Droplet Concentration Distribution Research Articles

A rapid method for prediction of airborne disease infection risks in an intercity bus

In an intercity bus, respiratory infectious diseases put passengers at high risk of getting infected by the droplets exhaled by an infected person, and the risk increases when exposed to more droplets. Here, to quickly determine the concentration distribution of droplets, to predict the infection risks in a closed space, and to enhance the reliability of the conventional steady-state particle tracking method for predicting the trajectory of droplets released by coughing or sneezing, an improved steady-state particle tracking method is proposed. In it, the momentum of released droplets previously ignored in the conventional steady-state particle tracking method was specifically incorporated using experimental data. Then, the improved method was combined with a random walk model and applied to investigate all possible trajectories of droplets released by different passengers inside a bus. Consequently, the concentration distribution of droplets was obtained from the trajectory information. Finally, the Wells–Riley equation was used to predict the infection risk of every passenger based on the evaluated number of droplets inhaled per passenger. The results show that the improved steady-state tracking method performs more accurately at predicting the concentration field of droplets and associated infection risk than the conventional steady-state particle tracking method. Furthermore, the relative cost of the improved steady-state tracking method is just 1% of the transient calculation method currently considered the most accurate.

Analysis of the spread of cough droplets and body deposition fraction in the smart classroom in different seasons

Smart classrooms are a relatively confined public space for college students. SARS-COV-2 and other respiratory viruses have been shown to pose a more significant threat to human health in relatively confined spaces. Using numerical simulation method to simulate the transmission and concentration distribution of virus-carrying droplets in smart classrooms in three different seasons (summer, winter, transitional seasons: spring and autumn). The Realizable k-ε model is used to simulate the airflow pattern in the smart classroom, and the Lagrangian method is used to simulate the transmission of droplets. The transmission process of droplets produced from the teacher standing on the platform and the student sitting on the seat is studied. The influence of three kinds of outdoor temperature on droplet transmission and the body deposition fraction of people in the smart classroom is analyzed. The results show that droplet transmission speed is maximum at the temperature of 5 degrees when the outdoor temperature is 5 °C, 20 °C, and 35 °C respectively. At 10 s, the transmission distance of droplets increases by 9.55% compared with that at 20 °C and 10.31% compared with that at 35 °C. In addition, the body deposition fraction is also affected by the location of the vent, with downwind contact being 6 times more likely than upwind contact. The research results can provide suggestions and measures for epidemic prevention and control in smart classrooms.

Characterization and size distribution of initial droplet concentration discharged from human breathing and speaking

A study of the aerodynamic characteristics of aerosol expiration is important for understanding the size distribution of droplet concentration in COVID-19 disease transmission. This study measured the initial concentration and size distribution of droplets released during four breathing processes: nose breathing, mouth breathing, reading alphabets and counting numbers. Influencing factors on droplet generation were studied by statistical analyses. Significant differences in droplet concentration among the four breathing activities in pairs were found ( p < 0.001), except for reading alphabets and numbers ( p = 1). The droplet concentration during nose breathing ( p < 0.05), but not mouth breathing ( p = 0.136), of male subjects, was found significantly higher than that of female subjects. The droplet concentration generated by speaking during letters reading with special phonemes, for example, /i:/, was significantly ( p < 0.05) higher than reading letters without phonemes. The discharged droplet sizes from four breathing activities were dominated by small droplets ( dp < 1.037 μm of over 50% and dp < 2.642 μm of over 80%). Basically, no particles larger than 8 μm were detected by the aerodynamic particle sizer. The inference indicates a possible aerosol transmission of disease during non-symptomatic aerosol-producing activities such as breathing or speaking and may elucidate the disease transmission pathway of COVID-19.

Experimental study on atomization characteristics of high-energy-density fuels using a fuel slinger

