The dust particles generated during coal mining and processing are generally removed by spray dust reduction. The dynamic process via which spray droplets wet and wrap coal dust can directly affect ...the spray dust reduction efficiency. Here, we analyzed the contact wetting and wrapping process of dust particles by droplets under different particle size ratios and different initial droplet velocities. The results show that the larger the particle size ratio θ of the droplets and dust particles, the greater the variation in the dimensionless spreading coefficient. When the particle size ratio is more than 2, the droplets can completely wrap the coal dust particles. If the initial velocity is too small, the droplets will shrink and return to form a single sheet after coming into contact with the coal dust. With increasing velocity, the dimensionless spreading coefficient increases rapidly, and the coal dust is moistened and rapidly wrapped. By experimenting with various spray atomization characteristics, when the spray pressure is 6 MPa，the droplet size distribution in the spray field accounting for the highest proportion is 10 μm. Under the optimized parameters, the settling efficiency of respirable dust at each measuring point of mine return air duct reaches 86.2% on average.
Display omitted
•The collision wetting process of coal dust by droplets with different particle size ratios was analyzed.•The droplets can completely wet and wrap the coal dust particles when the particle size ratio is more than 2.•It is of great significance to determine the parameters of spray dust and ensure the environmental sanitation of mines.
For many industrial applications dealing with gas-liquid flow, transporting gas and liquid simultaneously causes many challenges. Prediction of erosion in multiphase flow is a complex problem due to ...lack of accurate models for calculating particle impact velocities in multiphase flow. Computational Fluid Dynamics (CFD) studies of annular flow are being conducted to investigate annular flow behavior and particle impact characteristics. Volume of Fluid (VOF) method was employed for air-water flow simulation with high gas velocities and low liquid rates. Simulation results are compared with results obtained with Eulerian-Eulerian with Multi-Fluid VOF approach with the same flow rates. CFD simulations of annular flow were also compared and validated with experimental data previously obtained with wire-mesh sensors. The liquid film thickness in bends can significantly affect erosion, therefore the simulation results of liquid film thickness trends are also investigated based on the gas and liquid flow rates.
Particle tracking along with flow simulations have been solved simultaneously, and the maximum values of mass removal calculated with current erosion models were recorded in order to calculate the maximum erosion magnitudes. Particle impact characteristics including particle impact velocities, particle impact angles as well as particle impact number distribution along the bend were studied in several cases with different superficial gas and liquid velocities. Predicted erosion ratios are compared with available experimental data and show very good agreement.
•CFD was used to simulate multiphase flow and erosion under annular flow conditions.•Multi-fluid VOF and VOF method were able to model multiphase flow.•Particle impact characteristics along the bend were studied in several cases.•Simulation results were compared with experimental data and showed good agreement.
•Bubble deformation and oscillation in ultrasonic standing wave was studied.•The effects of sound pressure, wave number and viscosity ratio were studied.•The mechanism on bubble deformation and ...oscillation was analyzed.•A regime map predicting bubble deformation and shape oscillation was drawn.
To promote the development of science and technology such as underwater explosions, water cleaning, and medical treatment, it is necessary to understand the mechanism of the influence of ultrasonic fields on bubble dynamics. The authors investigated in depth the effects of the dimensionless sound pressure amplitude pa*, the dimensionless wave number k* and the liquid–gas viscosity ratio μl/μg on the micro–sized bubble deformation and shape oscillation using the volume of fluid (VOF) method. It was found that the bubble deformation and oscillation types can be classified into four categories: volume oscillation, shape oscillation, splitting oscillation and other types of oscillation. Both the dimensionless sound pressure amplitude pa* and the dimensionless wave number k* determine the bubble oscillation types. The regime map of predicting the micro–sized bubble deformation and shape oscillation was drawn based on pa* and k*. If the shape oscillation occurs in a micro–sized bubble, the dimensionless time τ*, when the micro–sized bubble experiences the shape oscillation, decreases with an increase in pa*, but increases with an increase in k* and μl/μg. The mode number n of shape oscillation increases with an increase in k* but decreases with an increase in μl/μg, and it is irrelevant to pa*. If the splitting oscillation happens to a micro–sized bubble, τ*, when a micro–sized bubble experiences splitting, decreases with an increase in pa* and increases with an increase in k*, while the liquid–gas viscosity ratio μl/μg has a negligible effect on this phenomenon.
