Gas Dynamics Research Articles

Gasification of mixed plastic-biomass pellets in an updraft fixed bed reactor: A simplified dynamic model

In the context of materials recycling, updraft gasifiers are promising small and middle-scale reactors to obtain building blocks and/or energy from waste plastic and biomasses. To this aim, these kinds of materials can also be mixed and reduced into pellets, while the needed heat enters the process with a heated carrier gas. In order to preliminarily design a gasifier to check its feasibility with an available feedstock, currently available models are inadequate. Thermodynamic ones are useless for the purposes of sizing, while too detailed rate-based models (e.g. based on fluid-dynamic modelling) are too substrate specific, need detailed input data and are extremely time consuming. A dynamic model of a fixed-bed reactor for biomass gasification is presented here. The gasifier is loaded continuously from the top with solid pellets and fed with counter-current air flow. The model considers: i) a one-step gasification kinetics, yielding a product spectrum which matches experimental data from the literature; ii) dynamic gas and solid energy balances and iii) steady-state energy balance for the furnace. The model has been applied to describe a tubular furnace (Ø 40 cm) which gasifies 500–1000 kg day−1 of mixed wastes using air heated up to 1200 °C: on the basis of the produced chemicals, the energy consumption was estimated as ca. 2 MJ per kg of solid feedstock. This simplified approach proved robust in describing the overall yields and start-up dynamics, showing higher reliability than equilibrium models in addressing the temperature profiles, at the cost of a simplified reaction kinetic and pellet description with respect to more complex simulation models. The model validation was done by comparison between the calculations results and available pilot-plant data. An overall good fit of the data can be concluded. The solid-gas heat transfer and the bed packing are the main computational criticalities to achieve a reliable process description.

An optimization based limiter for enforcing positivity in a semi-implicit discontinuous Galerkin scheme for compressible Navier–Stokes equations

We consider an optimization-based limiter for enforcing positivity of internal energy in a semi-implicit scheme for solving gas dynamics equations. With Strang splitting, the compressible Navier–Stokes system is split into the compressible Euler equations, which are solved by the positivity-preserving Runge–Kutta discontinuous Galerkin (DG) method, and the parabolic subproblem, which is solved by Crank–Nicolson in time with interior penalty DG method. Such a scheme is at most second order accurate in time, high order accurate in space, conservative, and preserves positivity of density. To further enforce the positivity of internal energy, we impose an optimization-based limiter for the total energy variable to post-process DG polynomial cell averages. The optimization-based limiter can be efficiently implemented by the popular first order convex optimization algorithms such as the Douglas–Rachford splitting method by using nearly optimal algorithm parameters. Numerical tests suggest that the DG method with Qk basis and the optimization-based limiter is robust for demanding low-pressure problems such as high-speed flows.

Dynamic gas emission during coal seam drilling under the thermo-hydro-mechanical coupling effect: A theoretical model and numerical simulations

The potential risk of coal and gas outbursts has increased with the depth of coal mining, posing the threat of heavy casualties, vast economic losses, and harm to the ecological environment. Considering the limitations of existing coal and gas outburst prediction methods, novel indicators or methods are needed. In this study, a fully coupled dynamic drilling gas emission THM model of gas diffusion, seepage, and flow is established, and the reliability of the model was verified by the measured data. The validated model was applied to single-factor and multi-factor impact analysis of dynamic drilling gas emission. The simulation results showed that (1) During the drilling process, the increase in permeability is mainly attributed to the coal seam damage and disturbance caused by the borehole and the coupling effects of the THM model, which increase the porosity around the borehole. Driven by the pressure gradient and temperature gradient, the gas pressure and temperature of the coal seam around the borehole show a conical distribution. (2) In the single-factor analysis, increasing the values of coal seam parameters (initial gas pressure, initial permeability, and initial coal temperature) and drilling parameters (borehole radius) will promote gas emissions in the drilling process to different degrees. The most significant promoting effect is the initial permeability, and the weakest is the initial coal temperature. (3) The response surface analysis results of the four factors show that the interaction between initial coal temperature C and borehole radius D is the least significant, and the interaction between initial gas pressure A and initial permeability B is the most significant. Meanwhile, the established multiple regression model characterizes the relationship between borehole gas emissions and initial coal seam gas pressure, and the idea of using drilling gas emission volume to predict coal and gas outbursts was proposed.