High-energy-density fuels are promising fuels for advanced aero engines, especially rotary engines, in which atomization characteristics play a crucial role in fuel practical application. This work experimentally investigated the basic spray characteristics mainly including spray angle, droplet concentration, velocity, and size distribution of two liquid fuels JP-10, HEF-1 and one slurry fuel HEF-2 by PDPA technique using a fuel slinger under 10000–25000 rpm. Effects of fuel physical properties (fuel type, density, viscosity, surface tension, etc) and control parameters (rotational speed and fuel flowrate) on atomization characteristics were discussed. The results showed that the droplet number was parabolic distribution for each fuel and significantly increased with the elevated rotational speeds. Slurry fuel had the largest droplet number near the spray center due to the fragmentation effect of nano-Al particles. For liquid fuels, viscosity determined the droplet breakup below 10000 rpm. Spray angle and droplet velocity negatively related to fuel viscosity. Besides, elevated rotational speeds promoted average droplet velocities, while lowered spray angles. Overall, Sauter mean diameter (SMD) of each fuel greatly depended on fuel viscosity was arch bridge-type distribution. Slurry fuel possessed a smoother distribution with much larger SMD (55–65 μm) than liquid fuels (20–45 μm). Furthermore, the average SMD (ASMD) of JP-10 and HEF-1 gradually decreased from 58 μm to 40 μm, and 44 μm–35 μm, respectively as rotational speeds increased. Additionally, they increased by less than 20% when flowrates increased from 0.15 Lpm to 1.20 Lpm. Nevertheless, slurry fuel HEF-2 had an ASMD around 65 μm which remained almost unchanged with varying rotational speeds and flowrates.

Spatial Distribution of Exhalation Droplets in the Bus in Different Seasons

ABSTRACT In closed buses, the spread of droplets with viruses/bacteria may cause the spread of respiratory infectious diseases. Discrete phase modeling is used to simulate the diffusion characteristics and concentration distribution of droplets at different temperatures and different exhalation positions by ANSYS FLUENT software. The integral concentration of droplets at different locations can be quantified, which leads to identification of low-risk areas and high-risk areas in the bus. Results show that a higher outdoor temperature leads to lower droplets’ diffusion speed and longer time until the droplets reach the driver. In addition, based on the integral concentration of droplets at the seats, regardless of whether a passenger exhales droplets in the front row of the bus, the position of the rear door or the last row of the bus, the seats in the last row of the bus away from the door belong to the low-risk area. In contrast, the seats near the door and the middle seat in the bus are higher risk areas. Consequently, this study proposed sitting on a seat in the low-risk area as a means to reduce the risk of passengers. Moreover, safety protection facilities around the driver should be modified to improve the isolation of the upper area of the driver’s location, so as to effectively prevent the droplet diffusion towards the driver, thereby effectively reducing the driver’s risk of infection.

A First Look at Cloud Inhomogeneity and Its Effect on Lightning Optical Emission

AbstractOptical emission observed from lightning is the result of in‐cloud scattering, lightning channel geometry, and optical energy of the source discharge. To better understand how scattering affects measured optical emission, we developed a Monte Carlo model using an inhomogeneous scattering profile from a simulated storm using two‐moment bulk microphysics in the Weather Research and Forecasting model, including the concentration and size distribution of cloud droplets and ice particles. Within the Monte Carlo model lightning discharges are simulated as point and line sources. These idealized simulations indicate that for a lightning discharge at 7 km altitude approximately 22% and 13% of light, emitted near and away (respectively) from a reflectivity core will reach cloud top in a thunderstorm. The vertical gradient of the microphysical environment impacts light reaching cloud top regardless of overall particle concentration. Increasing cloud ice modifies the gradient of the microphysical profile and modulates the profile gradient effect.

Evaporation of a sessile droplet on a slope

We theoretically examine the drying of a stationary liquid droplet on an inclined surface. Both analytical and numerical approaches are considered, while assuming that the evaporation results from the purely diffusive transport of liquid vapor and that the contact line is a pinned circle. For the purposes of the analytical calculations, we suppose that the effect of gravity relative to the surface tension is weak, i.e. the Bond number (Bo) is small. Then, we express the shape of the drop and the vapor concentration field as perturbation expansions in terms of Bo. When the Bond number is zero, the droplet is unperturbed by the effect of gravity and takes the form of a spherical cap, for which the vapor concentration field is already known. Here, the Young-Laplace equation is solved analytically to calculate the first-order correction to the shape of the drop. Knowing the first-order perturbation to the drop geometry and the zeroth-order distribution of vapor concentration, we obtain the leading-order contribution of gravity to the rate of droplet evaporation by utilizing Green’s second identity. The analytical results are supplemented by numerical calculations, where the droplet shape is first determined by minimizing the Helmholtz free energy and then the evaporation rate is computed by solving Laplace’s equation for the vapor concentration field via a finite-volume method. Perhaps counter-intuitively, we find that even when the droplet deforms noticeably under the influence of gravity, the rate of evaporation remains almost unchanged, as if no gravitational effect is present. Furthermore, comparison between analytical and numerical calculations reveals that considering only the leading-order corrections to the shape of the droplet and vapor concentration distribution provides estimates that are valid well beyond their intended limit of very small Bo.

