The effect of a perforated plate on the propagation of laminar hydrogen flames in a channel – A numerical study

Laminar hydrogen flame propagation in a channel with a perforated plate is investigated using 2D reactive Navies-Stokes simulations. The effect of the perforated plate on flame propagation is treated with a porous media model. A one step chemistry model is used for the combustion of the stoichiometric H₂–air mixture. Numerical simulations show that the perforated plate has considerable effect on the flame propagation in the region downstream from the perforated plate and marginal effect on the upstream region. It is found to squeeze the flame front and result in a ring of unburned gas pocket around the flame neck. The resulting abrupt change in flow directions leads to the formation of some vortices. Downstream of the perforated plate, a wrinkled “M”-shape flame is observed with “W” shape flame speed evolution, which lastly turns back to a convex curved flame front. Parametric studies have also been carried out on the inertial resistance factor, porosity, perforated plate length and its location to investigate their effects on flame evolution. Overall, for parameter range studied, the perforated plate has an effect of reducing the flame speed downstream of it.     

Experimental investigation on the self-acceleration of 10%H2/90%CO/air turbulent expanding premixed flame

The self-acceleration characteristics of a syngas/air mixture turbulent premixed flame were experimentally evaluated using a 10% H₂/90% CO/air mixture turbulent premixed flame by varying the turbulence intensity and equivalence ratio at atmospheric pressure and temperature. The propagation characteristics of the turbulent premixed flame including the variation in the flame propagation speed and turbulent burning velocity of the syngas/air mixture turbulent premixed flame were evaluated. In addition, the effect of the self-acceleration characteristics of the turbulent premixed flame was also evaluated. The results show that turbulence gradually changes the radius of the premixed flame from linear growth to nonlinear growth. With the increase of turbulence intensity, the formation of a cellular structure of the flame front accelerated, increasing the flame propagation speed and burning speed. In the transition stage, the acceleration exponent and fractal excess of the turbulent premixed flame decreased with increasing equivalence ratio and increased with increasing turbulence intensity at an equivalence ratio of 0.6. The acceleration exponent was always greater than 1.5.     

Large eddy simulations and rotational CARS/PIV/PLIF measurements of a lean premixed low swirl stabilized flame

This paper reports on numerical and experimental studies of a lean premixed low swirl stabilized methane/air flame. The burner is made up of a central perforated plate and an annular swirler. A premixed methane/air mixture at an equivalence ratio of 0.62 is injected to an ambient co-flow of air through the burner under atmospheric pressure and room temperature condition with a Reynolds number of 30,000. Stereoscopic Particle Image Velocimetry (PIV) and simultaneous OH/acetone Planar Laser Induced Fluorescence (PLIF) are used to characterize the flame front and the turbulence field downstream of the burner. The flame is stabilized in the low speed central region and in the inner shear-layer vortices, where ambient air dilution to the flame is found to eventually quench the reactions downstream. Rotational Coherent Anti-Stokes Raman Spectroscopy (RCARS) measurements are carried out to characterize the temperature field and the relative oxygen mole fraction field, which enables quantification of the air dilution to the flame. The experimental data provides a challenging test case for numerical simulation models owing to the stratification of the mixture and quenching of the flame. Large eddy simulations are carried out using a three-scalar level-set G-equation flamelet model, which is shown to capture the basic flame characteristics and quenching at the trailing edge of the flame.     

Turbulent flow with combustion in a moving bed

This paper presents a mathematical model for treating turbulent combusting flows in a moving porous bed, which might be useful to design and analysis of modern and advanced biomass gasification systems. Here, one explicitly considers the intra-pore levels of turbulent kinetic energy and the movement of the rigid solid matrix is considered to occur at a steady speed. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. The rate of fuel consumption is described by an Arrhenius expression involving the product of the fuel and oxidant mass fractions. Results indicate that fixing the gas speed and increasing the speed of the solid matrix pushes the flame front towards the end of the reactor. Also, since the rate of production of turbulence is dependent on the relative velocity between phases, as the solid velocity approaches that of the gas stream, the level of turbulence in the flow is reduced.     

