High Slip Velocity Research Articles

A numerical scale-up study of annular fluidized bed reactor with high centrifugal acceleration

A rotating fluidized bed imposes a strong centrifugal force to form the dense annular bed with high interphase slip velocity for significantly enhanced gas-solids contact. The fundamental fluidization behaviors in rotating fluidized beds have been well understood in small-scale systems. The current study focuses on the scale-up of a rotating fluidized bed from lab-scale to commercial-scale through computational fluid dynamics modeling. The computational model was first validated against a lab-scale horizontal rotating fluidized bed for the general flow behavior and pressure drop. The validated computational model with a column diameter of 0.12 m was then scaled up to 0.48 m and 1.92 m to investigate the flow hydrodynamics at larger scales. By comparing the solids distribution and other flow field variables, major limitations in the stable operation of a rotating fluidized bed at large scales were identified. The challenges faced in the scale-up of an annular fluidized bed with centrifugal acceleration are discussed and ways to overcome such challenges are explored.

Numerical modelling of earthquake sequences involving valving and pumping of fluids

SUMMARY Faults in the upper crust are sometimes thought to act as self-sealing valves, episodically releasing highly overpressured fluids trapped at greater depth during earthquakes. They are also often considered to be capable of actively pumping fluids into or out of faults in response to coseismic volumetric strain, thermal pressurization or other mechanisms. In this study, we investigate how these different types of behaviour (i.e. valve versus pump) are manifested in earthquakes. We do this using a 2-D plane strain model where frictional sliding on a thrust fault that is fed by a fluid source at its base is coupled to porous flow, thermal pressurization and strong variations in permeability. Our results show that thermal pressurization leads to dramatic dynamic weakening that produces earthquakes that propagate as slip pulses producing large stress drop, large slip and high slip velocities. On the other hand, valve-type behaviour typically produces smaller, less energetic earthquakes that commonly arrest before rupturing the entire fault. In some valve models, we observe complex compound ruptures and swarm activity, which is linked to the ascent of a propagating fluid pressure pulse driven by a large increase in permeability during sliding. Both pump and valve mechanisms can produce anomalously weak faults, though they are each associated with distinctly different fluid pressure and strength evolution over the seismic cycle and during rupture. Our models highlight the complex way in which fluids may interact with earthquakes, especially if valve and pump models coexist.

Dynamic shear behaviours of granite under coupled static and high-rate loadings

Understanding dynamic shear behaviours of rock under quasi-static confinement and dynamic loads is not only of great importance in underground engineering structures, but also beneficial to reveal earthquake mechanics regarding pre-stressed rocks fracturing from initial intact to rapid shear sliding. Here we propose a dynamic shear test setup and present full-field observations of stressed rock failure at high shear rate (up to γ˙ = 220 s−1) and sliding at high slip velocity over 10 m/s in the order of coseismic slip, with the integration of a novel triaxial Hopkinson bar (Tri-HB) and high-speed three-dimensional Digital Image Correlation (3D-DIC) techniques. Dynamic shear strength and friction coefficient are determined during the dynamic shear test under confinement, with different shear strain rates and pre-stresses. The dynamic shearing process can be divided into three stages, i.e. shear deformation, fracture degradation and dynamic frictional sliding. The high energy density of shear fracture band is observed, the rate and stress state dependences of rock shear strength and dynamic frictional coefficient are revealed. The dynamic shear strength of granite follows the Mohr-Coulomb criteria under varying pre-stresses and strain rates. The gouge materials with size of 1∼10 μm are generated within the high-rate shearing zone, and this appears to control the frictional resistance. The results could help determination of rock shear strength and post-shear failure, as well as explain the shear rupture and the resistance to slip mechanism of rock faults and strong near-surface earthquake events.

