Methane-oxygen Mixtures Research Articles

Detonation propagation over a wedge with two channels in a duct

An experimental study was performed in a rectangular duct to investigate the detonation diffraction process in stoichiometric methane-oxygen mixtures. A wedge was placed in the duct to form two channels between the wedge and the channel walls, i.e., an upper channel and a divergence channel. The upper channel heights were 27mm, 47mm and 67mm, and the deviation angles (α) were 60 ° and 30 °. On the bottom wall and side wall, soot foils were placed to record the cellular structure evolution. For the larger deviation angle (α =60 °), the re-initiation mechanisms can be varied, which are affected by the upper channel height. Two overdriven detonation regions are generated behind the wedge on the bottom wall as well as the side wall for more sensitive mixtures (characterized by smaller detonation cell sizes). However, for insensitive mixtures, the re-initiation process seems to be more complicated. More regions containing finer cells can be observed on the foils. In case of α =30 °, the detonation re-initiates at the divergence wall and propagates toward the side wall. Universally, the re-initiation is facilitated as the upper channel height increases, and equivalently, the re-initiation distance is reduced.

Experimental study of detonation limits in methane-oxygen mixtures: Determining tube scale and initial pressure effects

In this paper, detonation limits in stoichiometric methane-oxygen mixtures with varying tube inner diameter and initial mixture pressure were investigated. Detonations in tubes with different inner diameter (D = 36 mm, 25 mm, 20 mm and 13 mm) and low initial pressure from 3.5 to 18 kPa were studied. Smoked foils were applied to observe the evolution of the detonation cellular structure for various initial conditions. An alternate length scale at the limits is examined, Ldcs, which is the maximum length from the beginning of the test section after which cellular patterns can no longer be observed. Simultaneous local velocity measurements were obtained by photodiodes to complement the Ldcs results. The study also aims to reveal relation between the near-limit detonation dynamics, the tube geometry, and the thermodynamic properties of the mixture. Past the failure limit, Ldcs decreases with decreasing initial mixture pressure for a given tube diameter, and Ldcs decreases faster in a smaller diameter tube. In the D = 13 mm tube, galloping detonation mode is observed, and the length of the galloping cycle is reduced with an increase in initial pressure. To further characterize the onset of detonation limits, a scaling analysis of Ldcs with tube inner diameter (D) and detonation cell size (λ) was performed. The experimental results show that the decrease of Ldcs/D and Ldcs/λ are more abrupt in smaller diameter tubes with decreasing initial pressure. At low initial pressure, the boundary layer displacement thickness growth is significant in the flow structure. Since the distribution of global curvature over the whole detonation front is faster in smaller tube, it thus leads to a more abrupt decrease sensitive to initial pressure. For increasing pressure closer to the critical failure limit, the boundary layer displacement thickness is becoming less comparable to the tube diameter. The failure mechanism appears to be more dominant by the rate of transverse waves attenuation or cell disappearance. Lastly, by comparing the detonation cell size and the tube scale at the critical limits condition in different tubes, λ = π⋅D is shown to be an appropriate limit criterion of detonation propagation in agreement previous studies.

Purificación de biogás usando Cianobacterias


 Objetivo: Investigar técnicas biológicas para purificar biogás. Metodología: El biogás con bajo contenido de dióxido de carbono fue burbujeado en dos soluciones acuosas que contenían cianobacterias filamentosas de diferentes cepas de Leptolyngbya sp. Luego, los resultados obtenidos fueron comparados contra un blanco. Resultados y Concluiones: El biogás que fue parcialmente purificado redujo su contenido de dióxido de carbono en una proporción de 20 % a < 10 % luego de estar en contacto con cianobacterias. A la vez, el oxígeno producido durante la fotosíntesis se mantuvo por debajo de los límites de explosión para la mezcla metano-oxígeno. En contraste, el blanco usado en el ensayo se saturó de dióxido de carbono, causando una caída en el pH conforme pasaba el tiempo. El contenido de metano en el biogás purificado, cuya pureza fue medida con un método volumétrico, superó el 90 %. Las dos cepas de cianobacteria usadas tenían una composición en base seca de proteína ? 25 % y en lípidos < 2 %.

