Droplet Trajectories Research Articles

Spatiotemporal distribution prediction of coughing airflow at mouth based on machine learning—Part II: Boundary inference using neural network

In the post-epidemic era, the trajectory of pathogenic airflows and droplets generated by coughing have been widely studied. However, owing to the limitations of measurement methods, there is a lack of detailed data on their spatiotemporal distribution at the mouth during coughing, which are the basis of research and the critical boundary conditions for computational simulation. Previous experiments have determined the velocity distribution of coughing airflow in spaces located far from the mouth. This study aims to collect detailed data at the mouth for use as the Computational Fluid Dynamics (CFD) boundary conditions from the experimental data. In Part I of this study, the critical parameters that describe the boundary conditions at the mouth for CFD simulation were obtained. Based on these parameters, this part infers the detailed temporal and spatial distribution velocity data of the coughing airflow at the mouth using a neural network. We performed CFD simulation on the prediction results with V=10.76 and M=4, and got FAC2=0.56 compared with the experimental values. The results obtained provided a generic detailed boundary condition for coughing airflow at the mouth and appropriate machine-learning parameters. This study can provide more accurate boundary conditions for simulating the propagation of pathogenic airflow and a supplementary database for epidemic prevention research.

Effect of the relative arrangement of atomizers on composite liquid fuel spraying characteristics

The paper presents the experimental research findings for the characteristics of promising composite liquid fuel spraying with two atomizers, when varying the scheme of their relative arrangement. The nozzle arrangement was shown to have a significant effect on the spraying characteristics (droplet sizes and velocities), the volume and rate of the combustion chamber filling with aerosol. Following the experimental research, the relative efficiency indicators of spraying water and composite fuels were calculated. The research findings for the spraying of water and composite liquid fuels based on coal and coal processing waste were compared. It was established that in the interaction of sprayed liquid jets, it is possible to control the average size and, thus, number of secondary fragments by intensifying their transformation during coalescence and disruption. A mathematical processing of the experimental results was performed using a set of dimensionless criteria introduced. Schemes were shown that make it possible to provide minimum sizes of slurry droplets and maximum relative efficiency factor of spraying. • One- and two-atomizer spraying characteristics of fuels were compared. • Droplet number, sizes, velocities and trajectories were determined. • Fuel droplet sizes can be changed by the liquid jet interaction with air. • Maximum efficiency factor of fuel spraying was determined. • Spraying schemes for more complete filling of the chamber with fuel aerosol were proposed.

Similarity Analysis of Droplet Evaporation Trajectory in High-Temperature Gas Flow

In this study, a similarity analysis of droplet evaporation trajectory in high-temperature air crossflow based on numerical calculations was conducted. The procedure used for modeling the evaporation of a water droplet involved the use of a low boiling point liquid droplet. The trajectories of water and acetone droplets over an extensive temperature range of 695–1194 K were investigated, and the relationships for the corresponding parameters of the two different droplets, as well as the scale effect, were discussed. The results identify the existence of a comparative trajectory for different kinds of droplets, as well as the main dimensionless numbers governing the trajectory, by determining the Reynolds, Stokes, and Nusselt numbers. For each given gas flow Reynolds number, the droplet evaporation trajectory has a unique correspondence with the airflow temperature. Given this, a method of calculation to determine the modeling parameters of the droplet and crossflow was proposed. The applicability of the similarity modeling method of a single droplet to the spray droplets was examined, from which the spray structure in high-temperature crossflow was obtained.

Numerical simulation study on the structure optimization of liquid separation device in supersonic separator

ABSTRACT The effect of the supersonic separator on the condensation characteristics and fluid separation efficiency is not clear, and the structure of the liquid separation device still has room for optimization. Therefore, based on the rear-swirl type supersonic separator, three kinds of drainage ports and three kinds of liquid collection tank structures were proposed. The supersonic flow and separation process was studied numerically under nine different combination structures. The influence of the drainage port’s length and width and the inclination of the liquid collection tank on the liquefaction and separation process were analyzed. The separation trajectory of condensation droplets and the influence of particle size on the separation efficiency were clarified. The results show that the liquid separation device structure of the Em-Attached wall type can take condensation and separation efficiency into account, and its separation effect is the best. The optimal size of the key components was obtained: the length of the drainage port is 150 mm, the width of the drainage port is 3 mm, and the inclination of the liquid collection tank is 14 °. The particle size of the condensation droplets is in the range of 2 μm to 6 μm.