Concentration Distribution of Droplets in a Liquid‐Liquid Hydrocyclone and its Application

AbstractThe concentration distribution of droplets in liquid‐liquid hydrocyclones was studied using a phase Doppler particle analyzer. The effect of cone angle and inlet flow flux on the concentration distribution of droplets was determined. The cone angle of the hydrocyclone obviously influenced the maximum droplet concentration, especially that in the cone section. The hydrocyclone with a cone angle of 6° presented the best separation result. A higher inlet flow rate enhanced the separation of droplets from the water. The concentration profiles at five different axial heights presented a parabola law pattern in the radial direction. The optimized hydrocyclone was applied for the dewatering of vinyl chloride monomer (VCM). The amount of residual water could be greatly reduced.

Monitoring spray drift in aerial spray application based on infrared thermal imaging technology

The pesticide drift in aerial spray applications is harmful both to environment and human beings but hard to be measured. We demonstrated a new method for real-time evaluation of pesticide drift which used infrared thermal imaging technology to detect the thermal differences before and after spraying process and then measured the range and concentration distribution of droplets. The drift distribution and spray range are detected using infrared thermal imaging system combined with image processing algorithms. The experiments were carried out in both ground spray and aerial spray applications. The results showed that this method had the ability to detect the tiny thermal differences during spray application and thus to monitor the aerial drifts. Furthermore, the testing results using this method had well agreement with the water-sensitive paper method. Therefore, it is verified that infrared thermal imaging method combined with infrared image processing algorithms can be used to monitor the pesticide drift in aerial spray application with advantages of fast, non-contact and able to realize real-time measurement.

The construction of a hospital disease tracking and control system with a disease infection probability model.

With relatively short latency and rapid propagation, viral diseases could be transmitted through the air to medical personnel or the public during the incubation period. To reduce the possibilities of spread, this research creates an infection probability model based on the settling velocity and concentration distribution of infectious droplets. Then, radio frequency identification (RFID) technology is employed to track the travel history (time, date and place) of the infected patients. A tree structure algorithm and an infection probability model are applied to trace the transmission routes, discover the correlations between carriers and suspected cases, and finally calculate the infection probability on the basis of time interval. In case of an epidemic outbreak or once an infected case is confirmed, the disease tracking and control system could be initiated by accessing RFID logs to plot the carriers’ time of onset and to trace possible routes of transmission via tree diagrams. The disease tracking and control system developed in this research can assist hospitals in assessing the risk of infection among medical personnel, as well as in prompt implementation of infection prevention and control measures, in order to reduce hospital acquired infections and provide a safe health care setting.

Numerical simulation of macro- and micro-structures of intense convective clouds with a spectral bin microphysics model

By use of a three-dimensional compressible non-hydrostatic convective cloud model with detailed microphysics featuring spectral bins of cloud condensation nuclei (CCN), liquid droplets, ice crystals, snow and graupel particles, the spatial and temporal distributions of hydrometeors in a supercell observed by the (Severe Thunderstorm Electrification and Precipitation Study) STEPS triple-radar network are simulated and analyzed. The bin model is also employed to study the effect of CCN concentration on the evolution characteristics of the supercell. It is found that the CCN concentration not only affects the concentration and spectral distribution of water droplets, but also influences the characteristics of ice crystals and graupel particles. With a larger number of CCN, more water droplets and ice crystals are produced and the growth of graupel is restrained. With a small quantity of CCN the production of large size water droplets are promoted by initially small concentrations of water droplets and ice crystals, leading to earlier formation of small size graupel and restraining the recycling growth of graupel, and thus inhibiting the formation of large size graupel (or small size hail). It can be concluded that both the macroscopic airflow and microphysical processes influence the formation and growth of large size graupel (or small size hail). In regions with heavy pollution, a high concentration of CCN may restrain the formation of graupel and hail, and in extremely clean regions, excessively low concentrations of CCN may also limit the formation of large size graupel (hail).

