Search Results (63)

A methane-air premixed gas explosion is one of the most destructive disasters in the process of coal mining, and the dynamic coupling between the shock wave triggered by the explosion and the surrounding rock of the roadway can lead to the destabilization of the surrounding rock structure, the destruction of equipment, and casualties. The aim of this study is to systematically reveal the propagation characteristics of the blast wave, the spatial and temporal evolution of the wall load, and the damage mechanism of the surrounding rock by establishing a two-way fluid-solid coupling numerical model. Based on the Ansys Fluent fluid solver and Transient Structure module, a framework for the co-simulation of the fluid and solid domains has been constructed by adopting the standard $k-\epsilon$ turbulence model, finite-rate/eddy-dissipation (FR/ED) reaction model, and nonlinear finite-element theory, and by introducing a dynamic damage threshold criterion based on the Drucker–Prager and Mohr–Coulomb criteria. It is shown that methane concentration significantly affects the kinetic behavior of explosive shock wave propagation. Under chemical equivalence ratio conditions (9.5% methane), an ideal Chapman–Jouguet blast wave structure was formed, exhibiting the highest energy release efficiency. In contrast, lean ignition (7%) and rich ignition (12%) conditions resulted in lower efficiencies due to incomplete combustion or complex combustion patterns. In addition, the pressure time-history evolution of the tunnel enclosure wall after ignition triggering exhibits significant nonlinear dynamics, which can be divided into three phases: the initiation and turbulence development phase, the quasi-steady propagation phase, and the expansion and dissipation phase. Further analysis reveals that the closed end produces significant stress aggregation due to the interference of multiple reflected waves, while the open end increases the stress fluctuation due to turbulence effects. The spatial and temporal evolution of the strain field also follows a three-stage dynamic pattern: an initial strain-induced stage, a strain accumulation propagation stage, and a residual strain stabilization stage and the displacement is characterized by an initial phase of concentration followed by gradual expansion. This study not only deepens the understanding of methane-air premixed gas explosion and its interaction with the roadway’s surrounding rock, but also provides an important scientific basis and technical support for coal mine safety production. Full article

Based on a pre-constructed simplified chemical reaction mechanism for afterburning in exhaust plumes, this study integrates a gas–solid two-phase combustion flow model with numerical radiative transfer calculations to systematically explore the optimization of computational domains for exhaust plume simulations and reveal the regulatory mechanisms of flight parameters affecting on plume evolution. The results demonstrate that as altitude increases, the plume expands overall, the afterburning zone shifts rearward, and the peak radiation brightness is delayed but with a slight enhancement. Conversely, increasing flight velocity leads to axial elongation and radial compression of the plume, reduced afterburning intensity, and an overall decrease in radiative intensity. This study establishes a correlation between solid rocket motor flight parameters and plume dynamics, providing theoretical and practical guidance for suppressing infrared signature signals in solid rocket motors and designing multifunctional propellant formulations. Full article

In order to study the problem of obvious wall thinning in the wellbore caused by proppant backflow and sand production under throttling conditions in tight gas wells. Based on the gas-phase control equation, particle motion equation, and erosion model, the wellbore erosion model is established. The distribution law of pressure, temperature, and velocity trace fields under throttling conditions is analyzed, and the influences of different throttling pressures, particle diameters, and particle mass flows on wellbore erosion are analyzed. The flow field at the nozzle changes drastically, and there is an obvious pressure drop, temperature drop, and velocity rise. When the surrounding gas is completely mixed, the physical quantity gradually stabilizes. The erosion shape of the wellbore outlet wall has a point-like distribution. The closer to the throttle valve outlet, the more intense the erosion point distribution is. Increasing the inlet pressure and particle mass flow rate will increase the maximum erosion rate, and increasing the particle diameter will reduce the maximum erosion rate. The particle mass flow rate has the greatest impact on the maximum erosion rate, followed by the particle diameter. The erosion trend was predicted using multiple regression model fitting of the linear interaction term. The research results can provide a reference for the application of downhole throttling technology and wellbore integrity in tight gas exploitation. Full article

