Gas-dust Mixture Research Articles

A smoothed particle hydrodynamics algorithm for multigrain dust with separate sets of particles

ABSTRACT We present a method for simulating the dynamics of a mixture of gas and multiple species of large Stokes number dust grains, typical of evolved protoplanetary discs and debris discs. The method improves upon earlier methods, in which only a single grain size could be represented, by capturing the differential backreaction of multiple dust species on the gas. This effect is greater for large dust-to-gas ratios that may be expected in the later stages of the protoplanetary disc life. We benchmark the method against analytic solutions for linear waves, drag, and shocks in dust–gas mixtures, and radial drift in a protoplanetary disc showing that the method is robust and accurate.

A solution to the overdamping problem when simulating dust–gas mixtures with smoothed particle hydrodynamics

ABSTRACT We present a fix to the overdamping problem found by Laibe & Price when simulating strongly coupled dust–gas mixtures using two different sets of particles using smoothed particle hydrodynamics. Our solution is to compute the drag at the barycentre between gas and dust particle pairs when computing the drag force by reconstructing the velocity field, similar to the procedure in Godunov-type solvers. This fixes the overdamping problem at negligible computational cost, but with additional memory required to store velocity derivatives. We employ slope limiters to avoid spurious oscillations at shocks, finding the van Leer Monotonized Central limiter most effective.

Effect of low-concentration coal dust on gas explosion propagation law

In order to study the effect of low-concentration coal dust on gas explosion characteristics and gas propagation law in gas transmission pipelines in the field of gas power generation, a gas explosion experiment with the participation of low-concentration coal dust was performed by using the self-developed experimental system for coal dust-gas mixture explosion. The results show that the addition of low-concentration coal dust raises the gas explosion pressure and explosion flame propagation speed. The volatile content of coal dust and gas content have a great influence on the law of explosion. When the gas concentration is 9.5%, After adding coal dust, the explosion flame speed reaches the detonation level under the influence of reflection wave. The experimental data and scientific analysis can provide guidance for gas explosion and prevention under coal mine.

Morphological signatures induced by dust back reaction in discs with an embedded planet

ABSTRACT Recent observations have revealed a gallery of substructures in the dust component of nearby protoplanetary discs, including rings, gaps, spiral arms, and lopsided concentrations. One interpretation of these substructures is the existence of embedded planets. Not until recently, however, most of the modelling effort to interpret these observations ignored the dust back reaction to the gas. In this work, we conduct local-shearing-sheet simulations for an isothermal, inviscid, non-self-gravitating, razor-thin dusty disc with a planet on a fixed circular orbit. We systematically examine the parameter space spanned by planet mass (0.1Mth ≤ Mp ≤ 1Mth, where Mth is the thermal mass), dimensionless stopping time (10−3 ≤ τs ≤ 1), and solid abundance (0 < Z ≤ 1). We find that when the dust particles are tightly coupled to the gas (τs < 0.1), the spiral arms are less open and the gap driven by the planet becomes deeper with increasing Z, consistent with a reduced speed of sound in the approximation of a single dust–gas mixture. By contrast, when the dust particles are marginally coupled (0.1 ≲ τs ≲ 1), the spiral structure is insensitive to Z and the gap structure in the gas can become significantly skewed and unidentifiable. When the latter occurs, the pressure maximum radially outside of the planet is weakened or even extinguished, and hence dust filtration by a low-mass (Mp < Mth) planet could be reduced or eliminated. Finally, we find that the gap edges where the dust particles are accumulated as well as the lopsided large-scale vortices driven by a massive planet, if any, are unstable, and they are broken into numerous small-scale dust–gas vortices.

Morphology of the surface and magnetic resonance properties of lavsan modified by steel clusters by ion-plasma exposure

Polymeric materials are mainly dielectrics. The use of protective clothing made of such materials may occur accumulation of static electricity. The electrification of materials creates a fire hazard in the presence of dust and various gas mixtures. For reducing the accumulation of static electricity charges on the surface of protective clothing carried out the modification of polyethylene terephthalate by clusters of steel 12X18H10T. Modification was carried out in a vacuum using stationary metal plasma source operating in the mode of separations. The process was carried out by alternating periods of the source plasma (1 minute) and pause for tissue cooling (1 minute).The results of the study of the surface morphology of polyethylene terephthalate steel coated fabrics and magnetic resonance absorption measurements of microwave energy is given. The main regularities of the magnetic-resonance absorption spectra are revealed. Method of electronic magnetic resonance (EMR) found that the spectra are inhomogeneous broadened broad lines with effective g-factor value of 2.2±0.1 and the line width of 110.1 mTl. It was found that the non-resonance absorption of the electromagnetic field of microwave is insignificant.

