Simulation Of Gas Flow Research Articles

Simulation-based optimisation of thermodynamic conditions in the ESEM for dynamical in-situ study of spherical polyelectrolyte complex particles in their native state

We present a complex analysis and optimisation of dynamic conditions in the environmental scanning electron microscope (ESEM) to allow in-situ observation of extremely delicate wet bio-polymeric spherical particles in their native state. According to the results of gas flow and heat transfer simulations, we were able to develop an improved procedure leading to thermodynamic equilibrium between the sample and chamber environment. To quantify and hence minimise the extent of any sample deformation during specimen chamber pumping, a strength-stress analysis is used. Monte Carlo simulations of beam-gas, -water, and -sample interactions describe beam scattering, absorbed energy, interaction volume and the emission of signal electrons in the ESEM. Finally, we discuss sample damage as a result of drying and the production of beam-induced free radicals. Based on all experimental and simulation results we introduce a Delicate Sample Observation Strategy for the ESEM. We show how this strategy can be applied to the characterization of polyelectrolyte complex spherical particles containing immobilized recombinant cells E. coli overexpressing cyclohexanone monooxygenase, used as a model biocatalyst. We present the first native-state electron microscopy images of the viscous core of a halved polyelectrolyte complex capsule containing living cells.

Basis of the computational domain selection in the problems of modeling a viscous flow in lowflow stages of a centrifugal compressor

The paper examines the problem of definition of the computational domain of the gasflow part at numerical simulation (RANS-approach) of gas flow in the low-flow stage of a centrifugal compressor at conditional flow rate Ф = 0.008. Several variations of replacing the design assembly of the flow part of the simulation stage with a gasdynamic model are considered: main working path of the stage; main working path of the stage taking into account the inlet area (inlet pipe and cowling); main working path of the stage taking into account the adjacent areas with full and partial connection with the gasflow part; main working path of the stage taking into account the friction surfaces of the stage rotor hub. It is shown that in case of axial suction into the stage the inlet pipe and cowling don’t significantly affect the parameter profile in front of the stage rotor and the gasdynamic characteristics of the stage. Modeling without areas adjacent to the path gives unsatisfactory quantitative and qualitative results. The most relevant option to take into account the losses of disk friction and leaks is a full twoway connection of adjacent areas with the flow part along the stage rotor disks, because when connecting only at the outlet of the stage rotor, the friction and leak losses are simulated, but the effect on the flow in the stage rotor is not taken into account. In some cases, it is important to consider the additional areas involved in friction losses.

Двойственность вихревой структуры, возникающей при сверхзвуковом обтекании области сопряжения затупленного тела и пластины вязким газом

Results of numerical simulation of the supersonic gas flow past a fin-body mounted on the flat plate with developing laminar boundary layer are presented. The calculations cover flow cases with the freestream Mach number of 6.7 and two different Reynolds numbers: Re=12500, 15600. The structure of the horseshoe vortices arising in the body leading-edge region is analyzed. It has been revealed that for both the Reynolds numbers there exist two stable solutions, which correspond to different metastable states of the flow. The solutions differ in the number of the vortices forming in the separation region, as well as in the separation region length. In the smaller Reynolds number case, both solutions are steady-state, whereas in case of the larger Re value, one of the solutions remains steady-state, and the other one becomes quasi-periodic.

Lattice Boltzmann method for simulation of shale gas flow in kerogen nano-pores considering temperature dependent adsorption

Lattice Boltzmann method for simulation of shale gas flow in kerogen nano-pores considering temperature dependent adsorption

Lattice Boltzmann method for simulation of shale gas flow in kerogen nano-pores considering temperature dependent adsorption

Due to the combined action of gas adsorption, surface diffusion and slippage, classical simulation approaches based on Darcy's law may not be appropriate for simulating shale gas flow in shale. In this work, a novel lattice Boltzmann (LB) model is proposed to study shale gas flow in a kerogen pore by introducing temperature dependent thickness of adsorption layer. The surface diffusion, which is caused by gradient of adsorption density, is considered as the slippage velocity on the surface of adsorption layer. The proposed LB model was adopted to simulate shale gas flow in a nano-pore. The results show that adsorption can significantly decrease the permeability of nano-pores. Surface diffusion improves the gas movement in nano-pores at lower pressure. With the decrease of pore size, the adsorbed layer had more impacts on gas permeability. Increasing temperature improves gas flow ability in nano-pores when pore size is less than 10nm. [Received: December 18, 2017; Accepted: May 2, 2018]