•Numerical study of alumina droplet impact on flat, etched and realistic 3D grit-blasted (Ra=2.7 µm) and laser treated surfaces using the Volume-of-Fluid (VOF) method.•Splat cooling shrinkage reduces ...the final droplet diameter (without accounting for splashing finger lengths) by approximately 10 % when impacting on flat, etched and sharp surface features.•Solidification shrinkage influences primarily splat porosity when impacting on a surface with Ra=2.7µm: with splat porosity reaching 0.12 % while only 0.05 % is detected under a constant density model definition.•Increasing the surface roughness above >2.7 µm decreases the effect of droplet shrinkage on splat impact dynamics and heat transfer processes due to abrupt splat asymmetrical peripheral detachment/ejection from the substrate surface. Detected splat contact and covering areas reach ∼9 % and 13 %, respectively, irrespective of the density model.•Splashing, asymmetrical splat spreading, porosity formation, splat stretching and droplet interfacial separation associated to interfacial micro and macro-features are thoroughly described.
Thermal spray splat morphology and its microstructure are of significance to the coating resulting properties. Depending on the material, the changes in density as the droplet solidifies during splat development and solidification could have a substantial role in the particle deformation dynamics, formation of porosity and subsequent adhesion to the flat or rough target surface.
In the current study, the effect of shrinkage on the final splat shape is numerically investigated for collisions occurring under several high-speed impact conditions using the Volume-of-Fluid (VOF) method on real surface topography. This influence is also studied for the interaction of two successive impacting particles. The developed models study the splat impact, development and dynamics of heat transfer on flat, laser-patterned, grit-blasted surfaces and numerically designed features. The results of droplet material variable density cases are compared with those assuming constant density. Experimentally obtained single splat depositions are also used to validate the developed models and obtained numerical results.
Findings show that micro substrate surface features lead to splat stretching, porosity and limited finger creation subsequent to improved solidification through interfacial contact area increase. Macro surface characteristics, however, lead to splashing and separation from the substrate surface driven by fluid instabilities. Irrespective of the interface topography, shrinkage processes reduce droplet spreading and accentuate splat feature discontinuities such as porosity.
•AMM is used to study the accuracy of BIM/CBIM for compressible bubble dynamics.•CBIM has good accuracy in simulating compressible bubbles when Ma≲0.3.•Liquid compressibility plays a vital role even ...when the Mach number is less than 0.1.•Gas inertia is another factor that may affect the applicability of BIM/CBIM.•BIM can be used to simulate the bubble behaviors in the first bubble cycle.
The Boundary Integral Method (BIM) has been widely applied to simulate oscillating bubbles, for its high efficiency and accuracy. A conventional BIM assumes the fluid surrounding the bubble to be inviscid and incompressible. Wang & Blake (J. Fluid Mech., 659, 2010, 191–224) proposed an improved model for bubbles in a weakly compressible flow, which is referred to as CBIM. In this study, an all-Mach method (AMM) implemented in the free software program Basilisk for the simulation of compressible multiphase flows, and using a geometric Volume-of-Fluid (VoF), is employed to study and estimate the accuracy of BIM and CBIM at different Mach numbers. First, for a spherical bubble, an extended Rayleigh-Plesset equation, CBIM and AMM give very close results when Ma≲0.3. However, a deviation between these three schemes gradually becomes evident as Ma increases from 0.3 to 0.6. Second, for the nonspherical deformation of a bubble close to a wall, the results obtained from CBIM and AMM show many similarities, including the evolution of the nonspherical bubble morphology, jet impact velocity, and impact pressure on the wall. Apart from the liquid compressibility, the gas inertia/density is found to be another factor that may affect the applicability of CBIM. In addition, we compare the CBIM and BIM results against an experiment of a spark-generated cavitation bubble, in which the liquid compressibility is found to play a vital role. From the perspective of engineering applications, BIM can reproduce the main features of the bubble dynamics in the first cycle if the initial conditions are set properly. The new findings provide a reference for research of bubble dynamics in both fundamental and applied problems.
