Non-stationary Gas Flows Research Articles

Numerical homogenization of the gas filtration problem

The non-stationary gas flow equation in homogeneous and heterogeneous media is considered. Areas of this type are often encountered when considering gas production processes from a gas-bearing reservoir. The heterogeneous structure of the reservoir can strongly affect the extraction processes. For the numerical solution of the problem under consideration, we construct an approximation of the equations on a coarse grid using the numerical averaging method. The method allows us to solve the problem using less computational power and in a shorter time. A numerical comparison of the results of solving the model problem is carried out in a two-dimensional domain for the cases of linear and nonlinear variants of the equations, as well as in a homogeneous and heterogeneous medium. The finite element solution on a fine mesh was taken as a reference solution. The computational realization was performed using the FEniCS library. The construction of the geometric computational domain was performed in the program Gmsh. Paraview and the Matplotlib library in Python were used for data visualization.

Separation of binary gas mixture in a microchannel with oscillating barriers

The time-dependent flow of a neon–argon mixture in a microchannel interrupted by a row of oscillating barriers is numerically studied using the Direct Simulation Monte Carlo method in a range of Knudsen numbers from 0.1 to 10 and in a wide range of oscillation frequencies. The emphasis of the study is on the effect of mixture separation. It is demonstrated that in addition to a mid-frequency (“resonance”) regime, as discovered in the author's previous works [Kosyanchuk et al., “Numerical simulation of novel gas separation effect in microchannel with a series of oscillating barriers,” Microfluid. Nanofluid. 21, 116 (2017) and Kosyanchuk and Pozhalostin, “Non-stationary rarefied gas flow in a plane channel with a series of oscillating barriers,” Eur. J. Mech.-B/Fluids 92, 90–99 (2022)], two other enhanced separation regimes at very low and at very high oscillation frequencies are present. It is also demonstrated that the effect in the mid-frequency regime degrades with decreasing Knudsen number and is almost absent for Kn values around 0.1. The effect in the high-frequency regime is shown to be dictated both by the high frequency of barrier oscillations and by the high speed of barrier motion, and it is shown that with decreasing Knudsen number, the impact of barriers speed becomes dominant. The effect in the low-frequency regime is present for all Knudsen numbers and significantly depends on the phases of barrier motion, which is not observed in other regimes. The separation factor in the low-frequency regime also increases with the number of barriers but only up to the level of molecular diffusion. It was also shown that in the low-frequency regime, there is a trade-off between the separation factor and the gas flow rate.

ИЗУЧЕНИЕ ВЛИЯНИЯ СКОРОСТИ ВХОДЯЩЕГО ПОТОКА НА ЭФФЕКТИВНОСТЬ ЦИКЛОНОВ

A study into the influence of the speed of the incoming dust flow and a size of the inlet of a cyclone on its performance. The structure of the flow field has been investigated by calculations made using the Reynolds Stress Turbulence Model (MTNR) for a cyclone separator. The findings show that the maximum tangential velocity in the cyclone decreases when the entry size to the cyclone increases. No acceleration occurs within the cyclone body (maximum tangential velocity is almost constant throughout the cyclone). Increased size of the cyclone inlet reduces the drop of pressure. The cyclone's cutoff diameter increases with the cyclone's inlet size (therefore, the overall efficiency of the cyclone decreases due to a low vortex strength). The effect of altering the entry’s width is more significant than its height, especially for the cut-off diameter. The influence of simulations of velocity fluctuation on prediction of collection efficiency of cyclone separators has been numerically investigated using MTNR and large eddy simulation (MBV). The Euler-Lagrange modeling approach was used by Solidworks Flow Simulation to simulate 3D non-stationary turbulent gas and solid flows in the high-efficiency Starmand cyclone. The simulation results have been compared with available reference data. An analysis of the findings shows that the MTNR and MBV could adequately predict the mean flow field. The study shows that the (MBV) performs well in predicting fluctuating flow field and capture efficiency for each particle size. The results show that prediction of collection efficiency, especially for fine particles, is greatly influenced by simulation of velocity fluctuations in cyclones.

