Effects of radial air flow quantity and location of an air curtain generator on dust pollution control at fully mechanized working face

To better understand the effects of radial air flow quantity and the location of air curtain generator on dust pollution control, the 2–109₂ fully mechanized working face in Xinzhi coal mine (Huozhou Coal Electricity Group Co., Ltd., Shanxi, China) was numerically simulated in the present study. A full-scale physical model of the working face was established; then, based on airflow-dust particle two-phase flowing characteristics, the k-ε-Θ-k_p mathematical model was constructed. The comparison between simulation results and field measurements validated the model and the parameter settings. Furthermore, the effects of ventilation parameters on airflow migration and dust diffusion were numerically investigated using FLUENT. The results show that the increase of the radial air flow quantity (denoted as φ) and the distance of the air curtain generator from working face (denoted as d_w) is beneficial to the formation of a dust-control air curtain. At a constant d_w, the dust diffusion distance (denoted as D) decreases with the increase of φ. At a constant φ, D decreases with the increase of d_w when a dust-control air curtain is formed; otherwise, the increase of d_w leads to the increase of D. By analyzing the simulation results, the optimal ventilation parameters for 2–109₂ fully mechanized working face and those working faces under similar production conditions are determined as: φ = 240–270 m³/min and d_w = 20–30 m.     

Effects of particle–particle collisions on sudden expansion gas-particle flows via a two-phase kinetic energy-particle temperature model

In the framework of the gas–particle two-fluid mode, an improved gas–particle two-phase kinetic energy incorporating into a particles collision model (k–k_p–θ) is proposed to study the sudden expansion gas–particle turbulent flows in a cylindrical pipe section. Anisotropy of gas–solid two-phase stress and the interaction between two-phase stresses are considered by means of a transport equation of two-phase fluctuation velocity correlation. Xu and Zhou [10] experimental data is used to quantitatively validate k–k_p–θ and k–k_p model for analysis the effects of particle–particle collision. Numerical predicted results show that time-averaged velocity, fluctuation velocity of gas and particle and correlation of two-phase fluctuation velocity considering particles collision are better than those of the without particle temperature model and they are in good agreement with experimental data. Larger particle concentration and particle temperature located at shear layer adjacent to wall surface and re-circulation region. Energy dissipation due to smaller scale particles collision contributes to homogeneous distribution of Reynolds stress and affects the particle transportation behavior together with particle inertia.     

DEM study and machine learning model of particle percolation under vibration

This paper aims to understand model the effect of vibration on particle percolation. The percolation of small particles in a vibrated bed of big particles is studied by DEM. It is found the percolation velocity (V_p) decreases with increasing vibration amplitude (A) and frequency (f) when the size ratio of small to large particles (d/D) is smaller than the spontaneous percolation threshold of 0.154. Vibration can enable percolation when the size ratio is larger than 0.154, while V_p increases with increasing A and f first and then decreases. V_p can be correlated to the vibration velocity amplitude under a given size ratio. Previous radial dispersion model can still be applied while the dispersion coefficient is affected by vibration conditions and size ratio. Furthermore, a machine learning model is trained to predict V_p as a function of A, f and d/D, and is then used to obtain the percolation threshold size ratio as a function of vibration conditions.     

Effects of specularity and particle-particle restitution coefficients on the hydrodynamic behavior of dispersed gas-particle flows through horizontal channels

Specularity coefficient ($?$) and particle–particle restitution coefficient (e) are two important parameters governing the flow physics of dispersed gas-particle flows. In this work, a detailed numerical analysis is carried out to get an insight into the effects of these two parameters in the flow hydrodynamics of dispersed gas-particle flows through horizontal channels. Investigations have also been carried out to find the $?$-e pair for which the phase velocities become an extremum. It has been found that at a particular value of e, both gas and particle velocities at the centerline of the channel increase with increase in the value of $?$, whereas near the wall, they tend to decrease. At a fixed non-zero value of $?$, both gas and particle velocities tend to increase with increase in the value of e. For $?$ equal to zero, which corresponds to free-slip boundary condition for particle velocity, there is no significant variations in gas and particle velocities with changes in e. Out of all combinations of values of $?$ and e investigated herein, it is found that both gas and particle velocities attain a maximum value when both the values of $?$ and e are maximum.     