Temporal evolution of shear-induced dilatancy of rock fractures: controls from surface roughness and normal stress

SUMMARY Understanding the shear-induced dilatancy of rock fractures is important for assessing the permeability evolution and seismic hazard in shale and geothermal reservoirs. The displacement dependence of fracture dilation has been well studied, while the influence of slip velocity is poorly constrained. In this study, we combined displacement- and velocity-dependent aperture models to reproduce the transient shear-induced dilatancy of fractures in sandstone in 16 normal stress unloading tests. Our results show that the combined aperture model can describe the transient fracture aperture evolution during accelerating slip induced by normal stress unloading better than the model dependent only on slip displacement. Slip velocity could enhance the aperture increase on smoother fractures at lower normal stresses and higher slip velocities. Both the dilation factor and characteristic slip distance decrease with increasing normal stress and surface roughness, signifying reduced contribution of slip velocity to transient shear-induced dilatancy at higher normal stresses and surface roughness. The dilation angle increases with the increase of surface roughness, and this increase diminishes at higher normal stresses primarily attributable to more severe asperity wear. These findings highlight the importance of slip velocity in controlling the transient evolution of aperture and permeability of a rock fracture. Our study also provides constraints on the constitutive parameters in the combined aperture model for describing transient shear-induced fracture dilatancy. We suggest that it is crucial to incorporate the velocity-dependent aperture model to simulate the nonlinear evolution of fracture aperture in future analytical and numerical models involving coupled hydromechanical processes in geoenergy systems.

The effect of temperature on injection-induced shear slip of laboratory faults in sandstone

Fluid injection into subsurface reservoirs may cause existing faults/fractures to slip seismically. To study the effect of temperature on injection-induced fault slip, at a constant confining pressure of 10 MPa, we performed a series of injection-induced shear slip experiments on critically stressed sandstone samples containing saw-cut fractures (laboratory-simulated faults) under varying fluid pressurization rates (0.1 and 0.5 MPa/min, respectively) and temperatures (25, 80, and 140 °C, respectively). At 25 °C, slow fault slip events with a peak slip velocity of about 0.13 μm/s were observed on a tested sample in response to a low fluid pressurization rate of 0.1 MPa/min. In contrast, fluid injection with a high pressurization rate of 0.5 MPa/min caused fault slip events with a peak slip rate up to about 0.38 μm/s. In response to a given fluid pressurization rate, several episodes of slip events with a higher slip velocity were induced at an elevated temperature of 140 °C, indicating an appreciable weakening effect at elevated temperatures. We also experimentally constrained the rate-and-state frictional (RSF) parameters at varying effective normal stresses and temperatures by performing velocity-stepping tests. The obtained RSF parameters demonstrate that for a relatively high normal stress, increasing temperature tends to destabilize fault slip. Post-mortem microstructural observations reveal that elevated temperatures promote the generation of abundant fine-grained gouge particles associated with injection-induced shear slip. Our experiments highlight that injection-induced fault slip is affected by temperature-related wear production over the fault surface.

Nano‐bioconvective anisotropic slip flow in anisotropic porous medium with Coriolis force effects

AbstractThe present paper examines the impact of the Coriolis force and anisotropic Darcian porous medium on the nonsteady convective flow of bio‐nanofluid along a three‐dimensional stretchable surface. The problem has applications in extrusion processes and the geographical movement of tectonic plates under a moveable ocean. Emphasis is placed on the combined effects of anisotropic porous medium, anisotropic momentum slip and temperature, and microorganism slips at the wall. The pertaining equations are condensed to a set of similarity equations before being simulated using the Keller‐box method, an efficient numerical technique. The study reveals that the friction factor along the y‐axis decreases as the rotation parameter increases. Moreover, the heat and nanoparticle volume fraction movement gradient decrease along the y‐axis when the Darcy number and stretching rate increase. However, these rates increase with a higher velocity slip. In addition, the density of microorganisms in the bio‐nanofluid decreases when there are higher levels of unsteadiness, rotation, and Darcy numbers, as well as with increased bioconvection parameters.

Mapping surface defects in highly-filled wood fiber polymer composite extrusion from inline spectral analysis

Surface defects in highly-filled wood polymer composites (WPCs) are mapped in single-screw extrusion via inline optical spectral analysis for the first time. The effects of wood fiber content and drying on the dynamics of surface defects are spatio-temporally resolved via space–time inline optical imaging. Surface tearing appeared from the lowest shear rates investigated followed by a gradual decay in spectral intensity with increasing shear rates/slip velocities. This is accompanied by broadening of the surface tearing characteristic frequency while the average wavelength is estimated to remain constant within the experimental conditions. Increasing shear rates, drying and increasing wood fiber content showed mitigating effects on surface tearing. However, surface tearing in undried samples was still present even at the highest shear rates and high wall slip velocities. A regime where the extrudate surface is dominated by bubbles at high shear rates and low wood fiber contents in undried WPCs was identified.