Mathematical modeling of non-stationary gas flow modes along a linear section of a gas transmission system

Article demonstrates the applicability of modeling non-stationary non-isothermal gas flow along a linear section of a gas transmission system by means of using various numerically simulated models and sophisticated numerical techniques. There are described several models of non-stationary non-isothermal regimes of gas flow along the pipeline section. They are included in the considered general model and their comparative analysis is carried out by the virtue of numerical simulation. The finite difference algorithm is used to solve the simultaneous equations of the numerically simulated model for the pipeline section. The results of calculating the gas flow parameters using various models are presented: both with and without taking into account kinetic energy, as well as both with and without taking into account the Joule–Thompson effect. The matter of choosing the appropriate model is discussed. The obtained results can be used at the stage of transfer pipeline system operation in order to develop scientifically well-founded recommendations for improving the safety and efficiency of the pipeline transportation system.

Non-stationary rarefied gas flow in a plane channel with a series of oscillating barriers

Present paper is dedicated to the numerical study of transient free-molecular gas flow through the microchannel with a series of barriers, oscillating perpendicular to the axis of the channel. Phases of barriers motion are arranged in a specific way to obtain a “wave-like” motion of barriers system. Time-dependent flow rate pattern is studied for different values of problem parameters — oscillation frequencies, number of barriers, distance between them, pressure drop in the channel. It is obtained that pattern of gas flow rate significantly depends on the ratio of characteristic “wavespeed” of barriers system motion and the thermal speed of molecules. It is shown that frequency of output pulsatile flow may be significantly higher than the frequency of barriers mechanical oscillations, depending on the number of barriers, which can be used in practice for creation of high-frequency pulsatile flows. Similarly to authors previous paper Kosyanchuk, et al. (2017) it was obtained that motion of barriers system can lead to separation of gas mixtures with different molecular masses. In contrast to “quasi-stationary” (time-periodic) flow regime (at times around 100-1000 oscillation periods), it is shown that in transient regime (at times less than 10-100 oscillation periods) separation factor can be significantly higher. Moreover, separation effect in transient regime is observed even for the values of defining parameters, which do not grant separation in quasi-stationary flow regime.

MATHEMATICAL MODELING OF THERMOGASDYNAMIC PROCESSES IN PISTON ENGINES

A method for calculating the non-stationary gas flow in the exhaust pipeline of a piston engine by the method of solitary waves of finite amplitude is presented. The comparison of experimental data and calculation results by the method of characteristics and obtained when considering the process of propagation of solitary waves through the exhaust pipeline of a piston engine equipped with a pulsed turbocharging system is shown.

Identification of model flow parameters and model coefficients with the help of integrated measurements of pipeline system operation parameters

The problem of state and parameter estimation of natural gas pipeline system under stationary and non-stationary gas flow is considered. The initial information for solving the problem are pressure and flow measurements with standard measuring equipment. Measurement errors are considered random values that have a normal distribution with zero expectation. The developed methodology takes into account the whole complex of measured parameters in their relationship. The problem reduces to an optimization task; the objective function is derived from the maximum likelihood method. The effectiveness of the methodology was tested on a real object – a complex gas distribution system of a looped structure. The hydraulic efficiency coefficients for this facility are estimated in stationary and non-stationary modes. We use a non-standard model with lumped parameters, which allows us to switch from a system of partial differential equations to a system of ordinary differential equations connecting nodal pressures and flow rates at the arcs of the network graph. In a computational experiment using the gas pipeline branch as an example, the developed algorithm showed very fast convergence.

Improving diagnostics and forecasting of the technical condition of aircraft structural elements by taking into account the impact of non-stationary gas flows

This paper presents the results of a study of the effect of pulsating subsonic gas flow treatment on the corrosion resistance of structural metal materials used in aircraft structural elements. The disadvantage of even the most modern methods of increasing the structural strength of metal materials is an insufficient increase in corrosion resistance. An urgent task is to develop methods for increasing the corrosion resistance of aircraft structural elements throughout their volume without additional alloying, which not only increases the cost of the product material, but also, often, makes it less producible, which can be used during the restoration of both aviation and airfield equipment. The article describes a method developed by the authors to increase corrosion resistance using pulsed subsonic gas flows. The data obtained as a result of the conducted research will allow more effective control of the corrosion condition of aircraft by non-destructive methods, as well as increase the accuracy of forecasting the technical condition.

