Experimental and numerical study of the accuracy of flame-speed measurements for methane/air combustion in a slot burner

Measuring the velocities of premixed laminar flames with precision remains a controversial issue in the combustion community. This paper studies the accuracy of such measurements in two-dimensional slot burners and shows that while methane/air flame speeds can be measured with reasonable accuracy, the method may lack precision for other mixtures such as hydrogen/air. Curvature at the flame tip, strain on the flame sides and local quenching at the flame base can modify local flame speeds and require corrections which are studied using two-dimensional DNS. Numerical simulations also provide stretch, displacement and consumption flame speeds along the flame front. For methane/air flames, DNS show that the local stretch remains small so that the local consumption speed is very close to the unstretched premixed flame speed. The only correction needed to correctly predict flame speeds in this case is due to the finite aspect ratio of the slot used to inject the premixed gases which induces a flow acceleration in the measurement region (this correction can be evaluated from velocity measurement in the slot section or from an analytical solution). The method is applied to methane/air flames with and without water addition and results are compared to experimental data found in the literature. The paper then discusses the limitations of the slot-burner method to measure flame speeds for other mixtures and shows that it is not well adapted to mixtures with a Lewis number far from unity, such as hydrogen/air flames.     

Kinetic and radiative extinctions of spherical burner-stabilized diffusion flames

Extinction of steady, spherical diffusion flames stabilized by a spherical porous burner was investigated by activation energy asymptotics. An optically-thin radiation model was employed to study the effect of radiation on flame extinction. Four model flames with the same adiabatic flame temperature and fuel consumption rate but different stoichiometric mixture fraction and flow direction, namely the flames with fuel issuing into air, diluted fuel issuing into oxygen, air issuing into fuel, and oxygen issuing into diluted fuel, were adopted to understand the relative importance of residence time and radiation intensity. Results show that for a specified flow rate emerging from the burner, only the kinetic extinction limit at low Damköhler numbers (low residence times) exists. In the presence of radiative heat loss, extinction is promoted so that it occurs at a larger Damköhler number. By keeping the radiation intensity constant while varying the flow rate, both the kinetic and radiative extinction limits, representing the smallest and largest flow rates, between which steady burning is possible, are exhibited. For flames with low radiation intensity, extinction is primarily dominated by residence time such that the high-flow rate flames are easier to be extinguished. The opposite is found for flames suffering strong radiative heat loss. The kinetic extinction limit might occur at mass flow rates lower than what is needed to keep the flame outside of the burner and not observable. An extinction state on the radiative extinction branch can be either kinetic or radiative depending on the process.     

Nonunitary Lewis Number Effects on the Combustion of a Linear Array of Gaseous Fuel Pockets

The numerical solution for the combustion of an infinite linear array of gaseous fuel pockets in a stagnant oxidizing environment under microgravity conditions is discussed. The gas pocket combustion is described using the generalized Shvab-Zel'dovich formulation with nonunitary Lewis number. The combustion process is considered isobaric and the flow is induced by density gradients due to the heat and mass transfer processes (Stefan flow). The model is based on mass, momentum, excess enthalpy, and mixture fraction conservation equations and considers the Burke-Schumann reaction mechanism and ideal gas behavior. The thermophysical properties, except the density, are assumed constant. The finite-volume method is employed in the numerical solution, using a generalized system of coordinates. A nonstaggered grid is used and the SIMPLEC algorithm is employed to solve the modified pressure-velocity coupling. Nonunitary Lewis number and interaction effects on flame behavior and on the fuel consumption are analyzed. Results show that the nonunitary Lewis number can modify the interaction effects on gas pocket linear array combustion. During the combustion process, the flame can evolve from individual flames around each gas fuel pocket to a merged flame, surrounding the merged fuel region. However, under certain conditions, the merged flame and the merged fuel region can be broken, returning to individual flames around each gas fuel pocket.     

Numerical characterization of a premixed flame based annular microcombustor

Thermal inertia of the surrounding hardware or elaborate flow arrangement is used for external recirculation of heat in many microcombustors, increasing the weight and pressure losses. Recent research promotes hydrogen as a promising fuel for microcombustion due to its high heat of combustion. On this background, a hydrogen-fuelled microcombustor of simple construction was designed, which utilized the external thermal recirculation by a hollow nitrogen-filled tube inserted in the flame. The present paper reports stabilization and structure of a well stabilized stoichiometric H₂-air flame established in this microcombustor with the help of a detailed computational fluid dynamics model. Self-sustaining combustion could be achieved without any need for catalytic action. An asymmetric flame composed of two branches was stabilized on the walls at a location where the wall heat losses were balanced by the wall heat conduction. The flame thickness exceeded its characteristic one-dimensional value and flame zone broadened from the base to the tip due to heat losses and differential diffusion of hydrogen. Finally, the performance data for different inlet mass flow rates and wall thermal conductivities revealed useful operating points of the microcombustor for applications including micro-propulsion, heating and portable electric power generation.     