Performance of heat transfer and pressure drop in superconducting cable former

This study was performed of the heat transfer and the pressure drop in superconducting cable former of more realistic geometry. Liquid nitrogen passes through the former for cooling. Corrugated pipes are used as formers to ensure flexibility, and the ratio of pitch and depth of corrugation p/e is less than 5, which is out of the range of normal usage. Range of the test was p/e=1–15, e/D=0.039–0.118. The heat transfer and the pressure drop were proportional to the corrugation depth, which is the same with the results reported by previous researchers. But the effect of pitch was different from the results of the range p/e>10. The heat transfer and the pressure drop had the maximum values on the range of p/e=5–10. Based on the numerical results, discussion was made on the design of superconducting cable former.     

Photocarrier Radiometry for Non-contact Evaluation of Monocrystalline Silicon Solar Cell Under Low-Energy (< 200 keV) Proton Irradiation

A three-layer theoretical model is developed for the characterization of the electronic transport properties (lifetime τ, diffusion coefficient D, and surface recombination velocity s) with energetic particle irradiation on solar cells using non-contact photocarrier radiometry. Monte Carlo (MC) simulation is carried out to obtain the depth profiles of the proton irradiation layer at different low energies (<?200 keV). The monocrystalline silicon (c-Si) solar cells are investigated under different low-energy proton irradiation, and the carrier transport parameters of the three layers are obtained by best-fitting of the experimental results. The results show that the low-energy protons have little influence on the transport parameters of the non-irradiated layer, but high influences on both of the p and n-region irradiation layers which are consisted of MC simulation.     

Numerical study to predict the particle deposition under the influence of operating forces on a tilted surface in the turbulent flow

This study uses a v2-f turbulence model with a two-phase Eulerian approach. The v2-f model can accurately calculate the near wall fluctuations in y-direction, which mainly represent the anisotropic nature of turbulent flow. The model performance is examined by comparing the rate of particle deposition on a vertical surface with the experimental and numerical data in a turbulent channel flow available in the literature. The effects of lift, turbophoretic, electrostatic and Brownian forces together with turbulent diffusion are examined on the particle deposition rate. The influence of the tilt angle and surface roughness on the particle deposition rate were investigated. The results show that, using the v2-f model predicts the rate of deposition with reasonable accuracy. It is observed that in high relaxation time the effect of lift force on the particle deposition is very important. It is also indicated that decreasing the tilt angle from 90° to 0° enhances the deposition rate especially for large size particles. Furthermore, the results show that increasing the Reynolds number at a specific tilt angle decreases the rate of particle deposition and the tilt angle has insignificant impact on the particle deposition rate in high shear velocity or high Reynolds number.     

Spreading and solidification of metal-melt droplets under conditions of substrate submelting. Theory and model experiment

Spreading and solidification of metal-melt droplets upon their collision with a surface are of particular interest for thermal spraying, molding, melt spinning for obtaining amorphous and microcrystal powders, etc. The scenarios of the formation of splats (spread and solidified particles on the substrate) can be significantly different and are determined by thermophysical properties of materials of the particle and substrate, as well as by key physical parameters (KPPs): velocity u _p0, size D _p , and temperature T _p0 of the droplet and substrate T _b0 and the state of its surface. An attempt was made to theoretically and experimentally study the formation of splats for the case in which the substrate is submelted in the spot of the contact with the particle in the process of spreading and solidification of metal-melt droplets.     

Numerical simulation of flow field in three types of standard cyclone separators

The numerical simulation of the fluid flow and particle dynamics is presented by CFD techniques to characterize the performance of the three types of standard cyclones. The three types of cyclones named 1D3D, 2D2D and 1D2D. The length of cylindrical part of the body is equal to 1, 2 and 1 times of the body diameter, respectively; and the length of the conical part is 3, 2 and 2 times of the body diameter. The Reynolds averaged Navier–Stokes equations with Reynolds stress turbulence model (RSM) are solved by use of the finite volume method based on the SIMPLE pressure correction algorithm in the computational domain. The Eulerian–Lagrangian computational procedure is used to predict particles tracking in the cyclones. The velocity fluctuations are simulated using the Discrete Random Walk (DRW). The dependency of cyclone characteristics on its diameter is investigated and D₅₀ (Cut-Point) is calculated for different Particle Size Distributions (PSDs). The numerical results are compared with the experimental data and the theoretical model and good agreement is observed.     

