Underground coal mines use mechanical bolts in addition to other types of bolts to control the rib deformation and to stabilize the yielded coal ribs.Limited research has been conducted to understand the performance o...Underground coal mines use mechanical bolts in addition to other types of bolts to control the rib deformation and to stabilize the yielded coal ribs.Limited research has been conducted to understand the performance of the mechanical bolts in coal ribs.Researchers from the National Institute for Occupational Safety and Health(NIOSH)conducted this work to understand the loading characteristics of mechanical bolts(stiffness and capacity)installed in coal ribs at five underground coal mines.Standard pull-out tests were performed in this study to define the loading characteristics of mechanical rib bolts.Different installation torques were applied to the tested bolts based on the strength of the coal seam.A typical tri-linear load-deformation response for mechanical bolts was obtained from these tests.It was found that the anchorage capacity depended mainly on the coal strength.Guidelines for modeling mechanical bolts have been developed using the tri-linear load-deformation response.The outcome of this research provides essential data for rib support design.展开更多
This paper presents an optimization methodology for the geometric configuration of a room–and–pillar mining project,considering safety and operational restrictions while maximizing ore recovery.An underground mangan...This paper presents an optimization methodology for the geometric configuration of a room–and–pillar mining project,considering safety and operational restrictions while maximizing ore recovery.An underground manganese mine was chosen as a case study to investigate the capabilities of the presented methodology.A software package(OPTIMINE)was implemented to address the computational demand in an automated manner.Three–dimensional finite difference analyses were performed in FLAC3D and used as implicit functions to consider safety in terms of the factor of safety and room convergence.The obtained results showed that recovery could be increased from 44%to more than 80%in a safe manner.展开更多
In an arcjet thruster,the cathode and constrictor degrade with time,and the electrical arc discharge may become unsymmetrical.In this work,a three-dimensional numerical model of a hydrogen plasma arcjet is developed a...In an arcjet thruster,the cathode and constrictor degrade with time,and the electrical arc discharge may become unsymmetrical.In this work,a three-dimensional numerical model of a hydrogen plasma arcjet is developed and validated to study the effect of unsymmetrical electric arc discharge on thruster performance.The unsymmetrical arc discharge is realized by introducing a radial shift of the cathode so that the cathode tip offset is 80μm(25%of the constrictor radius).Simulations are conducted for both axially centered cathode(coaxial)and off-centered cathode(non-coaxial)configurations with identical propellant flow rates and input current.Simulations show asymmetrical arc discharge in the non-coaxial cathode configuration,resulting in azimuthally asymmetric Joule heating,species concentrations,and velocity field.This asymmetry continues as the plasma expands in the divergent section of the nozzle.Temperature,species concentrations,and axial velocity exhibit asymmetric radial distribution at the nozzle exit.The computed Joule heating was found to reduce with cathode shift,and consequently,the thrust and specific impulse of the thruster was decreased by about 6.6%.In the case of the non-coaxial cathode,geometric asymmetry also induces a small side thrust.展开更多
Sloshing is relevant in several applications like ship tanks,space and automotive industry and seiching in harbours.Due to the relationship between ship and sloshing motions and possibility of structural damage,it is ...Sloshing is relevant in several applications like ship tanks,space and automotive industry and seiching in harbours.Due to the relationship between ship and sloshing motions and possibility of structural damage,it is important to represent this phenomenon accurately.This paper investigates sloshing at shallow liquid depths in a rectangular container using experiments and RANS simulations.Free and forced sloshing,with and without baffles,are studied at frequencies chosen specifically in proximity to the first mode natural frequency.The numerically calculated free surface elevation is in close agreement with observations from experiments.The upper limit of the resonance zone,sloshing under different filling depths and roll amplitudes and sloshing with one,two and four baffles are also investigated.The results show that the extent of the resonance zone is reduced for higher filling depth and roll amplitude.It is also found that the inclusion of baffles moves the frequency at which the maximum free surface elevation occurs,away from the fundamental frequency.Finally,a submerged baffle is found to dissipate more energy compared to a surface piercing baffle and that the effect of several submerged baffles is similar to that of a single submerged baffle.展开更多