Effects of Lewis number on the burning intensity of Bunsen flames

Effects of Lewis number on the burning intensity of Bunsen flames have been elucidated for various combinations of fuel/inert gasoxygen systematically, paying attention to the effects of transport properties of the mixture such as thermal diffusivity and mass diffusivity. The mixtures studied are methane, propane, butane, ethylene, and hydrogen with air, argon-air, helium-air, and carbon dioxide-air. It is found that the burning intensity of Bunsen flames for almost all the mixtures studied can be estimated by the Lewis number especially for mixtures whose Lewis number is far from unity. However, there are some exceptions, especially for mixtures whose Lewis number is close to unity. Although the behavior of fuel/inert gasoxygen and fuel/inert gasoxygen mixtures qualitatively agrees with that of hydrocarbon/air mixture, the behavior of hydrocarbon/heliumoxygen is different from those three kinds of inert gases. Although the tip of the Bunsen flame is opened at rich hydrocarbon flames, the tip-opening of hydrogen flames always occurs in a mixture which is richer than the stoichiometric mixture but leaner than the maximum burning velocity mixture.     

Edge flame instability in low-strain-rate counterflow diffusion flames

Experiments in low-strain-rate methane-air counterflow diffusion flames diluted with nitrogen have been conducted to study flame extinction behavior and edge flame oscillation in which flame length is less than the burner diameter and thus lateral conductive heat loss, in addition to radiative loss, could be high at low global strain rates. The critical mole fraction at flame extinction is examined in terms of velocity ratio and global strain rate. Onset conditions of the edge flame oscillation and the relevant modes are also provided with global strain rate and nitrogen mole fraction in the fuel stream or in terms of fuel Lewis number. It is observed that flame length is intimately relevant to lateral heat loss, and this affects flame extinction and edge flame oscillation considerably. Lateral heat loss causes flame oscillation even at fuel Lewis number less than unity. Edge flame oscillations, which result from the advancing and retreating edge flame motion of the outer flame edge of low-strain-rate flames, are categorized into three modes: a growing, a decaying, and a harmonic-oscillation mode. A flame stability map based on the flame oscillation modes is also provided for low-strain-rate flames. The important contribution of lateral heat loss even to edge flame oscillation is clarified finally.     

A theoretical analysis of the extinction limits of a methane-air opposed-jet diffusion flame

A theoretical analysis is described for a methane-air diffusion flame stabilized in the forward stagnation region of a porous metal cylinder in a forced convective flow. The analysis includes effects of radiative heat loss from the porous metal surface and finite rate kinetics but neglects the effects of gravity. The theoretically predicted extinction limits compare well with experimentally observed extinction limits from the literature.After the predicted limits compared well with the experimental limits, a parametric study of the effect of fuel surface emissivity and Lewis number was conducted with the numerical model. It was found that the computed blowoff limit is independent of radiative heat loss for high fuel blowing velocities but is a strong function of Lewis number. At low fuel blowing velocities, the extinction limit varies with both radiative heat loss and Lewis number. It is discovered, however, that even if thermal losses from the fuel surface are absent, the flame can extinguish at the fuel surface independently of Lewis number due to excessive reaction zone thinning.     

The effect of Lewis and Damk?hler numbers on the flame propagation through micro-organic dust particles

In this study, the role of Lewis and Damköhler numbers on the premixed flame propagation through micro-organic dust particles is investigated. It is presumed that the fuel particles vaporize first to yield a gaseous fuel, which is oxidized in the gas phase. In order to simulate the combustion process, the flame structure is composed of four zones; a preheat zone, a vaporization zone, a reaction zone and finally a post flame zone, respectively. Then the governing equations, required boundary conditions and matching conditions are applied for each zone and the standard asymptotic method is used in order to solve these differential equations. Consequently the important parameters on the combustion phenomenon of organic dust particles such as gaseous fuel mass fraction, organic dust mass fraction and burning velocity with the various numbers of Lewis, Damköhler and the onset of vaporization are plotted in figures. This prediction has a reasonable agreement with experimental data of micro-organic dust particle combustion.     

