Investigation of products of thermal methane conversion in hydrocarbon mixtures by mass spectrometry

The influence of ethylene/acetylene admixture on catalytic methane pyrolysis is investigated. Mass spectrometric analysis of the products of reactions in synthesis of carbon structures by thermal chemical gas-phase deposition is carried out for different temperatures, gas mixture compositions, and catalyst concentrations. It is shown that in the presence of ethylene and acetylene, there occurs partial methane decomposition at lower temperatures.     

Modulation of a methane Bunsen flame by upstream perturbations

In this paper the effects of an upstream spatially periodic modulation acting on a turbulent Bunsen flame are investigated using direct numerical simulations of the Navier-Stokes equations coupled with the flamelet generated manifold (FGM) method to parameterise the chemistry. The premixed Bunsen flame is spatially agitated with a set of coherent large-scale structures of specific wave-number, K. The response of the premixed flame to the external modulation is characterised in terms of time-averaged properties, e.g. the average flame height ?H? and the flame surface wrinkling ?W?. Results show that the flame response is notably selective to the size of the length scales used for agitation. For example, both flame quantities ?H? and ?W? present an optimal response, in comparison with an unmodulated flame, when the modulation scale is set to relatively low wave-numbers, 4π/L ? K ? 6π/L, where L is a characteristic scale. At the agitation scales where the optimal response is observed, the average flame height, ?H?, takes a clearly defined minimal value while the surface wrinkling, ?W?, presents an increase by more than a factor of 2 in comparison with the unmodulated reference case. Combined, these two response quantities indicate that there is an optimal scale for flame agitation and intensification of combustion rates in turbulent Bunsen flames.     

Attachment structure of a non-premixed laminar methane flame

The stoichiometry and the flame structure of the leading edge, an anchor point, of a non-premixed methane flame were investigated. Local equivalence ratio at an anchor point was measured using local chemiluminescence spectra with a high spatial resolution of 17 × 450 μm. Spatially and spectrally resolved chemiluminescence measurements were carried out along the centerline and radius of the non-premixed laminar flame. The chemiluminescence spectra measured at the flame tip contained very strong luminous spectra, while these continuous background spectra disappeared at the blue flame tip region. The chemiluminescence spectra below the blue flame region were very similar to those measured in laminar premixed methane/air flames. Based on these results, the local equivalence ratio near the anchor point was calculated. Therefore, we measure the anchor point location, its shape, and stoichiometry using the flame spectra. At the anchor point, there was an island of lower equivalence ratio of 0.65, which can be estimated as the lower flammable limit of premixed laminar flame. The size of the anchor point was of horizontal elliptical shape less than 0.6 and 0.4 mm in vertical length, which located at 1.2 mm above the burner rim and inside of the rim.     

常压辉光等离子体催化CH4转化制C2烃在线诊断研究

利用发射光谱原位技术对旋转电极辉光等离子体作用下CH4-H2转化反应进行诊断研究,在300~700 nm波长范围内检测到了C,CH,C2,H和H2等激发态物种的发射谱线。利用H原子发射光谱,通过Boltzmann图解法计算了等离子体的激发温度,该激发温度在6 300~6 600 K之间。同时由谱线展宽计算了电子密度,其数量级在1020m-3。     

Study of the excited 0+ states through conversion electron spectroscopy of neutron rich Zr and Mo fission products

A windowless Si(Li) electron detector has been used in conjunction with the gas filled fission product separator JOSEF to study totally converted transitions in Zr and Mo isotopes. The existence of twoβ-decay modes in⁹⁶Y,⁹⁸Y and¹⁰⁰Nb is confirmed. The half-lives of theβ-decaying levels feeding O ₂ ⁺ states in⁹⁶Zr and⁹⁸Zr are 6.0 ± 0.4 sec and 0.60±0.05 sec respectively. A search for other first excited O ₂ ⁺ states shows that no such states are apparent in¹⁰⁰Zr and¹⁰⁴Mo.     

Transformation of photo-counting statistics by non-linear light conversion

It is known [1–3] that the statistical properties of the electrons emitted by a detector, which registers a light beam, contain information about light radiation statistics. This method has recently been widely used to measure the statistical properties of laser radiation [4]. In analysing non-linearly-transformed laser or thermal radiation, this method can give information about the non-linear process and also additional data on the statistics of the transformed radiation.     

