Tulip flame - the mechanism of flame front inversion

The paper explains the mechanism of tulip flame formation in horizontal combustion chambers closed at the ignition end. The explanations are based essentially on the PIV images and the direct visualization of the process. The obtained results demonstrate that the tulip flame is a purely hydrodynamic phenomenon which results from the competition between the backward movement of deflected burned gases expanding from the lateral flame skirts and the forward movement of unburned gases accelerated in the phase of finger-shaped flame. In some configurations a supplementary global movement imposed by the confinement (for example: acoustic waves) is superposed on the two above mentioned, and modifies the parameters of the process. The results also prove that the intrinsic instabilities of the flame front (Rayleigh–Taylor, Richtmyer–Meshkov or Darrieus–Landau) are not involved in this process. The convex shape of the flame front has no influence on the phenomenon.     

Radiation effects on flame stabilization on flat flame burners

This study investigates the impact of radiative heat transfer on the behavior of flat flame burners within the framework of a simplified one-dimensional model. Flat flame burners stabilize planar premixed flames downstream of a porous plug. Within this study, the porous plug is modeled as a thermally conducting, optically thick medium, allowing for both conductive and radiative heat transfer. Based on the simplified model, the impact of radiative heat exchange between the porous plug exit and the downstream environment is investigated. In “surface” combustion, flame stabilization occurs due to heat transfer between gas phase and porous solid. Results demonstrate that radiative heat transfer from a hot downstream environment to the porous plug significantly increases maximum attainable mass fluxes. For a cold downstream environment, plug properties do not affect the maximum supportable mass flux, although plug porosity and heat transfer between gas and solid have a significant impact on the “stand-off” distance between flame and plug exit. In addition, the model provides insight to a second “submerged” combustion mode, where the flame is stabilized within the porous plug of the burner. Here, increased flame temperatures lead to a dramatic increase of the maximum supportable mass flux. Overall, results show that radiative heat losses play a critical role in both combustion modes: in surface combustion, they are an important mode of heat dissipation, where they can prevent “flash-back” conditions with the flame moving into the porous matrix; in submerged combustion, they prevent flame stabilization close to inlet and exit faces and enable a “slow” solution branch that does not exist without radiative losses.     

Dynamics of upstream flame propagation in a hydrogen-enriched premixed flame

An unconfined strongly swirled flow is investigated to study the effect of hydrogen addition on upstream flame propagation in a methane-air premixed flame using Large Eddy Simulation (LES) with a Thickened Flame (TF) model. A laboratory-scale swirled premixed combustor operated under atmospheric conditions for which experimental data for validation is available has been chosen for the numerical study. In the LES-TF approach, the flame front is resolved on the computational grid through artificial thickening and the individual species transport equations are directly solved with the reaction rates specified using Arrhenius chemistry. Good agreement is found when comparing predictions with the published experimental data including the predicted RMS fluctuations. Also, the results show that the initiation of upstream flame propagation is associated with balanced maintained between hydrodynamics and reaction. This process is associated with the upstream propagation of the center recirculation bubble, which pushes the flame front in the upstream mixing tube. Once the upstream movement of the flame front is initiated, the hydrogen-enriched mixture exhibits more unstable behavior; while in contrast, the CH₄ flame shows stable behavior.     

Numerical analysis of the Cambridge stratified flame series using artificial thickened flame LES with tabulated premixed flame chemistry

Detailed comparisons of LES results against measurement data are presented for the turbulent lean and rich stratified Cambridge flame series. The co-annular methane/air burner with a central bluff body for flame stabilization has been investigated experimentally by Sweeney et al.  and . Three cases with varying levels of stratification in the lean and rich combustion regime are taken into account. Turbulent combustion is modeled by using the artificial thickened flame (ATF) approach in combination with flamelet generated manifolds (FGM) lookup tables. The model is adapted for stratified combustion and an alternative formulation for the flame sensor is presented. Three different grids are used to investigate the influence of the filter width and the sub-filter modeling on the overall results. Velocities, temperatures, equivalence ratios, and major species mass fractions predictions are compared with measurements for three different stratification rates and an overall good overall agreement was found between simulation and experiment. Some deviations occur near the bluff body, which are analyzed further by evaluation of atomic and species mass fractions. The stratified combustion process was further investigated and characterized by probability density functions extracted from the simulation results.     