In this study, the passage of waves through pile groups with different arrangements is investigated using a three-dimensional(3D)numerical model. For the simulations, waves of three different heights of 36, 58, and 81...In this study, the passage of waves through pile groups with different arrangements is investigated using a three-dimensional(3D)numerical model. For the simulations, waves of three different heights of 36, 58, and 81 mm, a fixed period of 0.88s, and a fixed wave length of 1.128 m were used. To simulate the waves and flow pattern through the piles, Reynolds-averaged Navier–Stokes(RANS) equations of fluid motion were solved based on the finite volume method(FVM). Piles were defined as obstacles in the rectangular domain using the fractional area/volume obstacle representation(FAVOR) method. The volume-of-fluid(VOF) and re-normalization group(RNG) methods were used to simulate the free surface and turbulence phenomenon, respectively. By performing different numerical simulations, the effect of coastal pile arrangements on wave pattern was studied and was compared with existing experimental data, and an acceptable agreement was achieved.展开更多
Mitchell's solution is commonly used to determine the required strength of vertically exposed cemented backfill in mines. Developed for drained backfill, Mitchell model assumed a zero friction angle for the backfi...Mitchell's solution is commonly used to determine the required strength of vertically exposed cemented backfill in mines. Developed for drained backfill, Mitchell model assumed a zero friction angle for the backfill. Physical model tests were performed. Good agreements were obtained between the required strengths predicted by the analytical solution and experimental results. However, it is well-known that zero friction angle can only be possible in terms of total stresses when geomaterials are submitted to unconsolidated and undrained conditions. A revisit to Mitchell's physical model tests reveals that both the laboratory tests performed for obtaining the shear strength parameters of the cemented backfill and the box stability tests were conducted under a condition close to undrained condition. This explains well the good agreement between Mitchell's solution and experimental results. Good agreements are equally obtained between Mitchell's experimental results and FLAC3 D numerical modeling of shortterm stability analyses of exposed cemented backfill.展开更多
Drying is a complicated physical process which involves simultaneous heat and mass transfer in the removal of solvents inside propellants.Inappropriate drying techniques may result in the formation of a hard skin laye...Drying is a complicated physical process which involves simultaneous heat and mass transfer in the removal of solvents inside propellants.Inappropriate drying techniques may result in the formation of a hard skin layer near the surface to block the free access of most solvent through for long stick propellants with large web thickness,which lead to lower drying efficiency and worse drying quality.This study aims to gain a comprehensive understanding of drying process and clarify the mechanism of the blocked layer near the propellant surface.A new three-dimensional coupled heat and mass transfer(3D-CHMT)model was successfully developed under transient conditions.The drying experiment results show that the 3DCHMT model could be applied to describe the drying process well since the relative error of the content of solvent between simulation and experiment values is only 5.5%.The solvent behavior simulation demonstrates that the mass transfer process can be divided into super-fast(SF)and subsequent minorfast(MF)stages,and the SF stage is vital to the prevention of the blocked layer against the free access for solvent molecules inside propellant grains.The effective solvent diffusion coefficient(Deff)of the propellant surface initially increases from 3.4×10^(-6)to 5.3×10^(-6)m^(2)/s as the temperature increases,and then decreases to 4.1×10^(-8)m^(2)/s at 60-100 min.The value of Deffof surface between 0-1.4 mm has a unique trend of change compared with other regions,and it is much lower than that of the internal at100 min under simulation conditions.Meanwhile,the temperature of the propellant surface increases rapidly at the SF stage(0-100 min)and then very slowly thereafter.Both the evolution of Deffand temperature distribution demonstrate that the blocked layer near the propellant surface has been formed in the time period of approximately 0-100 min and its thickness is about 1.4 mm.To mitigate the formation of blocked layer and improve its drying quality of finial propellant products effectively,it should be initially dried at lower drying temperature(30-40℃)in 0-100 min and then dried at higher drying temperature(50-60℃)to reduce drying time for later drying process in double base gun propellants.The present results can provide theoretical guidance for drying process and optimization of drying parameters for long stick propellants with large web thickness.展开更多