Minimum explosion concentration of coal dusts in air with small amount of CH4/H2/CO under 10-kJ ignition energy conditions

A 20-L spherical explosion chamber was used to investigate the minimum explosion concentration (MEC) of coal dusts with small amount of flammable gas which is lower than its lower explosion limit (LEL). Two dust samples (anthracite coal and bituminous coal) and three flammable gases (CH4, H2 and CO) were tested. Two methods respectively based on overpressure and combustion duration time were used to determine the MEC of the hybrid mixtures. Experimental results show that the explosion of hybrid mixtures occurs when both dust and gas concentrations are lower than the LEL or MEC of the single substances. As flammable gas concentration increases, either explosion pressure (Pex) and explosion pressure rise (dp/dt)ex) increase or the MEC decreases for all the hybrid gas-dust mixtures as a general trend, showing a strong concentration effect. At the same concentration of coal dusts, the addition of CH4 poses a higher explosion risk than the other two flammable gases. Moreover, it was found that the results of MEC determined by both methods agree each other well, suggesting that both methods are valid to determine MEC of hybrid mixture in the synergic explosion region. The distribution of experimental data in the explosion regimes shows that the restricted areas defined by empirical formulas are insufficient from safety considerations.

Анализ состояния пылегазовоздушных смесей в атмосфере горных выработок в окрестности очагов самонагревания

Stationary process of dusty-gas mixtures flow in the mine workings in the presence of the heat supply zone caused by spontaneous heating centers located near the workings is considered in the article. Based on the laws of conservation of mass, impulse and energy the formulas are obtained for determining the number of characteristics (speed, density, pressure, temperature, Mach number) of the dust-gas-air mixture flow, which describe the intensity of the moving front of the dust-gas-air mixture, which crossing the zone of influence of the spontaneous heating points. These expressions contain the plus or minus sign in front of the radical, so each sought function has two significantly different meanings. The problem is formulated on the influence of the thermal field caused by the points of spontaneous heating on the flow of dust-gas-air mixtures in the room. It was identified that for any Damkeler number representing the dimensionless amount of heat input, each of the functions found, that characterizes the stationary flow of mixtures along the working, consists of two sections. Their boundaries are two critical Mach numbers, and, moreover, the flow of mixtures in the section between them is unsteady. The larger the Damkeler number is, the longer is the section between the critical Mach numbers, and the more unsteady nature of the dust-gas mixture flow is manifested. The conditions for the unsteady flow of mixtures are revealed. Plots are constructed that characterize the changes in the parameters of dust-gas-air mixtures crossing the zone of influence of spontaneous heating points. At critical values of heat input, the relative values of pressure and density of mixture increase monotonically with increasing the critical Mach numbers. With a Mach number equal to one, the Damkeler number assumes zero value. This indicates the impossibility of supplying heat to the dust-gas mixture when it moves in a stationary mode.

On the nature of the resonant drag instability of dust streaming in protoplanetary disc

Abstract The recently discovered resonant drag instability (RDI) of dust streaming in protoplanetary disc is considered as the mode coupling of subsonic gas-dust mixture perturbations. This mode coupling is coalescence of two modes with nearly equal phase velocities: inertial wave (IW) having positive energy and a streaming dust wave (SDW) having negative energy as measured in the frame of gas environment being at rest in vertical hydrostatic equilibrium. SDW is a trivial mode produced by the bulk streaming of dust, which transports perturbations of dust density. In this way, settling combined with radial drift of the dust makes possible coupling of SDW with IW and the onset of the instability. In accordance with the concept of the mode coupling, RDI growth rate is proportional to the square root of the coupling term of the dispersion equation, which itself is proportional to mass fraction of dust, f ≪ 1. This clarifies why RDI growth rate ∝f1/2. When SDW has positive energy, its resonance with IW provides an avoided crossing instead of the mode coupling. In the high wavenumber limit RDI with unbounded growth rate ∝f1/3 is explained by the triple mode coupling, which is coupling of SDW with two IW. It coexists with a new quasi-resonant instability accompanied by bonding of two oppositely propagating low-frequency IW. The mode coupling does not exist for dust streaming only radially in a disc. In this case RDI is provided by the obscured mechanism associated with the inertia of solids.