Collecting finely-dispersed particles from the gas flow in a centrifugal separator with coaxially arranged pipes

The article deals with the problem of increasing the efficiency of dedusting the gas flow from the finely dispersed particles smaller than 10 μm. In order to solve this problem, a design of centrifugal separator with coaxially arranged pipes is proposed. The described principle of operation includes the large values of centrifugal forces, which take place inside the device when the flow is swirled, and these forces throw the finely dispersed particles to the walls of device. This scientific paper shows a numerical simulation of gas flow dedusting process by means of ANSYS Fluent software package. The efficiency of dedusting the gas from the finely dispersed particles of up to 10 μm in the device is on average within the range of 53.8–76.7%. The exponential function, describing the changes in the pressure loss from the input gas rate, is obtained. In the course of studies, it was found that the pressure loss in the device is not more than 800 Pa at the input gas rate from 3 to 19 m/s.

Second-order slip condition considering Langmuir isothermal adsorption for rarefied gas microflows

Effect of the slip boundary condition on rarefied gas flow simulations plays an important role to understand the behaviour of gas microflows in MEMS. Several second-order slip conditions were proposed by the models of the kinetic theory of gases to simulate the rarefied gas microflows, in which the so-called classical second-order slip condition was derived from the Karniadakis et al. model. In this paper, a new second-order slip condition is proposed to employ with the Navier-Stokes-Fourier equations for simulating the rarefied gas flows in microchannels. It is derived by combining the Langmuir isothermal adsorption and the Karniadakis et al. model, with the aim of achieving a more realistic physical model. The pressure-driven back-forward-step, the Couette and pressure-driven Poiseulle rarefied gas flows in microchannels are investigated to validate our new second-order slip condition. Slip velocities using our new second-order slip condition are better than those using the conventional Maxwell and the so-called classical second-order slip conditions, and are in very good agreement with the DSMC data for all cases considered.

Numerical Modelling of Microchannel Gas Flows in the Transition Flow Regime Using the Cascaded Lattice Boltzmann Method.

In this article, a lattice Boltzmann (LB) method for studying microchannel gas flows is developed in the framework of the cascaded collision operator. In the cascaded lattice Boltzmann (CLB) method, the Bosanquet-type effective viscosity is employed to capture the rarefaction effects, and the combined bounce-back/specular-reflection scheme together with the modified second-order slip boundary condition is adopted so as to match the Bosanquet-type effective viscosity. Numerical simulations of microchannel gas flow with periodic and pressure boundary conditions in the transition flow regime are carried out to validate the CLB method. The predicted results agree well with the analytical, numerical, and experimental data reported in the literature.

A comprehensive model for simulating gas flow in shale formation with complex fracture networks and multiple nonlinearities

Shale gas reservoirs are characterized by gas adsorption and multiple flow mechanisms in shale matrix and complex fracture networks after hydraulic fracturing stimulation. It is of significance to capture the dynamic pressure-dependent properties, including gas PVT properties, gas desorption, flow mechanisms, and stress-dependent fracture permeability. In this paper, this problem is handled by proposing a novel Green element method, which is a good extension of the traditional boundary element method. The domain is discretized into many Cartesian grids, and boundary element method is applied to each element. Discrete fractures are flexibly handled by embedding the fractures into the background grids for the domain, and finite difference method is applied to model flow within the fracture system. The flow between the fracture system and matrix system is coupled to obtain the eventual matrix equation, and Picard successive approximation method is applied to obtain the solution of the nonlinear system. The solution of the novel Green element method is validated by the commercial simulator Eclipse for a fractured tight gas well model. A field case from Longmaxi formation in Southwestern China is used to verify the utility of the model. Moreover, the effects of nonlinear parameters on the performance of the well are analyzed. The results show that the nonlinear factors in shale gas flow model have a great impact on the eventual results of gas production forecasts, especially gas PVT properties and stress-sensitive fracture conductivity.

