Flame-vortex Interaction Research Articles

Effects of CO 2 dilution on the interactions of a CH 4–air nonpremixed jet flame with a single vortex

We carried out numerical simulations to understand how CO 2 dilution in either fuel or oxidizer stream changes the flame-vortex interactions in terms of hydrodynamic effects in CH 4–air nonpremixed jet flames. The simulation used a time-dependent, axisymmetric computational model and a low Mach number approximation. Reaction rates were calculated from a two-step global reaction mechanism that considered six species. Studies were conducted with fixed initial velocities for three different cases of CO 2 introduction: (1) without dilution, (2) dilution in a fuel stream, and (3) dilution in an oxidizer stream. A single vortex was generated by an axisymmetric jet driven of cold fuel, after a flame development was reached to quasi steady-state condition. The simulation shows that CO 2 dilution in a fuel stream leads to a slightly increased vortex radius and more entrainment of surrounding fluids compared to the other dilution methods tested. Thus, dilution of CO 2 in a fuel stream enhances the mixing inside a single vortex and increases the stretching of the flame surface. The vorticity transport equation budgets were examined to reveal the mechanisms of vortex formation in the presence of CO 2. In the stage of vortex formation, vortex production due to baroclinic torque and vortex destruction due to volumetric expansion were found to be greater in the case of CO 2 dilution in a fuel stream than in the other dilution cases. However, after vortex formation, there terms showed the opposite tendencies.

Turbulent premixed flame modeling using artificial neural networks based chemical kinetics

The applicability of Artificial Neural Networks (ANN) approach as a chemistry integrator for Large Eddy Simulations (LES) of reactive flows is evaluated with special emphasis on generating training tables independent of the computation of interest. An ANN code based on back-propagation algorithm is developed with a new approach for self-determining the model coefficients adaptively with respect to the error surface topology. The training table is constructed with an independent flame study, and the trained networks are used in LES of turbulent Flame–Vortex Interaction (FVI) studies at different equivalence ratios and turbulence levels. It is shown that once the ANN is well trained, it can successfully predict the reaction rates in both memory and time efficient manner compared to traditional look-up table approach and stiff ODE solvers, respectively.

Extinction and interruption of diffusion flame interacting with a large scale vortex

Instantaneous and simultaneous measurements of two-dimensional temperature and OH LIF profiles by combining Rayleigh scattering with laser-induced fluorescence (LIF) were demonstrated in nitrogen-diluted hydrogen (H2 30%+N2 70%) diffusion flame interacting with a large scale vortex induced by the continuous acoustic excitation on the fuel side. It is investigated to analyze dynamic behavior of flame extinction and interruption during the flame–vortex interaction processes. The results obtained are described as follows. (1) The reaction zone of the closed dome type is formed due to fuel jet and remaining air at vortex front. The upper OH layer above this region is disintegrated near the central axis. The temperature and the OH LIF at the reaction zone become significantly lowered at the convex and circumferential part of the vortex where temperature layer of the vortex exists and the thinning of the OH LIF and temperature layer and the consequent flame extinction are induced. (2) By curling up motion of the vortex, the burnt gas is pulled up from the upstream reaction zone and makes axial high-temperature region between the vortex and fuel jet. The burnt gas reaches the circumferential reaction zone and the low temperature layer between circumferential reaction zone and the burnt gas dissipates locally with time. The dissipated burnt gas reduces the supply of fuel into the circumferential reaction zone and interruption of the reaction zone is induced.