Evolution of Permeability and Sensitivity Analysis of Gas-Bearing Coal under Cyclic Dynamic Loading

It is imperative to conduct experimental studies on the seepage behavior of gas-bearing coal under cyclic dynamic loading conditions. This paper focuses on the evolution of coal permeability under the combined effects of dynamic loading, static loading, and gas adsorption. The principal conclusions are as follows: (1) As the frequency and amplitude of dynamic loading increase, the development of pore and fissure structures within the coal body becomes increasingly pronounced during dynamic loading cycles, resulting in a gradual rise in permeability. Notably, as the coal approaches its yielding stage, the permeability can increase by up to 47%. (2) The permeability curve is divided into four regions: the compaction reduction zone, the oscillation zone, the gradual recovery zone, and the abrupt failure increase zone. Ultimately, in the failure phase, the permeability surges dramatically, potentially reaching four to five times the initial permeability. (3) When the static loading stage and dynamic load are constant, the rate of change in coal permeability decreases with increasing adsorption amounts. When the adsorption amount is constant, the rate of change in permeability of the coal under dynamic loading increases with the increase in the static loading stress stage, with the maximum increase reaching 75.2%. It can be concluded from the rate of change in permeability and the dynamic loading sensitivity coefficient that the permeability of coal is highly sensitive to cyclic dynamic loading, with increased sensitivity associated with larger static loading stages and decreased sensitivity with greater adsorption amounts.

Kinetics and dynamics of Gas-liquid separation and bubble generation in surfactant solutions: Role of bulk/interfacial properties and hydrodynamic conditions

Understanding the kinetics and dynamics of gas–liquid separation and bubble generation in surfactant solutions is important for many industrial applications. To explore the potential mechanisms affecting the physical properties (expansion ratio, bubble size, and foam stability) of foams and bubbles, the surface tension of the solution, including the equilibrium and dynamic properties, was investigated. Then, the morphology of the surfactant aggregates was explored by cryo-transmission electron microscopy (cryo-TEM). Based on these experimental results, the effects of various physical and chemical factors (including the relative concentration of surfactant, dynamic surface tension, surface coverage, surface elasticity, surface mobility, aggregate morphology, etc.) on the expansion ratio and bubble size were analysed to identify which “universal” parameters can explain the phenomenon for all aqueous solutions in the gas–liquid separation process. Research has shown that the morphology of aggregates in a solution largely determines the surface properties of the solution at 1.5 ms (surface tension, surface coverage, surface elasticity, and so on). These surface properties significantly affect the expansion ratio. However, no good correlation was found between bubble size and these surface properties because surfactant vesicles can directly affect bubble size. In addition, the liquid flow rate and gas–liquid ratio have a significant impact on the expansion ratio and bubble size. Ultimately, we found that the foam stability, bubble size, and expansion ration can be described by a simple linear relationship. Our research provides new opinions for further understanding the effects of bulk/interfacial properties and hydrodynamic conditions on the physical properties of bubbles in the gas–liquid separation process.

Theory and Observation of Winds from Star-Forming Galaxies

Galactic winds shape the stellar, gas, and metal content of galaxies. To quantify their impact, we must understand their physics. We review potential wind-driving mechanisms and observed wind properties, with a focus on the warm ionized and hot X-ray-emitting gas. Energy and momentum injection by supernovae (SNe), cosmic rays, radiation pressure, and magnetic fields are considered in the light of observations: ▪Emission and absorption line measurements of cool/warm gas provide our best physical diagnostics of galactic outflows.▪The critical unsolved problem is how to accelerate cool gas to the high velocities observed. Although conclusive evidence for no one mechanism exists, the momentum, energy, and mass-loading budgets observed compare well with theory.▪A model in which star formation provides a force ∼L/c, where L is the bolometric luminosity, and cool gas is pushed out of the galaxy's gravitational potential, compares well with available data. The wind power is ∼0.1 of that provided by SNe.▪The very hot X-ray-emitting phase may be a (or the) prime mover. Momentum and energy exchange between the hot and cooler phases is critical to the gas dynamics.▪Gaps in our observational knowledge include the hot gas kinematics and the size and structure of the outflows probed with UV absorption lines. Simulations are needed to more fully understand mixing, cloud–radiation, cloud–cosmic ray, andcloud–hot wind interactions, the collective effects of star clusters, and both distributed andclustered SNe. Observational works should seek secondary correlations in the wind data thatprovide evidence for specific mechanisms and compare spectroscopy with the column density–velocity results from theory.

