Scale Flame Research Articles

Measurement on instantaneous flame front structure of turbulent premixed CH4/H2/air flames

Instantaneous flame front structure of turbulent premixed CH4/H2/air flames (hydrogen fraction of 0%, 5%, 10% and 20% by mole fraction) was investigated quantitatively using a nozzle-type Bunsen burner. Hot wire anemometer and OH-PLIF were used to measure the turbulent flow and detect the instantaneous flame front structure, respectively. Turbulent burning velocity, ST, flame surface density, Σ, and mean flame volume, Vf, were calculated by processing the OH-PLIF images. Results show that the flame front structures of the turbulent premixed flames are the wrinkled flame front and it becomes much finer with the increase of turbulence intensity as well as hydrogen fraction. The value of ST/SL significantly increases with the increase of u′/SL and it slightly increases with the increase of hydrogen fraction. Flame surface density profile are symmetric and gives its maximum value at about 〈c〉=0.5. Hydrogen addition slightly enhances the Σ and the tendency is more obvious under higher turbulence intensity. The decrease of Σ with the increase of turbulence intensity is mainly due to the effect of flame volume. The mean flame volume of flame region obviously increases with the increase of turbulence intensity within the experimental range due to the increase in depth of the large scale flame wrinkles and flame height. Hydrogen addition is not a predominant factor within the hydrogen fraction range in this study.

LES of a premixed jet flame DNS using a strained flamelet model

Turbulent premixed flames in the thin and broken reaction zones regimes are difficult to model with Large Eddy Simulation (LES) because turbulence strongly perturbs subfilter scale flame structures. This study addresses the difficulty by proposing a strained flamelet model for LES of high Karlovitz number flames. The proposed model extends a previously developed premixed flamelet approach to account for turbulence’s perturbation of subfilter premixed flame structures. The model describes combustion processes by solving strained premixed flamelets, tabulating the results in terms of a progress variable and a hydrogen radical, and invoking a presumed PDF framework to account for subfilter physics. The model is validated using two dimensional laminar flame studies, and is then tested by performing an LES of a premixed slot-jet direct numerical simulation (DNS). In the premixed regime diagram this slot-jet is found at the edge of the broken reaction zones regime. Comparisons of the DNS, the strained flamelet model LES, and an unstrained flamelet model LES confirm that turbulence perturbs flame structure to leading order effect, and that the use of an unstrained flamelet LES model under-predicts flame height. It is shown that the strained flamelet model captures the physics characterizing interactions of mixing and chemistry in highly turbulent regimes.

Measurement of the instantaneous flame front structure of syngas turbulent premixed flames at high pressure

Instantaneous flame front structure of syngas turbulent premixed flames including the local radius of curvature, the characteristic radius of curvature, the fractal inner cutoff scale and the local flame angle were derived from the experimental OH-PLIF images. The CO/H2/CO2/air flames as a model of syngas/air combustion were investigated at pressure of 0.5MPa and compared to that of CH4/air flames. The convex and concave structures of the flame front were detected and statistical analysis including the PDF and ADF of the local radius of curvature and local flame angle were conducted. Results show that the flame front of turbulent premixed flames at high pressure is a wrinkled flame front with small scale convex and concave structures superimposed with large scale flame branches. The convex structures are much more frequent than the concave ones on flame front which reflects a general characteristic of the turbulent premixed flames at high pressure. The syngas flames possess much wrinkled flame front with much smaller fine cusps structure compared to that of CH4/air flames and the main difference is on the convex structure. The effect of turbulence on the general wrinkled scale of flame front is much weaker than that of the smallest wrinkled scale. The general wrinkled scale is mainly dominated by the turbulence vortex scale, while, the smallest wrinkled scale is strongly affected by the flame intrinsic instability. The effect of flame intrinsic instability on flame front of turbulent premixed flame is mainly on the formation of a large number of convex structure propagating to the unburned reactants and enlarge the effective contact surface between flame front and unburned reactants.

Evaluation of simple models of flame radiation in the frame of fire propagation

Simple models for radiative transfer evaluation in the frame of flame propagation are discussed and evaluated through comparisons with a complete absorbing–emitting volume flame model. The cases of the solid flame model and of the optically thin flame are considered as they are usually involved in fire propagation studies. It is shown that the solid flame model provides satisfactory results in some applications, but only for flames with sufficiently high optical thickness (above three at least). However, its inability to model the intensity distribution when shifting from the normal to the flame surface and its inaccuracy when addressing the case of low or even moderate optical thicknesses are two weaknesses that should be kept in mind when using this model. Regarding the optically thin flame model, it is demonstrated that its use should be restricted to really low optical thicknesses, far below one, which makes the application of such models quite unrealistic unless for small scale flames. The true absorbing–emitting flame treatment should be preferred, with moderate supplementary cost regarding computational integration, if combined with dedicated methods like a reciprocal Monte Carlo Method as presented here for example.

