Blowoff characteristics of bluff-body stabilized conical premixed flames with upstream spatial mixture gradients and velocity oscillations

This experimental study concerns determination of blowoff equivalence ratios for lean premixed conical flames for different mixture approach velocities ranging from 5 to 16 m/s in the presence of spatial mixture gradients and upstream velocity modulation. Conical flames were anchored on a disk-shaped bluff body that was attached to a central rod in the burner nozzle. A combustible propane-air mixture flowed through a converging axisymmetric nozzle with a concentric insert, allowing radial mixture variation by tailoring the composition in the inner and outer parts of the nozzle. The radial mixture profiles were characterized near the location of the flame holder by laser Rayleigh light scattering. Additionally, a loudspeaker at the nozzle base allowed introduction of periodic velocity oscillations with an amplitude of 9% of the mean flow velocity up to a frequency of 350 Hz. The flame blowoff equivalence ratio was experimentally determined by continuously lowering the fuel flow rates and determining the flame detachment point from the flame holder. Flame detachment was detected by a rapid reduction of CH* emission from the flame base imaged by a photomultiplier detector. It was found that the flame blowoff is preceded by progressive narrowing of the flame cone for the case of higher inner jet equivalence ratios. In this case, the fuel-lean outer flow cannot sustain combustion, and clearly this is not a good way of operating a combustor. Nevertheless, the overall blowoff equivalence ratio is reduced by inner stream fuel enrichment. A possible explanation for this behavior is given based on the radial extent of the variable-equivalence-ratio mixture burning near the flame stabilization region. Fuel enrichment in the outer flow was found to have no effect on blowoff as compared to the case of uniform mixture. The results were similar for the whole range of mean flow velocities and upstream excitation frequencies.     

Blowoff mechanism of two dimensional bluff-body stabilized turbulent premixed flames in a prototypical combustor

Near blowoff dynamics and characteristics of turbulent premixed flames stabilized by a triangular flame holder in the midspan of a rectangular duct were studied using high speed imaging and simultaneous particle imaging velocimetry and OH planar laser-induced fluorescence. Near blowoff dynamics manifested by the onset of asymmetric vortex shedding and local extinction were observed. It has been proposed that a partial or total extinction of the flame sheet along the shear layers is the major factor that determines the final blowoff event. Observation of flame kernels within the recirculation zone, which under stable conditions contain only combustion products, is further evidence of the shear layer extinction. For a stably burning planar V flame away from blowoff, the flame front envelopes the Kelvin Helmholtz vortices. Near blowoff, the two flame fronts become more aligned with the flow direction due to reduction in flame speed and interact with the vortices emanating from the shear layer. This overlap induces high local stretch rates that exceed the extinction stretch rates, resulting in local flame extinction along the shear layers. Following extinction, fresh reactants are entrained into and react within the recirculation zone, with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for time scales of about one hundred milliseconds, potentially reigniting the shear layers such that the entire flame is re-established for a short period. This shear layer extinction and re-ignition event can happen several times before final blowoff, which occurs when the flame kernel fails to reignite the shear layers before being extinguished itself, thus leading to global flame extinction.     

Upstream flow dynamics of a laminar premixed conical flame submitted to acoustic modulations

This study is concerned with the response of conical flames to acoustic modulations. It deals with the dynamics of the velocity field in the fresh gases feeding the flame. Experiments are carried out to determine the gain and phase shift between the excitation signal and the axial velocity signal. This information, combined with PIV data, is used to identify the propagation mode in the fresh stream. Experiments indicate that three ranges can be defined based on a Strouhal number St involving the burner diameter and the upstream flow velocity. When this number is sufficiently low (St?1), the response consists in a convective wave featuring a phase velocity close to that of the mean flow. As St is augmented (1?St?Stc), where Stc depends on the flame geometry, the phase difference between the velocity oscillation and the imposed signal nearly vanishes in a finite region adjacent to the burner exhaust indicating that the perturbation propagates at the speed of sound. Further away from the burner, velocity perturbations exhibit convective features again. In the third frequency range, corresponding to higher modulation frequencies (St?Stc), velocity perturbations are dominated by acoustics in most of the experimental domain. It is shown that this behavior results from the upstream influence of the flame wrinkling. The region of influence may be deduced by considering the velocity potential associated with the flame motion. When this perturbation potential takes large values, the flow is dominated by the convective wave. This suitably reproduces experimental observations.     

