The effect of monodispersed water mists on the structure, burning velocity, and extinction behavior of freely propagating, stoichiometric, premixed, methane-air flames

We present a computational model to describe the two-phase thermal and chemical interactions between a freely propagating premixed flame and fine droplets of water. The objective is to develop a fundamental understanding of flame structure and extinction in the presence of a water mist. The model and the computational algorithm must accommodate strong coupling between the droplet dynamics and the gaseous flow. The gas-phase conservation equations, which include elementary chemistry, are discretized and solved on an adaptive Eulerian mesh, while the droplet dynamics are represented in a Lagrangian framework. A modified arclength-continuation method is used to follow the solutions through the extinction turning point and thus predict flame-extinction limits. The model predicts how burning velocity and extinction conditions depend on droplet size and number density. The results compare very favorably with previously published theoretical analyses.     

Extinction limits and associated uncertainties of nonpremixed counterflow flames of methane,ethylene, propylene and n-butane in air

Fundamental flame characteristics derived from counterflow flames are routinely used in chemical kinetic model optimization and validation. This paper reports an experimental and computational investigation aimed at understanding and quantifying the source of uncertainties associated with such characterization of extinction limits of fuel–air mixtures, ranging from low extinction strain rate methane–air flames to high-extinction strain rate ethylene–air flames. In the experiments, two pairs of convergent nozzles with exit diameters of 7.9 mm and 14.5 mm were used to introduce opposed jets of nonpremixed fuel and air to establish a planar flame in the counterflow mixing region. Velocity profiles and extinction data were measured using both LV and PIV setups. Experiments were conducted at various nozzle separation distances to investigate potential differences in axial velocity profiles along the axial and radial directions and the corresponding local extinction strain rates. The slope of axial velocity in the axial and radial directions at the air outlet boundary was found to increase with decreasing nozzle separation distance. The variation of local extinction strain rate with changes in separation distance was within the uncertainty of experimental data. Using a C1–C4 chemical kinetic model, quasi one-dimensional computations have been performed to quantify the experimentally determined boundary condition effects on the predicted extinction strain rate of counterflow flames.     

Laminar flame speeds and kinetic modeling of hydrogen/chlorine combustion

In this study, laminar flame speeds at atmospheric pressure are accurately measured for H₂/Cl₂/N₂ mixtures at different equivalence ratios and N₂ mole fractions by the counterflow flame technique. A kinetic mechanism based on recently published and evaluated rate constants is developed to model these measured laminar flame speeds as well as the literature data on the concentrations of H₂, Cl₂, and HCl species in flat-burner flames and the ignition delay times from shock tube experiments. The kinetic model yields satisfactory comparison with these experimental data, and suggests that the reactions involving excited HCl(v) species and energy branching are not of substantial significance in combustion situations, and that the use of accurate elementary rate constants is instead crucial to the accuracy of the reaction mechanism.     

Transient model and experimental validation of low-stretch solid-fuel flame extinction and stabilization in response to a step change in gravity

A transient stagnation point numerical model was developed that includes gas-phase and solid-phase radiation and solid-phase coupling to describe the dynamic transition from a flame at higher stretch to a flame at lower stretch. To validate the model, low-stretch experiments using PMMA samples were performed in NASA Glenn's Zero Gravity Facility. When the final stretch rate is sufficiently low, the flame transitions to extinction. Above the critical stretch rate, the flame reaches a new steady state with larger flame standoff distance. But the transient process is very dynamic. The model captures the transient behavior of the experimental flame. A parametric study of the surface temperature and standoff distance demonstrates that the flame standoff overshoot at the beginning of the drop is the result of the faster response of the gas phase and the slower response of the solid layer immediate beneath the surface sample. The predicted surface energy balance shows that as the feedback from the flame decreases, the importance of the ongoing heat losses becomes greater, and extinction is observed when these losses represent 80% or more of the flame feedback. Extinction is attributable to insufficient heat feedback to the surface to compensate for existing heat losses under these low-stretch conditions. There is good agreement between the model and both the drop tower and previous buoyant low-stretch experiments in terms of a limiting stretch rate. This work supports the hypothesis that buoyant experiments with large burners can be used to evaluate the low-gravity, low-stretch flammability limits of a material.     