Conditional Generative Adversarial Networks for modelling fuel sprays

In this study, the probabilistic, data driven nature of the generative adversarial neural networks (GANs) was utilized to conduct virtual spray simulations for conditions relevant to aero engine combustors. The model consists of two sub-modules: (i) an autoencoder converting the variable length droplet trajectories into fixed length, lower dimensional representations and (ii) a Wasserstein GAN that learns to mimic the latent representations of the evaporating droplets along their lifetime. The GAN module was also conditioned with the injection location and the diameters of the droplets to increase the generalizability of the whole framework. The training data was provided from highly resolved 3D, transient Eulerian–Lagrangian, large eddy simulations conducted with OpenFOAM. Neural network models were created and trained within the open source machine learning framework of PyTorch. Predictive capabilities of the proposed method was discussed with respect to spray statistics and evaporation dynamics. Results show that conditioned GAN models offer a great potential as low order model approximations with high computational efficiency. Nonetheless, the capabilities of the autoencoder module to preserve local dependencies should be improved to realize this potential. For the current case study, the custom model architecture was capable of conducting the simulation in the order of seconds after a day of training, which had taken one week on HPC with the conventional CFD approach for the same number of droplets (200,000 trajectories).

Experimental and numerical study on the length of LZVV in the bubble separator for molten salt reactor

The bubble separator using centrifugal technology is the key device to remove the fission gas from the molten salt in the molten salt reactor. The Locus of Zero Vertical Velocity (LZVV) is an important characteristic of swirling flow. In this paper, a new measuring method is introduced to obtain the length of the LZVV of the bubble separator by tracking the droplet trajectories and analyzing the particular droplets’ location at the single-phase condition. The effects of the flowrate, split ratio and the flowrate distribution of the two Light Phase Outlets (defined as LPO ratio) on the length of LZVV are studied. Moreover, numerical simulation at single-phase condition is carried out using the Reynolds stress model and it shows a good agreement with the experimental result. The experimental and numerical results show that the velocity distribution does not change with the flowrate and the larger split ratio leads to a longer length of LZVV and stronger swirl strength.

Scaling formulation of multiphase flow and droplet trajectories with rime ice accretion on a rotating wind turbine blade

In this paper, scaling and similitude are investigated for ice accretion on a rotating wind turbine blade. Numerical CFD icing simulations are performed using FENSAP ICE software to test the scaling methods and verify the results. A small blade section is scaled six times, and each case is tested at specific flow conditions. Scaled conditions for velocity (streamwise and rotational), droplet size, and icing time are examined. CFD solutions for the flow field (air and droplet) are obtained in terms of the velocity, droplet trajectories, pressure coefficient distributions, ice thickness, and ice shapes, by quantifying the significant parameters involved in the icing process. Recommendations for parameters to be used for rime ice scaling on a rotating blade are presented, and numerical results are reported to support those recommendations. Numerical results and test conditions are obtained at sea level in wind tunnel facilities for experimental investigation. This paper presents the formulation of non-dimensionlized governing equations for scaling of the flow field, droplet trajectories, and ice accretion on a rotating blade model. It derives a set of non-dimensional groups which govern the flow parameters for a rotating blade, including the Strouhal number, Froude number, drag coefficient of droplets and droplet based Reynolds number, droplet inertia parameter, and two other new parameters. The scaling methodology can also be utilized to determine alternative test conditions to predict rime ice conditions on a rotating blade. The study provides valuable insight to predict ice accretion on large wind turbine blade sections in the field based on scaled smaller blade sections tested in a laboratory setting.