Insights into non-adiabatic-equilibrium flame temperatures during millimeter-size vortex/flame interactions

Previous experimental and numerical studies have demonstrated that local flame temperatures can significantly increase above or decrease below the adiabatic-equilibrium flame temperature during millimeter-size vortex/flame interactions. Such large excursions in temperature are not observed in centimeter-size vortex/flame interactions. To identify the physical mechanisms responsible for these super- or sub-adiabatic-equilibrium flame temperatures, numerical studies have been conducted for millimeter-size vortex/flame interactions in a hydrogen-air, opposing-jet diffusion flame. Contrary to expectations, preferential diffusion between H₂ and O₂ and geometrical curvature are not responsible for these variations in local flame temperature. This was demonstrated through simulations made by forcing the diffusion coefficients of H₂ and O₂ to be equal and thereby eliminating preferential diffusion. Propagation of flame into small (∼1 mm) vortices suggested that the amount of reactant carried by such a small vortex is not sufficient to feed the flame with fresh reactant during the entire vortex/flame interaction process. Various numerical experiments showed that the reactant-limiting characteristics associated with the millimeter-size vortices and the local Lewis number (not preferential diffusion) are responsible for the generation of flame temperature that is different from the adiabatic-equilibrium value. The reactant-deficient nature of the millimeter-size vortices forces the combustion products to be entrained into the vortex. While a greater-than-unity Lewis number results in pre-heating of the reactant through the product entrainment, a less-than-unity Lewis number causes cooling of the reactant. Contrary to this behavior, a centimeter-size large vortex wraps and maintains the flame around its outer perimeter by feeding the flame with fresh reactant throughout the interaction process, thereby rendering the flame unaffected by the Lewis number. Since turbulent flames generally involve interactions with small-size vortices, the physical mechanisms described here should be considered when developing mathematical models for turbulent flames.     

Analysis of flame stabilization mechanism in a hydrogen-fueled reacting wall-jet flame

In this study, the novel conservative representation of chemical explosive mode analysis is augmented to analyze the key flame features in the Burrows-Kurkov flames simulated by both Reynolds-Averaged Navier-Stokes (RANS) and large eddy simulation (LES). Subtle difference are revealed in flame stabilization mechanisms resulting from the difference in modeling and spatial resolution. RANS shows that, ahead of the flame onset location, the composition diffusion and shock wave compression play dominant roles in chemical explosion indicating that the flame is stabilized by the assisted-ignition combustion mode. In contrast, LES shows that the flame is stabilized by the auto-ignition mode since the nonchemical contribution counteracts chemical reaction during the development of ignited flame kernels. For RANS, the radical pool builds up through the unphysical back diffusion near the flame stabilization front, which reveals the limitation of RANS method in the resolution and characterization of the key flame features in Burrows-Kurkov flames.     

Hydrogen-enriched non-premixed jet flames: Analysis of the flame surface,flame normal,flame index and Wobbe index

A non-premixed impinging jet flame is studied using three-dimensional direct numerical simulation with detailed chemical kinetics in order to investigate the influence of fuel variability on flame surface, flame normal, flame index and Wobbe index for hydrogen-enriched combustion. Analyses indicate that the fuel composition greatly influences the H₂/CO syngas combustion, not only on the important local stoichiometric iso-mixture fraction surface distribution but also on the vortical structures in the flow field. As a result of CO addition to hydrogen-rich combustion, changes of the reaction zone in the flammable layer, shift of peak flame surface density distribution, shift of non-premixed regions, formation of widely populated scalar dissipation distribution rate with respect to tangential strain and reduction of global heat release are all found to appear. In particular, the CO addition induces a micromixing process which appears to be an important factor for the modelling investigation of turbulence/chemistry interaction especially for combustion modelling of H₂-rich syngas fuels.     

