Simulation Of Gas Flow Research Articles

Multi-step chemical vapor synthesis reactor based on a microplasma for structure-controlled synthesis of single-walled carbon nanotubes

To address the need for the high precision synthesis of nanomaterials with the potential for high productivity, we present a novel design of a multi-step chemical vapor synthesis (CVS) reactor, which centers on the use of an atmospheric, continuous flow microplasma. Based on computational fluid dynamic gas flow simulations, the CVS reactor was designed to provide abrupt interaction steps to provide a well-defined and highly controlled start and stop to the nanomaterial (e.g., nanoparticle) growth. By applying this framework to gas-phase single-walled carbon nanotube (SWCNT) synthesis, the precise start and stop of the catalyst nanoparticle nucleation process can be done using the microplasma reactor (start) and carbon reactant gas to suppress further aggregation/promote SWCNT growth (stop), respectively. In so doing, we demonstrated the generation of controlled size of catalyst nanoparticles and SWCNTs despite high particle densities. We envision that our approach demonstrates the feasibility of fine structural control of nanomaterials through a multi-step CVS process with precisely regulating the aggregation time for high density particle flow, which overcomes the obstacle of both structural control and productivity.

Numerical Simulation of Gas Flow Passing through Slots of Various Shapes in Labyrinth Seals

Labyrinth seals are widely used in centrifugal compressors, turbines, and many other pneumatic systems due to their simplicity of design, reliability, and low cost. The calculation scheme for the movement of the working medium in a labyrinth seal is constructed by analogy with the movement of the working medium through holes with a sharp edge. Annular and flat slots, holes, and such a factor as the shaft rotation with a calculated sector of 3 degrees were studied. The purpose of the study is to determine the flow coefficient when the working medium flows through slots of various shapes. To achieve this purpose, modeling of the working medium flow in the FlowVision software was performed. The mass flow and flow coefficients are determined for the studied slot shapes. The convergence of the calculation results was determined by comparing the values of the mass flow rate at the inlet and outlet of the slot. Differences in visualizations of the flow for the studied variants of slots were established. The resulting difference should be taken into account in practical calculations of the working medium mass flow through the slot using a conditional flow rate factor which is determined by the slot design.

Semi-analytical modeling of transient pressure behaviour for a multifractured horizontal well in a gas reservoir with a complex fracture network by considering effects of slippage, stress-sensitivity, and gas adsorption/desorption

A semi-analytical technique has been developed, validated, and applied to quantify the pressure behaviour of a multifractured horizontal well (MFHW) in a gas reservoir with discrete fracture networks and an arbitrary boundary. Considering stress-sensitivity and the slippage effect in the matrix subsystem, a pseudo-pressure is incorporated into the gas flow equations. To weaken the strong nonlinearity from such a combined effect, the dual-reciprocity boundary element method (DRBEM) is applied to efficiently and effectively linearize the governing equations by replacing the nonlinear function with a series of particular solutions. Comparing with the boundary element method (BEM), not only can the DRBEM be used to obtain the accurate solution in any space and time domains, but also it is flexible to deal with the nonlinearity restriction in the governing equations. The finite volume method (FVM) is then adopted to simulate gas flow behaviour in the fracture subsystem with different fracture geometries. Not only can the proposed model be applied to a complex fracture network and operating schedules, but also capture complex gas flow behaviour including the Langmuir sorption, slippage effect, and stress-sensitive effect in a gas reservoir with a discrete fracture network and an arbitrary boundary. The mathematical formulations have been validated and then extended to field applications. A strong stress-sensitive effect is found to result in a large pressure drop and offset the permeability-enhancing effect from the slippage effect. In addition, a decrease in pressure drop derived from gas desorption would restrict the stress-sensitive effect. Furthermore, it can be found that each individual factor imposes a significant impact on the linear flow and boundary dominated flow regimes. On the basis of a mutual interference, the combined effect (i.e., stress-sensitivity, slippage effect, and gas adsorption/desorption) may be weaker than that of each individual factor. In the late boundary-dominated flow regime, the effect of boundary shape becomes more obvious on the pressure distribution curves since the pressure wave would reach the boundaries near fractures within a relatively short time.