Effects of specularity and particle-particle restitution coefficients on the recirculation characteristics of dispersed gas-particle flows through a sudden expansion

We numerically investigate the effects of restitution and specularity coefficients on the characteristics of dispersed gas-particle flows through a sudden expansion. The studies are carried out using an indigenous finite volume flow solver in a collocated framework with two-fluid model. Parametric studies are performed to gain insights into the differences in recirculation patterns that arise due to variations in restitution and specularity coefficients. The simulations show that particle-particle interactions, quantified by restitution coefficient (e) have a greater impact on recirculation characteristics than particle-wall interactions, which are quantified by specularity coefficient ($?$). Studies reveal that the recirculation lengths tend to decrease as particle collisions become more elastic (as e tends to unity) while they increase, as the value of $?$ increases. However, the changes in recirculation length are very gradual and less pronounced when only particle-wall interactions are considered as compared to particle-particle interactions. From the range of parametric variations studied in this work, the maximum recirculation length has been found when the value of $?$ is maximum and that of e is minimum.     

Experimental and theoretical study on hydrodynamic characteristics of tapered fluidized beds

A novel fluidized bed ash cooler was developed for circulating fluidized bed boilers based on a proposed modified tapered fluidized bed. A cold model was built to study the hydrodynamic characteristics of the modified tapered fluidized bed, and its critical superficial gas velocity u_mf and critical velocity for full fluidization u_mff were particularly studied. The effects of taper angle α, static bed height H, air inlet section width δ and particle size d_p on the u_mf and u_mff were experimentally investigated. Furthermore, a theoretical model and an empirical correlation have been proposed to predict the u_mf and u_mff, respectively. The predicting capabilities of the model and correlation have been experimentally discussed. And the predicting capability of the model has also been compared with that of an existing representative model. It is found that both the u_mf and u_mff increase with the increase of taper angle α, static bed height H and particle size d_p, but decrease with the increase of air inlet section width δ, respectively. Additionally, the predicted values of u_mf and u_mff compare well with the experimental data, and the model has a better capability than the existing representative model in predicting the u_mf of the modified bed.     

The 3-D spread of saltation sand over a flat bed surface in aeolian sand transport

This paper investigated the 3-D motion of saltation sand by high-speed photography and stereo particle image velocimetry (SPIV). By the high-speed camera, the sand particle trajectories in the transverse plane near bed surface have been obtained. It could be found that the collision between the particle and the bed surface would in principle cause the transverse motion of the particle regardless of the change of the wind direction. Based on SPIV, The three-dimensional velocities of the sand particles in the wind–sand flow have been obtained by combining the velocity data from double CCD cameras. The three-dimensional velocity of the sand particle was resolved into three component velocities in the paper, i.e. the streamwise velocity u, the vertical velocity v and the transverse velocity w. The distribution of the transverse velocities w of the sand particles approximated symmetrical. The peak value of the PDF (probability density function) of velocity w fell down obviously with the increase of the wind speed when the sand sizes were equal to or less than 125 μm. When the sand sizes were larger than 125 μm, the peak value of the PDF of w was almost constant with the change of the wind speed. Although the increments of v with the increase of the wind speed were larger than that of w, the velocity w of the sand particle appeared to be much larger than its velocity v in general. Moreover, it was near one fourth probability that the quantity of the particle velocity w was one order higher than that of its velocity v. The inclination angle between w and u of the sand would be less than 60° and the inclination angle between the v and u would be less than 20° integrally.     

