甲烷/乙烷/空气预混火焰层流燃烧速度与火焰稳定性的数值研究(英文)

A numerical study on premixed methane/ethylene/air flames with various ethylene fractions and equivalence ratios was conducted at room temperature and atmospheric pressure. The effects of ethylene addition on laminar burning velocity, flame structure and flame stability under the condition of lean burning were investigated. The results show that the laminar burning velocity increases with ethylene fraction, especially at a large equivalence ratio. More ethylene addition gives rise to higher concentrations of H, O and OH radicals in the flame, which significantly promotes chemical reactions, and a linear correlation exists between the laminar burning velocity and the maximum H + OH concentration in the reaction zone. With the increase of ethylene fraction, the adiabatic flame temperature is raised, while the inner layer temperature becomes lower, contributing to the enhancement of combustion. Markstein length and Markstein number, representative of the flame stability, increase as more ethylene is added, indicating the tendency of flame stability to improve with ethylene addition.     

Measurement of laminar burning velocity of dimethyl ether-air premixed mixtures

Measurement of laminar burning velocity of dimethyl ether-air mixtures was taken under different initial pressures and equivalence ratios using a constant volume bomb and high-speed schlieren photography. The stretched laminar burning velocity increases with the increase of stretch rate. At equivalence ratio of 1.0, low initial pressure gives high stretched flame speed. At initial pressure less than 0.1 MPa, the stoichiometric mixture gives the higher value of stretched flame speed than those at ? = 1.2 and ? = 0.8. The Markstein numbers decrease with the increase of equivalence ratio, and this reveals that lean mixture will maintain higher stability of flame front surface than that of rich mixture in dimethyl ether-air premixed flames.     

Laminar burning velocities and Markstein numbers of syngas-air mixtures

In the currently reported work, three typical mixtures of H₂, CO, CH₄, CO₂ and N₂ have been considered as representative of the producer gas coming from wood gasification. Laminar burning velocities have been determined from schlieren flame images at normal temperature and pressure, over a range of equivalence ratios within the flammability limits. The study of the effects of flame stretch rate was also performed. Combustion demonstrates a linear relationship between flame radius and time for syngas-air flames. The maximum value of syngas-air flame speeds is observed at the stoichiometric equivalence ratio, while lean or rich mixtures have lower flame speeds. The higher is the syngas heat value the higher is the laminar burning velocity of the syngas mixture. Markstein numbers show that typical syngas-air flames are generally unstable. Karlovitz numbers indicates that typical syngas-air flames are little influenced by stretch rate. Based on the experimental data, a formula for calculating the laminar burning velocities of syngas-air flames is proposed. The magnitude of laminar burning velocity for typical syngas compositions is comparable to that of a simulated mixture comprising 5% H₂/95% CO and proved to be similar to methane, although somewhat slower than propane.     

预测生物质颗粒热解时热损失影响的分析模型(英文)

This paper presents the combined influence of heat-loss and radiation on the pyrolysis of biomass parti-cles by considering the structure of one-dimensional, laminar and steady state flame propagation in uniformly pre-mixed wood particles. The assumed flame structure consists of a broad preheat-vaporization zone where the rate of gas-phase chemical reaction is small, a thin reaction zone composed of three regions：gas, tar and char combustion where convection and the vaporization rate of the fuel particles are small, and a broad convection zone. The analy-sis is performed in the asymptotic limit, where the value of the characteristic Zeldovich number is large and the equivalence ratio is larger than unity （i.e. u 1？ ≥ ）. The principal attention is made on the determination of a non-linear burning velocity correlation. Consequently, the impacts of radiation, heat loss and particle size as the de-termining factors on the flame temperature and burning velocity of biomass particles are declared in this research.     

Flame structure and laminar burning speeds of JP-8/air premixed mixtures at high temperatures and pressures

Jet propellant 8 (JP-8)/air laminar burning speed was experimentally measured and its flame structure was studied at high temperatures and pressures using a high-speed camera. The experimental facilities included a spherical vessel, used for the measurement of burning speed, and a cylindrical vessel, used in a shadowgraph system to study flame shape and structure and to measure burning speed. A thermodynamic model was developed to calculate burning speeds using the dynamic pressure rise in the vessel due to the combustion process. The model consists of a central burned gas core of variable temperature surrounded first by a reaction sheet, then by an unburned gas shell with uniform temperature and lastly by thermal boundary layers at the wall and electrodes. Radiation from burned gases to the walls was also included in the model. Burning speeds of laminar flames of JP-8/air were calculated for a wide range of conditions. A Power law correlation was developed to calculate laminar burning speed at temperatures ranging from 500-700 K, pressures of 1-6 atm and equivalence ratios of 0.8-1. Flame structure and cell formations were observed using an optical system. Experimental results showed that pressure and the fuel-air equivalence ratio have a strong influence on flame structure.     