Simultaneous water vapor concentration and temperature measurements in unsteady hydrogen flames

Techniques applicable to visualization and scalar measurements in large-scale hydrogen fires are highly desired, but lacking. In this research, planar thermal images of a buoyancy-driven unsteady laminar hydrogen/air flame and line images of a thin filament stretched across this flame were obtained using an infrared (IR) camera at a sampling frequency of 348 Hz. A pulsing frequency of 11 Hz was measured. Transient line measurements of temperature ( T) and water vapor mole fraction ( X H 2 O ) have been achieved using inverse radiation calculations. Instantaneous X H 2 O and T distributions during a flame–vortex interaction cycle were obtained with temporal and spatial resolutions of 3 ms and 1.7 mm, respectively. The instantaneous distributions of X H 2 O and T were effected by preferential diffusion and the altered velocity field of the flame. Vortices caused more scalar fluctuations outside the flame surface than inside. The present X H 2 O and T measurements are consistent with detailed chemistry calculations of the flame and laser diagnostic measurements of similar flames. With some modifications, the thermal imaging based technique can be extended to large-scale hydrogen fire applications.

Time-resolved conditional flow field statistics in extinguishing turbulent opposed jet flames using simultaneous highspeed PIV/OH-PLIF

Time-resolved particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), both at 5 kHz, were applied simultaneously on extinguishing turbulent opposed jet flames. This repetition rate allowed tracking of transient extinction events in turbulent combustion. The additional information acquired about time history enabled a study of the evolution of vortex-flame interactions leading to extinction from individual events. A newly introduced multidimensional conditioning technique to avoid spatial- and temporal-smearing of important flow field information was developed in order to compare individual extinction events in a meaningful, statistical manner. The conditional statistics show that vortices tend to align around the flame and generate regions of high strain in the region where the flame is about to extinguish.

A Study of Flame-Vortex Interactions in the Presence of Residual Gases

In this work the effects of residual combustion products from a prior injection on flame extinction characteristics in a Diesel fuel pulse are investigated through a fundamental study. A numerical code with sixth-order accurate spatial discretization and multiple-step chemical kinetics is used to simulate flame-vortex interactions under Diesel conditions in the presence of residual combustion product mixtures. The results indicate that there is a balance between the heating effect of the high-temperature combustion products that enhance flame stability and the dilution effect of the reduced oxygen concentration, which promotes extinction. These counteracting aspects of the entrained combustion products can lead to an ideal dwell time between multiple injection events for optimum benefits in flame stability.

A 2-D DNS investigation of extinction and reignition dynamics in nonpremixed flame–vortex interactions

Two-dimensional (2-D) DNS investigations of extinction and reignition dynamics during interactions of laminar nonpremixed flames with counterrotating vortex pairs are performed. The length and velocity scales chosen for the vortices are representative of those in the near fields of high-Reynolds-number jets such as those occurring in Diesel engines. The governing equations are solved with sixth-order spatial discretization and fourth-order time integration. Chemistry is modeled as an irreversible single-step reaction. Local extinction along the symmetry axis, followed by reignition, is observed. The extinction is characterized by strong unsteady effects, which are captured well by 1-D transient diffusion flamelet libraries, provided the time-history of the instantaneous scalar dissipation rate is taken into account. On the other hand, reignition is essentially a 2-D phenomenon involving flame–flame interactions, which are favored for smaller vortices and increasing flame curvature. The effects of unsteadiness and curvature on extinction and reignition are carefully assessed through parametric studies involving a range of vortex and flame characteristics. The interaction outcomes are summarized on Reynolds–Damköhler number (Re–Da) diagrams, which show the combined effects of unsteadiness and curvature on extinction and reignition. The implications of the observed interaction outcomes for turbulent combustion modeling in the near fields of jet diffusion flames are discussed.