用光学纹影摄影术观察分析定容燃烧室内氢燃料的燃烧过程

论述了采用纹影摄影术和高速摄影法观察分析氢气和空气预混合燃料在定容燃烧室内的火花点火燃烧过程,定性地分析了预混合氢气燃料的火焰形态和变化过程,以及燃烧室内的初始压力和空燃比对火焰传播速度及其燃烧压力的影响,通过采用纹影摄影术方法,初步揭示了预混合氢气燃料在定容燃烧室内燃烧时火焰初期紊流产生的机理,以及由开始的层流状火焰发展到湍流状火焰的过程,研究结果表明,预混合氢气燃料燃烧的火焰传播速度及燃烧压力明显地受燃烧室内的初始压力和空燃比的影响。     

Numerical study on laminar flame velocity of hydrogen-air combustion under water spray effects

In the context of hydrogen safety and explosions in hydrogen-oxygen systems, numerical simulations of laminar, premixed, hydrogen/air flames propagating freely into a spray of liquid water are carried out. The effects on the flame velocity of hydrogen/air flames of droplet size, liquid-water volume fraction, and mixture composition are numerically investigated. In particular, an effective reduction of the flame velocity is shown to occur through the influence of water spray.To complement and extend the numerical results and the only scarcely available experimental results, a “Laminar Flame Velocity under Droplet Evaporation Model” (LVDEM) based on an energy balance of the overall spray-flame system is developed and proposed. It is shown that the estimation of laminar flame velocities obtained using the LVDEM model generally agrees well with the experimental and numerical data.     

Experimental analysis of the effects of porous wall on flame stability and temperature distribution in a premixed natural gas/air combustion

In the present analysis, the flame stabilization and temperature distribution within a premixed burner contain porous wall are studied experimentally. The effects of inner diameter, length, and pore density of the porous wall, thermal load, equivalence ratio, and the inlet velocity of the fuel‐air mixture on these are studied. The fuel used in this study is natural gas and the porous wall is SiC (silicon carbide) ceramic foam. The experimental results clearly indicate that the axial temperature along the porous wall increases when the inner diameter of the porous wall decreases and its length increases. The porous wall temperature with an inner diameter of 40 mm, length of 66 mm, and pore density of 30 PPI (pores per inch) has the highest temperature among the examined states. The results of studying the effect of the porous wall on flame stability show that the flame stability limit has a direct relationship with the length and pore density of porous wall and an inverse relationship with the inner diameter of the porous wall. Also, it is found that the porous wall has the highest temperature causes the maximum flame stability limit.     

Investigations of swirl flames in a gas turbine model combustor: II. Turbulence–chemistry interactions

The thermochemical states of three swirling CH₄/air diffusion flames, stabilized in a gas turbine model combustor, were investigated using laser Raman scattering. The flames were operated at different thermal powers and air/fuel ratios and exhibited different flame behavior with respect to flame instabilities. They had previously been characterized with respect to their flame structures, velocity fields, and mean values of temperature, major species concentrations, and mixture fraction. The single-pulse multispecies measurements presented in this article revealed very rapid mixing of fuel and air, accompanied by strong effects of turbulence–chemistry interactions in the form of local flame extinction and ignition delay. Flame stabilization is accomplished mainly by hot and relatively fuel-rich combustion products, which are transported back to the flame root within an inner recirculation zone. The flames are not attached to the fuel nozzle, and are stabilized approximately 10 mm above the fuel nozzle, where fuel and air are partially premixed before ignition. The mixing and reaction progress in this area are discussed in detail. The flames are short (<50 mm), especially that exhibiting thermoacoustic oscillations, and reach a thermochemical state close to adiabatic equilibrium at the flame tip. The main goals of this article are to outline results that yield deeper insight into the combustion of gas turbine flames and to establish an experimental database for the validation of numerical models.     

