Transient Gas Flow Research Articles

Simulation of transient gas flow using the orthogonal collocation method

Proper evaluation of the dynamics of the transmission system is the key element in the design and operation of natural gas pipelines. Basic equations describing the transient flow of gas in pipes are derived from the Euler equations. The orthogonal collocation technique is employed as the mathematical method for the numerical solution of the governing equations. This method leads to a set of non-linear ordinary differential equations which can be solved by the Runge–Kutta–Fehlberg method. The performance of the proposed method is tested using two practical examples. The predicted results clearly demonstrate that the proposed method can successfully simulate the isothermal and non-isothermal unsteady flow in gas transmission systems.

Transient flow of rarefied gas through an orifice

A transient rarefied gas flow through a thin orifice is studied on the basis of the direct simulation Monte Carlo method. The mass flow rate and flow field are calculated as a function of the time in the transitional and hydrodynamic regimes with respect to the gas rarefaction. Two values of the pressure ratio, i.e., 0.1 and 0.5, are considered. A characteristic time equal to that needed to cross the orifice radius with the most probable molecular speed is introduced. It is found that it takes about ten characteristic times in order the flow rate to reach its steady value. The flow field past the orifice reaches the steady state during the same time in the transitional regime and it takes a longer time in the hydrodynamic regime.

Transient modeling of gas flow in pipelines following catastrophic failure

This paper simulates transient compressible adiabatic gas flows in a long pipeline following a catastrophic failure using an implicit high order finite difference scheme as a discretisation technique for convective terms. A rupture is assumed to be occurred accidentally at a distance from the feeding point of a pipeline where the pipeline being divided into two distinct sections, high and low pressure segment. The results show that at early times after the rupture, considerable pressure loss will occur at the breakpoint with a chocked flow at this point. After expansion waves reach the closed end of the pipe, a reversal flow will constitute in the pipe. In the high pressure segment, the gas feeding continues and flow rate reaches a steady state chocked flow. In the low pressure segment, the flow rate decreases towards zero until the entire gas of the low pressure segment be evacuated.

An Analysis of Axial Tubing Vibration in a High-Pressure Gas Well

This article considers the influence of flow-induced vibrations on gas-filled tubing system used in high-pressure gas wells, adopts the fluid–structure interaction four-equation model, combined with data of field example of high-pressure gas well, and utilizes the method of characteristics to simulate the gas and tubing string response to the valve-closure transient excitation at the wellhead. The simulation results show that the tubing alternating stress and tubing reciprocating motion, caused by natural gas transient flow, aggravate the tubing fatigue failure, leakage, and wear damage.

Simultaneous Three-dimensional Analysis of Gas–Solid Flow in Blast Furnace by Combining Discrete Element Method and Computational Fluid Dynamics

Low reducing agent operation of the blast furnace is an essential method for mitigating CO2 emissions in ironmaking. Because the coke rate is reduced in low reducing agent operation, gas permeability tends to deteriorate. Recently, blast furnaces with inner volume larger than 5000 m3 have become usual not only in Japan, but also in other Asian nations. Under these conditions, detailed information on in-furnace phenomena is required to attain stable operation.In the present study, a combination model using the discrete element method and computational fluid dynamics (DEM-CFD) was introduced to understand the fully three-dimensional in-furnace phenomena in the whole blast furnace. Due to the limitations of computational resources, the number of DEM particles must be reduced when applying DEM to the whole blast furnace. On the other hand, small cells must be used in the continuum model in order to calculate the gas flow in detail. Thus, mutual conversion between the location of particles in DEM and the property of the cells in the continuum model is needed. In this study, a method of converting information on the locations of cluster-approximated particles treated in DEM to continuum cells was proposed. Furthermore, optimization in which cells could be obtained without conversion parameters was performed to avoid losing local information obtained by DEM calculations. Simulations of solid movement and gas flow were successfully carried out with this coupled DEM-CFD model. As a result, it became possible to understand the three-dimensional stress field among particles under gas flow, transient gas flow and pressure distribution caused by charging of the burden materials, as well as solid motion.