Experimental study on electrostastic particle deposition from gas–solids two-phase flow based on tribo-electrification

Electrostastic particle deposition on a target embeded in a substrate under a locally applied voltage has been investigated experimentally based on the tribo-electrification of particles. Initially particles deposit mainly on the edge of the target because of the contact potential difference between substrate (poly(methyl methacrylate)) and target (brass). The thickness of particle layer formed by the deposited particles increases with time, but gradually saturates. Since then almost no additional particles deposit on the target. When high voltage is applied to the target, and orange peel phenomenon is observed on the surface of the particle layer. In the central region of the target, the particles under high particles. The larger the Coulombic force parameter K_E, the higher the effective deposition velocity is. The velocity increases dramatically for K_E 6 × 10⁻⁶ and 20 < St < 60. The larger Stokes number makes the coefficient of variation larger for the thickness of the particle layer, i. e. the deposition under higher Stokes number gives a less uniform deposition layer. However, for St < 10, particles deposit almost uniformly. For the deposition under a larger Coulombic force parameter, onset of the rebound and resuspension is suppressed, and the region with a uniform deposition layer is shifted to a higher Stockes number. It is also found that particles can successfully deposit on a non-conductive target of a dielectric substance through setting a grounded guard electrode around the target.     

Dilute phase pneumatic conveying of whey protein isolate powders: Particle breakage and its effects on bulk properties

Breakage of dairy powder during pneumatic conveying negatively affects the end-customer properties (scoop uniformity and reconstitution). A dilute phase pneumatic conveying system was built to conduct studies into this problem using whey protein isolate powder (WPI) as the test material. Effects of conveying air velocity (V), solid loading rate (S_L), pipe bend radius (D), and initial particle size (d) on the level of attrition were experimentally studied. Four quality characteristics were measured before and after conveying: particle size distribution, tapped bulk density, flowability, and wettability. The damaged WPI agglomerates after conveying give rise to many porous holes exposed to the interstitial air. V is the most important input variable and breakage levels rise rapidly at higher airspeeds. The mean volume diameter D[4,3] decreased by around 20% using the largest airspeed of 30 m/s. Powder breakage is also very sensitive to particle size. There appears to be a threshold size below which breakage is almost negligible. By contrast, S_L and D show lesser influence on powder breakage. Reflecting the changes in particle size due to breakage, tapped bulk density increases whereas wettability decreases as a result of an increase in conveying air velocity. However, breakage does not show a significant effect on powder flowability as powder damage not only decreases particle size but also changes the particle’s surface morphology.     

Deagglomeration of airborne nanoparticles in a decelerating supersonic round jet

The size and the agglomeration state of nanoparticles (NPs) play a very important role in nanoparticle applications due to the effect of the dispersion level of NPs on their toxicity, pharmaceutical activity, and catalytic activity. In this study, a novel two-stage nanoparticle disperser system was proposed. We focused on the effects of the aspect ratio (L/D, length (L) and diameter (D)) of the round jet (a straight pipe) on the nanoparticle size distribution (PSD) and we attempted to explain how the jet wakes induced the NP deagglomeration based on the numerical results. The median diameters, mode diameters, and geometric mean diameters (GMD) of the PSDs that were obtained using the second-stage were significantly smaller than those achieved using the first-stage disperser alone. Overall, the GMD, mode diameters, and median diameters could all reach sub-70 nm, which was approaching close to the NP primary diameter (30–40 nm). The particle sizes decreased with the increasing dimensionless nozzle diameter (D* = D/D_p × 10^?3, D_p represents the particle diameter). The increase in L/D for a constant D hardly affected the PSDs but led them to be closer to a standard lognormal distribution. Regarding the NP traveling path and its considerable inertia as it penetrated the Mach disc, these particular field distributions of the jet-wake might explain why the suspended sub-micro NP agglomerates could be highly deagglomerated. Namely, the nano NP agglomerates would suffer the severe shear stress around the Mach disc.     

Effect of pulse frequency on heat transfer at the stagnation point of an impinging turbulent jet

Heat transfer in an impact pulsed air jet is numerically studied using the Reynolds stress model. It is shown that both enhancement and suppression in the heat transfer are possible in an impinging pulsed jet as compared with a steady flow. The heat transfer intensifies with the pulse frequency at a stagnation point in the region of small distances between a pipe exit cross section and an obstacle (H/D ≥ 6), while an increase in the pulse frequency causes a decrease in the heat transfer for H/D > 8. An increase in the Reynolds number causes a deintensification of the heat transfer, and the data for all frequencies approach the single-phase flow mode. A comparison with available data by other authors is made, and satisfactory agreement is obtained with respect to the pulse frequency effect on the heat transfer between a gas jet and the impact surface.     