Study on nonpremixed methane/air combustion from flame structure and NOX emission aspect for different burner head structures

In the present study, the air turbulator, which is a part of a nonpremixed burner, is investigated numerically in terms of its effects on the diffusion methane flame structure and NO_X emissions. A computational fluid dynamics (CFD) code was used for the numerical analysis. At first, four experiments were conducted using natural gas fuel. In the experimental studies, the excess air ratio was taken constant as 1.2, while the fuel consumption rate was changed between 22 and 51 Nm³/h. After the experimental studies, the CFD studies were carried out. Pure methane was taken as fuel for the simulations. The nonpremixed combustion model with the steady laminar flamelet model (SFM) approach was used in the combustion analyses. Methane‐air extinction mechanism with 17 species and 58 reactions was used for the simulations. The results obtained from the CFD studies were confronted with the measurements of the flue gas emissions in the experimental studies. Then, a modified burner head was analysed numerically for the different air turbulator blade numbers and angles. The CFD results show that increasing the air turbulator blade number and angle causes the thermal NO emissions to be reduced in the flue gas by making the flame in the combustion chamber more uniform than the original case. This new flame structure provides better mixing of the fuel and combustion air. Thus, the diffusion flame structure in the combustion chamber takes the form of the partially premixed flame structure. The maximum reduction in the thermal NO emissions in the flue gas is achieved at 38% according to the original case.     

TIME-DEPENDENT MODEL OF A BUOYANT DOWNWARD FLAME SPREAD OVER A THERMALLY THIN SOLID FUEL

A time-dependent model was developed and solved numerically to study a purely buoyant downward flame spread over a thermally thin solid fuel in various gravity environments. According to the specified burn-out solid density and no retained ash, the flame propagation behavior over a thin solid fuel surface could be simulated. At ignition, the flame is premixed. After several transition burnouts the flame transitions into a self-sustained steady spread diffusion flame. When the gravity level was varied, the Damkohler number effects were verified. An unstable flame spread was noted near the extinction limit at which the flame spread rate decelerated. Blow-off extinction was predicted after the flame spread a short distance. The ignition delay time increased with increasing gravity level. Compared with the experimental measurements, the predicted blow-off extinction limit was closer than that predicted by the steady combustion model.     

A computational study of spherical diffusion flames in microgravity with gas radiation Part I: Model development and validation

The overarching goal of this study is to improve our understanding of the extinction characteristics of spherical diffusion flames in microgravity. In particular, one of the key objectives is to assess the effects of gas radiation as a means to promote flame extinction. To investigate these phenomena, a one-dimensional computational model was developed to simulate the evolution of a spherical diffusion flame with consideration of detailed chemistry and transport properties. The model formulation was described along with the detailed numerical method. Radiation model was discussed with two aspects: radiation property model and radiative transfer model. Various levels of radiation models were implemented and the results were compared with experimental measurements of flame radius and temperature profiles. It was shown that the statistical narrow band model (SNB) combined with the discrete ordinate method (DOM) reproduced the experimental results with highest accuracy, and this combination of the radiation models were adopted in the subsequent parametric studies in Part II. Computational issues to optimize numerical accuracy and efficiency are also discussed.     

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.     

Tubular premixed and diffusion flames: Effect of stretch and curvature

Tubular flames are ideal for the study of stretch and curvature effects on flame structure, extinction, and instabilities. Tubular flames have uniform stretch and curvature and each parameter can be varied independently. Curvature strengthens or weakens preferential diffusion effects on the tubular flame and the strengthening or weakening is proportional to the ratio of the flame thickness to the flame radius. Premixed flames can be studied in the standard tubular burner where a single premixed gas stream flows radially inward to the cylindrical flame surface and products exit as opposed jets. Premixed, diffusion and partially premixed flames can be studied in the opposed tubular flame where opposed radial flows meet at a cylindrical stagnation surface and products exit as opposed jets. The tubular flame flow configurations can be mathematically reduced to a two-point boundary value solution along the single radial coordinate. Non-intrusive measurements of temperature and major species concentrations have been made with laser-induced Raman scattering in an optically accessible tubular burner for both premixed and diffusion flames. The laser measurements of the flame structure are in good agreement with numerical simulations of the tubular flame. Due to the strong enhancement of preferential diffusion effects in tubular flames, the theory-data comparison can be very sensitive to the molecular transport model and the chemical kinetic mechanism. The strengthening or weakening of the tubular flame with curvature can increase or decrease the extinction strain rate of tubular flames. For lean H₂-air mixtures, the tubular flame can have an extinction strain rate many times higher than the corresponding opposed jet flame. More complex cellular tubular flames with highly curved flame cells surrounded by local extinction can be formed under both premixed and non-premixed conditions. In the hydrogen fueled premixed tubular flames, thermal-diffusive flame instabilities result in the formation of a uniform symmetric petal flames far from extinction. In opposed-flow tubular diffusion flames, thermal-diffusive flame instabilities result in cellular flames very close to extinction. Both of these flames are candidates for further study of flame curvature and extinction.     