Fundamental burning velocities of meso-scale propagating spherical flames with H2, CH4 and C3H8 mixtures

This study is performed to experimentally examine the fundamental burning velocity characteristics of meso-scale outwardly propagating spherical laminar flames in the range of flame radius rf approximately from 1 to 5mm for hydrogen, methane and propane mixtures, in order to make clear a method for improving combustion of micro–meso scale flames. Macro-scale laminar flames with rf>7mm are also examined for comparison. The mixtures have nearly the same laminar burning velocity (SL0=25cm/s) for unstretched flames and different equivalence ratios ϕ. The radius rf and the burning velocity SLl of meso-scale flames are estimated by using sequential schlieren images recorded under appropriate ignition conditions. It is found that SLl of hydrogen and methane premixed meso-scale flames at the same rf or the Karlovitz number Ka shows a tendency to increase with decreasing ϕ, whereas SLl of propane flames increases with ϕ. However, SLl tends to decrease with the Lewis number Le and the Markstein number Ma, irrespective of the type of fuel and ϕ. It also becomes clear that the optimum flame size and Ka to improve the burning velocity exist for some mixtures depending on Le and fuel types.

A Filtered Tabulated Chemistry model for LES of stratified flames

The model called Filtered Tabulated Chemistry for LES (F-TACLES), recently developed for turbulent premixed combustion [1], is extended in the present article to stratified flames where the equivalence ratio is not spatially uniform. The principle is to generate a look-up table where thermo-chemical variables, computed from a set of filtered laminar unstrained premixed flamelets, are stored as a function of the filtered progress variable Y˜c, the filtered mixture fraction c˜, and the mixture fraction subgrid scale variance z″2∼ for a given value of the LES filter size Δ. From this database and using a classical subgrid scale flame surface wrinkling model, a closure model for the filtered progress variable balance equation is then proposed. As only three balance equations (respectively for Y˜c, z˜ and z″2∼) are added to the Navier–Stokes system, this turbulent combustion model is straightforward to implement and is not expensive in terms of CPU time. After preliminary tests on 1-D planar flames, the proposed strategy is applied to a 3-D simulation of a swirled turbulent stratified flame. The agreements observed between numerical and measured data validate the modeling strategy. Finally, it is observed that the flame position cannot be captured without accounting for the subgrid scale mixture fraction heterogeneities that impact on the flame propagation. This result highlights the importance of the mixture fraction filtered density function on the filtered chemical reaction rate.

Large eddy simulation of a growing turbulent premixed flame kernel using a dynamic flame surface density model

The relevant description of a flame kernel growth in homogeneous isotropic turbulence environment is a key ingredient for the simulation of combustion in spark-ignited internal combustion engine. A small flame kernel burns at an increasing rate as it grows and becomes progressively more and more wrinkled by the prevailing turbulence. Large eddy simulation (LES) models based on algebraic expressions for the turbulent flame speed, the flame surface density or the flame wrinkling factor cannot reproduce such a situation because they assume an equilibrium between turbulence and flame wrinkling which is generally not reached during the simulation as the flame is laminar at early stages. A possible solution is to solve an additional balance equation for the flame surface density or for the wrinkling factor but a promising alternative is to implement a dynamic model where the wrinkling factor, measuring the subgrid scale flame surface, is automatically determined along the computation from the known instantaneous resolved flame front. A Germano-like identity expresses the conservation of the total flame surface at filter and test filter scales to automatically adjust the parameter of a fractal-like wrinkling factor model from the instantaneous resolved flame front characteristics. This model is tested against the experimental data of Renou et al. (2000) [33] to reproduce the temporal evolution of the flame kernel growth. For a range of ignition parameters, preliminary results are in good agreements with experimental data and show that the model parameter evolves with time and depends on operating conditions, validating the dynamic procedure.

