Effects of initial temperature on the structure of laminar CH4-air premixed flames

The growing popularity of natural gas as a eco-friendly fuel, is of paramount motivation of present investigation. In the present paper, the effect of initial temperature on the flame structure have been investigated in which laminar one-dimensional planar propagating flames of CH₄/air mixtures is simulated numerically using detailed chemical kinetic scheme and realistic transport models. The burning velocities are fundamentally important in developing models to predict progress of combustion. Hence, the burning velocities as a function of initial temperature of unburnt gas have been computed for stoichiometric mixture. The present predictions of burning velocities are compared with reported experimental data of Stone et al. [Combust. Flame. 114 (1998) 546], Hill and Huang [Combust. Sci. Technol. 60 (1980) 7] and Rallis and Garforth [Combust. Flame 31 (1978) 53]. The present prediction lies within the scatter of experimental data. A correlation in the form of S_u/S_u,0=(T_u/T_u,0)^1.575 has been developed to describe the dependence of initial temperature on the burning velocity for stoichiometric mixture. The structures of flame are investigated in details for initial temperature of 300 and 600 K which clearly indicate that detailed chemical kinetics are essential for prediction of the effects of initial temperature on the burning velocities. The present study will help in designing and developing the regenerative combustion systems.     

Computational studies of turbulent premixed flame based dump combustor

The present work reports the computational studies of turbulent premixed flame based dump combustor. The effects of various flow parameters such as inlet Reynolds number, inlet turbulence intensity and expansion ratio on important flow quantities like axial velocity, turbulent kinetic energy and turbulent dissipation rate have been studied extensively. It was found out that the axial velocities and the radial velocities within the flowfield are many times higher as compared to the cold flow case due to the heat release and volumetric expansion. The reattachment length for the reacting flow case is found to be lower than that of the cold flow cases due to higher recirculation velocities. A maximum reduction of 64.6% compared to the cold flow reattachment length was observed in the case of turbulence intensity, I = 10%, equivalence ratio,  = 1.00 with an expansion ratio of 2. In contrast, minimum of 41.51% reduction in recirculation length was observed in the case of I = 5%,  = 0.5 for the same expansion ratio. This reduction was quite significant at higher inlet turbulence intensities and at higher equivalence ratios.     

Desulfurization in the gas-continuous impinging stream gas-liquid reactor

An investigation is made to evaluate the flue gas desulfurization (FGD) by absorption in a gas-continuous impinging stream gas-liquid reactor recently developed for systems involving fast reaction(s) in liquid. The mixture of air and SO₂ was used as the pseudo-flue gas and Ca(OH)₂-water suspension as the absorbent. By employing horizontal two-impinging streams, the reactor is simple in structure with few internal parts, while exhibits satisfied overall performance for FGD. Under moderate conditions, the content of SO₂ in the cleaned gas can achieve a level much lower than that permitted, while the pressure drop across the device is about 400 Pa only. The influences of some operating and structural parameters, such as V_L/V_G, Ca/S mole ratio, SO₂ concentration in flue gas, impinging distance S, and nozzle location, etc., are examined. The gas-film mass transfer coefficient, k_G, is determined based on Sauter mean diameter of spray droplets. The results show that k_G is essentially independent of concentration of SO₂ in flue gas, implying the process can be considered to be controlled by diffusion through gas film. The relationship between k_G and impinging velocity, u₀, is fitted to be with the standard deviation of , suggesting u₀ is a strong effecting variable on mass transfer, and, consequentially, important operating variable. In the range of u₀ from 5.53 to , the values determined for the volumetric mass transfer coefficient, k_Ga, are 0.577-, and those for k_G are ranged from 0.00641 to .     

Effects of oxygenate concentration on species mole fractions in premixed n-heptane flames

Atmospheric pressure, laminar, premixed, fuel-rich flames of n-heptane/oxygen/argon and n-heptane/oxygenate/oxygen/argon were studied at an equivalence ratio of 1.97 to determine the effects of oxygenate concentration on species mole fractions. The oxygen weight percents in n-heptane/oxygenate mixtures were 2.7 and 3.4. Three different fuel oxygenates (i.e. MTBE, methanol, and ethanol) were tested. A heated quartz micro-probe coupled to an on-line gas chromatography/mass spectrometry has been used to establish the identities and absolute concentrations of stable major, minor, and trace species by the direct analysis of samples, withdrawn from the flames. The oxygenate addition has increased the maximum flame temperatures and reduced the mole fractions of CO, low-molecular-weight hydrocarbons, aromatics, and polycyclic aromatic hydrocarbons. The reduction in mole fractions of aromatic and polycyclic aromatic hydrocarbon species by an increase in oxygenate concentration was more significant.     