Small-scale flame acceleration and application of medium and large-scale flame speed correlations

The flame acceleration plays a major role on the explosion effects. Then, it is of importance to understand the flame acceleration process and to predict explosion effects in open and congested areas for industrial safety reasons. In this aim, small-scale deflagration experiments were performed in cylindrical congested volumes of hydrogen – air mixtures varying from 1.77 L to 7.07 L. The influence of the reactivity was studied since the equivalence ratio of hydrogen – air mixtures were ranging from 0.5 to 2. The congestion was realized with varying numbers of grid layers and configurations. Experimental results, in term of flame speeds, were compared to results from correlations of the literature. Correlations were also adapted to the small-scale and modified to take into account the volume and the reactivity of the combustible mixtures.     

Three-dimensional flame displacement speed and flame front curvature measurements using quad-plane PIV

In this work we demonstrate the use of a quad-plane particle image velocimetry technique in order to investigate the three-dimensional behavior of several important quantities for combustion research such as the flame displacement speed and the flame front curvature. In results from a premixed methane flame stabilized in a diffuser burner, a comparison of three-dimensional and two-dimensional data is made in order to critically analyze the error of the usually performed planar measurements. It is shown that two-dimensional measurements can only give an estimate of the real situation under certain circumstances, such as with mainly spherical structures in the flame and the perfect alignment of both the flame propagation and flow direction to the measurement plane. However, in turbulent flames, this alignment can never be achieved due to fluctuations stemming from turbulence. The application of two crossed planes leads to significant improvements and good agreement with the three-dimensional quantities can be observed, although no perfect match is achieved. Flame displacement speeds ranging from ?0.4 s_L to 4.5 s_L with a mean of 1.1 s_L were recorded, but were not correlated with the flame curvature or strain rate.     

Effect of preferential diffusion and flame stretch on flame structure and laminar burning velocity of syngas Bunsen flame using OH-PLIF

By using OH-PLIF technique, experiments were conducted for laminar Bunsen flame of premixed CO/H₂/air mixtures with equivalence ratio ranging from 0.5 to 1.8. Reynolds number was varied from 800 to 2200, X_H2 = H₂/(H₂+CO) in the mixture was varied from 20% to 100% to study the effects of both preferential diffusion and flame curvature on flame structures and laminar flame burning velocities. Results showed that the combined effects of preferential diffusion and curvature gave an interesting phenomenon of the flame OH radical distributions on high hydrogen content flames. Furthermore, with the increase of H₂ fraction in fuel mixture, the effects of both preferential diffusion and flame curvature were increased. Interpretation of flame stretch effect on laminar burning velocity is also provided in this paper.     

The cylindrical stretched flame

For sufficiently cool remote gases, the cylindrical stretched flame shows classical ignition-extinction behavior. For remote gas temperatures close to the adiabatic flame temperature, the flame response is qualitatively different, with negative flame speed solutions which may be physically accessible.     

A tubular flame theory

The axisymmetric tubular flame established in a rotating flow field was studied theoretically to explain the effects of flow rate and equivalence ratio observed in the previous experiment. The incompressible viscous flow field was solved exactly in terms of an axisymmetric similar solution. The predicted flame structure and stability behavior correlate satisfactory with those of the experiment.     

Determination of particle temperatures in a silica-generating counterflow flame via flame emission measurements

We propose a simple technique to measure particle temperatures in a particle generating counterflow flame. The silica particle temperature was derived from flame light emission measurements. This technique allows the non-intrusive measurement of particle temperatures over 2000 K. In addition, the OH concentration distribution in the hydrogen–oxygen flame was estimated from flame emission spectra in the ultraviolet region. A numerical model of the combustion processes, which included the reactions of SiCl₄ leading to the formation of silica particles, verified that the measured particle temperatures and OH concentration were close to the theoretical values.     

