Stoichiometric Methane-oxygen Research Articles

Gasification of Liquid Hydrocarbon Waste by the Ultra-Superheated Mixture of Steam and Carbon Dioxide: A Thermodynamic Study

The thermodynamic modeling of waste oil (WO) gasification by a high-temperature gasification agent (GA) composed of an ultra-superheated H2O/CO2 mixture is carried out. The GA is assumed to be obtained by the gaseous detonation of fuel–oxidizer–diluent mixture in a pulsed detonation gun (PDG). N-hexadecane is used as a WO surrogate. Methane or the produced syngas (generally a mixture of H2, CO, CH4, CO2, etc.) is used as fuel for the PDG. Oxygen, air, or oxygen-enriched air are used as oxidizers for the PDG. Low-temperature steam is used as a diluent gas. The gasification process is assumed to proceed in a flow-through gasifier at atmospheric pressure. It is shown that the use of the detonation products of the stoichiometric methane–oxygen and methane–air mixtures theoretically leads to the complete conversion of WO into a syngas consisting exclusively of H2 and CO, or into energy gas with high contents of CH4 and C2-C3 hydrocarbons and an LHV of 36.7 (fuel–oxygen mixture) and 13.6 MJ/kg (fuel–air mixture). The use of the detonation products of the stoichiometric mixture of the produced syngas with oxygen or with oxygen-enriched air also allows theoretically achieving the complete conversion of WO into syngas consisting exclusively of H2 and CO. About 33% of the produced syngas mixed with oxygen can be theoretically used for PDG self-feeding, thus making the gasification technology very attractive and cost-effective. To self-feed the PDG with the mixture of the produced syngas with air, it is necessary to increase the backpressure in the gasifier and/or enrich the air with oxygen. The addition of low-temperature steam to the fuel–oxygen mixture in the PDG allows controlling the H2/CO ratio in the produced syngas from 1.3 to 3.4.

On the role of transverse detonation waves in the re-establishment of attenuated detonations in methane–oxygen

This investigation concerns the problem of detonation attenuation in stoichiometric methane-oxygen and its re-establishment following its interaction with obstacles, using high resolution numerical simulation. The main focus was on the role of the transverse detonation on the re-establishment of the detonation wave, and the importance of applying a numerical combustion model that responds appropriately to the thermodynamic state behind the complex shock wave dynamics. We applied an efficient thermochemically derived four-step global combustion model using an Euler simulation framework to investigate the critical regimes present. While past attempts at using one- or two- step models have failed to capture transverse detonations, for this scenario, our simulations have demonstrated that the four-step combustion model is able to capture this feature. We thus suggest that to correctly model detonation re-initiation in characteristically unstable mixtures, an applied combustion model should contain at least an adequate description to permit the correct ignition and state variable response when changes in temperature and pressure occur, i.e., behind shocks. Our simulations reveal that (1) there is a relationship between the critical outcomes possible and the mixture cell size, and (2) while pockets of unburned gas may exist when a detonation re-initiates, it is not the direct rapid consumption of these pockets that gives rise to transverse detonations. Instead, the transverse detonations are initiated through pressure amplification of reaction zones at burned/unburned gas interfaces whose combustion rates have been enhanced through Richtmyer–Meshkov instabilities associated with the passing of transverse shock waves, or by spontaneous ignition of hot spots, which can form into detonations through the Zeldovich gradient mechanism. In both situations, non-uniform ignition delay times are found to play a role. Finally, we found that the transverse detonations are in fact Chapman–Jouguet detonations, but whose presence contributes to overdriving the re-initiated detonation along the Mach stem.