LES model for sooting turbulent nonpremixed flames

In this work, an integrated Large Eddy Simulation (LES) model is developed for sooting turbulent nonpremixed flames and validated in a laboratory scale flame. The integrated approach leverages state-of-the-art developments in both soot modeling and turbulent combustion modeling and gives special consideration to the small-scale interactions between turbulence, soot, and chemistry. The oxidation of the fuel and the formation of gas-phase soot precursors is described by the Flamelet/Progress Variable model, which has been previously extended to account for radiation losses. However, previous DNS studies have shown that Polycyclic Aromatic Hydrocarbons (PAH), the immediate precursors of soot particles, exhibit significant unsteady effects due to relatively slow chemistry. To model these unsteady effects, a transport equation is solved for a lumped PAH species. In addition, due to the removal of PAH from the gas-phase, alternative definitions of the mixture fraction, progress variable, and enthalpy are proposed. The evolution of the soot population is modeled with the Hybrid Method of Moments (HMOM), an efficient statistical model requiring the solution of only a few transport equations describing statistics of the soot population. The filtered source terms in these equations that describe the various formation, growth, and destruction processes are closed with a recently developed presumed subfilter PDF approach that accounts for the high spatial intermittency of soot. The integrated LES model is validated in a piloted natural gas turbulent jet diffusion flame and is shown to predict the magnitude of the maximum soot volume fraction in the flame relatively accurately, although the maximum soot volume fraction is shown to be rather sensitive to the subfilter scalar dissipation rate model.

A fractal-based flame propagation model for large eddy simulation

A novel combustion model for large-eddy simulation (LES) for gasoline engines has been developed. Unlike conventional models based on Reynolds-averaged Navier–Stokes (RANS) models, the new model takes a unique approach; it is described by the fractal characteristics of flame front and a universal expression for the subgrid scale (SGS) flame speed. The present fractal combustion model was applied to calculations of a spark ignition engine. Both the 0–10 per cent and 10–90 per cent combustion periods agree well with the experimental data. Because the modelling of the SGS turbulent speed is based on fractal analysis with experimental observations, the SGS combustion model is able to apply a wide range of engine operating conditions. The present model was applied to a multi-cycle simulation of a single-cylinder engine. The fluctuations at the instant when the heat release rate peaked were compared with data that was obtained experimentally. The calculated magnitude of the fluctuations was found to be close to the experimental values. It is thought that the flow variation generated during the intake stroke significantly influences the cyclic variations.

Bisphenol-1,2,3-triazole (BPT) Epoxies and Cyanate Esters: Synthesis and Self-Catalyzed Curing

Novel bisphenol-1,2,3-triazole (BPT) resins, including epoxy (DGE-BPT) and cyanate ester (BPTCE) systems, were prepared and used in curing chemistry with no added reagents or catalysts. The materials were characterized relative to conventional epoxies and cyanate esters, in terms of curing and thermal properties, and the curing process and products were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), pyrolysis combustion flow calorimetry (PCFC), and small scale flame tests. These novel BPT-based resins cure in the absence of added catalysts and exhibit exceptionally low heat release and high char residue. Despite the absence of antiflammable additives (either halogenated or inorganic compounds), BPTCE itself was found to be nonignitable, with extremely high char residue value (67%) and extraordinarily low heat release capacity (HRC of ∼10 J/(g K)).

通常スケールの火炎端特性から考察するマイクロフレームの安定性

通常スケールの火炎端特性から考察するマイクロフレームの安定性

Development and analysis of a LTCC micro stagnation-point flow combustor

A micro stagnation-point flow burner was fabricated using low-temperature co-fired ceramic (LTCC) tapes. Methane/oxygen counterflow micro diffusion flames with luminous zones of less than 1 mm in length and 250 µm in width were stabilized in the reaction channel of the burner and analyzed using microscopic imaging spectroscopy. The burner was built with 25 layers of LTCC tapes which were pre-laminated into seven blocks. Integrated sapphire windows and sub-millimeter sized internal channels provide optical accessibility and reactant feeds, respectively. Spatial distributions of CH* and C*2 species were measured and compared with those obtained from multi-dimensional reacting flow calculations. Results show that the location and size of the diffusion flame may be controlled by flow velocity, or strain rate, just as in larger centimeter scale flames, and that the chemical structure of the micro flames was in agreement with those predicted by the numerical simulations. In this paper, we show that LTCC tape technology may be used to fabricate microburners for continuous flow steady-state homogeneous gaseous combustion and that integrated optical windows may be incorporated into the fabrication process for in situ optical diagnostics. The understanding and analysis of the present reacting flowfield, because of its simplicity, is important to the development of more complicated microreactors and microburners, in which mixing and variable strain rates exist.