Three-dimension isothermal flows of polymer melt in the kneading blocks of triangularly-arranged and parallelly-arranged intermeshing co-rotating three-screw extruders are simulated using the finite element package PO...Three-dimension isothermal flows of polymer melt in the kneading blocks of triangularly-arranged and parallelly-arranged intermeshing co-rotating three-screw extruders are simulated using the finite element package POLYFLOW. Based on the velocity fields calculated, the particle trajectories in both machines are visualized using particle tracking technique. The numerical results indicate that the flow patterns in three-screw extruders are similar to those in twin-screw extruders. The triangularly-arranged three-screw extruder has the largest pumping capacity and also the highest extrusion stability in terms of flowrate fluctuation with screw rotation. The instantaneous mixing and cumulative residence time distribution （RTD） characteristics are also analyzed and compared with traditional intermeshing co-rotating twin-screw extruders. It is shown that the start section of the cumulative RTD curve for the triangularly-arranged machine has a small shoulder, which is attributed to the faster flow in the central region of this type of extruder.展开更多
Accurately estimating load distributions and ground responses around underground openings play a significant role in the safety of the operations in underground mines.Adequately designing pillars and other support mea...Accurately estimating load distributions and ground responses around underground openings play a significant role in the safety of the operations in underground mines.Adequately designing pillars and other support measures relies highly on the accurate assessment of the loads that will be carried by them,as well as the load-bearing capacities of the supports.There are various methods that can be used to approximate mining-induced loads in stratified rock masses to be used in pillar design.The empirical methods are based on equations derived from large databases of various case studies.They are implemented in government approved design tools and are widely used.There are also analytical and numerical techniques used for more detailed analysis of the induced loads.In this study,two different longwall mines with different panel width-to-depth ratios are analyzed using different methods.The empirical method used in the analysis is the square-decay stress function that uses the abutment angle concept,implemented in pillar design software developed by the National Institute for Occupational Safety and Health(NIOSH).The first numerical method used in the analysis is a displacement-discontinuity(DD)variation of the boundary element method,LaModel,which utilizes the laminated overburden model.The second numerical method used in the analysis is Fast Lagrangian Analysis of Continua(FLAC)with the numerical modeling approach recently developed at West Virginia University which is based on the approach developed by NIOSH.The model includes the 2D slice of a cross-section along the width of the panel with the chain pillar system that also includes the different stratigraphic layers of the overburden.All three methods gave similar results for the shallow mine,both in terms of load percentages and distribution where the variation was more obvious for the deep cover mine.The FLAC3D model was observed to better capture the stress changes observed during the field measurements for both the shallow and deep cover cases.This study allowed us to see the shortcomings of each of these different methods.It was concluded that a numerical model which incorporates the site-specific geology would provide the most precise estimate for complex loading conditions.展开更多
文摘Underground coal mines use mechanical bolts in addition to other types of bolts to control the rib deformation and to stabilize the yielded coal ribs.Limited research has been conducted to understand the performance of the mechanical bolts in coal ribs.Researchers from the National Institute for Occupational Safety and Health(NIOSH)conducted this work to understand the loading characteristics of mechanical bolts(stiffness and capacity)installed in coal ribs at five underground coal mines.Standard pull-out tests were performed in this study to define the loading characteristics of mechanical rib bolts.Different installation torques were applied to the tested bolts based on the strength of the coal seam.A typical tri-linear load-deformation response for mechanical bolts was obtained from these tests.It was found that the anchorage capacity depended mainly on the coal strength.Guidelines for modeling mechanical bolts have been developed using the tri-linear load-deformation response.The outcome of this research provides essential data for rib support design.
文摘This paper presents an optimization methodology for the geometric configuration of a room–and–pillar mining project,considering safety and operational restrictions while maximizing ore recovery.An underground manganese mine was chosen as a case study to investigate the capabilities of the presented methodology.A software package(OPTIMINE)was implemented to address the computational demand in an automated manner.Three–dimensional finite difference analyses were performed in FLAC3D and used as implicit functions to consider safety in terms of the factor of safety and room convergence.The obtained results showed that recovery could be increased from 44%to more than 80%in a safe manner.
