微重力燃烧研究进展

认识燃烧过程是安全、高效、洁净地利用能源的基础. 但是, 常重力条件下的浮力对流和重力沉降使得燃烧现象变得复杂. 而微重力条件下这些影响几乎消失, 这简化了对燃烧的研究. 在加深对地面燃烧过程和载人航天器火灾安全问题的认识的推动下, 经过近半个世纪特别是最近10多年的发展, 微重力燃烧研究已经涵盖了预混气体、气体扩散、液滴、颗粒和粉尘燃烧、燃料表面的火焰传播等燃烧学科的各个领域. 研究中实现了球对称液滴燃烧、不受沉降影响的粉尘燃烧、静止或低速对流环境中的燃烧, 观察到了火球、自熄灭火焰等现象,阐明了碳黑形成中的热泳力效应、可燃极限与火焰稳定性等机理. 加深了对燃烧现象,特别是对辐射效应的理解: 在预混气体、气体扩散、液滴等多种火焰中, 除了停留时间过短引起的吹熄极限外, 还存在辐射热损失过大引起的冷熄极限, 后者只能在微重力条件下观测到. 部分研究成果已经进入教材. 而火焰在微重力下不同于常重力下的现象, 对载人航天器火灾安全具有重要意义. 考虑到我国的现实情况和国内外的研究现状, 建议将煤炭颗粒和粉尘的燃烧、与碳黑相关的机理、辐射效应、化学动力学等作为我国微重力燃烧的主要研究方向.      

Turbulent combustion modeling using a flamelet generated manifold approach——a validation study in Open FOAM

An Open FOAM based turbulence combustion solver with flamelet generated manifolds(FGMs) is presented in this paper. A series of flamelets, representative for turbulent flames, are calculated first by a one-dimensional(1D) detailed chemistry solver with the consideration of both transport and stretch/curvature contributions. The flame structure is then parameterized as a function of multiple reaction control variables. A manifold, which collects the 1D flame properties, is built from the 1D flame solutions.The control variables of the mixture fraction and the progress variable are solved from the corresponding transport equations. During the calculation, the scalar variables, e.g.,temperature and species concentration, are retrieved from the manifolds by interpolation.A transport equation for NO is solved to improve its prediction accuracy. To verify the ability to deal with the enthalpy loss effect, the temperature retrieved directly from the manifolds is compared with the temperature solved from a transport equation of absolute enthalpy. The resulting FGM-computational fluid dynamics(CFD) coupled code has three significant features, i.e., accurate NO prediction, the ability to treat the heat loss effect and the adoption at the turbulence level, and high quality prediction within practical industrial configurations. The proposed method is validated against the Sandia flame D,and good agreement with the experimental data is obtained.     

Effects of nitrates on hydrocarbon-air flames and detonations

Abstract. The subject of hydrocarbon sensitization by nitrates under conditions of a heterogeneous spray has been of interest due to its applicability in promoting ignition. To gain insight into the mechanisms of the nitrate sensitization effect, the present work was limited to vapour phase studies at elevated temperatures in order to avoid the influence of heterogeneous factors. The experiments performed included studies of flammability, flame propagation, shock ignition and detonation. The mixtures used were composed of air, hexane, and isopropyl nitrate (IPN) with IPN concentrations ranging from 0 to 100 mole % in hydrocarbon-IPN. In addition, methane and propane were also included in the flame experiments. For the shock ignition and detonation experiments, the measured ignition delay and detonation cell size had minimum values for IPN-air and maximum values for hexane-air. With increases in the IPN concentration, the ignition delay and detonation cell size fell monotonically between the values for hexane and IPN. This monotonic behaviour was explained to be the result of mixing the hydrocarbon with the more sensitive nitrate whose energetics are larger than or comparable to the hydrocarbon when mixed with air. For the slow combustion mode, the results also confirmed the monotonic behavior and showed that the lean flammability limit and the flame velocity fell between those of the hydrocarbon and IPN. Received 10 September 1999 / Accepted 27 July 2000     

Effect of swirl intensity on the flow and combustion of a turbulent non-premixed flat flame

