Formation mechanisms of a dust-removal air curtain in a fully-mechanized excavation face and an analysis of its dust-removal performances based on CFD and DEM

A high concentration of dust in a fully-mechanized excavation face is a serious threat to the safety of production underground and miners’ health. This paper discusses the use of a novel air curtain generator and proposes a novel dust control and prevention technique. Based on the k-ε two-equation turbulence model, Hertz-Mindlin model and the CFD-DEM coupled interface compiled with C++ language, this paper firstly constructs a simulation model of the coupling between airflows and dust in a fully-mechanized excavation face, and then simulates the airflow fields and dust fields under forced/exhaust ventilation conditions with and without a novel air curtain generator being utilized. The results show that when only the forced/exhaust ventilation was used, a high concentration of dust spread throughout the entire tunnel space and no effective air curtain was formed. Furthermore, after the air curtain generator was turned on, as the radial-to-axial forced air ratio (P_FQ) increased, the horizontal vortex in the front of the head-on section weakened gradually, and the originally disordered airflows behind the heading machine moved uniformly towards the head-on section. As the P_FQ further increased, the distance (d) between the formed air curtain and head-on section decreased overall; through a curve fitting, this relationship can be written as: d = ?5.247 ln(P_FQ) + 13.569. When the P_FQ > 5:5, the average negative-pressure-induced dust-exhaust capacity increased, the distance between the formed air curtain and the head-on section decreased, and the re-entrainment of dust did not take place in a straightforward manner. Finally, some field measurements were carried out in order to validate the simulated results, with the subsequent comparison showing that the numerical simulated results were basically accurate.     

Evaluation of thermal conductivity in air permeable concrete for dynamic breathing wall construction

Air permeable concrete (APC) is potentially useful as a dynamic insulator. The dynamic function is achieved by passing air through the material in the direction of heat flow to facilitate heat recovery. An APC sample of 200 mm length with 60% cement filling of large voids (between 0.5 and 5 mm), was tested between 5 and 10 Pa differential pressures; permeabilities were 0.28–0.32 m²/Pa h, confirming its suitability as a dynamic insulator. To characterise properties it is necessary to determine the static thermal conductivity, i.e., no air flow. A one-dimensional heat flow model for predicting the effective thermal conductivity (λ_e) of APC is developed using as variables the fractions of voids, aggregate and cement paste comprising the material. Measured values of λ_e were 0.7–1.4 W/m K. A theoretical model predicts and further improves the performance and formulation of APC. The water/cement ratio (w/c) also controls the λ_e. Increasing w/c increases the volume of micropores, adding resistance to heat flow.     

Influence of Eu substitution for Gd on the structure and photoluminescent properties of (Gd1−xEux)2Ti2O7 pyrochlore solid solutions

This paper presents a study of the synthesis and structural properties of the pyrochlore-type titanates (Gd_1?xEu_x)₂Ti₂O₇. Six compositions with 0 ≤ x ≤ 1 were prepared by solid-state reaction with thermal treatments at 1000 and 1200 °C under atmospheric pressure conditions. All the products were systematically characterized by X-ray powder diffraction (XRD), Raman spectroscopy, photoluminescence (PL) and photoluminescent excitation spectra (PLE). Structural refinements of X-ray powder diffraction data using Rietveld method show that all compounds of the (Gd_1?xEu_x)₂Ti₂O₇ solid solution crystallize in a pyrochlore structure. The lattice parameters increase linearly with increasing Eu content in agreement with Vegard's rule. PL spectra show that the characteristic peaks correspond to the f–f transition ⁵D₀–⁷F_J (J = 1, 2, 3 and 4) of Eu³⁺. The effect of Eu³⁺ ions concentration on the optical properties, namely, photoluminescence emission, is measured and discussed. The oxides exhibit higher PL intensity with the Eu³⁺ ion concentration and strong orange emission at 589 nm (⁵D₀–⁷F₁) corresponding to the magnetic dipole transition.     

