Gas Flow Characteristics Research Articles

Experimental study on the characteristics of buoyant gas flow produced by vertical gas leakage in tunnels under natural ventilation

In recent years, there have been reports of gas leakage accidents during the transportation of gas tankers in tunnels, potentially leading to destructive explosions. However, the overall distribution of gas concentration in the entire tunnel has not been experimentally investigated. This study presents a series of brine-water experiments to explore the characteristics of buoyant gas flow in tunnel leakage accidents. The gas distribution is categorized into a stably stratified longitudinal current and a source region. Gas concentration in the tunnel is correlated with the dimensionless reduced gravity, which increases with the source buoyancy flux but decreases with the source release velocity. A model is proposed to estimate the maximum gas concentration of the longitudinal current. The spatial range of the explosive region is influenced by the source buoyancy flux and the leakage velocity. However, the propagation velocity of the longitudinal current is primarily controlled by the source buoyancy flux, with the source momentum having a negligible effect. The thickness of the longitudinal gas flow, δ, linearly decreases from the source region to the tunnel ends. A dimensionless parameter, λ, is introduced to characterize δ. When λ < 1.25, δ rapidly increases with λ and then remains approximately constant when λ exceeds 1.25. This study contributes to a better understanding of buoyant gas flow in tunnel leakage accidents.

Spatter-flow interaction behavior under protective gas flow parameters and scanning directions

Spattering is a common issue in selective laser melting (PBF-LB), causing contamination of the powder bed and negatively affecting product quality. One solution to this problem is introducing an inert gas flow that can remove spatter particles. The gas flow's characteristics and interaction with spatter particles depend on process parameters and involve a multi-physical field coupling process. However, current research mainly focuses on spattering behavior under single process conditions and lacks systematic exploration of gas flow characteristics and their interaction with spattering. This paper investigates the gas flow-spatter interaction process in PBF-LB, constructing a coupled model of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM). The model investigates the effects of inlet height, inlet flow velocity and laser scanning direction on the deposition and removal of spattered particles. The study reveals that flow velocity and inlet height affect the dynamic behavior of spattering by altering the flow distribution, while the laser scanning direction influences spatter dynamics by changing the direction of spatter ejects. The rate of spatter removal increases as the angle between the scanning direction and flow direction, and the flow velocity, increase. Conversely, it decreases as the inlet height increases. This work provides theoretical understanding of the interaction between gas flow characteristics and spattering, which is significant for optimizing printing process parameters and improving printing quality.

Analysis of gas flow and PM movement characteristics inside diesel particulate filter channel

Based on experimental data from engine after-treatment tests, the diesel particulate filter (DPF) microscopic channel model was constructed using UG, and the accumulation of particulate matter (PM) in the deep bed filter layer and carbon deposit layer was analyzed through computational fluid dynamics (CFD). A comprehensive study was conducted on the exhaust gas flow characteristics and PM movement within the DPF channel, and focus on the effects of PM distribution type on capture and regeneration characteristics. The results show that with the increase in particle size in the exhaust, the distribution of PM concentration along the axial position of the channel shows a trend of gradually shifting from 0.2 to 0.25 m (rear section) to 0.125–0.2 m (front section). In particular, PM with 1 μm particle size has a large motion inertia, which is easy to move out of the airflow and deposit in the middle part of the channel. For PM in the front and middle sections of the channel, the airflow obstruction is relatively small, but the heat transfer loss is large, which is not beneficial for PM regeneration in the rear section. Therefore, with the distribution of LID, PD-1, and PD-2, the regeneration efficiency is relatively low, at 50.84%, 55.01%, and 54.59%, respectively.

