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.     

Experimental study on the laminar flame speed of hydrogen/carbon monoxide/air mixtures

Carbon monoxide and hydrogen are two important components in the syngas. In this study, the laminar flame speed of hydrogen/carbon monoxide fuel mixtures is measured over a large range of fuel compositions (0-100% volume fraction for hydrogen in the mixture) by using a Bunsen burner. The reaction zone area is used to calculate the laminar flame speed. The equivalence ratio covers from lean conditions to rich conditions. The experimental results show that by using the Bunsen flame, the laminar flame speed calculated with the reaction zone area is reliable. Based on the experimental results, empirical equations are derived which can be readily employed to calculate the laminar flame speeds of hydrogen, carbon monoxide, and hydrogen/carbon monoxide mixtures.     

Measurement and analysis of ion distribution in laminar flame using an electrostatic probe

An electrostatic probe is utilized to determine the ion distribution in a laminar flame. The probe is developed to be implemented in the ion measurement of combustion process contained in a solid container, and therefore the probe fabrication and the construction of a whole measurement system are explained in detail. Also, a numerical analysis is conducted to compare experimental measurement with the computed temperature and concentration. The ion distribution of four different flames of various fuel/air ratios is measured to analyze the distribution for understanding the combustion process. An explicit method is implemented to solve a set of partial differential equations of material and energy balances. Comparison between the measured distribution and the computed distribution of temperature indicates that the ion formation is closely related to the temperature distribution.     

甲醇-空气层流火焰速度的数值研究和预测模型

收集总结分析了现有的甲醇层流火焰速度的实验数据,比较了三种典型的描述甲醇氧化的详细化学反应机理(Li机理、USC Mech-Ⅱ和Burke机理)对层流火焰传播速度的预测精度。结果表明三种机理均能定性反映甲醇层流火焰速度的变化规律,但在富燃料侧,机理计算值明显高于实验结果。反应动力学分析表明甲醇脱氢反应对层流火焰速度的影响至关重要。根据文献中的最新成果,修正了Li机理中甲醇脱氢反应的速率常数,提高了Li机理对甲醇-空气层流火焰速度的预测精度。针对工程需求,给出了两个甲醇层流火焰速度的快速预测模型,经过校核,所提出的预测模型能够较为准确地预测不同初始温度和压力下甲醇层流火焰速度。     

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.     

Gas-phase synthesis of nanoparticles: scale-up and design of flame reactors

The scale-up and design of flame aerosol reactors are investigated for synthesis of silica and titania nanoparticles. In specific, coflow burners of different dimensions are studied at various precursor, fuel, and oxygen flow rates. The influence of the flame enthalpy content on product primary particle size is investigated by changing the fuel from methane to propane or hydrogen. Operation lines relating product particle size with reactant outlet conditions, burner size, and flame enthalpy are developed, showing how the different reactors can produce silica or titania nanoparticles of the same size. A scale-up procedure developed for fumed silica is extended to the synthesis of titania nanoparticles covering production rates of 2-200 g/h. At high fuel-oxidant velocity difference at the burner outlet, the operation of diffusion flame reactors converges to that of premixed ones.     

Foundations of laminar chaotic mixing and spectral theory of linear operators

This article addresses the potentiality and the range of application of the spectral theory of mixing as a simple and objective way to define in a rigorous way mixing performance and stirring efficiency. Attention is mainly oriented to the class of laminar flows which is the typical condition occurring in micromixers and flow microdevices. The application of the theory is highlighted by considering the cavity flow as a case study.     