COMPARISON OF SHORT-PERIOD SPECTRAL LEVEL OF CRUSTAL EARTHQUAKES IN JAPAN ESTIMATED BY DIFFERENT METHODS

We estimated short-period spectral level A of crustal earthquakes in Japan by three methods. The A of earthquakes after 1996 estimated by the method proposed by Dan et al.(2001) is 1/3 to 1/4 of the A of the Dan et al.’s relation. This difference is caused that long-period waveform inversion source models used by Dan et al. for earthquakes before 1995 have much shorter rise time and higher slip velocity than those for earthquakes after 1996. The A estimated by spectral inversion and from broadband source models for earthquakes after 1996 almost agree with the Dan et al.’s relation.

Effect of combining multi-jet component with axial swirl blade on evaporation in a spouted bed

To improve the fluidization behavior and the heat and mass transfer process in a spouted bed, a multi-jet–axial-swirl-blade spouted bed (MJ-ASB SB) was developed. The water evaporation process of the MJ-ASB SB was simulated and compared with those of the conventional spouted bed (CSB) and an integral multi-jet spout-fluidized bed (IMJSFB). The simulation results showed that the MJ-ASB SB combined the staged spouting action of multi-jet with the swirling action of the axial swirl blade, which promoted particle turbulence in the annulus region and ensured effective particle mixing. The swirl number of the MJ-ASB SB ranged from 0.0816 to 2.7239 with enhanced vortex intensity, thus promoting momentum and heat transfer of gas and particles in the spouted bed. The MJ-ASB SB had a higher slip velocity than the other two bed types, which indicates that the combined internal structure could improve the fluidization state of the bed and intensify the movement and mixing of phases in the spouted bed. The three-phase temperature, water evaporation rate, and gas humidity of the MJ-ASB SB were higher than those of the CSB and IMJSFB, and water evaporation occurred in an enlarged region in the MJ-ASB SB. The mass transfer intensification factors I of the MJ-ASB SB (2.62) and IMJSFB (1.92) were 91% and 161% higher than that of the CSB (1), respectively, indicating that the combined internal structure of the MJ-ASB SB significantly contributed to the water evaporation process.

CO2 Injection-Induced Shearing and Fracturing in Naturally Fractured Conventional and Superhot Geothermal Environments

This study elucidates the possibility of CO2 injection-induced formation of a complex cloud-fracture network (CFN) in granite, along with shearing (shear slip) of natural pre-existing fractures under conventional (~ 150–300 °C) and superhot (> ~ 400 °C) geothermal conditions, potentially providing additional connecting flow paths between the stimulated natural fractures. Here, we conduct a set of experiments under triaxial stress states at 150 °C and 450 °C on cylindrical granite samples containing a sawcut (representing a natural fracture) inclined 45° from the sample’s axis. CO2 injection induced dynamic shear slip on the sawcut, with higher slip velocities at 150 °C owing to the higher elasticity of the rock than that at 450 °C. The lower viscosity of CO2 also allowed it to more uniformly pressurize the sawcut, resulting in higher slip velocities and slip displacements compared with those based on water injection in the 150 °C experiments. This implies that under conventional geothermal conditions, CO2 injection can stimulate the same volume of rock as that of water injection at lower injection-well pressures. The CFNs then formed via CO2 injection at pressures similar to those predicted by the Griffith failure criterion as the sawcut shear slip progressed at both experimental temperatures. The experiments also revealed potential challenges associated with CO2 injection in naturally fractured geothermal environments, such as narrow aperture CFNs, owing to decreasing differential stress during their formation; all these factors should be addressed in future research.

