Effect of trimethylphosphate additives on the flammability concentration limits of premixed methane-air mixtures

The effect of small additives of trimethylphosphate (TMP) on the lean and rich flammability concentration limits of CH₄/air gas mixtures were studied using an opposed-flow burner and numerical modeling based on detailed kinetic mechanisms. TMP was found to narrow the flammability concentration limits of premixed CH₄/air mixtures. Modeling using a previously developed model for flame inhibition by phosphorus compounds showed that the model provides a satisfactory fit to experimental results on the effect of TMP additives on the lean concentration limit. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 12–21, January–February, 2008.     

Experimental and numerical study of CH4/CH3Cl/O2/N2 premixed flames under oxygen enrichment

A comprehensive experimental and numerical study has been conducted to understand the influence of CH₃Cl addition on CH₄/O₂/N₂ premixed flames under oxygen enrichment. The laminar flame speeds of CH₄/CH₃Cl/O₂/N₂ premixed flames at room temperature and atmospheric pressure are experimentally measured using the Bunsen nozzle flame technique with a variation in the amount of CH₃Cl in the fuel, equivalence ratio of the unburned mixture, and level of oxygen enrichment. The concentrations of major species and NO in the final combustion products are also measured. In order to analyze the flame structure, a detailed chemical kinetic mechanism is employed, the adopted scheme involving 89 gas-phase species and 1017 elementary forward reaction steps. The flame speeds predicted by this mechanism are found to be in good agreement with those deduced from experiments. Chlorine atoms available from methyl chloride inhibit the oxygen-enhanced flames, resulting in lower flame speeds. This effect is more pronounced in rich flames than in lean flames. Although the molar amount of CH₃Cl in the methane flame is increased, the temperature at the post flame is not significantly affected, based on the numerical analysis. However, the measured concentration of NO is reduced by about 35% for the flame burning the same amount of methyl chloride and methane at the oxygen enrichment of 0.3. This effect is due to the reduction of the concentration of free radicals related to NO production within the flame. In the numerical simulation, as CH₃Cl addition is increased, the heat flux is largely decreased for the oxygen-enhanced flame. It appears that the rate of the OH + H₂ → H + H₂O reaction is reduced because of the reduction of OH concentration. However, the function of CH₃Cl as an inhibitor on hydrocarbon flames is weakened as the level of oxygen enrichment is increased from 0.21 to 0.5. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 103–111, November–December, 2006.     

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.     

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.     

Numerical comparison of premixed laminar flame velocity of methane and wood syngas

The laminar flame velocity of a synthetic gas is calculated numerically with PREMIX and is compared to methane laminar flame velocity. The calculations are performed at different equivalence ratios, initial mixture temperatures and pressures. For each fuel, a correlation for the laminar flame velocity is presented in the form . The low heating value syngas yields a slower laminar flame velocity than methane, especially around stoichiometry. The laminar flame velocities of methane and wood residue syngas react similarly to the effect of pressure, while numerical results suggest that the laminar flame velocity of syngas is more sensitive to the increase of mixture initial temperature.     

The effect of diluent on flame structure and laminar burning speeds of JP-8/oxidizer/diluent premixed flames

Experimental studies have been performed to investigate the flame structure and laminar burning speed of JP-8/oxidizer/diluent premixed flames at high temperatures and pressures. Three different diluents including argon, helium, and a mixture of 14% CO₂ and 86% N₂ (extra diluent gases), were used. The experiments were carried out in two constant volume spherical and cylindrical vessels. Laminar burning speeds were measured using a thermodynamics model based on the pressure rise method. Temperatures from 493 to 700 K and pressures from 1 to 11.5 atm were investigated. Extra diluent gases (EDG) decrease the laminar burning speeds but do not greatly impact the stability of the flame compared to JP-8/air. Replacing nitrogen in the air with argon and helium increases the range of temperature and pressure in the experiments. Helium as a diluent also increases the temperature and pressure range of stable flame as well as the laminar burning speed. Power law correlations have been developed for laminar burning speeds of JP-8/air/EDG and JP-8/oxygen/helium mixtures at a temperature range of 493-700 K and a pressure range of 1-10 atm for lean mixtures.     

Promotion and inhibition of a hydrogen—oxygen flame by the addition of trimethyl phosphate

This paper gives results of numerical modeling of a laminar hydrogen—oxygen flame doped with trimethyl phosphate at various pressures and compositions of the combustible mixture. The calculations were performed using the PREMIX and CHEMKIN-II software packages. It was found that phosphorus-containing additives promoted the flame at subatmospheric pressures and inhibited it at atmospheric pressure. Kinetic analysis showed that catalytic recombination reactions were responsible for both phenomena. In the case of subatmospheric pressures, the promoting effect and its enhancement with increasing additive concentration were related to a flame temperature rise in the chemical-reaction zone due to catalysis of the recombination reactions by phosphorus-containing compounds. Increasing the additive concentration led to an increase in both the rate of the branching reaction H + O₂ = OH + O and the rate of the chain termination reaction, but the increase in branching reaction rate prevails, resulting in an increase in the flame velocity. In the case of atmospheric-pressure flames, where the reaction-zone temperature is close to the adiabatic equilibrium value, the additive led to an increase in the rate of decay of active flame species and, hence, to a decrease in the flame propagation velocity with increasing additive concentration. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 3–13, September–October, 2006.     