Numerical Simulation of Transmission LineIcing Based on Computational Fluid Dynamics

In recent years, transmission line icing disasters occur frequently, which poses a great threat to the operation of power grid. Most of the existing studies do not consider the impact of time-varying meteorological conditions on conductor icing. In order to effectively predict the growth of line icing, it provides an important reference for transmission line deicing and galloping early warning. The wind tunnel model of the transmission line is established by using the computational fluid dynamics simulation software, and the air flow field distribution around the transmission line is calculated by using the hybrid grid division. By establishing the motion equation of cooling water droplets in the air and analyzing the motion trajectory of water droplets, the local collision coefficient between water droplets and the transmission line is obtained, and the icing surface of the transmission line is discretized into a control body, Through the establishment of thermodynamic model, the mass balance and energy balance equations are established to calculate the icing quality of transmission line. According to the environmental temperature, wind speed, transmission line diameter and other factors, the quantitative influence degree of different factors on the icing form of transmission line is output, so as to provide technical support for the safe and stable operation of important transmission channels.

Influence of the ventilation strategy on the respiratory droplets dispersion inside a coach bus: CFD approach

The airborne transmission of the COVID-19 virus was considered the main cause of infection. The increasing concern about the virus spread in confined spaces, characterized by high crowding indexes and an often-inadequate air exchange system, pushes the scientific community to the design of many studies aimed at improving indoor air quality. The risk of transmission depends on several factors such as droplet properties, virus characteristics, and indoor airflow patterns. The main transmission route of the SARS-CoV-2 virus to humans is the respiratory route through small (<100 μm) and large droplets. In an indoor environment, the air exchange plays a fundamental role on the dispersion of the droplets. In this study, an integrated approach was developed to evaluate the influence of the ventilation strategy on the dispersion of respiratory droplets emitted inside a coach bus. There are no specific guidelines and standards on the air exchange rate (AER) values to be respected in indoor environments such as coach buses. The aim of this work is to analyse the influence of ventilation strategy on the respiratory droplet concentration and distribution emitted in a coach bus. Ansys FLUENT was used to numerically solve the well-known transient Navier-Stokes equations (URANS equations), the energy equation and using the Lagrangian Discrete Phase Model (DPM) approach to construct the droplet trajectories. The geometry is representative of an intercity bus, a vehicle constructed exclusively for the carriage of seated passengers. The 3D CAD model represented a coach bus with an HVAC system, within which an infected subject was present. The positions of exhaust vents and air-conditioning vents were chosen to ensure complete air circulation throughout the bus. The infected subject emitted droplets with a well-defined size distribution and mass through the mouth. The air exchange is provided in two different ways: general ventilation (from air intakes positioned along the bus windows and top side of central corridor) and personal ventilation (with air intakes for each passenger). For the general ventilation a single AER value was set (0.3 m3 s−1). The first results obtained showed a slight particle dispersion in the computational domain due to the airflow rate entered through the HVAC system, but a still elevated level of particle concentration tended to accumulate on the area near to infected subject. Additional analysis was executed to evaluate the beneficial effects linked to further addition of airflow through personal air-conditioning vents placed above every passenger’s head. The results show the importance of the use of the ventilation system inside a coach bus, highlighting how the contribution linked to of the personal air exchange rate can lead to a significant reduction of droplet concentration exposure and consequently a reduction of the risk of infection from airborne diseases.

Numerical Research of an Ice Accretion Delay Method by the Bio-Inspired Leading Edge

The accumulation of ice on aircraft is a typical meteorological issue. The ice accretion on the wing’s leading edge can cause an earlier stall and significantly increase the safety risks. Because the equivalent shape of the wing will change based on the ice pattern on the leading edge, it is crucial to predict the ice pattern of the aircraft and design the anti-icing device. The ice accretion is predicted in the present work through a multi-shot approach. In the current study, a bio-inspired leading edge that can generate multiple pairs of counter-rotating vortices is used to alter the trajectory of the water droplets. This results in a lowering of the ratio of droplet attachment on the leading edge, hence and the ice accretion time, which is an indication of hazardous flight conditions, can be delayed. As a result, the spanwise continuous ice transforms into the discontinuous ice. Meanwhile, the Procrustes analysis provides a result for the thickness of the ice pattern on the wing model based on a variety of parameters for the leading edge.