Features of haloalkane effect on the concentration limits and induction time for the ignition of methane–oxygen mixtures

A strong difference in the character of admixture effect on ignition by shock wave and spark initiation has been revealed using the haloalkane effect on the ignition of methane–oxygen mixtures as an example. It has been shown that the difference is determined by the presence of considerable concentrations of reactive intermediate species in an initial mixture, the qualitative difference in gas heating modes, and the strong difference between the temperature ranges of the process for two these methods.

Effect of the angle between gas flow direction and electrode on single-chamber SOFC stacks

Single-chamber solid oxide fuel cell (SC-SOFC) stacks with different angles between the gas flow direction and the electrode were fabricated and tested in a diluted methane-oxygen mixture. The gas tubes with separated gas vents were used for transmission of the reactant gasses. Experimental results showed that the performance of the two-cell stack at a gas flow angle (θ) of 45° was greatly enhanced compared with that at 0 and 90°. At a furnace temperature of 700 °C and a CH4/O2 ratio of 2, the maximum output power of the anode-first configuration was 154 mW at θ = 45°, which was 51% higher than that obtained at θ = 90°. Additionally, the maximum power of the anode-first stack was higher than that of the cathode-first one at θ = 45°, which was contrary to the results obtained at θ = 90°. The stack with an angled configuration of θ = 45° will generate a more attractive power for portable power applications.

Ignition characteristics of dual-fuel methane-n-hexane-oxygen-diluent mixtures in a rapid compression machine and a shock tube

Ignition delay times of methane-n-hexane-oxygen-dulent mixtures were studied experimentally and numerically in a wide temperature range (640–2335 K) using both a rapid compression machine (RCM) and a shock tube (ST). The RCM results demonstrated a two-stage ignition and negative temperature coefficient (NTC) behavior. Increasing n-hexane concentration, pressure and equivalence ratio shortened the ignition delay time. For the ST experiments, the addition of 10% n-hexane (relative to methane) can reduce the ignition delay time dramatically. However, no further reduction effect can be achieved with increasing addition of n-hexane from 10% to 20%. In addition, increasing equivalence ratio reduces the effect of n-hexane addition on ignition delay time. Three detailed chemical mechanisms, CaltechMech, GalwayMech and LLNLMech, were evaluated based on a quantitative error analysis. LLNLMech and CaltechMech demonstrated the best performance in the RCM and ST temperature ranges, respectively. Chemical kinetic analyses showed that the addition of n-hexane to methane provides some chemical pathways not available for methane oxidation which result in the production of active radicals and eventually accelerate the ignition of the methane-oxygen mixtures. The crucial intermediate species for the ignition process are H2O2 and H under RCM and ST conditions, respectively.

Effect of an Additional Turbulence Source in a Flow Combustion Chamber on the Combustion Mode of Rich Methane-Oxygen Mixtures

In this paper, we experimentally study the effect of turbulent combustion modes on the conversion of initial components and main product yields in the noncatalytic partial gas-phase oxidation of natural gas in a flow reactor with a critical pressure gradient and an oxidizer excess ratio of α = 0.3. Where the design of the turbulizer (an additional turbulence source in the flow combustion chamber) is changed, two combustion modes leading to significantly different product distributions are found.

Enhanced performance of a single-chamber solid oxide fuel cell with dual gas supply method

Single-chamber solid oxide fuel cell made of conventional materials with two gas tubes located at the different sides of the cell was fabricated and tested in a diluted methane-oxygen mixture to evaluate the influence of various operating parameters on cell performance. The traditional gas supply method was also studied for comparison. Experimental results showed that the cell performance was greatly enhanced by using the dual gas supply method. At a furnace temperature of 700 °C, the maximum power density was 459.2 mW cm−2 for a CH4/O2 ratio of 1.5, which was 67% higher than that of the traditional gas supply method. Additionally, the dual gas supply method could provide the required reactant gas for each electrode by changing the gas composition of both the gas tubes separately. The highest power density of 493.9 mW cm−2 was obtained at the anode and cathode CH4/O2 ratios of 1.5 and 1, respectively. A cell with dual gas supply method will generate a more attractive power output than that of the traditional method in a single-chamber condition.