GAS TURBINE ENGINES IMPELLERS FORCED VIBRATION AND STRESS-STRAIN STATE INVESTIGATION

The method of the gas turbine engine impeller forced vibration and stress-strain state parameters calculation is given. Using the finite element method, a refined mathematical model was developed for the several types of impellers most widespread in the practice of gas turbine engines building. The developed mathematical model takes into consideration the impeller blades geometric parameters and the construction of blades connectors. The results of its forced vibration frequencies calculation, caused by the influence of non-stationary gas flow are given for different types of the blades connectors. The dependencies of the impeller blades stress-strain state from the value its feather geometric perameters have been studied too.

Control of the Formation of a Transonic Region in an Axisymmetric Supersonic Flow with the Use of a Jet and a Near-Wall Energy Supply

The control action of a throttling air jet produced by a gas generator and of the energy sources positioned at the walls of an axisymmetric channel, which models the combustion-chamber unit of a ramjet in which a gas flow is decelerated, on the shock-wave structure of a supersonic gas flow in the channel was investigated for the purpose of formation of a lengthy transonic region in it. A heat energy was supplied to the gas flow in the indicated channel in the pulse-periodic regime, and the pressure in the gas generator was also changed periodically. The range of energies corresponding to a stable transonic flow regime in the channel was determined on the basis of solution of the nonstationary Euler equations in dimensionless variables. For comparison, the combustion of a gaseous fuel inflowing to the channel through a slot in the channel wall upstream of the control air jet was considered. A nonstationary gas flow in the channel was calculated on the basis of the Reynolds-averaged Navier–Stokes equations with the use of the k–ω SST model of turbulence. It is shown that a controllable energy release in the channel, providing the obtaining of a transonic flow regime in it analogous to that obtained on the basis of solution of the Euler equations, can be realized with the use of a control transverse air jet.

Kinetic Methods for Solving Unsteady Problems with Jet Flows

The study of nonstationary rarefied gas flows is currently paid much attention. Such interest to these problems is caused by the creation of pulsed jets used for the deposition of thin films and special coatings on solid surfaces. However the problems of nonstationary rarefied gas flows have not been studied sufficiently fully because of their large computational complexity. In this paper the computational aspects of investigating the nonstationary flows of a reflected gas from a wall and flowing through a suddenly formed gap is considering. The objective of this study is to analyze the possible numerical kinetic approaches for solving such nonstationary problems and to identify the difficulties encountered in solving.When studying the gas flows in strong rarefaction regimes one should consider the Boltzmann kinetic equation, but its numerical implementation is rather laborious. In order to use more simple approaches based for example on approximation kinetic equations (Ellipsoidal-Statistical model, Shakhov model), it is important to estimate the difference of the solutions of the model equations and the Boltzmann equation. For this purpose two auxiliary problems are considered: reflection of the gas flow from the wall and outflow of the free jet into the rarefied background gas. Numerical solution of these problems shows a weak dependence of the solution on the type of the collision operator in the rarefied region, but a strong dependence on the velocity grid step . The detailed velocity grid is necessary to avoid non-monotonous behavior of macroparameters caused by the “ray effect”. To reduce numerical costs on detailed grid a hybrid method based on the synthesis of model equation and the Boltzmann equation is proposed. Such approach can be promising since it reduces the domain in which the Boltzmann collision integral should be used.The results presented in this paper were obtained using two different software packages Unified Flow Solver (UFS) [13] and Nesvetay 3D [14-15]. Note that UFS uses the discrete ordinate method for velocity space on a uniform grid and a hierarchical adaptive mesh refinement in physical space. The possibility of calculating both the Boltzmann equation and model equations is realized. The Nesvetay 3D complex was created to solve the Shakhov model equation, (S-model) and makes it possible to calculate on non-structured non uniform grids in velocity and physical spaces.Translated from Russian. Original text: Mathematics and Mathematical Modeling. 2018. no. 4. Pp. 27-44.