Quantitative analysis on friction stress of hot-extruded AZ31 magnesium alloy at room temperature

Mg alloy AZ31 with ～79% (volume fraction of scattering less than 30°) basal-fiber texture through hot extrusion exhibits strong grain-size dependent yield strength. Samples with grain sizes varying from 4.5 to 22.3 μm were obtained by altering annealing time durations. The Hall-Petch relations of tension and compression are σ_0.2 = 86+200d^?1/2 and σ_0.2 = 17 + 327d^?1/2, respectively. Considering the correlation between grain orientation and deformation modes, a novel weighted average method of calculating friction stress σ₀ was proposed, and results of calculation agreed with the experimental ones, which can reasonably understand the yielding behavior in tension and compression.     

Ferromagnetic element microalloying and clustering effects in high Bs Fe-based amorphous alloys

Fe_(83)(Co_x,Ni_y)(B_(11)Si_2P_3C_1)_(1-x,y/17)(x,y=1–3)amorphous alloys with high saturation magnetic flux density(B_s)and excellent soft-magnetic properties were developed and then the microalloying and clustering effects were explored.The microalloying of Co and Ni improves the B_sfrom 1.65 T to 1.67–1.72 T and 1.66–1.68 T,respectively.The Ni-doped alloys exhibit better soft-magnetic properties,containing a low coercivity(H_c) of about 5.0 A/m and a high Effective permeability(μ_e)of(8–10)×10~3,whereas the microalloying of Co leads to a deteriorative H_c of 5.0–13.0 A/m and a μ_eof(5–8)×10~3.Moreover,microalloying of Ni can increase the ductile-brittle transition(DBT)temperature of the ribbons,while a totally opposite effect is found in the Co-doped alloys.The formation of dense α-Fe(Co,Ni)clusters during annealing process is used to explain the distinct effects of Co and Ni microalloying on the magnetic properties and bending toughness.     

Super-toughening polyamide-612 by controlling dispersed phase domain size: Essential work of fracture assessment

A new pathway to super-toughen polyamide-612 (PA-612) by incorporating domains of soft poly(octene-co-ethylene)-g-maleic anhydride (POE-g-MA) via melt blending leading to more than ∼1100% increase in notched Izod impact strength vis-à-vis fracture toughness enhancement is demonstrated. Fourier transform infra red (FTIR) studies showed effective phase interactions between PA-612 and POE-g-MA whereas dynamic mechanical analysis (DMA) revealed a reduction in loss-peak intensity at ∼45 °C with increase in the soft phase fraction. The optimal dependence of fracture-toughness (in plane-stress) on domain-size (D_n) of dispersed-phase in the form of a reduction in resistance to crack initiation indicated by essential work of fracture (w_e) and linear increase in resistance to crack propagation indicated by non-essential work of fracture (βw_p) of the blends ⩾10 wt% of POE-g-MA content is correlated to an increase in domain-size ⩾∼0.3 μm. Fracture surface morphology indicated crazing to be responsible for the transition in fracture behavior, i.e. remarkable toughening of PA-612 at the critical rubber phase domain size range of ∼0.2–0.3 μm.     

Optical constants and crystal chemical parameters of Sc2W3O12

The refractive indices of Sc₂W₃O₁₂, measured at wavelengths of 435.8–643.8 nm, were used to calculate n_a = 1.7331, n_b = 1.7510, n_c = 1.7586 at λ = 589.3 nm and n_∞ values at λ = ∞ from a one-term Sellmeier equation. Mean refractive indices, 〈n_D〉, and mean dispersion values, 〈A〉, are, respectively, 1.7475 and 110 × 10^?16 m². Total electronic polarizabilities, α_obs, were calculated from n_∞ and the Lorenz–Lorentz equation. The unusually large difference between the observed polarizability of 28.415 Å³ and the calculated total polarizability α_T of 26.74 Å³ (Δ = +6.3%) is attributed to (1) a large M–O–W angle, and (2) a high degree of W 5d–O 2p and Sc nd–O 2p hybridization, where n signifies unspecified Sc d orbitals.     