This article reviews and analyzes a number of numerical methods to track interfaces in multiphase flows. Several interface tracking methods can be found in literature: the level-set method, the ...marker particle method, the front tracking method and the volume of fluid method (VOF) to name a few. The volume of fluid method has an advantage of being conceptually simple, reasonably accurate and phenomena such as interface breakup and coalescence are inherently included. Over the years a number of different techniques to implement the VOF method have been devised.
This article gives a basic introduction to the VOF method and focuses on four VOF methods: flux-corrected transport (FCT) by Boris et al. J.P. Boris, D.L. Book, Flux-corrected transport. I: SHASTA, a fluid transport algorithm that works, J. Comput. Phys. 11 (1973) 38–69, Lagrangian piecewise linear interface construction (L-PLIC) by van Wachem and Schouten B.G.M. van Wachem, J.C. Schouten, Experimental validation of 3-d Lagrangian VOF model: bubble shape and rise velocity, AIChE 48 (12) (2002) 2744–2753, Compressive interface capturing scheme for arbitrary meshes (CICSAM) by Ubbink O. Ubbink, Numerical prediction of two fluid systems with sharp interfaces, Ph.D. Thesis, Imperial College of Science, Technology and Medicine, 1997 and inter-gamma scheme by Jasak and Weller H. Jasak, H.G. Weller, Interface-tracking capabilities of the InterGamma differencing scheme, Technical Report, Imperial College, University of London, 1995. A detailed description of these schemes is given and implemented into an in-house fully coupled solver. Further, the performance of these schemes is examined employing a number of tests to analyze their strengths and weaknesses. Their advantages and limitations are discussed.
•Melting of a NePCM in partially filled horizontal elliptical capsules is numerically investigated.•Different aspect ratios and nanoparticles volumetric concentrations are considered.•The simulations ...are carried out using the enthalpy-porosity technique and VOF model.•Increasing nanoparticles enhances the melting rate but decreases the NePCM volume change.•Melting performance of the capsules with AR ≠ 1 is higher than that with AR = 1.
Melting of a Nano-enhanced Phase Change Material (NePCM), i.e., n-octadecane paraffin dispersed with Cu nanoparticles, in partially-filled horizontal elliptical capsules for a Rayleigh number of 1.744 × 106 is investigated numerically using the collocated finite volume method. To accommodate the increase in the NePCM volume during melting, a 15% air void within the capsule is considered. The simulations are carried out using the enthalpy-porosity technique and the Volume of Fluid (VOF) model. In this work, three volumetric concentrations (Φ = 0, 1 and 3 vol%) of nanoparticles and various aspect ratios (AR = 2.0, 1.0 and 0.5) of the capsule are adopted. The accuracy of the numerical procedure is validated through the comparison of two test cases with those available in the literature. The results are demonstrated in terms of the isotherms, streamlines, melting interface and the air-NePCM interface. It is concluded that for a given AR, the presence of nanoparticles enhances the melting rate and decreases the volume change of NePCM as compared to the pure PCM case. In addition, the highest and the lowest melting rates are associated with the AR = 2.0 and AR = 1.0, respectively.
Display omitted
Display omitted
•Established validity of sub-grid scale model for fluid flow field in hydrocyclone.•Particle classification simulated using one way, two way and coupled MPPIC-VOF approaches.•Accurate ...particle performance prediction of hydrocyclone found with Coupled VOF-MPPIC.•Turbulence dispersion forces improves fine size particle classification significantly.•At low solids content the fluid turbulence increases at high solids content it reduces.