Mechanism and characteristics of thrust generated by a submerged detonation tube for underwater propulsion

The detonation engine, which can produce high specific impulse during the underwater detonation process (UDP), has become the forefront of underwater propulsion. In this paper, the thrust mechanism conducted in UDP and the propagation characteristics of the complex pressure waves are numerically studied, and the correlation between those two features is analyzed. The thrust from UDP is generated in a submerged detonation tube (SDT) and driven by the stoichiometric methane-oxygen mixture. The results show that detonation of the pre-filled combustible gas mixture gives rise to complex pressure waves and delivers several force impulses to the SDT. The impulses present different effects on the thrust performance, which is divided into two stages. In the first stage, before the detonation wave collides with the exterior water, the thrust is provided by the persistent back pressure effect of the detonation product. When the detonation wave propagates through the SDT exit and strikes the gas–water interface, a transmitted shock wave and a reflected shock wave are formed, which produce the impulses dominating the second stage. The reflected shock wave eventually impinges on the inner wall, imposing a force impulse on it. The pressure disturbance on the annular wall caused by the transmitted shock wave and subsequent detonation gas jet leads to another two thrust impulses. Finally, a comparison between the thrust of the SDT and its counterpart in the air is conducted to characterize the influence of UDP, and the effects of dimensional parameters of the SDT are also investigated.

Interactions Between Flame Topology and Turbulent Transport in High-Pressure Premixed Combustion

Direct numerical simulations of a turbulent premixed stoichiometric methane-oxygen flame were conducted. The chosen combustion pressure was 20 bar, to resemble conditions encountered in modern rocket combustors. The chemical reactions followed finite rate detailed mechanism integrated into the EBI-DNS solver within the OpenFOAM framework. Flame geometry was thoroughly investigated to assess its interaction with the transport of turbulent properties. The resulting flame front was remarkably thin, with high density gradients and moderate Karlovitz and Damköhler numbers. At mid-flame positions, the variable-density transport mechanisms dominated, leading to the generation of both vorticity and turbulence. A reversion of this trend towards the products was observed. For intermediate combustion progress, vorticity transport is essentially a competition between the baroclinic torque and vortex dilatation. The growth of turbulent kinetic energy is strongly correlated to this process. A geometrical analysis reveals that the generation of enstrophy and turbulence is restricted to specific topologies. Convergent and divergent flame propagation promote turbulence creation due to pressure fluctuation gradients through different physical processes. The possibility of modeling turbulence transport based on curvature is discussed along with the inherent challenges.

Ignition behavior and the onset of quasi-detonation in methane-oxygen using different end wall reflectors

The shock wave focusing technique is beneficial to the development of detonation-based engines, which can shorten the long run-up distance that is usually required for a detonation, reducing the weight of the detonation engine. In this work, we performed experiments using different end wall reflectors to facilitate the formation and intensity of the combustion wave in a stoichiometric methane-oxygen mixture diluted by argon. By changing shock wave intensity, the effects of the incident shock Mach number (Ma) on the ignition delay times in two reflectors are systematically investigated. The experimental results show that the conical reflector creates an abrupt pressure rise in the apex, resulting in a 64.5% rise in the acceleration of the reflected shock velocity compared with the planar reflector. The introduction of a conical reflector significantly shortens the ignition delay time by an order of magnitude and facilitates the amalgamation of the leading reflected shock wave and the subsequent combustion wave. The experimental results also illustrate that conical reflectors promote the formation of the second ignition. However, only a weak deflagration wave is formed in a planar reflector, and a strong deflagration wave and quasi-detonation wave are observed by using a conical reflector. Our observation sheds light on the mechanism of ignition modes induced by the shape of reflectors.

Experimental study on the propagation characteristics of detonation wave in annular channels with convergent cross-sections

Methane (CH4) is an environmental-friendly fuel with a wide range of applications. The explosion and detonation hazards with respect to methane in tubes with different shapes should be paid in special attention. In this study, experiments were performed to investigate the propagation characteristics of detonation waves in annular channels with convergent cross-sections in stoichiometric methane-oxygen mixture at sub-atmospherical pressures (10 kPa-40 kPa). The annular channels with convergent cross-sections were assembled by inserting 0.5-m long cones into a 48-mm circular tube. Three conicities (dend/L) were employed, i.e., dend/L = 0.048, 0.072, 0.096, in which dend and L represent the bottom diameter and the length of the cone, respectively. The detonation velocity was obtained based on the time of arrival of the wave front. Smoked foils were used to register the detonation cellular evolution in the annular channels. The results show that the convergence of the channel has a great impact on the detonation velocity and cell sizes. Depending on the conicity, the detonation velocity and cellular structure exhibit different behaviors. For the smallest conicity, the continuous velocity deficit can be observed after detonation propagating into the annular tube. However, the characteristic of detonation velocity evolution shows distinct with the increase of the conicity, which is governed by the chemical scale (the detonation cell size) as well as the physical geometry (the channel width). The interaction between Mach reflection, boundary layer effect and the wall can be served as the dominating reason for the velocity behavior.

