Homogeneous Gas Mixture Research Articles

Quasi-Gasdynamic Model and Numerical Algorithm for Describing Mixtures of Different Fluids

An elegant and easy-to-implement numerical algorithm for simulating flows of homogeneous gas mixtures with component temperatures and velocities assumed to be equal is constructed and tested. The algorithm yields monotone density profiles for the components even if their specific heat ratios are widely different. The algorithm can be used to simulate some flows of gas–liquid mixtures.

On properties of aggregated regularized systems of equations for a homogeneous multicomponent gas mixture

Two aggregated regularized systems of equations for a multicomponent homogeneous gas mixture are considered. An entropy balance equation with a non‐negative entropy production is derived for them in the presence of diffusion fluxes. The existence, uniqueness and L2‐dissipativity of weak solutions to an initial‐boundary value problem for the systems linearized on a constant solution are established. The Petrovskii parabolicity and the local in time classical unique solvability of the Cauchy problem are also proved for the aggregated systems themselves.

On the Properties of a Quasihydrodynamic System of Equations for a Homogeneous Gas Mixture with a Common Regularizing Velocity

On the Properties of a Quasihydrodynamic System of Equations for a Homogeneous Gas Mixture with a Common Regularizing Velocity

Start-up behavior and the ignition limit of passive hydrogen recombiners with various catalytic elements

The paper gives the results of experiments aimed to study start-up characteristics and the ignition limit of recombiners containing two various types of catalysts, namely ceramic rods and high porosity cell material. Both types of catalysts were produced by Redkino Catalyst Company. Experiments were carried out using a 15 m3 BM-P chamber located at the FSUE “RFNC-VNIITF named after Academ. E.I. Zababakhin” test site. The recombiner was tested in a homogeneous hydrogencontaining gas mixture under the quasi-stationary conditions with a stepwise increase in hydrogen concentration.Experimental results demonstrated that similar physical processes were observed for both types of catalysts. The recombiner was shown to start up when natural convection flow was activated through its case. Ignition was proved to be caused not only by a heated catalyst, but also by particles escaping from catalytic surface due to spallation and thus decreasing the ignition limit from 10% down to 5%.

Properties of an Aggregated Quasi-Gasdynamic System of Equations for a Homogeneous Gas Mixture

Properties of an Aggregated Quasi-Gasdynamic System of Equations for a Homogeneous Gas Mixture

Свойства агрегированной квазигидродинамической системы уравнений гомогенной газовой смеси с общей регуляризующей скоростью

We study a quasi-hydrodynamic system of equations for a homogeneous (with common velocity and temperature) multicomponent gas mixture in the absence of chemical reactions, with a regularizing velocity common for the components. We derive the entropy balance equation with a non-negative entropy production taking into account the diffusion fluxes of the mixture components. In the absence of diffusion fluxes, a system of equations linearized at a constant solution is constructed by a new technique, In the absence of diffusion fluxes, a system of equations linearized on a constant solution is constructed by a new technique. It is reduced to a symmetric form, the L^2-dissipativity of its solutions is proved, and a degeneration (with respect to the densities of the mixture components) of the parabolicity property for the original system is established. Actually, the system has the composite type. The obtained properties strictly reflect its physical correctness and dissipative nature of the quasi-hydrodynamic regularization.