Numerical study of multi-component flow and mixing in a scaled fission product venting system

Numerical simulation of isothermal multi-component gas flow was conducted to study fluid flow and mixing in a scaled low-pressure, low-temperature test facility for the fission product venting system (FPVS) of a gas cooled fast reactor (GFR). The geometry of FPVS test facility was an open loop including gradual expansion coupling and two 90°pipe elbows.First, the validation of large eddy simulation (LES) wall-adapting local eddy-viscosity (WALE) turbulent model was performed for transitional flow in a circular pipe with gradual expansion. The reactingFOAM solver in OpenFOAM v8 was employed. By introducing appropriate turbulence disturbance at the flow inlet, such as turbulence intensity, the numerical results showed good agreement in (1) the velocity profile downstream of the pipe expansion measured by particle image velocimetry (PIV) in this study, and (2) the reattachment length reported in the literature.Using this validated model, the numerical simulation of flow and component mixing was performed for the FPVS with an inlet flowrate corresponding to a Reynolds number of 2,400 to investigate the flow behavior and component mixing. Standard deviation of the mass fraction of component, referred to as absolute mixing index (AMI), was calculated to quantify the mixing of components, showing that the component mixing length determined by AMI is related to the reattachment length downstream of the expansion. Finally, this study was able to identify the best location in the FPVS test facility to measure the velocity and concentration profies where multicomponent flow is well mixed.

Efficiency of rocket engine thrust vector control by solid obstacle on the nozzle wall

The thrust vector control of a rocket engine by disturbing the supersonic flow in its nozzle is used for missile development for various purposes in different countries. Disturbance of the supersonic flow in the jet engine nozzle can be caused by various obstacles on the nozzle wall: solid obstacle, liquid or gas jet, combinations of solid obstacle with injected jets. The simplest and most effective way to create a disturbance is to disturb it by setting a solid cylindrical obstacle on the nozzle wall. The high efficiency is explained by the lack of the working fluid consumption on board the aircraft to create a control force, or its minimum amount necessary to protect the obstacle from the high-temperature oncoming gas flow in the rocket engine nozzle. This paper presents the study results of gas flow simulation with cylindrical obstacle perturbation on the wall of the Laval rocket engine nozzle in its subsonic and supersonic parts. The optimal placement in the nozzle is determined to obtain the maximum lateral control force. As a result of research, it was found that the perturbation of a supersonic flow in a rocket engine nozzle by a cylindrical obstacle has practically the same character when its position changes along the length of the nozzle. In the subsonic part of the nozzle in the median plane, the perturbed pressure on the wall has a positive sign, and on the obstacle wall its sign-alternating. When an obstacle is in the subsonic part of the nozzle, the integral value of the lateral force is negative in comparison with positive for the supersonic part.

A lattice Boltzmann simulation on the gas flow in fractal organic matter of shale gas reservoirs

The gas flow in the organic matter with multiple flow regimes is critical to the shale gas production from shale gas reservoirs, but simultaneously simulating the full flow regimes of shale gas in the nanopores of the fractal shale organic matter is still unavailable. This paper presents a modified lattice Boltzmann model with effective relaxation time to simulate the shale gas micro-flow behaviors in fractal organic matter for permeability prediction. Firstly, some fractal shale organic matters are reconstructed by the quartet structure generation set (QSGS) algorithm, and the structural parameters such as fractal dimension, lacunarity and average pore diameter are calculated to characterize the complexity of the fractal shale organic matter. Secondly, effective viscosity and Knudsen layer are introduced into the effective relaxation time to formulate a modified lattice Boltzmann model. Thirdly, this modified lattice Boltzmann model is verified by comparing with the theoretical permeability models for the gas flow in the body centered cubic (BCC) arrays. Finally, a series of gas flow simulations in reconstructed fractal shale organic matter are conducted to explore the effects of structure parameters on permeability. Numerical simulations show that the modified lattice Boltzmann model can simulate the full gas flow regimes and predict the permeability of the fractal shale organic matter. The logarithm of permeability is negatively linearly correlated with fractal dimension and positively linearly correlated with lacunarity of the shale organic matter. This permeability exponentially increases with the increase of average pore diameter and a power law is observed between the fractal dimension and average pore diameter of these fractal shale organic matters. The anisotropic structures have great impacts on the directional permeability of shale organic matter. The permeability kx in the horizontal direction increases first and then decreases with the increase of anisotropic ratio (AR) and reaches the peak when AR is about 20. The permeability ky in the vertical direction monotonically decreases with the increase of AR.