An Experimental Study on the Seepage Characteristics of Rough Fractures in Coal under Stress Loading

Fracture and stress environments significantly affect the flow of coalbed methane. Under stress, fracture deformation and damage occur, which change the original fracture characteristics and lead to changes in gas flow characteristics. The change in gas pressure gradient makes the fluid flow obviously nonlinear. Using linear flow theory to describe the fracture flow leads to a large error in predicting coalbed methane productivity. In this study, seepage tests on fractured coal are carried out under different stresses and gas pressure gradients, the nonlinear flow and changes in related parameters are analyzed, and the applicability of the nonlinear flow equation is evaluated. The resulting seepage of the gas flow in the fracture under stress is obviously nonlinear, which gradually increases with increasing effective stress and gas pressure gradient. When the Forchheimer equation is used to characterize the nonlinear seepage in fractures, the coefficients increase with increasing effective stress. The permeability, nonlinear factor, and critical Reynolds number decrease with increasing effective stress. When the Izbash equation is used for this case, the linear coefficient ranges from 1015 to 1016, and the nonlinear coefficient ranges from 1.064 to 1.795. The coefficients are related to the effective stress through a power function. Both the Forchheimer and Izbash equations can characterize the flow in rough fractures in coal during stress loading. However, the Forchheimer equation better reveals the mechanism of flow transformation from linear to nonlinear in fractures.

МОДЕЛИ ТУРБУЛЕНТНОСТИ В РАСЧЕТАХ АЭРОДИНАМИЧЕСКИХ ХАРАКТЕРИСТИК ГАЗОВЫХ ПОТОКОВ

Abstract. The review of the most common turbulence models in engineering calculations, which allow approximate description of turbulent flows using the Reynolds equations of motion, is carried out, and their areas of application are shown. To calculate turbulent flows by solving the Reynolds equations, a closure hypothesis is adopted for apparent turbulent stresses and other variables included in the equations.
 It is noted that Businex is considered one of the main ones, which is confirmed by experimental studies of turbulence, which show that the turbulent flow is characterized by the presence of an energy cascade from large to smaller scales.
 In addition, the authors considered: the semi-empirical Prandtl theory based on the assumption that turbulent motion consists of an infinite number of vortices of various scales that interact with each other and transfer energy from large scales to smaller ones, models specially adapted to a specific type of problem, for example, LES for modeling the interaction of turbulent flows, a generalized theory of developed turbulence (GDT, Generalized Theory of Turbulence), which is a theory, developed for the description and simulation of turbulent flows in various technical devices.
 It is proved that one of the most common approaches is to use models with different levels of detail, starting with simpler models and moving on to more complex ones, if necessary to achieve the required accuracy. This approach is called the "ladder of models" or RANS/LES method. 
 Subject of research: turbulence models for calculating aerodynamic characteristics of gas flows.
 Materials and methods: the basis of the performed research is a number of scientific works of domestic and foreign researchers in this field and the results obtained by the authors in the course of the work.
 A systematic approach is used, meaning consideration and study of the object of research as an integrated system consisting of many interrelated elements.
 Results: The authors note that one of the most common approaches is to use models with different levels of detail, starting with simpler models and moving on to more complex ones if necessary to achieve the required accuracy. This approach is called the "ladder of models" or RANS/LES method. 
 Conclusions: The choice of a turbulence model for specific cases depends on a number of factors, including flow characteristics, available data, accuracy requirements and computational capabilities. The selection criteria include: physical properties and flow mode; the degree of accuracy of calculations and calculation capabilities.

Optimization of simulation model adaptability and CFD of particles of Geldart‐B in a gas–solid fluidized bed for silicone monomer synthesis

AbstractThe flow characteristics of gas–solid in fluidized bed reactors constrain or promote the reaction process by affecting gas–solid mixing homogeneity, heat transfer timeliness, and fluidization stability. To improve the production efficiency of silicone monomers, this work shows a comparative analysis of the effects of geometric modelling in different dimensions, flow state, and the interphase exchange coefficient on the characteristics of particles of Geldart‐B in a gas–solid fluidized bed for silicone monomer synthesis based on two‐fluid models (TFM). The results of different dimensional simulations show that the third dimension plays an important role in the bubble behaviour and the particle volume fraction spatial distribution within the fluidization zone, and the three‐dimensional simulation results are closer to the experimental data, and the maximum average error of the simulation results is 4.52% lower than that of the two‐dimensional simulation results. Meanwhile, the flow model has a greater influence on the bubble behaviour during the flow process, whereas the interphase force model has little effect on the flow state of small‐diameter particles. However, for large‐diameter particles, the interphase force model is closely related to the bubble behaviour and the direction and size of the vortex, and the lift model prediction of bubble breakage and coalescence behaviours are higher than the experimental results from the power spectrum results. The inlet velocity has a greater effect on the formation of vortices near the wall; with the same inlet velocity, the more dispersed the particle distribution in the bed, and the more uniform the gas–solid mixing.