Gas mixing caused by interacting heat sources PART I: Experiments

The four experiments presented in this paper investigate the influence of the plumes released by the operation of one or two heat sources located at different elevations, on the erosion of a stratified gas atmosphere in the presence of steam condensation on vertically oriented cooled pipes. The heat sources are located at two different elevations in a vessel with 4 m diameter and 8 m height, and they represent Passive Autocatalytic Recombiners (PARs) in a Light Water Reactor (LWR) containment and the heat PARs would release. The condensation on the cooled tubes represents the condensation which would take place during postulated accident scenarios at the concrete wall of a real containment. The condensation on the tubes contributes to the large-scale convection loops and enhances the gas mixing caused by the rising plumes from the heat sources. The stratification with a helium-rich layer – the helium represents hydrogen in a real containment – was created by a vertical steam and steam-plus-helium jet interacting with a circular disk located 1 m above the jet exit. Temperature, gas concentration and velocity field measured during the experiments showed that the activation of only the upper heater created a homogeneous gas mixture above the level of the heater, while forming a sharp concentration gradient below it. By operating only the lower heater, the emerging plume homogenized the entire vessel at the end of the test, showing that it was more effective in gas mixing. The simultaneous activation of the upper and the lower heaters accelerated the breakup of the helium stratification. The experiments were performed in the PANDA facility in Switzerland in the frame of the OECD/NEA HYMERES project which is focused on the hydrogen safety issues related to Light Water Reactors, specifically to hydrogen distribution in the containment. The experiments presented in the present paper were analyzed with the GOTHIC code which is the subject of Part II of this article.

Multidimensional analysis of the impact of motion variation on exergetic performance in a free piston linear hydrogen engine

Linear engine is a promising new energy conversion device which supports variable stroke (or compression ratio) operation for thermal optimization. Previous studies have shown that hydrogen fuel is well adapted to the new engine due to fast combustion. In order to further explore the energy efficiency optimization path of linear hydrogen engine (LHE), this study proposes a multidimensional simulation to investigate the effect of motion stroke variation on exergetic performance of a two-stroke LHE. Meanwhile, a comparative evaluation of exergy between the LHE and a crankshaft hydrogen engine (CHE) is also presented. The results demonstrate that, the irreversible exergy of LHE is slightly higher, but the peak thermomechanical exergy, final chemical exergy, heat transfer exergy and work transfer exergy are lower than CHE, which is attributed to its more homogeneous gas mixture, lower pressure and temperature. The 68 mm stroke achieves more complete combustion and higher temperature for LHE, which promotes the reduction of the final chemical exergy, but inevitably leads to more heat transfer exergy. Shortening the stroke keeps the heat transfer loss of LHE within an acceptable range and effectively reduce the irreversible exergy loss, thereby maximizing the work transfer exergy and increasing the engine efficiency.

Thermodynamic cycles variability of TJI gas engine with different mixture preparation systems

Gas engines are a viable source of propulsion due to the ecological indicators of gas fuels and the large amount of the needed natural resources. Combustion of lean homogeneous gas mixtures allows achieving higher thermal efficiency values, which is a key factor in current engine development trends. Using the spark-jet ignition system (also called as Turbulent Jet Ignition or Two-stage combustion) significantly improves the efficiency and stability of the combustion process, especially in the part-load operation on lean or very lean mixtures. This paper presents the impact of using two different fuel injection methods: Port Fuel Injection or Mixer on the operation stability of a gas engine designed for LDVs. Comparative studies of two different mixture preparation systems were carried out on a single-cylinder AVL 5804 test engine. By re-cording the cylinder pressure for a significant number of engine cycles, it became possible to determine the repeatability of engine operation and to correlate the results with the mixture formation system and the air-fuel ratio. In the performed research the beneficial effect of the mixer system application on the engine operation stability in the part-load conditions was found.

Sensitive Mass Spectrometer for Time-Resolved Gas-Phase Chemistry Studies at High Pressures.

We report a new experimental apparatus for direct time-resolved probing of high-pressure gas-phase chemical reactions by photoionization mass spectrometry. The apparatus uses a laser photolysis flow reactor, capable of operating at P = 0.3-100 bar and T = 300-1000 K. We initiate reactions in homogeneous gas mixtures by the photolysis of an appropriate radical precursor using laser pulses at repetition rates of 1-10 Hz. The reacting mixture is continuously sampled into a vacuum chamber, ionized by vacuum-ultraviolet (VUV) photons from laboratory-based discharge lamps or from a synchrotron beamline, and analyzed by a custom mass spectrometer. Soft near-threshold ionization by tunable synchrotron radiation enables spectroscopic quantification of many key intermediates and products of chemical reactions. A novel ionization scheme in the high-density region of the sample jet expansion increases the experimental sensitivity 100-fold, compared with the existing instruments, without compromising mass resolution. A 40 kHz pulsed reflectron time-of-flight mass spectrometer achieves simultaneous detection of all ionized species with 25 μs time resolution. We demonstrate the capability of this apparatus by probing the ethyl radical oxidation reaction using very dilute (<1012 molecules·cm-3) ethyl concentrations at pressures up to 25 bar.

