Simulation Of Gas Flow Research Articles

A study on pressure-driven gas transport in porous media: from nanoscale to microscale

Gas flow in porous media can be seen in various engineering devices such as catalytic converters and fuel cells. It is important to understand transport phenomena in porous media for improvement of the performance of such devices. Porous media with pores as small as the mean free path of gas molecules are used in such devices as proton exchange membrane fuel cells. It is difficult to measure molecular transport through such small pores in the experimental approach. In addition, even when using theoretical or numerical approaches, gas flow through nanoscale pores must be treated by the Boltzmann equation rather than the Navier–Stokes equations because it cannot be considered as a continuum. Thus, conventional analyses based on the continuum hypothesis are inadequate and the transport phenomena in porous media with nanoscale pores are not yet clearly understood. In this study, we represented porous media by randomly arranged solid spherical particles and simulated pressure-driven gas flow through the porous media by using the direct simulation Monte Carlo (DSMC) method based on the Boltzmann equation. DSMC simulations were performed for different porosities and different sizes of solid particles of porous media. It was confirmed that Darcy’s law holds even in the case of porous media with micro-/nanoscale pores. Using the obtained results, we constructed expressions to estimate the pressure-driven gas transport in porous media with micro-/nanoscale pores and porosity ranging from 0.3 to 0.5. The flow velocities estimated by using the constructed expressions agreed well with those obtained in the DSMC simulations.

A new direct simulation Monte Carlo implementation for more efficient simulation of hypersonic flow over arbitrarily shaped bodies using dynamic cells

Methods are reported for less computationally expensive and more accurate implementations of the direct simulation Monte Carlo (DSMC) method for the simulation of high speed gas flows over arbitrarily shaped bodies. A new particle-tracking algorithm with a saving of computational time of up to 10% is reported in which tracking of particles is done with the help of big triangles having vertices lying on the boundary curves. An algorithm has been developed to generate DSMC cells for collision and sampling that contain a prescribed number of molecules. This algorithm is able to generate over 90% cells having the optimum number of seven or eight molecules for simulating collisions. Sampling for macroscopic properties is done on dynamic cells that contain a number of particles varying spatially as a function of the local number density. A criterion for finding the number of particles in sampling cells is presented. This criterion has been found to result in accurate and fast simulation of two-dimensional hypersonic flows of argon over a wedge, and argon and nitrogen over a circular cylinder.

Lattice Boltzmann accelerated direct simulation Monte Carlo for dilute gas flow simulations

Hybrid particle-continuum computational frameworks permit the simulation of gas flows by locally adjusting the resolution to the degree of non-equilibrium displayed by the flow in different regions of space and time. In this work, we present a new scheme that couples the direct simulation Monte Carlo (DSMC) with the lattice Boltzmann (LB) method in the limit of isothermal flows. The former handles strong non-equilibrium effects, as they typically occur in the vicinity of solid boundaries, whereas the latter is in charge of the bulk flow, where non-equilibrium can be dealt with perturbatively, i.e. according to Navier-Stokes hydrodynamics. The proposed concurrent multiscale method is applied to the dilute gas Couette flow, showing major computational gains when compared with the full DSMC scenarios. In addition, it is shown that the coupling with LB in the bulk flow can speed up the DSMC treatment of the Knudsen layer with respect to the full DSMC case. In other words, LB acts as a DSMC accelerator.This article is part of the themed issue 'Multiscale modelling at the physics-chemistry-biology interface'.

Bridging the gaps at the physics-chemistry-biology interface.

It is commonly agreed that the most challenging problems in modern science and engineering involve the concurrent and nonlinear interaction of multiple phenomena, acting on a broad and disparate spectrum of scales in space and time. It is also understood that such phenomena lie at the interface

Numerical simulation of shale gas flow in three-dimensional fractured porous media

In this study, a Computational Fluid Dynamics (CFD) solver able to simulate shale gas flow as fluid flow in a porous medium on the macro level is presented. The shale gas flow is described by means of a tailored governing equation with both fluid properties and permeability expressed as a function of the effective pore pressure (stress effect) and with Knudsen effects included through an apparent permeability. This CFD solver, developed in the OpenFoam framework, allows for the simulation of three-dimensional fractured geometries without limitations on the shape of the domain. The solver was assessed and validated against literature data showing good agreement in terms of both recovery rate and pressure field profiles. The solver was then used to explore two different phenomena affecting shale gas dynamics: the diffusion behaviour and the influence of fracture geometry. It was shown that shale gas flow, on the macro level, is a diffusion-dominated phenomenon, and its behaviour can also be qualitatively represented by a diffusion equation. It was also shown that the early behaviour of shale gas flow is dictated by the fracture geometry, and that the reservoir dimensions have no effect on the flow at early times. Finally, a newly developed “dual-zone” solver, where the shale matrix and the fracture network are modelled as two distinct domains interacting through the common boundaries, is presented and discussed.