Temperature measurement of a premixed radially symmetric methane flame jet using the Mach-Zehnder Interferometry

The temperature field of a premixed methane symmetric laminar flame jet is visualized by studying the interferograms of the flame, using the Mach-Zehnder Interferometry. Two kinds of oxidizers are chosen for combustion: industrially pure oxygen and oxygen-enriched air. The flame is chosen to be both lean, and rich. For the lean oxygen-enriched combustion (OEC), the equivalence ratio was held constant at 0.5, and the oxygen enrichment was adjusted to 0.5 and 0.6, and for rich OEC, equivalence ratio is chosen to be 1.2 while the oxygen enrichment was 0.7 and 0.8. For methane/oxygen combustion, the equivalence ratio varied from 0.35 to 0.55 for the lean flame, and 1.3 and 1.7 for the rich flame. Attempt was made to keep the Reynolds number unchanged at 500, for OEC, and 1000, for methane/oxygen flame. In the present study a non-contact method is successfully developed to measure the temperature field of a premixed radially symmetric laminar methane flame jet. The effect of oxygen enrichment and equivalence ratio on temperature field is also investigated and depicted.     

Direct conversion of solid hydrocarbons in a molten carbonate fuel cell

Electrical characteristics of a molten carbonate fuel cell allowing direct electrochemical oxidation of dispersed hydrocarbons have been examined. As the fuel, graphite, anthracite, and cannel coal samples were used. Data illustrating the effect of electrolyte temperature, fuel type and dispersion, and also reactant gas mixture composition on the performance characteristics of the fuel cell, were obtained. Correlation between the specific characteristics of the fuel cell and the hydrogen content of fuel material was established. The maximum current-density values were achieved with hydrogen-rich cannel coal. For dispersed fuel samples, interparticle contact losses were found to have influence on the cell-generated voltage. The maximum cell opencircuit voltage was reached with stoichiometric oxygen-carbon dioxide mixture blown into the cathode. Yet, the largest current-density values were obtained when carbon dioxide lean mixtures were used. Even at zero carbon dioxide concentration the range of cathode polarizations was less than that observed with stoichiometric mixture. The processes proceeding in the cathode and anode packs of the fuel cell are believed to be interrelated processes. In a model fuel cell fueled with dispersed coal, current densities up to 140 mA/cm² and specific powers up to 70 mW/cm² were achieved.     

An experimental study of the rich premixed ethylbenzene flame at low pressure

A slightly sooting premixed ethylbenzene flame with an equivalence ratio of 1.90 was investigated at low pressure (4.0 kPa) using molecular-beam mass spectrometry (MBMS) and tunable synchrotron vacuum ultraviolet (VUV) photoionization. Basing on the ionization threshold measurements of photoionization efficiency (PIE) spectra, combustion intermediates up to C₁₉H₁₂ were identified, including a number of radicals and isomeric species. Mole fraction profiles of observed flame species were evaluated from the measurements of burner scan at the photon energies near ionization thresholds. Besides, the flame temperature profile was measured by a Pt/Pt-13%Rh thermocouple. From the intermediate identification and mole fraction measurements, the degradation of ethylbenzene, as well as the formation of some interested polycyclic aromatic hydrocarbons (PAHs), was discussed in detail. It is suggested that the formation of most typical PAHs observed in this work can be related to the H-abstraction/C₂H₂-addition (HACA) mechanism. Furthermore, the high concentration levels of intermediates in this flame is ascribed to the weak C-C bonds in the sidechain of ethylbenzene, which provides a potential explanation of the high sooting tendencies of ethylbenzene and other monocyclic aromatic fuels with complex sidechain structure. This study is anticipated to be constructive for combustion investigations of aromatic fuels, and the discussion is hoped to be helpful for further modeling studies concerning PAHs formation in combustion process.     

Inversionless amplification of light by a two-level medium as a result of mechanical-to-electromagnetic atomic energy conversion

Nonreciprocal effects in the acousto-optical interaction in gyrotropic cubic crystals with electroinduced anisotropy are studied. It is shown that the presence of optical gyrotropy leads to the doubling of the number of peaks of amplitude nonreciprocity, whereas an external electric field causes their shift and enhancement. It is found that the direction of lasing in a ring laser can be reversed by changing the direction of the control field. The results obtained can be used for the development and optimization of polarization-and electro-controlled acousto-optical nonreciprocal elements on the basis of cubic gyrotropic crystals.     