Improved curved-boundary scheme for lattice Boltzmann simulation of microscale gas flow with second-order slip condition

An improved curved-boundary scheme with second-order velocity slip condition for multiple-relaxation-time-lattice Boltzmann (MRT-LB) simulation of microgas flow is proposed. The proposed interpolation bounce-back (IBB)-explicit counter-extrapolation (ECE) scheme adopts the IBB method to describe the curved boundary, while the ECE method is employed to predict the slip velocity on gas-solid interface. To incorporate the effect of second-order velocity slip term and the influence of boundary curvature, a slip velocity model is also derived, from which the gas slip velocity is captured by the ECE discretization method. The influence of fictitious slip velocity can be eliminated by adopting the present ECE method, and the influence of actual offset between the lattice node and the physical boundary can be well considered by the IBB method. The proposed IBB-ECE boundary scheme is then implemented with the MRT-LB model and tested by simulations of force-driven gas flow in horizontal (inclined) microchannel, gas flow around a micro-cylinder, and Couette flow between two micro-cylinders. Numerical results show that the proposed IBB-ECE scheme improves the computational accuracy of gas slip flow (0.001<Kn≤0.1) when compared with other boundary schemes reported in the literature, and provides a precise and easy implementing scheme for curved boundary with second-order slip condition.

Conserved method for specified heat flux boundary in UGKS simulation of microscale gas flow

The unified gas kinetic scheme (UGKS) is a powerful method to simulate microscale gas flows. However, due to the lack of corresponding specified heat flux boundary (SHFB) treatment, there is great difficulty for UGKS to simulate problem with SHFB. In this paper, a SHFB treatment for UGKS is proposed based on gas kinetic theory. In the proposed boundary treatment, the heat flux term is directly added into the gas kinetic conservation equation of heat flux on boundary. In this conservation equation, the heat flux transferring from gas to the boundary is calculated using an extrapolated distribution function of inside gas, and the heat flux transferring form boundary to the gas is calculated using an equilibrium rebounding distribution function. Using the conservation equation of mass on boundary to close conservation equations, the gas density and temperature terms for rebounding gas distribution function can be calculated simultaneously. Distribution function on SHFB is reconstructed using rebounding gas distribution function and extrapolated distribution function of inside gas. Fluxes of gas distribution function and macro variables on SHFB is calculated with the reconstructed distribution function. Typical benchmark cases including steady and unsteady Fourier flow, steady Couette flow, pressure-driven flow, cavity flow and supersonic flow around square cylinder are tested to validate the proposed boundary treatment. The validations prove the accuracy, reliability and universality of the proposed method.

Modified LB model for simulation of gas flow in shale pore systems by introducing end effects and local effective mean free path

Natural gas flow in shale pore systems determines the accumulation and production of shale gas. Under the conditions of reservoir, gas flow in such shale pores is significantly different from that in conventional reservoirs. Studies of gas flow under such conditions are usually limited to simple pore models with tube/slit geometries or bundle of tubes/slits, which constructs important theoretical basis, however, cannot represent the pore networks of shales. For directly simulating gas flow in complex media, we proposed a modified microscale lattice Boltzmann (MM-LB) model, in which the local effective mean free path (MFP) at any location of the pore system with complex geometry corrected by a custom two-dimensional (2D) eight-direction wall function was considered to capture the effective relaxation time for each lattice node in LB model. What's more, to consider slip velocity at solid boundaries, a combined bounce-back specular-reflection (BSR) scheme was adopted. The MM-LB model was firstly validated in 2D pore systems with simple slit/channel geometries and complex geometries (square and triangular cylinder flows) and in a 3D Sierpinski carpet, where good agreements with linearized Boltzmann solutions, molecular dynamics (MD) simulation and the direct simulation BGK (DSBGK) method results were found. Using the MM-LB model, we quantified the end effect on gas flow through pore throat and found that the end effect is caused by not only the streaming bending but also the variance of MFP (i.e., viscosity) throughout the pore throat. Finally, we applied this model to simulate shale gas flow in three digital reconstructions of kerogen pore systems and compared the results of apparent permeability prediction with Klinkenberg model and Beskok-Karniadakis (B–K) model. It is showed that the apparent permeability of gas flow in shales decreases with the decrease of temperature or the increase of pressure, and the pressure has a greater effect than temperature. For similar pore structures, the apparent permeability increases with the width of pore body and pore throat, especially the pore throat, which dominates the overall flow velocity of the entire flow field. According to the comparison and analysis of the results of MM-LB model with Klinkenberg and B–K model, it can be inferred that permeability prediction based on models of tube/slit (or bundle of tube/slit) pores misestimate the apparent permeability due to ignoring the connectivity and the end effect, especially for the conditions of lower pressures and nanoscale pore spaces.