Exploration of structural,thermal, vibrational and spectroscopic properties of new noncentrosymmetric double borate Rb3NdB6O12

New noncentrosymmetric rare earth borate Rb₃NdB₆O₁₂ is found in the ternary system Rb₂O–Nd₂O₃–B₂O₃. The Rb₃NdB₆O₁₂ powder was fabricated by solid state synthesis at 1050 K for 72 h and the crystal structure was obtained by the Rietveld method. Rb₃NdB₆O₁₂ crystallized in space group R32 with unit cell parameters a = 13.5236(4), c = 31.162(1) Å, Z = 3. From DSC measurements, the reversible phase transition (I type) in Rb₃NdB₆O₁₂ is observed at 852–936 K. The 200 μm thick tablet is transparent over the spectral range of 0.3–6.5 μm and the band gap is found as E_g ～ 6.29 eV. Nonlinear optical response of Rb₃NdB₆O₁₂ tested via SHG is estimated to be higher than that of K₃YB₆O₁₂. Blue shift of Nd luminescent lines is found in comparison with other borates. The vibrational parameters of Rb₃NdB₆O₁₂ are evaluated by experimental methods.     

Investigations on the MnO2-Fe2O3 system roasted in air atmosphere

Solid-state reaction method is a common and effective technique to synthesize ferrites. This work investigated the phase transformation of MnO₂ and Fe₂O₃ system roasted at 500–1400 °C in air atmosphere to understand the formation process of manganese ferrite. The results showed that the formation of manganese ferrite (Mn_xFe_3?xO₄) was derived from the reaction between Fe₂O₃ and Mn₃O₄ (the decomposition product of MnO₂). Below 900 °C, MnO₂ firstly decomposed to Mn₂O₃ and then to Mn₃O₄, and Fe₂O₃ was seldom reacted with Mn₂O₃ and Mn₃O₄. When the temperature went up to 1000 °C, Fe₂O₃ easily reacted with Mn₃O₄ to generate manganese ferrite. The reaction degree was enhanced dramatically with the rising of temperature. Moreover, the x value in the Mn_xFe_3?xO₄ increased from 0 to 1 from 900 °C to 1400 °C. In other words, the higher the temperature was, the closer the Mn_xFe_3?xO₄ was to MnFe₂O₄. Thermodynamic analysis of MnO₂-Fe₂O₃ system under different O₂ partial pressures was carried out to further explain the formation mechanism.     

Contact properties determination of macroscopic fine disperse glass particles via compression tests in normal direction

The paper describes the deformation behavior of spherical, dry and non-porous particles during a single particle compression test in normal direction. Therefore a compression tester was built. Industrial used soda lime glass particles with two macroscopic fine disperse sizes (d_1,50,3 = 284.30 μm and d_2,50,3 = 513.20 μm) were applied as model material to investigate the micromechanical contact behavior. In order to influence the elastic-plastic contact properties of particles, the surfaces were altered with chemical modification by means of silanization.The determination of various micromechanical contact properties (e.g. adhesion force, modulus of elasticity and contact stiffness) was carried out model-based with the contact model ‘stiff particles with soft contacts’ by means of a back-calculation.It could be shown that the model-based determination of material properties was a good alternative compared to the comprehensive tensile tests and pull-off force measurements.In addition to the gained normal force-displacement data in normal direction, the friction limits for tangential loading and rolling with the load-dependent adhesion force were model-based determined.     

Entropy generation in a heat exchanger working with a biological nanofluid considering heterogeneous particle distribution

Entropy generation rates considering particle migration are evaluated for a biologically produced nanofluid flow in a mini double-pipe heat exchanger. The nanofluid is used in tube side and hot water flows in annulus side. Silver nanoparticles synthesized through plant extract method from green tea leaves are utilized. Particle migration causes non-uniform concentration distribution, and non-uniformity intensifies by increase in Reynolds number and concentration. The results indicate that at high concentrations and Reynolds numbers, particle migration can have a great effect on entropy generation rates. For water inlet temperature of 308 K, the contribution of friction in nanofluid entropy generation is much more than that of heat transfer. However, as the water inlet temperature increases to 360 K, the heat transfer contribution increases such that at low Reynolds numbers, the thermal contribution exceeds the frictional one. For total heat exchanger, Bejan number is smaller than 0.2 at water inlet temperature of 308 K, while Bejan number has a large value at water inlet temperature of 360 K. Furthermore, entropy generation at the wall has an insignificant contribution, such that for Re = 1000 and φ_m = 1%, the total entropy generation rates for the nanofluid, wall, and water are 0.098810, 0.000133, and 0.041851 W/K, respectively.     