The response of buoyant laminar diffusion flames to low-frequency forcing

Buoyant jet diffusion flames are frequently used to investigate phenomena associated with flares or fires, such as the formation and emission of soot, polycyclic aromatic hydrocarbons (PAH), and carbon monoxide (CO). To systematically investigate the influence of transient vortex–flame interactions on these processes, laminar jet flames may be periodically forced. Previous work has demonstrated that forcing the fuel stream at a (low) frequency close to the natural buoyant instability frequency will trigger the production of vortices on the air side of the high-temperature reaction zone, coupling the overall flame response to the forcing frequency. In the work reported here, measurements in methane/air and ethylene/air slot flames show that over a substantial range of forcing frequencies and amplitudes, the dominant, air-side vortex production is locked at precisely one-half the excitation frequency of the fuel stream. This phenomenon is examined in detail through the utilization of several laser diagnostic techniques, yielding measurements of both the frequency response of the flames and phase-locked images of the internal flame structure. Under some conditions the subharmonic response of the flame leads to transient separation of the PAH and soot layers from the surrounding high-temperature flame zone, potentially affecting the soot formation and radiation processes. This data should provide useful information for comparison with detailed modeling aimed to improve the understanding of the complex nature of the buoyant instability in jet flames.

Simultaneous stereo particle image velocimetry and double-pulsed planar laser-induced fluorescence of turbulent premixed flames

A new technique was developed and applied to the study of flame structure and flame–vortex interaction in turbulent premixed flames. Turbulent premixed flames were probed using simultaneous stereo particle image velocimetry (PIV) and a double-pulsed acetone planar laser-induced fluorescence system (PLIF). Two double-pulsed Nd:YAG lasers operating at 532 and 266 nm were used for the PIV and acetone PLIF measurements, respectively. The stereo PIV images were acquired using two double-frame CCD cameras, and two ICCD cameras were used to capture the PLIF signal. The diagnostic system was applied to study turbulent methane–air stoichiometric premixed flames at relatively high Reynolds numbers. Flame merging and the creation of pockets of both products and reactants were detected, and a very strong interaction between the flame front and the vortex structures was suggested in the simultaneous PIV/PLIF images. Double-pulsed PLIF data obtained for different time delays allowed statistical study of flame development. Three-dimensional turbulent fluxes of mean progress variable were obtained. It was shown that the fluxes obey the gradient diffusion hypothesis. The proposed diagnostic increases flexibility and range of measurements available for premixed flames.

A regime diagram for premixed flame kernel-vortex interactions

Direct numerical simulations of flame kernel-vortex interactions are implemented in an axisymmetric configuration using a two-step global mechanism to study the different combustion regimes of the interactions. Four combustion regimes have been identified. They include: (1) the “laminar kernel” regime, (2) the “wrinkled kernel” regime, (3) the “breakthrough” regime, and (4) the “global extinction” regime. Transitions from different regimes are achieved through variations of the vortex strength, and operation in each regime is governed by two key parameters, the ratio of the vortex translational velocity to the laminar flame speed and the ratio of the kernel size to the vortex size at the onset of the interactions. Qualitative and quantitative comparisons between the flame responses in the different regimes are presented. A regime diagram is constructed based on the key parameters that control transition between the different regimes. The diagram bears some similarities with other diagrams based on planar flame-vortex interactions. However, it also offers additional features that constitute refinements to the existing diagrams of which the role of interaction of a vortex with an already curved flame is important.

Dynamic reduction of a CH4/air chemical mechanism appropriate for investigating vortex–flame interactions

AbstractThis paper describes two methods, piecewise reusable implementation of solution mapping (PRISM) and dynamic steady‐state approximation (DYSSA), in which chemistry is reduced dynamically to reduce the computational burden in combustion simulations. Each method utilizes the large range in species timescales to reduce the dimensionality to the number of species with slow timescales. The methods are applied within a framework that uses hypercubes to partition multidimensional chemical composition space, where each chemical species concentration, plus temperature, is represented by an axis in space. The dimensionality of the problem is reduced uniquely in each hypercube, but the dimensionality of chemical composition space is not reduced. The dimensionality reduction is dynamic and is different for different hypercubes, thereby escaping the restrictions of global methods in which reductions must be valid for all chemical mixtures. PRISM constructs polynomial equations in each hypercube, replacing the chemical kinetic ordinary differential equation (ODE) system with a set of quadratic polynomials with terms related to the number of species with slow timescales. Earlier versions of PRISM were applied to smaller chemical mechanisms and used all chemical species concentrations as terms. DYSSA is a dynamic treatment of the steady‐state approximation and uses the fast–slow timescale separation to determine the set of steady‐state species in each hypercube. A reduced number of chemical kinetic ODEs are integrated rather than the original full set. PRISM and DYSSA are evaluated for simulations of a pair of counterrotating vortices interacting with a premixed CH4/air laminar flame. DYSSA is sufficiently accurate for use in combustion simulations, and when relative errors are less than 1.0%, speedups on the order of 3 are observed. PRISM does not perform as well as DYSSA with respect to accuracy and efficiency. Although the polynomial evaluation that replaces the ODE solver is sufficiently fast, polynomials are not reused sufficiently to enable their construction cost to be recovered. © 2007 Wiley Periodicals, Inc. 39: 204–220, 2007