Molecular beam scattering of a diatomic molecule from a solid surface in case of strong rotational non-equilibrium

In this work, a numerical study on molecular beam scattering of a nitrogen molecule from a graphite surface has been performed. The study was carried out using the molecular dynamics method. The focus of the study is mainly placed on investigating the scattering dynamics in the case of strong rotational non-equilibrium, defined here as a state in which rotational temperature of a molecule strongly deviates from the room temperature. To that end, the incident beam velocity and initial rotational energy of nitrogen molecules have been varied greatly in order to capture a broad range of possible initial states. The obtained results provide valuable insight into the nature of energy transfer occurring during collisions and help to quantify the intensity of rotational–translational coupling between inner kinetic modes in gas–surface collisions. Consequently, the collected data can potentially be used for more accurate characterization of the respective phenomena and improve the quality of boundary models used in rarefied gas flow simulations.

Analysis of nonlinear effects in fluid flows through porous media

This article examines the nonlinear effects of fluid flow in a porous medium, governed by a new semi-analytical equation, from three aspects: equation derivation, experimental verification, and macroscale simulation modelling. The rigorous derivation of the new equation is presented with a semi-analytical approach in which the gas slippage effect and inertial forces are described. The latter effect is controlled by Fochheimer number, which is defined as a product of tortuosity and Reynolds number. The new equation successfully predicts the deviations from Darcy’s law in low-permeability media when the gas slippage effect occurs. The Klinkenberg gas slippage factor is obtained as a function of porous media’s structural parameter (porosity and intrinsic permeability) and gas property (mean free path of gas molecules). The equation validations are performed by core flow experiments for a wide range of reservoir properties, which yield good matching relationship between modelled and observed values. In addition, the proposed semi-analytical equation is used to simulate gas flow in the radial model.

Parametric study of the influence of location and extension height of the rod interceptor on jet engine nozzle characteristics

Modern aircraft need increased maneuverability to maintain a competitive position. One way to improve this characteristic is to use methods to adjust the thrust vectors of a jet engine nozzle. One method of controlling nozzle thrust is the use of nozzle interceptors devices of various shapes that extend into the nozzle channel and cause an uneven distribution of pressure along the walls of the channel. The extension of the rod interceptor near the critical section of the nozzle creates a greater lateral force than the extension of the rod near the exit section. The dependence of the lateral force on the extension height is nonlinear. The article presents a description of a nozzle interceptor of a cylindrical shape extended from the channel wall. Numerical simulation of gas flow in the nozzle with a cylindrical interceptor was carried out. The patterns of gas flow in the channel and in the environment outside the nozzle are presented. The plots of the lateral and axial components of the thrust force against the interceptor rod length are also presented for both cases of the rod location. Rod interceptors can be used in combination with other correcting devices.