Gas voids inevitably form during the solidification of phase change materials (PCMs) due to volumetric contraction and thus deteriorate the thermal conductivity of solidified PCMs. In this work, the gas void morphology and distribution in solidified pure paraffin within a cubic thermal energy storage unit are experimentally studied. The three-dimensional structure of the solidified pure paraffin is reconstructed via computed tomography (CT) scanning with a resolution of up to 25 µm. Four distinct morphological types of gas voids are found, including irregular elliptical gas voids, elongated “needle-like” gas voids, micro gas voids, and large circular gas voids. The formation mechanisms of each type are analyzed. The morphology and distribution of gas voids indicate that the solidified pure paraffin structure is anisotropic. The effective thermal conductivity (ETC) of this solid–gas structure is numerically evaluated using lattice Boltzmann simulations, and a two-term power equation is fitted. The results show that the ETC in the vertical direction is significantly lower than in the horizontal direction and the ETC could be reduced by as much as 31.5% due to the presence of gas voids. Full article

The bypass dual-throat nozzle is based on the dual-throat nozzle, which is a fluidic thrust vector nozzle suitable for integration into rocket motors in a symmetrical manner. As the effects of gas–solid two-phase flows are essential for solid rocket motors (SRMs), this study employs the RNG k–ε turbulence model and a particle trajectory model to numerically simulate the three-dimensional flow field inside a fixed-geometry axisymmetric bypass dual-throat nozzle to investigate its two-phase flow characteristics and thrust vectoring performance. Numerical results reveal that the smaller-diameter particles exhibit better flow-following characteristics and have a more significant impact on nozzle performance. As particle size increases, particle trajectories gradually rise within the cavity and converge toward the nozzle axis until a critical value is exceeded, after which the distribution tends to disperse. Particle deposition occurs at the bends of the bypass channel, the upstream converging section of the nozzle, and the converging section of the cavity, underscoring the need for a reinforced geometric design and thermal protection. In addition, the introduction of the particle phase into the flow reduces the thrust-vectoring angle of the nozzle and results in a loss of thrust coefficient. This research has the potential to guide the design of engines according to the incorporation of metal powder in propellants and combustion control. Full article

Polymetallic nodules and REE-rich mud under the seabed of 5500–5700 m water depth around Minamitorishima island are promising and attractive for exploration and development. Following our previous research, numerical analysis was used to investigate the unsteady flow characteristics and the lifting performance of a commercial production system using an air-lift pump for hybrid lifting, lifting both polymetallic nodules and REE-rich mud. Gas–liquid–solid three-phase flow and gas–liquid two-phase flow in the system were analyzed using the one-dimensional drift–flux model. First, the reliability of the schemes and program was verified by comparing the numerical results with the experimental ones. Next, numerical simulations were conducted, in which the model’s dimensions were related to a commercial production system operated in the deep sea around Minamitorishima island, and the conditions fit the expected production rate. The results revealed the unsteady flow characteristics under the operations, such as start-up, shut-down, feed of polymetallic nodules and REE-rich mud, and those associated with disturbances, such as feed rate fluctuations. We demonstrate that the program and the schemes can simulate the unsteady flow characteristics and the lifting performance of a commercial production system with an air-lift pump well, and they can derive useful information and know-how in advance for the safe and continuous operation of the system. Full article