含氨燃料预混火焰的层流火焰速度及NO排放特性

将甲烷或氢气与氨气共燃可以克服NH₃火焰的点火能量高、燃烧速度慢的缺点。为了解NH₃作为燃料的燃烧特性,对含NH₃燃料进行一维层流预混火焰数值模拟,研究其层流火焰速度及NO排放特性。采用文献中5个简化反应机理进行数值计算,发现Okafor机理模拟NH₃/CH₄/air火焰精度更高;Xiao机理模拟NH₃/H₂/air、NH₃/air精度适中,计算时间较短。此外,开展了当量比、燃料混合物组分比例、压力等参数对含NH₃燃料燃烧时烟气中NO浓度影响的研究。研究发现：含NH₃燃料燃烧时NO主要通过OH、H、O自由基和O₂分子的消耗而生成,主要通过与NH_i（i=0, 1, 2）自由基反应消耗;含NH₃燃料在富燃状态下燃烧可有效减少NO排放,但富燃燃烧效率低,可采用富燃-贫燃分级燃烧技术来提高燃烧效率,同时保持NO的低排放;掺有较多NH₃的含NH₃燃料在中高压下燃烧时可有效减少NO排放。     

Propagation and quenching of premixed flames in narrow channels

The shape and propagation of unsteady premixed flames in narrow channels with adiabatic and isothermal walls are numerically investigated in the present study. The flame chemistry is modelled by an one-step overall reaction, which simulates the reaction of a stoichiometric acetylene-air mixture. The numerical results show that both ignition methods and thermal boundary conditions affect flame formation and its shape. The flame keeps a single tulip shape in the whole process of propagating through the channel if plane ignition is used and a single mushroom shape if spark ignition is used. For isothermal cold walls, the flame cannot keep a single tulip shape or mushroom shape all the time. Under plane ignition, a transition from a tulip-shaped flame to a mushroom-shaped flame occurs as flames propagate from one end of channel to the other. Under spark ignition, the process is just the contrary. It is also shown that the heat loss at the cold walls has a dual effect on the formation of a tulip-shaped flame. Flame propagation and quenching in narrow channels of different heights are analyzed systematically, and a criterion is proposed to judge the flame states of partial extinction and total extinction. It is called the criterion of flame propagation and quenching in narrow channels. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 27–36, May–June, 2006.     

Laser-based investigations of flow fields and OH distributions in impinging flames of domestic cooker-top burners

A thermal performance of impinging flames of widely-used cooker-top burners depends on a complex interaction between flow field and combustion. The critical situation is placed on two typical cooker-top burners having almost the same configurations, but yielding about 10% point difference in thermal efficiency. Therefore, in situ flame measurements of averaged velocity using particle image velocimetry (PIV) and image of averaged OH radicals using planar laser-induced fluorescence (OH-PLIF) are required to obtain insight into the coupling mechanisms during the combustion and heat transfer, and to develop a comprehensive experimental data base to support a phenomenological understanding of the differences in flames of the two burners. A combined study of flow field and combustion forms the basis of a much better understanding of the casual links that couple fluid mechanics, combustion and heat transfer in the two burners. This can provide guidelines for efficient burner design to meet future regulatory requirements.     

Comparison of the structures of methane-air and propane-air partially premixed flames

A comparison of flame structures between methane-air and propane-air laminar partially premixed flames has been made through the centerline concentration distributions of selected species measured using gas chromatography. The concentrations of fuel, major species like O₂, CO and CO₂ and those of the intermediate hydrocarbons like C₂H₆, C₂H₄, C₂H₂ and CH₄ (for the propane flame only) have been compared. Distinct double flame structures are observed for the experimental conditions under study. With approximately the same equivalence ratio and jet velocity for the primary mixture, the height of the inner flame for propane is less than that of methane. The peak concentration of C₂H₆ in the propane flame is found to be only a little higher than that in the methane flame, while the peak concentrations of C₂H₄ and C₂H₂ are much greater in the propane flame than in the methane flame. In a methane partially premixed flame, the hydrocarbon concentrations drop from their peak values very rapidly at the inner flame tip, but in the propane flames it is more gradual. In a methane partially premixed flame, CO is formed at the inner flame and burns at the outer flame to CO₂. Similar distributions of CO and CO₂ are found in the propane flame also. However, the peak CO concentration in the propane flame is found to be higher than in methane flame. A radial measurement of species distribution for a particular case in the propane partially premixed flame is also done to ascertain the species distributions across the flame.     