Large eddy simulation of particle deposition and resuspension in turbulent duct flows

Particle deposition and resuspension in a horizontal, fully developed turbulent square duct flow at four flow bulk Reynolds numbers (10,320, 83k, 215k and 250k) is simulated by applying large eddy simulation coupled with Lagrangian particle tracking technique. Forces acting on particles includes drag, lift, buoyancy and gravity. Four particle sizes are considered with the diameters of 5?μm, 50?μm, 100?μm and 500?μm. Results obtained for the fluid phase are in good agreement with the available experimental and numerical data. Predictions for particles show that particle size, flow Reynolds number and the duct (celling, floor and vertical) walls play important roles in near-wall particle deposition and resuspension. For the smallest particle (5?μm), the particle deposition rates in duct ceiling, floor and vertical walls are found to be similar with each other and all increase with the flow Reynolds number, while the particle resuspension tends to occur in the middle wall regions and corners of the duct with less influenced by the flow Reynolds number. The ceiling deposition rate gradually decreases with particle size while the floor and vertical wall deposition rates both increase with particle size. The ceiling particle deposition rate increases with Reynolds number while the floor deposition rate decreases with it. The vertical deposition rate for the small particles (5–50?μm) increases with the flow Reynolds number obviously, while for the large particles (100–500?μm) that becomes insensitive. In addition, the flow Reynolds number is found to have an obvious effect on particle resuspension while the effect of particle size on particle resuspension decreases with Reynolds number. Eventually, a dynamic analysis was conducted for particles deposition and resuspension in turbulent duct flows.     

Growth and characterization of CuInSe2 thin films prepared by successive ionic layer adsorption and reaction method with different deposition temperatures

CuInSe₂ films were prepared at different deposition temperatures (T_D) by successive ionic layer adsorption and reaction method with chelating solutions. Influence of T_D on film growth, morphology, crystal structure, and band gap energy was investigated. Results showed that elevation of T_D mainly enhanced reaction kinetics and ionic diffusion velocity, resulting in fast growth rate of CuInSe₂ films, and maximum 20-30 nm/cycle depended upon T_D were acquired. Films with 60 dip-cycles could grow from 180 nm to 1000 nm by elevating T_D from 30 °C to 90 °C. Surface roughness of CuInSe₂ films was closely related to dip-cycle times and T_D. Accelerated growth rate by T_D could reduce the dip-cycle times for a required film thickness, which improved quality of film morphology.     

Relation entre la granulométrie d'une couche métallique discontinue et les mesures de son épaisseur et de sa conductance

The purpose of this paper is to determine the repartition of the metal in a discontinuous metallic deposit (DMD) on a carbon film from two experimental parameters: its thickness e_D and the electric current I_D which passes through its plane under a constant voltage. To this end, the theories of electrical conduction within the DMD, by the tunnel effect and by the thermionic emission, were applied to the case where the substrate is not an insulator but an amorphous conductor. Using measured values of e_D and I_D, we then employed these theories to calculate the parameters which describe the repartition of the metal on its substrate. If we took the values of I_D and e_D obtained after stabilization of the repartition by a covering of SiO or by exposure to the atmosphere, we were than able to compare the calculated repartition parameters with those derived by statistical means from a dimensional analysis of an electron micrograph of the DMD. The agreement is fairly satisfactory.     

Experimental and numerical study on collapse of quasi-two-dimensional bilayer granular column

The collapse of granular columns has been widely investigated numerically and experimentally. However, a column consisting of single-sized particles is typically modeled; thus, the influence of the column composition needs to be analyzed. In this study, quasi-two-dimensional discrete element simulations and experimental tests were performed to assess the collapse of two-layered granular columns with different particle sizes in each layer. The effects of the particle sequence, column aspect ratio (AR), percentage of basal particles (H_l/H₀), and particle size ratio between large and small particles (d_L/d_S) were evaluated. The results showed that the small particles moved slower than the large particles. The movement of the large particles in the upper layer was restricted because of the slow response of the small particles in the lower layer. Furthermore, the normalized final height, normalized run-out distance, peak value of average velocity, normalized kinetic energy, and normalized potential energy were analyzed in detail.