The influence factors of mean particle size and positive quality control of bacterial filtration efficiency system

Summary Background The Coronavirus Disease 2019 (COVID-19) has swept the whole world with high mortality. Since aerosol transmission is the main route of transmission, wearing a mask serves as a crucial preventive measure. An important parameter to evaluate the performance of a mask is the bacteria filtration efficiency (BFE). Aerosol mean particle size (MPS) and positive quality control value are two key indexes of BFE system. Aim To study the major influence factors of the mean particle size of bacterial aerosols and positive quality control value of BFE system. Method and Results In this study, we investigated the influence of Anderson sampler, spray flow, medium thickness, and peristaltic pump flow on the MPS of bacterial aerosols and positive quality control value of BFE system, respectively. The results show that the machining accuracy of Anderson sampler has great influence on aerosol MPS and positive quality control value. With the increase of aerosol spray flow rate, the positive quality control value will increase gradually, but the effect on aerosol MPS is not a simple linear relationship. As the agar medium thickness increased, the positive quality control value and aerosol MPS increased gradually. With the increase of peristaltic pump flow, the positive quality control value increased gradually, while the aerosol MPS was basically in a downward trend. When the peristatic pump flow rate was 0.1mL/min, the spray flow rate was 7.2L/min, the agar plate thickness was 27mL, and the Anderson sampler of Beijing Mingjie was used for the experiment, the aerosol MPS and positive quality control value were both within the acceptable range and were the optimal parameters. Conclusions This study provides guidance for the manufacturers of the BFE system and improves the protective performance of masks, which is important for the human health, especially during the occurrence of viral pandemics such as "COVID-19".

Welding Parameters Optimization in Plunging and Dwelling Phase of FSW Medium Thickness 2219 Aluminum Alloy

The influence of welding parameters on temperature distribution in plunging and dwelling phase of friction stir welding (FSW) medium thickness 2219 aluminum alloy is blank. Improper selection of welding parameters will result in uneven temperature distribution along the thickness of the weldment, which will lead to welding defects and ultimately affect the mechanical properties of the weldment. To realize the prediction of temperature distribution and achieve the optimization of welding parameters, a simulation model of FSW 18mm thick 2219 aluminum alloy is built based on DEFORM. The validity of the simulation model is verified by temperature measurement experiments. With the minimum temperature difference in the core area of the weldment as target value, and weldable temperature range of 2219 aluminum alloy as constraint conditions, orthogonal experiments are conducted considering the rotational speed, the press amount, the tool tilt angle, the plunging traverse speed and the dwelling time. The results of variance analysis show that the rotational speed and the dwelling time are significant factors affecting temperature field during plunging and dwelling phase. Through single factor simulation, the welding parameters in plunging and dwelling phase are optimized. This study provides a reference for realizing high-quality welding of a heavy rocket fuel tank.

Temperature Distribution And Mechanical Properties of FSW Medium Thickness 2219 Aluminum Alloy

Friction stir welding (FSW) is a solid-state jointing technology, which has the advantages of high joint strength, low residual stress aXnd small deformation after welding. During the process of FSW, the welding temperature has an important effect on the quality of the weldment. Therefore, the heat generation model of FSW of medium thickness 2219 aluminum alloy is established based on the friction heat generation at the interface between the tool and the workpiece and the plastic deformation heat generation of the weldment material near the tool. The heat transfer model is set under the premise of considering heat conduction, thermal convection, and thermal radiation. Using JMatPro technology, the temperature-related material parameters of 2219 aluminum alloy are calculated based on the material composition, and the heat generation model is imported into the ABAQUS simulation software based on the DFLUX subroutine, and the establishment of the FSW thermodynamic model is realized. The effectiveness of the model is verified by FSW experiments. The thermodynamic model takes into account both heat generation (friction heat generation and plastic deformation heat generation) and heat transfer (heat conduction, thermal convection and thermal radiation), so it has a high prediction accuracy. Based on the FSW thermodynamic model, the influence of welding parameters on temperature distribution is explored, subsequently the influence of welding temperature on mechanical properties of welded joint are also studied. The research can provide guidance for predicting and characterizing the temperature distribution and the improvement of mechanical performance of FSW.

Development and research of new technologies for precision plasma cutting of metals