A new cut-cell algorithm for DSMC simulations of rarefied gas flows around immersed moving objects

Direct Simulation Monte Carlo (DSMC) is a widely applied numerical technique to simulate rarefied gas flows. For flows around immersed moving objects, the use of body fitted meshes is inefficient, whereas published methods using cut-cells in a fixed background mesh have important limitations. We present a novel cut-cell algorithm, which allows for accurate DSMC simulations around arbitrarily shaped moving objects. The molecule–surface interaction occurs exactly at the instantaneous collision point on the moving body surface, and accounts for its instantaneous velocity, thus precisely imposing the desired boundary conditions. A simple algorithm to calculate the effective volume of cut cells is presented and shown to converge linearly with grid refinement. The potential and efficiency of method is demonstrated by calculating rarefied gas flow drag forces on steady and moving immersed spheres. The obtained results are in excellent agreement with results obtained with a body-fitted mesh, and with analytical approximations for high-Knudsen number flows.

Parallel Fokker–Planck-DSMC algorithm for rarefied gas flow simulation in complex domains at all Knudsen numbers

A major challenge for the conventional Direct Simulation Monte Carlo (DSMC) technique lies in the fact that its computational cost becomes prohibitive in the near continuum regime, where the Knudsen number (Kn)—characterizing the degree of rarefaction—becomes small. In contrast, the Fokker–Planck (FP) based particle Monte Carlo scheme allows for computationally efficient simulations of rarefied gas flows in the low and intermediate Kn regime. The Fokker–Planck collision operator—instead of performing binary collisions employed by the DSMC method—integrates continuous stochastic processes for the phase space evolution in time. This allows for time step and grid cell sizes larger than the respective collisional scales required by DSMC. Dynamically switching between the FP and the DSMC collision operators in each computational cell is the basis of the combined FP-DSMC method, which has been proven successful in simulating flows covering the whole Kn range. Until recently, this algorithm had only been applied to two-dimensional test cases. In this contribution, we present the first general purpose implementation of the combined FP-DSMC method. Utilizing both shared- and distributed-memory parallelization, this implementation provides the capability for simulations involving many particles and complex geometries by exploiting state of the art computer cluster technologies.

Fundamental solutions to moment equations for the simulation of microscale gas flows

Fundamental solutions (Green’s functions) to Grad’s steady-state linearised 13-moment equations for non-equilibrium gas flows are derived. The creeping microscale gas flows, to which they pertain, are important to understanding the behaviour of atmospheric particulate and the performance of many potential micro/nano technologies. Fundamental solutions are also derived for the regularised form of the steady-state linearised 13-moment equations, due to Struchtrup & Torrilhon (Phys. Fluids, vol. 15 (9), 2003, pp. 2668–2680). The solutions are compared to their classical and ubiquitous counterpart: the Stokeslet. For an illustration of their utility, the fundamental solutions to Grad’s equations are implemented in a linear superposition approach to modelling external flows. Such schemes are mesh free, and benefit from not having to truncate and discretise an infinite three-dimensional domain. The high accuracy of the technique is demonstrated for creeping non-equilibrium gas flow around a sphere, for which an analytical solution exists for comparison. Finally, to demonstrate the method’s geometrical flexibility, the flow generated between adjacent spheres held at a fixed uniform temperature difference is explored.