An experiment in a turbulent non-premixed flat flame was carried out in order to investigate the effect of swirl intensity on the flow and combustion characteristics. First, stream lines and velocity distribution in the flow field were obtained using PIV (Particle Image Velocimetry) method in a model burner. In contrast with the axial flow without swirl, highly swirled air induced streamlines going along the burner tile, and its backward flow was generated by recirculation in the center zone of the flow field. In the combustion, the flame shape with swirled air also became flat and stable along the burner tile with increment of the swirl number. Flame structure was examined by measuring OH and CH radicals intensity and by calculating Damkohler number (Da) and turbulence Reynolds number (Re _T ). It appeared that luminescence intensity decreased at higher swirl number due to the recirculated flue gas, and the flat flames were comprised in the wrinkled laminar-flame regime. Backward flow by recirculation of the flue gas widely contacted on the flame front, and decreased the flame temperature and emissions concentration as thermal NO. The homogeneous temperature field due to the widely flat flame was obtained, and the RMS in the high temperature region was rather lower at higher swirl number. Consequently, the stable flat flame with low NO concentration was achieved.     

Modelling of a Bluff-Body Nonpremixed Flame using a Coupled Radiation/Flamelet Combustion Model

A coupled radiation/flamelet combustion modelling technique is applied to the simulation of a bluff-body flame. Radiation heat transfer is incorporated into the laminar flamelet model for turbulent combustion through the enthalpy defect. A new method is developed for generating flamelet library with enthalpy defect. The radiation within the flame is modelled using a raytracing approach based on the discrete transfer method. The predicted results are compared with the reported experimental data. Comparison shows that the effects of radiative heat transferr on the temperature and major species are small for the flame considered. However, a significant improvement in the prediction of OH is achieved when radiation heat transfer is included. This revised version was published online in July 2006 with corrections to the Cover Date.     

Prediction of a Turbulent Non-Premixed Natural Gas Flame in a Semi-Industrial Scale Furnace using a Radiative Flamelet Combustion Model

A mixedness-reactedness flamelet combustion model coupled with a comprehensive radiation heat transfer model based on the discrete transfer method of solution of the radiative transport equation is applied for the simulation of a 3 MW non-swirling turbulent non-premixed natural gas flame in the experimental furnace at the International Flame Research Foundation. In the calculation, turbulence is represented by the standard k − ε and a differential Reynolds-stress model. Predictions are compared with measurements of mean gas velocity, temperature, major species concentrations and incident radiation wall flux. The radiative mixedness-reactedness flamelet combustion model, irrespective of the model for turbulence, is able to reproduce the basic structure of the experimental flame, which is stabilised downstream of the burner nozzle. In the near burner region, encompassing the non-reacting lift-off zone, good quality predictions are obtained using both the turbulence models, whereas further downstream, within the combusting zone of the jet, the Reynolds-stress turbulence model generates better predictions at and about the furnace axis. The nitric oxide (NO) formation via the thermal- and prompt-NO routes was also calculated and compared with in-flame and flue-gas NO data. The measured NO level at the furnace exit is well reproduced in the calculation, however discrepancies exist near the burner where NO concentrations around the furnace axis are overpredicted.     

Investigation over the recirculation influence on the combustion of micro organic dust particles

This paper investigates the role of recircnlation and non-unity Lewis number on the combustion of organic dust particles. Since recirculation effect is more noticeable in micro-combustors, it is necessary to propose a modeling approach of this phenomenon to better simulate the performance of micro-combustors. In this research, in order to model the combustion of organic dust particles, it is assumed that the dust particles va- porize first to yield a known chemical structure which is oxidized in the gas phase, and the chemical structure of this gaseous fuel is assumed methane. To study the flame structure and solve the governing equations, it is considered that the flame structure consists of three zones titled the preheat-vaporization zone, the narrow reaction zone and finally the post flame zone. The recirculation phenomenon is evaluated by entering the exhausted heat from the post flame zone into the preheat zone. The solution is based on the follow- ing approach. First, the governing equations in each zone are nondimensionalized. Then the needed boundary and matching conditions are applied in each zone. After that, these equations and the required boundary and matching conditions are simultaneously solved with the analytical model. Consequently, the remarkable effects of recirculation and non- unity Lewis number on the combustion characteristics of the organic dust particles such as burning velocity and temperature profiles for different particle radii are obtained. The results show reasonable agreement with published experimental data.     