Features of Haloalkane Effect on the Concentration Limits and Induction Time for the Ignition of Methane–Oxygen Mixtures

Features of Haloalkane Effect on the Concentration Limits and Induction Time for the Ignition of Methane–Oxygen Mixtures

On inefficiency of suppression of methane-oxygen mixtures autoignition by halogenated hydrocarbons

On inefficiency of suppression of methane-oxygen mixtures autoignition by halogenated hydrocarbons

Interaction of the flames of diluted methane-oxygen mixtures with obstacles of conical shape

Interaction of the flames of diluted methane-oxygen mixtures with obstacles of conical shape

Investigation on the detonation propagation limit criterion for methane-oxygen mixtures in tubes with different scales

The detonation propagation limits of CH4-O2 mixtures (φ = 0.25–2) in a macro-channel (D = 36 mm) and in micro-channels with heights of 2 mm, 4.5 mm and 7 mm were studied. The critical pressure (pc) below which detonation cannot maintain steady propagation was measured experimentally for each mixture in different channels. At the condition of pc, the scaling between detonation cell size (λ) and tube dimension (DH) were also analyzed to explore the detonation failure mechanism in channels of macro- and micro-scale. The experimental results show that the detonation propagation limit criterion λ = πD (D is the inner diameter) holds in a macro-channel for the mixture with larger value of instability; however, the detonation instability is reduced as the mixture tends to fuel-rich condition, where this typical relation breaks down. The results indicate that the propagation limit criterion in macro-channel depends less on the boundary conditions of the tube and more on the detonation instability. In the micro-channels, because of the losses from the walls and mass divergence into the boundary layers, detonation propagates in a thinner channel with larger velocity deficit (VD), thereby increasing both the reaction zone length and the detonation cell size; therefore, λ = πD is unsuitable as a criterion of the detonation propagation limit, which suggests that for detonation propagating in the micro-channels, the propagation mechanism is mainly governed by the boundary conditions.

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.

Experimental Study of Methane Combustion over Metallic Palladium upon Flame Penetration through Obstacles

In this paper, the flame penetration of a dilute methane–oxygen mixture through obstacles containing fine-meshed iron grids with Pd-wire turns is studied, using high-speed recording. It is shown that the Pd catalyst can, under certain conditions, suppress the development of flame propagation in a dilute methane–oxygen mixture since the Pd surface is highly efficient in the termination of active reaction centers. Therefore, kinetic factors can be important even under conditions of high turbulence. Numerical simulation using the Navier–Stokes equations for a compressible reaction medium in the low Mach-number approximation shows a qualitative agreement with a number of experimental regularities. The results obtained are of interest for the model development of turbulent flows in reactive media and in matters relating to explosion safety.

Effect of the Calorific Intensity of Combustion Chamber on Production of Synthesis Gas in Partial Oxidation of Methane–Oxygen Mixtures in the Combustion Mode

Optimal design of a flow-through chemical reactor with increased calorific intensity was experimentally sought for in partial oxidation of natural gas by oxygen at oxidant excess factors in the range 0.27 < α < 0.4. It was shown that this reactor with a chamber for additional turbulent mixing of the starting components, turbulizer, and supercritical pressure difference at the outlet from the combustion chamber can provide a combustion mode close to the process in the plug-flow reactor. The increase in the calorific intensity of the combustion chamber of the reactor as a result of a decrease in its volume leads to full conversion of the starting reagents and to lower carbon-black formation.

Effect of acoustically absorbing wall tubes on the near-limit detonation propagation behaviors in a methane–oxygen mixture

Methane (CH4) is the main component of natural gas and produces less carbon dioxide for each unit of heat released and more heat per unit mass than other hydrocarbon fuels, and it is thus considered to be an environmental-friendly fuel. The explosion and detonation hazards associated with methane mixtures deserve special attention because of their potential safety hazards. Acoustically absorbing materials are effective in damping out the transverse wave of a detonation structure. A detonation could be attenuated or prohibited after passing over this material, but the failure mechanism of the detonation still needs further exploration. In this study, tubes made of an acoustically absorbing material (hole diameters of the wall are from 30 μm to 300 μm) are inserted in the smooth rigid wall tube to investigate the effect of the porous-walled material on the detonation propagation at the near-limit conditions. Porous-walled tubes with three different scales (L/D = 3.85, 7.69, and 15.38) are studied. Photodiodes and smoked foils are employed to simultaneously measure the local velocity of the combustion waves and record the cellular detonation structures, respectively. The results show that, for shorter porous-walled tubes, the prohibition effect of the absorbing material on the detonation propagation is only prominent at the critical and sub-critical conditions, but the material has a minor effect on the detonation propagation at the super-critical condition. In addition, the prohibition effect of the porous material on the detonation propagation becomes more evident with the increased length of the acoustically absorbing material. This outcome occurs because a transverse wave plays an important role in the propagation of a self-sustained detonation, as it is partially damped during the transmission of the detonation through the porous-walled tube; thus, extending the length of the porous-walled section results in increasing the losses of the incident and reflected shock waves due to a greater expansion and mass divergence into porous material. Therefore, the velocity of the combustion wave decreases faster in the downstream of the porous material with an increased length. On the other hand, the methane-oxygen mixture has a highly irregular cellular pattern and is characterized as an unstable mixture with a high degree of instability. The strong instability leads to an enhanced ability to generate new transverse waves in the far downstream, and therefore, at the super-critical condition (a relatively higher initial pressure), the instability partly compensates for the negative effect of the acoustically absorbing material on the detonation propagation.