Experimental and theoretical investigation of the mechanism of flame propagation above the surface of combustible liquid moving under the action of an oxidant in microchannels

Flames propagating above the surface of n-butanol in mini and microchannels have been investigated experimentally. It was determined that when oxygen is used as an oxidant, the critical channel size is about 0.3 mm, and according to the estimates this corresponds to the critical size for a premixed homogeneous gas mixture. In order to understand the mechanism of flame propagation above the surface of liquid, in this study, we attempted to simulate the processes, occurring in liquid. The problem of “thermal wave” propagation in a horizontal rectangular channel, when the liquid film moves under the action of gas flow, was studied theoretically. The analysis was carried out taking into account evaporation, specific heat release, and radiation and absorption processes in the gas and on the free surface, as well as the formed zone of strong heating above the liquid film surface for low oxidant velocities in the channel. Based on data obtained, it can be concluded that at high concentration of oxygen and low oxidant velocities, the experimental value of velocity coincides with the calculated value with satisfactory accuracy.

Effects of nozzle structure on the gas mixture uniformity of marine gas engine

Natural gas is a preferred fuel choice owing to its easy availability, clean combustion, and low emission levels, especially in the field of marine engines. Recently, manifold multi-point gas injection technology was adopted by engine manufactures to improve the response of the marine gas engine to load changes and the consistency of gas fuel supplied to each cylinder. The characteristics of manifold gas injection have a significant influence on the combustion and emissions of marine gas engines. It is therefore vital to investigate the effect of gas nozzle structures on the gas mixture formation and combustion process. In this study, based on the validation through experimental method, a computational fluid dynamics simulation method was adopted to analyze the influence of the nozzle structure on the uniformity of the intake gas mixture in a natural gas engine, and the combustion process was analyzed to show the further effects of nozzle structure on combustion characteristics. The results show that, at high engine load, the structure of the cross multi-hole gas nozzle can help achieve a more homogeneous gas mixture, which is beneficial to the in-cylinder combustion; it yet yields higher NO emission. The single-hole gas nozzle structure yields an inhomogeneous mixture during the intake stroke and has negative effects on combustion. However, at lower engine load, employing a cross multi-hole gas nozzle results in the deposition of gas fuel residue in the intake port during the intake process, thereby inhibiting complete combustion. The structure of the single-hole gas nozzle can provide higher kinetic energy, which has positive effects on fuel intake efficiency and combustion intensity, although there is still an increased NO emission associated with it, owing to a higher cylinder temperature.