Air-lift characteristics of rock core mass in gas–liquid–solid mixed flow

The air-lift reverse circulation drilling method enables efficient and continuous core extraction. However, the flow characteristics of gas–liquid–solid mixtures during core mass transport are complex, and developing a model to predict performance remains challenging. The flow characteristics of gas–liquid–solid mixtures inside the drill pipe are numerically analyzed by calculating the mixing transport of the core during the airlift process. By solving the momentum equations for multiphase flow, the distribution of physical quantities of three phases and pressure along the depth was calculated, and the laws of influence of the operating parameters on the transport characteristics of the core were analyzed. The results show that the drilling performance is related to the geometric parameters of the pipe and core, drilling speed, and depth. When the submergence ratio is increased to 0.9, the maximum liquid-phase superficial velocity is increased by 109% compared with that when the submergence ratio is 0.6. The size of the annular space gap 67 formed between the core and the pipe wall is the key to the core's mechanical properties, and the differential pressure gradient force increases with increasing core diameter. An increase in well depth reduces the fluid flow. In addition, increasing the pipe diameter improves drilling performance due to reduced friction losses, which elevates the air flow rate required for air-lift reverse circulation operation.

Study on characteristics of gas–liquid two-phase flow in pump as turbine using multiple-size group model

The multi-size group (MUSIG) model is employed in this paper to simulate the gas–liquid two-phase flow in pump as turbine (PAT) since the traditional Eulerian–Eulerian two-fluid model is unable to take into account the phenomena of breakup and coalescence of bubbles. First, the simulation of gas–liquid two-phase flow in a square column is compared with the experiment to verify the accuracy of the MUSIG model. Then, the results of gas–liquid two-phase flow in PAT simulated by the MUSIG model are compared with those by the conventional uniform bubble (UB) model and find that the MUSIG model is more favorable to capture the flow pattern at high gas content compared to the UB model. Based on the MUSIG model, the internal flow characteristics, pressure fluctuation, and bubble size distribution of the PAT are analyzed. The rotation of the blades breaks a part of big bubbles into small bubbles in the volute, resulting in a smaller diameter of the bubbles entering the impeller. As the gas content increases, the number and size of vortices in the impeller flow channel increase. The vortex is formed at locations where the gas phase distribution in the impeller flow channel is concentrated. The outlet of the impeller is more prone to bubble consolidation under high gas content conditions. In conclusion, the MUSIG model can well predict the complex flow characteristics of gas–liquid two-phase inside the PAT and identify the key influencing factors of energy acquisition, which can provide support for improving the performance of the PAT design.

Impact of Laval nozzle structure on the flow characteristics of supersonic gas-solid two-phase flow

This study introduces a mathematical coupling model within the Euler-Lagrange framework to conduct numerical simulations for supersonic gas-particles flow. After validating the model, the gas flow characteristics and particle dynamics within supersonic gas-particle flow under various nozzle configurations are investigated. The findings are as follows: (1) A Laval nozzle with a smoothly connected throat suppresses the generation of fan-shaped Prandtl-Meyer expansion waves and other complex wave systems, effectively mitigating significant changes in gas pressure at the throat and enhancing the stability of supersonic jets. (2) The presence of a particle-free zone in the nozzle divergent segment reduces the collision frequency between particles and the walls, thereby restricting the radial expansion of particles. (3) Utilizing an internally smooth-lined nozzle and expanding the radial divergence angle to 2.5–2.8 times, while ensuring particles maintain consistent axial penetration performance, increases the reflective contact area. This leads to an improvement in the utilization efficiency of particles. These findings provide valuable insights for optimizing nozzle configurations and enhancing the performance of supersonic gas-particle systems.