Nanosecond volume discharge in the flow with diffracting shock wave in the rectangular channel

New effects of plasma redistribution shock wave (M = 2.8–3.4) diffraction on an obstacle at flow ionization with pulse volume discharge are presented. The obstacle has size 48.0 × 6.2x1.9 mm3 and consist of forward facing and backward facing steps on the wall. Localization of the nanosecond combined discharge plasma (with pre-ionization in the volume) in non-stationary gas flow was visualized with plasma glow imaging. It is shown that the discharge images visualize flow evolution structure, mainly - zones of reduced density. Image are compared with 2D CFD simulation.

A Generalized Approach to the Analysis of Feed Unevenness in Displacement Pumps

A generalized approach to the analysis of feed unevenness in positive displacement pumps is considered. A procedure for calculating the working processes in chambers of constant or variable volume in a discharge line as well as a nonstationary one-dimensional gas flow in a discharge pipe is developed. Boundary conditions are defined. Nonstationary one-dimensional flow is calculated by the method of “characteristics.” By means of a numerical experiment, variations in the pressure in the working chambers and along the length of the pipe as a function of the turning angle of the shaft are established. On the basis of pressure fluctuation in a discharge pipe, it is proved that the feed unevenness significantly exceeds the magnitude of the feed unevenness determined when the pressure fluctuation is not taken into account.

A new principle of separation of gas mixtures in non-stationary transitional flows

The paper studies non-stationary gas flow in a transitional regime in a region with a moving boundary. Interest in this problem is due to the search for new principles of gas separation, which can be implemented in practice as a microelectromechanical system (MEMS). The diminutiveness of such systems makes them relevant for aerospace applications. To solve the problem, the method of event-driven molecular dynamics (EDMD) was used, which was extended by the authors to the case of computational domain with moving boundaries (simulating moving elements of the device). A new principle of operation of the separation device was proposed and investigated. It was obtained that the separation coefficient of proposed device can be significantly higher than that of the diffusion method.

The simulation of the atmospheric flow with emmisions

The aim of this work is to simulate the flow of the passive gas mixture and estimate the resulting concentration of the potentionally dangerous pollutant emmisions. We assume non-stationary compressible gas flow in the gravitational field described by the system of the RANS equations, equipped with the equation for the concentration of additional specie. Due to gravitational force effect we modify the boundary conditions and data resulting admissible situations. The real simulations of the atmospheric flow require the use of the wall functions for the boundary condition simulating the ground surface with given roughness. We show the modification of such wall functions. The computational results are obtained with the own-developed CFD code for the compressible turbulent mixture flow. The originality of this work lies with the special handling of the boundary conditions, and own computational code.