Skeletal reaction models based on principal component analysis: Application to ethylene–air ignition, propagation, and extinction phenomena

The development of a skeletal reaction model based on Principal Component Analysis of local Sensitivity (PCAS) coefficients is reported. The analysis presented is comprehensive in the sense that it includes sensitivity coefficients from three distinct canonical reacting configurations, namely ignition, flame propagation, and flame extinction phenomena. To minimize the computational effort involved in constructing sensitivity coefficients, and with the objective of accurately predicting global features or target functions such as ignition delay, burning velocity and extinction strain rates, optimal temporal and spatial locations to perform the local sensitivity are identified. Furthermore, it is shown that the sensitivity coefficients of temperature and heat release, and/or global flame properties (or eigenvalues) associated with burning velocity and extinction strain rate, are sufficient to extract an accurate skeletal model to predict stated target functions. Application of the PCAS approach to a C₁–C₄ hydrocarbon kinetic model consisting of 111 species and 784 reversible reactions, with ethylene as the fuel of interest, is presented. The results clearly indicate that the smallest skeletal model that can be developed is dictated by non-premixed extinction phenomenon that has been neglected in previous analyses using various reduction approaches.     

Numerical investigation of extinction in a counterflow nonpremixed flame perturbed by a vortex

The features of extinction in a CH₄/N₂-air strained counterflow nonpremixed flame perturbed by a vortex were investigated numerically. First, the extinction behaviors using two augmented reduced mechanisms (ARM) and their original full reaction mechanisms (the Miller and Bowman mechanism and GRI-Mech 3.0) were investigated with the numerical results for a steady counterflow flame and unsteady flamelet equations. The modified ARM, based on Miller and Bowman's mechanism (MB-ARM), and adjusted to predict an extinction limit reasonably, was the most suitable mechanism for the unsteady simulation, taking into consideration computational cost, stiffness during the ignition process, and prediction performance for an unsteady flame with sinusoidal transient disturbances. The unsteady 2D computations with the modified MB-ARM showed that fuel- and air-side vortices caused the unsteady effect, and a flame interacting with a vortex was extinguished at a much higher scalar dissipation rate than a steady flame. Moreover, an air-side vortex extinguished the flame more rapidly than a fuel-side vortex, since the air-side vortex was much stronger than the fuel-side vortex, given the same vortex jet velocity conditions. In addition, the degree of the unsteady effect experienced by a flame could be clearly understood by introducing characteristic time scales for the flame, vortex, and convective-diffusive layer.     

A model relating extinction of the opposed-flow diffusion flame to reaction kinetics

A theoretical analysis is developed for the extinction of a homogeneous counterflow diffusion flame due to limitations in the reaction kinetics. With increasing mass flux of reactants into the diffusion flame a maximum in the consumption rate of fuel and oxidizer in the reaction zone occurs. The analysis shows that experimental measurements of the extinction conditions (“apparent flame strengths”) can be used to evaluate quantitatively such kinetic parameters as the overall activation energy, the pre-exponential Arrhenius term, and the order of reaction. The theoretical model is applied to the propane-oxygen-nitrogen system, for which an activation energy of 21 ± 2 kcal/mole is calculated on the basis of experimental flame extinction data.     