高浓度丙酮VOCs高温预热近极限贫燃预混火焰特性的数值分析

为进一步对一种丙酮挥发性有机化合物（VOCs）焚烧炉进行设计优化和运行参数调节,本文对其在不同的燃料当量比、预热温度下的火焰特性进行了数值模拟,分析了其绝热火焰温度、着火延迟时间、火焰传播速度和一维火焰产物分布特性。研究结果表明：典型当量比（约0.113）下的绝热火焰温度为850~900℃,属于中低温燃烧,绝热火焰温度随预热温度和当量比（0.06~0.4）的升高均线性升高。预热温度和化学当量比对着火延迟时间的影响十分敏感。在其典型贫燃条件下,层流火焰传播速度随预热温度升高呈指数函数关系增大,随化学当量比增大而缓慢升高,且其层流火焰传播速度不超过150cm/s。反应过程首先发生丙酮的分解和部分氧化,并持续时间较长,仅当混合物的温度升高一定程度后才发生较剧烈的CO氧化。     

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.     

Experimental determination of laminar burning velocity for butanol and ethanol iso-octane blends

The potential of butanol as an additive in iso-octane used as gasoline fuel was characterized with respect to laminar combustion, and compared with ethanol. New sets of data of laminar burning velocity are provided by using the spherical expanding flame methodology, in a constant volume vessel. This paper presents the first results obtained for pure fuels (iso-octane, ethanol and butanol) at an initial pressure of 0.1 MPa and a temperature of 400 K, and for an equivalence range from 0.8 to 1.4. New data of laminar burning velocity for three fuel blends containing up to 75% alcohol by liquid volume are also provided. From these new experimental data, a correlation to estimate the laminar burning velocity of any butanol or ethanol blend iso-octane-air mixture is proposed.     

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.     

Experimental study on the laminar flame speed of hydrogen/natural gas/air mixtures

Laminar flame speeds of hydrogen/natural gas/air mixtures have been measured over a full range of fuel compositions (0–100% volumetric fraction of H₂) and a wide range of equivalence ratio using Bunsen burner. High sensitivity scientific CCD camera is use to capture the image of laminar flame. The reaction zone area is employed to calculate the laminar flame speed. The initial temperature and pressure of fuel air mixtures are 293 K and 1 atm. The laminar flame speeds of hydrogen/air mixture and natural gas/air mixture reach their maximum values 2.933 and 0.374 m/s when equivalence ratios equal to 1.7 and 1.1, respectively. The laminar flame speeds of hydrogen/natural gas/air mixtures rise with the increase of volumetric fraction of hydrogen. Moreover, the increase in laminar flame speed as the volumetric fraction of hydrogen increases presents an exponential increasing trend versus volumetric fraction of hydrogen. Empirical formulas to calculate the laminar flame speeds of hydrogen, natural gas, and hydrogen/natural gas mixtures are also given. Using these formulas, the laminar flame speed at different hydrogen fractions and equivalence ratios can be calculated.     

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.     

Determination of the laminar burning velocity and the Markstein length of powder–air flames

This work deals with the determination of the laminar burning velocity and introduces the Markstein length of powder–air mixtures. A powder burner was used to stabilize laminar cornstarch–air dust flames and the laminar burning velocity was determined by means of laser Doppler anemometry. The dust concentration was varied from 0.26 to 0.38 kg m⁻³. The measured laminar burning velocities were found to be sensitive to the shape of the flame. With the same dust concentration, parabolic flames were found to have a laminar burning velocity, which was almost twice that of a planar flame (ca. 30 cm s⁻¹ for the latter as compared with ca. 54 cm s⁻¹ for the former). From this discrepancy and the flame curvature, the Markstein length could be determined. It was found to have a value of 11.0 mm. This Markstein length was subsequently used to correct the measured laminar burning velocities at various dust concentrations in order to obtain the unstretched laminar burning velocity. The unstretched laminar burning velocity lies between 15 and 30 cm s⁻¹ and is thought to be a property of the dust and of the concentration.     