A two-way coupling for modeling thermoacoustic instabilities in a flat flame Rijke tube

Thermoacoustic instabilities are a serious problem for lean premixed combustion systems. Due to different time and length scales associated with the flow field, combustion, and acoustics, numerical computations of thermoacoustic phenomena are conceptually challenging. This work presents a coupled method for the simulation of thermoacoustic instabilities in low Mach number reacting flows. The acoustics are represented by a reduced order model that can be obtained from network techniques or finite element computations. A detailed chemistry finite-difference zero Mach number solver is used for the small scale flame dynamics. Under the assumption that the pressure is continuous across the flame, the acoustic model can be reduced to a time-domain relation mapping the velocity perturbation downstream of the flame to that upstream. Closure is obtained by the flame code, which delivers the jump in velocity across the combustion zone. The method is applied to an experimental laminar premixed burner-stabilized flat flame Rijke tube, that exhibits strong thermoacoustic oscillations associated with the 5λ/4 mode of the geometrical set-up. In addition to the fundamental oscillation, a significant subharmonic response of the flame is observed. Results from the coupled simulation are compared to the experimental data. Good qualitative and quantitative agreement is found.

Analytical predictions of shapes of laminar diffusion flames in microgravity and earth gravity

Flame shape is an important observed characteristic of flames that can be used to scale flame properties such as heat release rates and radiation. Flame shape is affected by fuel type, oxygen levels in the oxidiser, inverse burning and gravity. The objective of this study is to understand the effect of high oxygen concentrations, inverse burning, and gravity on the predictions of flame shapes. Flame shapes are obtained from recent analytical models and compared with experimental data for a number of inverse and normal ethane flame configurations with varying oxygen concentrations in the oxidiser and under earth gravity and microgravity conditions. The Roper flame shape model was extended to predict the complete flame shapes of laminar gas jet normal and inverse diffusion flames on round burners. The Spalding model was extended to inverse diffusion flames. The results show that the extended Roper model results in reasonable predictions for all microgravity and earth gravity flames except for enhanced oxygen normal diffusion flames under earth gravity conditions. The results also show trends towards cooler flames in microgravity that are in line with past experimental observations. Some key characteristics of the predicted flame shapes and parameters needed to describe the flame shape using the extended Roper model are discussed.

Numerical analysis of hydrogen deflagration mitigation by venting through a duct

This paper presents a model and simulation results for the mitigation of a hydrogen–air deflagration by venting through a duct. A large eddy simulation (LES) model, applied previously to study both closed-vessel, and open atmosphere hydrogen–air deflagrations, was developed further to model a hydrogen–air explosion vented through a duct. Sub-grid scale (SGS) flame wrinkling factors were introduced to model major phenomena which contribute to the increase of flame surface area in vented deflagrations. Simulations were conducted to validate the model against 20% hydrogen–air mixture deflagrations (vent diameters 25 and 45 cm) and 10% hydrogen–air mixture deflagration (vent diameter 25 cm). There was reasonable correlation between the simulations and the experimental data. The comparative importance of different physical phenomena contributing to the flame wrinkling is discussed.

Flame front analysis of high-pressure turbulent lean premixed methane–air flames

An experimental study on lean turbulent premixed methane–air flames at high pressure is conducted by using a turbulent Bunsen flame configuration. A single equivalence ratio flame at Φ = 0.6 is explored for pressures ranging from atmospheric pressure to 0.9 MPa. LDA measurements of the cold flow indicate that turbulence intensities and the integral length scale are not sensitive to pressure. Due to the decreased kinematic viscosity with increasing pressure, the turbulent Reynolds numbers increase, and isotropic turbulence scaling relations indicate a large decrease of the smallest turbulence scales. Available experimental results and PREMIX code computations indicate a decrease in laminar flame propagation velocities with increasing pressure, essentially between the atmospheric pressure and 0.5 MPa. The u′/ S L ratio increases therefore accordingly. Instantaneous flame images are obtained by Mie scattering tomography. The images and their analysis show that pressure increase generates small scale flame structures. In an attempt to generalize these results, the variance of the flamelet curvatures, the standard deviation of the flamelet orientation angle, and the flamelet crossing lengths have been plotted against Re t 1 / 2 which is proportional to the ratio between the integral and Taylor length scales, and which increases with pressure. These three parameters vary linearly with the ratio between large and small turbulence scales and clearly indicate the strong effect of this parameter on premixed turbulent flame dynamics and structure. An obvious consequence is the increase in flame surface density and hence burning rate with pressure, as confirmed by its direct determination from 2D tomographic images.