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.     

Effect of laser radiation and electric field on combustion of hydrocarbon-air mixtures

Publications on combustion of hydrocarbon-air mixtures under the action of a weak electric field and laser radiation are analyzed. A specific feature of the authors’ experimental study is a pulsed-periodic action of an electric field and focused laser radiation, which does not lead to electric discharge or optical breakdown. Numerous experiments reveal a noticeable effect of weak electric fields on combustion processes. Application of an electric field may result in expansion of the limits of flammability and combustion of fuel-air mixtures, changes (decrease or increase) in the burning rate, shifting of flame stabilization toward lean mixtures, reduction of the concentration of hazardous substances (for instance, NO _x and CO) in combustion products, etc. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 77–85, July–August, 2009.     

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。     

Determination of schmidt number of mixed fuels by the characteristics of laminar lifted jet flames

A method to determine the Schmidt number of fuel is proposed from the behavior of laminar lifted jet flames. Based on the observation of a laminar lifted flame edge, the flame stabilization point is located along the stoichiometric contour in the mixing layer of fuel and air in laminar jets, since a tribrachial (triple) flame structure exists which is composed of a diffusion flame, a rich premixed flame and a lean premixed flame, all extending from a single location. For the flame edge to be stationary, the axial velocity at the edge should balance with the propagation speed of the tribrachial flame. Since the region between the flame stabilization point and the nozzle exit can be treated as a cold jet, the jet theory of momentum and species can be applied to obtain the correlation of liftoff height with jet velocity and nozzle diameter of . Using this relation, the mixture of fuels having Sc<1 and Sc>1 are tested. The dependence of liftoff height on jet velocity is curve-fitted to extract the effective Schmidt number of mixed fuels. Experimentally determined Schmidt numbers agree satisfactorily with the theoretical predictions from the kinetic theory.     

Measurements of Markstein numbers and laminar burning velocities for liquefied petroleum gas-air mixtures

Experimental test for premixed laminar combustion of liquefied petroleum gas-air mixtures is conducted in a constant volume combustion bomb. Spherically expanding flames have been employed to measure laminar flame speeds over wide equivalence ratios, at the initial pressures of 0.05, 0.1 and 0.15 MPa, and preheat temperatures from 300 to 400 K. To study the effects of stretch on burning velocity, various Markstein numbers for both strain and curvature have been measured and the effects of initial temperature and pressure on these parameters have been discussed. Following the linear relation between flame speeds and flame stretches, one has then obtained the corresponding unstretched laminar burning velocity after omitting the effect of stretches imposed on these flames. Over the ranges studied, laminar 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 discussed.     

Influence of organophosphorus inhibitors on the structure of atmospheric lean and rich methane-oxygen flames

The inhibition of atmospheric laminar methane-oxygen flames of various compositions by trimethyl phosphate was studied experimentally and by numerical modeling using mechanisms based on detailed kinetics. The H and OH concentration profiles in flames with and without the addition of trimethyl phosphate were measured and calculated. It was shown that the addition of the inhibitor reduced the maximum (in the reaction zone) concentrations of H and OH in lean and rich flames. The concentration reduction was higher in rich flames than in lean flames. The concentration profiles of the phosphorus-containing products PO, PO₂, HOPO, HOPO₂, and (HO)₃PO in lean and rich flames stabilized on a flat burner were measured and calculated. Tests of the previously developed model of flame inhibition by phosphorus compounds showed that the model provides adequate predictions of many experimental results. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 23–31, March–April, 2007.     

Modeling combustion of lycopodium particles by considering the temperature difference between the gas and the particles

The effect of the temperature difference between the gas and the particles on propagation of premixed flames in a combustible mixture containing volatile fuel particles uniformly distributed in an oxidizing gas mixture is analyzed in this paper. It is presumed that the fuel particles vaporize first to yield a gaseous fuel, which is oxidized in the gas phase. The analysis is performed in the asymptotic limit, where the value of the characteristic Zel’dovich number is large, which implies that the reaction term in the preheating zone is negligible. Required relations between the gas and the particles are derived from equations for premixed flames of organic dust. Subsequently, the governing equations are solved by an analytical method. Finally, the variation of the dimensionless temperatures of the gas and the particles, the mass fraction of the particles, the equivalence ratio ϕ _g as a function of ϕ _u , the flame temperature, and the burning velocities of the gas and the particles are obtained. The analysis shows that the calculated value of ϕ _g is smaller than unity for certain cases, even though ϕ _u ⩾1. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 49–57, May–June, 2009.     