煤粉燃烧中NOx和SOx生成的详细反应机理模拟

采用详细化学反应机理与CFD方法耦合的煤粉燃烧数值模拟程序,计算了煤粉燃烧过 程中CO, NOx和SOx等的浓度分布特性. 通过与实验对照,验证了数值模拟方法的 可靠性,并分析了燃烧温度和煤粉细度等因素对于NO生成的影响. 结果表明温度对于NO的 生成影响较大,随着温度的增加,NO的生成浓度明显增加;对于不同煤种,可能存在一个温 度点,这时NO的生成速率将非常快;在计算范围内,温度对SO2的最终生成浓度没有 明显影响,而只影响SO2的释放过程. 煤粉细度对于NO生成有较小的影响.     

The effect of magnetic field on a microgravity single droplet combustion

The effects of magnetic field on the microgravity combustion characteristics of a single methanol droplet in homogeneous flow are numerically investigated to develop an effective magnetic control method for microgravity droplet combustion and spray combustion systems. First, governing equations of microgravity single methanol droplet combustion under a homogeneous magnetic field based on an unsteady two-dimensional, spherically symmetric model including single-step chemistry are presented. Employing numerical modeling, several combustion behaviors are calculated taking into account the effect of the unsteady magnetic field profiles at the flame front. It is found that the flame front becomes deformed and is elongated in the direction of the magnetic field due to the inhomogeneous magnetic pressure distribution at the interface between the fuel vapor phase and the oxidizer phase. This nonuniformity of magnetic pressure is caused by the transient deformation of the magnetic field with refraction of magnetic flux at the flame front due to the difference of magnetic susceptibility between the diamagnetic fuel vapor phase and the paramagnetic oxidizer phase.     

Flame/Wall Interactions: Laminar Study of Unburnt HC Formation

This work presents a numerical study dedicated to the formation of unburnt hydrocarbon. Two configurations: head-on quenching (HOQ) on a planar wall and in crevices, are considered. It is well known that they contribute for an important part to the sources of hydrocarbon (HC) emission in a combustion chamber. The aim of this work is to use laminar flame simulations (LFS) to understand how the unburnt HC are produced near walls in gasoline engine. A skeletal mechanism (29 species and 48 reactions) mimicking iso-octane combustion is used. In the HOQ configuration, the flame front propagates toward the cold wall where quenching occurs. The numerical procedure and the chemical scheme used in this study are first validated by comparisons with literature results for the 1D case. Several aspects of flame wall quenching such as oxidation of unburnt HC, wall heat flux, quench distances as well as HC families are investigated by varying parameters like wall temperature and equivalence ratio. In a second part, crevices are considered to study the impact of wall imperfections in combustion chambers. Configurations with different geometrical and thermodynamic properties are tested. It leads to a wide range of flame properties and HC production modes. When incomplete combustion occurs, total HC (fuel + HC) concentration can reach very high levels at the wall. When the crevice is not wide enough, the flame cannot propagate and the quantity of HC is smaller than in the case where the flame can propagate (but the fuel is not oxidizing). If the crevice is wide enough for the flame to propagate, HOQ occurs at the bottom of the crevice and HC accumulate in the corners. The computational results obtained in this work demonstrate the ability of LFS to reproduce incomplete combustion mechanisms that are responsible for a major part of HC production in gasoline engines.     

甲烷/煤尘复合爆炸火焰的传播特性

为揭示甲烷/煤尘复合爆炸火焰的传播机理,利用气粉两相混合爆炸实验系统,在低于甲烷爆炸下限条件下,采用高速摄影机记录火焰传播图像,通过热电偶采集火焰温度,研究了煤尘种类以及甲烷体积分数对甲烷/煤尘复合火焰传播特性的影响。结果表明:挥发分是衡量煤尘燃烧特性的主导因素;随着煤尘挥发分的升高,燃烧反应增强,火焰传播速度升高,火焰温度升高;挥发分含量差异较小时,水分含量越低,燃烧反应越剧烈;在相同条件下,焦煤的燃烧反应强度最高,其次为长焰煤,最后为褐煤;随着甲烷体积分数的增加,煤尘颗粒的燃烧可由释放挥发分的扩散燃烧转变为气相预混燃烧,燃烧反应增强,火焰传播速度和火焰温度显著升高;热辐射和热对流作用促进煤尘颗粒热解,释放挥发分进行燃烧反应,维持复合火焰的持续传播;随着混合体系中甲烷体积分数的增加,混合爆炸机制由粉尘驱动型爆炸转为气体驱动型爆炸,燃烧反应增强;甲烷/煤尘复合爆炸火焰可由未燃区、预热区、气相燃烧区、多相燃烧区和焦炭燃烧区5部分组成,湍流扰动导致燃烧介质空间分布存在差异,使得燃烧区无规则交错分布。     