Non-linear evolution of instabilities between dust and sound waves

ABSTRACT We study the non-linear evolution of the acoustic ‘resonant drag instability’ (RDI) using numerical simulations. The acoustic RDI is excited in a dust–gas mixture when dust grains stream through gas, interacting with sound waves to cause a linear instability. We study this process in a periodic box by accelerating neutral dust with an external driving force. The instability grows as predicted by linear theory, eventually breaking into turbulence and saturating. As in linear theory, the non-linear behaviour is characterized by three regimes – high, intermediate, and low wavenumbers – the boundary between which is determined by the dust–gas coupling strength and the dust-to-gas mass ratio. The high and intermediate wavenumber regimes behave similarly to one another, with large dust-to-gas ratio fluctuations while the gas remains largely incompressible. The saturated state is highly anisotropic: dust is concentrated in filaments, jets, or plumes along the direction of acceleration, with turbulent vortex-like structures rapidly forming and dissipating in the perpendicular directions. The low-wavenumber regime exhibits large fluctuations in gas and dust density, but the dust and gas remain more strongly coupled in coherent ‘fronts’ perpendicular to the acceleration. These behaviours are qualitatively different from those of dust ‘passively’ driven by external hydrodynamic turbulence, with no back-reaction force from dust on to gas. The virulent nature of these instabilities has interesting implications for dust-driven winds in a variety of astrophysical systems, including around cool stars, in dusty torii around active-galactic-nuclei, and in and around giant molecular clouds.

The resonant drag instability (RDI): acoustic modes

Recently, Squire & Hopkins (2017) showed any coupled dust-gas mixture is subject to a class of linear 'resonant drag instabilities' (RDI). These can drive large dust-to-gas ratio fluctuations even at arbitrarily small dust-to-gas mass ratios. Here, we identify and study both resonant and new non-resonant instabilities, in the simple case where the gas satisfies neutral hydrodynamics and supports acoustic waves ($\omega^{2}=c_{s}^{2}\,k^{2}$). The gas and dust are coupled via an arbitrary drag law and subject to external accelerations (e.g. gravity, radiation pressure). If there is any dust drift velocity, the system is unstable. The instabilities exist for {\em all} dust-to-gas ratios $\mu$ and their growth rates depend only weakly on $\mu$ around resonance, as $\sim\mu^{1/3}$ or $\sim \mu^{1/2}$ (depending on wavenumber). The behavior changes depending on whether the drift velocity is larger or smaller than the sound speed $c_{s}$. In the supersonic regime a 'resonant' instability appears with growth rate increasing without limit with wavenumber, even for vanishingly small $\mu$ and values of the coupling strength ('stopping time'). In the subsonic regime non-resonant instabilities always exist, but their growth rates no longer increase indefinitely towards small wavelengths. The dimensional scalings and qualitative behavior of the instability do not depend sensitively on the drag law or equation-of-state of the gas. The instabilities directly drive exponentially growing dust-to-gas-ratio fluctuations, which can be large even when the modes are otherwise weak. We discuss physical implications for cool-star winds, AGN-driven winds and torii, and starburst winds: the instabilities alter the character of these outflows and could drive clumping and/or turbulence in the dust and gas.