Characterization of the aerosol produced from an aerated jet

Faucet aerators that form aerated water jets generate aerosols, which can constitute a risk of infection if the water is contaminated, particularly for vulnerable individuals near the sink. In this study, we characterize the size and trajectory of water droplets produced from an aerated jet. The detected particle diameter ranged from 3 to 150μm. The concentration of droplets in the air varied from near-zero to a maximum of 2×1011particles/m3, depending on the location relative to the jet. We found four relevant categories of droplets based on their trajectories following their emission at the jet’s free surface: particles with inertia high enough to escape the immediate vicinity of the jet (category 1), particles moving towards the jet (category 2), particles drawn into the aerator, which only included particles with a diameter smaller than 50μm (category 3), and particles with a near-vertical trajectory (category 4). Tracing category 1 particles to their generation location on the water interface shows a higher emission rate near the aerator. Finally, we employ a numerical model to compute the subsequent trajectories of droplets detected at the limits of the sampled domain. We find that particles whose diameter is smaller than 55μm completely dry and become airborne. Larger droplets deposit within a radius of 7cm around the jet, assuming a surface is located 20cm below the aerator tip. These results increase the fundamental understanding of the emission mechanisms of droplets in aerated jets and their fate in the sink environment.

Deep-learning based analysis of metal-transfer images in GMAW process

Gas metal arc welding (GMAW) is a widely used metal-joining method in industrial manufacturing. In this method, the metal-transfer process plays an important role in determining welding quality. It is common industrial practice to optimize GMAW waveforms and process parameters using high-speed videography. The assessment of videos is currently done by subjective human interpretation, which is time consuming and prone to errors. Computer vision techniques before deep learning have not been able to overcome the confounding aspects of the images. This work uses deep learning segmentation models to isolate droplets in the video footage of the GMAW metal-transfer process. Segmentation masks are used to compute the geometric and kinematic properties of the droplet to illustrate the dynamic characterization process. The proposed deep learning model is a fully convolutional network (FCN) approach. Several architectures are considered and compared here. The main result shows that the FCN-based approach can reliably segment droplets within an image with the benefit of processing thousands of images within minutes. The image features isolated in this work allow for the calculation of valuable process variables such as droplet trajectory, velocity, acceleration, and detachment frequency, which agree with those found in the literature.

Influence of jumping-droplet condensation on the properties of separated flow in an air-cooled condenser tube: An Euler-Lagrange approach

An Air-cooled condenser (ACC), which finds popularity in a steam power plant in arid areas, is usually less efficient as film condensation occurs inside the condenser tube. Recent research is directed towards eliminating the thermally insulating liquid film with the application of novel superhydrophobic surfaces. The self-cleaning property of such surfaces facilitates easy condensate drainage in the form of jumping droplets exposing favourable nucleation sites, thereby significantly promoting dropwise condensation. The present study explores the characteristics of jumping droplet condensation in finite condenser tubes using computational fluid dynamics (CFD). The wall-heat-flux for condensation is modelled here by a uniform suction boundary condition. The strength of suction is quantified by a suction Reynolds number Re s. We mainly focus on the zone corresponding to 2.3 &lt; Re s &lt; 10, where no previous solution exists. In a long horizontal tube, the progressive realization of a self-similar region starting from the developing regions is demonstrated. We examine the characteristics of the developing region based on the sign of the pressure gradient. The results of three-dimensional CFD simulations illustrate the variations of droplet trajectories with the inception size and coordinates of jumping droplets determined locally by the relative contributions of various force components, viz. gravity, axial drag in the vapour core, suction induced radial drag and Saffman lift. The present study also predicts the effect of pipeline inclination on condensate drainage. Ultimately, considering multiple jumps, we found that the maximum condensate emission can be obtained for small droplets (5−15 μm), while medium-sized droplets (20−50 μm) are most advantageous for isolated jumps.