We discuss the implementation of a coupled Multi-phase particle in cell method (MPPIC) and Volume of Fluid (VOF) flow solver to simulate particles in a fluid flow field having a free surface. Using the solver developed using OpenFOAM 4.1 library, we study the flow of particles inside a highly anisotropic turbulent flow field of a hydrocyclone. The free surface from the air-core inside the hydrocyclone is resolved using the VOF method coupled with the LES turbulence model. We test multiple sub-grid scale model and find the dynamic k-equation model to have the best-predicting capability of the mean and turbulent flow field as well as the air-core shape. Using the established flow field, we study the particle flow properties and its effect on the fluid flow field using a four-way coupled description between MPPIC and VOF. Important flow properties such as the particle flow field velocity and turbulence modulation with varying particle concentration were studied. It was found that even with low solid concentration (0.3–1.5% by volume) there is a quite substantial increase in the fluctuating flow field (around 12%) and with a further increase (1.8%) we observe a decrease in the turbulent levels by approximately 6%.
Display omitted
•A new mesoscale 3D CFD model is proposed to predict the liquid flow in an RPB.•Detailed liquid flow patterns in the RPB are obtained.•Liquid holdup, percentage of droplets and ...interfacial area in the RPB are analysed.•New correlations for liquid holdup and effective interfacial area are developed.•Parametric sensitivity analyses of the RPB for influencing CO2 capture are performed.
Rotating packed beds (RPBs), as a type of process intensification technology, are promising to be employed as high-efficiency CO2 absorbers. However, the detailed understanding of the liquid flow in the RPB is still very limited. The complex and dense packing of the bed and the multiscale of the RPB make it very difficult to perform numerical simulations in detail, in particular for full 3D simulations. In this paper, a mesoscale 3D CFD modelling approach is proposed which can be used to investigate the liquid flow in both laboratory- and large-scale RPBs in detail and accuracy. A 3D representative elementary unit of the RPB has been built and validated with experimental observations, and then it is employed to investigate the gas–liquid flows at different locations, across a typical RPB, so that the overall characteristics of the liquid flow in the RPB can be assembled. The proposed approach enables the detailed prediction of the liquid holdup, droplets formation, effective interfacial area, wetted packing area and specific surface area of the liquid within real 3D packing structures throughout the bed. New correlations to predict the liquid holdup, effective interfacial area, and specific surface area of the liquid are proposed, and the sensitivities of these quantities to the rotational speed, liquid flow rate, viscosity and contact angle have been investigated. The results have been compared with experimental data, previous correlations and theoretical values and it shows that the new correlations have a good accuracy in predicting these critical quantities. Further, recommendations for scale-up and operation of an RPB for CO2 capture are provided. This proposed model leads to a much better understanding of the liquid flow behaviours and can assist in the RPB optimisation design and scaling up.
•The particle area beneath the bubble can be divided into three parts: stable bubble wake region, fluctuating bubble wake region, and vortex tail region, respectively.•The entrained particle number ...depended on the initial particle entrainment ability of jet and vortex shedding in the bubble wake range.•Particle concentration of the stable bubble wake region was unaffected by the vortex shedding motion, while vortex shedding resulted in the zigzag path distribution of axial particles in the fluctuating bubble wake region and vortex tail region.
The wake structure and particle entrainment behavior during a single bubble ascent in a liquid–solid system were investigated through numerical simulations. A combined volume-of-fluid and discrete phase model method was used for the numerical simulation. Three-phase hydrodynamics are systematically investigated and discussed. The results showed that the bubble wake with entrained particles comprised a stable bubble wake region, fluctuating bubble wake region, and vortex tail region. Vortex shedding with the bubble oscillation caused particles in the fluctuating wake region to be de-entrained. Analysis of the wake regions and particle distribution revealed the stable bubble wake region as the key for entrainment. For the selected micron particles (dp = 1–100 μm), most of entrained particles were enriched in the stable bubble wake region, and the enrichment effect was gradually significant with the increase in bubble size. Compared with bubble sizes, particle properties such as density and diameter had a weaker effect on entrainment.
PDF