Laminar flame propagation of atmospheric iso-cetane/air and decalin/air mixtures

Laminar flame speeds of iso-cetane/air and decalin/air mixtures were measured in the counterflow configuration at atmospheric pressure and an elevated unburned mixture temperature of 443 K. Axial flow velocities were measured along the stagnation streamline using the digital particle image velocimetry. The laminar flame speeds were determined by determining the variation of a reference flame speed as a function of strain rate and computationally assisted non-linear extrapolations. The data are the first to be reported in the literature, and they were modeled using a recently developed kinetic model that includes 187 species and 6086 elementary reactions. In general, the computed results were found to be in close agreement with the data. In order to get insight into kinetic effects on flame propagation, detailed sensitivity and reaction path analyses were performed using the computed flame structures. The results revealed that at the same equivalence ratio, laminar flame speeds of iso-cetane/air mixtures are lower than those of n-hexadecane/air mixtures. Additionally, it was found that the laminar flame speeds of iso-cetane/air and decalin/air mixtures are sensitive largely to C₀–C₄ kinetic subset, and that the lower reactivity of iso-cetane compared to n-hexadecane could be attributed to the higher production of relatively stable intermediates.     

Fluid dynamic-chemical interactions at the lean flammability limit

Laminar flame propagation at the lean flammability limit is analyzed in terms of a four step kinetic model that accounts for the competition of the chain branching reaction H + O₂ → OH + O with the chain breaking reaction H + O₂ + M → HO₂ + M. Assuming large activation energies for the chain-branching reaction, the structure of the reactive-diffusive zone is derived. It consists of a thin fuel consumption layer where radicals are formed and an even thinner radical consumption layer. Within the fuel consumption layer the radical profiles are described by the steady state approximation. The radical consumption layer lies ahead of the fuel consumption layer toward the burned mixture at the position where the rate of the chain breaking reaction equals that of the chain-branching reaction. When the temperature decreases by a lowering of the fuelair ratio, the radical consumption layer moves toward the burned gas and takes all radicals out of the reaction zone. This leads to a vanishing flame velocity. Thus the kinetic model describes chemical extinction. The same effect is produced by a negative temperature perturbation for flames close to the flammability limit. It is shown how those flames will eventually be extinguished by flame stretch or heat loss.     

Rapidly mixed combustion of hydrogen/oxygen diluted by N2 and CO2 in a tubular flame combustor

In this study hydrogen flames have been attempted in a rapidly mixed tubular flame combustor for the first time, in which fuel and oxidizer are individually and tangentially injected into a cylindrical combustor to avoid flame flash back. Three different cases were designed to examine the effects of fuel and oxidizer feeding method, diluent property, oxygen content and equivalence ratio on the characteristics of hydrogen flame, including the flame structure, lean extinction limit, flame stability and temperature. The results show that by enhancing mixing rate through feeding system, the range of equivalence ratio for steady tubular flame can be much expanded for the N₂ diluted mixture, however, at the oxygen content of 0.21 (hydrogen/air) the steady tubular flame is achieved only up to equivalence ratio of 0.5; by decreasing oxygen content, the lean extinction limit slightly increases, and the upper limit for steady tubular flame establishment increases significantly, resulting in an expanded tubular flame range. For CO₂ diluted mixture, the stoichiometric combustion has been achieved within oxygen content of 0.1 and 0.25, for which the burned gas temperature is uniformly distributed inside the flame front; as oxygen content is below 0.21, a steady tubular flame can be obtained from the lean to rich limits; and the lean extinction limit increases from 0.17 to 0.4 as oxygen content decreases from 0.21 to 0.1, resulting in a shrunk tubular flame range. Laminar burning velocity, temperature and Damköhler number are calculated to examine the differences between N₂ and CO₂ diluted combustion as well as the requirement for hydrogen-fueled tubular flame establishment.     