PBTCA及马-丙共聚物对碳酸钙垢阻垢机理的动力学研究

用加晶种于过饱和溶液中的方法测定了ＣａＣＯ３晶体结晶生长的动力学中的两种得到了不加阻垢剂及加阻垢剂两种条件下的结晶生长反应速率常数。结果表明,ＰＢＴＣＡ及同济大学测试中心研制的丙烯酸马来酸共聚物（ＡＡ－ＭＡ）能明显地阻止溶液中ＣａＣＯ３在晶种表面的结晶生长,研究发现,阻垢的机理主要是阻垢剂对ＣａＣＯ３晶种表面具有吸附作用,从而抑制了ＣａＣＯ３的结晶生长,这种吸附作用符合Ｌａｎｇｍｕｉｒ等温吸附特征     

Effects of temperature and composition on the laminar burning velocity of CH4 + H2 + O2 + N2 flames

This work summarises available measurements of laminar burning velocities in CH₄ + H₂ + O₂ + N₂ flames at atmospheric pressure performed using a heat flux method. Hydrogen content in the fuel was varied from 0% to 40%, amount of oxygen in the oxidiser was varied from 20.9% down to 16%, and initial temperature of the mixtures was varied from 298 to 418 K. These mixtures could be formed when enrichment by hydrogen is combined with flue gas recirculation. An empirical correlation for the laminar burning velocity covering a complete range of these measurements is derived and compared with experiments and other correlations from the literature.     

Numerical simulation of the effect of the addition of NO and NO<Subscript>2</Subscript> on a rich hydrogen flame using the tracer method

The effect of the addition of nitric oxides (NO and NO₂) on rich hydrogen-air flames was studied using the tracer method in numerical simulation. It is shown that the effects of these additives are not similar. Both oxides promote the formation of OH and H₂O in the low-temperature zone of the front. The addition of NO reduces the first maximum of the OH profile and the burning velocity. The addition of NO₂ increases the first maximum of the OH profile and does not change the burning velocity. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 19–25, May–June, 2009.     

Effect of inert and reactive admixtures on the initiation and propagation of laminar spherical flames

The initial stage of the propagation of discharge-initiated laminar spherical flame in stoichiometric natural gas- and isobutylene-oxygen mixtures containing krypton or carbon dioxide and in the hydrogen-air mixture at atmospheric pressure in a constant-volume bomb has been investigated by high-speed color cinematography. Dilution of the combustible mixtures with the admixtures increases the steady-state flame front formation time by a factor larger than 10. The phlegmatizing effect of carbon dioxide on hydrocarbon combustion is stronger than that of krypton. For given combustible mixtures, the ratio of steady-state flame front formation times is inversely proportional to the ratio of the temperature rise values for the flames. The introduction of a minor amount of a reactive admixture (1.2% isobutylene) into the stoichiometric hydrogen-air mixture greatly increases the steady-state flame front formation time.     

Effect of Montmorillonite Clay on the Burning Rate of High-Impact Polystyrene

The effect that montmorillonite clay has on the burning rate of high impact polystyrene (HIPS) is examined. Two clays are considered a pristine montmorillonite (MMT-Na⁺) and a montmorillonite clay intercalated with triphenyl phosphite (TPP). In addition to the burning rate tests, mechanical, impact and rheological properties are determined. Results show an increase of 55% in the burning rate of the nanocomposite when the neat MMT-Na⁺ clay is added. Thermal measurements indicate a diminishing HIPS decomposition temperature as the MMT-Na⁺ clay concentration rises. On the other hand, when TPP is incorporated in MMT-Na⁺ (MMT-i-TPP) and the clay is added to HIPS, a substantial reduction (42%) of the burning rate is observed, while the mechanical properties are kept at the level of those of the original HIPS. The system HIPS-MMT-i-TPP presents a steeper reduction in the viscosity as compared to HIPS in the high-temperature range.     

隧道窑冷却带喷嘴布置对窑内气流动的影响

利用计算流体力学软件Fluent中的标准紊动能-紊动能耗散率(k-ε)模型和多孔介质模型,对隧道窑冷却带内气体流场进行了三维数值模拟,研究喷嘴的数量和布置方式对窑内气体流动的影响。研究表明,喷嘴的安装布置应为:上排喷嘴靠近窑顶,下排喷嘴靠近窑车台面;当维持喷入总风量和喷嘴喷出速度不变时,喷嘴个数应有一定范围,不能太多;上排喷嘴和下排喷嘴应对称安装。