Effects of jet in crossflow on flame acceleration and deflagration to detonation transition in methane–oxygen mixture

The fluidic jet turbulator has been a novel perturbation generator in the pulse-detonation engines research field for the past few years. In this paper, an experiment is performed to study the deflagration to detonation transition (DDT) process in a detonation chamber with a reactive transverse methane–oxygen mixture jet in crossflow (JICF). The jet injection arrangement is fundamentally investigated, including single jet and various double jets patterns. Corresponding two-dimensional direct numerical simulations with a multistep chemical kinetics mechanism are employed for analyzing details in the flow field, and the interaction between the vortex and flame temporal evolution is characterized. Both the experiments and simulations demonstrate that the JICF can distinctly accelerate flame propagation and shorten the DDT time and distance. The vortex stream induced by the jet distorts and wrinkles the flame front resulting in local flame acceleration. Moreover, the double jet patterns enhance flame acceleration more than the single jet injection because of the intrinsic counter-rotating vortex pairs and enhanced turbulence intensity.

The influence of iodinated fire suppressants on shock-induced ignition of acetylene– and methane–oxygen mixtures

ABSTRACTIn this work, the influence of additives of iodinated hydrocarbons trifluoroiodomethane (CF3I) and 1-iodoperfluoropropane (C3F7I) on shock-induced ignition of acetylene and methane was experimentally investigated. The observed temperature dependencies of induction times indicate that both suppressants show noticeable inhibiting activity in acetylene, but reduce the ignition delay time in methane, similarly to previously studied halogenated fire suppressants CCl4, CF3H, and C2F4Br2.

A Compact Reaction Mechanism of Methane Oxidation at High Pressures

A skeletal methane kinetic mechanism is developed for conditions relating to the combustion of undiluted methane–oxygen mixtures at high pressures. The new skeletal mechanism is based on the detailed mechanism of oxidation of alkanes by Zhukov (2009). The skeletal model has been created by eliminating unimportant species and reactions from the detailed mechanism. The reduction technique is based on the reaction path and sensitivity analyses. They allow one to determine the reactions and species that play important roles in combustion in rocket combustion chambers. The skeletal mechanism consists of 23 species and 51 reactions. The final and intermediate versions of the skeletal mechanism are compared with the parent detailed mechanism, with other reduced kinetic models and with experimental data on the ignition of methane at high pressures. This comparison shows that the developed skeletal mechanism has a better performance than other kinetic mechanisms in terms of accuracy and required computational power.

Calculated Impacts of Diluents on Flame Temperature, Ignition Delay, and Flame Speed of Methane–Oxygen Mixtures at High Pressure and Low to Moderate Temperatures

Oxy-fuel combustion is one method to produce concentrated streams of carbon dioxide for subsequent sequestration. An additional benefit of oxy-firing is a reduction in NOx formation. The high combustion temperatures resulting from oxy-firing are typically controlled by exhaust gas recirculation. In this work, we performed chemical kinetic (CHEMKIN) calculations using a mechanism validated for these conditions to study the effects of dilution by either carbon dioxide or water vapor on methane oxy-combustion, and to compare the results with methane air-combustion (N2 as the diluent). The study was performed under adiabatic conditions at a pressure of P = 30 atm, an equivalence ratio of φ = 1, and initial temperatures of T = 800–1200 K, which mimic the inlet conditions of many gas turbines and flameless combustors. The calculations show that H2O addition at low initial temperatures and high pressure leads to considerable reduction in the ignition delay time. This result is mainly due to changes in the radica...