CFD modeling of a industrial-scale steam methane reforming furnace

Hydrogen is a required key material for petroleum refineries that convert crude oil into products with higher economic value and is often produced by the steam methane reforming (SMR) process, which synthesizes hydrogen and carbon oxides from methane and superheated steam in the presence of a nickel-based catalyst network in a steam methane reformer. To investigate methods for improving profits for a reformer while avoiding costly on-site parametric studies, a high-fidelity model of a steam methane reformer can be investigated. Motivated by this, the present work focuses on developing a computational fluid dynamics (CFD) model of an industrial-scale steam methane reformer that consists of 336 reforming reactors, 96 burners and 8 flue gas tunnels. The motivation for choosing the modeling strategies used in the industrial-scale steam methane reformer CFD model is discussed and is based on expected transport phenomena and chemical reactions within the reformer. Specifically, the finite rate/eddy dissipation turbulence-chemistry interaction model, global kinetic models of hydrogen/methane combustion, global kinetic model of the SMR process and standard k-∊ turbulence model with the ANSYS Fluent enhanced wall treatment function are used to simulate the formation and consumption rates of all chemical components of the system. In addition, an empirical correlation for estimating the radiative properties of a homogeneous gas mixture, Kirchhoff’s law, Lambert Beer’s law and the discrete ordinate method are employed to simulate radiative heat transfer in the furnace side of an industrial-scale steam methane reformer. Moreover, the modeling strategies of the reforming tubes developed in our previous work are adopted to model 336 reforming tubes in the reformer. Subsequently, the boundary conditions (i.e., the reforming tube feed, burner feed and the energy leakage through the combustion chamber refractory wall) of the industrial-scale reformer CFD model are derived based on typical plant data. The simulation results produced by the industrial-scale reformer CFD model are shown to be in agreement with typical plant data reported in the SMR literature, with the simulation data generated by an industrial-scale reforming tube CFD model and with the simulation data generated by a reforming Gibbs reactor model, which validates the chosen modeling strategies and allows the CFD data to be considered to represent actual plant data with sufficient accuracy for use in industrial operating parameter studies.

Optical determination of temperature and concentrations of homogeneous turbulent gas mixtures

In this study, a previously developed inverse radiation model for a single gas species is extended to mixtures of the combustion gas species CO2, H2O and CO. Time-averaged transmissivities and their root-mean-square (rms) values are successfully related to time-averaged temperatures, species concentrations and their rms values by considering interactions between turbulence and radiation (TRI). The sensitivity of the rms transmissivity spectra to all the parameters to be retrieved are examined and optimal wavenumber ranges for retrieving turbulent scalars of the gas mixtures are discussed. To validate the models, measured spectra are synthesized for different combinations of species concentrations, temperatures, turbulent intensities, pressure path length and turbulent length scales. Results show that mean and rms temperature and species concentrations and other turbulent scalars can be retrieved accurately from turbulent transmissivity measurements as long as the turbulent intensity is below 15% and the pressure path length for the gas mixture is less than 60bar·cm.

Shock wave dispersion of gas–particle mixtures

The decay of a discontinuity in a two-component homogeneous gas mixture and the dispersion of a gas–particle mixture with a two-component carrier medium are numerically simulated. The mathematical model of the dynamics of heterogeneous media takes into account the interphase force interaction and interphase heat exchange. Experimental results known from the literature are compared with numerical results describing the dispersion of a gas–particle mixture in a shock tube.

Combustion of liquid fuel in rectangular mini and microchannels

It is shown that flame spread rate can be high and comparable with velocities of flame propagation in the stoichiometric homogeneous gas mixture. The flame spread rate depends on velocity of oxidizer. It can either increase or decrease with arise of oxidizer velocity, depending on the oxygen content. The flame surface is significantly distorted with increase in average flame spread rate. It is shown that the flame spread rate can be significant and comparable with the laminar burning velocity of the stoichiometric homogeneous gaseous mixture.

Impact of Finite Mixing on Gas-Phase Partial Oxidation for Selective Removal of Deposit Precursors from Gasifier Outlet Streams

A coupled computational fluid dynamics and detailed kinetics study was performed to investigate the impact of finite mixing on the gas-phase partial oxidation of a gasifier outlet stream to selectively remove deposit precursors. The results showed that deposit precursors can be selectively removed as previously demonstrated on the basis of homogeneous gas mixture experiments and kinetic modeling. However, the finite mixing causes substantial degradation of the predicted performance. A detailed rate analysis showed that the imperfect mixing changes the kinetics of the gas-phase reactions. The reaction rates are much faster in the highly localized regions, where the O2 concentration is high, and thus H2 consumption by oxidation reactions become more competitive than H2 regeneration. As a result, the imperfect mixing leads to a significantly less desirable outcome than the ideal mixing case. The performance can be improved by increasing the number air inlets to enhance the degree of mixing. The relative removal amount of deposit precursors can be increased if the initial O2 concentration is increased, but it can cause substantial dilution by N2 from air at the same time. Therefore, one should adjust the initial O2 concentration depending upon the needs of a target system.