Modeling and simulation of plasma gas flow driven by a single nanosecond-pulsed dielectric barrier discharge

A simplified (7 species and 9 processes) plasma kinetic model is proposed to investigate the mechanism of the plasma aerodynamic actuation driven by nanosecond-pulsed dielectric barrier discharge (NS-DBD). The governing equations include conservation equations for each species, the Poisson equation for the electric potential, and Navier-Stokes equations for the gas dynamic flow. Numerical simulations of plasma discharge and flow actuation on NS-DBD plasma actuators have been carried out. Key discharge characteristics and the responses of the quiescent air were reproduced and compared to those obtained in experiments and numerical simulations. Results demonstrate that the reduced plasma kinetic model is able to capture the dominant species and reactions to predict the actuation in complicated hydrodynamics. For the one-dimensional planar and two-dimensional symmetric NS-DBD, the forming of the sheath collapse is mainly due to the charge accumulation and secondary emission from the grounded electrode. Rapid species number density rise and electric field drop occur at the edge of the plasma sheath, where the space charge density gradient peaks. For the aerodynamic actuation with typical asymmetry electrodes, discharge characteristics have a core area on the right edge of the upper electrode, where the value can be much higher. The formation and propagation of the compression waves generated through rapid heating have also been performed and compared to those measured in a recent experiment. Energy release leads to gas expansion and forms a cylindrical shock wave, centering at the upper electrode tip with low gas acceleration. For the present single pulsed 12 kV case, the mean temperature of gas heating reaches about 575 K at 1 μs and decreases to about 460 K at 10 μs.

Prospects for the use of security air flow to prevent ion-molecule reactions in the ionization and drift zone in classical IMS

The effective transfer of sample problem is relevant in modern analytical equipment. The paper considered a problem in detection trace concentrations of explosives by Ion Mobility Spectrometry (IMS). The investigation deals with sample adsorption on the walls of transport tubes, the ion drift chamber and the chamber of the ion source in ion mobility spectrometer. The sample losses on inlet channel surface and diffusion through penetrable gas channels are comparable with the quantity of sample itself at the sensitivity level of 10-14 g / cm3. The trinitrotoluene (TNT) sorption in different channel materials depending on their sorption properties is analyzed. A new approach preventing sorption of the substance on the chamber walls by security airflow is presented. The study includes gas flow simulation and experiments of protective gas flow setup.

Numerical simulation of gas flow in jet nozzle

We determined traction and hydraulic characteristics of the jet nozzle of a gas turbine engine, taking into account the geometry of the internal structural elements and flow swirl in the inlet section according to the results of numerical simulation.

A new approach for chemical reaction simulation of rarefied gas flow by DSMC method

Present study concerns the development of a new approach for chemical reaction modelling in DSMC technique. The conventional quantum-kinetic (QK) method based on the VHS collision model is usually applied by the DSMC algorithm to simulate chemical reactions in rarefied gas flowfield. In this paper, a modified quantum-kinetic model (MQK) is presented to remove some limitation of the QK chemical model. The MQK method uses the GSS collision model based on the Stockmayer potential function which can capture both the attractive and repulsive intermolecular forces and polarization effect of the molecules in the reactive flow. To investigate the accuracy and capability of the proposed model, firstly, dissociation of the nitrogen (non-polar gas) and water vapour (polar gas) are simulated in a single cell DSMC for equilibrium and non-equilibrium conditions. The results of the MQK are assessed with those of the QK methods, analytical models and experimental data. Secondly, dissociation of these two types of gases is studies in the rarefied gas flowfield along the stagnation line of a typical hypersonic atmospheric blunt body problem. The flowfield properties including the density, velocity, transient, vibrational and total temperature along the stagnation line and surface properties including the surface pressure and heatflux at the stagnation point obtained by the MQK model are compared with results of the QK method and available analytical and experimental data. The results show that the MQK model provides more accurate results when the polarization effects and both the attractive-repulsive intermolecular forces are dominant especially at high temperature. Also, comparison of the computational cost between two models indicates higher efficiency using the proposed method rather than the QK model due to reduction of number of the collision pairs.

New Type of Smoluchowski Temperature Jump Condition Considering the Viscous Heat Generation

The effect of the sliding friction has been important in calculating the heat flux of gas flow from the surface because there is some slip over the surface. There is not any temperature jump condition including the sliding friction part (viscous heat generation) in its derivation so far. In this paper, the Smoluchowski temperature jump condition is modified by considering viscous heat generation. This new temperature jump condition will be evaluated for three configurations in high-speed rarefied gas flow simulations using the computational fluid dynamics method, which solves the Navier–Stokes equations within the OpenFOAM framework: 1) the flat plate, with Mach number 2) the NACA0012 microairfoil, with Mach number ; and 3) the sharp 25–55 deg biconic case, with Mach number . The working gases are air and nitrogen. The airfoil case is simulated with various Knudsen numbers from 0.026 to 0.26, and the angles of attack are from 10 to 20 deg. The surface gas temperatures predicted by the new temperature jump condition give good agreement with the direct simulation Monte Carlo data, especially the NACA0012 microairfoil cases with the high Knudsen numbers ( and ) with an angle of attack of 20 deg.