Measurement of Gas Permeability in Geosynthetic Clay Liners in Transient Flow Mode

Abstract Geosynthetic clay liners (GCLs) are often selected as a part of the barrier layer for cover systems in solid waste landfills to prevent infiltration of rainfall and migration of biogas into the atmosphere. To address the ability of GCLs to mitigate gas flow through cover layers, this paper presents an apparatus and testing method to measure the gas permeability of non-saturated and strained GCLs in the laboratory. The test is performed following a transient gas flow regime, and the test results are interpreted using a simple analytical solution. A series of gas permeability tests was performed on a needle-punched GCL with a degree of saturation between 32 % and 47 %. The specimens were tested under a 20 kPa load and 1.3 % strain in the radial direction, whereas the gas flow was induced by a gas differential pressure of less than 4 kPa. The results show that both the apparatus and the testing method can provide a reliable, fast, and simple method to measure the gas permeability of GCLs.

Analytical Solutions to Gas Flow Problems in Low Permeability Porous Media

In most of conventional porous media the flow of gas is basically controlled by the permeability and the contribution of gas flow due to gas diffusion is ignored. The diffusion effect may have significant impact on gas flow behavior, especially in low permeability porous media. In this study, a dual mechanism based on Darcy flow as well as diffusion is presented for the gas flow in homogeneous porous media. Then, a novel form of pseudo pressure function was defined. This study presents a set of novel analytical solutions developed for analyzing steady-state and transient gas flow through porous media including effective diffusion. The analytical solutions are obtained using the real gas pseudo pressure function that incorporates the effective diffusion. Furthermore, the conventional assumption was used for linearizing the gas flow equation. As application examples, the new analytical solutions have been used to design new laboratory and field testing method to determine the porous media parameters. The proposed laboratory analysis method is also used to analyze data from steady-state flow tests of three core plugs. Then, permeability (k) and effective diffusion coefficient (D e) was determined; however, the new method allows one to analyze data from both transient and steady-state tests in various flow geometries.

The accuracy and efficiency of a reduced-order model for transient flow analysis in gas pipelines

A reduced-order modeling approach for natural gas transient flow in pipelines is proposed. The Euler equations are considered as the governing equations and solved numerically using the implicit Steger–Warming flux vector splitting method (FSM). Next, the linearized form of the equations is derived and the corresponding eigensystem is obtained. Then, a few dominant flow eigenmodes are used to construct an efficient reduced-order model. Two well-known test cases are presented to demonstrate the accuracy and the computational efficiency of the proposed method. The results obtained using the proposed reduced-order model, are in good agreement with those of the direct numerical method. Moreover, it is shown that the proposed reduced-order model is more efficient than the conventional numerical techniques for transient flow analysis of natural gas in pipelines.

Sensitivity of pipeline gas flow model to the selection of the equation of state

Real gas effects exert a significant influence on the hydraulics of natural gas transmission pipelines. In this article the implications of the selection of the equation of state for the pipeline gas flow model are investigated. A non-isothermal transient gas flow model with AGA-8 and SGERG-88 equations of state was studied. Models with Soave-Redlich-Kwong and Benedict-Webb-Rubin equations of state were solved to illustrate the overall gas flow model inaccuracies. The effect of the selection of different equations of state on the flow parameters is demonstrated and discussed.

Methane Oxidation over Alumina and Ceria Supported Platinum

Oxidation of methane over Pt/Al2O3 and Pt/CeO2 catalysts was studied by transient gas-flow reactor and in situ FTIR spectroscopy experiments. The Pt/CeO2 catalyst is generally more active, indicating more efficient CH4 dissociation, especially during oxygen pulsing, which likely is connected to the higher oxygen storage capacity/mobility of ceria.