Methane flow in nanopores: Analytical approximation based on MD simulations

In the current study by means of molecular dynamics (MD) computer simulation, we have investigated pressure-driven flow of methane at high temperature and pressures (398.15 K and 400–500 bar) in circular, square, and triangular nanopores of different size and different fluid–wall interaction strength. To generalize the results obtained in MD simulation, we have proposed an analytical approximation (ansatz) of fluid flow dependence on pore size, while the numerical coefficients have been derived from the fitting of computer simulation data. The formulae obtained take into consideration such nanoflow features as adsorption of fluid on the pore walls and high slip velocity, which are usually neglected in petrophysical simulators operating with continuum fluid approach. These analytical formulae can be then incorporated into various larger scale models used for calculation of fluid flow in nanoporous media. This study is aiming the investigation of general phenomena occurring near the fluid–wall interfaces at nanoscale.

Hydrodynamics-reaction-coupled simulations in a low-scale batch FCC regenerator: Comparison between an annular and a free-bubbling fluidized beds

Although annular fluidized beds (AFB) has been widely studied, a direct comparative evaluation of its reactive performance with bubbling fluidized beds (BFB) under similar operating conditions is still lacking. In this study, a comparative simulation using Euler–Lagrange MP-PIC method is performed under varying superficial gas velocities to assess their chemical reaction performance in relation to coke combustion rate, flue gas and temperature profiles in both reactors. The macroscale hydrodynamics of transport phenomenon, pressure fluctuation, solids holdup, slip velocity, and radial nonuniformity index (RNI) are also evaluated in detail. The results show, compared to BFB, the AFB shows big hydrodynamic differences, as indicated by the stronger cross-flow of solids, higher pressure fluctuation magnitudes, nonuniform lateral solids distribution, higher slip velocity and RNIs. However, reaction performance in relation to coke burning pattern and rates, flue gas distribution and temperature profile in the two beds are close. Under the constant bed inventory and air flowrate criteria, bigger changes in hydrodynamics are needed to affect the reaction performance significantly in the two small reactors due to the batch operation mode and the slow coke burning rate.

Large dynamic range, high resolution optical heterodyne readout for high velocity slip events.

We present a free-space optical displacement sensor for measuring geological slip event displacements within a laboratory setting. This sensor utilizes a fiberized Mach-Zehnder based optical heterodyne system coupled with a digital phase lock loop, providing a large dynamic range (multiple centimeters), high displacement resolution (with an amplitude spectral density of <10-10 m/Hz for frequencies above 100Hz), and high velocity tracking capabilities (up to 4.96m/s). This displacement sensor is used to increase the displacement and the time sensitivity for measuring laboratory-scale earthquakes induced in geological samples by using a triaxial compression apparatus. The sensor architecture provides an improved displacement and time resolution for the millisecond-duration slip events, at high containment and loading pressure and high temperatures. Alternatively, the sensor implementation can be used for other non-contact displacement readouts that required high velocity tracking with low noise and large dynamic range sensing. We use 13 high-velocity slip events in Fontainebleau sandstone to show the large dynamic range displacement tracking ability and displacement amplitude spectral densities to demonstrate the optical readout's unique sensing capabilities.

Hydrodynamics of inverse liquid-solid circulating fluidized bed

Hydrodynamics of inverse liquid–solid circulating fluidized bed (ILSCFB) is comprehensively studied with five types of low-density particles in a long experimental column of 5.4 m tall. The flow patterns observed mirror those of upright LSCFB under similar conditions. Axial distribution of solids holdup is uniform and radial solids holdup is also generally uniform but with some dilute region existing near the wall. The average solids holdup increases with Us while decreases with Ul. The effect of particle properties on the fluidization appears to be less extensive. The relationship between slip velocity and solids holdup in ILSCFB and inverse conventional fluidized bed is different. Clustering effect in liquid–solid circulating fluidized beds is first investigated. Particles with small particle Reynolds number tend to have more severe clustering as shown by the higher slip velocity. A modified Richardson-Zaki equation is developed for the prediction of solids holdup in inverse liquid–solid circulating fluidized bed.