Ubiquitous instabilities of dust moving in magnetized gas

Squire & Hopkins (2017) showed that coupled dust-gas mixtures are generically subject to 'resonant drag instabilities' (RDIs), which drive violently-growing fluctuations in both. But the role of magnetic fields and charged dust has not yet been studied. We therefore explore the RDI in gas which obeys ideal MHD and is coupled to dust via both Lorentz forces and drag, with an external acceleration (e.g., gravity, radiation) driving dust drift through gas. We show this is always unstable, at all wavelengths and non-zero values of dust-to-gas ratio, drift velocity, dust charge, 'stopping time' or drag coefficient (for any drag law), or field strength; moreover growth rates depend only weakly (sub-linearly) on these parameters. Dust charge and magnetic fields do not suppress instabilities, but give rise to a large number of new instability 'families,' each with distinct behavior. The 'MHD-wave' (magnetosonic or Alfven) RDIs exhibit maximal growth along 'resonant' angles where the modes have a phase velocity matching the corresponding MHD wave, and growth rates increase without limit with wavenumber. The 'gyro' RDIs are driven by resonances between drift and Larmor frequencies, giving growth rates sharply peaked at specific wavelengths. Other instabilities include 'acoustic' and 'pressure-free' modes (previously studied), and a family akin to cosmic ray instabilities which appear when Lorentz forces are strong and dust streams super-Alfvenically along field lines. We discuss astrophysical applications in the warm ISM, CGM/IGM, HII regions, SNe ejecta/remnants, Solar corona, cool-star winds, GMCs, and AGN.

Tricolor Technique for Visualization of Spatial Variations of Polydisperse Dust in Gas-Dust Flows

The aim of this work is to construct an algorithm for visualizing a polydisperse phase of solid particles (dust) in an inhomogeneous flow of a two-phase gas-dust mixture that would allow us to see, within one plot, the degree of polydispersity of the dust phase and the difference in the spatial distributions of individual fractions of dust particles in the computational domain. The developed technique allows us to reproduce concentrations from one to three fractions of dust particles in each cell in the computational domain. Each of the three fractions of dust particles is mapped to one of the main channels of the RGB palette. The intensity of the color shade is set to be proportional to the relative concentration of dust particles in this fraction. The final image for a polydisperse mixture is obtained by adding images in each of the three color channels. To visualize the degree of polydispersity, we propose depicting the spatial distribution of the entropy of the dust mixture. The definition of the entropy of a mixture is generalized to take into account the states of a mixture with zero number of particles in the mixture. They correspond to dust-free sections of the computational domain (voids). The proposed method for visualizing the polydispersity of a mixture of particles is demonstrated using the example of dynamic numerical modeling of the spatial features of dust structures formed in turbulent gas-dust flows and in flows with shock waves.

Resonant drag instabilities in protoplanetary discs: the streaming instability and new, faster growing instabilities

We identify and study a number of new, rapidly growing instabilities of dust grains in protoplanetary disks, which may be important for planetesimal formation. The study is based on the recognition that dust-gas mixtures are generically unstable to a Resonant Drag Instability (RDI), whenever the gas, absent dust, supports undamped linear modes. We show that the "streaming instability" is an RDI associated with epicyclic oscillations; this provides simple interpretations for its mechanisms and accurate analytic expressions for its growth rates and fastest-growing wavelengths. We extend this analysis to more general dust streaming motions and other waves, including buoyancy and magnetohydrodynamic oscillations, finding various new instabilities. Most importantly, we identify the disk "settling instability," which occurs as dust settles vertically into the midplane of a rotating disk. For small grains, this instability grows many orders of magnitude faster than the standard streaming instability, with a growth rate that is independent of grain size. Growth timescales for realistic dust-to-gas ratios are comparable to the disk orbital period, and the characteristic wavelengths are more than an order of magnitude larger than the streaming instability (allowing the instability to concentrate larger masses). This suggests that in the process of settling, dust will band into rings then filaments or clumps, potentially seeding dust traps, high-metallicity regions that in turn seed the streaming instability, or even overdensities that coagulate or directly collapse to planetesimals.

Novel method for hybrid gas-dust cloud ignition using a modified standard minimum ignition energy device