Detonation Re-initiation behind an Aluminum Foam Plate in Stoichiometric Methane-oxygen Mixture

ABSTRACT Porous materials can be used for accidental explosion prevention and suppression in process industries. In this study, an experimental study was conducted to study the effect of an aluminum foam plate on stoichiometric methane-oxygen detonation propagation behavior at different initial pressures (i.e., 10 kPa to 50 kPa). Experiments were carried out in a 4.4-m long, 48-mm diameter tube. Downstream of the foam plate, the detonation velocity was measured using even-spaced photodiodes, and the detonation structure evolution was recorded using the soot foil technique. The density of the aluminum foam was 1.1 g/cm3 and the thicknesses were 10 mm and 15 mm. Results demonstrate that behind the foam plate, the detonation fails first and re-initiates after a certain distance (re-initiation distance). At the end of the tube, the detonation velocity can be steady although the velocity fluctuation is significant downstream of the plate. The soot pattern indicates that the onset of detonation is achieved via the interaction between shock waves and wall, and fast turbulence mixing. For the case of the 10-mm foam plate, the single-headed mode can be observed. Generally, the re-initiation distance increases with the cell size. When the single-headed spin appears, the re-initiation distance is decreased notably.

Effects of inert gas jet on the transition from deflagration to detonation in a stoichiometric methane-oxygen mixture

Detonation is an energetic combustion mode augmenting high flow momentum and thermodynamic efficiency, it has been applied in detonation engines, such as pulse detonation engines (PDEs) and rotating detonation engines (RDEs), they have become potential aerospace propulsion equipment. Recently, fluidic jet-in-cross flow (JICF) has been demonstrated experimentally and numerically that can accelerate the deflagration-to-detonation transition (DDT) process. Nonetheless, most of previous studies focused on the jets using combustible mixture or oxygen, which may bring additional risk for turbulence-generated system in detonation engines. In this study, a more safe and controllable inert gas (i.e., Ar) is applied for JICF, experiments are carried out to investigate effects of argon jet as an enhancement method on promoting the DDT in a stoichiometric methane-oxygen mixture. The effects of local argon concentration, turbulence intensity and injection position on the DDT process are systematically examined. Two-dimensional numerical simulations are also performed to elucidate the details of the injection evolution. The experimental results show that turbulence generated by the argon injection can promote flame acceleration and the onset of detonation only in the fast deflagration regime. The enhancing effect is more prominent at higher turbulence intensity by increasing jet injection pressure and shorter injection time. Too long injection duration increases argon local concentration that leads to an adverse effect prohibiting the DDT occurrence. During the initial laminar flame acceleration, referred to as the slow deflagration regime, no enhancement by the argon jet on DDT can be observed. By looking numerically at the flow structure of the argon jet, the vortical features enhance the transport and mixing between reactants and products. The interaction between the reactive travelling wave and the jet structure further induces turbulence and thus accelerates the chemical reaction rate. With the time elapsed, the injected argon entrains largely and dilutes the ambient combustible mixture, and restrains the DDT. Furthermore, a novel dimensionless criterion and a characteristic parameter Turc are proposed, quantitatively analyzing the dominate mechanism in flame propagation and the initial stage of DDT as inert jet is introduced.

Experimental study on the effects of different fluidic jets on the acceleration of deflagration prior its transition to detonation