Experimental study of cellular instability and extinction of non-premixed opposed-flow tubular flames

An experimental study of cellular instability of non-premixed opposed-flow tubular flames was conducted burning H₂ diluted with CO₂ flowing against air. The transitions to cellularity, cellular structures, and extinction conditions were determined as a function of the initial mixture strength, stretch rate, and curvature. The progression of cellular structures from the onset of cells through extinction was analyzed by flame imaging using an intensified CCD camera. Three different procedures of decreasing the Damköhler number (forward process), as well as using those same procedures in the opposite progression of increasing the Damköhler number (backward process) were completed. Significant flame hysteresis was seen and the forward transition occurred at a lower Damköhler number than the backward transition. Mechanical perturbations were conducted to show that the onset of cellularity could be realized at a higher Damköhler number than without perturbations. Once cellular instability was induced, it was possible to perturb the flame into multiple stable cellular states and extinguish the flame at a much higher Damköhler number than without perturbations. Images are shown of rotating cellular flames and a cellular instability regime at an initial mixture strength greater than unity and away from extinction conditions. A qualitative explanation of flame rotation and a general categorizing of three distinct flame regimes is given.     

Pressure effects on nonpremixed strained flames

This article deals with the effect of pressure on the structure and consumption rate of nonpremixed strained flames. An analysis based on the fast chemistry limit indicates that the flame thickness is inversely proportional to the square root of pressure and that the flame structure may be described in terms of a similarity variable that scales like the product of pressure and the strain rate to the power 1/2. This scaling rule also applies to flames submitted to a time-variable strain rate provided that the frequencies characterizing these changes are low compared to the mean strain rate. It is also confirmed that reactants consumption rates per unit flame surface vary like the square root of pressure and that this rule holds for time-variable strain rates of arbitrary nature. Complex chemistry calculations carried out over a broad range of operating pressures indicate that the pressure dependences deduced analytically are remarkably accurate and can be used for a broad range of strain rates, excluding values in the near vicinity of extinction conditions, where finite rate chemistry effects become important and influence the flame response to pressure. Thus, it appears that the pressure exponent characterizing the heat release rate in nonpremixed strained flames is essentially constant and equal to 1/2. This exponent is independent of finite rate chemistry effects, except when conditions are close to extinction.     

The structure of partially premixed methane flames in high-intensity turbulent flows

Direct numerical simulations (DNS) are conducted to study the structure of partially premixed and non-premixed methane flames in high-intensity two-dimensional isotropic turbulent flows. The results obtained via “flame normal analysis” show local extinction and reignition for both non-premixed and partially premixed flames. Dynamical analysis of the flame with a Lagrangian method indicates that the time integrated strain rate characterizes the finite-rate chemistry effects and the flame extinction better than the strain rate. It is observed that the flame behavior is affected by the “pressure-dilatation” and “viscous-dissipation” in addition to strain rate. Consistent with previous studies, high vorticity values are detected close to the reaction zone, where the vorticity generation by the “baroclinic torque” was found to be significant. The influences of (initial) Reynolds and Damköhler numbers, and various air-fuel premixing levels on flame and turbulence variables are also studied. It is observed that the flame extinction occurs similarly in flames with different fuel-air premixing. Our simulations also indicate that the CO emission increases as the partial premixing of the fuel with air increases. Higher values of the temperature, the OH mass fraction and the CO mass fraction are observed within the flame zone at higher Reynolds numbers.     

Flame structures and behaviors of opposed flow non-premixed flames in mesoscale channels

An opposed flow non-premixed flame (OFNPF) in a narrow channel was chosen as a model of a non-premixed flame in a mesoscale combustion space or micro-combustor. The stabilization limits and behaviors of methane-air flames and propane-air flames were compared for various experimental parameters such as flow velocity, nozzle distance, nozzle width, channel gap, and fuel dilution. Flames could be stabilized in a wide range of strain rates (0.9–150 s⁻¹) and dilution ratios (∼80% nitrogen at the fuel side). The flame extinction limits were classified into three types and their mechanisms were investigated: higher-strain-rate (HSR) extinction limit determined by the flame stretch, lower-strain-rate (LSR) extinction limit determined by the conductive or convective heat loss from the flame, and fuel-dilution-ratio (FDR) extinction limit determined by the decrease in the heat release rate from the flames. The HSR extinction limits in mesoscale channels could be explained with a modified strain rate, and the LSR extinction limits could be explained by employing a premixed quenching theory in which the heat loss through the dead space near the wall was considered as a major extinction mechanism. Finally, the variation of the extinction limits with the FDR in both the HSR and the LSR conditions could be explained with a modified global reaction rate in which the variations in flame temperature and species concentrations were reflected. This study provides an essential model for the stabilization and extinction of non-premixed flames in mesoscale combustion spaces.     