Prediction of autoignition in a lifted methane/air flame using an unsteady flamelet/progress variable model

An unsteady flamelet/progress variable (UFPV) model has been developed for the prediction of autoignition in turbulent lifted flames. The model is a consistent extension to the steady flamelet/progress variable (SFPV) approach, and employs an unsteady flamelet formulation to describe the transient evolution of all thermochemical quantities during the flame ignition process. In this UFPV model, all thermochemical quantities are parameterized by mixture fraction, reaction progress parameter, and stoichiometric scalar dissipation rate, eliminating the explicit dependence on a flamelet time scale. An a priori study is performed to analyze critical modeling assumptions that are associated with the population of the flamelet state space.For application to LES, the UFPV model is combined with a presumed PDF closure to account for subgrid contributions of mixture fraction and reaction progress variable. The model was applied in LES of a lifted methane/air flame. Additional calculations were performed to quantify the interaction between turbulence and chemistry a posteriori. Simulation results obtained from these calculations are compared with experimental data. Compared to the SFPV results, the unsteady flamelet/progress variable model captures the autoignition process, and good agreement with measurements is obtained for mixture fraction, temperature, and species mass fractions. From the analysis of scatter data and mixture fraction-conditional results it is shown that the turbulence/chemistry interaction delays the ignition process towards lower values of scalar dissipation rate, and a significantly larger region in the flamelet state space is occupied during the ignition process.     

A comparative experimental and computational study of methanol, ethanol, and n-butanol flames

Laminar flame speeds and extinction strain rates of premixed methanol, ethanol, and n-butanol flames were determined experimentally in the counterflow configuration at atmospheric pressure and elevated unburned mixture temperatures. Additional measurements were conducted also to determine the laminar flame speeds of their n-alkane/air counterparts, namely methane, ethane, and n-butane in order to compare the effect of alkane and alcohol molecular structures on high-temperature flame kinetics. For both propagation and extinction experiments the flow velocities were determined using the digital particle image velocimetry method. Laminar flame speeds were derived through a non-linear extrapolation approach based on direct numerical simulations of the experiments. Two recently developed detailed kinetics models of n-butanol oxidation were used to simulate the experiments. The experimental results revealed that laminar flame speeds of ethanol/air and n-butanol/air flames are similar to those of their n-alkane/air counterparts, and that methane/air flames have consistently lower laminar flame speeds than methanol/air flames. The laminar flame speeds of methanol/air flames are considerably higher compared to both ethanol/air and n-butanol/air flames under fuel-rich conditions. Numerical simulations of n-butanol/air freely propagating flames, revealed discrepancies between the two kinetic models regarding the consumption pathways of n-butanol and its intermediates.     

Numerical investigation of turbulent combustion with hybrid enrichment by hydrogen and oxygen

In this study, the NG + H₂/air + O₂ turbulent flame is numerically investigated using the Computational Fluid Dynamics CFD code. The modulation of combustion and radiation is performed respectively by the Eddy Dissipation Model and the Discrete Ordinate Model. The turbulence modeling is carried out by Shear Stress Transport (SST/k-ω) turbulence model. The H₂ amount in the fuel mixture varies under constant volumetric fuel flow between 0 and 60% and the oxidant is composed by 80% air and 20% pure oxygen. The results obtained show the hydrogen addition to Natural Gas improves the mixing between the reactants, reduces their residence time and reduces the length and thickness of the flame. On the other hand, the hydrogen enrichment minimizes the CO₂ and CO production and increases the NO_x level.     

高温高压下掺氢天然气的燃烧特性

在定容燃烧弹内研究了高温(450,K)、高压(0.75,MPa)条件下天然气-氢气-空气混合气的火焰传播过程,获得了不同掺氢比和不同当量比下掺氢天然气的无拉伸层流燃烧速率,并分析了火焰的稳定性。结果表明,高温高压下随着掺氢比的增加,掺氢天然气的燃烧速率增加,且增长速率逐渐加快;马克斯坦长度则随着掺氢比的增加而减小,即火焰的稳定性下降。随着当量比的增加,无拉伸层流燃烧速率呈现先增大后减小的趋势,且最大无拉伸层流燃烧速率所对应当量比的位置随着掺氢比的提高而向浓混合气移动;马克斯坦长度随当量比的增加而增大,即火焰稳定性随当量比的增加而提高。     