Dynamic grid emission factors and export limits reduce emission abatement and cost benefits of building PV systems

Photovoltaic (PV) installations in the building sector are expected to play a crucial role in Switzerland's efforts to achieve decarbonization targets. Nevertheless, most analyses on PV system design prioritize economic factors, and overlook the impact of angle-dependent PV generation, dynamic grid greenhouse gas (GHG) emissions, and export limitations on their emission abatement potential during operation. Our study sheds light on the cost- and emission-optimal orientation and sizing of PV systems in buildings, factoring in hourly grid GHG emission intensity and curtailment measures. We employ mixed-integer linear programming (MILP) and illustrate the methodology for a case study building under different scenarios, aiming to minimize costs and emissions by considering a combination of rooftop and façade PV systems, and a battery. We show that assuming a constant annual grid GHG emission intensity may overestimate annual emission abatement potential of PV generation by a factor of two, emphasizing the need for dynamic grid GHG emission intensity. When considered, cost-optimal solutions favour larger systems that maximize total production. In contrast, emission-optimal solutions increase self-consumption via inclusion of battery systems. Curtailment due to export limitations reduces the cost and emission benefits of excess generation (from feed-in tariff payments and carbon credits), prompting a trend towards smaller PV installations with steeper panel tilts. Thus, the emission abatement potential of PV generation heavily depends on the option to export excess electricity production.

Multiscale simulation of rarefied gas dynamics via direct intermittent GSIS-DSMC coupling

The general synthetic iterative scheme (GSIS) has proven its efficacy in modeling rarefied gas dynamics, where the steady-state solutions are obtained after dozens of iterations of the Boltzmann equation, with minimal numerical dissipation even using large spatial cells. In this paper, the fast convergence and asymptotic-preserving properties of the GSIS are harnessed to remove the limitations of the direct simulation Monte Carlo (DSMC) method. The GSIS, which leverages high-order constitutive relations derived from DSMC, is applied intermittently, which not only rapidly steers the DSMC towards steady state, but also eliminates the requirement that the cell size must be smaller than the molecular mean free path. Several numerical tests have been conducted to validate the accuracy and efficiency of this hybrid GSIS-DSMC approach.

Research on multi-parameter precise prediction of borehole gas extraction under negative pressure drive

Accurate prediction of multi-parameter gas extraction in mines is vital for intelligent gas extraction regulation. This study establishes a linear and nonlinear multi-parameter prediction model to address excessive prediction residuals resulting from parameter interactions. Initially, a combined linear-nonlinear model is constructed using ARIMA and RBFNN. Real-time multi-parameter data under varying negative pressures is collected via a custom experimental platform. After preprocessing and inspection, the linear models is obtained: ARIMA (0,1,1) for gas concentration, ARIMA (0,1,1) for flow rate, ARIMA (2,1,2) for air leakage volume. Finally, the prediction residuals were optimised using RBFNN to obtain the combined model predictions. Results demonstrate closer proximity of combined model predictions to actual values, with gas extraction multi-parameter R2 surpassing 0.7. Additionally, negligible values of MAE、MSE and MAPE attest to the model’s robust performance, laying theoretical groundwork for dynamic gas extraction regulation and control.

