Droplet Dispersion Characteristics Research Articles

Optimization of Cabin Virus Transmission Suppression Technology Based on Hanging Curtain Physical Isolation

This study presents an innovative physical isolation measure for commercial scenarios, namely, hanging curtains, to prevent the spread of respiratory infections. Using computational fluid dynamics simulation techniques, the closed spaces within cruise cabins were modeled and numerically analyzed, focusing on the dispersion characteristics of droplets. Additionally, orthogonal methods were employed to investigate various arrangements of hanging curtains and their effects on droplet dispersion based on spatial positioning. The research findings indicated that hanging curtains can effectively alter the airflow within a space, realizing the innovative concept of localized pollutant containment. It was found that the spatial partitioning method based on the location of individuals contributes more to reducing droplet dispersion than other methods. Moreover, the sag height of curtains emerges as the most influential factor on individual infection risk, while the scheme for hanging curtain positions has the least impact. Finally, the optimal configuration recommendation is provided: a curtain bottom coordinate of Z = 2.3 m and a top coordinate of Z = 2.8 m when the infection source was positioned at the center of the space. This configuration has also been validated by varying the location of the infection source. The research findings provide valuable insights for formulating preventive measures for passengers on cruise ships and for pandemic control in similar scenarios.

Modeling of primary breakup considering turbulent nozzle flow, internal turbulence and surface instability of liquid jet using turbulence decay theory

In heat engines utilizing fuel injection, the processes of atomization and spray formation have a significant impact on the combustion process, thereby determining both efficiency and emission characteristics. Accurate prediction and control of spray formation in fuel injection systems play a key role in improving the efficiency and environmental performance of thermal engines, especially with the emergence of carbon-neutral fuels. To achieve accurate prediction of spray mixture formation, it is imperative to refine the atomization model for the liquid jet within numerical simulations. This requires a phenomenological representation of the atomization process that avoids reliance on computational constants obtained from spray experimental results. Consequently, the present study attempts to mathematically model the turbulent nozzle flow and liquid jet atomization process, leading to the development of a novel primary breakup model. The construction of the primary breakup model involves an analysis of the turbulence at the nozzle inlet. By merging this turbulence with the turbulence resulting from wall shear flow within the nozzle, the model provides insight into the internal turbulence and surface instability of the liquid jet, encompassing the turbulence spectrum. Consequently, the influence of nozzle length on the turbulent flow within the nozzle can be understood, and the droplet formation characteristics of the liquid jet can be predicted along with its multi-wavelength dispersion characteristics. The model effectively captures the experimental results in terms of breakup length and droplet dispersion characteristics, thus adding a higher level of accuracy to numerical simulations. Ultimately, the in-depth study of this model, coupled with its comparison with experimental results, enhances the understanding of the liquid jet atomization process.

Droplet dispersion characteristics during human walking in a queue

The dispersion of respiratory droplets is strongly influenced by the complex airflow induced by human activities, such as walking in a queue. Understanding the relationship between local airflow disturbances during queue walking and droplet dispersion is crucial. This study investigates the effects of following distance (1.0, 1.5, 2.0, and 2.5 m), walking speed (0.8, 1.0, 1.2, and 1.4 m/s), and droplet diameter (1, 10, 50, 80, and 120 μm) on droplet dispersion. The findings reveal that the interaction between wake vortex and forward airflow provides a foundation for cross-infection among individuals. An increased following distance leads to an initial rise and subsequent decrease in the concentration in the breathing zone of the susceptible individual. The social distances of 1.0 and 1.5 m are insufficient to mitigate the risk of cross-infection, warranting a recommended following distance of at least two meters. The effect of walking speed on droplet dispersion varies depending on the scenario. In cases involving standing and walking cycles, the infection risk of the susceptible individual gradually increases with higher walking speeds. Conversely, when individuals walk continuously in a queue, the infection risk of the susceptible individual decreases with increased walking speed. Moreover, intermediate-sized droplets play a critical role in the transmission of respiratory infectious diseases and demand heightened attention. This study expounds the intricate airflow dynamics during queue walking and emphasizes the significance of following distance, walking speed, and droplet diameter in minimizing the risk of cross-infection.