Plasma torches of Russian origin concede to import metal-cutting plasma facilities by several parameters, in particular energy efficiency, quality of cut, degree of automation. To increase efficiency and safety of domestic electro-plasma technologies itь is necessary to perfect methods of analysis of known design solutions to provide new developments. The results of the plasma torch­es design widely used in metallurgical and machine-building technologies for air-plasma cutting of metals presented. It was noted that productivity, cutting quality and reliability of plasma equipment should be chosen as the main criteria of efficiency for plasma cutting technology. It was shown that special attention should be paid to improving the gas-vortex stabilizing method for the plasma forming gas, which ensures the efficiency of both the plasma torch and the plasma cutting process as a whole. Results of studies of a complex system for arranging the flow of plasma-forming gas through the channels of the gas-air path in the plasma torch and the subsequent creation of new systems for stabilizing the arc discharge of metal cutting DC plasma torches with high technological capabilities presented. The study was carried out by the methods of numerical simulation of gas-dynamic and thermo-physical pro­cesses for various designs of plasma torches. Optimized designs of plasma torches with various variants of gas-vortex stabilization systems were elaborated. A series of elaborated torches, which includes a single-stream plasma torches ПМВР-5 for precision cutting of medium thickness metals, as well as two-stream plasma torches ПМВР-9 for cutting metals of small and medium thicknesses presented. The last plasma torches use the technology of narrow-jet or compressed plasma and have no domestic analogues. A meth­odology of metal-cutting plasma torches designing by gas-dynamic, thermo-physical and acoustic criteria was elaborated. The results of experimental studies showed that the use of new ПМВР-5 plasma torches allows to obtain precision cuts corresponding to the 1st and 2nd quality classes according to GOST 14792-80 on steels of the 09Г2С type of medium thickness. It was also shown that the use of new plasma torches makes it possible to perform precision finishing plasma cutting on low-carbon steels of medium thickness (without additional mechanical processing of the cutting edge) in the production technologies of welded joints.

Comparison of Central Corneal Thickness Measured by Tono-pachymetry and Ultrasound Pachymetry

Purpose: We compared the accuracy of central corneal thickness (CCT) measurements according to CCT measured by noncontact Tono-pachymetry and ultrasound pachymetry (USP). Methods: CCT was measured in 90 eyes of 90 subjects by two optometrists. The CCT measurements were compared and the correlations between the measurements were analyzed. To evaluate whether the measurements varied depending on CCT, the subjects were classified into three groups according to CCT (group 1: thin thickness group; group 2: medium thickness group; and group 3: thick thickness group). The differences in CCT obtained by the two devices (△CCT = Tono-pachymetry-USP) were compared and analyzed among the three groups. Results: The average CCT measurements by Tono-pachymetry and USP were 523.26 ± 32.93 μm and 527.08 ± 37.33 μm, respectively. CCT by Tono-pachymetry was significantly thinner than by USP (△CCT= -3.82 ± 15.34, p = 0.020). The two measurements were strongly correlated (r = 0.912, p < 0.001). The △CCT values were 5.40 ± 12.13 μm in group 1, -6.37 ± 15.07 μm in group 2, and -10.50 ± 14.39 μm in group 3 (p < 0.001). Conclusions: CCT measured by tono-pachymetry was thinner on average compared to the value measured by USP and the differences in measurements between the two devices were different according to CCT. The thinner the CCT, the thicker the measurement, and the thicker the CCT, the thinner the measurement. Therefore, this trend should be considered when interpreting tono- pachymetry results in clinical practice.

Analysis of Equivalent Thickness of Geological Media for Lab-scale Study of Radon Exhalation

Geological media are widely distributed in nature. Lab-scale tests are frequently employed in radon studies for these media. Thus, it is critical to find the thickness of the medium at an experimental scale that is equivalent to the medium thickness in a real geological system. Based on the diffusion–advection transport of radon, theoretical models of the surface radon exhalation rate for homogeneous semi-infinite and finite-thickness systems were derived (denoted as Jse and Jfi, respectively). Analysis of the equivalency of Jse and Jfi was subsequently carried out by introducing several dimensionless parameters, including the ratio of the exhalation rates for the semi-infinite and finite-thickness models, ε, and the number of diffusion lengths required to achieve a desired ε value, n. The results showed that when radon transport in geological media is dominantly driven by diffusion, if n > 3.6626, then ε > 95% (and if n > 5.9790, then ε > 99.5%). When radon migration is dominantly driven by advection, if n > 2.5002, then ε > 95% (and if n > 4.0152, then ε > 99.5%). Therefore, if the thickness of the geological media (x0) is greater than a certain n times the radon diffusion length of the media (L), the media can be modeled as semi-infinite. To validate the model, a pure radon diffusion experiment (no advection) was developed using uranium mill tailings, laterite, and radium-bearing rocklike material with different thicknesses (x0). The theoretical model was demonstrated to be reliable and valid. This study provides a basis for determining the appropriate thickness of geological media in lab-scale experiments of radon exhalation.

Vegetation Survival in Green Roofs without Irrigation

The implementation of green roofs as sustainable urban drainage systems provides benefits for stormwater control and the environment and is more and more encouraged. A model for the estimation of the probability of vegetation survival without irrigation is proposed. The model, developed through a probabilistic analytical derivation procedure, can also consider the effects of chained rainfall events, without the need of continuous simulation of hydrological processes. The model equations can be useful in the design of green roofs, allowing to determine the growing medium thickness in terms of an assumed risk of vegetation withering in dry periods. The proposed model is also able to identify the optimal thickness of the growing medium, over which the survival performances can be increased only with irrigation. Model performances were tested by the application to two case studies in Italy. Comparison between the probabilities and the cumulative frequencies from a continuous simulation of water content in the growing medium shows a good agreement and provide a first confirmation of reliability.