DSMC simulations of neutral gas flow in the DTT particle exhaust system

Divertor Tokamak Test Facility (DTT) is a new European superconducting tokamak, currently under final design, addressed to investigate alternative power exhaust solutions for DEMO. Although the divertor system is not finalized yet, the machine and port geometry set limitations on the divertor pumping system operational space. A numerical study of neutral gas dynamics in the divertor region is performed based on the DSMC method by applying the DIVGAS code. The study includes both single-null (SN) and double-null (DN) divertor configurations. For both configurations, the SolEdge2D–EIRENE plasma simulations have been performed for a deuterium plasma with neon seeding and the extracted information about the neutral particles on the predefined interfaces is imposed as incoming boundary conditions for DIVGAS simulations. In the SN case, two plasma puffing scenarios and three candidate pumping port arrangements have been considered. The divertor dome influence on the pumped fluxes can reach 50%. An increase of the capture coefficient six times leads to a decrease in the pressure at the pumping openings by a factor of about 4.5–7. The influence of the size of the lower vertical opening has been studied showing that the enlarged vertical port may establish as the main pumping opening. In the DN case, when the pumping is performed from both lower and upper divertor the overall pumped fluxes at the upper divertor are always higher than the corresponding ones for the lower divertor by a factor of 2–2.5, mainly due to the difference in the pumping areas. In both SN and DN cases, the neutrals outflux toward the X-point dominates the particle transport in the private flux region. The operational space provided by this first assessment is relatively stable against modified classical divertor geometries and allows a more thorough assessment of the pumping technology of the DTT fusion device in the future.

Hydrogen production by solar fluidized bed reactor using ceria: Euler-Lagrange modelling of gas-solid flow to optimize the internally circulating fluidized bed

To perform solar thermochemical conversion, by utilizing high-temperature solar heat as an energy source and redox metal oxide particles as a chemical source, fluidized bed reactor has been developed to produce clean fuels. In this study, an Euler-Lagrange model has been developed for the simulation of particulate and gas flows in fluidized bed reactor for hydrogen production, by two-step water splitting cycles, using solar beam down concentrating system. The solid phase is modelled by Discrete Element Method (DEM) using soft-sphere approach and the gas phase is modelled as continuum by Navier-Stokes equations. The flow behavior of newly developed fine ceria particles has been analyzed for various conditions using the 30 kWth fluidized bed reactor prototype. The effect of particle size on the flow-dynamics at the spout, fountain periphery and annulus of the internally circulating fluidized bed has been examined. The results indicate that the particle size distribution should be minimized as much as possible to avoid the segregation behavior of different size particles.

Numerical simulation of supersonic gas flow in a conical nozzle with local plasma heating

The results of calculations and experiments on plasma heating of the supersonic flow of gases in the conical nozzle by an external high-frequency inductor are given. The simulation was carried out in order to increase the specific impulse and efficiency of the liquid rocket engine. The results of numerical simulations are qualitatively consistent with the experiments conducted in a water-cooled quartz reactor. Keywords: Low-temperature plasma, specific high-frequency power, skin layer.

Unified Gas-Kinetic Wave-Particle Methods VI: Disperse Dilute Gas-Particle Multiphase Flow

In this paper, a unified gas-kinetic wave-particle scheme (UGKWP) for the disperse dilute gas-particle multiphase flow is proposed. The gas phase is always in the hydrodynamic regime. However, the particle phase covers different flow regimes from particle trajectory crossing to the hydrodynamic wave interaction with the variation of local particle phase Knudsen number. The UGKWP is an appropriate method for the capturing of the multiscale transport mechanism in the particle phase through its coupled wave-particle formulation. In the regime with intensive particle collision, the evolution of solid particle will be followed by the analytic wave with quasi-equilibrium distribution; while in the rarefied regime the non-equilibrium particle phase will be captured through particle tracking and collision, which plays a decisive role in recovering particle trajectory crossing behavior. The gas-kinetic scheme (GKS) is employed for the simulation of gas flow. In the highly collision regime for the particles, no particles will be sampled in UGKWP and the wave formulation for solid particle with the hydrodynamic gas phase will reduce the system to the two-fluid Eulerian model. On the other hand, in the collisionless regime for the solid particle, the free transport of solid particle will be followed in UGKWP, and coupled system will return to the Eulerian-Lagrangian formulation for the gas and particle. The scheme will be tested for in all flow regimes, which include the non-equilibrium particle trajectory crossing, the particle concentration under different Knudsen number, and the dispersion of particle flow with the variation of Stokes number. A experiment of shock-induced particle bed fluidization is simulated and the results are compared with experimental measurements. These numerical solutions validate suitability of the proposed scheme for the simulation of gas-particle multiphase flow.