Hybrid dust-gas explosions are a persistent problem in the process industries because of their low ignition energy and serious consequences. The probability of ignition of a hybrid dust-gas mixture is quantified by its minimum ignition energy (MIE). This study aims at improving the MIE measurement of hybrid dust-gas systems using a modified Kühner MIKE3 MIE apparatus. The modification includes an add-on device in order to purge the Hartmann tube with the gas of the hybrid mixture before the dust dispersion and ignition attempt. This ensures the gas composition in the Hartmann tube is the same as that of the gas used for dispersing the dust.In this study, the MIE for a hybrid system consisting of Pittsburgh Pulverized Coal (PPC), methane and air was examined. MIE testing was conducted for the two cases: case (a) followed the ASTM E2019-03 standard procedure, while case (b) applied a pre-dispersion purge of the Hartmann tube with the methane-air mixture. In addition, this hybrid system MIE study was conducted with two different particle sizes of PPC, having equivalent polydispersity. Certified cylinders consisting of methane (1, 2, and 3 vol %) blended with ultra-high purity (UHP) air (21.00% oxygen and 79.00% nitrogen) were used.Significant differences in the MIE values of the hybrid dust-gas system were found between the two cases for both particle sizes. The hybrid MIE values obtained with a pre-dispersion purge of the Hartmann tube were found to be lower than those without the purge (which is the most common testing approach); while a smaller particle size also resulted in a lower hybrid MIE. Additionally, it was found that for experiments with or without pre-dispersion purge, and for both particle sizes, the greater the methane concentration, the lower the MIE.This study proves that many of the previous hybrid MIE studies in the Hartmann tube are non-conservative. Future studies should be conducted by including a pre-dispersion purge in the testing procedure. This work lays the foundation for improving the existing ASTM E2019-03 or developing a standalone standard for hybrid dust-gas system MIE testing.

Phantom: A Smoothed Particle Hydrodynamics and Magnetohydrodynamics Code for Astrophysics

AbstractWe presentPhantom, a fast, parallel, modular, and low-memory smoothed particle hydrodynamics and magnetohydrodynamics code developed over the last decade for astrophysical applications in three dimensions. The code has been developed with a focus on stellar, galactic, planetary, and high energy astrophysics, and has already been used widely for studies of accretion discs and turbulence, from the birth of planets to how black holes accrete. Here we describe and test the core algorithms as well as modules for magnetohydrodynamics, self-gravity, sink particles, dust–gas mixtures, H2chemistry, physical viscosity, external forces including numerous galactic potentials, Lense–Thirring precession, Poynting–Robertson drag, and stochastic turbulent driving.Phantomis hereby made publicly available.

Turbulent burning velocity of methane–air–dust premixed flames

Investigation of turbulent burning velocity (ST) of methane–air–dust premixed flames with different dust types (coal, sand and sodium bicarbonate) and dust concentrations (λp = 0–75 g/m3) were conducted at three methane–air pre-mixture equivalence ratios (ϕg = 0.8, 1.0 and 1.2) and different turbulent intensities (urms′ = 0.65, 0.72 and 0.88 m/s). Experiments were conducted in a dust Bunsen burner set-up at constant pressure conditions to study stabilized premixed flames. The results indicate that based on the particle type, the variation of turbulent burning velocity with an increase in the particle concentration differs. In general, coal and sodium bicarbonate result in the heterogeneous effect of absorbing heat and releasing volatiles; whereas sand particles just absorb heat from the flame zone. The detailed time scale analysis conducted shows that the presence of particles in the concentration range considered tends to slightly enhance the cold flow turbulence whereas with the presence of flame zone, an increase in the turbulent intensity results in increasing the vaporization rate of the particles. This effects in decreasing the turbulent burning velocity of methane–air mixtures with coal and sodium bicarbonate particles at higher concentrations and turbulent intensities. Out of three dusts examined, sodium bicarbonate addition results in the lowest ST due to the release of CO2 and H2O. Between coal and sand, at fuel lean and stoichiometric conditions, ST values with coal are greater than sand due to the equivalence ratio promotion with the release of CH4. But, as the turbulent intensity increases and for ϕg = 1.0–1.2, ST values with sand becomes comparable to or greater than that of coal. Model coefficients are generated from the experimental data to estimate the turbulent burning velocity in these conditions and the results show a clear distinction in the model coefficients for gaseous and gas–dust mixtures.

STUDY OF THE PHASE-AND-CHEMICAL COMPOSITION OF SILICON PRODUCTION FURNACE SLAGS

Metallurgical grade silicon produced in ore-smelting furnaces (OSF) generates a negligible amount of a furnace slag compromising the valuable smelted product quality. The purpose of the study was to analyze the factors of furnace slag formation with its further passing into silicon when released from the ore-smelting furnace, as well as to study the chemical composition of samples. The computer program was developed to forecast the yield of smelting products (crude silicon, dust-gas mixture, furnace slag) depending on the amount and chemical composition of loaded charge material, i.e. siliceous raw ore and carbonaceous reductants. The paper provides the results of studying OSF slag samples from the existing plant by metallographic, X-ray, and X-ray diffraction methods and electron probe microanalysis. The study showed the presence of entrapped silicon prills with intermetallic inclusions (Fe(Ti)Si, CaSi2), silicon carbide SiC, incompletely reduced α-SiO2-cristobalite and Al2O3, and formed complex oxides of impurity elements (CaO·SiO2). It was found that the amount of formed furnace slag and thus the chemical composition of smelted silicon depend on the optimal excess of solid carbon in charge (within 103–111 %) and stable electrical operation of the ore-smelting furnace.