Post-combustion CO2 capture and recovery by pure activated methyldiethanolamine in crossflow membrane contactors having coated hollow fibers

Gas absorption and stripping using membrane contactors is one of a number of carbon capture and storage technologies being investigated for separating CO2 from flue gas. Energy consumption in CO2 absorption-stripping employing amine-containing aqueous absorbent solutions is strongly influenced by demanding stripping conditions involving higher temperatures. The utility of using pure methyldiethanolamine (MDEA) activated by piperazine as a reactive absorbent was studied using a compact hollow fiber device for the absorber as well as the stripper. Water needed for reactive absorption of CO2 in tertiary amine MDEA was obtained from simulated humidified flue gas stream which is saturated with moisture in actual practice. This study investigated also the absorption and stripping performances of aqueous absorbent solutions containing 80% and 90% activated MDEA (aMDEA). Considerable CO2 stripping from CO2-loaded pure aMDEA absorbent was achieved at 92 °C while absorption was carried out at 25–46 °C. Further, the CO2 stripping rate was far higher for pure MDEA compared to the other two aMDEA solutions with water. Results are reported for the performance of the absorption-stripping process as a function of humidified simulated flue gas flow rate and the absorbent flow rate. High values of the overall mass transfer coefficients (MTCs) have also been reported for absorption. The highest volumetric gas phase based overall MTC obtained was 0.504 sec−1. This system has a much lower absorbent circulation load, eliminates the energy needed to heat and evaporate water present in aqueous absorbent solutions and benefits from the absence of excess water during stripping.

Comparison of computational fluid dynamics–discrete element method and discrete element method simulations for a screw conveyor

AbstractFlow behavior of particles is simulated by means of discrete element method (DEM) in a screw conveyor. The gas flow is considered by the Euler model. The subgrid scale model turbulence model is applied to solve gas turbulence. A detailed description of the model equations applied has been presented. The comparison between computational fluid dynamics (CFD)‐DEM (i.e., considering gas flow) and DEM (i.e., not considering gas flow) simulation results is performed. Predicted mass flow rate with CFD‐DEM model is in better agreement with experiments by Roberts and Wills comparing with DEM results. The effects of incline angle and filling rate on the flow behavior of particles are predicted. The simulation results show that the gas flow has a major impact on particle flow behavior at a high rotation speed, whereas the velocity of particles increases with an increase of rotation speed. As the filling rate rises, both the translational granular temperature and the rotational granular particle temperature decline, which means that the particles fluctuation turns weak. While with an increase of incline angle, from the horizontal to vertical, the translational granular temperature is increased, but the rotational granular temperature reduces and then goes up. In brief, translational motion is a main mode of motion in a screw conveyor.

Limitations of Lattice Boltzmann Modeling of Micro‐Flows in Complex Nanopores

The multiscale transport mechanism of methane in unconventional reservoirs is dominated by slip and transition flows resulting from the ultra‐low permeability of micro/nano‐scale pores, which requires consideration of the microscale and rarefaction effects. Traditional continuum‐based computational fluid dynamics (CFD) becomes problematic when modeling micro‐gaseous flow in these multiscale pore networks because of its disadvantages in the treatment of cases with a complicated boundary. As an alternative, the lattice Boltzmann method (LBM), a special discrete form of the Boltzmann equation, has been widely applied to model the multi‐scale and multi‐mechanism flows in unconventional reservoirs, considering its mesoscopic nature and advantages in simulating gas flows in complex porous media. Consequently, numerous LBM models and slip boundary schemes have been proposed and reported in the literature. This study investigates the predominately reported LBM models and kinetic boundary schemes. The results of these LBM models systematically compare to existing experimental results, analytical solutions of Navier‐Stokes, solutions of the Boltzmann equation, direct simulation of Monte Carlo (DSMC) and information‐preservation DSMC (IP_DSMC) results, as well as the numerical results of the linearized Boltzmann equation by the discrete velocity method (DVM). The results point out the challenges and limitations of existing multiple‐relaxation‐times LBM models in predicting micro‐gaseous flow in unconventional reservoirs.