Numerical Simulation and Process Enhancement of the Hydrolysis of 2-Chlorobenzal Chloride

Hydrolysis of 2-Chlorobenzal chloride is the primary production method of 2-chlobrobenzaldehyde in industry, but the reactor technologies and reaction processes of this method are traditional and underdeveloped, in which the dichotomous leaf hydrolysis agitator is commonly used, resulting in low efficiency during the hydrolysis process. In this article, ANSYS software was utilized to enhance the hydrolytic reactors used in industrial production by simulating and analyzing the dispersion characteristics of droplets in the reactor. The particle size distribution of dispersed phase droplets and the Sauter mean diameter (D32) were used to characterize the dispersion effect, and the dispersion characteristics of the dichotomous leaf agitator and the three-bladed back-curved impeller, three-bladed propeller impeller, open turbine impeller, and crescent impeller were compared and studied. The results showed that there was a log-linear relationship between the D32 of the dispersed phase and the stirring speed in different systems; the number of tiny droplets in the three-bladed back-curved impeller system increased remarkably, and the droplets size distribution width decreased significantly, achieving mass transfer enhancement in the hydrolysis reaction system. The reliability of the simulation results was verified by the measurement experiment using the Sauter mean diameter (D32) of dispersed phase droplets. The results showed that the Sauter mean diameter (D32) variation trend is consistent with that of the simulation, and the error between them is less than 10%. A lab-scale hydrolytic reactor was designed and produced based on the scheme of an industrial hydrolytic reactor equipped with a three-bladed back-curved impeller, the effect of the hydrolytic reaction was verified, and the process conditions were strengthened under the same conditions. The results showed that, compared with the case in the reactor using a dichotomous leaf hydrolysis agitator at the same reaction temperature and catalyst conditions, the reaction time of hydrolysis completion reduced to 58.33% at the lab-scale, and to 63.89% at the industrial-scale at the stirring rate of 446 r/min, using a three-bladed back-curved impeller. These results provide a technical scheme and a basis for process strengthening for improving the production efficiency of 2-chlorobenzaldehyde production equipment in industry.

Mass transfer process intensification for SO2 absorption in a commercial-scale wet flue gas desulfurization scrubber

To intensify SO2 absorption process in wet flue gas desulfurization (WFGD) spray scrubber, a comprehensive CFD model coupled transfer process with chemical reactions is established by Eulerian‒Lagrangian method and verified in a commercial-scale device of 330 MW coal-fired power unit. The desulfurization performance is significantly affected by both the property of each droplet and the dispersion of droplet swarm, which are determined by the spray behaviors. Three dominant factors, including droplet diameter, injection direction and spray morphology, are explored to optimize the spray conditions for intensifying SO2 absorption process. Differing from the ideal plug flow, the optimal diameter of droplet in non-ideal flow is obtained by considering the synergistic effect of the uniformity of gas–slurry distribution and the dynamics of SO2 mass transfer. Compared with the single-direction spray with downward or upward injection, the dual-direction spray reaches a great balance between the competitive effects of the uniformity of gas‒slurry flow and the interfacial area for mass transfer. The influence of spray morphology on mass transfer process is essentially the difference of dispersion characteristics of droplets in the middle part of scrubber. A design scheme combining full-cone and hollow-cone nozzles is suggested for improving the overall SO2 removal efficiency.

