OH concentration fluctuations in turbulent natural gas jet flames

OH radical concentrations in a turbulent non-premixed natural gas flame were measured using laser-induced fluorescence. Instantaneous concentration profiles along a line were obtained using a diode array camera. Investigation of the molecular transitions during laser excitation shows that concentrations, where calibrated with a one-dimensional laminar premixed flame, are biased with a factor of about two. This bias is similar for different flames, so that results of different flames can be compared with an accuracy of about 20%. Three different flames were studied, with fuel jet Reynolds numbers of 9.7 × 10³, 6.8 × 10³ and 4.9 × 10³. Average concentrations and probability density functions show that concentrations close to the nozzle in the flame with highest turbulence are low, which may indicate local extinction. Integral length scales and Taylor micro scales, derived from spatial correlation, exhibit minima at radical locations where OH fluctuations exhibit maxima.     

The stability of turbulent hydrogen jet flames with carbon dioxide and propane addition

In the study the lift-off, blow-out and blow-off stability limits of hydrogen/propane flames and hydrogen/carbon dioxide flames were tested in three different mixing arrangements. The first was to premix hydrogen with carbon dioxide or propane to form a jet flame. The second was to add the gas as an annular jet around the hydrogen flame. The third was to inject into the centre of the hydrogen flame. Propane and carbon dioxide have the same density but create very different chemical kinetic changes when added to hydrogen flames. The results showed that when premixed with hydrogen, propane is more effective in flame lift-off and blow-out. The analysis of kinetic mechanisms revealed that the propane is the dominating fuel in determining the burning rate of the hydrogen/propane while carbon dioxide mainly acted to dilute the hydrogen/CO₂ mixture. Comparing the three mixing arrangements, the experiments showed that hydrogen flame can be effectively lifted or blown out when gases were in annular flow around the hydrogen flame. The isothermal mixing process of the co-flow configuration was discussed.     

Measurements of velocity and concentration for a two-phase turbulent jet

The velocity and concentration distributions for an axisymmetric air jet containing sand particles are determined in the fully developed region. Similarity of the profiles is seen beyond 40 nozzle radii downstream from the jet exit and entrainment is gradually suppressed with increase in the initial particle concentration.     

Velocity and concentration field measurements in a turbulent, impinging flammable jet

The impingement of a jet on a flat surface is a flow configuration of interest in many engineering applications. Of particular interest in this study, it is a generic flow encountered in assessments of the consequences of releases of flammable material on process plant. Experimental data, obtained using laser Doppler anemometry, are presented for the turbulent velocity field in the radial wall jet formed by a high momentum release of methane impinging normally on a flat surface. Mean and fluctuating concentration data obtained using laser Raman spectroscopy are also described, and compared with mean data gathered using aspirated hot film probes. Velocity results are generally in good agreement with data on impinging air jets obtained using particle image velocimetry techniques, although significant differences do occur with earlier hot-wire anemometer measurements. Measured concentrations are in similar agreement with earlier data on impinging air jets obtained using a Mie scattering technique, although differences again occur between mean concentrations obtained using both laser-based techniques and aspirating probes. The provision of a single data set that contains information on both the velocity and conserved scalar fields is of value for improving understanding and in permitting the formulation and validation of RANS models of impinging flows.     

Simultaneous temperature and concentration measurements in hydrogen-air flames using a cars spectrometer

A relatively simple CARS spectrometer has been designed taking advantage of the pure rotational spectra of hydrogen. The spectrometer was used for simultaneous measurement of instantaneous temperatures and concentrations of fuel in hydrogen jets and flames with a high spatial resolution. Data on the fluctuation structure of the flow are reported. Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 5, pp. 34–37, September–October, 1993.     

Modelling of instabilities in turbulent swirling flames

A large eddy simulation-based data analysis procedure is used to explore the instabilities in turbulent non-premixed swirling flames. The selected flames known as SM flames are based on the Sydney swirl burner experimental database. The governing equations for continuity, momentum and mixture fraction are solved on a structured Cartesian grid and the Smagorinsky eddy viscosity model with dynamic procedure is used as the sub-grid scale turbulence model. The thermo-chemical variables are described using the steady laminar flamelet model. The results show that the LES successfully predicts the upstream first recirculation zone generated by the bluff body and the downstream second recirculation zone induced by swirl. Overall, LES comparisons with measurements are in good agreement. Generated power spectra and snapshots demonstrate oscillations of the centre jet and the recirculation zone. Snapshots of flame SM1 showed irregular precession of the centre jet and the power spectrum at a downstream axial location situated between the two recirculation zones showed distinct precession frequency. Mode II instability defined as cyclic expansion and collapse of the recirculation zone is also identified for the flame SM2. The coupling of swirl, chemical reactions and heat release exhibits Mode II instability. The presented simulations demonstrate the efficiency and applicability of the LES technique to swirl flames.     

The formation and destruction of NO in turbulent propane diffusion flames

This paper describes a study of the formation and destruction of NO in turbulent propane diffusion flames with recourse to both experiments and modelling. Detailed in-flame measurements of local mean gas species concentrations of O₂, CO, CO₂, unburnt hydrocarbons and NO_x and local mean gas temperature have been performed for three flames — two of them with the same Froude number and two with the same Reynolds number. These experimental data have been analysed with the aid of a mathematical model. For the NO calculations, three reaction schemes have been used: the Zeldovich reactions, an overall approximate prompt reaction, and a 27 reaction scheme, which includes the thermal NO and the prompt NO reactions and the NO to HCN recycle via fuel NO reactions. The main conclusions are that in the present flames: (1) the prompt NO (or Fenimore) mechanism is the dominant route for the NO formation; and (2) the reactions between NO and hydrocarbon radicals, recycling NO to HCN via the fuel NO reactions, play an important role in the global NO reduction.     