Smoothed particle hydrodynamics simulations of gas and dust mixtures

We present a 'two-fluid' implementation of dust in smoothed particle hydrodynamics (SPH) in the test particle limit. The scheme is able to handle both short and long stopping times and reproduces the short friction time limit, which is not properly handled in other implementations. We apply novel tests to verify its accuracy and limitations, including multi-dimensional tests that have not been previously applied to the drag-coupled dust problem and which are particularly relevant to self-gravitating protoplanetary discs. Our tests demonstrate several key requirements for accurate simulations of gas-dust mixtures. Firstly, in standard SPH particle jitter can degrade the dust solution, even when the gas density is well reproduced. The use of integral gradients, a Wendland kernel and a large number of neighbours can control this, albeit at a greater computational cost. Secondly, when it is necessary to limit the artificial viscosity we recommend using the Cullen & Dehnen (2010) switch, since the alternative, using ${\alpha} \sim 0.1$, can generate a large velocity noise up to ${{\sigma}_v} \lesssim 0.3 c_s$ in the dust particles. Thirdly, we find that an accurate dust density estimate requires $>400$ neighbours, since, unlike the gas, the dust particles do not feel regularization forces. This density noise applies to all particle-based two-fluid implementations of dust, irrespective of the hydro solver and could lead to numerically induced fragmentation. Although our tests show accurate dusty gas simulations are possible, care must be taken to minimize the contribution from numerical noise.

Numerical simulation of an internal flow field in a uniflow cyclone separator

The characteristics of a particle separator were numerically investigated using the concept of a uniflow cyclone. The objective of the current study was to predict the internal flow field and to study the effect of flow streams on particle movement in a uniflow cyclone separator. The motion of solid particles in a flow field was simulated using the Eulerian–Lagrangian approach. Inlet temperature (Tin=300–1100K) and pressure (Pin=1–9bar) were varied for the initial conditions. Dust–gas mixtures (i.e., particle-laden flow) were injected into the separator inlet at uin=3–15m/s. Calculation results showed that the Eulerian–Lagrangian approach was useful for modifying the two-phased viscous turbulence flow. The recirculation zone was predicted under a vortex finder, while a helical flow developed in the carrier gas outlet. Separation efficiency decreased with an increase in dust–gas temperature and pressure and increased with an increase in particle loading, inlet velocity, and particle diameter.

Interstellar medium composition through X-ray spectroscopy of low-mass X-ray binaries

The diffuse interstellar medium (ISM) is an integral part of the evolution of the entire Galaxy. Metals are produced by stars and their abundances are the direct testimony of the history of stellar evolution. However, the interstellar dust composition is not well known and the total abundances are yet to be accurately determined. We probe ISM dust composition, total abundances, and abundance gradients through the study of interstellar absorption features in the high-resolution X-ray spectra of Galactic low-mass X-ray binaries (LMXBs). We use high-quality grating spectra of nine LMXBs taken with XMM-Newton. We measure the column densities of O, Ne, Mg, and Fe with an empirical model and estimate the Galactic abundance gradients. The column densities of the neutral gas species are in agreement with those found in the literature. Solids are a significant reservoir of metals like oxygen and iron. Respectively, 15-25 % and 65-90 % of the total amount of O I and Fe I is found in dust. The dust amount and mixture seem to be consistent along all the lines-of-sight (LOS). Our estimates of abundance gradients and predictions of local interstellar abundances are in agreement with those measured at longer wavelengths. Our work shows that X-ray spectroscopy is a very powerful method to probe the ISM. For instance, on a large scale the ISM appears to be chemically homogeneous showing similar gas ionization ratios and dust mixtures. The agreement between the abundances of the ISM and the stellar objects suggests that the local Galaxy is also chemically homogeneous.