A constitutive equation for droplet distribution in unidirectional flows of dilute emulsions for low capillary numbers

The concentration distribution of droplets in the unidirectional flow of an emulsion for small capillary numbers (Ca) can be written as a balance between the drift flux arising from droplet deformation and the flux due to shear induced migration. The droplet drift flux is modeled using the O(Ca) theoretical results of Chan and Leal [J. Fluid Mech. 92, 131 (1979)], while the flux due to shear-induced migration is modeled using the suspension balance approach of Nott and Brady [J. Fluid Mech. 275, 157 (1994)], whereby particle migration is ascribed to normal stress gradients in the flowing dilute emulsion. In the limit of vanishingly small capillary numbers, the leading order contribution of the normal stresses in dilute emulsions arises from droplet-droplet interaction and thus scales as ϕ2τ, where ϕ is the droplet volume fraction and τ is the local shear stress. In our model, the normal stress calculations of Zinchenko [Prikl. Mat. Mekh. 47, 56 (1984)] are connected to our gradient diffusivity data computed from droplet trajectories [M. Loewenberg and E. J. Hinch, J. Fluid Mech. 338, 299 (1997)] via a reduced droplet mobility to derive the droplet flux due to shear-induced migration. As an example, the model is applied to the tube Poiseuille flow of a dilute emulsion at small Ca. It is demonstrated that the unsteady concentration distribution of droplets resulting from arbitrary time-dependent average velocity obeys a self-similar solution, provided the thickness of the droplet-depleted region near the walls is always nonzero.

Experimental Investigation of Exhaled Aerosol Transport Under Two Ventilation Systems

Although the importance and implications of studying expiratory droplet concentration distribution in indoor environments are obvious, experimental measurements are very scarce and incomplete. In the work described herein, spatial and temporal aerosol concentrations generated by a modeled expiratory process were studied experimentally. Two heated manikins were placed inside a chamber with either a displacement ventilation system or a ceiling supply and ceiling return ventilation system. One of the manikins emitted 0.05 micrometer monodisperse polystyrene microspheres mimicking the generation of expiratory droplets. Flow characteristics were measured by a hot-film anemometry system before and after the momentary emission of aerosols at three locations. The temporal concentration profiles at eight locations were also measured. The results show that a high-speed air jet easily penetrates the boundary layer of the manikin regardless of the ventilation system used. Transient ventilation effectiveness was also ...

CFD modeling of droplet dispersion in a Venturi scrubber

A modified two-dimensional mathematical model was developed to simulate droplet dispersion through a cylindrical Venturi scrubber based on Eulerian approach. Droplet concentration distribution was evaluated using the equation of mass balance of droplets. The velocity field of gas flow was determined using k– ɛ turbulence model. Droplet velocity distribution was calculated by means of the equations derived using momentum balance of droplets. Gas eddy diffusivity which has already been determined based on constant Peclet number in the previous models was calculated using turbulent characteristic velocity and length. Fathikalajahi et al. approach was applied to calculate eddy diffusivity of droplets (Fathikalajahi et al., 1995 [7]). The mean diameter of generated droplets was determined by Boll's correlation (Boll et al., 1974 [16]). Rosin–Rammler distribution function was used to take into account the distribution of droplet size. In order to verify the results of the new model a set of experiments was performed on a pilot-scale cylindrical Venturi scrubber with axial liquid injection. During these experiments, the flow rates of liquid droplets were measured at several points of throat section end from center line to the wall. The comparison between the results and experimental data showed that the droplet concentration distribution predicted using the present model were in better agreement with experimental data than that predicted using the previous models which were based on constant Peclet number. Also it was concluded the distribution parameter of Rosin–Rammler function, n RR , could not be considered as a constant parameter and it depended on the L/ G (gas to liquid flow rate ratio) and V g0 (gas throat velocity).

SIMULATION OF SO2 ABSORPTION IN A VENTURI SCRUBBER

In this study, a three-dimensional mathematical model, based on a nonuniform droplet concentration distribution, has been developed to simulate gas absorption in a venturi scrubber. The mass transfer process was illustrated by assuming the liquid phase as a combination of droplets and film. The flow, just as the annular two-phase flow, includes a flow of liquid film layer on the walls and a flow of gas and liquid drops in the core. Peclet number was determined using experimental data reported by Viswanathan et al. (1984) for distribution of droplets across the cross section of the scrubber. The mathematical model for gas absorption was justified by comparing the theoretical predictions with the experimental data reported by Johnstone et al. (1954) and the Office of Air Programs, U.S. Environmental Protection Agency (Wen and Fan, 1975), for SO2 removal by alkaline solution and H2O.