A Study on the Steady and Dynamic Performance of Dry Gas Seals with Combined Superelliptical Grooves and Holes on the End Face

Background:: Enhancing the stability and leakage control of Dry Gas Seals (DGS) under high parameters has been a crucial research focus. The design of end-face groove structures and surface texture shapes has been an essential aspect of DGS studies aimed at improving performance. Objective:: The proposed end-face gas seal utilizes superelliptical grooves and holes to improve its performance, aiming to obtain a patent. The use of superelliptical curves allows for a more precise and efficient geometric representation, resulting in a better sealing effect. Methods:: A theoretical analysis model for the steady and dynamic behavior of the seal is established using the lubrication theory and perturbation methods. The study investigates the distribution patterns of gas film pressure on the sealing end-face and gas flow characteristics within the film. This approach provides a new perspective for understanding seal performance and offers a theoretical basis for optimizing seal design. Results:: The results indicate that the combined end-face structure with grooves and holes ensures good sealing stability and effectively enhances leakage control performance. By optimizing the design of the superelliptical groove and holes on the end face, the performance of DGS can be significantly improved. Conclusion:: Within the parameter range studied, better steady-state performance is achieved for θ=40~80°, v=1.3~1.4, u=1~2, β=0.6~0.7, and λ=1.0~1.5. In addition, better dynamic performance is observed for θ=80~120°, v=1.1~1.2, u=2~3, β=0.9~1.0, and λ=2.0~2.5.

Experimental Investigation on Pressure Drops of Purge Gas Helium in Packed Pebble Beds for Nuclear Fusion Blanket

The flow characteristics of purge gas helium in the pebble bed of the tritium breeding blanket are important in analyzing the tritium purging process and optimizing the design of the solid breeder blanket. Therefore, the flow characteristics of helium gas in randomly packed pebble beds are investigated experimentally with a focus on the analysis of the pressure loss distribution. The results show that gas velocity, bed dimension, and pebble diameters have an obvious influence on the helium flow characteristics in pebble beds. With the increase in the inlet helium gas velocity, the pressure drop gradient of helium in the pebble bed gradually increased. With increases in the pebble bed dimension, the pressure drop gradient of helium in the pebble bed gradually increased. With the increase in the pebble diameter, the pressure drop gradient gradually decreased. In addition, the effect of temperature on the pressure drop of helium in the pebble bed was also preliminarily investigated. The pressure drop gradient of the helium through the pebble bed obviously increased with the increase in the helium and the bed temperature. The experimentally obtained pressure loss characteristics can be used for the validation of the simulation of a blanket pebble bed and as input parameters in the thermo-hydraulic analysis of solid-tritium breeder blankets.

Investigation of Wall Effect on Packing Structures and Purge Gas Flow Characteristics in Pebble Beds for Fusion Blanket by Combining Discrete Element Method and Computational Fluid Dynamics Simulation

In a tritium-breeding blanket of a fusion reaction, helium, used as a tritium-purging gas, will purge the tritium breeder pebble beds to extract the tritium in blanket. The purge gas flow characteristics will affect the tritium extraction efficiency. The effect of the fixed wall on the pebble packing structures and purge gas flow characteristics was investigated by combining the discrete element method (DEM) and computational fluid dynamics (CFD) method. The results indicate that the fixed wall leads to a regular packing of the pebbles adjacent to the fixed wall in association with drastic fluctuations in the porosity of the pebble bed, which can affect the purge gas flow behaviors. Further analyses of helium flow behaviors show that the helium pressure in the pebble bed decreases in a linear manner along the flow direction, whereas the pressure drop gradient of helium increases gradually with an increase in the packing factor. The reduction in porosity in the pebble bed leads to a notable escalation in helium flow velocity. Concerning the direction perpendicular to the helium gas flow, the evolution of the cut-plane averaged velocity of helium is similar to that of the porosity, except in the region immediately adjacent to the wall. The pressure drop and flow characteristics obtained in this study can serve as input for the thermohydraulic analysis of the tritium blowing systems in the tritium-breeding blanket of a fusion reactor.

Research on mechanism of gas leakage in microchannels of steel containment vessels for nuclear power plants

Steel containment vessels for nuclear power plants can experience gas leakage due to minute defects such as cracks, corrosion, and aging, leading to gas leakage. A gas leakage model for microchannels is established to elucidate the mechanism underlying gas leakage within microchannels caused by these defects, specifically addressing the issue of unidirectional gas flow. Computational Fluid Dynamics (CFD) and the UK R6 method are employed to calculate the gas leakage rate within microchannels. Furthermore, the characteristics of gas flow within microchannels are explored, including the factors affecting the gas leakage rate. Validation of the calculation results is verified experimentally. The results indicate that the gas mass flow rate exhibits a linear decrease with decreasing internal pressure and a non-linear decline as temperature increases. Additionally, the gas mass flow rate demonstrates a negative correlation with the microchannel length but a positive association to its hydraulic diameter. The primary influencing factors on gas leakage rates are hierarchically ranked as follows: pressure difference, microchannel cross-sectional area, temperature, microchannel length, and microchannel hydraulic diameter.

