIR) and its effectiveness in mitigating HCW's exposure to airborne infection. The computational fluid dynamics (CFD) analysis of the airflow patterns and the flu virus dispersal behavior in a mock AIIR is conducted using the room geometries and layout (room dimensions, bathroom dimensions and details, placement of vents and furniture), ventilation parameters (flow rates at the inlet and outlet vents, diffuser design, thermal sources, etc.), and pressurization corresponding to that of a traditional ceiling-mounted ventilation arrangement observed in existing hospitals. The measured data shows that ventilation rates for the AIIR are about 12 air changes per hour(ach). However, the numerical results reveals incomplete air mixing and that not all of the room air is changed 12 times per hour. Two life-sized breathing human models are used to simulate a source patient and a receiving HCW. A patient cough cycle is introduced into the simulation and the airborne infection dispersal is tracked in time using a multiphase flow simulation approach. The results reveal air recirculation regions that diminished the effect of air filtration and prolong the presence of flu-contaminated air at the HCW's zone. Immediately after the patient coughs (0.51 s), the cough velocity from the patient's mouth drives the cough aerosols toward the HCW standing next to patient's bed. Within 0.7 s, the HCW is at risk of acquiring the infectious influenza disease, as a portion of these aerosols are inhaled by the HCW. As time progresses (5 s), the aerosols eventually spread throughout the entire room, as they are carried by the AIIR airflow patterns. Subsequently, a portion of these aerosols are removed by the exhaust ventilation. However, the remaining cough aerosols reenter and recirculate in the HCW's zone until they are removed by the exhaust ventilation. The infectious aerosols become diluted in the HCW's region over a period of 10 s because of the fresh air supplied into the HCW's zone. The overall duration of influenza infection in the room (until the aerosol count is reduced to less than 0.16% of the total number of aerosols ejected from the patient's mouth) is recorded as approximately 20 s. With successive coughing events, a near-continuous exposure would be possible. Hence, the ceiling-ventilation arrangement of the mock AIIR creats an unfavorable environment to the HCW throughout his stay in the room, and the modeled AIIR ventilation is not effective in protecting the HCW from infectious cough aerosols. The CFD results suggest that the AIIR ceiling ventilation arrangement has a significant role in influencing the flu virus transmission to the HCW.

Computational fluid dynamics study on the influence of an alternate ventilation configuration on the possible flow path of infectious cough aerosols in a mock airborne infection isolation room.

When infectious epidemics occur, they can be perpetuated within health care settings, potentially resulting in severe health care workforce absenteeism, morbidity, mortality, and economic losses. The ventilation system configuration of an airborne infection isolation room is one factor that can play a role in protecting health care workers from infectious patient bioaerosols. Though commonly associated with airborne infectious diseases, the airborne infection isolation room design can also impact other transmission routes such as short-range airborne as well as fomite and contact transmission routes that are impacted by contagion concentration and recirculation. This article presents a computational fluid dynamics study on the influence of the ventilation configuration on the possible flow path of bioaerosol dispersal behavior in a mock airborne infection isolation room. At first, a mock airborne infection isolation room was modeled that has the room geometry and layout, ventilation parameters, and pressurization corresponding to that of a traditional ceiling-mounted ventilation arrangement observed in existing hospitals. An alternate ventilation configuration was then modeled to retain the linear supply diffuser in the original mock airborne infection isolation room but interchanging the square supply and exhaust locations to place the exhaust closer to the patient source and allow clean air from supply vents to flow in clean-to-dirty flow paths, originating in uncontaminated parts of the room prior to entering the contaminated patient's air space. The modeled alternate airborne infection isolation room ventilation rate was 12 air changes per hour. Two human breathing models were used to simulate a source patient and a receiving health care worker. A patient cough cycle was introduced into the simulation, and the airborne infection dispersal was tracked in time using a multi-phase flow simulation approach. The results from the alternate configuration revealed that the cough aerosols were pulled by the exhaust vent without encountering the health care worker by 0.93 s after patient coughs and the particles were controlled as the aerosols' flow path was uninterrupted by an air particle streamline from patient to the ceiling exhaust venting out cough aerosols. However, not all the aerosols were vented out of the room. The remaining cough aerosols entered the health care worker's breathing zone by 0.98 s. This resulted in one of the critical stages in terms of the health care worker's exposure to airborne virus and presented the opportunity for the health care worker to suffer adverse health effects from the inhalation of cough aerosols. Within 2 s, the cough aerosols reentered and recirculated within the patient and health care worker's surroundings resulting in pockets of old contaminated air. By this time, coalescence losses decreased as the aerosol were no longer in very close proximity and their movement was primarily influenced by the airborne infection isolation room airflow patterns. In the patient and health care worker's area away from the supply, the fresh air supply failed to reach this part of the room to quickly dilute the cough aerosol concentration. The exhaust was also found to have minimal effect upon cough aerosol removal, except for those areas with high exhaust velocities, very close to the exhaust grill. Within 5-20 s after a patient's cough, the aerosols tended to break up to form smaller sized aerosols of less than one micron diameter. They remained airborne and entrained back into the supply air stream, spreading into the entire room. The suspended aerosols resulted in the floating time of more than 21 s in the room due to one cough cycle. The duration of airborne contagion in the room and its prolonged exposure to the health care worker is likely to happen due to successive coughing cycles. Hence, the evaluated alternate airborne infection isolation room is not effective in removing at least 38% particles exposed to health care worker within the first second of a patient's cough.

Numerical Investigation of Sheath and Aerosol Flows in the Flow Combination Section of a Baron Fiber Classifier.

The Baron fiber classifier is an instrument used to separate fibers by length. The flow combination section (FCS) of this instrument is an upstream annular region, where an aerosol of uncharged fibers is introduced along with two sheath flows; length separation occurs by dielectrophoresis downstream in the flow classification section. In its current implementation at NIOSH, the instrument is capable of processing only very small quantities of fibers. In order to prepare large quantities of length-separated fibers for toxicological studies, the throughput of the instrument needs to be increased, and hence, higher aerosol flow rates need to be considered. However, higher aerosol flow rates may give rise to flow separation or vortex formation in the FCS, arising from the sudden expansion of the aerosol at the inlet nozzle. The goal of the present investigation is to understand the interaction of the sheath and aerosol flows inside the FCS, using computational fluid dynamics (CFD), and to identify possible limits to increasing aerosol flow rates. Numerical solutions are obtained using an axisymmetric model of the FCS, and solving the Navier-Stokes equations governing these flows; in this study, the aerosol flow is treated purely aerodynamically. Results of computations are presented for four different flow rates. The geometry of the converging outer cylinder, along with the two sheath flows, is effective in preventing vortex formation in the FCS for aerosol-to-sheath flow inlet velocity ratios below ~ 50. For higher aerosol flow rates, recirculation is observed in both inner and outer sheaths. Results for velocity, streamlines, and shear stress are presented.