DNS analysis of partially premixed combustion in spray and gaseous turbulent flame-bases stabilized in hot air

Direct numerical simulations of weakly turbulent-lifted flame bases are examined in the case of both gaseous and spray fuel jet injection. Simplified transport properties and an adjustable single-step chemistry that matches the flame response to equivalence ratio are used. The flames are stabilized within a coflowing stream of heated air. The properties of the zone where burning starts are found to strongly depend on the type of fuel injection. The gaseous flame base is essentially composed of an edge flame, with a large contribution of partially premixed combustion. This partially premixed flame takes two different forms, a nearly stoichiometric propagating kernel and a rich trailing flame whose burning rate is diffusion controlled. The rich premixed flame is parallel to the stoichiometric line, along which a diffusion flame burns the fuel left by this rich trailing flame, up to the very leading edge of the flame base. In the spray case, a nonnegligible amount of oxidizer is entrained within the dilute spray, also leading to an important contribution of partially premixed burning. However, diffusion and premixed burning are found more distributed in space in the spray case than with gaseous injection. A progress variable that is generalized to partially premixed combustion is discussed and the relative contributions of the terms of its balance equation are analyzed from the DNS. A flame partitioning into premixed and diffusion types is then examined and the stabilization zone is decomposed into basic flame prototypes. A subgrid scale flame decomposition is further discussed from a direct filtering of DNS and some a priori tests of subgrid scale modeling are reported.

A power-law flame wrinkling model for LES of premixed turbulent combustion Part I: non-dynamic formulation and initial tests

A model of turbulent sub-grid scale flame speed for premixed combustion is proposed and tested in Large Eddy Simulation (LES). The model is based on writing the unresolved flame surface density in terms of a general power-law expression that involves an inner cutoff scale. This scale is derived from an equilibrium assumption of flame-surface production and destruction. The flame-surface production term is modeled using a parameterization of the effective flame stretch, obtained from a spectral superposition of earlier DNS results of single vortex-flame interactions [8]. The model is implemented in a LES combustion code in the context of Thickened Flame LES (TF-LES) using an empirically chosen (non-dynamic) value for the power-law exponent. Three-dimensional simulations of premixed flame embedded in a time decaying isotropic turbulent flow are performed in several different parameter ranges. Comparisons between direct numerical simulation (DNS) and LES using different resolutions and thickness factors show that the LES reproduces the total reaction rate of the DNS quite well and independently on the thickness factor, resolution, and sub-grid scale model used for the turbulent eddy viscosity. Comparisons between the predicted overall turbulent flame speed s T as function of the r.m.s. velocity and experimental data show good agreement over a significant range of parameters.

Dynamics of Macroprocesses in a Flame and Heated Jet

Based on the spectral and correlation analysis of three oscillatory processes (hydrodynamic, acoustic, and electric) generated during operation of a meteotron, some regular features of the behavior of a large–scale flame and a heated jet were revealed. The frequency range under study was 0.03—20 Hz.

A Cellular Automata Approach To Cfd Flame Spread Modelling

This paper focuses on the growth and spread stage of full-scale fires in enclosures, where a localised flame spreads across a single fuel item, increasing the heat release rate, thereby resulting in increased fire hazard. The development of a numerical model for fire spread over a solid fuel surface, and its integration into a CFD model, is described. The conflicting demands of resolving the small scale flame front phenomena while simultaneously accounting for the large-scale enclosure flow phenomena are addressed. The surface of the fuel is descretised with a regular square array, and flame spread occurs as a series of ignitions of surface elements. Ignition of an element is determined by a combination of critical surface ignition temperature and cellular automata techniques. Regression of the combusting fuel surface is modelled, with a fine grid retained at the fuel surface by allowing the grid to collapse with the surface. A grid transformation is applied to restore orthogonality of the collapsed grid, for simple computation of the heat equation. The flame spread model has been designed to be independent of geometry, although experimental validation exercises are carried out for radial spread over horizontal surfaces only. Predictions are made for the burning of a slab of polyurethane foam in a full-scale multi-room experimental building-fire facility. The predictions compare favourably with the experiments, indicating the broad validity of the methods used in flame spread model.