Numerical study on the flow characteristics of ultrahigh pressure gas affected by valve opening time based a real gas model

Numerical study on the flow characteristics of ultrahigh pressure gas affected by valve opening time based a real gas model

High‐order models for hydro‐mechanical coupling problems in multiscale porous media

AbstractAn accurate prediction of nonlinear hydro‐mechanical (HM) coupling in subsurface structures with pronounced heterogeneity at multiple spatial scales is still an open topic and crucial for numerous engineering applications, for example, hydraulic fracturing and enhanced geothermal systems. In this study, novel high‐order multi‐scale asymptotic solutions are developed to accurately capture the locally oscillating characteristics of gas flow and solid displacement fields at multiple scales. First, the formal macro‐meso two‐scale asymptotic expansion is performed to establish the homogenized solutions and macro‐meso high‐order models which can predict the HM coupling problems at the macroscale and mesoscale. By directly expanding the cell functions defined at the mesoscale to the microscopic levels, the high‐order two‐scale expansion for the mesoscopic cell functions is built, and the upscaled relations for the flow parameters and constitutive coefficients from the microscale to the mesoscale and macroscale are developed correspondingly. The multi‐scale low/high‐order models are established by combining the high‐order expansion of all cell functions at the mesoscale and the developed macro‐meso two‐scale models. The main contributions are that the present approaches follow the reverse thought processes of reiteration homogenization methods, and offer a very innovative and efficient way to establish high‐accuracy high‐order models for the nonlinear HM coupling problems in the heterogeneous porous medium with any number of spatial scales. The effectiveness and accuracy of the present solution are validated by several representative cases with different constitutive coefficient contrasts. The results demonstrate that the high‐order solutions provide an accurate prediction for gas transport and solid deformation of heterogeneous porous media with multi‐scale configurations.

DUST COLLECTION EFFICIENCY UNDER IN-PHASE TURBULENCE CONDITIONS IN THE PRESENCE OF PHASE TRANSITIONS

The article presents the results of a study of the dust collection efficiency in a regular packing layer of a rotoclone-type hybrid apparatus on the particle inertia index in the presence of phase transition processes. The effect of phase transitions on the efficiency of gas purification processes under inphase turbulence conditions is considered. Based on the conditions of stability of the in-phase vortex formation regime of the gas phase in a regular packing layer, the dominant influence of turbulent diffusion mechanisms on the intensity of phase transition processes and, as a consequence, on the dust collection efficiency has been established. In this case, the predominant influence is in the area of highly dispersed and finely dispersed dust fractions. The results of the experimental studying the patterns of dust collection efficiency depending on the particle inertia index in the presence of phase transition processes are presented. The studies were carried out for three phase transition modes: condensation of vaporsfrom a vapour-gas mixture, evaporation of a spraying liquid as well as a neutral mode. Using A.N. Kolmogorov’s hypothesis,the basic equations and results of calculating the numerical values of local characteristics of a turbulent gas flow in a regular packing layer are presented under the assumption of isotropic turbulence realized in a layer of a regular packing, whose local characteristics do not depend on coordinates and direction.

Continuous Positive Airway Pressure vs. Nasal Intermittent Positive Pressure Ventilation in Preterm Neonates: What if Mean Airway Pressures were Equivalent?

Respiratory support for preterm neonates in modern neonatal intensive care unit is predominantly with the use of non-invasive interfaces. Continuous positive airway pressure (CPAP) and nasal intermittent positive pressure ventilation (NIPPV) are the prototypical and most commonly utilized forms of non-invasive respiratory support, and each has unique gas flow characteristics. In meta-analyses of clinical trials till date, NIPPV has been shown to likely reduce respiratory failure and need for intubation compared to CPAP. However, a significant limitation of the included studies has been the higher mean airway pressures used during NIPPV. Thus, it is unclear to what extent any benefits seen with NIPPV are due to the cyclic pressure application vs. the higher mean airway pressures. In this review, we elaborate on these limitations and summarize the available evidence comparing NIPPV and CPAP at equivalent mean airway pressures. Finally, we call for further studies comparing non-invasive respiratory support modes at equal mean airway pressures.