The effect of stretch on cellular formation in non-premixed opposed-flow tubular flames

Cellular formation in non-premixed flames is experimentally studied in an opposed-flow tubular burner. This burner allows independent variation of the global stretch rate and overall flame curvature. In opposed-flow flames formed by 21.7% hydrogen diluted in carbon dioxide versus air, cells are formed near extinction with a low fuel Lewis number and a low initial mixture strength. Using an intensified CCD camera, the flame chemiluminescence is imaged to study cellular formation from the onset of cells to near extinction conditions. The experimental onset of cellular instability is found to be at or at a slightly lower Damköhler number than the numerically determined extinction limit based on a two-point boundary value solution of the tubular flame. For fuel Lewis numbers less than unity, concave curvature towards the fuel retards combustion and weakens the flame and convex curvature towards the fuel promotes combustion and strengthens the flame. In the cell formation process, the locally concave flame cell midsection is weakened and the locally convex flame cell ends are strengthened. With increasing stretch rate, the flame breaks into cells and the cell formation process continues until near-circular cells are formed with no concave midsection. Further increase in the stretch rate leads to cell extinction. With increasing stretch rate, the flame thickness at the cell midsection decreases similar to a planar opposed-flow flame while the flame thickness at the cell edges is unchanged and can even increase due to the strengthening effect of convex curvature at the flame edges toward the low Lewis number fuel. The results show the existence of cellular flames well beyond the two-point boundary value extinction limit and the importance of local flame curvature in the formation of flame cells.     

On a converging spherical flame front

The paper deals with the frontal propagation velocity of a spherically symmetric flame converging to a focussing center. Such a problem can describe burning in a disunited zone in the laminar theory of a turbulent flame. It is shown that at Lewis number exceeding unity, the temperature in the reaction zone increases together with propagation velocity as the front approaches the center. By contrast, at Lewis numbers below unity, propagation is slowed down and the temperature in the reaction zone increases to extinction.

The problem is solved under the assumption of strong temperature-dependence for the reaction rate.

The asymptotic approach developed in solving this problem can then be applied to investigate the normal flame propagation stability. The article intends to show that at Lewis number exceeding unity propagation is unstable and a self-oscillation regime sets in.     

燃气轮机燃料轴向分级燃烧室的数值分析

为改善燃气轮机燃烧室的火焰筒壁温较高以及污染物排放等问题,提出了在火焰筒的壁面增加二次燃料喷口的轴向分级燃烧模式。利用ANSYS CFX软件并根据化学反应机理计算和分析了燃气轮机轴向分级燃烧室的流场和温度场,并与非分级燃烧室的结果进行了比较。结果表明:增大二次燃料比例可以使火焰筒壁面温度降低、出口污染物质量分数及出口不均匀系数减小,但出口平均温度会随之降低,导致做功能力减小。过量空气系数会影响火焰筒壁温、出口平均温度与NO质量分数。合理的二次燃料比例区间取决于多个条件。     

Mixing and stabilization study of a partially premixed swirling flame using laser induced fluorescence

A laboratory-scale swirling burner, presenting many similarities with gas turbines combustors, has been studied experimentally using planar laser induced fluorescence (PLIF) on OH radical and acetone vapor in order to characterize the flame stabilization process. These diagnostics show that the stabilization point rotates in the combustion chamber and that air and fuel mixing is not complete at the end of the mixing tube. Fuel mass fraction decays exponentially along the mixing tube axis and transverse profiles show a gaussian shape. However, radial pressure gradients tend to trap the fuel in the core of the vortex that propagates axially in the mixing tube. As the mixing tube vortex enters the combustion chamber, vortex breakdown occurs through a precessing vortex core (PVC). The axially propagating vortex shows a helicoidal trajectory in the combustion chamber which trace is observed with transverse acetone PLIF. As a consequence, the stabilizing point of the flame in the combustion chamber rotates with the PVC structure. This phenomenon has been observed in the present study with a high speed camera recording spontaneous emission of the flame. The stabilization point rotation frequency tends to increase with mass flow rates. It was also shown that the coupling between the PVC and the flame stabilization occurs via mixing, explaining one possible coupling mechanism between acoustic waves in the flow and the reaction rate. This path may also be envisaged for flashback, an issue that will be more completely treated in a near future.     

Flame-sound interaction in tubes with nonslip walls

Flame interaction with sound is studied for a premixed flame propagating to the closed end of a tube with nonslip walls. The flow geometry is similar to that in the classical Searby experiments on flame-acoustic interaction [Combust. Sci. Technol. 81 (1992) 221]. The problem is solved by direct numerical simulations of the combustion equations. The flame-sound interaction strongly influences oscillations of the flame front. Particularly, sound noticeably increases the oscillation amplitude in comparison with that in an open tube with nonreflecting boundary conditions at the ends studied previously. Oscillations become especially strong in the second part of the tube, where flame pulsations are in resonance with the acoustic wave. Parameters of the flame oscillations are investigated for different values of the tube width and length. It is demonstrated that the oscillations are stronger in wider tubes, though the investigated tube width is limited by the computational facilities. In sufficiently wide tubes, violent folding of a flame front is observed because of the flame-acoustic resonance. By increasing the Lewis number, one also increases the oscillation amplitude.     