Kinetic Methods for Solving Non-stationary Jet Flow Problems

The study of non-stationary rarefied gas flows is, currently, attracting a great deal of attention. Such an interest arises from creating the pulsed jets used for deposition of thin films and special coatings on the solid surfaces. However, the problems of non-stationary rarefied gas flows are still understudied because of their large computational complexity. The paper considers the computational aspects of investigating non-stationary movement of gas reflected from a wall and flowing through a suddenly formed gap. The study objective is to analyse the possible numerical kinetic approaches to solve such problems and identify the difficulties in their solving. When modeling the gas flows in strong rarefaction one should consider the Boltzmann kinetic equation, but its numerical implementation is rather time-consuming. In order to use more simple approaches based, for example, on approximation kinetic equations (Ellipsoidal-Statistical model, Shakhov model), it is important to estimate the difference between the solutions of the model equations and of the Boltzmann equation. For this purpose, two auxiliary problems are considered, namely reflection of the gas flow from the wall and outflow of the free jet into the rarefied background gas.A numerical solution of these problems shows a weak dependence of the solution on the type of the collision operator in the rarefied region, but at the same time a strong dependence of a behavior of the macro-parameters on the velocity grid step. The detailed velocity grid is necessary to avoid a non-monotonous behavior of the macro-parameters caused by so-called ray effect. To reduce computational costs of the detailed velocity grid solution, a hybrid method based on the synthesis of model equations and the Boltzmann equation is proposed. Such an approach can be promising since it reduces the domain in which the Boltzmann collision integral should be used.The article presents the results obtained using two different software packages, namely a Unified Flow Solver (UFS) [13] and a Nesvetay 3D software complex [14-15]. Note that the UFS uses the discrete ordinate method for velocity space on a uniform grid and a hierarchical adaptive mesh refinement in physical space. The possibility to calculate both the Boltzmann equation and the model equations is realized. The Nesvetay 3D software complex was created to solve the Shakhov model equation (S-model) for calculations based on non-structured non-uniform grids, both in velocity space and in physical one.

UNSTEADY FRICTION AND HEAT TRANSFER IN THE INITIAL PIPELINE SECTION AT THE RESET OF THERMAL LOAD

The results of numerical and experimental study of wall friction and heat transfer in a short cylindrical channel are presented. Hydrodynamic and thermal characteristics of nonstationary high-temperature gas flow are determined. The results are summarized in the framework of boundary layer theory.

Dependence of energy characteristics of ascending swirling air flow on velocity of vertical blowing

In the model of a compressible continuous medium, for the complete Navier-Stokes system of equations, an initial boundary problem is proposed that corresponds to the conducted and planned experiments and describes complex three-dimensional flows of a viscous compressible heat-conducting gas in ascending swirling flows that are initiated by a vertical cold blowing. Using parallelization methods, three-dimensional nonstationary flows of a polytropic viscous compressible heat-conducting gas are constructed numerically in different scaled ascending swirling flows under the condition when gravity and Coriolis forces act. With the help of explicit difference schemes and the proposed initial boundary conditions, approximate solutions of the complete system of Navier-Stokes equations are constructed as well as the velocity and energy characteristics of three-dimensional nonstationary gas flows in ascending swirling flows are determined.

Mathematical modeling of non-stationary gas flow in gas pipeline

An analysis of the operation of the gas transportation system shows that for a considerable part of time pipelines operate in an unsettled regime of gas movement. Its pressure and flow rate vary along the length of pipeline and over time as a result of uneven consumption and selection, switching on and off compressor units, shutting off stop valves, emergence of emergency leaks. The operational management of such regimes is associated with difficulty of reconciling the operating modes of individual sections of gas pipeline with each other, as well as with compressor stations. Determining the grounds that cause change in the operating mode of the pipeline system and revealing patterns of these changes determine the choice of its parameters. Therefore, knowledge of the laws of changing the main technological parameters of gas pumping through pipelines in conditions of non-stationary motion is of great importance for practice.

Measurement of Non-Stationary Gas Flow Parameters Using Diode Laser Absorption Spectroscopy at High Temperatures and Pressures

The layout of an absorption spectrometer with diode lasers for contactless measurement of the temperature and water-vapor concentration in gas flows with mixture pressures of up to 3 atm and temperatures of 300–2000 K has been designed. The technique is based on the rapid tuning of the radiation wavelength of two lasers, the registration of the absorption lines of water molecules that are located in the tuning range, and the fitting of the experimental absorption spectra by theoretical ones that have been simulated using spectroscopic databases. The original components of the spectrometer and different algorithms of the processing of experimental spectra are described. The performance of the spectrometer and processing methods were tested in the laboratory with a cuvette at a pressure of 1 atm and temperatures of 300–1500 K. The different processing algorithms give a reasonable coincidence of data on hot zone parameters that were obtained by the method of diode laser absorption spectrometry, and the temperature that was measured using standard sensors. The designed layout of the spectrometer passed the first tests on the T-131 experimental stand at the TsAGI (Central Aerohydrodynamics Institute).