The direct Monte Carlo simulation of microchannel flows for a large Knudsen number range

In recent years, porous materials containing micro- and nano-scale pores have found widespread applications. As the pore size decreases in such materials, rarefaction effects become significant in the pore flow, making the study of flow characteristics under higher Knudsen number conditions particularly crucial. In this work, through a direct simulation Monte Carlo (DSMC) method, an in-depth investigation is conducted into the gas flow characteristics and Klinkenberg effect in porous media with pore sizes ranging from 1 nm to 50 μm and Knudsen numbers spanning from 0.02 (slip flow) to 1200 (free molecular flow). The feasibility of using the DSMC method to simulate an internal free molecular flow in a porous medium under extreme rarefaction conditions with a Knudsen number of 1200 is validated. Furthermore, the impact of the gas pressure and porous medium pore size on the permeability is examined. The results reveal that with an increase in the Knudsen number, the dominant forces in the flow field transition from viscous forces to Knudsen diffusion, leading to a gradual increase in permeability. A comparative analysis reveals that existing apparent permeability models only provide satisfactory descriptions under certain Knudsen number conditions. Re-fitting the coefficient of the Kawagoe model and incorporating viscosity corrections leads to an apparent permeability model that can provide good predictions over a broader range of Knudsen numbers.

Analysis of methods for modeling operating modes of compressor equipment that ensures the operation of field low-temperature separation units

Objective. A booster compressor station (BCS) is one of the key technological facilities necessary to ensure the effective operation of low-temperature separation technology for natural gas in field gas preparation systems for transportation. Ensuring promising operating modes of booster compressor stations with high efficiency is an urgent task. Method. One of the tools for solving this problem is mathematical modeling of the operating modes of centrifugal compressors (hereinafter - CPC), which are part of gas pumping units (GPU). The basis of the mathematical model is the gas dynamic characteristic (GDC) of the pulp and paper mill. The existing regulatory documentation presents simplified methods for modeling HDC, which are suitable for low-pressure pulp and paper mills with a pressure ratio of up to 1.5 and the number of compression stages of no more than three, but are not suitable for multi-stage high-pressure pulp and paper mills. Result. The results of a comparison of three methods of modeling the hydrodynamic characteristics (method of reduced characteristics, refined method of reduced characteristics, method of two-parameter approximation) of high-pressure pulp and paper mills are presented using the example of a pulp and paper mill with a pressure ratio of up to 2.0, intended for equipping a gas pumping unit as part of field booster compressor stations. An analysis and comparison of the obtained modeling results with actual data was carried out. Conclusion. When using the UMPH-2D modeling method, the smallest errors were obtained (no more than 2.0%), which, in turn, indicates its highest accuracy among the considered methods for recalculating the gas flow characteristics of high-pressure and multi-stage compressors.

Flow and heat transfer characteristics of high-pressure natural gas in the gaps of high-speed motors with a high radius ratio

A motor stator-gap-rotor model is established based on the numerical heat transfer theory by using the finite volume method. The flow evolution of high-pressure natural gas in the gap with a radius ratio of 0.971 is investigated. The results demonstrate that the flow patterns of high-pressure natural gas in the motor gap can be categorized into turbulent, spiral Taylor-Couette, and turbulent Taylor-Couette flow; the flow ranges are determined based on the Ta/Re2. Then, the flow and heat transfer characteristics of the cooling medium in the gap under different flow regimes as well as the mechanism of locally enhanced heat transfer in the gap by the Taylor-Couette flow are explored. Finally, the mathematical expressions for the Nusselt number of motor gap are determined in terms of the Reynolds number, Taylor number, and Prandtl number by fitting using the Levenberg-Marquardt and global optimization methods. Using these expressions, the flow and heat transfer characteristics in the motor gap can be predicted. Overall, this study provides useful and novel insights on the design of cooling systems for high-speed motors.