基金the Indian Space Research Organization(VSSC-ISRO)for funding this research through ISRO-IITM Cell。
文摘In an arcjet thruster,the cathode and constrictor degrade with time,and the electrical arc discharge may become unsymmetrical.In this work,a three-dimensional numerical model of a hydrogen plasma arcjet is developed and validated to study the effect of unsymmetrical electric arc discharge on thruster performance.The unsymmetrical arc discharge is realized by introducing a radial shift of the cathode so that the cathode tip offset is 80μm(25%of the constrictor radius).Simulations are conducted for both axially centered cathode(coaxial)and off-centered cathode(non-coaxial)configurations with identical propellant flow rates and input current.Simulations show asymmetrical arc discharge in the non-coaxial cathode configuration,resulting in azimuthally asymmetric Joule heating,species concentrations,and velocity field.This asymmetry continues as the plasma expands in the divergent section of the nozzle.Temperature,species concentrations,and axial velocity exhibit asymmetric radial distribution at the nozzle exit.The computed Joule heating was found to reduce with cathode shift,and consequently,the thrust and specific impulse of the thruster was decreased by about 6.6%.In the case of the non-coaxial cathode,geometric asymmetry also induces a small side thrust.
基金Open access funding provided by NTNU Norwegian University of Science and Technology(incl St.Olavs Hospital Trondheim University Hospital)。
文摘Sloshing is relevant in several applications like ship tanks,space and automotive industry and seiching in harbours.Due to the relationship between ship and sloshing motions and possibility of structural damage,it is important to represent this phenomenon accurately.This paper investigates sloshing at shallow liquid depths in a rectangular container using experiments and RANS simulations.Free and forced sloshing,with and without baffles,are studied at frequencies chosen specifically in proximity to the first mode natural frequency.The numerically calculated free surface elevation is in close agreement with observations from experiments.The upper limit of the resonance zone,sloshing under different filling depths and roll amplitudes and sloshing with one,two and four baffles are also investigated.The results show that the extent of the resonance zone is reduced for higher filling depth and roll amplitude.It is also found that the inclusion of baffles moves the frequency at which the maximum free surface elevation occurs,away from the fundamental frequency.Finally,a submerged baffle is found to dissipate more energy compared to a surface piercing baffle and that the effect of several submerged baffles is similar to that of a single submerged baffle.
文摘In this study, the passage of waves through pile groups with different arrangements is investigated using a three-dimensional(3D)numerical model. For the simulations, waves of three different heights of 36, 58, and 81 mm, a fixed period of 0.88s, and a fixed wave length of 1.128 m were used. To simulate the waves and flow pattern through the piles, Reynolds-averaged Navier–Stokes(RANS) equations of fluid motion were solved based on the finite volume method(FVM). Piles were defined as obstacles in the rectangular domain using the fractional area/volume obstacle representation(FAVOR) method. The volume-of-fluid(VOF) and re-normalization group(RNG) methods were used to simulate the free surface and turbulence phenomenon, respectively. By performing different numerical simulations, the effect of coastal pile arrangements on wave pattern was studied and was compared with existing experimental data, and an acceptable agreement was achieved.
基金financial support from China Scholarship Council(CSC)under the Grant CSC No.201406460041financial support from the Natural Sciences and Engineering Research Council of Canada(NSERC 402318)+4 种基金the Institut de Recherche Robert-Sauvéen Santéet en Sécuritédu Travail(IRSST 2013-0029)Fonds de Recherche du Québec-Nature et Technologies(FRQNT 2015-MI-191676)the industrial partners of Research Institute on Mines and Environment(RIME UQAT-Polytechnique)The financial support from the National Science and Technology Support Program of China(No.2013BAB02B02)the Scientific Research Fund of Beijing General Research Institute of Mining and Metallurgy of China(No.YJ201507)
文摘Mitchell's solution is commonly used to determine the required strength of vertically exposed cemented backfill in mines. Developed for drained backfill, Mitchell model assumed a zero friction angle for the backfill. Physical model tests were performed. Good agreements were obtained between the required strengths predicted by the analytical solution and experimental results. However, it is well-known that zero friction angle can only be possible in terms of total stresses when geomaterials are submitted to unconsolidated and undrained conditions. A revisit to Mitchell's physical model tests reveals that both the laboratory tests performed for obtaining the shear strength parameters of the cemented backfill and the box stability tests were conducted under a condition close to undrained condition. This explains well the good agreement between Mitchell's solution and experimental results. Good agreements are equally obtained between Mitchell's experimental results and FLAC3 D numerical modeling of shortterm stability analyses of exposed cemented backfill.