Experimental study and kinetic modeling of benzene oxidation in one-dimensional laminar premixed low-pressure flames

One-dimensional laminar premixed benzene-oxygen-argon flames with equivalence ratios of 2, 1, and 0.7, stabilized at low pressure (45 mbar) on a flat flame burner are studied. Gas sampling is performed by a conical quartz nozzle, at different positions in the flames. Identification and monitoring of chemical species is performed by gas chromatography. These measurements should complete experimental data on rich and sooting benzene flames available in the literature and will be of particular help for further improvements of benzene oxidation mechanisms. A comparison of experimental results with data simulated with the use of two recent kinetic models highlights their inability to predict stoichiometric and lean benzene combustion. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 53–66, July–August, 2009.     

Experiment study of oxygenates impact on n-heptane flames with tunable synchrotron vacuum UV photoionization

In order to determine the effects of oxygenates on the fuel combustion, the experiments reported here investigated the premixed n-heptane flame chemistry. Heptane typifies the large alkanes that comprise the bulk of most hydrocarbon fuels. The specific flames were low-pressure (25 Torr), laminar, premixed flames of n-heptane/oxygen/argon and n-heptane/oxygenate/oxygen/argon at an equivalence ratio of 1.0. Two different fuel oxygenates (i.e. MTBE and ethanol) were tested, these are the main oxygenates used to improve motor vehicle fuel properties. The experiment was performed with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry. Major species on the centerline of each flame were identified by measurements of the photoionization mass spectrum and photoionization efficiency (PIE) spectra. Mole fraction profiles of these species were derived at the selected photon energies near the ionization thresholds. A large amount of oxygenated intermediates was detected in the oxygenate containing flames. The species measurements indicated that MTBE and ethanol enhanced the heptane oxidation via different routes, and reduced the mole fractions of aromatics and cycloalkenes in varying degrees. The results are a useful databases for testing detailed chemical kinetic mechanism of fuel decomposition.     

交流和直流电场对天然气贫燃火焰的影响

对常温、常压下定容燃烧弹中的甲烷/空气预混贫燃火焰的传播和燃烧特性进行了研究,对比了相同电压有效值下15 kHz高频交流电压和直流电压对火焰的影响。结果表明：两种电压加载方式下,火焰形状均发生变形,当过量空气系数一定时,交流电场作用下的火焰在水平方向上的拉伸比直流电场下的剧烈,且混合气越稀,两者差异越明显;与未加载电压相比,当过量空气系数为1.2、1.4和1.6时,交流电场作用下的平均火焰传播速度分别提高49.14%、76.54%、117.65%,直流电场下分别提高41.38%、58.02%、62.75%,交流电场下压力峰值增幅分别为9.48%、11.48%、14.20%,直流电场下增幅分别为4.46%、5.25%、8.76%。因此,相同电压有效值下,15 kHz高频交流电场对火焰的促进作用较直流电场更明显。     

Laminar burning velocity predictions by meso-scale flames in an annular diverging tube

Flame behavior in an annular diverging tube (ADT) consisting of an outer quartz tube and a tapered inner core column was investigated as a basic model for small combustion devices of various combustion space scales. Flames can be stabilized at suitable locations where the mean flow velocity is matched to the spatial average propagation velocity (SAPV). Transient variations of wall temperature and the SAPV were compared for various experimental parameters: inner core materials, burner configurations, and flow rates. It was found that a critical propagation velocity (CPV) exists that is least affected by the flow rates. The CPVs of methane, propane, and dimethyl ether (DME) were measured and a good agreement was shown between the measured CPVs and the laminar burning velocities presented in the literatures. Therefore, the ADT method can be a model for small combustion devices of various combustion space scales; furthermore, this study can be beneficial in designing and operating small combustion devices. The ADT method can also be applied in the field for in situ monitoring of the burning velocities.     

平板狭缝间C1~C4烷烃/空气预混射流火焰的燃烧特性

对平行平板狭缝间C₁~C₄烷烃预混射流火焰进行了实验研究,考察了壁面温度、狭缝间距、当量比、燃料种类等对火焰形态和稳定性的影响,并利用高速相机获得了狭缝间的火焰图像。结果表明,随着狭缝间距的减小,火焰经历了稳定、脉动和熄火3个阶段。其中,火焰脉动发生在大于熄火间距的狭小范围内,其脉动频率随着壁面温度的升高而增加。对同一种燃料,当预混气当量比和壁面温度保持恒定时,火焰的脉动频率在脉动发生的区域内保持不变。对比C₁~C₄烷烃预混火焰的脉动频率及脉动火焰持续距离范围,发现甲烷预混火焰的均最小,而其他3种燃料则比较接近。     