This study aims to investigate the characteristics of ash accumulation and slagging in boilers during low- and medium-load operation and to analyse the migration pattern of alkali metals in high-alkali coal. In this paper, the ash accumulation characteristics and slagging trend of the furnace interior under a 500 MW load were investigated using numerical simulation by comparing the ash accumulation and slagging characteristics under two different burner configurations, and analysing the slagging trend of the furnace with upper burner arrangement and lower burner arrangement by taking the deposition location on the furnace wall and the deposition rate and the temperature of the furnace wall as the indices. The existing formation of sodium in Jundong coal at different temperatures was investigated using computational methods; SiO₂, Al₂O₃, and kaolin were doped separately; and the migration and transformation characteristics of their different additives on the sodium-based compounds in Jundong coal were explored. The results showed that, under a 500 MW load, the size of the tangent circle formed in the furnace by commissioning the upper burner condition was larger than the lower burner, and the main combustion zone was larger than the lower burner. The ash accumulation of coal ash particles in the boiler was mainly concentrated in the hearth region, and the deposition rate was higher at the height regions of 10 m and 25 m in the hearth. The solid-phase NaCl transition temperature was reduced to 350 °C after the doping of SiO₂ in Jundong coal, and the doping of Al₂O₃ inhibited the transition of solid-phase NaCl, promoted the generation of gas-phase NaCl, and had certain inhibitory effects on the generation of sodium-based silica–aluminium compounds, the content of which at all temperatures was inversely proportional to the proportion of doping. The doping of kaolin promotes the transformation of solid-phase NaCl and inhibits the generation of gas-phase NaCl. Full article

High-temperature solid granules are by-products produced by various industrial processes and contain an obvious quantity of waste heat. Therefore, recovering their heat can not only reduce energy costs but also prevent polluting the environment, which has a significantly valuable sense of sustainable development. Computational fluid dynamics (CFD) technology is widely used to solve challenges involving heat recovery, which can simulate the heat and mass transfer processes of the gas–solid two-phase flow. Herein, a review about the mass flow analysis methods, including the Euler–Euler and Euler–Lagrange methods, as well as heat transfer mechanisms, covering heat conduction, heat convection and heat radiation, is made. Meanwhile, the bases of numerical models, mass flow and heat transfer are also summarized. In addition, at the end of the paper, a prospect about this research field is proposed. This article not only reviews common research methods but also summarizes relevant new models and methods that have emerged in recent years. Based on existing work, it both fully demonstrates the widespread application of CFD technology in the field of recovering heat from high-temperature solid granule fields and summarizes the development trends and further utilization prospects of the technology. Full article

The pressure oscillation caused by vortex–acoustic coupling is one of the main gain factors that results in the combustion instability of motors. Focusing on a solid rocket motor with a backward step configuration that can generate a corner vortex, this study aims to investigate the pressure oscillation characteristics in a combustion chamber under two-phase flow interactions through numerical simulations. The two-phase flow discrete phase model (DPM) was chosen to study particle motion and two-phase interactions. The numerical methodology was hence established by coupling the DPM with the large eddy simulation (LES) method. Taking the Clx motor as a reference and introducing aluminum oxide particles, two important particle parameters (diameter and concentration) and the key geometric parameters of the backward step were numerically studied. The numerical results show that both increased particle diameter and concentration can decrease the frequency and amplitude of pressure oscillations; additionally, the effects of geometric parameters on the pressure oscillations of the backward step, such as the downstream aspect ratio, the expansion ratio, and the step position, are basically consistent under both pure gas and two-phase flows. The influences of those geometric parameters are mainly reflected in defining the space for the development of upstream flow instability and the motion of downstream vortices. Compared with the pure-gas flow, the presence of aluminum oxide particles in two-phase flow globally decreases the vortex shedding frequency, the primary frequency, and the amplitude of pressure oscillations. It can also weaken the effects of vortex–acoustic coupling due to increased turbulent viscosity, which hinders the orderly development of vortices. Full article