Tandem droplet locomotion in a uniform electric field

An isolated charge-neutral droplet in a uniform electric field experiences no net force. However, a droplet pair can move in response to field-induced dipolar and hydrodynamic interactions. If the droplets are identical, the centre of mass of the pair remains fixed. Here, we show that if the droplets have different properties, the pair experiences a net motion due to non-reciprocal electrohydrodynamic interactions. We analyse the three-dimensional droplet trajectories using asymptotic theory, assuming spherical droplets and large separations, and numerical simulations based on a boundary integral method. The dynamics can be quite intricate depending on the initial orientation of the droplets line-of-centres relative to the applied field direction. Drops tend to migrate towards a configuration with line-of-centres either parallel or perpendicular to the applied field direction, while either coming into contact or indefinitely separating. We elucidate the conditions under which these different interaction scenarios take place. Intriguingly, we find that in some cases droplets can form a pair (tandem) that translates either parallel or perpendicular to the applied field direction.

Electrostatic fields for the control of evaporating charged fuel sprays

The current socio-economic shift towards electrification of the transport sector and development of hybrid thermal–electric propulsion systems provides new opportunities for the development of ‘clean’ aviation technologies. In this work, the use of electrostatic fields to control the location of electrically charged fuel droplets is proposed as a novel technology to enhance pre-evaporation of liquid sprays in confined spaces. An electrospray in cross-flow is numerically investigated using large-eddy simulations for a range of flow and droplet conditions in order to study the feasibility of the approach. A deterministic model is further introduced to compute the trajectory of single droplets in a steady cross-flow. This enables a separation of the effects of turbulence, droplet repulsion and evaporation through comparison with data obtained from the large-eddy simulations, and at the same time provides a cheap computational tool to explore a wider range of operating conditions. It is shown that external electrostatic fields below the breakdown threshold of air can significantly change the trajectory of charged droplets at moderate flow velocities. Moreover, electrostatic forces acting in the opposite direction of the mean cross-flow can potentially be used to stabilise the spray position within a confined region, hence allowing for an increase of the residence time available for full evaporation. The application and modulation of such electrostatic forces is envisioned as a new paradigm to achieve ‘targeted evaporation’ in next-generation hybrid thermal–electric aero-engines and to improve the fuel-oxidiser mixing quality. The electrical power associated with the external electrostatic field to achieve droplet stabilisation is negligible compared to the thermal power released by complete combustion of the injected fuel. In addition, it is shown that stabilisation of the droplets enhances the evaporation rate (by more than 30%) and mixing quality due to an increase of the relative velocity between the droplets and the gas flow, as well as the turbulence induced by the stagnating spray cloud. The results of this work offer new insights for the development of advanced fuel injection strategies based on electrohydrodynamics.

Mathematical and computer simulation of particle redistribution and inertial swarming in dispersed systems

&lt;span&gt;This work is devoted to modeling the motion of a dispersed phase in a gas flow in two cases: i) the dynamic characteristics of the gas flow (velocity components, stream functions and vorticity, ii) are found by numerically solving the Navier-Stokes equations with the subsequent construction of the droplet trajectory; study of the coalescence of droplets in a given model gas flow with analysis of the swarming of dispersed particles. In the first approach, the size and location of vortex and stagnant zones in a channel with a streamlined plate and with a turning flow are determined by numerically solving the equations of gas dynamics in Helmholtz variables. To study the coalescence of drops that occurs when the trajectories of two or more drops come into contact, a second approach has been developed-a computer model of inertial swarming in dispersed systems using the Delphi10 programming environment based on stochastic lattice algorithms and computable lattice enlargement. This approach is needed to pre-define the media flow rate profile. Also in this article, were analyzed the grouping and redistribution of particles for the manifestation of the "swarm of drops" effect.&lt;/span&gt;

Experimentally validated high-fidelity simulations of a liquid jet in supersonic crossflow