The effect of oxidant flow rate on a coaxial oxy-fuel flame

The effect of oxidant flow rate on temperature, heat transfer, and NO_x formation of an oxy-fuel flame is investigated using numerical simulation. The finite volume approach is used to solve the governing equations. The realizable k-ε turbulence model and β-PDF model are adopted to simulate the turbulence-combustion interaction. There is a good agreement between the present numerical results and the reference experimental data. The exhaust temperature is decreased with decreasing the oxidant inlet flow rate. As an important result, the adiabatic flame temperature can be considered a key parameter in the oxy-fuel flame applications. Also, it is observed that increasing oxygen inlet angle causes a decrease in NO formation.     

Numerical Study of Turbulent Jet Ignition in a Lean Premixed Configuration

Direct numerical simulations (DNS) of a hot combustion product jet interacting with a lean premixed hydrogen-air coflow are conducted to fundamentally investigate turbulent jet ignition (TJI) in a three-dimensional configuration. TJI is an efficient method for initiating and controlling combustion in ultra-lean combustion systems. Fully compressible gas dynamics and species equations are solved with high order finite difference methods. The hydrogen-air reaction is simulated with a reliable detailed chemical kinetics mechanism. The physical processes involved in the TJI-assisted combustion are investigated by considering the flame heat release, temperature, species concentrations, vorticity, and Baroclinc torque. The complex turbulent flame and flow structures are delineated in three main: i) hot product jet, ii) burned-mixed, and iii) flame zones. In the TJI-assisted combustion, the flow structures and the flame features such as flame speed, temperature, and species distribution are found to be quite different than those in “standard” turbulent premixed combustion due to the existence of a high energy turbulent hot product jet. The flow structures and statistics are also found to be different than those normally seen in non-isothermal non-reacting jets.     

Study on structure of singular points of laminar flame system

IntroductionAcombustiondeviceandcombustionprocedurearenormallyverycomplex ,forsimplicityandwithoutloseofgenerality ,thephysicalmodelforacombustionissimplifiedintoalaminarflamesystem .Accordingtothethermodynamictheory[1,2 ],thelaminarflamesystemisdescribed…     

Validation of a FBC model for co-firing of hazelnut shell with lignite against experimental data

Performance of a comprehensive system model extended for modelling of co-firing of lignite and biomass was assessed by applying it to METU 0.3 MW_t Atmospheric Bubbling Fluidized Bed Combustor co-firing lignite with hazelnut shell and validating its predictions against on-line temperature and concentration measurements of O₂, CO₂, CO, SO₂ and NO along the same test rig fired with lignite only, lignite with limestone addition and lignite with biomass and limestone addition. The system model accounts for hydrodynamics; volatiles release and combustion, char combustion, particle size distribution for lignite and biomass; entrainment; elutriation; sulfur retention and NO formation and reduction, and is based on conservation equations for energy and chemical species. Special attention was paid to different devolatilization characteristics of lignite and biomass. A volatiles release model based on a particle movement model and a devolatilization kinetic model were incorporated into the system model separately for both fuels. Kinetic parameters for devolatilization were determined via thermogravimetric analysis. Predicted and measured temperatures and concentrations of gaseous species along the combustor were found to be in good agreement. Introduction of biomass to lignite was found to decrease SO₂ emissions but did not affect NO emissions significantly. The system model proposed in this study proves to be a useful tool in qualitatively and quantitatively simulating the processes taking place in a bubbling fluidized bed combustor burning lignite with biomass.     

Monte Carlo method of radiative transfer applied to a turbulent flame modeling with LES

Radiative transfer plays an important role in the numerical simulation of turbulent combustion. However, for the reason that combustion and radiation are characterized by different time scales and different spatial and chemical treatments, the radiation effect is often neglected or roughly modelled. The coupling of a large eddy simulation combustion solver and a radiation solver through a dedicated language, CORBA, is investigated. Two formulations of Monte Carlo method (Forward Method and Emission Reciprocity Method) employed to resolve RTE have been compared in a one-dimensional flame test case using three-dimensional calculation grids with absorbing and emitting media in order to validate the Monte Carlo radiative solver and to choose the most efficient model for coupling. Then the results obtained using two different RTE solvers (Reciprocity Monte Carlo method and Discrete Ordinate Method) applied on a three-dimensional flame holder set-up with a correlated-k distribution model describing the real gas medium spectral radiative properties are compared not only in terms of the physical behavior of the flame, but also in computational performance (storage requirement, CPU time and parallelization efficiency). To cite this article: J. Zhang et al., C. R. Mecanique 337 (2009).     