The anchoring mechanism of a bluff-body stabilized laminar premixed flame

The objective of this work is to investigate the mechanism of the laminar premixed flame anchoring near a heat-conducting bluff-body. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane–air combustion. No artificial flame anchoring boundary conditions were imposed. Simulations show a shear-layer stabilized flame just downstream of the bluff-body, with a recirculation zone formed by the products of combustion. A steel bluff-body resulted in a slightly larger recirculation zone than a ceramic bluff-body; the size of which grew as the equivalence ratio was decreased. A significant departure from the conventional two-zone flame-structure is shown in the anchoring region. In this region, the reaction zone is associated with a large negative energy convection (directed from products to reactants) resulting in a negative flame-displacement speed. It is shown that the premixed flame anchors at an immediate downstream location near the bluff-body where favorable ignition conditions are established; a region associated with (1) a sufficiently high temperature impacted by the conjugate heat exchange between the heat-conducting bluff-body and the hot reacting flow and (2) a locally maximum stoichiometry characterized by the preferential diffusion effects.     

The influence of reactant temperature on the dynamics of bluff body stabilized premixed flames

The impact of increased reactant temperature on the dynamics of bluff-body stabilized premixed flows is investigated using numerical simulation. A two-dimensional triangular bluff body is considered. Flow compressibility is assumed to exist at the low Mach number limit and combustion is fast and robust such that a flamesheet representation is assumed to apply. In this formulation, reactant temperature variations are represented via corresponding temperature ratio and flame speed variations. The Lagrangian, Transport Element Method is used to provide the numerical solutions. Results indicate that as reactant temperature increases, the fluid dynamics transition from a low amplitude, broadband, coarsely symmetric (about the bluff-body centerline) behavior, to a high amplitude, tonal and asymmetric one that bears similarities to the corresponding non-reacting flow. The reasons for this are that as the reactant temperature increases, (i) the temperature ratio across the flame is reduced, thus reducing combustion exothermicity, and (ii) the flame speed increases causing the flame to propagate away from the bluff-body wake. In both cases the ability of the two main combustion-driven fluid dynamical processes, namely volumetric expansion and baroclinic generation to impact the bluff body generated vorticity is reduced. Reduction in baroclinic generation enables to wake to survive futher downsteam and makes the flow susceptible to the wake instability. As reactant temperature is increased the location of the onset of the instability moves upstream. At very high reactant temperatures even the near field symmetrizing effect of volumetric expansion is overwhelmed and asymmetric vortex shedding is witnessed at the bluff body. Even in this regime, the flow differs from the non-reacting flow in that it is susceptible to bifurcations in vortex shedding behavior that are linked to local flame-vortex interactions. Results also show that in the general case, knowledge of the fluid dynamics alone is not sufficient to characterize the flame dynamics, as the flame position in relation to the vorticity field is critical to the unsteady flame response. Specifically, the flame exhibits a similar transition from a broadband to a tonal response but the amplitude is not monotonically increasing. Rather it experiences a regime of decreasing response for intermediate reactant temperatures where the flame propagation away from the wake appears to dominate the increase in fluid dynamical induced oscillations due to the enhancement of the asymmetric mode.     

Effect of dilatation on scalar dissipation in turbulent premixed flames

The scalar dissipation rate signifies the local mixing rate and thus plays a vital role in the modeling of reaction rate in turbulent flames. The local mixing rate is influenced by the turbulence, the chemical, and the molecular diffusion processes which are strongly coupled in turbulent premixed flames. Thus, a model for the mean scalar dissipation rate, and hence the mean reaction rate, should include the contributions of these processes. Earlier models for the scalar dissipation rate include only a turbulence time scale. In this study, we derive exact transport equations for the instantaneous and the mean scalar dissipation rates. Using these equations, a simple algebraic model for the mean scalar dissipation rate is obtained. This model includes a chemical as well as a turbulence time scale and its prediction compares well with direct numerical simulation results. Reynolds-averaged Navier-Stokes calculations of a test flame using the model obtained here show that the contribution of dilatation to local turbulent mixing rate is important to predict the propagation phenomenon.     