A Computational Fluid Dynamics Study on the Gas Mixing Capabilities of a Multiple Inlet System

The mixing behavior of three gas streams was investigated numerically by computational fluid dynamics (CFD) for 16 different geometries to gain insight for the construction of soot measuring systems. The overall goal was to find the design that leads to the fastest mixing of all incoming gas components for a given pipe length by numerical simulations. For this purpose, a main pipe with two symmetrically arranged side inlet pipes was considered, where the angle of inclination of the side pipes and the inflow conditions were varied. Upon the change of the angle of inclination, a transition from a conform to a counter flow is observed. As a variant of the simulation setup, the junction of the three pipes was enclosed by a spherical mixing chamber. The dependency on the angle is much less pronounced in the presence of the additional spherical chamber, which, however, in most cases results in a slower mixing of the gas streams. We found, in general, that the required pipe length to reach a sufficiently homogeneous gas mixture decreases with increasing inclination angles exhibiting the best performance at obtuse angles.

Reaction of a Particle Suspension in a Rapidly‐Heated Oxidizing Gas

AbstractThe reaction of a suspension of solid particles in a rapidly‐heated oxidizing gas is relevant to metalized explosives and propellants, as well as to combustion of solid fuel‐particle suspensions in premixed‐gaseous‐fuel clouds encountered in accidents within the mining and process industries. A simplified model is considered, using a constant‐volume approximation, which assumes that non‐volatile particles react heterogeneously via a one‐step surface reaction. The resulting unified particle reaction rate includes both kinetic and diffusive reaction resistances. It is shown that the onset of the chemical reaction in a rapidly heated particulate suspension may occur by two different physical mechanisms. The first mechanism, realized in a dilute suspension of particles, is defined by the ignition of a single particle, i.e., by the critical phenomenon associated with the rapid transition from a kinetically‐ to diffusively‐limited reaction regime. The second mechanism dominates the reaction onset in a dense particulate suspension and occurs in a similar manner to the reaction onset in a rapidly‐heated homogeneous gas mixture, where the highly‐activated reaction occurs in an explosion‐like manner after some time delay and preheating. Unlike the single particle ignition phenomenon, the second mechanism lacks criticality and is not limited to particles above a certain size. The interplay between these two reaction‐onset mechanisms leads to a nontrivial dependence of the total reaction time on the particle size and solid‐fuel concentration within the suspension.

A correlation of the lower flammability limit for hybrid mixtures

Hybrid mixtures are widely encountered in industries such as coal mines, paint factories, pharmaceutical industries, or grain elevators. Hybrid mixtures explosions involving dust and gas can cause great loss of lives and properties. The lower flammability limit (LFL) is a critical parameter when conducting a hazard assessment or developing mitigation methods for processes involving hybrid mixtures. Unlike unitary dust or gas explosions, which have been widely studied in past decades, only minimal research focuses on hybrid mixtures, and data concerning hybrid mixtures can rarely be found. Although methods to predict the LFL have been developed by using either Le Chatelier's Law, which was initially proposed for homogeneous gas mixtures, or the Bartknecht curve, which was adopted for only certain hybrid mixtures, significant deviations still remain. A more accurate correlation to predict an LFL for a hybrid mixtures explosion is necessary for risk assessment. This work focuses on the study of hybrid mixtures explosions in a 36 L dust explosion apparatus including mixtures of methane/niacin, methane/cornstarch, ethane/niacin and ethylene/niacin in air. By utilizing basic characteristics of unitary dust or gas explosions, a new formula is proposed to improve the prediction of the LFL of the mixture. The new formula is consistent with Le Chatelier's Law.