Моделирование течения газа в каналах переменного сечения при различных режимах течения методом решеточных уравнений Больцмана (LBM)

All contact-free vacuum pumps operate in a very wide pressure range. Therefore, the calculation of flows through the slot channels is associated with the need to take into account the laws of all three modes of gas flow: viscous, transitional and molecular. Most of channels of contact-free pumps are formed by curved walls, which are slits of variable cross-section in the direction of gas flow, having a minimum gap in some place. The paper considers the basic methods of calculating flows in channels of variable cross-section: the Monte Carlo method for molecular mode, the numerical solution of Navier --- Stokes equations for viscous mode and the Lattice Boltzmann method (LBM) for a wide range of pressures. The results of gas flow simulation calculated in COMSOL Multiphysics with LBM method are presented. The influence of the gas flow mode on the velocity profile in the channel is discussed. Based on the simulation results, the conductivity of channels of different geometries was calculated at various pressures at the inlet and outlet of the channel. The graphs of conductivity dependence on the Knudsen number for the method of angular coefficients, the model of lattice Boltzmann equations and experimental data are presented. It is shown that for slit channels of variable cross-section, the LBM model agrees well with the experiment under any gas flow modes.

Simulation and measurement of rarefied gas flow and neutral density profiles through a large multiaperture multigrid negative ion accelerator

In large multi-aperture, multi-grid negative-ion accelerator for fusion application, the background gas density in the electrostatic accelerator causes a loss of negative-ion current and the formation of dangerous stray particles. In addition, to sustain in dynamic equilibrium a sufficient gas pressure in the plasma ion-source, a very large pumping speed is typically necessary. This paper presents and compare simulations and measurements of gas flows and background pressure profiles, through the electrostatic accelerator of the SPIDER beam source, the full-size prototype source of the ITER neutral beam injectors. The gas pressure profiles through the ion accelerator were measured by multiple movable capacitive pressure gauges, mapping the uniformity of the profiles along nine beamlets at three different filling pressures. The gas flow simulations in molecular flow regime were performed with a large-scale view-factor model, and with a single-aperture periodic test particle Monte Carlo model. The models reproduced the measured pressure profiles but additionally, provided also the profiles of local gas density. The gas conductance and the profiles calculated with the full-scale gas flow model correctly reproduce the measured profiles, and the transversal non-uniformities over the 1.6 m × 0.6 m cross section. The calculated gas density profile is also verified by comparing the two different numerical approaches. The gas flow model validated at room temperature can be used to simulate the pressure profile during operation, in presence of gas heating and dissociation by the source plasma.

Origin of spurious oscillations in lattice Boltzmann simulations of oscillatory noncontinuum gas flows.

Oscillatory noncontinuum gas flows at the micro and nanoscales are characterized by two dimensionless groups: a dimensionless molecular length scale, the Knudsen number Kn, and a dimensionless frequency θ, relating the oscillatory frequency to the molecular collision frequency. In a recent study [Shi et al., Phys. Rev. E 89, 033305 (2014)10.1103/PhysRevE.89.033305], the accuracy of the lattice Boltzmann (LB) method for simulating these flows at moderate-to-large Kn and θ was examined. In these cases, the LB method exhibits spurious numerical oscillations that cannot be removed through the use of discrete particle velocities drawn from higher-order Gauss-Hermite quadrature. Here, we identify the origin of these spurious effects and formulate a method to minimize their presence. This proposed method splits the linearized Boltzmann Bhatnagar-Gross-Krook (BGK) equation into two equations: (1) a homogeneous "gain-free equation" that can be solved directly, containing terms responsible for the spurious oscillations; and (2) an inhomogeneous "remainder equation" with homogeneous boundary conditions (i.e., stationary boundaries) that is solved using the conventional LB algorithm. This proposed "splitting method" is validated using published high-accuracy numerical solutions to the linearized Boltzmann BGK equation where excellent agreement is observed.

A BGK model for high temperature rarefied gas flows

High temperature gases, for instance in hypersonic reentry flows, show complex phenomena like excitation of rotational and vibrational energy modes, and even chemical reactions. For flows in the continuous regime, simulation codes use analytic or tabulated constitutive laws for pressure and temperature. In this paper, we propose a BGK model which is consistent with any arbitrary constitutive laws, and which is designed to make high temperature gas flow simulations in the rarefied regime. A Chapman-Enskog analysis gives the corresponding transport coefficients. Our approach is illustrated by a numerical comparison with a compressible Navier-Stokes solver with rotational and vibrational non equilibrium. The BGK approach gives a deterministic solver with a computational cost which is close to that of a simple monoatomic gas.