Effects of premixed flames on turbulence and turbulent scalar transport

Experimental data and results of direct numerical simulations are reviewed in order to show that premixed combustion can change the basic characteristics of a fluctuating velocity field (the so-called flame-generated turbulence) and the direction of scalar fluxes (the so-called countergradient or pressure-driven transport) in a turbulent flow. Various approaches to modeling these phenomena are discussed and the lack of a well-elaborated and widely validated predictive approach is emphasized. Relevant basic issues (the transition from gradient to countergradient scalar transport, the role played by flame-generated turbulence in the combustion rate, the characterization of turbulence in premixed flames, etc.) are critically considered and certain widely accepted concepts are disputed. Despite the substantial progress made in understanding the discussed effects over the past decades, these basic issues strongly need further research.     

Three-dimensional temperature gradients in premixed turbulent flamelets via crossed-plane Rayleigh imaging

A new technique for obtaining instantaneous, high-resolution, three-dimensional thermal structure data from turbulent flames, crossed-plane Rayleigh imaging is described and then demonstrated. Quantitative Rayleigh imaging measurements are made simultaneously in two orthogonal, intersecting laser-sheet illumination planes. At points along the line of intersection of the two laser sheets, instantaneous, three-dimensional temperature gradient data are measured. The technique has higher resolution than parallel plane measurement techniques, which have limited resolution in the direction orthogonal to the parallel planes. The technique is used to measure temperature gradient data for a lean, premixed, methane-air turbulent V-flame with an equivalence ratio of Φ=0.7, and normalized turbulence intensity (, where u^′ is the turbulence intensity and is the unstretched laminar flame speed. Measurements are also presented for a laminar V-flame and a laminar Bunsen flame for comparison. Finally, an unstretched laminar flame calculation is made. Quantitative estimates of the experimental uncertainty are presented. The primary source of uncertainty in the data is due to shot noise. Measured temperature gradient data for laminar flames differ from that of the unstretched laminar flame calculation, especially in the oxidation layer. Turbulent flame temperature gradient data indicate that the turbulent V-flame thermal structure is not significantly perturbed from the measured laminar V-flame structure. For the flame studied, the flamelet approximation is valid if the flamelet used is based on the measured laminar flame structure of the V-flame. Isothermal surface orientation data are presented and are close to parallel for most realizations. Isothermal surface density is calculated from the distribution of isothermal surface orientations and from conditional averages of the magnitude of the temperature gradient. Isothermal surface density does not vary for different isothermal surfaces.     

Blowoff dynamics of bluff body stabilized turbulent premixed flames

This article concerns the flame dynamics of a bluff body stabilized turbulent premixed flame as it approaches lean blowoff. Time resolved chemiluminescence imaging along with simultaneous particle image velocimetry and OH planar laser-induced fluorescence were utilized in an axisymmetric bluff body stabilized, propane-air flame to determine the sequence of events leading to blowoff and provide a quantitative analysis of the experimental results. It was found that as lean blowoff is approached by reduction of equivalence ratio, flame speed decreases and the flame shape progressively changes from a conical to a columnar shape. For a stably burning conical flame away from blowoff, the flame front envelopes the shear layer vortices. Near blowoff, the columnar flame front and shear layer vortices overlap to induce high local stretch rates that exceed the extinction stretch rates instantaneously and in the mean, resulting in local flame extinction along the shear layers. Following shear layer extinction, fresh reactants can pass through the shear layers to react within the recirculation zone with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for a few milliseconds and can reignite the shear layers such that the entire flame is reestablished for a short period. This extinction and reignition event can happen several times before final blowoff which occurs when the flame kernel fails to reignite the shear layers and ultimately leads to total flame extinguishment.     