Effect of radiation losses on very lean methane/air flames propagating upward in a vertical tube

The stationary upward propagation of a very lean methane/air flame in a long vertical tube open at the bottom and closed at the top is simulated numerically using a single overall chemical reaction to model combustion and assuming an optically thin gas and a transparent or non-reflecting tube wall to approximately account for radiation losses from CO₂${\text{CO}}_2$ and H₂O${\text{H}}_2\text{O}$. Buoyancy plays a dominant role in the propagation of these flames and causes a large region of low velocity of the burnt gas relative to the flame to appear below the flame front when the equivalence ratio is decreased. The size of this region scales with the radius of the tube, and its presence enhances the effect of radiation losses, which would be otherwise negligible for a standard flammability tube, given the small concentration of radiating species. Heat conduction is found to be important in the low velocity region and to lead to a conduction flux from the flame to the burnt gas that causes extinction at the flame tip for a value of the equivalence ratio near the flammability limit experimentally measured in the standard tube. The effect of radiation losses decreases with the radius of the tube. Numerical results and order-of-magnitude estimates show that, in the absence of radiation, a very lean flame front fails to propagate only after recirculation of the burnt gas extends to its reaction region and drastically changes its structure. This condition is not realized for the standard flammability tube, but it seems to account for the flammability limit measured in a tube of about half the radius of the standard tube.     

Experimental study of spontaneous ignition induced by sudden hydrogen release through tubes with different shaped cross-sections

The shock wave dynamics, spontaneous ignition and flame variation during high-pressure hydrogen release through tubes with different cross-section shapes are experimentally studied. Tubes with square, pentagon and circular cross-section shapes are considered in the experiments. The experimental results show that the cross-section shape of the tube has no great difference on the minimum burst pressure for spontaneous ignition in our tests. In the three tubes with length of 300 mm, spontaneous ignition may occur when overpressure of shock wave is 0.9 MPa. When the spontaneous ignition is induced in a non-circular cross-section tube, the possible turbulent flow in the corner of the tube increases can promote the mixing of hydrogen and air, thus producing more amount of the hydrogen/air mixture. As a result, both the peak light signal and flame duration detected in the non-circular cross-section tubes are more intense than those in the circular tube. The smaller angle of the corner leads to a more intensity flame inside tube. When the hydrogen flame propagates to the tube exit from the circular tube, the ball-like flame developed near tube exit is relatively weak. In addition, second flame separation outside the tube is observed for the cases of non-circular cross-section tubes.     

Experimental determination of laminar burning velocities and Markstein lengths for 75% hydrous-ethanol, hydrogen and air gaseous mixtures

Hydrogen-rich mixtures generated by the on-board reforming of biomass-derived hydrous-ethanol can be used as a potential alternative fuel (i.e., reformed ethanol fuel, RE fuel). In this paper, outwardly propagating spherical flames were employed to observe the laminar flame characteristics of the gaseous mixtures composed of simulated RE fuel (mixture of 75% hydrous-ethanol and hydrogen) and air in a constant-volume combustion vessel at an initial temperature of 383 K, a pressure of 0.1 MPa, a hydrogen fraction from 0% to 80%, and an equivalence ratio from 0.6 to 1.6. The results show that the unstretched flame propagation speeds and burning velocities increase with increasing hydrogen fraction, especially when the fraction is above 40%. When the hydrogen fraction is less than 40%, the Markstein length and flame instability decrease and increase with the equivalence ratio, respectively, while the reverse holds when the hydrogen fraction is greater than 40%. At an equivalence ratio below 1.4, the Markstein length decreases with increasing hydrogen fraction, indicating a positive correlation between the flame instability and hydrogen fraction. At an equivalence ratio above 1.4, a negative relationship is observed. Finally, it is concluded that a hydrogen fraction of approximately 40% in simulated RE fuel is feasible for spark ignition engines by comparing the laminar burning characteristics of ethanol-air mixtures.     