Study of Gas Flow Characteristics in Tight Porous Media with a Microscale Lattice Boltzmann Model.

To investigate the gas flow characteristics in tight porous media, a microscale lattice Boltzmann (LB) model with the regularization procedure is firstly adopted to simulate gas flow in three-dimensional (3D) digital rocks. A shale digital rock and a sandstone digital rock are reconstructed to study the effects of pressure, temperature and pore size on microscale gas flow. The simulation results show that because of the microscale effect in tight porous media, the apparent permeability is always higher than the intrinsic permeability, and with the decrease of pressure or pore size, or with the increase of temperature, the difference between apparent permeability and intrinsic permeability increases. In addition, the Knudsen numbers under different conditions are calculated and the results show that gas flow characteristics in the digital rocks under different Knudsen numbers are quite different. With the increase of Knudsen number, gas flow in the digital rocks becomes more uniform and the effect of heterogeneity of the porous media on gas flow decreases. Finally, two commonly used apparent permeability calculation models are evaluated by the simulation results and the Klinkenberg model shows better accuracy. In addition, a better proportionality factor in Klinkenberg model is proposed according to the simulation results.

A simulation of gas flow: The dependence of the tangential momentum accommodation coefficient on molecular mass

The tangential momentum accommodation coefficient (TMAC) influences the rate of pressure driven flow of a gas in a channel. The manner in which TMAC depends on the molecular mass is of importance as it influences the extent to which gas flow rates are affected by their mass, but there are conflicting opinions in the literature concerning the extent and even the sign of this dependence. We simulate the flow of the noble gases He, Ne, Ar, Kr, and Xe using molecular dynamics with Lennard-Jones potentials. The interaction with the wall is made realistic by simulating five layers of mobile atoms and allowing for adsorbed gas on the wall. With increasing mass of the gas, the TMAC exhibits asymptotic behaviour in approaching the value assumed to apply for an entrapped atom. Either increasing or decreasing TMAC with respect to an increasing molecular mass is produced, depending on the assumed TMAC of an entrapped atom. This resolves a conflict in the literature, where both increasing and decreasing trends are observed with mass.

Transient pressure and production dynamics of multi-stage fractured horizontal wells in shale gas reservoirs with stimulated reservoir volume

Multi-stage fracturing in unconventional shale gas reservoirs always creates complex fracture network around horizontal wellbore. The existence of the stimulated reservoir volume (SRV) has significant impact on performance of fractured horizontal well, and various composite models are introduced to simulate gas flow caused by fractured horizontal wells with SRV. However, most of the existing models do not simultaneously taken into account fracture conductivity, non-uniform properties of hydraulic fractures, SRV region, multiple flowing mechanisms of shale gas.This paper extends previous work by presenting a more comprehensive model to simulate the performance of multi-stage fractured horizontal well in shale gas reservoirs. The model takes into account not only the existence of SRV, but also the influence of fracture conductivity, multiple flowing mechanisms (desorption, transient state and pseudo-steady state viscous flow and diffusion) in shale matrix as well as the effect of non-uniform fracture distribution. By employing the source function theory, Laplace transformation and discretization method, a relatively simple semi-analytical solution is obtained. After the verification of the presented model with other published results, the transient pressure and production dynamics are compared and analyzed. The influences of relevant parameters on performance of fractured horizontal wells are also discussed. It is found that the performance of fractured horizontal wells in shale gas reservoirs is mainly affected by the parameters related to SRV (the size of SRV, the density of fractures in SRV, the configuration of primary fractures in SRV), conductivity and distribution of hydraulic fractures, and desorption and diffusion in shale matrix. The model presented in this paper can be easily simplified to simulate simpler cases, such as performance of fractured horizontal wells in tight reservoirs. The model and corresponding conclusions presented in this study will be helpful to understand transient performance of multi-stage fractured horizontal wells in shale gas reservoirs and thus to effectively develop unconventional shale gas reservoirs.