Hybrid Continuum/Molecular Simulations of Transient Gas Flows with Rarefaction

An adaptively coupled continuum-molecular approach for compressible viscous flows in transient calculations is presented. The continuum domain is described by the unsteady compressible Navier―Stokes equations, and the molecular domain is solved by direct simulation Monte Carlo. A strategy is described to extend the overlapped Schwarz method with Dirichlet―Dirichlet boundary-condition coupling procedure to transient simulations. The method has been successfully validated against full direct simulation Monte Carlo results for transient simulations of a one-dimensional shock tube and a two-dimensional pressure-driven slit flow. A sensitivity analysis showed that the used overlapped Schwarz coupling method with Dirichlet―Dirichlet boundary conditions is only weakly sensitive to various parameters (e.g., the exact position of the continuum/molecular interface, the size of the overlap region, and the scatter in the molecular solution), which is a clear advantage over the more commonly used flux-based coupling technique. Another advantage of the chosen coupling method is that the continuum and molecular time steps can be decoupled and that, in general, a coupling time step can be used that is much larger than the molecular time step. The study performed also highlighted a limit of the method: it was found to be necessary to keep the Courant number (based on the coupling time step, the cell size in the continuum region, and the molecular most-probable velocity) below 1 to avoid instabilities.

An implicit box scheme for subsonic compressible flow with dissipative source term

We investigate the stability and convergence of an implicit box scheme for subsonic flows modelled by scalar conservation laws with dissipative and possibly stiff source terms. The scheme is proposed for solving transient gas flow problems in pipeline networks. Such networks are operated in the subsonic flow region and are characterized by pressure losses due to dissipative friction terms. We verify the properties stated by Kružkov’s theorem (Kružkov, Math. USSR-Sb. 10:217–243, 1970) for the approximate solution and prove its convergence to the entropy solution.

Gas flow to a vertical gas extraction well in deformable MSW landfills

Active gas control systems are commonly used in municipal solid waste (MSW) landfills and the design of such systems requires thorough understanding of the gas flow pattern. A model is developed to predict the two-dimensional radial transient gas flow to a vertical gas extraction well in deformable MSW landfills. Variations of gas storage include time-dependent compression of the refuse, dissolution of gas components and porosity enlargement due to organic matter degradation. Mechanical compression of solid skeleton is coupled with gas pressure using K–H rheological model which is capable of reproduce the evolution of settlement for MSW landfills. The new analytical solution obtained in Laplace transform domain can be used to determine excess gas pressure fields, gas fluxes in the well and through the top cover as well as landfill settlements. The solution is validated by comparison with field measurements and numerical simulations. It demonstrates that the gas storage variation term becomes predominant only during early times. Long-term gas flow is controlled by the gas generation rate and the quasi-steady solution is valid. Parametric studies indicate that the solution given in this paper is useful for the prediction of gas fluxes, for the choice of the optimum spacing between wells, and for the determination of the final cover properties as well as appropriate vacuum pressure imposed in the extraction well.

Evaluation of a new computational fluid dynamics model for internal combustion engines using hydrogen under motoring conditions

The present work conducts a preliminary evaluation of a new CFD (computational fluid dynamics) model, which is under development at the authors' laboratory. Using this model, it is feasible to understand how the intake manifold and in-cylinder geometry affect the in-cylinder flow field and the mixing processes taking place in an Otto (spark-ignition) engine. The model is applied on a high-swirl, two-valve, four-stroke, transparent combustion chamber engine running under motoring conditions. To investigate the fuel–air mixing process, hydrogen is injected in the intake manifold. To evaluate the model three case studies are examined. First, the model is applied to simulate the external mixing in the intake manifold with a tee-mixer injection system. Secondly, the transient gas flow field in the intake manifold and engine cylinder is examined over the complete engine cycle. Finally, the transient mixing process in the intake manifold and the spatial and temporal distribution of species concentrations inside the cylinder are numerically computed using the developed model. To validate the model, the results obtained through the test cases examined are compared either with available experimental data or with simulated results, which are obtained using a commercially available CFD code applied under the same conditions.