Analysis of the properties of the mixture’s components on the basis of construction and polymeric materials waste

The paper presents the technologies of obtaining composite structures on the basis of secondary polymeric and construction materials filled with coordination compounds, got by the mixture’s coextrusion through the coextrusion head as well as by the extrusion pressing-out through the fillister head. It has been found out that with the decrease in the size of the layer’s boundary and with the increase of the layers’ durability the friction coefficient diminishes. Besides, reaching the friction coefficient’s value at the established stage happens during a longer period than for a single-layer structure. The value of the tear-and-wear for a multilayer composite structure is much less than for a single-layer one. A multilayer composite structure is recommended to be used as an antifriction material under low pressures at a high slip velocity. It is demonstrated that due to the addition of the filler on the basis of coordination compounds into separate layers during the coextrusion molding it is possible to obtain the material with the improved wear resistance.

Homogeneous ignition and volatile combustion of single solid fuel particles in air and oxy-fuel conditions

The ignition and volatile combustion of single coal particles were investigated under laminar conditions. Relevant physico-chemical processes were analyzed under conventional and oxy-fuel atmospheres with varying O2 contents in experiments and simulations. An optically accessible laminar flow reactor with well-defined boundary conditions measured with PIV and quantitative OH-LIF was employed. Multi-parameter optical diagnostics were conducted, including OH-LIF, luminescence imaging, and backlight illumination. Simultaneously acquired experimental data allowed for the evaluation of particle size, ignition delay time, and volatile combustion duration for individual particles. A statistical analysis revealed the improved accuracy of OH-LIF compared to luminescence imaging regarding ignition detection. Simulations within an Eulerian–Lagrangian framework were introduced and validated against experiments. On this basis, particle temperatures, local gas temperatures, and fuel mass fraction were evaluated, providing insights into the devolatilization. Both experimental and numerical results indicated that increasing particle sizes significantly retarded homogeneous ignition and volatile consumption. When increasing the O2 content, a shorter ignition delay time and volatile combustion duration were observed experimentally, which was more significant for larger particles. High slip velocities accelerated convective transport resulting in an earlier ignition and faster volatile combustion. An atmosphere change from N2 to CO2 showed an earlier ignition and increased volatile combustion duration for larger particles, whereas the differences were insignificant for small particles. Simulation results suggested that the local heat transfer was improved by CO2, mainly due to the lower temperature sink close to particles and hence higher volatile release rates. As the initial ambient temperatures were similar, the introduction of CO2 favored homogeneous ignition and slowed down the volatile consumption.

The effect of a gouge layer on rupture propagation along brittle shear fractures in deep and high-stress mines

The presence of fault gouge and the generation of wear material between two sliding rock surfaces plays a critical role in slip weakening and propagation of ruptures along underground brittle shear fractures forming ahead of tabular excavations in deep and high stress gold mining. We performed two types of friction experiments: one with a fault gouge layer between two sliding surfaces, and the other without a fault gouge layer ‘rock-on-rock’, both under room dry conditions at slip velocities ranging from ~1.0 mm/s to 1200 mm/s. These friction experiments revealed a remarkable difference in the frictional weakening behaviour, e.g., rock-on-rock friction experiments show weakening behaviour at lower slip velocity (~5 mm/s) and generally has lower frictional strength than those with the intervening fault gouge between sliding surfaces. This study shows that the existence of the fault gouge layer between sliding rock surfaces delays the onset of fault weakening (i.e., slip weakening displacement of gouge layer experiments is larger compared to rock-on-rock experiments). It is proposed that flash heating may be the main active weakening mechanism within both our gouge and rock-on-rock experiments, and provides a feasible account for the observed weakening. The observed slip weakening displacement (Dc) differences may be attributed to the presence of a gouge layer between sliding surfaces, which has many more contacts during sliding compared to rock-on-rock experiments, thus reducing the average slip velocity per contact, consequently, the potential for activation of flash heating which delays the onset of weakening. Here we suggest that we may be able to describe brittle shear fracture rupture propagation process along underground brittle shear zones by conducting low, intermediate and high slip velocity friction experiments with and without an intervening fault gouge between sliding rock surfaces. These findings should have important implications for the modelling of rupture propagation processes in underground shear zones, a phenomenon that influences the severity of rockbursts, and hence the safety of mine workers and mining operations.