Selective Diffusion during Flame Propagation and Quenching in a Porous Medium

The propagation of propane-air flames in an inert high-porosity medium with nitrogen dilution and oxygen enrichment of the mixture was studied experimentally. It is shown that variation in the nitrogen or oxygen concentration (in the gas phase) leads to a more significant variation in the flame propagation velocity than in the laminar burning velocity; with the addition of nitrogen, the rate of increase in the flame velocity with the initial pressure becomes lower and the concentration range of flame propagation becomes narrower. At the flame propagation limit, the Peclet number obtained from the laminar burning velocity of the initial mixture is not constant but depends on the fuel-to-oxidizer ratio and the nitrogen content in the mixture. The results are interpreted from a physical point of view based on the hypothesis of selective diffusion. It is shown that accounting for the effects of the Lewis numbers of the fuel and oxidizer allows flame propagation in inert porous media to be described quantitatively over wide parameter ranges using a unified relation. At the flame propagation limit, the Peclet number constructed from the laminar burning velocity taking into account these effects is a constant.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 4, pp. 50–59, July–August, 2005.     

天然气成分波动对其预混火焰传播特性的影响

利用定容燃烧弹试验平台和CHEMKIN PRO气相化学动力学软件,研究了常温常压和化学计量比下天然气成分变化对其层流燃烧速度和火焰不稳定性的影响规律。结果表明,天然气的层流燃烧速度随乙烷、丙烷和正丁烷含量的增加而上升,且乙烷的影响效果最为显著。天然气-空气火焰的不稳定性随着乙烷、丙烷和正丁烷含量的增加而降低,正丁烷对火焰综合不稳定性的抑制能力与丙烷相近,且都强于乙烷。火焰结构分析表明,天然气成分波动时H基浓度峰值的变化最为显著,天然气的层流燃烧速度与火焰中OH基和H基浓度之和的最大值之间有较强的相关性。层流燃烧速度敏感性分析和净反应速率分析表明,天然气成分变化会影响其燃烧过程中重要基元反应的进行,通过正影响的基元反应和负影响的基元反应之间的竞争,火焰中H基的浓度峰值发生变化,乙烷含量变化对H基浓度的影响最大。     

Effect of the equivalence ratio on the effectiveness of inhibition of laminar premixed hydrogen-air and hydrocarbon-air flames by trimethylphosphate

This paper reports results of an experimental study and numerical simulation of the effect of the equivalence ratio (φ = 0.6–1.6) on the burning velocity of laminar, premixed atmospheric methane-air and propane-air flames without additives and with 0.06% trimethylphosphate (TMP). The effect of the equivalence ratio (φ = 0.7–4.5) on the burning velocity of hydrogen-air flames without additives and with 0.1% TMP was studied by simulation. The experimental and simulation results show that, in hydrocarbon flames doped with TMP, the inhibition effectiveness decreases sharply with a growth in φ from 1.2–1.3 to 1.4–1.6 and in hydrogen-air flames, the inhibition effectiveness increases with a rise in φ from 1.5 to 4.5. The reactions determining the dependence of the inhibition effectiveness on the equivalence ratio were found by analyzing the flame velocity sensitivity coefficients to changes in reaction rate constants. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 2, pp. 14–22, March–April, 2008.     

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.     

Laminar burning velocity and explosion index of LPG-air and propane-air mixtures

The determination of burning velocity is very important for the calculations used in hazardous waste explosion protection and fuel tank venting, which has a direct impact on environmental protection. The scope of the present study encompass an extensive study to map the variations of the laminar burning velocity and the explosion index of LPG-air and propane-air mixtures over wide ranges of equivalence ratio (Φ = 0.7-2.2) and initial temperature (T_i = 295-400 K) and pressure (P_i = 50-400 kPa). For this purpose a cylindrical combustion bomb was developed. The reliability and accuracy of the built up facility together with the calculation algorithm are confirmed by comparing the values of the laminar burning velocity obtained for a standard fuel (propane at normal pressure normal temperature conditions, NPT) with those available in the literature. The burning velocity was determined using different models depending on the pressure history (P-t) of the central ignition combustion process at the minimum ignition energy.The data obtained for the laminar burning velocity is correlated to S_L = S_L0(T/T₀)^α(P/P₀)^β where S_L0 is the burning velocity at NPT, α and β are the temperature and pressure exponents respectively. The value of β is observed to slightly vary with the equivalence ratio for both fuels. However, propane exhibits higher pressure dependency than that of LPG. The maximum laminar burning velocity found for propane is nearly 455 mm/s at Φ = 1.1, while that for LPG is nearly 432 mm/s at 4.5% fuel percent (Φ ≈ 1.5). The maximum explosion index, commonly called the “explosion severity parameter”, is calculated from the determined laminar burning velocity and is found to be 93 bar m/s for propane, and nearly 88 bar m/s for LPG.     