A filtered-laminar-flame PDF sub-grid-scale closure for LES of premixed turbulent flames: II. Application to a stratified bluff-body burner

Large eddy simulation of the two stratified nonswirling configurations of the Cambridge burner studied by Sweeney et al. (2012) is presented. The sub-grid-scale combustion closure relies on a physical space filtering operation with a filter size determined locally depending on the resolved and sub-grid-scale flame properties, which is discussed in a companion paper. Similarly to the premixed configuration of the same burner, the modeling reproduces the differential diffusion effects leading to accumulation of carbon and an enhancement of mixture fraction in the recirculation zone, an effect that is less pronounced than in the fully lean premixed case, because of the modification of the topology of the reaction zone that is induced by the mixture stratification. The study of the LES combustion regimes shows that the reaction zones develop under a quite large range of flame topologies, from wrinkled flamelets up to thin reaction zones. Instantaneous and time-averaged LES data were analyzed to extract information concerning the degree of stratification and the orientation of flame and mixing vectors. A decomposition of the flame response into premixed, diffusion, and partially premixed flamelets is performed, to conclude that the premixed mode dominates close to the burner, with a partially premixed burning regime further downstream. Overall, the length scales associated with stratification were found to be much larger than that of the reaction zone and flame, resulting in a quasi-homogeneous propagation, predominantly in a back supported stratified combustion regime. Overall good agreement between simulation and measurements was obtained for either configurations.     

Effects of position and frequency of obstacles on turbulent premixed propagating flames

This paper studies the effects of the number and location of solid obstacles on the rate of propagation of turbulent premixed flames. A vented explosion chamber is constructed where controlled premixed flames are ignited from rest to propagate past grids or baffles plates as well as other solid obstacles strategically positioned in the chamber. Laser Induced Fluorescence (LIF) is used to image OH which is used as an indicator of the reaction zone while pressure transducers are used to obtain pressure-time traces. Single grids or baffle plates located at different distances from the ignition source are tested. Two as well as three baffle plates are also investigated in varying configurations. It is found that while the peak overpressure increases with increasing number of grids or baffle plates, a limit is reached where the pressure starts to decrease. The location of the obstacles is found to have a significant effect on the overpressure and the flame structure. Higher overpressures are obtained when the baffle plates and obstacles are stacked closer together hence not allowing turbulence to decay. LIF images for OH show that the reaction zones become more contorted with increasing number of baffle plates in the flame path.     

Nonlinear combustion instability analysis based on the flame describing function applied to turbulent premixed swirling flames

Instability analysis of swirling flames is of importance in the design of advanced combustor concepts for aircraft propulsion and powerplant for electricity production. Thermoacoustic instabilities are analyzed here by making use of a nonlinear representation of flame dynamics based on a describing function. In this framework, the flame response is determined as a function of frequency and amplitude of perturbations impinging on the combustion region. This model is adapted to the case of confined swirling flames comprising an upstream manifold, an injection unit equipped with a swirler and a cylindrical flame tube. The flame describing function is experimentally determined and is combined with an acoustic transfer matrix representation of the system to provide growth rates and oscillation frequencies as a function of perturbation amplitude. These data can be used to determine regions of instability, frequency shifts with respect to the acoustic eigenfrequencies and they also yield amplitude levels when self-sustained oscillations of the system have reached a limit cycle. This equilibrium is obtained when the amplitude dependent growth rate equals the damping rate in the system. This requires an independent determination of this last quantity which is here based on measurements of the combustor resonance response curve, together with numerical estimates of the flame contribution to the system response. The geometrical parameters of the upstream manifold and flame tube are varied and the corresponding operating regimes are compared with those predicted with the FDF framework. The present demonstration of the FDF framework in a generic configuration indicates that this can be used in more general situations of technological interest.     