Concentration fluctuations in a round turbulent free jet

The root-mean-square fluctuating concentration is supposed to obey a parabolic differential equation containing terms for convection, diffusion, generation and dissipation. This equation is solved numerically, together with similar simultaneous equations for the time-mean concentration, the longitudinal time-mean velocity, the kinetic energy of the turbulent motion, and a measure of the fluctuations of vorticity; the initial and boundary conditions are appropriate to the steady injection of fluid from a nozzle of circular cross-section into a reservoir containing stagnant fluid of equal density.     

Mass transfer in a turbulent radial wall jet

Experimental and theoretical coefficients are reported for mass transfer in a turbulent radial wall jet initiated by an impinging free jet. The hydrodynamic solution was obtained by the momentum integral technique, and mass transfer was predicted by analogy. Point mass transfer data were obtained for air-naphthalene and cinnamic acid-water systems. At low Schmidt numbers, experimental coefficients were slightly above the theoretical prediction, and followed the theoretical trend with radial distance and nozzle Reynolds number from 10,000 to 60,000. Coefficients at high Schmidt numbers showed large positive deviations from theory, which decreased with radial distance and increased with Schmidt number. These discrepancies were attributed to surface roughness effects.     

圆湍射流中的拟序结构和颗粒弥散

引言 气固两相圆射流广泛存在于各种工程应用当中.仅以能源工程领域来说,煤粉的输送、分离、燃烧效率及污染物的生成控制等过程都跟煤粉颗粒在湍流气流中的弥散方式有关.预测并控制颗粒在气流中的弥散对于实现稳定燃烧、强化传热、降低噪声等有十分重要的意义.为此,需要深入理解气固两相射流中拟序结构的演化特性和不同颗粒的弥散规律.     

Experimental studies of the role of chemical kinetics in turbulent flames

Flames of di-t-butyl-peroxide (DTBP) decomposition in a 0.376DTBP + 1.0N₂ mixture are studied in laminar and turbulent media. The observed values of unstretched laminar burning velocity are in reasonable agreement with the value obtained from the Zel’dovich-Semenov-Frank-Kamenetsky theory. Turbulent explosions in this particular mixture are characterized by a number of features that are believed to be common for all developing turbulent flames and have relevance to spark-ignition engine combustion of lean mixtures. Flame propagation is unsteady and is characterized by a mass burning rate that increases in time. The rate of the flame acceleration varies from one explosion to another. If the burning rate is related to the average flame radius, however, it exhibits much smaller variations. This phenomenon bears a striking resemblance to cycle-to-cycle variations in a spark-ignition engine. Comparisons of the present results with mixtures of significantly different composition, chemical kinetics, and exothermicity, but with similar laminar flame speed and Lewis number show that the data obtained in closed-volume explosions are in good agreement if the unsteady character of the flame is taken into account. The differences in details of the kinetic mechanisms and thermochemistry appear to be responsible for the flame behaviour only near the limit of extinction by turbulence. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 43–52, July–August, 2009.     

A visual study on flame behavior in tone-excited non-premixed jet flames

A visualization study on the effect of forcing amplitude in tone-excited jet diffusion flames has been conducted. Visualization techniques are employed using optical schemes, which are a light scattering photography. Flame stability curve is attained according to Reynolds number and forcing amplitude at a fuel tube resonant frequency. Flame behavior is globally grouped into two from attached flame to blown-out flame according to forcing amplitude; one sticks the tradition flame behavior which has been observed in general jet diffusion flames and the other shows a variety of flame modes such as the flame of a feeble forcing amplitude where traditionally well-organized vortex motion evolves, a fat flame, an elongated flame, and an in-burning flame. Particular attention is focused on an elongation flame, which is associated with a turnabout phenomenon of vortex motion, and on a reversal of the direction of vortex roll-up. It is found that the flame length with forcing amplitude is the direct outcome of the evolution process of the formed inner flow structure. Especially the negative part of the acoustic cycle under the influence of a strong positive pressure gradient causes the shapes of the fuel stem and fuel branch part and even the direction of vortex roll-up to dramatically change.     

气体组分对绝热甲烷化反应温度和总转化率的影响研究

近年来,受天然气需求增加和环保压力影响,煤制天然气和焦炉煤气制天然气成为能源领域研究热点,而甲烷化技术是煤制天然气和焦炉煤气制天然气相关技术的核心之一。采用Aspen Plus模拟软件,模拟选取7组典型甲烷化反应原料气,研究了原料气组分变化对甲烷化反应温度和总碳转化率的影响。研究结果表明:绝热甲烷化反应器出口温度随着H2、CO的浓度增加而增加,随着CH4、CO2、N2和H2O浓度增加而降低,其中CH4和H2O的变化影响较为显著,所以在工艺流程设计和现场装置操作时,选取CH4和H2O作为甲烷化反应的主要控制手段。∑CO+CO2的总碳转化率随着原料气中CO、CO2浓度的增加而降低,随H2浓度增加而快速增加,而与N2、CH4和H2O的浓度影响较小。研究结果既可作为甲烷工艺设计的技术基础,也可对甲烷化现场装置的安全操作提供技术指导,促进煤制气产业的健康、快速发展。