The glow of axion quark nugget dark matter. Part I. Large scale structures

Axion quark nuggets (AQN) are hypothetical, macroscopically large objects with a mass greater than a few grams and sub-micrometer size, formed during the quark-hadrontransition. Originating from the axion field, they offer a possible resolution of the similarity between visible and dark components of the Universe, i.e. ΩDM ∼ Ωvisible and observed matter-antimatter asymmetry. These composite objects behave as cold dark matter, interacting with ordinary matter and resulting in pervasive electromagnetic radiation throughout the Universe. This work aims to predict the electromagnetic signature in large-scale structures from this AQN-baryon interaction, accounting for thermal and non-thermal radiations.We use Magneticum hydrodynamical simulations to describe the realistic distribution and dynamics of gas and dark matter at cosmological scales. We construct a light cone encompassing a 1.4 square degree area on the sky, extending up to redshift z = 5.4, and we calculate the electromagnetic signature across a wide range of frequencies from radio, starting at ν ∼ 1 GHz, up to a few keV X-ray energies. We find that the AQNs electromagnetic signature is characterized by global (monopole) and fluctuation signals. The amplitude of both signals strongly depends on the average nugget mass and the ionization level of the baryonic environment, allowing us to identify a most optimistic scenario and a minimal configuration. The signal of our most optimistic scenario is often near the sensitivity limit of existing instruments, such as FIRAS in the ν = [100-500] GHz range and the South Pole Telescope for high-resolution ℓ > 4000 at ν = 95 GHz. Fluctuations in the Extra-galactic Background Light caused by the axion quark nuggets in the most optimistic scenario can also be tested with space-based imagers Euclid and James Webb Space Telescope. In general, our minimal configuration is still out of reach of existing instruments, but future experiments might be able to pose some constraints.We conclude that the axion quark nuggets model represents a viable model for dark matter, which does not violate the canons of cosmology nor existing observations. A reanalysis of existing data sets could provide some evidence of axion quark nuggets if our most optimistic configuration is correct. The best chances for testing the model reside in 1) ultra-deep infrared and optical surveys, 2) future experiments to probe the frequency spectrum of the cosmic microwave background, and 3) low-frequency (1 GHz < ν < 100 GHz) and high-resolution (ℓ ≳ 104) observations.

Innovative Metal Powder Production Using CFD with Convergent-Divergent Nozzles in Wire Arc Atomization

This study aims to enhance the production of metal powders using a novel approach that integrates computational fluid dynamics (CFD) with convergent-divergent (C-D) nozzles in wire arc spraying atomization (WASA). The primary objective is to investigate the influence of nozzle design on particle size distribution and production efficiency. Utilizing the ANSYS CFD Fluent program, simulations were conducted to analyze the effects of various parameters, including throat diameter and divergent angles, on gas dynamics and metal droplet behavior. The findings reveal that C-D nozzles facilitate the acceleration of gas flow to supersonic speeds, significantly improving the shear force acting on the molten metal, thereby promoting the fragmentation of droplets into smaller particles. Notably, the optimized nozzle configuration achieved a median particle size (D50) of 44.42 µm, suitable for additive manufacturing applications. The novelty of this work lies in its comprehensive simulation framework that allows for rapid virtual testing, potentially leading to significant improvements in the efficiency and quality of metal powder production processes. This research addresses critical gaps in the existing literature and provides a robust foundation for future studies in the field of metal powder manufacturing. Doi: 10.28991/HIJ-2024-05-03-02 Full Text: PDF

Suppressed Cosmic-Ray Energy Densities in Molecular Clouds from Streaming Instability-regulated Transport

Cosmic rays (CRs) are the primary driver of ionization in star-forming molecular clouds (MCs). Despite their potential impacts on gas dynamics and chemistry, no simulations of star cluster formation following the creation of individual stars have included explicit cosmic-ray transport (CRT) to date. We conduct the first numerical simulations following the collapse of a 2000M ⊙ MC and the subsequent star formation including CRT using the STAR FORmation in Gaseous Environments framework implemented in the GIZMO code. We show that when CRT is streaming-dominated, the CR energy in the cloud is strongly attenuated due to energy losses from the streaming instability. Consequently, in a Milky Way–like environment the median CR ionization rate in the cloud is low (ζ ≲ 2 × 10−19 s−1) during the main star-forming epoch of the calculation and the impact of CRs on the star formation in the cloud is limited. However, in high-CR environments, the CR distribution in the cloud is elevated (ζ ≲ 6 × 10−18), and the relatively higher CR pressure outside the cloud causes slightly earlier cloud collapse and increases the star formation efficiency by 50% to ∼13%. The initial mass function is similar in all cases except with possible variations in a high-CR environment. Further studies are needed to explain the range of ionization rates observed in MCs and explore star formation in extreme CR environments.