Flame brush characteristics and burning velocities of premixed turbulent methane/air Bunsen flames

The flame brush characteristics and turbulent burning velocities of premixed turbulent methane/air flames stabilized on a Bunsen-type burner were studied. Particle image velocimetry and Rayleigh scattering techniques were used to measure the instantaneous velocity and temperature fields, respectively. Experiments were performed at various equivalence ratios and bulk flow velocities from 0.7 to 1.0, and 7.7 to 17.0 m/s, respectively. The total turbulence intensity and turbulent integral length scale were controlled by the perforated plate mounted at different positions upstream of the burner exit. The normalized characteristic flame height and centerline flame brush thickness decreased with increasing equivalence ratio, total turbulence intensity, and longitudinal integral length scale, whereas they increased with increasing bulk flow velocity. The normalized horizontal flame brush thickness increased with increasing axial distance from the burner exit and increasing equivalence ratio. The non-dimensional leading edge and half-burning surface turbulent burning velocities increased with increasing non-dimensional turbulence intensity, and they decreased with increasing non-dimensional bulk flow velocity when other turbulence statistics were kept constant. Results show that the non-dimensional leading edge and half-burning surface turbulent burning velocities increased with increasing non-dimensional longitudinal integral length scale. Two correlations to represent the leading edge and half-burning surface turbulent burning velocities were presented as a function of the equivalence ratio, non-dimensional turbulence intensity, non-dimensional bulk flow velocity, and non-dimensional longitudinal integral length scale. Results show that the half-burning surface turbulent burning velocity normalized by the bulk flow velocity decreased as the normalized characteristic flame height increased.     

Investigation of the nonlinear response of turbulent premixed flames to imposed inlet velocity oscillations

Acoustically forced lean premixed turbulent bluff-body stabilized flames are investigated using turbulent combustion CFD. The calculations simulate aspects of the experimental investigation by Balachandran et al. [R. Balachandran, B. Ayoola, C. Kaminski, A. Dowling, E. Mastorakos, Combust. Flame 143 (2005) 37-55] and focus on the amplitude dependence of the flame response. For the frequencies of interest in this investigation an unsteady Reynolds-averaged Navier-Stokes (URANS) approach is appropriate. The combustion is represented using a modified laminar flamelet approach with an algebraic representation of the flame surface density. The predictions are compared with flame surface density (FSD) and OH^∗ chemiluminescence measurements. In the experiments the response of the flame has been quantified by means of a number of single-frequency, amplitude-dependent transfer functions. The predicted flame shape and position are in good agreement with the experiment. The dynamic response of the flame to inlet velocity forcing is also well captured by the calculations. At moderate frequencies nonlinear behavior of the transfer functions is observed as the forcing amplitude is increased. In the experiments this nonlinearity was attributed in part to the rollup of the reacting shear layer into vortices and in part to the collision of the inner and outer flame sheets. This transition to nonlinearity is also observed in the transfer functions obtained from the predictions. Furthermore, the vortex shedding and flame-sheet collapse may be seen in snapshots of the predicted flow field taken throughout the forcing cycle. The URANS methodology successfully predicts the behavior of the forced premixed turbulent flames and captures the effects of saturation in the transfer function of the response of the heat release to velocity fluctuations.     

Effects of dimethyl methylphosphonate on premixed methane flames

The impact of dimethyl methylphosphonate (DMMP) was studied in a premixed methane/oxygen/N₂-Ar flame in a flat flame burner slightly under atmospheric pressure at two different equivalence ratios: rich and slightly lean. CH₄, CO, CO₂, CH₂O, CH₃OH, C₂H₆, C₂H₄, and C₂H₂ profiles were obtained with a Fourier Transform Infrared (FTIR) spectrometer. Gas samples, analyzed in the FTIR, were extracted from the reaction zone using a quartz microprobe with choked flow at its orifice. Temperature profiles were obtained by measuring the probe flow rate through the choked orifice. Flame calculations were performed with two existing detailed chemical kinetic mechanisms for organophosphorus combustion. DMMP addition caused all profiles except that of CH₃OH to move further away from the burner surface, which can be interpreted as a consequence of a reduction in the adiabatic flame speed. Experimentally, the magnitude of the shift was 50% greater for the near-stoichiometric flame than for the rich flame. Experimental CH₃OH profiles were four to seven times higher in the doped flames than in the undoped ones. The magnitude of this effect is not predicted in the calculations, suggesting a need for further mechanism development. Otherwise, the two mechanisms are reasonably successful in predicting the effects of DMMP on the flame.     