Fluidic jet-in-crossflow (JICF) is a reliable technique facilitating the flame acceleration and the transition from deflagration to detonation, most previous investigations on this project have focused on JICF using combustible mixtures but there are relatively few studies explored the mechanism of non-reactive gas JICF in the process of deflagration-to-detonation transition (DDT). In this study, three non-reactive gases (i.e., He, N2 and CO2) are considered as the medium of fluidic jets, the detailed mechanism of JICF enhancing deflagrations in stoichiometric methane-oxygen mixture is discussed. We experimentally demonstrate that JICF prominently enhances the propagation characteristics of deflagration, i.e., the 1st shock wave (1st SW), the 2nd shock wave (2nd SW), the combustion wave (CW) and the formation of precursor shock wave. Non-reactive gas JICF has concurrently positive turbulence-enhancement as well as negative inert gas-inhibition effects, which simultaneously influence the propagation behavior of deflagration. By adjusting the pressure difference (i.e., pJ0/p0) in JICF and test mixture, the performance of non-reactive gas jet on the propagation characteristics of deflagration is varied and mainly dependent on the attributes of the JICF. Our observation sheds light on the competing mechanism of JICF in the acceleration of a deflagration.

The precursor shock wave and flame propagation enhancement by CO2 injection in a methane-oxygen mixture

Successful initiation of a detonation in a confined combustor with limited length is a formidable task for designing the detonation-based engines, shortening the length for the run-up distance of a detonation greatly reduces the weight of the detonation engine, rendering the detonation propulsion become possible in the advanced aerospace propulsion systems. In this work, we proposed a new method that using a non-reactive gas jet (CO2) to facilitate the formation of the precursor shock wave and the flame propagation speed in a deflagration wave in a stoichiometric methane-oxygen mixture, by changing a series of the injection parameters, i.e., the initial pressure of mixture, the jet delay time, the jet pressure and its position, the effects of those parameters on the deflagration propagation are systematically investigated. The experimental results show that, the development processes of pressure rise are divided into two stages prior to the DDT: 1) slow deflagration stage, which characterized by two discrete shock waves named as the 1st shock wave and 2nd shock wave; 2) Fast deflagration stage, the two discrete waves merge into a strong precursor shock wave, and the velocities of precursor shock and flame achieve to the “quasi-detonation” state. The experimental results also illustrate that, introducing the CO2 jet in the first stage prior to the DDT promotes the formation of precursor shock wave and accelerates the flame speed, which speeds up the transformation of a detonation in a shorter distance. By comparing different jet parameter combinations, it is found that high jet pressure and long jet time increase the local concentration of CO2, which adversely affect the formation of a detonation. Thereby there exists an optimal condition of injection, whose injection pressure ratio is 15, jet delay time is 20 ms and jet position is found to be 0.6 m, those parameters are verified to have prominent detonation-enhancement effects on DDT, the underlying mechanisms for those optimal conditions are explored and explained in this study. This study provided new experimental evidences supporting the theory of turbulence-enhancement effect on the initiation of a detonation.

Role of instability on the limits of laterally strained detonation waves

The present work examines the role of instability and diffusive phenomena in controlling the limits of detonations subject to lateral strain. Experiments were conducted in mixtures with varying levels of cellular instability, i.e., stoichiometric methane–oxygen (CH4/2O2), ethylene–oxygen (C2H4/3O2), and ethane–oxygen (C2H6/3.5O2). These detonations were propagated in channels with exponentially enlarging cross-sections, following the recent works of Radulescu and Borzou (2018) and Xiao and Radulescu (2020). Steady detonation waves were obtained at the macro-scale, with the near-limit reaction zone structures characterized by significant unreacted gas pockets. The turbulent flame burning velocity of these pockets was evaluated to be 30 m/s to 70 m/s, which is larger than the theoretical laminar value by a factor of 2 to 7 and smaller than the CJ deflagration speed by a factor of 2 to 3. For all the mixtures tested, the characteristic D−K relationships, relating the detonation mean propagation speed with lateral flow divergence, were obtained directly from experiments and as well from the generalized ZND model with lateral strain rates using detailed chemical kinetics. The results showed that the degree of departure between experiments and the theoretical predictions increases significantly with the detonation instability level. As compared to the laminar ZND wave, the more unstable detonations are much more detonable than the more stable detonations, with substantially larger limiting divergence rates and maximum velocity deficits. Such enhanced detonability with detonation instability can be manifested in the significantly enhanced global rates of energy release with the notably suppressed thermal character of ignition for the more unstable detonations. This globally enhanced burning mechanism is found to be realized by the intensified auto-ignition assisted by the turbulent diffusive burning of the unreacted gas pockets, substantially shortening the characteristic reaction zone lengths. Finally, the relatively good universality of the D/DCJ−Keffλ relations implicitly suggests that cell size is also a function of the detonation wave stability, since it is controlled by the reaction zone thickness of unstable detonations.