Partially invariant solution with an arbitrary surface of blow-up for the gas dynamics equations admitting pressure translation

We applied a method of symmetry reduction to the gas dynamics equations with a special form of the equation of state. This equation of state is a pressure represented as the sum of a density and an entropy functions. The symmetry Lie algebra of the system is 12-dimensional. One, two and three-dimensional subalgebras were considered. In this article, four-dimensional subalgebras are considered for the first time. Specifically, invariants are calculated for 50 four-dimensional subalgebras. Using invariants of one of the subalgebras, a symmetry reduction of the original system is calculated. The reduced system is a partially invariant submodel because one gas-dynamic function cannot be expressed in terms of the invariants. The submodel leads to two families of exact solutions, one of which describes the isochoric motion of the media, and the other solution specifies an arbitrary blow-up surface. For the first family of solutions, the particle trajectories are parabolas or rays; for the second family of solutions, the particles move along cubic parabolas or straight lines. From each point of the blow-up surface, particles fly out at different speeds and end up on a straight line at any other fixed moment in time. A description of the motion of particles for each family of solutions is given.

Macroscopic modeling of gas permeability in hierarchical micro/nanoporous media: A unified characterization of rarefaction using Klinkenberg theory and equivalent diameter

Estimating gas transport through a hierarchical micro/nanoporous system is challenging due to non-equilibrium gas dynamics. The primary difficulty lies in determining the rarefaction level, because identifying a representative flow dimension in a complex porous system with multiple pore scales is not straightforward. Our study performed a pore-level analysis for gas permeability in dual-scale porous media with varying porosity, throat size, and secondary pore size under different rarefaction conditions. We found that secondary porosity negatively affects permeability due to increased friction forces, with this influence growing as the secondary pore size and porosity increase until the secondary pore becomes comparable to the throat. However, rarefaction reduces the effects of secondary pores due to boundary slip. Traditional Knudsen number (Kn) calculations based on Darcy-defined height failed to accurately describe the rarefaction effects on gas permeability. Instead, we introduced an equivalent diameter to calculate the Kn, which provided an accurate normalization of apparent gas permeability independent of pore geometry. The extended Kozeny–Carman–Klinkenberg model developed in our previous study successfully yielded a macroscopic model for apparent gas permeability in hierarchical micro/nanoporous systems as a function of the traditional Darcy height and porosity.

Modeling the influence of the duration of water vapor pulses on the properties of hafnium oxide synthesized by atomic layer deposition method

The work experimentally and theoretically analyzes the process of atomic layer deposition of hafnium oxide with the participation of water vapor as an oxidizing agent. The study of infrared absorption, Auger spectroscopy and luminescence shows that with increasing water pulse duration, the concentration of oxygen vacancies decreases and the composition tends to stoichiometric, which is given by the formula HfO2. Modeling of gas dynamics and kinetics of film synthesis by atomic layer deposition was performed, which showed the dependence of the coverage of the film surface with oxygen on the pulse duration and deposition temperature. A comparison of calculations with experimental results made it possible to estimate the magnitude of the kinetic coefficients of the synthesis processes that describe the observed experimental dependences and to show that the larger of the two temperatures of the atomic layer deposition window is associated with water desorption.

Magnetically Driven Turbulence in the Inner Regions of Protoplanetary Disks

Given the important role turbulence plays in the settling and growth of dust grains in protoplanetary disks, it is crucial that we determine whether these disks are turbulent and to what extent. Protoplanetary disks are weakly ionized near the midplane, which has led to a paradigm in which largely laminar magnetic field structures prevail deeper in the disk, with angular momentum being transported via magnetically launched winds. Yet, there has been little exploration of the precise behavior of the gas within the bulk of the disk. We carry out 3D, local shearing box simulations that include all three low-ionization effects (ohmic diffusion, ambipolar diffusion, and the Hall effect) to probe the nature of magnetically driven gas dynamics 1–30 au from the central star. We find that gas turbulence can persist with a generous yet physically motivated ionization prescription (order unity Elsässer numbers). The gas velocity fluctuations range from 0.03 to 0.09 of the sound speed c s at the disk midplane to ∼c s near the disk surface, and are dependent on the initial magnetic field strength. However, the turbulent velocities do not appear to be strongly dependent on the field polarity, and thus appear to be insensitive to the Hall effect. The midplane turbulence has the potential to drive dust grains to collision velocities exceeding their fragmentation limit, and likely reduces the efficacy of particle clumping in the midplane, though it remains to be seen if this level of turbulence persists in disks with lower ionization levels.

