Stoichiometric Methane-oxygen Mixture Research Articles

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.

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.

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.

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.

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.

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.

Processes of chemoionization in the course of inflammation of a methane-oxygen mixture by a high-current gliding surface discharge in a closed chamber

Results are presented from experiments on the inflammation of a stoichiometric methane-oxygen mixture by a high-current multielectrode spark-gap in a closed cylindrical chamber. It is shown that, in both the preflame and well-developed flame stages, the gas medium is characterized by a high degree of ionization (n e ≈ 1012 cm−3) due to chemoionization processes and a high electron-neutral collision frequency (ν e0 ≈ 1012 s−1).

Detonability of natural gas–air mixtures

Direct initiation experiments were carried out in a 105 cm diameter tube to study detonation properties and evaluate the detonability limits for mixtures of natural gas (NG) with air. The natural gas was primarily methane with 1.5–1.7% of ethane. A stoichiometric methane–oxygen mixture contained in a large plastic bag was used as a detonation initiator. Self-supporting detonations with velocities and pressures close to theoretical CJ values were observed in NG–air mixtures containing from 5.3% to 15.6% of NG at atmospheric pressure. These detonability limits are wider than previously measured in smaller channels, and close to the flammability limits. Detonation cell patterns recorded near the limits vary from large cells of the size of the tube to spiral traces of spin detonations. Away from the limits, detonation cell sizes decrease to about 20 cm for 10% NG, and are consistent with existing data for methane–air mixtures obtained in smaller channels. Observed cell patterns are very irregular, and contain secondary cell structures inside primary cells and fine structures inside spin traces.

Use of the constant-volume bomb technique for measuring burning velocity

The method for computing normal burning velocity, called S u, from observed data in constant-volume bomb measurements is developed from first principles. The theory accounts approximately for effects due to the finite velocity of sound, and shows how radius/time measurements may be used to determine S u for quite fast-burning mixtures. It is shown that the degree of completeness of combustion, important in computing S u, cannot be determined with any accuracy from bomb data, even with simultaneous pressure and radius measurement. Experimental apparatus suitable for constant-volume bomb work at super-atmospheric initial pressures is described, and measurements of flame speed are reported for a stoichiometric methane-oxygen mixture for pressures from 0·1 to 2 atmospheres. Values of S u computed from the data on flame speed are in fair agreement with values obtained by burner methods.