Laminar flame speed measurements of dimethyl ether in air at pressures up to 10 atm

Laminar flame speed measurements of dimethyl ether/air mixtures were made at 1, 5, and 10 atm with equivalence ratios ranging from 0.7 to 1.6. All experiments were performed in a large cylindrical constant-volume bomb with optical access. A new method for converting flame images into flame radii was used. Results reported in other studies were investigated, and some explanations on the disparities found are presented. A full uncertainty analysis was performed combining precision errors from data scatter with predicted systematic errors. Uncertainties ranging between 4.2% and 8.6% were found depending on the equivalence ratio and initial pressure. Experimental results agreed well with some other spherical flame experiments and counterflow flame measurements, but were found to be much lower than PIV-based stagnation flame results. Also, two spherical flame studies deviated significantly both in magnitude and trend. Critical radii and Peclet numbers, defined by the onset of rapid flame acceleration, were recorded for all high-pressure experiments. Markstein lengths were measured and showed a decreasing trend with increasing equivalence ratio. Three different methods were used to define the laminar flame thickness, and large disparities were found between them. In this study, the modeled temperature gradient method for the definition of flame thickness is preferred over other methods. Modeling was performed with the latest version of a C3 chemical kinetics mechanism. Good agreement is seen between the experimental results and the model at all pressures. Emphasis is placed in this paper on reporting experimental uncertainties, calculated density ratios, flame temperatures, and flame radii ranges used for data analysis, and the results resolve some discrepancies seen in the literature for dimethyl ether flame speeds.     

On the threshold nature of erosive burning

A physical explanation for the existence of the threshold of erosive burning is proposed. It is shown that this type of combustion occurs when the thickness of the laminar sublayer in the turbulent boundary layer becomes smaller than the thickness of the laminar combustion zone. In this case, turbulent flame in the gas phase is formed. Relations are obtained linking the critical (threshold) velocity of the blowing flow and the critical Vilyunov number to the properties of the propellant and the gas resulting from propellant decomposition. Simple exponential dependences on the blowing velocity are found for the burning rate. The simplest representation of the erosive burning rate is obtained using the Bulgakov-Lipanov number, whose threshold value is equal to unity. A new mechanism for the occurrence of negative erosion is proposed, according to which the burning rate decreases during blowing because the boundary layer is displaced, resulting in a decrease in the heat flux from the flame zone to the solid-phase decomposition surface. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 61–71, May–June, 2008.     

Effect of biomass particle size and air superficial velocity on the gasification process in a downdraft fixed bed gasifier. An experimental and modelling study

A one-dimensional stationary model of biomass gasification in a fixed bed downdraft gasifier is presented in this paper. The model is based on the mass and energy conservation equations and includes the energy exchange between solid and gaseous phases, and the heat transfer by radiation from the solid particles. Different gasification sub-processes are incorporated: biomass drying, pyrolysis, oxidation of char and volatile matter, chemical reduction of H₂, CO₂ and H₂O by char, and hydrocarbon reforming. The model was validated experimentally in a small-scale gasifier by comparing the experimental temperature fields, biomass burning rates and fuel/air equivalence ratios with predicted results. A good agreement between experimental and estimated results was achieved. The model can be used as a tool to study the influence of process parameters, such as biomass particle mean diameter, air flow velocity, gasifier geometry, composition and inlet temperature of the gasifying agent and biomass type, on the process propagation velocity (flame front velocity) and its efficiency. The maximum efficiency was obtained with the smaller particle size and lower air velocity. It was a consequence of the higher fuel/air ratio in the gasifier and so the production of a gas with a higher calorific value.