Simulation of microscale gas flow in heterogeneous porous media based on the lattice Boltzmann method

A microscale multi-relaxation-time lattice Boltzmann model with the regularization procedure is adopted to simulate gas flow in different porous media. The diffuse reflection boundary condition is used to deal with the random solid boundaries. Because of the complex geometry of the pores, the characteristic length is no longer a constant but a function of the pore locations for the porous media. A rational method is proposed to obtain the local characteristic lengths of the porous media for the microscale gas flow simulations. The simulation results show that gas flow characteristics in different flow regions are notably different. In the continuum flow region and slip flow region, the gas flow abilities in different pores are quite different. The effect of heterogeneity of the porous media on gas velocity distribution is very obvious. As the Knudsen number increases, the differences of gas flow abilities in different pores decrease. For gas flow in the strong transition flow region and free molecular flow region, the gas flow abilities in small pores are similar to those in large pores and the effect of heterogeneity becomes small. Such phenomena are mainly caused by the different gas flow mechanisms in different flow regions. In addition, two commonly used apparent permeability calculation models are evaluated by the simulation results and a better coefficient for the Klinkenberg model is proposed according to the simulation results.

Numerical simulation of gas flow in artificial fracture coal by three-dimensional reconstruction based on computed tomography

The influences of spatial geometric shape and position of a fracture on gas seepage were analyzed to explore the law of gas flow in the fracture structure of coals. First, artificial fracture surfaces with different shapes were preset in the computed tomography (CT) image. A coal reconstruction model with a single artificial fracture with different shapes and angles was established by translating the artificial fracture plane using three-dimensional reconstruction technology. Finally, the internal gas seepage law was analyzed together with the finite element method. The simulation on the same spatial shape of the fracture structure demonstrated that the seepage velocity at the coal section did not show evident variation law with the angle. Based on further extraction of the penetration velocity on the artificial fracture section, the penetration velocity increased with the expansion of the fracture angle under the same fracture shape. The penetration velocity on different shapes of the fracture surface increased with the fracture edges (from square to circular) under the same angle.

Extended application of lattice Boltzmann method to rarefied gas flow in micro-channels

Abstract Simulation of rarefied gas flow in micro-channels is of great interest owing to its diverse applications in many engineering fields. In this study, a multiple-relaxation-time lattice Boltzmann (MRT-LB) model with a general second-order slip boundary condition is presented to investigate the behaviour of gas flow with a wide range of Knudsen number in micro-channels. With the aid of a Bosanquet-type effective viscosity, the effective relaxation time is correlated with local Knudsen number (Kn) to account for the varying degree of rarefaction effect. Unlike previous studies, the derived accommodation coefficient r for the combined bounce-back/diffusive reflection (CBBDR) boundary condition is dependent on the local Kn, which allows more flexibility to simulate the slip velocity along the channel walls. When compared with results of other methods, such as linearised Boltzmann equation, experimental data, direct simulation Monte Carlo (DSMC) and Information Preservation DSMC (IP-DSMC), it is found that the LB model is capable of capturing the flow behaviour, including the velocity profile, flow rate, pressure distribution and Knudsen minimum of rarefied gas with Kn up to 10. The effect of Knudsen layer (KL) on the velocity of gas flow with a wide range of Kn is also discussed. It is found that KL effect is negligible in the continuum flow and y -independent in the free molecular flow, while in the intermediate range, especially in transition flow, KL effect is significant and particular efforts should be made to capture this effect.

DEM-CFD simulation of purge gas flow in a solid breeder pebble bed

Solid tritium breeding blanket applying pebble bed concept is promising for fusion reactors. Tritium bred in the pebble bed is purged out by inert gas. The flow characteristics of the purge gas are important for the tritium transport from the solid breeder materials. In this study, a randomly packed pebble bed was generated by Discrete Element Method (DEM) and verified by radial porosity distribution. The flow parameters of the purge gas in channels were solved by Computational Fluid Dynamics (CFD) method. The results show that the normalized velocity magnitudes have the same damped oscillating patterns with radial porosity distribution. Besides, the bypass flow near the wall cannot be ignored in this model, and it has a slight increase with inlet velocity. Furthermore, higher purging efficiency becomes with higher inlet velocity and especially higher in near wall region.