Characteristics of liquid ethanol diffusion flames from mini tube nozzles

A series of experiments was conducted to explore the combustion characteristics of a diffusion flames from mini tubes fueled by liquid ethanol with visual observations of the flame shape, the dynamic liquid-vapor interface during phase change inside the capillary tubes and the tube outer surface temperature using CCD and IR cameras. As the fuel supply rate increased, the interface location rose to the tube exit and the temperature gradient on the outer tube surface increased, consequently the evaporating became much stronger and the interface tended to be unstable. The combustion characteristics are closely related to the rapid phase change and violent evaporation and interfacial dynamics, with the violent evaporation, actually explosive boiling, inducing an explosive flame. The intensity of the explosive flame became stronger as the flowrate increased with the maximum flame height, interface location movement, and sound intensity all significantly increasing. The periodicity of the explosive flame was directly proportional to the interface moving distance and inversely proportional to the fuel flow rate.     

The asymptotic structure of premixed tubular flames

The steady isobaric combustion of premixed tubular flames undergoing a direct one-step irreversible Arrhenius-type exothermic global reaction with a constant but general Lewis number is studied in the physically interesting limit of large activation energy. This analysis applies the combustion approximation and differs from previous asymptotic analyses of tubular flames by applying the Hirschfelder boundary condition at the burner exit for chemical species and the application of the delta-function closure scheme. The analysis yields a solution for flame sheet position, flame temperature, heat loss rate to the burner, temperature in the burned region, and stabilization limits as functions of the mass flow rate supplied to the burner and temperature of the burner surface in the near equidiffusional flame limit. Results predict the existence of dual flame behavior consistent with other investigations on the planar and cylindrical burner-stabilized flame. Two stabilization limits are identified, one for approaching flames, consistent with previous studies, and one for receding flames that has not been reported to date. Flame temperature profiles predict a nonmonotonic response, unique to the tubular flame. Consistent with the excess enthalpy of the tubular flame, the results demonstrate a strong dependence of a Lewis number differing from unity. Previous asymptotic analyses of tubular flames with a plug flow boundary condition for mass fraction are reanalyzed with the application of the delta-function closure scheme. Unlike previous results, the analysis predicts an interesting dual response for the temperature in the burned region.     

Physical Insight and Modelling for Lewis Number Effects on Turbulent Heat and Mass Transport in Turbulent Premixed Flames

The effects of global Lewis number on the behavior of Reynolds heat and mass fluxes in turbulent premixed flames are studied based on three-dimensional direct numerical simulation (DNS) of a number of statistically planar turbulent premixed flames with a global Lewis number ranging from Le = 0.34 to 1.2. For the same values of initial turbulent flow field parameters and duration of flame-turbulence interaction, it has been found that both Reynolds heat and mass fluxes may exhibit countergradient transport for flames with a Lewis number significantly smaller than unity; whereas predominantly gradient-type transport is obtained for flames with a Lewis number closer to unity. It is demonstrated that strong flame normal acceleration due to greater heat release in the low Lewis number flames acts to promote countergradient transport, and that the magnitude of the flame normal acceleration decreases with increasing Lewis number. Algebraic models for Reynolds heat and mass fluxes are proposed in which the effects of the Lewis number on flame normal acceleration are explicitly taken into account. The predictions of the new models are compared with DNS data, and the models are found to capture the influence of the Lewis number on turbulent scalar flux in a satisfactory manner for all the flames considered in this study.     

Modeling curvature effects in diffusion flames using a laminar flamelet model

The goal of this paper is to investigate the effects of curvature of mixture fraction iso-surfaces on the transport of species in diffusion flames. A general flamelet formulation is derived mathematically considering both curvature effects and differential diffusion effects. These theoretical results suggest that curvature does not play a role in the transport process irrespective of the flame curvature if species transport is described with a unity Lewis number. On the other hand, a curvature-induced term becomes explicit when differential diffusion effects are considered, and it acts as a convective term in mixture fraction space. It is found that this term needs to be taken into account when the radius of curvature is comparable or smaller than the local flame thickness. For the proper integration of the flamelet equations, the scalar dissipation rate and curvature dependences on mixture fraction are modeled by considering two basic curved one-dimensional flame configurations. The flamelet equations accounting for curvature effects are solved with various prescribed curvature values. Results indicate that the mass fraction profiles of species with very small or large Lewis numbers are shifted significantly in mixture fraction space by the inclusion of curvature. Differential diffusion effects are enhanced by negative curvature values and suppressed by positive curvature values. In cases where flame curvature is not uniform, the curvature-induced convective term generates gradients along mixture fraction iso-surfaces, which enhance tangential diffusion effects. Budget analysis is performed on an axisymmetric laminar coflow diffusion flame to highlight the importance of the curvature-induced convective term compared to other terms in the full flamelet equation. A comparison is made between full chemistry simulation results and those obtained using planar and curved flamelet-based chemistry tabulation methods.