Computational study on lean and rich combustion of flame ball,counterflow flame and planar flame: Their limits and stoichiometries

To investigate (fuel-)lean/rich limits and essential stoichiometries, i.e., the borders of lean/rich combustion, one-dimensional steady computations with detailed chemistry for flame balls, counterflow flames, and stretch-free planar flames were conducted using a CH₄/O₂/Xe mixture that has been used in microgravity experiments. As continuous converged solutions were obtained under lean/rich conditions, it was suggested that the existence of flame ball not only under lean but also under rich condition. Flame radii and temperatures of flame balls decreased and increased toward the lean/rich limits from their maximum and minimum values, respectively. The lean limits were wider in the order of the flame ball, counterflow flame, and stretch-free planar flame. Therefore, the lean flammability limit corresponded to the lean limit of the flame ball in the mixture. Conversely, the rich limits were wider in the order of the counterflow flame, stretch-free planar flame, and flame ball. Thus, the rich flammability limit corresponded to the rich limit of the counterflow flame in the mixture. Essential stoichiometry, which represents the actual stoichiometry depending on the dominant transport in near-flame front, was not uniquely determined as conventional stoichiometry (ϕ = 1); it was located between the equivalence ratio of ϕ = 1 and ϕ_c, where ϕ _c denotes the critical equivalence ratio is evaluated using the fuel and oxidizer Lewis number of a target mixture. The results indicated that the essential stoichiometry of the stretch-free planar flame corresponded to ϕ = 1, that of the flame ball corresponded to ϕ = ϕ _c, and that of the stretched flame was located between ϕ = 1 and ϕ _c depending on the stretch rate.     

LES/CMC modelling of ignition and flame propagation in a non-premixed methane jet

The Large Eddy Simulation (LES) / Conditional Moment Closure (CMC) model with detailed chemistry is used for modelling spark ignition and flame propagation in a turbulent methane jet in ambient air. Two centerline and one off-axis ignition locations are simulated. We focus on predicting the flame kernel formation, flame edge propagation and stabilization. The current LES/CMC computations capture the three stages reasonably well compared to available experimental data. Regarding the formation of flame kernel, it is found that the convection dominates the propagation of its downstream edge. The simulated initial downstream and radial flame propagation compare well with OH-PLIF images from the experiment. Additionally, when the spark is deposited at off-centerline locations, the flame first propagates downstream and then back upstream from the other side of the stoichiometric iso-surface. At the leading edge location, the chemical source term is larger than others in magnitude, indicating its role in the flame propagation. The time evolution of flame edge position and the final lift-off height are compared with measurements and generally good agreement is observed. The conditional quantities at the stabilization point reflect a balance between chemistry and micro-mixing. This investigation, which focused on model validation for various stages of spark ignition of a turbulent lifted jet flame through comparison with measurements, demonstrates that turbulent edge flame propagation in non-premixed systems can be reasonably well captured by LES/CMC.     

Experimental investigation of flame spreading over pure methane hydrate in a laminar boundary layer

Flame spreading over pure methane hydrate in a laminar boundary layer is investigated experimentally. The free stream velocity (U_∞) was set constant at 0.4 m/s and the surface temperature of the hydrate at the ignition (T_s) was varied between ?10 and ?80 °C. Hydrate particle sizes were smaller than 0.5 mm. Two types of flame spreading were observed; “low speed flame spreading” and “high speed flame spreading”. The low speed flame spreading was observed at low temperature conditions (T_s = ?80 to ?60 °C) and temperatures in which anomalous self-preservation took place (T_s = ?30 to ?10 °C). In this case, the heat transfer from the leading flame edge to the hydrate surface plays a key role for flame spreading. The high speed flame spreading was observed when T_s = ?50 and ?40 °C. At these temperatures, the dissociation of hydrate took place and the methane gas was released from the hydrate to form a thin mixed layer of methane and air with a high concentration gradient over the hydrate. The leading flame edge spread in this premixed gas at a spread speed much higher than laminar burning velocity, mainly due to the effect of burnt gas expansion.     

Quantitative measurements of one-dimensional OH absolute concentration profiles in a methane/air flat flame by bi-directional laser-induced fluorescence

The one-dimensional(1D) spatial distributions of OH absolute concentration in methane/air laminar premixed flat flame under different equivalence ratios at atmospheric pressure are investigated by using bi-directional laser-induced fluorescence(LIF) detection scheme combined with the direct absorption spectroscopy. The effective peak absorption cross section and the average temperature at a height of 2 mm above the burner are obtained by exciting absorption on the Q1(8)rotational line in the A2Σ+(υ= 0) ← X2Π(υ = 0) at 309.240 nm. The measured values are 1.86×10-15cm2and1719 K, respectively. Spatial filtering and frequency filtering methods of reducing noise are used to deal with the experimental data, and the smoothing effects are also compared using the two methods. The spatial distribution regularities of OH concentration are obtained with the equivalence ratios ranging from 0.8 to 1.3. The spatial resolution of the measured result is 84 μm. Finally, a comparison is made between the experimental result of this paper and other relevant study results.