Nonlinear thermoacoustics of ducted premixed flames: The influence of perturbation convection speed

When a premixed flame is placed within a duct, acoustic waves induce velocity perturbations at the flame’s base. These travel down the flame, distorting its surface and modulating its heat release. This can induce self-sustained thermoacoustic oscillations. Although the phase speed of these perturbations is often assumed to equal the mean flow speed, experiments conducted in other studies and Direct Numerical Simulation (DNS) conducted in this study show that it varies with the acoustic frequency. In this paper, we examine how these variations affect the nonlinear thermoacoustic behaviour. We model the heat release with a nonlinear kinematic G-equation, in which the velocity perturbation is modelled on DNS results. The acoustics are governed by linearised momentum and energy equations. We calculate the flame describing function (FDF) using harmonic forcing at several frequencies and amplitudes. Then we calculate thermoacoustic limit cycles and explain their existence and stability by examining the amplitude-dependence of the gain and phase of the FDF. We find that, when the phase speed equals the mean flow speed, the system has only one stable state. When the phase speed does not equal the mean flow speed, however, the system supports multiple limit cycles because the phase of the FDF changes significantly with oscillation amplitude. This shows that the phase speed of velocity perturbations has a strong influence on the nonlinear thermoacoustic behaviour of ducted premixed flames.     

Experimental study on the behaviors and shape changes of premixed hydrogen-air flames propagating in horizontal duct

The behaviors and shape changes of premixed hydrogen-air flames at various equivalence ratios propagating in half-open and closed horizontal ducts are experimentally investigated using high-speed schlieren imaging and pressure sensors. The study shows that the premixed hydrogen-air flame undergoes more complex shape changes and exhibits more distinct characteristics than that of other gaseous fuels. One of the outstanding findings is that obvious distortion happens to tulip flame after its full formation when equivalence ratio ranges from 0.84 to 4.22 in the closed duct. The salient tulip flame distortions are specially scrutinized and distinguished from the classical tulip collapse and disappearance. The dynamics of distorting tulip flame is different from that of classical tulip flame. The normal tulip flame can be reproduced after the first distortion followed by another distortion. The initiation of flame shape changes coincides with the deceleration both of pressure rise and flame front speed for flames with tulip distortions. And the formation and dynamics of tulip/distorting tulip flames depend on the mixture composition.     

The combined dynamics of swirler and turbulent premixed swirling flames

The dynamics of premixed confined swirling flames is investigated by examining their response to incident velocity perturbations. A generalized transfer function designated as the flame describing function (FDF) is determined by sweeping a frequency range extending from 0 to 400 Hz and by changing the root mean square fluctuation level between 0% and 72% of the bulk velocity. The unsteady heat release rate is deduced from the emission intensity of OH^* radicals. This global information is complemented by phase conditioned Abel transformed emission images. This processing yields the distribution of light emission. By assuming that the light intensity is proportional to the heat release rate, it is possible to deduce the distribution of unsteady heat release rate in W m⁻³ and see how it evolves with time during the modulation cycle and for different forcing frequencies. These data can be useful for the determination of regimes of instability but also give clues on the mechanisms which control the swirling flame dynamics. It is found from experiments and demonstrated analytically that a swirler submitted to axial acoustic waves originating from the upstream manifold generates a vorticity wave on its downstream side. The flame is then submitted to a transmitted axial acoustic perturbation which propagates at the speed of sound and to an azimuthal velocity perturbation which is convected at the flow velocity. The net result is that the dynamical response and unsteady heat release rate are determined by the combined effects of these axial and induced azimuthal velocity perturbations. The former disturbance induces a shedding of vortices from the injector lip which roll-up the flame extremity while the latter effectively perturbs the swirl number which results in an angular oscillation of the flame root. This motion is equivalent to that which would be induced by perturbations of the burning velocity. The phase between incident perturbations is controlled by the convective time delay between the swirler and the injector. The constructive or destructive interference between the different perturbations is shown to yield the low and high gains observed for certain frequencies.