Upward Propagation of Very Lean Methane-Air Flames in Vertical Tubes

The upward propagation of a lean methane-air flame front in a vertical tube is investigated. The velocity of the flame front has been measured with an array of photodiodes set along the tube wall and, independently, from photographic records. A PIV system triggered by a photodiode signal has been used to measure the velocity of the flow induced by the flame front in a vertical plane through the axis of the tube. The contour of the luminous region of the flame front, assumed cylindrically symmetric, has been extracted from the recorded images. As expected, the shape and velocity of a very lean flame front, and the velocity of the fresh gas relative to the front, are similar to those of a bubble rising in the tube. The flow of the burnt gas features a region of low velocity (relative to the flame front) which enhances radiation losses and seems to play an important role in the extinction of the flame at the flammability limit. This limit is found to depend very sensitively on the temperature of the tube wall. A simple model is proposed of the flow around the axis of the tube and the combustion around the tip of the flame front. This model uses the measured gas velocity at the axis of the tube together with simplified conservation equations to compute the temperature and species concentrations along the axis for a given kinetic scheme and radiation law. The results for a single overall Arrhenius reaction and for a four-step reduced scheme, both in an optically thin gas, are in reasonable agreement with our experimental data and shed some light on the roles played by radiation losses and kinetic effects on the flammability limit measured in the standard flammability tube. The model can also be used to test other kinetic schemes and radiation laws.     

Radiation Preheating can Trigger Transition from Deflagration to Detonation

In this article the possibility of radiation heat transfer to trigger transition to detonation is studied. It is assumed that the premixed deflagration front is able to emit and the unburnt mixture to absorb radiation heat effectively. Under this assumption the ability of the flame to significantly preheat the unburnt mixture and to form a promoting temperature gradient is investigated. First, we estimate the temperature rise due to the radiation preheating of the unburnt mixture, when it is traveling through deep flame wrinkles. Subsequently, we carry out numerical simulations of premixed gaseous combustion in a tube. The simulations confirm the possibility of formation of promoting temperature gradients within flame folds and initiation of the detonation waves. They demonstrate the plausibility of the proposed mechanism of the deflagration to detonation transition.     

Stability of detonation combustion with respect to the variation in the hydrogen concentration at the supersonic nozzle inlet

Detonation combustion of hydrogen-air mixtures entering an axisymmetric convergent-divergent nozzle at a supersonic velocity is considered. The nozzle geometry does not ensure gas self-ignition; for this reason, forced ignition is used, which, under certain conditions, leads to the formation of stationary detonation combustion in the case of both uniform and nonuniform hydrogen distribution at the channel entry. The nonlinear problem of the stability of these combustion regimes against periodic disturbances of the hydrogen concentration in the oncoming flow is numerically solved. The study is performed on the basis of the two-dimensional gasdynamic Euler equations for a multicomponent reacting gas. A detailed model of chemical reactions is used.     

Entropy analysis in electrical magnetohydrodynamic(MHD) flow of nanofluid with effects of thermal radiation,viscous dissipation,and chemical reaction

The unsteady mixed convection flow of electrical conducting nanofluid and heat transfer due to a permeable linear stretching sheet with the combined effects of an electric field, magnetic field, thermal radiation, viscous dissipation, and chemical reaction have been investigated. A similarity transformation is used to transform the constitutive equations into a system of nonlinear ordinary differential equations.The resultant system of equations is then solved numerically using implicit finite difference method.The velocity, temperature, concentration, entropy generation, and Bejan number are obtained with the dependence of different emerging parameters examined. It is noticed that the velocity is more sensible with high values of electric field and diminished with a magnetic field. The radiative heat transfer and viscous dissipation enhance the heat conduction in the system. Moreover, the impact of mixed convection parameter and Buoyancy ratio parameter on Bejan number profile has reverse effects. A chemical reaction reduced the nanoparticle concentration for higher values.