Lattice-Boltzmann and Eulerian Hybrid for Solid Burning Simulation

We propose a new hybrid simulation method to model burning solid interactions. Unlike gas fuel, fire and smoke interactions that have been relatively well studied in the past, simulations of solid fuel combustion processes remain largely unaddressed. These include pyrolysis/smoldering, interactions with oxygen and flow inside porous solid. To advance this simulation problem, we designed a new hybrid of the Lattice-Boltzmann method (LBM) and a Eulerian grid based Navier-Stokes equation (NSE). It uses the LBM, which has symmetrical directions of particle velocities in a cell, for inside the solid fuel and the NSE, which has a representative velocity in a cell, for outside the solid. At the interface where the two methods join, we develop a novel method to exchange physical quantities and show a natural transition between the two methods. Since LBM allows us to directly manage the quantity of exchanges from the microscopic perspective, that is, between lattice points, we can easily simulate the burning speed and the shape change of burning an inhomogeneous solid. Also, we derive an LBM version of the previously proposed porous Navier-Stokes equation to simulate gas flow inside the porous solid. In addition, we use the NS solver outside the solid where macroscopic behavior is much more dominant and, hence, LBM is less efficient than NS solver. Our results show us the physical stability and accuracy and visual realism.

New slip boundary condition in high-speed rarefied gas flow simulations

In this paper, we propose a new slip boundary condition in hypersonic gas flow simulations. It is derived by considering the Langmuir isotherm adsorption into the Kaniadarkis et al. model of the kinetic theory of gas. Moreover, the motion of the adsorbed molecules over the surface (i.e. surface diffusion) is considered for the calculation of the mean free path in new slip condition. Three aerodynamic configurations are selected for evaluating new slip condition such as (1) the sharp-leading-edge flat plate, (2) circular cylinder in cross-flow, and (3) the sharp 25°–55° biconic cases. Hypersonic gas flows have the Mach number ranging from 6.1 to 15.6, and the working gases are argon and nitrogen. The simulation results show that new slip condition predicts better slip velocity than the Maxwell slip condition and gives good agreement with the direct simulation Monte-Carlo data for all cases considered in the present work.

Pore network model for retrograde gas flow in porous media

Gas well deliverability in retrograde gas reservoirs is affected by the appearance of a liquid phase near the wellbore when the bottom-hole pressure is below the dew point. At low enough pressure gradients, the liquid phase may bridge the pore throats, blocking the gas phase flow, leading to what is known as condensate blockage. If the pressure gradient is above a critical value, the capillary forces cannot block the gas flow and the condensate bridges are displaced. Gas production can be estimated using reservoir-scale simulation of the retrograde gas flow near producing wells using relative permeability curves that take into account the different physical phenomena and their dependence on local pressure, temperature and pressure gradient (capillary number). Pore-scale simulators can be used to model retrograde gas flow through porous media and to determine the gas and liquid phases relative permeability curves as a function of different flow parameters. Here, a fully-implicit isothermal compositional pore-scale network model is presented for retrograde gas flow in porous media at high capillary number, at which capillary forces are neglected. The flow pattern in each capillary changes from a single-phase gas flow to a two-phase core-annular flow regime when the pressure and the fluid composition reach a critical condition based on the thermodynamics of the fluid mixture. Newton's method is used to solve the non-linear flow and thermodynamics equations to calculate the pressure distribution and the flow rate of each component through the network. The model was used to predict gas-liquid relative permeability at high capillary number from a depletion process in a 2D network. The results show the evolution of the condensate saturation at different conditions and how the gas permeability falls abruptly at a certain condensate saturation.

Numerical simulation of a two-component gas flow near a highway

Numerical simulation of the air flow near a highway was carried out taking into account injection of ethane through a system of low sources. Numerical model is based on full physical and mathematical models of continuum media and takes into account the phenomena of turbulent mixing of gas components in the surface boundary layer. Based on the calculation results, the fields of velocity distribution, turbulent parameters and impurity concentrations in characteristic sections near the road are obtained.