Defect detection in CFRP by infrared thermography with CO2 Laser excitation compared to conventional lock-in infrared thermography

This paper presents a NDT by a CO₂ Laser infrared thermography applied to defect detection in CFRP. The CO₂ Laser is an infrared laser with the wavelength of 10.6 μm. This excitation has a controllable heating beam by a geometric relation D = 0.01575·d, which allows to heat the samples at a specific position (placed at the distance “d”) and area (of a diameter “D”). The PPT interpretation principle was used to reduce the non-uniformity’s effect of the excitation causing inhomogeneous heat. The test with this excitation is much faster than the tests with conventional lock-in thermography method.     

Study of high-pressure air curtain and combined dedusting of gas water spray in multilevel ore pass based on CFD-DEM

In order to solve the problem of dust pollution caused by ore unloading in ore pass, this paper, taking Li Lou Mining as a case study, conducted the wind speed variation law in the fluid domain and the impact of the collision between the ore in the unloading process on the fluid to determine the key dust control point based on the CFD-DEM coupling software. By Fluent software, the air curtain dust-proof efficiency under the action of unloading airflow is analyzed, and the relationship between the dust-control wind speed and the impinging airflow is known. And an experimental model of gas water spray is established to analyze the effect of spray dust removal. By analyzing the impact airflow and dust migration caused by ore unloading and the effect of air curtain dust control through numerical simulation, it can be seen that when the ore discharging quantity M_o = 4000 kg, the dust production is mainly concentrated in the fourth middle section. By high-pressure air shield assisting dust removal, dust diffusion can be better controlled when the ratio of impact wind speed of ore pass wellhead (denoted as λ) to high-pressure air curtain wind speed (denoted as ζ) is at least 1:8. When the dust removal effect is optimal, the ratio δ of the water supply amount q_l and the gas supply amount Q_g is determined by the gas water spray dust control experimental platform.     

High energy-storage properties of Bi0.5Na0.5TiO3-BaTiO3-SrTi0.875Nb0.1O3 lead-free relaxor ferroelectrics

New lead-free ferroelectric (0.94-x)Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-xSrTi_0.875Nb_0.1O₃ (BNBT-STN, x = 0 and 0.2) are synthesized by using a solid state reaction process. In this work, an obvious evolution of dielectric relaxation behavior and slim P–E hysteresis loops with high P_max and low P_r is observed for BNBT-0.2STN, indicating the dominant of ergodic relaxor phase with dynamic polar nano-regions (PNRs). A relatively large recoverable energy density (W_rec = 1.17 J/cm³) with high energy efficiency (η = 91%) is obtained. Furthermore, it shows small variation (9%) in the temperature range of 30–150 °C and fatigue-free behavior, which can be attributed to the absence of ferroelectric domain in the relaxor phase. The achievement of these characteristics provides that tailoring by B-site vacancies is a potential route when designing a new energy-storage system for BNT-based relaxor ferroelectric materials.     

Thickness dependence of optical parameters for Ge–Se–Te thin films

The present work deals with thickness dependent study of the thin films of Ge₁₀Se_90 − xTe_x (x = 0, 10) chalcogenide glasses. Bulk samples of Ge₁₀Se₉₀ and Ge₁₀Se₈₀Te₁₀ have been prepared by melt quenching technique. Thin films (thickness d = 800 nm and 1100 nm) of the prepared samples have been deposited on glass substrate using vacuum evaporation technique. The optical parameters i.e. optical band gap (E_g^opt), absorption coefficient (α), refractive index (n) and extinction coefficient (k) are calculated from the transmission spectrum in the range 400–1500 nm. The optical band gap decreases with the increase of thickness from 1.87 ± 0.01 eV (d = 800 nm) to 1.80 ± 0.01 eV (d = 1100 nm) for Ge₁₀Se₉₀ and from 1.62 ± 0.01 eV (d = 800 nm) to 1.48 ± 0.01 eV (d = 1100 nm) for Ge₁₀Se₈₀Te₁₀ thin films.     