交流和直流电场对天然气贫燃火焰的影响

对常温、常压下定容燃烧弹中的甲烷/空气预混贫燃火焰的传播和燃烧特性进行了研究, 对比了相同电压有效值下15 kHz高频交流电压和直流电压对火焰的影响。结果表明:两种电压加载方式下, 火焰形状均发生变形, 当过量空气系数一定时, 交流电场作用下的火焰在水平方向上的拉伸比直流电场下的剧烈, 且混合气越稀, 两者差异越明显;与未加载电压相比, 当过量空气系数为1.2、1.4和1.6时, 交流电场作用下的平均火焰传播速度分别提高49.14%、76.54%、117.65%, 直流电场下分别提高41.38%、58.02%、62.75%, 交流电场下压力峰值增幅分别为9.48%、11.48%、14.20%, 直流电场下增幅分别为4.46%、5.25%、8.76%。因此, 相同电压有效值下, 15 kHz高频交流电场对火焰的促进作用较直流电场更明显。     

烟气排放连续监测系统在烧结锅炉脱硫中的应用

对石家庄钢铁厂烧结锅炉安装的加热式直接抽取法烟气连续排放监测系统的安装目的、工作原理、测量范围、采样位置和方法等做了介绍,对CEMS的各种指标做了全面测定研究,确定了系统的理想操作条件和性能指标,认为系统采集的数据结果可信,响应灵敏,监测结果与手工测试结果有良好的相关性,达到了国家标准的要求。     

Determination of laminar burning velocities for natural gas

Spherically expanding flames of natural gas-air mixtures have been employed to measure the laminar flame speeds, at the equivalence ratios from 0.6 to 1.4, initial pressures of 0.05, 0.1 and 0.15 MPa, and preheat temperatures from 300 to 400 K. Following Markstein theory, one then obtains the corresponding unstretched laminar burning velocity after omitting the effect of stretch imposed at the flame front. Over the ranges studied, the burning velocities are fit by a functional form u_l=u_l0(T_u/T_u0)^α_T(P_u/P_u0)^β_P, and the dependencies of α_T and β_P upon the equivalence ratio of mixture are also given. The effects of dilute gas on burning velocities have been studied at the equivalence ratios from 0.7 to 1.2, and the explicit formula of laminar burning velocities for dilute mixtures is achieved.     

An analysis of the ignition of premixed gases by a hot spherical surface with catalytic reforming reaction☆

A study of the unsteadiness problem of the ignition of static premixed gases that contain CH₄ and steam by a catalytic hot sphere and a non-catalytic hot sphere were conducted, and a comparison between calculated and experimental results was done in the paper. The catalytic reforming reaction of CH₄ with steam on the surface of the sphere produced a small amount of H₂, CO and CO₂, at the same time there occur oxidizing reactions of CH₄, H₂ and CO in the space. Both experimental and calculated results show that a small quantity of H₂ produced by catalytic reforming reaction can greatly reduce the ignition temperature. In traditional catalytic combustion precious metals is applied to catalyse oxidizing reaction between oxygen and fuel to reduce ignition temperature. In this paper, a study on a ‘indirect’ catalytic combustion is conducted. The cheap catalytic material of Ni with rare earth is used and reforming reaction between steam and fuel is catalyzed, so hydrogen is generated on the surface of hot sphere and utilized to improve combustion. Calculation indicates that the high reactivity and high diffusivity of H₂ remarkably affect ignition.     

Analysis of gas-particle flow characteristics in impinging streams

A three dimensional Euler–Lagrange model for the gas-particle two-phase impinging streams (GPIS) is developed based on the direct simulation Monte Carlo (DSMC) method with consideration of particle rotation and collision. The gas-particle flow characteristics involved in GPIS as well as the effects of inlet gas velocity and particle rotation are analyzed. The results indicate that two pairs of counter-rotating gas vortices are developed at two sides of the opposite jet flows, which is able to entrain the particles and thus greatly weaken the deposition of particles. Interparticle collisions in the impingement zone produce two effects on the particle behaviors: the direct escaping of particles from impingement zone and the progressive accumulation of particles in impingement zone. Under the same inlet particle mass flow rate, the particle concentration in the impingement zone decreases with increasing inlet velocity of gas due to the increasing impinging reaction of interparticle collisions and growing entrainment of gas vortices. In addition, the rotation of particle provides an additional driving force to push the particles away from the impingement zone, leading to the higher speed of escaping particles and smaller maximum particle concentration at the center of impingement zone than those without particle rotation.     

Selective oxidation of hydrogen in rich hydrogen-methane-air flames

Numerical simulation studies of flame propagation using the tracer method have shown that in flames of rich hydrogen, methane, and air mixtures there is selective oxidation of the initial molecular hydrogen. The combustion products contain superequilibrium concentrations of water formed from the initial molecular hydrogen and hydrogen formed from methane in the reaction zone, resulting in superadiabatic temperatures of the flames considered. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 5, pp. 3–11, September–October, 2007.