基金supported by the National Natural Science Foundation of China(Grant No.22075146)。
文摘Drying is a complicated physical process which involves simultaneous heat and mass transfer in the removal of solvents inside propellants.Inappropriate drying techniques may result in the formation of a hard skin layer near the surface to block the free access of most solvent through for long stick propellants with large web thickness,which lead to lower drying efficiency and worse drying quality.This study aims to gain a comprehensive understanding of drying process and clarify the mechanism of the blocked layer near the propellant surface.A new three-dimensional coupled heat and mass transfer(3D-CHMT)model was successfully developed under transient conditions.The drying experiment results show that the 3DCHMT model could be applied to describe the drying process well since the relative error of the content of solvent between simulation and experiment values is only 5.5%.The solvent behavior simulation demonstrates that the mass transfer process can be divided into super-fast(SF)and subsequent minorfast(MF)stages,and the SF stage is vital to the prevention of the blocked layer against the free access for solvent molecules inside propellant grains.The effective solvent diffusion coefficient(Deff)of the propellant surface initially increases from 3.4×10^(-6)to 5.3×10^(-6)m^(2)/s as the temperature increases,and then decreases to 4.1×10^(-8)m^(2)/s at 60-100 min.The value of Deffof surface between 0-1.4 mm has a unique trend of change compared with other regions,and it is much lower than that of the internal at100 min under simulation conditions.Meanwhile,the temperature of the propellant surface increases rapidly at the SF stage(0-100 min)and then very slowly thereafter.Both the evolution of Deffand temperature distribution demonstrate that the blocked layer near the propellant surface has been formed in the time period of approximately 0-100 min and its thickness is about 1.4 mm.To mitigate the formation of blocked layer and improve its drying quality of finial propellant products effectively,it should be initially dried at lower drying temperature(30-40℃)in 0-100 min and then dried at higher drying temperature(50-60℃)to reduce drying time for later drying process in double base gun propellants.The present results can provide theoretical guidance for drying process and optimization of drying parameters for long stick propellants with large web thickness.
文摘Three-dimension isothermal flows of polymer melt in the kneading blocks of triangularly-arranged and parallelly-arranged intermeshing co-rotating three-screw extruders are simulated using the finite element package POLYFLOW. Based on the velocity fields calculated, the particle trajectories in both machines are visualized using particle tracking technique. The numerical results indicate that the flow patterns in three-screw extruders are similar to those in twin-screw extruders. The triangularly-arranged three-screw extruder has the largest pumping capacity and also the highest extrusion stability in terms of flowrate fluctuation with screw rotation. The instantaneous mixing and cumulative residence time distribution （RTD） characteristics are also analyzed and compared with traditional intermeshing co-rotating twin-screw extruders. It is shown that the start section of the cumulative RTD curve for the triangularly-arranged machine has a small shoulder, which is attributed to the faster flow in the central region of this type of extruder.
文摘Accurately estimating load distributions and ground responses around underground openings play a significant role in the safety of the operations in underground mines.Adequately designing pillars and other support measures relies highly on the accurate assessment of the loads that will be carried by them,as well as the load-bearing capacities of the supports.There are various methods that can be used to approximate mining-induced loads in stratified rock masses to be used in pillar design.The empirical methods are based on equations derived from large databases of various case studies.They are implemented in government approved design tools and are widely used.There are also analytical and numerical techniques used for more detailed analysis of the induced loads.In this study,two different longwall mines with different panel width-to-depth ratios are analyzed using different methods.The empirical method used in the analysis is the square-decay stress function that uses the abutment angle concept,implemented in pillar design software developed by the National Institute for Occupational Safety and Health(NIOSH).The first numerical method used in the analysis is a displacement-discontinuity(DD)variation of the boundary element method,LaModel,which utilizes the laminated overburden model.The second numerical method used in the analysis is Fast Lagrangian Analysis of Continua(FLAC)with the numerical modeling approach recently developed at West Virginia University which is based on the approach developed by NIOSH.The model includes the 2D slice of a cross-section along the width of the panel with the chain pillar system that also includes the different stratigraphic layers of the overburden.All three methods gave similar results for the shallow mine,both in terms of load percentages and distribution where the variation was more obvious for the deep cover mine.The FLAC3D model was observed to better capture the stress changes observed during the field measurements for both the shallow and deep cover cases.This study allowed us to see the shortcomings of each of these different methods.It was concluded that a numerical model which incorporates the site-specific geology would provide the most precise estimate for complex loading conditions.