Numerical study and acceleration of LBM-RANS simulation of turbulent flow

The coupled models of LBM(Lattice Boltzmann Method) and RANS(Reynolds-Averaged Navier–Stokes) are more practical for the transient simulation of mixing processes at large spatial and temporal scales such as crude oil mixing in large-diameter storage tanks. To keep the efficiency of parallel computation of LBM, the RANS model should also be explicitly solved; whereas to keep the numerical stability the implicit method should be better for RANS model. This article explores the numerical stability of explicit methods in 2D cases on one hand, and on the other hand how to accelerate the computation of the coupled model of LBM and an implicitly solved RANS model in 3D cases. To ensure the numerical stability and meanwhile avoid the use of empirical artificial limitations on turbulent quantities in 2D cases, we investigated the impacts of collision models in LBM(LBGK, MRT)and the numerical schemes for convection terms(WENO, TVD) and production terms(FDM, NEQM) in an explicitly solved standard k–ε model. The combination of MRT and TVD or MRT and NEQM can be screened out for the 2D simulation of backward-facing step flow even at Re = 10~7. This scheme combination, however, may still not guarantee the numerical stability in 3D cases and hence much finer grids are required, which is not suitable for the simulation of industrial-scale processes. Then we proposed a new method to accelerate the coupled model of LBM with RANS(implicitly solved). When implemented on multiple GPUs, this new method can achieve 13.5-fold acceleration relative to the original coupled model and 40-fold acceleration compared to the traditional CFD simulation based on Finite Volume(FV) method accelerated by multiple CPUs. This study provides the basis for the transient flow simulation of larger spatial and temporal scales in industrial applications with LBM–RANS methods.     

Laminar flame speeds and extinction limits of conventional and alternative jet fuels

Experimental results of laminar flame speeds and extinction stretch rates for the conventional (Jet-A) and alternative (S-8) jet fuels are acquired and compared to the results from our earlier studies for neat hydrocarbon surrogate components, including n-decane and n-dodecane. Specifically, atmospheric pressure laminar flame speeds are measured using a counterflow twin-flame configuration for Jet-A/O₂/N₂ and S-8/O₂/N₂ mixtures at preheat temperatures of 400, 450, and 470 K and equivalence ratios ranging from 0.7 to 1.4. The flow field is recorded using digital particle image velocimetry. Linear extrapolation is then applied to determine the unstretched laminar flame speed. Experimental data for the extinction stretch rates of the nitrogen diluted jet fuel/oxidizer mixtures as a function of equivalence ratio are also obtained. In addition, the experimental data of Jet-A are compared to the computed values using a chemical kinetic mechanism for a kerosene surrogate reported in literature. A sensitivity analysis is further performed to identify the key reactions affecting the laminar flame speed and extinction stretch rate for this kerosene surrogate.     

Experimental evaluation of flame and flamelet spread over cellulosic materials using the narrow channel apparatus

Originally conceived as an apparatus to study near‐limit flames and their breakup into flamelets and later modified to function as a microgravity simulation apparatus, the narrow channel apparatus serves also as a facility for examining long time flame spread and material flammability in on‐earth (terrestrial) applications. These applications include flame spread in narrow gaps, persistence of flame in heat‐loss environments, and flame‐to‐flamelet front transition. The narrow channel apparatus tests described here measure behavior of the spreading flame and features of the flame‐to‐flamelet transition. Measured quantities include flow, flame and flamelet velocities in normal and inverted tests, flow deceleration and acceleration rates with associated flame or flamelet response, flame‐to‐flamelet transition times, and influences of fuel thickness. The principal goal of this research was to ascertain the capacity of the narrow channel apparatus to produce data for phenomena observed in both (1) simulated microgravity flame spread and (2) terrestrial flame spread in narrow gaps and channels. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

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.     

Image analysis of flame behavior for polyolefins and polystyrene in vertical flame test

Although the UL-94 vertical flame test is often used to evaluate the flammability of polymers based on ignition time and combustion duration parameters, a significant amount of information regarding the time course of combustion is difficult to analyze in detail. Herein, image analysis of the time course of the upper and lower end heights, total area, and color division of flame was performed for polyolefins and polystyrene with different molecular weights in the vertical flame test. The combustion process was classified into two stages: before and after the first drip. In the first stage, the upward spread velocity, oscillation, and color of flame were analyzed. It was assumed that the difference in the fuel production rates or thermal decomposition products depends on the molecular structure. In the second stage, the melt flowability, flame position, and combustion continuity differed vastly depending on the molecular structure or molecular weight.     

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.     