On the detonation behavior of methane-oxygen in a round tube filled with orifice plates

Abstract In this study, the detonation behaviors of stoichiometric methane-oxygen mixture were examined in a round tube filled with orifice plates. The blockage ratio was 0.56 and the plate spacings were 1, 1.5 and 2 times the tube diameter. Detonation velocity measurement and soot foils were adopted to study the propagation characteristics. Experimental results show that the abrupt velocity jump was only observed for the obstacle configuration with S = 2 D , where S and D are the plate spacing and the tube diameter. For a fixed plate spacing, the detonation velocity as well as the detonation limits are independent of the orifice geometry, since the effective diameters ( d e f f ) of the orifices are almost the same. The ratios of d e f f to the detonation cell size ( λ ) were found to be 0.4–0.5. For sensitive mixtures, the soot foils indicate that the detonation re-initiation occurs via hot spots formed on the wall. As the initial pressure decreases, the detonation can travel without hot spots generation. At the limits, no indication of detonation can be observed for obstacle configurations with smaller spacings in spite of the higher velocity (compared with the isobaric sound speed of the products). The propagation mechanism near the limits is left, however, for further investigation.

Promotion of methane ignition by the laser heating of suspended nanoparticles

The influence of laser heated iron and carbon nanoparticles on ignition of 20 vol% stoichiometric methane–oxygen mixture in argon was studied experimentally in shock tube reactor. The concentration of nanoparticles 0.3–2.0 ppm was measured by laser light extinction. The particles were heated by Nd:Yag laser pulse operated at wavelength 1064 nm. The ignition delay times were registered by increase of OH chemiluminescence and pressure rise. The temperatures of laser heated particles and their sizes were measured by laser induced incandescence technique. The significant decrease of ignition delay times were found at addition of iron particles heated by laser pulse to the combustible mixture at the temperatures less than 1400 K. Analysis performed has shown that the effect supposedly involves catalytic reactions of methane decomposition on the surface of heated particles and allowed estimating their effective activation energy.

Methane and hydrogen ignition with ethanol and butanol admixtures

This work is devoted to the investigation of combustion of simple and complex gaseous fuels: methane and hydrogen with admixtures of the most promising alcohols: ethanol and butanol. The process of ignition of investigated blends behind reflected shock waves in the temperature range of 1000–1600 K and pressure range of 4.5–6 bar was studied. The temperature dependences of ignition delay times for stoichiometric methane–oxygen–ethanol (or butanol) and hydrogen–oxygen–ethanol (or butanol) mixtures diluted in argon were obtained. The possible kinetic description is discussed.

Reduced detonation kinetics and detonation structure in one- and multi-fuel gaseous mixtures

Two-step approximate models of chemical kinetics of detonation combustion of (i) one-fuel (CH4/air) and (ii) multi-fuel gaseous mixtures (CH4/H2/air and CH4/CO/air) are developed for the first time. The models for multi-fuel mixtures are proposed for the first time. Owing to the simplicity and high accuracy, the models can be used in multi-dimensional numerical calculations of detonation waves in corresponding gaseous mixtures. The models are in consistent with the second law of thermodynamics and Le Chatelier’s principle. Constants of the models have a clear physical meaning. Advantages of the kinetic model for detonation combustion of methane has been demonstrated via numerical calculations of a two-dimensional structure of the detonation wave in a stoichiometric and fuel-rich methane-air mixtures and stoichiometric methane-oxygen mixture. The dominant size of the detonation cell, determines in calculations, is in good agreement with all known experimental data.