Numerical study on turbulent dispersion of diesel sprays under ultrahigh injection pressure using large eddy simulation

ABSTRACT The modern diesel engines equipped with an advanced fuel injection system up to ultrahigh injection pressure of 300 MPa are under development at present. The impact of injection pressure on fuel spray characteristics such as spray penetration and spray cone angle has been widely concerned in previous studies. However, it is still lack of the detailed investigation on the turbulent dispersion of small droplets produced by a high-pressure injector due to the limited capability of optical diagnostic techniques to simultaneously measure the two phases. Droplet dispersion due to turbulence plays a major role in high-velocity fuel spray mixing and combustion processes involving group evaporation of droplets. Thus, the effect of different injection pressures on droplet dispersion in nonevaporating diesel sprays was investigated using the Euler-Lagrange framework with large eddy simulation (LES) implemented in an open-source code OpenFOAM. Size-dependent instantaneous radial motion and dispersion characteristics of droplets dominated by large-scale turbulent structures within a polydisperse downstream spray were analyzed in detail. Results show that increasing injection pressure can modestly promote the radial dispersion of most droplets with diameter of 2 ~ 12 µm further downstream at the late stage of injection. This could be the result of the increasing interaction time of droplets with larger turbulent structures due to the increase of spray penetration with increasing injection pressure. However, increasing injection pressure has no significant impact on the strength of droplet-turbulence interaction in terms of slip velocity between droplet and gas phases and this could be quantitatively explained by the droplet Stokes number (St) estimated in this study. The most droplets with 0.1 < St < 10 tend to move and concentrate further radially outward, leading to droplet clustering. Thus, the effect of injection pressure on droplet-turbulence interaction is much less than its influence on droplet size for high-velocity fuel sprays, indicating that the ultrahigh injection pressure has much less important impact on the spatial inhomogeneity of droplet distribution than its influence on the evaporation rate of individual droplets.

Droplet dispersion characteristics of the hollow cone sprays in crossflow

The flows in a jet entering a crossflow are unsteady and the droplet movement is of vital importance for the two-phase mixing effect. In this paper, the hollow cone spray into the crossflow is investigated experimentally by using the PIV visualization system and the image-processing techniques. The experiments are carried out inside a rectangular duct (95mm×95mm in cross-section) at the ambient temperature and pressure. Different nozzle injection angles and crossflow velocities are experimented on. The instantaneous droplet distributions and the velocity vector fields are obtained. Our results show that the flow field falls into three main domains and their effects on the movement and distribution of the droplet are varied. The coherent structure which breaks the stability of the upper counter-rotating vortex pair (CVP) structure is induced on the interface between the upper CVP and the mainstream zone. When the spray is against the crossflow the larger coherent structures are induced and impose greater influences on the mixing process. The turbulence intensity on the shear layer increases and the dispersion of the droplet is promoted. The experimental findings will benefit the understanding of the mixing mechanism of the hollow cone spray in the crossflow and the achievement of an optimum mixing.

Large-eddy simulation of evaporating spray in a coaxial combustor

Large-eddy simulation of an evaporating isopropyl alcohol spray in a coaxial combustor is performed. The Favre-averaged, variable density, low-Mach number Navier-Stokes equations are solved on unstructured grids with dynamic subgrid scale model to compute the turbulent gas-phase. The original incompressible flow algorithm for LES on unstructured grids by [Mahesh et al., J. Comp. Phys. 197 (2004) 215–240] is extended to include density variations and droplet evaporation. An efficient particle-tracking scheme on unstructured meshes is developed to compute the dispersed phase. Experimentally measured droplet size distribution and size-velocity correlation near the nozzle exit are used as the inlet conditions for the spray. The predictive capability of the LES approach on unstructured grids together with Lagrangian droplet dynamics models to capture the droplet dispersion characteristics, size distributions, and the spray evolution is examined in detail. The mean and turbulent quantities for the gas and particle phases are compared to experimental data to show good agreement. It is shown that for low evaporation rates considered in the present study, a well resolved large-eddy simulation together with simple subgrid models for droplet evaporation and motion provides good agreement of the mean and turbulent quantities for the gas and droplet phases compared to the experimental data. This work represents an important first step to assess the predictive capability of the unstructured grid LES approach applied to spray vaporization. The novelty of the results presented is that they establish a baseline fidelity in the ability to simulate complex flows on unstructured grids at conditions representative of gas-turbine combustors.