Exhaust Gas Flow Study of Electric Turbo Compounding (ETC) to Determine the Potential Electrical Energy Recovery from Exhaust Emission

Electric turbo compounding (ETC) emerges as a pivotal energy recovery solution, seamlessly combining an advanced turbocharger with a high-speed generator to harvest electrical energy from exhaust gas dynamics. This research delves into the practical application of ETC in vehicles, employing a comprehensive approach blending simulation techniques with experimental validation. The investigative process involves meticulous data collection on vehicle muffler dimensions, subsequent device modelling based on this data, and rigorous flow simulations, followed by thorough analysis. The study's key findings highlight a noteworthy 9% increase in engine back pressure at 850 rpm, counterbalanced by a 7% reduction at 2000 rpm. Despite the integration of the ETC mechanism, electric current measurements remain consistently within the range of 1.4 to 1.6 amperes at 1200-2000 rpm. This research not only unveils the tangible effects of ETC on engine performance but also underscores its viability as an efficient energy recovery solution in the automotive sector, setting the stage for advancements in sustainable transportation.

Evolution and interaction of δ-waves in the zero-pressure gas dynamics system

Evolution and interaction of δ-waves in the zero-pressure gas dynamics system

A note on higher-order and nonlinear limiting approaches for continuously bounds-preserving discontinuous Galerkin methods

In Dzanic (2024) [12], a limiting approach for high-order discontinuous Galerkin schemes was introduced which allowed for imposing constraints on the solution continuously (i.e., everywhere within the element). While exact for linear constraint functionals, this approach only imposed a sufficient (but not the minimum necessary) amount of limiting for nonlinear constraint functionals. This short note shows how this limiting approach can be extended to allow exactness for general nonlinear quasiconcave constraint functionals through a nonlinear limiting procedure, reducing unnecessary numerical dissipation. Some examples are shown for nonlinear pressure and entropy constraints in the compressible gas dynamics equations, where both analytic and iterative approaches are used.

Research on the transient dynamic characteristics of the low-density polyethylene compressors shaft system with operating pressure exceeding 180 MPa

The safe and stable function of hyper-compressors with operating pressures exceeding 180 MPa is a guarantee for the low-density polyethylene production process, and the transient dynamics of the compressor shaft system under stable operating conditions are an important concern for the design, maintenance, and fault detection of such compressors. Therefore, this paper presents a finite-element model of the hyper-compressor shaft system and analyzes the dynamic stress and fatigue life of the crankshaft, the connecting rod, the crosshead, and the plunger. In order to verify the accuracy of the model, all stages of the compressor's plunger stress and the torque of the crankshaft–motor connection end were tested in the field. The dynamic gas pressure of each stage cylinder was obtained by the tested stress of the corresponding plunger, then it was set as the load input of the finite-element model. The results show that: the average errors of the simulated plunger stress at two stages are 0.14% and 0.21%, respectively; the mean, peak, valley, and range errors of the torque at the crankshaft–motor connection end are 3.80%, 12.37%, and 3.49%, respectively; the simulated first-order torsional natural frequency of the shaft system is 78 Hz with an error of 8.3%. In the occurrence of the first-order torsional resonance, the maximum torsional stress appears at the crankshaft–motor connection end. The maximum dynamic stress alternating amplitude of the hyper-compressor shaft system under stable operating conditions is 103.7 MPa with a minimum life of 3.309 × 109, which occurs at the crankshaft–motor connection end and is converted into 31.48 years. The finite-element model, test, and simulation data presented in this paper can provide a reference for the fault detection and optimization design of hyper-compressors.