Due to the orderly arrangement of tubular propellant, the permeability of combustion gases is improved, which is beneficial for enhancing the safety of the combustion system. However, current internal ballistic gas-solid flow calculation methods adopt a quasi-fluid assumption, which cannot accurately account for the characteristics of long tube shapes. Additionally, tubular propellants exhibit both overall movement and parameter distribution characteristics, necessitating the decoupling of gas and solid phases. These two deficiencies in previous studies have limited the effectiveness of gas-solid flow simulations for tubular propellant. This paper proposes a numerical calculation model suitable for tubular propellant charging based on the particle element method for internal ballistic two-phase flow. Firstly, considering the overall movement characteristics of tubular propellants, the concept of blank particle elements is introduced to represent pure gas phase regions. Then, based on computational requirements, the tubular propellants are divided to form the lumped element method and the multiple-element method. The moving boundary method is used to calculate the movement process of the propellant bed particle group and is compared with experimental results to verify the applicability of the two methods in tubular propellant beds. Analysis results show that the particle element method can effectively capture changes in the flow field inside the chamber and the position of tubular propellants. The lumped element method can quickly obtain the flow field distribution characteristics inside the chamber, while the multiple-element method can capture parameter distribution characteristics at different positions of the tubular propellants while ensuring overall movement. Full article

The nozzle, as a critical jet component in dry powder fire extinguishing systems, significantly affects jet characteristics through its geometric configuration. To explore the influence of structural parameters on ultrafine dry powder gas-solid two-phase jet characteristics, a bidirectional coupled numerical model based on the SST k-ω turbulence model and the Discrete Phase Model is employed. This study examines how variations in the semi-expansion angle (α) and semi-contraction angle (β) of the nozzle affect compressible gas flow behavior and particle distribution trajectories through a combination of simulations and experiments. The results indicate that when α = 2°, the gas jet is in an under-expanded state, leading to increased particle dispersion due to the stripping effect of the surrounding high-speed airflow. Within the range of x = 0–180 mm, the dry powder exhibits a diffusion trend. When α = 4.5°, the gas jet core region is the longest, providing optimal particle acceleration. Under constant inlet pressure, reducing α enhances particle collimation. The reduction of α alters the gas jet state, with α = 2° showing better powder diffusion compared to α = 6°. However, an excessively small α is detrimental to increasing the range of dry powder. With consistent structural parameters, the diffusion and range of dry powder remain the same across different β values, and variations in β have a relatively minor impact on supersonic jet characteristics. These findings offer theoretical guidance for optimizing and improving nozzles in ultrafine dry powder fire extinguishing systems. Full article

In order to solve the problem where the traditional intelligent inspection robot only has a single inspection function, we studied the use of a dry powder (including an ultra-fine dry powder) as a fire-extinguishing medium for the first time. In fire-extinguishing robots, the spray pressure is difficult to control, and there are several other issues. For integrated inspection, an intelligent, nitrogen-driven fire-extinguishing robot using a dry powder in a pressure-controlled spray was developed. On this basis, in order to investigate nitrogen-driven dry powder particle spraying as a gas–solid two-phase mechanism, as well as the flow characteristics and the influence of relevant parameters on the spraying effect, a nitrogen-driven dry powder particle spraying system was established as part of a gas–solid two-phase computational fluid dynamics model. The flow field of the spraying system and the particle motion characteristics were analyzed to explore the micro-mechanisms of the influence of different driving pressures, pipe diameters, and nozzle configurations on the spraying of the dry powder. In order to investigate the macroscopic effect of dry powder spraying where the gas–solid two-phase micro-mechanisms could not be revealed, an experimental platform was set up, and the experiments verified the accuracy of the numerical simulation results. We also investigated the dry powder spraying effect under different driving pressures, pipe diameters, nozzle configurations, and loading ratios. Finally, an orthogonal test was designed based on the results of the single-factor experiments to find the best combination of parameters required to achieve the optimal spraying effect. The research results can provide a theoretical and technical reference for the design and development of nitrogen-driven dry powder spraying systems. Full article