Effects of hydrogen addition on flame structure and forced flame response to velocity modulation in a turbulent lean premixed combustor

Flame transfer function measurements were performed in a turbulent premixed lean combustor with various blends of hydrogen and natural gas. The fuel mixture was completely premixed with air upstream of a choked inlet to the combustor to avoid equivalence ratio fluctuations. A variable speed siren was used to modulate fluctuations in the inlet velocity, which was measured using a hot wire anemometer as an input parameter of the flame transfer function. Heat release oscillations as an output function were determined using chemiluminescence measurements from whole flames. Stable flame images were captured to understand general flame behavior over a range of operating conditions and fuel blends. Experimental results showed that the stable flames’ COMs (centers of mass) laid along a common path in a 2-D plane for all of the operating conditions and tested fuel compositions at a given injector geometry, and that variations in the stable flame shape could be characterized by the location of the common path of the flame’s COM. It was also shown that changes in the fuels significantly affected the flame shape; as a result, flame dynamics varied with changes in flame geometry. Accordingly, flames that were close together on the characteristic flame COM curve were shown to have similar forced flame responses.     

Effects of pressure and inlet temperature on coaxial gaseous methane/liquid oxygen turbulent jet flame under transcritical conditions

This study has investigated numerically turbulent flames of cryogenic oxygen and methane under supercritical pressures relevant to liquid propellant rocket engines. A real-fluid version of the flamelet equations is employed to accommodate simultaneously non-equilibrium chemistry of hydrocarbon fuel and non-ideal thermodynamics in local flame structures while the effect of turbulent fluctuations is accounted for via a presumed probability density functions. The present model reproduced qualitatively well the experimentally observed unique feature of a transcritical flame of coaxial gaseous methane/liquid oxygen injector, which is characterized by sudden flame expansion, abruptly terminated flame tip, and expansion induced flow recirculation. Numerical results reveal that pseudo-boiling phenomena occurred in the transcritical mixing layer between the cryogenic oxygen core and the surrounding hot gas play a crucial role in mixing and combustion processes. It is also found that the transcritical flame structure is drastically affected by elevating the chamber pressure or increasing the oxygen inlet temperature in terms of flame length, sudden expansion angle, and reverse flow strength. Detailed discussions are made for effects of the real-fluid behaviors on the turbulent flame field as well as on the local flame structure in mixture fraction space.     

氨气/甲烷贫预混旋转湍流火焰稳定性及NO生成

为了研究氨气/甲烷掺混燃气在贫预混旋转湍流状态下的火焰稳定性及NO的排放特性,设计建造了一个可视化的旋转湍流燃烧装置,开展了一系列的实验测量研究。研究表明：随着当量比增大,氨气火焰稳定燃烧的范围有所扩大,但当氨气掺混比大于0.60时火焰出现上下振荡现象,继续增加将导致火焰吹熄;NO的排放水平随当量比增加而提高;但在相同的当量比下,NO的排放随氨气掺混比的增加先升高再下降。此外,分别采用化学反应器网络（CRN）方法和一维层流预混火焰计算方法,对相应的火焰状态进行了数值计算分析,虽然计算结果与实验结果误差较大,但其预测的NO排放特性随氨气掺混比、当量比的变化趋势是一致的,对三者之间误差的来源进行了分析。     

磷酸酯阻燃剂市场现状分析及展望

介绍了几种磷酸酯阻燃剂,包括烷基磷酸酯、缩合磷酸酯、苯基磷酸酯、环状磷酸酯、笼型及螺状磷酸酯的国内外开发进展,分析了阻燃剂市场特点和现状并对今后国内阻燃剂的发展方向和规模作出展望。     

氨气/甲烷贫预混旋转湍流火焰稳定性及NO生成

为了研究氨气/甲烷掺混燃气在贫预混旋转湍流状态下的火焰稳定性及NO的排放特性,设计建造了一个可视化的旋转湍流燃烧装置,开展了一系列的实验测量研究。研究表明：随着当量比增大,氨气火焰稳定燃烧的范围有所扩大,但当氨气掺混比大于0.60时火焰出现上下振荡现象,继续增加将导致火焰吹熄;NO的排放水平随当量比增加而提高;但在相同的当量比下,NO的排放随氨气掺混比的增加先升高再下降。此外,分别采用化学反应器网络（CRN）方法和一维层流预混火焰计算方法,对相应的火焰状态进行了数值计算分析,虽然计算结果与实验结果误差较大,但其预测的NO排放特性随氨气掺混比、当量比的变化趋势是一致的,对三者之间误差的来源进行了分析。