The effects of large scale perturbation-generating obstacles on the propagation of detonation filled with methane–oxygen mixture

Orifice plates with large blockage ratios (BR=0.691, 0.826, 0.923 and 0.962) are placed in the entrance of the detonation transmission to investigate the artificially large-scale perturbations on the detonation propagation mechanism. Obstacles create large perturbations, thereby generating flow instability to affect the detonation transmission. In this study, stoichiometric methane–oxygen is examined because it has a wide application in the industry; therefore, its safety concerns must be addressed. Furthermore, the methane–oxygen mixture is characterized by a highly irregular cellular detonation front, and it is the representative of those potentially combustible mixtures used in practical aerospace application. Velocity behaviors after the detonation passes through the obstacles with different BR near the critical condition are analyzed; particular attention is paid to the cases for which BR is larger than 0.9 (i.e., 0.923 and 0.962), and the results are also compared with relatively smaller BR cases (BR=0.691 and 0.826). The effect of the diffraction, shock reflection and detonation instability on the propagation of detonation is examined. Finally, the analysis of critical length scale between the cell size (λ/d) and BR when the detonation fails or succeeds as it passes the obstacles is also performed, this work aims to explore the mechanism of critical perturbation scale on the detonation propagation.

Relative contribution of gas dynamic and chemical factors to flame penetration through small openings in a closed cylindrical reactor

The ignition of a dilute stoichiometric methane–oxygen mixture (total pressure, up to 200 Torr) occurs behind a single opening on the transition of a laminar flow to a turbulent one rather than after a delay period of ignition.

Ignition delays in methane–oxygen mixture in the presence of small amount of iron or carbon nanoparticles

The influence of small additions (0.3-2 ppm) of iron or carbon nanoparticles on ignition delay times in stoichiometric mixture of 20% (methane + oxygen) diluted in argon was investigated. The experiments were performed in 50 mm diameter shock tube behind reflected shock waves. The nanoparticles were synthesized in pyrolysis of 0.5-1% Fe(CO)5 and 1-2% of C6H6 diluted in argon in the experiment before the ignition test. The residual nanoparticles were pulled into the flow behind incident and reflected shock wave from the shock tube walls and their volume fraction was measured by laser light extinction at the wavelength 633 nm. Additions of 0.3-2 ppm of iron nanoparticles to stoichiometric methane-oxygen mixture resulted in twofold decrease of ignition delays at temperatures below 1400 K relatively to calculated and experimental data for the mixture without nanoparticle addition. At additions of 0.4-1 ppm of carbon nanoparticles to stoichiometric methane-oxygen mixture a weak decrease of ignition delay relatively to the calculated data for the mixture without additives of carbon nanoparticles was observed.

Viscous solution of the triple-shock reflection problem

The reflection of a triple-shock configuration was studied numerically in two dimensions using the Navier-Stokes equations. The flow field was initialized using three shock theory, and the reflection of the triple point on a plane of symmetry was studied. The conditions simulated a stoichiometric methane-oxygen detonation cell at low pressure on time scales preceding ignition, when the gas was assumed to be inert. Viscosity was found to play an important role on some shock reflection mechanisms believed to accelerate reaction rates in detonations when time scales are small. A small wall jet was present in the double Mach reflection and increased in size with Reynolds number, eventually forming a small vortex. Kelvin-Helmholtz instabilities were absent and there was no Mach stem bifurcation at Reynolds numbers corresponding to when the Mach stem had travelled distances on the scale of the induction length. Kelvin-Helmholtz instabilities are found to not likely be a source of rapid reactions in detonations at time scales commensurate with the ignition delay behind the Mach stem.

Fast combustion waves and chemi-ionization processes in a flame initiated by a powerful local plasma source in a closed reactor.

Results are presented from experimental studies of the initiation of combustion in a stoichiometric methane-oxygen mixture by a freely localized laser spark and by a high-current multispark discharge in a closed chamber. It is shown that, preceding the stage of 'explosive' inflammation of a gas mixture, there appear two luminous objects moving away from the initiator along an axis: a relatively fast and uniform wave of 'incomplete combustion' under laser spark ignition and a wave with a brightly glowing plasmoid behind under ignition from high-current slipping surface discharge. The gas mixtures in both the 'preflame' and developed-flame states are characterized by a high degree of ionization as the result of chemical ionization (plasma density n(e)≈10(12) cm(-3)) and a high frequency of electron-neutral collisions (ν(en)≈10(12) s(-1)). The role of chemical ionization in constructing an adequate theory for the ignition of a gas mixture is discussed. The feasibility of the microwave heating of both the preflame and developed-flame plasma, supplementary to a chemical energy source, is also discussed.