On the transition from a highly turbulent curved flame into a tulip flame

Experimental and numerical investigations of premixed flame propagation behaviour associated with vortex interactions due to planar pressure waves crossing a curved flame front have been carried out. The resulting “tulip flame” formation in such a closed tube has been studied by Schlieren visualization. The “tulip flame” phenomenon was observed only in closed tubes, while cellular flame fronts appeared in half-open tubes. A physical model has been developed and implemented in a discrete vortex method combined with a flame tracking algorithm. The numerical method has been applied to model and understand the processes that cause the flame to change from a curved to a tulip shape. The results of the simulation are in good agreement with the experimental observations. We find that the rotational flows causing the tulip formation in our experimental case originate from the baroclinic effect — an interaction of non-parallel density and pressure gradients. Pressure waves were generated ahead of the accelerating and highly turbulent flame front. In closed tubes the pressure waves were reflected and crossed the curved flame front. As a result we saw the “tulip flame”. Within 0.5 ms, the flame front velocity reversed from about 50 m/s to about −20 m/s.     

Postulations of flame spread mechanisms

An experimental study was conducted to explore the flame spread mechanism over thin solid fuel sheets. Flame spread rates over paper at various inclined angles were measured, and Schlieren photography was used to qualitatively assess heat transfer to the unburnt material in front of the pyrolysis zone. Two different types of flame spread were observed. One is te downward flame spread observed in the range of ?90 to ?30 deg from the horizontal. In this region, flame spread rate was almost constant with time, although it increased slightly with increasing angle. The other type of spread observed was the accelerative upward flame spread at angles of zero to 90 deg. Flame spread at angles from ?30 deg to zero seemed unsatable and increased by repetitive acceleration and deceleration In the range of inclined angles from ?90 to ?30 deg, heat transfer from the flame zone to the unburnt material seemed to take place mainly through the gas phase in the region 0.2 ～ 0.4 cm in front of the pyrolysis zone. In this case, the direction of the gas stream could be considered to oppose that of the flame spread. In the case of upward flame spread, the unburnt material in front of the pyrolysis zone seemed to be heated by convection of the bottom side, where the direction of the gas stream was obviously parallel with that of flame spread.     

The fast-response flame ionization detector

The fast-response flame ionization detector has become a widely used instrument for time-resolved hydrocarbon measurement in internal combustion engines. The characteristics of and working experience with the instrument are reviewed. In particular, the sampling system and its performance for isolating the pressure pulsation in in-cylinder and in engine exhaust measurements are described. Results from different applications are given to illustrate the utilities of the instrument.     

Intensity calibrated hydrogen flame spectrum

The detection of hydrogen fires is important to the aerospace community. The National Aeronautics and Space Administration (NASA) has devoted significant effort to the development, testing, and installation of hydrogen fire detectors based on ultraviolet, near-infrared, mid-infrared, and/or far-infrared flame emission bands. Yet, there is no intensity calibrated hydrogen-air flame spectrum over this range in the literature and consequently, it can be difficult to compare the merits of different radiation-based hydrogen fire detectors. In this paper we present an intensity calibrated irradiance spectrum for a low pressure hydrogen flame burning in air from 200 nm to 13.5 microns that varies by more than six orders of magnitude. The results resolve relative intensity errors between spectral bands that appear within the literature. The impact of the measured spectrum on the choice of radiation-based hydrogen fire detectors is discussed.     

The influence of fuel-air swirl intensity on flame structures of syngas swirl-stabilized diffusion flame

Flame structures of a syngas swirl-stabilized diffusion flame in a model combustor were measured
using the OH-PLIF method under different fuel and air swirl intensity.The flame operated under
atmospheric pressure with air and a typical low heating-value syngas with a composition of 28.5%
CO,22.5% H2 and 49% N2 at a thermal power of 34 kW.Results indicate that increasing the air swirl
intensity with the same fuel,swirl intensity flame structures showed little difference except a small
reduction of flame length;but also,with the same air swirl intensity,fuel swirl intensity showed great
influence on flame shape,length and reaction zone distribution.Therefore,compared with air swirl
intensity,fuel swirl intensity appeared a key effect on the flame structure for the model
combustor.Instantaneous OH-PLIF images showed that three distinct typical structures with an
obvious difference of reaction zone distribution were found at low swirl intensity,while a much
compacter flame structure with a single,stable and uniform reaction zone distribution was found at
large fuel-air swirl intensity.It means that larger swirl intensity leads to efficient,stable combustion of
the syngas diffusion flame.