Experimental study of limit lean methane/air flame in a standard flammability tube using particle image velocimetry method

Lean limit methane/air flame propagating upward in a standard 50 mm diameter and 1.8 m length tube was studied experimentally using particle image velocimetry method. Local stretch rate along the flame front was determined by measured gas velocity distributions. It was found that local stretch rate is maximum at the flame leading point, which is in agreement with earlier theoretical results. Similar to earlier observations, extinction of upward propagating limit flame was observed to start from the flame top. It is stated that the observed behavior of the extinction of the lean limit methane/air flame can not be explained in terms of the coupled effect of flame stretch and preferential diffusion. To qualitatively explain the observed extinction behavior, it is suggested that the positive strain-induced flame stretch increases local radiation heat losses from the flame front. An experimental methodology for PIV measurements in a round tube is described.     

Effect of the composition of the hot product stream in the quasi-steady extinction of strained premixed flames

The extinction of premixed CH₄/O₂/N₂ flames counterflowing against a jet of combustion products in chemical equilibrium was investigated numerically using detailed chemistry and transport mechanisms. Such a problem is of relevance to combustion systems with non-homogeneous air/fuel mixtures or recirculation of the burnt gases. Contrary to similar studies that were focused on heat loss/gain, depending on the degree of non-adiabaticity of the system, the emphasis here was on the yet unexplored role of the composition of counterflowing burnt gases in the extinction of lean-to-stoichiometric premixed flames. For a given temperature of the counterflowing products of combustion, it was found that the decrease of heat release with increase in strain rate could be either monotonic or non-monotonic, depending on the equivalence ratio φ_b of the flame feeding the hot combustion product stream. Two distinct extinction modes were observed: an abrupt one, when the hot counterflowing stream consists of either inert gas or equilibrium products of a stoichiometric premixed flame, and a smooth extinction, when there is an excess of oxidizing species in the combustion product stream. In the latter case four burning regimes can be distinguished as the strain rate is progressively increased while the heat release decreases smoothly: an adiabatic propagating flame regime, a non-adiabatic propagating flame regime, the so-called partially-extinguished flame regime, in which the location of the peak of heat release crosses the stagnation plane, and a frozen flow regime. The flame structure was analyzed in detail in the different burning regimes. Abrupt extinction was attributed to the quenching of the oxidation layer with the entire H-OH-O radical pool being comparably reduced. Under conditions of smooth extinction, the behavior is different and the concentration of the H radical decreases the most with increasing strain rate, whereas OH and O remain comparatively abundant in the oxidation layer. As the profile of the heat release rate thickens, the oxidation layer is quenched and the attack of the fuel relies more heavily on the OH radicals.     

Extinction of diffusion flames burning diluted methane and diluted propane in diluted air

A theoretical and experimental investigation of the extinction limits of counterflow diffusion flames burning methane and propane is outlined. A diffusion flame is stabilized between counterflowing streams of a fuel diluted with nitrogen and air diluted with nitrogen. Extinction limits for such flames were measured over a wide parametric range. Results for methane and propane were found to be in approximate agreement with previous measurements.The experimental results are interpreted by use of activation energy asymptotic theories developed previously. The gas-phase chemical reaction is approximated as a one step, irreversible process with a large value for the ratio of the activation energy characterizing the chemical reaction to the thermal energy in the flame. Equilibrium dissociation of products is neglected. The theoretical predictions are compared with experimental results, and the overall chemical kinetic rate parameters characterizing the gas-phase oxidation of methane and propane in a diffusion flame are deduced. The overall chemical kinetic rate parameters deduced by use of this procedure are valid only at flame temperatures where equilibrium dissociation is negligible. The scalar dissipation rate at extinction is predicted over a wide range.