Investigation on flow dynamics and temperatures of solid fuel particles in a gas-assisted oxy-fuel combustion chamber

Flow dynamics and temperatures of solid fuel particles strongly influence flame stabilization, local heat release and fuel conversion inside pulverized solid fuel combustors. To investigate these phenomena, experiments are carried out under well-controlled inflow and boundary conditions inside a gas-assisted, swirled oxy-fuel combustion chamber. Flow fields of small particles that represent the gas phase velocity are determined in the near-burner region by PIV using a particle separation algorithm. Trajectories of large solid fuel particles are evaluated in a two-dimensional plane using a combined high-speed PIV/PTV approach. Particle temperatures and particles sizes are measured at different levels downstream the burner exit to reveal different stages of combustion. Therefore, a two-color pyrometer is used that dissolve single particles to achieve local particle temperature and particle size distributions. Two oxy-fuel operation conditions with an oxygen fraction of 33%V and a reference operation point in air are investigated within this study. In the flow fields of the gas phase the impact of the atmosphere is clearly visible in the spatial expansion of the internal recirculation area. Regions of high slip velocities and high heat release could be identified by analyzing particle trajectories in terms of direction, velocity and acceleration. Residence times of small and large particles are estimated from the flow fields. Significantly larger residence times are observed for large particles which leads to higher burn out rates in the near-burner region. Furthermore, particle temperature measurements reveal similar particle temperatures for the investigated oxy-fuel and air operation conditions.

The Influence of an Operating Conditions on the Friction Coefficient in Transportation Machines Drives

Abstract The publication presents the construction of the station for testing the friction coefficient under high pressure and low slip velocity values. These conditions of cooperation of friction pairs occur in the drives of transport machines such as belt conveyors and suspended monorails. The value of the friction coefficient, depending on the operating conditions such as moisture and rock or coal dust pollution, has a significant impact on the correct and efficient operation of such drives. The features of the station allow mapping the operating conditions on a laboratory scale. As part of the research, appropriate friction samples made of rubber or polyurethane were prepared. Referring to the conditions of conveyor operation and conditions of contact of the conveyor belt with the drive drum, the values of unit pressures and values of slip speed occurring in the drum drive of the belt conveyor were determined. A series of laboratory tests were carried out for the friction pair rubber drum lining – conveyor belt cover. The tests were carried out for four different states of friction vapor surfaces, namely for dry and wet samples, as well as for samples in clean or contaminated with stone dust. As a result of the tests, the values of friction coefficients for various surface states were determined, which will be valuable information for designers of friction drive systems of transport devices.

Correlating Interlayer Spacing and Separation Capability of Graphene Oxide Membranes in Organic Solvents.

Membranes synthesized by stacking two-dimensional graphene oxide (GO) hold great promise for applications in organic solvent nanofiltration. However, the performance of a layer-stacked GO membrane in organic solvent nanofiltration can be significantly affected by its swelling and interlayer spacing, which have not been systematically characterized. In this study, the interlayer spacing of the layer-stacked GO membrane in different organic solvents was experimentally characterized by liquid-phase ellipsometry. To understand the swelling mechanism, the solubility parameters of GO were experimentally determined and used to mathematically predict the Hansen solubility distance between GO and solvents, which is found to be a good predictor for GO swelling and interlayer spacing. Solvents with a small solubility distance (e.g., dimethylformamide, N-methyl-2-pyrrolidone) tend to cause significant GO swelling, resulting in an interlayer spacing of up to 2.7 nm. Solvents with a solubility distance larger than 9.5 (e.g., ethanol, acetone, hexane, and toluene) only cause minor swelling and are thus able to maintain an interlayer spacing of around 1 nm. Correspondingly, GO membranes in solvents with a large solubility distance exhibit good separation performance, for example, rejection of more than 90% of the small organic dye molecules (e.g., rhodamine B and methylene blue) in ethanol and acetone. Additionally, solvents with a large solubility distance result in a high slip velocity in GO channels and thus high solvent flux through the GO membrane. In summary, the GO membrane performs better in solvents that are unlike GO, i.e., solvents with large solubility distance.