Investigation of velocity field and oil distribution in an oil–water hydrocyclone using a particle dynamics analyzer

A non-intrusive particle dynamics analyzer (PDA) was used to investigate the distribution of velocity field and oil droplets in a 50 mm oil–water separation hydrocyclone. Axi-asymmetry distribution of the flow field was observed in the hydrocyclone, which was perhaps attributed to the use of single tangential inlet in the hydrocyclone. The average size of oil droplets decreased from 43–126 μm at Z = 0 mm (the top) to 19–21 μm at Z = 990 mm (the bottom) at an inlet flow rate of 2000 L/h and flow rejection ratio of 20%. The average size of oil droplets at Z = 990 mm decreased from approximately 40–20 μm with the increase of flow rate from 1300 to 2000 L/h, showing the important impacts of the inlet flow rate on the removal of small oil droplets by the hydrocyclone. The concentration distribution of oil droplets in the upper cylindrical sections was characterized by the existence of a “concentration valley” surrounding the oil core. The “concentration valley” disappeared from Z = 590 mm. Results show that PDA is a powerful instrument for the characterization of oil–water separation hydrocyclones because it can present the velocity field and the distribution of oil droplets in the hydrocyclone simultaneously.

A New Method of Eddy Diffusivity Calculation for Droplets of a Venturi Scrubber

AbstractAn analytical model using eddy diffusivity is applied for predicting droplet concentration distribution and liquid film formation in a Venturi scrubber. By comparing experimental data of film formation reported in literature (Viswanathan et al., 1984) and the results obtained from this model, a semi‐empirical correlation for liquid droplets eddy diffusivity is obtained. The validity of this correlation is confirmed by obtaining good agreement between theoretical and experimental data of droplet concentration distribution and film formation in a Venturi scrubber (Viswanathan, 1998; Viswanathan et al., 1984).

Simulation of a spray scrubber performance with Eulerian/Lagrangian approach in the aerosol removing process

In this study, a mathematical model has been developed to simulate the performance of a spray scrubber in an industrial ammonium nitrate plant. The model is based on the Lagrangian approach for the droplets movement and particle source in cell (PSI-CELL) model for calculating the droplet concentration distribution. Consequently, unlike former research, the emphasis is on the droplet dynamic behavior. In the current study, for approaching a realistic model, a droplet size distribution rather than average diameter, and also liquid film formation rather than uniform and constant droplet flow rate has been applied. Also, the Eulerian method has been used for the calculation of the particles removal efficiency and energy balance has been applied on the gas to estimate the droplet size distribution. In the experimental section, the concentration of particles and their size distribution in both inlet and outlet gas of the studied scrubber has been measured for the validation of the predicted particles collection efficiency. In addition, the temperature of the gas at inlet, outlet and in the middle of the tower has been measured for the confirmation of the predicted droplet size distribution in the tower. A good consistency between the model and data has been observed. After the model is validated, it is used to investigate the various variable profiles such as liquid film, total projected surface area of the droplets, velocity profile of the droplets and some of the other parameters in the spray scrubbers.

Effects of salinity and clay type on oil-mineral aggregation.

Effects of salinity and clay type on the aggregation process of crude oil and clay particles (oil-mineral aggregates--OMA) were studied in the laboratory. OMA were generated by shaking various oil/clay mixtures in water at 20 degrees C and at a pre-determined energy level. Shape, median and maximum sizes, size distribution and concentration of oil droplets forming OMA were measured using epi-fluorescence microscopy. Results showed that the median and maximum sizes and the concentration of mineral-stabilized droplets increase rapidly when salinity increases from zero to a critical aggregation salinity in the range of 1.2-3.5 ppt. The magnitude of the increase of droplet size is controlled by clay type, while the increase of droplet concentration is influenced by oil type. Size distributions of oil droplets are self-similar, but their magnitudes depend on salinity and oil type. The effect of salinity on droplet size distribution is strongly influenced by clay type. Empirical relationships are derived to calculate concentration and size distribution of mineral-stabilized droplets.

Simulation of an orifice scrubber performance based on Eulerian/Lagrangian method

A mathematical model based on Eulerian/Lagrangian method has been developed to predict particle collection efficiency from a gas stream in an orifice scrubber. This model takes into account Eulerian approach for particle dispersion, Lagrangian approach for droplet movement and particle-source-in-cell (PSI-CELL) model for calculating droplet concentration distribution. In order to compute fluid velocity profiles, the normal k− ε turbulent flow model with inclusion of body force due to drag force between fluid and droplets has been used. Experimental data of Taheri et al. [J. Air Pollut. Control Assoc. 23 (11) (1973) 963] have been used to test the results of the mathematical model. The results from the model are in good agreement with the experimental data. After validating the model the effect of operating parameters such as liquid to gas flow rate ratio, gas velocity at orifice opening, and particle diameter were obtained on the collection efficiency.