Reduced Order Modeling of Natural Gas Transient Flow in Pipelines

Reduced Order Modeling of Natural Gas Transient Flow in Pipelines

Formation and structure of circular-disc assemblies of double-walled carbon nanotubes from a catalytic CVD reaction

A large-size circular-disc (CD) assembly of carbon nanotubes (CNTs) was formed from injection catalytic chemical vapor deposition (CVD) reactions. The CNT-CD assembly, which has an outer diameter of 830 nm, consists of closely packed rings of double-walled carbon nanotubes (DWNTs), in twenty-eight circles, and stacked in five layers. A structural interpretation suggests that the CNT-CD assembly is coiled from a “tape-like” bundle of DWNTs, whose coiling to form the CNT-CD resembles the rolling up of tape to form a disc. The large CD assembly was formed within milliseconds in the transient gas flow reactions, and the coiling of nanotubes or “nucleation” of the circular assembly was associated with shears generated by gas disturbances in the vapor phase.

Computation of the integral parameters of turbulent structures for the transient gas flows in pipes using wavelet transforms

Turbulent structures of simulated two-dimensional gas flows in a convergent-tube assembly of a complex pipe are analyzed by computing the morphometric parameters for the texture of two-dimensional energy fields using wavelet transforms. A new computer-aided procedure based on the decomposition of the original image of a gas flow into low-and high-frequency components using a discrete wavelet transform, calculation of the textural parameters for the high-frequency component, binary decomposition, and calculation of the morphometric parameters for the textured images is developed to analyze the flow pattern of the turbulence of transient gas flows in the assemblies of complex pipes. A new system of integral parameters computed on the basis of the binary decomposition of the textured images of gas-flow characteristics and the morphometric measurements of the parameters of flow objects in binarization sections is proposed to characterize the turbulent structure of transient gas flows in pipes.

Computer simulation of transient gas flows in complex round pipes

A computer model and algorithms for predicting and analyzing the transient incompressible gas flows in standard assemblies of complex round pipes are developed. It is shown that the vibrational safety and reliability of complex pipes are most reasonable to provide by using the three-dimensional computer simulation of the distribution of velocities, pressures, and temperatures over different sections of the pipe. It is shown that the developed three-dimensional computer model and obtained results can be used for calculating the integral indices of transient gas flows in assemblies of complex pipes.

Simulations of transient shock motion within a biological contoured-shock-tube system

This study is motivated by the author’s interest in developing needle-free powdered vaccine/drug delivery systems. One system configuration is called the Contoured Shock Tube (CST). Of great importance is the behaviour of a transonic gas flow with a strongly nonlinear starting process, which accelerates powdered vaccines in micro-form to a sufficient momentum to penetrate the outer layer of human skin or mucosal tissue. In this paper, an established Modified Implicit Flux Vector Splitting (MIFVS) solver for the Navier–Stokes equations is extended to numerically study these transient transonic gas flows. A low Reynolds number k – ϵ turbulence model, with the compressibility effect considered, is integrated into the MIFVS solver to predict the turbulent structures and interactions with inherent shock systems. The MIFVS is first calibrated for NASA validation case, NPARC, and the resulting flow characteristic are compared with experimental date and simulations published. The MIFVS calculation with the modified k – ϵ model shows the best agreement. Subsequently, the MIFVS is applied to model the transient gas flow within a biolistic CST prototype. Comparison with experimental pressure traces shows the MIFVS captures gas flow mechanics with more accuracy than calculations with a commercial code (Fluent). This illustrates that the MIFVS is well-suited to model the strongly nonlinear fluid dynamics associated with the CST biolistic particle delivery system.

Simulation of transient gas flow at pipe‐to‐pipe intersections

AbstractWe numerically investigate a well‐established mathematical model for gas flow in pipeline networks. The emphasis is given to the description of coupling conditions at pipe‐to‐pipe intersections. The accurate prediction of these coupling conditions is essential in order to achieve good performance and reliable numerical simulations of network simulations. Nowadays, a variety of theoretical results for a reduced geometry exists. We conduct a numerical simulation of the two‐dimensional representation at a pipe intersection. The results are used for verification and comparison with known theoretical results. High‐order and non‐oscillatory discretizations are used for solving the governing equations of gas flow. Verifications are conducted for two types of junctions to analyse the most adequate coupling conditions. Copyright © 2007 John Wiley & Sons, Ltd.