Analysis of turbulent combustion in inert porous media

The objective of this paper is to present an extension of a simplified reaction kinetics model that, combined with a thermo-mechanical closure, entails a full-generalized turbulent combustion model for flow in porous media. In this model, one explicitly considers the intra-pore levels of turbulent kinetic energy. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. The rate of fuel consumption is described by an Arrhenius expression involving the product of the fuel and oxidant mass fractions. These mass fractions are double decomposed in time and space and, after applying simultaneous time-and-volume integration operations to them, distinct terms arise, which are here associated with the mechanisms of dispersion and turbulence. Modeling of these extra terms remains an open question and the derivations herein might motivate further development of models for turbulent combustion in porous media.     

Investigation of the structural and reactants properties on the thermal characteristics of a premixed porous burner

Porous burners offer attractive features such as competitive combustion efficiency, high power ranges, and lower pollutant emissions. In the present study, the thermal characteristics of a porous burner are numerically investigated for a range of operating conditions and design specifications within a practical range. The premixed flame propagation of a methane/air mixture in a ceramic porous medium is simulated through an unsteady, one-dimensional model. The combustion process is modeled using a suitable single-step chemical kinetics. The reaction location is not predetermined, thus the flame is allowed to float within the solid matrix or to run off from either side of the porous medium. The numerical results indicate that flame stability and thermal characteristics of the burner are strongly dependent on the inlet mixture specifications and the solid matrix structural properties. For a fixed value of the inlet firing rate, the combustion products temperature will increase by an increase in the inlet gas temperature, an increase in the matrix porosity, or by a decrease of the matrix pore density. Among the geometrical properties, the burner length has virtually no effect on the burner performance. An increase in the solid matrix porosity or burner firing rate will increase the efficiency of the preheating zone, while increasing the inlet gas temperature or matrix pore density will cause a reduction in this efficiency. Simulation results also suggest that in order to prevent flame blow-out or flash-back, critical values of the burner settings and design parameters must be avoided.     

初始温度对氢-空气预混诱导湍流燃烧特性的影响

通过在火焰传播路径上布置孔板实现诱导湍流燃烧,利用纹影技术和压力采集系统研究了初始温度对孔板诱导氢-空气预混湍流燃烧特性的影响。试验结果表明:穿越孔板前火焰传播速度略有下降,穿越孔板后火焰被诱导为湍流燃烧,火焰发展进程加快;随着初始温度的升高,最高燃烧压力和最大压升率减少,两者出现的时刻提前,添加孔板后的燃烧持续期变化率降低,但穿越孔板后的火焰传播速度的差异不显著。     

多孔介质中预混火焰猝熄及自稳定性研究

分析了多孔介质中预混火焰的猝熄效应,试验测定了一系列工况下泡沫陶瓷的猝熄直径和自稳定范围,为多孔介质燃烧器的开发设计提供了依据。通过分析发现,猝熄直径受到多个参数的影响,包括：混合气体的流速u、预混气体的层流火焰传播速度SL、燃烧室空管Re、预混气体的导温系数a、当量比φ以及多孔介质固体温度Ts。通过对多孔介质中燃烧的自稳定性试验研究,发现了多孔介质燃烧器中火焰稳定极限(吹脱极限和回火极限)与多孔介质平均孔径和气流速度及燃烧当量比的关系。     

Experimental and numerical investigation of premixed acetylene flame

In this paper, the mean velocity, turbulence intensity and temperature profiles in different cross-sections of premixed acetylene flame are given. A mathematical model for prediction of velocity, temperature and concentration fields of axisymmetric free premixed turbulent flame is presented in this paper. A second-order closure for turbulent reacting flows is used. Special attentions is paid to model behavior with the respect to the prediction correlation coefficients of turbulent diffusion of the scalar components. Conditional and unconditional statistics of the LDA signals were performed using jet and/or air seed. Compared to commonly used unconditional statistics, conditional statistics of velocity fluctuations can give us more data about intensity of turbulent mixing in the flame.