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The subject of the research is modern marine turbomachines which differ in many types, designs, purposes, materials and working bodies. It is noted that this diversity is guaranteed by the use of modern computer technologies from the stage of preliminary preparation of production to the release of final products. The design stage of turbine units taking into account the external characteristics of the turbine stage - power, speed, efficiency, shaft torque and others is considered. The efficiency of the stage is expressed by the efficiency coefficient, which is determined by the energy losses available in the stage. Losses, in turn, are expressed by losses in each individual element of the turbine stage design. At the same time, it is noted that the losses in the nozzle apparatus are decomposed into friction losses, edge and end losses. The object of the study is the nozzle apparatus of a low-consumption inflow turbine. The subject of the study is the velocity coefficient of the nozzle apparatus of a low-consumption inflow turbine. The research method is numerical simulation of gas flow using computational gas dynamics. The purpose of the study is to compare the value of the speed coefficient of the nozzle apparatus obtained during a physical experiment with the results of numerical modeling of the nozzle apparatus with subsonic (narrowing) channels. The conducted studies have shown that low-consumption inflow turbines are characterized by small sizes that do not allow us to fully carry out a physical experiment. A graph of the dependence of the velocity coefficient of the nozzle apparatus on the Mach number is provided in the study. A good convergence of the values of the velocity coefficient of the nozzle apparatus obtained by the numerical method with the results of the physical experiment has been established. The velocity fields of the flow part of the nozzle apparatus are obtained in the range of the Mach number ranging from 0.66 to 0.88. The possibility of using numerical modeling for this type of nozzle apparatus is concluded.

Ar/SF6 plasma simulation for dual-frequency capacitively coupled plasma incorporating gas flow simulation and secondary electron emission

We developed the coupled calculation of plasma and gas flows in simulations for dual-frequency excited Ar/SF6 plasma. By focusing on the effect of secondary electron emission (SEE), we varied SEE coefficient γ and determined γ = 0.04 from the comparison of calculation results with the experimental results. The dependence of electron density on spatial distribution and SF6 gas partial pressure was compared between calculation and experimental results. As a result, at SF6 = 5.0 sccm, the calculated electron densities at the center and edge were almost the same as the experimental results. Furthermore, at SF6 = 2.5 sccm, the error from the experiment including the spatial distribution was in the range of −11.03 to 4.11%, and the results of coupled calculation of plasma and gas flows in simulations can reproduce the experimental results under at a SF6 partial pressure in the range from 2.5 to 5.0 sccm.

Coupled modeling and numerical simulation of gas flows laden with solid particles in de Laval nozzles

Coupled modeling and numerical simulation of gas flows laden with solid particles in de Laval nozzles

Керування газо-масопотоками в струминному обладнанні

Based on a numerical simulation of gas flows in an ejector unit and an analysis of grinding chamber acoustic signals, this paper shows ways to increase the efficiency of jet grinding. To prevent ejector speed-up tube wear and to obtain a ground product without impurities, the effect of feeding an additional energy carrier flow on the flow pattern in the speed-up tube of a jet mill was studied. A comparative analysis of the ejector flow pattern as a function of the presence of an additional feed and the speed-up tube shape was carried out. It was shown that the use of a conical nozzle offers a more uniform flow at the ejector outlet. The additional energy carrier feed provides a uniform increase in flow speed and reduces speed-up tube wall wear. The acoustic signals of the mill working zones were related to the jet grinding process parameters, around which a ground product quality control method was developed. The paper presents a technique for determining the material particle size in the energy carrier flow from the results of acoustic monitoring of the process. The technique uses the established relationship between the dispersion of the acoustic signal characteristic frequency and the mass of the corresponding fracture of the mixture in in-flow material transportation. The technique speeds up material particle size determination and improves the finished product quality. An automatic system was developed to control the grinding process by controlling the loading process according to the characteristics of the grinding zone acoustic signals. An operating model of a controlled hopper of a gas jet mill was made. The operability of the control system was verified on a simulation model, which includes a control objet (mill) model and a control system model. It was shown that the system of mill loading automatic control by the characteristics of the grinding zone acoustic signals offers an up to 10 percent increase in mill capacity, which was verified in industrial conditions at Vilnohorsk Mining and Metallurgical Plant.