Utilizing recent advancements in computational schemes for compressible, multiphase flows , this work features a parametric study of a pure liquid jet in supersonic crossflow that involves simulating the atomization process for four values of momentum-flux ratio. These simulations are validated against experimental results measured with high-speed X-ray imaging, which confirm the accuracy of the numerical approach. Also, the effect of numerical resolution on some flow behavior is investigated, revealing convergence of the jet shape and surface instability wavelength. Analysis of the resulting sprays includes statistical descriptions of the liquid distribution, liquid structures created through breakup, interfacial instabilities, and dominant flow features. As the flowrate increases, the spray penetrates further, it becomes more disperse, and less liquid impacts the wall, but the droplet size distribution changes little. The wavelength of instabilities on the windward side of the jet diverges from measured trends in subsonic crossflows. In a visualization of the time-averaged flow, counter-rotating vortices are observed along the jet core and in the wake, affecting the process of primary atomization and early droplet trajectories. • Large Eddy Simulations of liquid jet in supersonic crossflow at high Weber number. • Quantitative validation against experimental results using equivalent path length. • Shear-driven instability wavelengths differ from subsonic crossflow analogy. • Analysis of spray statistics and mean flow features.

Electric-Field-Assisted Ultrasonic Spray Pyrolysis of Solid State Electrolytes

Solid polymer electrolytes (SPEs) hold the potential to revolutionize energy storage by enabling secondary batteries containing lithium metal anodes, which have ultra-high theoretical capacity of 3860 mAh g−1, compared to a typical graphite anode’s theoretical capacity of 372 mAh g−1. The bulk of SPE research focuses on optimizing SPE chemistry, while the manufacturing process receives little attention. In this interdisciplinary study, as shown in Figure 1, an electric-field-assisted ultrasonic spray pyrolysis (EFAUSP) setup is proposed and integrated with a cyclone-separator, which was simulated ANSYS Fluent, then built based on the results. Digital in-line holography is performed to determine the droplet size distribution and investigate the droplet trajectories near the substrate surface. This newly proposed EFAUSP setup is based on a 3D printer controller, promising macrostructure and microstructure control in future work. Future work will build upon these results to investigate the mechanical and electrochemical properties of the deposited SPEs. Figure 1

Velocity and size quantification of drops in single and collective bursting bubbles experiments

Droplet production from bursting bubbles has been extensively studied for single bubbles but remains sparsely quantified in controlled collective settings. This article explores how the trajectories of droplets produced by interacting bursting bubbles can be used to track back the mode of production, for clean and contaminated water. Noticeably, it compares velocity-size relationships, reviewed for jet and film drops from individual bubbles, with the measurements made in collective experiments.

Investigation of bypass-coupled double-pulsed directed energy deposition of Al–Mg alloys

Wire and arc additive manufacturing (WAAM) is a promising directed energy deposition (DED) technique for Al–Mg alloy parts. However, excessive workpiece heat input may limit its high productivity application. Hence, a bypass-coupled double-pulsed WAAM (BCDP-WAAM) method is developed to control the workpiece heat input without deteriorating productivity. High-speed camera images demonstrated that the arc and droplet trajectory were deflected towards the molten pool side in BCDP-WAAM. The observed strong-to-weak change in the arc necessitated a periodic insertion of the bypass-cathode wire into the molten pool for heat absorption. Spray and bridging transfers occurred in the anode and bypass-cathode wires, respectively. Current–voltage waveforms indicated that a dynamic current distribution between the workpiece and bypass-cathode wire was achieved in BCDP-WAAM, decreasing the workpiece current. Consequently, the workpiece heat input was decreased by 38.2 %. Owing to the twin-melted wires, the height and effective area were increased by 134.1 % and 149.0 % in BCDP-WAAM, respectively. The obtained electron back-scattered diffraction (EBSD) results indicated that the average grain size was reduced to 21 µm in BCDP-WAAM compared to a grain size of 36 µm in double-pulsed WAAM. This is attributed to the reduction in the workpiece heat input and the change in solidification induced by the bypass-cathode wire. The BCDP-WAAM enhanced the mechanical strength of the part. Furthermore, the ultimate tensile strength (UTS) and yield strength (YS) of the optimized part increased to 256 and 124 MPa in BCDP-WAAM, representing increments of 7.1 % and 14.8 %, respectively. The average micro-hardness was enhanced to 88.2 HV, representing an increase of 16.1 %. • The arc and droplet trajectory were deflected towards the molten pool side. • Bypass-cathode wire periodically inserted into the molten pool for absorbing heat. • A dynamic current distribution between workpiece and bypass-cathode wire occurred. • Reduced workpiece heat input and improved productivity simultaneously occurred.