Stochastic modeling of atomizing spray in a complex swirl injector using large eddy simulation

Large-eddy simulation of an atomizing spray issuing from a gas-turbine injector is performed. The filtered Navier–Stokes equations with dynamic subgrid scale model are solved on unstructured grids to compute the swirling turbulent flow through complex passages of the injector. The collocated grid, incompressible flow algorithm on arbitrary shaped unstructured grids developed by Mahesh et al. ( J. Comp. Phys. 197 (2004) 215–240) is used in this work. A Lagrangian point-particle formulation with a stochastic model for droplet breakup is used for the liquid phase. Following Kolmogorov’s concept of viewing solid particle-breakup as a discrete random process, the droplet breakup is considered in the framework of uncorrelated breakup events, independent of the initial droplet size. The size and number density of the newly produced droplets is governed by the Fokker–Planck equation for the evolution of the pdf of droplet radii. The parameters of the model are obtained dynamically by relating them to the local Weber number and resolved scale turbulence properties. A hybrid particle-parcel is used to represent the large number of spray droplets. The predictive capability of the LES together with Lagrangian droplet dynamics models to capture the droplet dispersion characteristics, size distributions, and the spray evolution is examined in detail by comparing it with the spray patternation study for the gas-turbine injector. The present approach is computationally efficient and captures the global features of the fragmentary process of liquid atomization in complex configurations.

Experimental analysis of a spray impinging on a conical surface

The details of droplet–droplet and droplet–liquid film interactions on solid surfaces are believed to have significant role in spray impingement phenomena, yet details of this interaction have not been clearly identified. After impact, droplet interactions affect droplet collisions, coalescence and liquid splashing; this interaction affects secondary atomization and droplet dispersion characteristics of the impingement process. In this study detailed measurements are performed in liquid sprays with a phase-Doppler velocimeter: simultaneous measurements of droplet size and velocity are obtained. The analysis is oriented towards secondary droplets behaviour. One general conclusion of this paper is that, working with a mean dimensionless film thickness between 1 and 3, the size and velocity distribution of secondary droplets are sensitive to the film thickness.

Experimental and numerical study of a water spray in the wake of an axisymmetric bluff body

An experimental and numerical study was made of a water spray issuing into a recirculating flow behind a bluff body disk mounted in a nozzle. A two-component laser-Doppler/phase-Doppler anomometry system was used to characterize the mean and turbulent dispersed phase. The numerical prediction of the hollow-cone spray was based on an Eulerian-Lagrangian stochastic hybrid model. The continuous gas flow field was predicted using a differential Reynolds stress transport model whereas the particulate droplet flow field was predicted using an improved Lagrangian stochastic dispersion model. Comparison of numerical predictions and experimental measurements was carried out for droplet mean and fluctuating velocities, number mean diameters, and mass fluxes. Results indicate that the droplet dispersion characteristics are strongly influenced by the presence of flow recirculation due to different particle Stokes numbers, yielding recirculation or penetration of particles through the separated flow region. Predictions of droplet axial and radial rms velocities obtained with the Lagrangian stochastic model are less satisfactory than those of other simple gas spray flows.

Effect of external forcing on droplet dispersion in a developing shear layer

The dynamics and dispersion of nonevaporating droplets in a forced transitional shear layer are studied. The shear layer is formed by two coflowing parallel streams downstream of a splitter plate. The effects of periodic forcing on the dynamics of large-scale vortical structures and on the dispersion characteristics of droplets in a spatially and temporally developing shear layer are investigated. Results indicate that by forcing the shear layer at the first subharmonic of the fundamental mode, the droplet dispersion can be enhanced significantly. The forcing at the fundamental mode increases droplet dispersion in the initial part, but then decreases dispersion farther downstream. These results are consistent with the previously published experimental and numerical results on the effect of forcing on the shear layer growth. It is also observed that the forcing causes a relatively larger gain in dispersion for intermediate size particles compared to that for gas particles, implying that the centrifugal mechanism may be strengthened by forcing. The dispersion enhancement is more pronounced in the initial part of the shear layer, which is perhaps the more important region for improved droplet dispersion and mixing in spray applications.