Microstructure characterization,mechanical properties,compressibility and sintering behavior of Al-B4C nanocomposite powders

In the present work, Al-xB₄C nanocomposite (x = 0, 1, 2, 3, 4 and 5 in wt%, having the average B₄C size of 50 nm) were prepared using a high-energy ball mill. The milling times up to 16 h were applied. Then, the microstructural evolutions, mechanical properties, compressibility and sintering behavior of nanocomposites were investigated. The changes in powders morphology and microstructure during the milling process were characterized by laser diffraction particle size analyzer (LDA), SEM, XRD, EDS and TEM techniques. Compressibility and sintering behavior of milled powders compacted under different pressures (100–900 MPa) and at different sintering temperatures (500, 550 and 600 °C) were also studied. The pressing behavior of the nanocomposites was analyzed using linear compaction equations developed by Heckel, Panelli-Filho and Ge. The results showed the significant effects of B₄C amounts and sintering temperatures on the compressibility and sintering behavior of nanocomposites. The increase in the B₄C amount led to a decrease in both the compressibility rate and the sinterability of specimens. The maximum compression strength of 265 MPa and Vickers hardness of 165 VHN were obtained for Al-5 wt.% B₄C nanocomposite milled for 16 h followed by sintering at 600 °C.     

Ultra-high-temperature tensile properties and fracture behavior of ZrB2-based ceramics in air above 1500 °C

Nearly fully dense ZrB₂–SiC–graphite composites were fabricated from commercially available powder at 1900 °C by hot pressing. The tensile strength of ZrB₂-based ceramics was measured in air up to 1750 °C, which is the first reported tensile strength measurement in air above 1500 °C. A mechanical testing apparatus capable of testing material in ultra-high temperature under air atmosphere was built, evaluated, and used. Tensile strength was measured as a function of temperature up to 1750 °C in air. The respective average values of the tensile strength measured at 1550 °C, 1650 °C, and 1750 °C are 58.4, 44.8, and 21.8 MPa, which are 49.4%, 37.9%, and 18.4% of their room-temperature strength (118.2 MPa), respectively. Moreover, the tensile fracture behaviors and mechanism of ZrB₂-based ceramics at different testing temperatures were discussed based on microstructure characterization.     

Analysis of micro-tubular SOFC stability under ambient and operating temperatures

The stability of micro-tubular solid oxide fuel cell(MT-SOFC)is predicted at ambient and operating temperatures via simulation method.The results reveal that as long as the anode failure probability satisfies the failure criterion of 1E-6 at ambient temperature,the anode will retain its structural integrity at operating temperature.For the electrolyte or cathode,the stress strength ratio at operating temperature is significantly higher than that at ambient temperature.For an inappropriate component thickness,the cathode maybe fractures at operating temperature.In order to ensure the stability of MT-SOFC,the cathode thickness must be smaller than the maximum cathode thickness(t_(max–cathode)),which is derived from:t_(max–cathode)=5.49+5.54 te     

The origin of defects formation in nanosized zirconia

In the present work, we provide evidence of an increase in the Zr³⁺ paramagnetic center concentration under colloidal transformation in ZrO₂ prepared by the totally inorganic sol–gel method. The samples of thermally treated (100–950 °C in air) ZrO₂ precipitates and dried sol–gel products—xerogels—were examined by TEM, XRD, DTA, EPR and IR-spectroscopy. Under the same heat treatment condition, the concentration of Zr³⁺ (axially symmetric signal at g_⊥ = 1.977–1.979 and g₆ = 1.958–1.963) became higher in the xerogel samples rather than in precipitate samples and reached the maximum (∼ 10¹⁸ g^− 1 in xerogel) after calcinations at 500 °C in air.