Traditional fossil energy sources still dominate the world energy structure. And fully utilizing biomass is a viable approach for energy transition. A bubbling fluidized bed has better heat and mass transfer, while particle agglomeration limits the development of its industrial application. In this paper, two-phase flow characteristics of a bubbling fluidized bed are investigated by combining numerical simulations and fluidized bed gasification experiments. Numerical simulations found that the bed fluidization height reached twice the initial fluidization height at the 0.054 m initial fluidization height with uniform particle distribution. Fluidized bed gasification experiments found that syngas yield increased with increasing temperature. The carbon conversion efficiency reached 79.3% and the effective gas production was 0.64 m³/kg at 850 °C. In addition, when the water vapor concentration reached 15%, the carbon conversion efficiency and effective gas production reached the maximum values of 86.01% and 0.81 m³/kg, respectively. Full article

Dual fluidized bed (DFB) reactor systems are widely used in gas–solid two-phase flow applications, whose gas–solid flow characteristics have a significant effect on the performance of many kinds of technologies. A numerical simulation model was established on the basis of a large-scale DFB reactor with a maximum height of 21.6 m, and numerical simulations focused on gas–solid flow characteristics were carried out. The effects of the superficial gas velocity of both beds and the static bed height and particle size on the distribution of the pressure and solid suspension density and the solid circulation rate were studied. The simulation results were in good agreement with the experimental data. With the strong support of the experimental data, the gas–solid flow characteristics of large-scale DFB reactors were innovatively evaluated in this numerical simulation study, which effectively makes up for the shortcomings of the current research. The results showed that the superficial gas velocity of both beds and the static bed height have different degrees of influence on the gas–solid flow characteristics. Specifically, for 282 μm particles, when the superficial gas velocity of both beds and the static bed height were 4.5 m/s, 2.5 m/s, and 0.65 m, respectively, under typical working conditions, the bottom pressure of the two furnaces was 3412.42 Pa and 2812.86 Pa, respectively, and the solid suspension density was 409.44 kg/m³ and 427.89 kg/m³, respectively. Based on the simulation results, the empirical formulas of the solid circulation rate were fitted according to different particle sizes. Under similar conditions, the solid circulation rates of particles with a particle size of 100 μm, 282 μm, 641 μm, and 1000 μm were 2.84–13.28, 0.73–4.91, 0.024–0.216, and 0.0026–0.0095 kg/(m²s), respectively. It can be found that the influence of the particle size on the solid circulation rate is the most significant among all parameters. Full article

A Gas–solid two-phase flow coupling simulation is widely used to study the working process of pneumatic seed dischargers. Due to the demand for deterministic particle orbit numerical calculation models, seeds are mostly modeled using the particle aggregation method, where the seed model is formed through particle aggregation bonding without overlapping. The smaller the radius and the more filled ball particles used in this method, the closer they resemble the real morphology of the seed. However, this results in the over-consumption of simulation computational resources and simulation time growth. In this study, we used wheat seeds as the research object, studied the effect of seed models with different filled ball radii on the kinetic response characteristics between the particles, and searched for the optimal number of filled ball particles for the seed model. With the help of three-dimensional scanning and inverse fitting methods to obtain the seed profile, we used different radii (0.2 mm, 0.24 mm, 0.28 mm, 0.32 mm, 0.36 mm, and 0.4 mm) to fill the ball particles, and formed a wheat particle bonding model for a gas–solid coupling simulation. We used a combination of real tests and simulation measurements of bottomless cylinder-lifting and slip-stacking. The interspecies static and dynamic friction factors in seed models with different radii of filled spherical particles were first calibrated using the angle of repose as an index. Then, the parameters were verified using bottomless cylinder lifting and slip stacking tests, which used the coefficient of variation for the simulation test’s angle of repose as an index. Our results show that the smaller the radius of the filled ball, the closer the simulation results were to the real value. Validation was conducted using a gas–solid coupling simulation of an air-blown wheat seed discharger, with the seed filling rate as an index. Our results showed that the simulation length and simulation accuracy were optimal when the radius of the filling particle was 0.32 mm. Full article