Transport and control of droplets: A comparison between two types of local ventilation airflows

Numerical Simulation of an Oil Mist Particle Emission and Gas-Oil Separation Device of a Closed Machine Tool in the Post-Environmental Area

Owing to the airflow field within airtight machines, oil mist particles escape with the airflow from the machine shell gaps and are emitted externally to the post-environmental area, causing air pollution and threatening workers' health. The existing local exhaust system is ineffective in capturing oil mist particles. This study proposes a gas-oil separation device that can "in-situ control" the oil mist particles in situ and weaken their outgoing emission and that uses numerical simulations to compare and analyse the emission characteristics of oil mist particles, before and after the addition of the separation device at different exhaust air volumes and particle emission speeds, and to design the structural parameters of the device to improve the separation efficiency of oil mist particles. The structural parameters of the proposed device are designed to improve the separation efficiency of oil mist particles. Studies have shown that for every 200 m³/h increase in exhaust air volume, the capture efficiency increases by around 3%, and the particle concentration at the gap in the machine loading door decreases from 9.4 × 10^-7 kg/m³ to 7.7 × 10^-7 kg/m³. The overall escape rate of oil mist particles is in the range of 10-13% after the addition of a pressure relief device. Numerical simulations are performed to analyse the effects of inlet airflow velocity, folding plate spacing, and folding plate angle on the separation efficiency of oil mist particles. Results show that an increase in the inlet velocity of the airflow increases the particle separation efficiency. The most suitable structural parameters for the separation device and the machine are as follows: 60° angle of the folding plate and 30 mm distance between plates, where the separation efficiency is above 80%, and the average separation efficiency is about 86%. The results of this study can be used as a reference for the study of the emission of oil mist particles from enclosed mechanical cutting machines.

Numerical simulation for spatial distribution of water aerosol produced from nozzle spray and health risk related to Legionella pneumophila in spray scenarios

Water spray facilities are widely used in public places for sprinkling or beautifying the environment. However, the potential health risk induced by water aerosols increasingly calls for attention. In this study, the spatial distribution of water aerosols was investigated through the molecular sieve adsorption method, and predicted by discrete phase model (DPM). On this basis, the health risk regarding Legionella pneumophila for specific spray scenarios was evaluated by quantitative microbial risk assessment (QMRA). The results showed that the original droplet size can be described by the Rosin_Rommaler distribution (R²>0.99). The spatial distribution of water aerosols produced from a nozzle spray can be well predicted by the DPM. The concentration of water aerosols showed a sharp decline within 5 m from the nozzle and was not significantly different within 5 m (p>0.05) as for various spray scenarios. However, the difference was significant outside 5 m (p<0.05). Furthermore, a safe contact distance of exceeding 8 m is proposed in spray scenarios considering the risk threshold of 0.0001. Sensitivity analysis demonstrated the concentration of Legionella pneumophila in water aerosols as the critical factor affecting the health risk.

Cross-Regional Dynamic Transfer Characteristics of Liquid Oil Contamination Induced by Random Contact in Machining Workshops in Shanghai, China

In industrial sites, the movement and contact behaviors of workers are random, but their frequency and statistical characteristics can be determined. Particularly in machining workshops, metalworking fluids (MWFs) cause liquid oil contamination on the processed workpieces, and the contamination spreads to the entire workshop given the random contact of workers or the handling of workpieces. This study proposes a contact transmission model based on the Markov chain to quantify oil contamination transfer. First, the transfer efficiency between the glove and the workpiece, which is regarded as a key model parameter in this research, was determined through experiments. The model was used to characterize and predict the spread of oil contamination across different regions, including production and assembly areas. Specifically, the oil contamination concentrations on workbench surfaces in seven locations of a machining workshop in Shanghai GKN HUAYU Driveline Systems Co., Ltd. (SDS) were measured on-site. Findings showed that the model could feasibly depict the transfer process of oil contamination across different surfaces. Then, the variation law of oil contamination concentration on the workbench surfaces over time was analyzed, the oil contamination distribution map of the entire workshop plane was drawn, and the effectiveness of two cleaning measures to reduce oil contamination concentrations was compared. The proposed contact transmission model offers a basis for identifying highly polluted surfaces in machining workshops and controlling the spread of liquid oil contamination.

Transmission risk of infectious droplets in physical spreading process at different times: A review

Droplets provide a well-known transmission media in the COVID-19 epidemic, and the particle size is closely related to the classification of the transmission route. However, the term "aerosol" covers most particle sizes of suspended particulates because of information asymmetry in different disciplines, which may lead to misunderstandings in the selection of epidemic prevention and control strategies for the public. In this review, the time when these droplets are exhaled by a patient was taken as the initial time. Then, all available viral loads and numerical distribution of the exhaled droplets was analyzed, and the evaporation model of droplets in the air was combined with the deposition model of droplet nuclei in the respiratory tract. Lastly, the perspective that physical spread affects the transmission risk of different size droplets at different times was summarized for the first time. The results showed that although the distribution of exhaled droplets was dominated by small droplets, droplet volume was proportional to the third power of particle diameter, meaning that the viral load of a 100 μm droplet was approximately 10⁶ times that of a 1 μm droplet at the initial time. Furthermore, the exhaled droplets are affected by heat and mass transfer of evaporation, water fraction, salt concentration, and acid-base balance (the water fraction > 98%), which lead them to change rapidly, and the viral survival condition also deteriorates dramatically. The time required for the initial diameter (d_o) of a droplet to shrink to the equilibrium diameter (d_e, about 30% of d_o) is approximately proportional to the second power of the particle diameter, taking only a few milliseconds for a 1 μm droplet but hundreds of milliseconds for a 10 μm droplet; in other words, the viruses carried by the large droplets can be preserved as much as possible. Finally, the infectious droplet nuclei maybe inhaled by the susceptible population through different and random contact routes, and the droplet nuclei with larger d_e decompose more easily into tiny particles on account of the accelerated collision in a complex airway, which can be deposited in the higher risk alveolar region. During disease transmission, the infectious droplet particle size varies widely, and the transmission risk varies significantly at different time nodes; therefore, the fuzzy term "aerosol" is not conducive to analyzing disease exposure risk. Recommendations for epidemic prevention and control strategies are: 1) Large droplets are the main conflict in disease transmission; thus, even if they are blocked by a homemade mask initially, it significantly contains the epidemic. 2) The early phase of contact, such as close-contact and short-range transmission, has the highest infection risk; therefore, social distancing can effectively keep the susceptible population from inhaling active viruses. 3) The risk of the fomite route depends on the time in contact with infectious viruses; thus, it is important to promote good health habits (including frequent hand washing, no-eye rubbing, coughing etiquette, normalization of surface cleaning), although blind and excessive disinfection measures are not advisable. 4) Compared with the large droplets, the small droplets have larger numbers but carry fewer viruses and are more prone to die through evaporation.