Numerical simulations of flame propagation and DDT in obstructed channels filled with hydrogen–air mixture

We study flame acceleration and deflagration-to-detonation transition (DDT) in channels with obstacles using 2D and 3D reactive Navier–Stokes numerical simulations. The energy release rate for the stoichiometric H 2–air mixture is modeled by a one-step Arrhenius kinetics. Computations show that at initial stages, the flame and flow acceleration is caused by thermal expansion of hot combustion products. At later stages, shock–flame interactions, Rayleigh–Taylor, Richtmyer–Meshkov, and Kelvin–Helmholtz instabilities, and flame–vortex interactions in obstacle wakes become responsible for the increase of the flame surface area, the energy-release rate, and, eventually, the shock strength. Computations performed for different channel widths d with the distance between obstacles d and the constant blockage ratio 0.5 reproduce the main regimes observed in experiments: choking flames, quasi-detonations, and detonations. For quasi-detonations, both the initial DDT and succeeding detonation reignitions occur when the Mach stem, created by the reflection of the leading shock from the bottom wall, collides with an obstacle. As the size of the system increases, the time to DDT and the distance to DDT increase linearly with d 2. We also observe an intermediate regime of fast flame propagation in which local detonations periodically appear behind the leading shock, but do not reach it.

A ghost-fluid method for large-eddy simulations of premixed combustion in complex geometries

In this paper, a new ghost-fluid method for interfaces of finite thickness is described. It allows to compute efficiently turbulent premixed flames with a finite thickness in low-Mach flows. A level set algorithm is used to track accurately the flame and to define the overlapping region where the burned and unburned gases satisfy the jump conditions. These algorithms are combined with a fractional-step method to alleviate the acoustic CFL constraint. The full algorithm is verified for simple flame–vortex interactions and it is validated by computing a turbulent flame anchored by a triangular flame-holder. Finally, the algorithm is applied in the LES of an industrial lean-premixed swirl-burner.

Optical Analysis and Qualitative Discrimination of Vortex-Flame Interaction in a Plane Premixed Shear Layer

To obtain practical schemes of vortex–flame interactions, a series of organized eddies formed in the plane premixed shear layer is investigated, instead of a single vortex ring or a single vortex tube. The plane premixed shear layer is first formed between two parallel uniform propane–air mixture streams. For getting clear qualitative pictures of vortex–flame interactions in the plane premixed shear layer, two extreme ignition points are assigned; one is assigned at the center of an organized eddy where the vortex motion plays an important role, the other at the midpoint between two adjacent organized eddies where the rolling-up motion prevails. A premixed flame is initiated by an electric discharge at one of the two assigned points and propagates either in the large scale organized eddy or along the interface between two uniform mixture streams. Propagation and deformation processes of the flame are observed using the simultaneously two-directional and high-speed Schlieren photography. The tangential velocity of organized eddy and the equivalence ratio of premixed shear flow are varied as two main parameters. The outline of propagating flame after the midpoint ignition is numerically analyzed by superposing the flame propagation having a constant burning velocity on the vortex flow field simulated with the discrete vortex method. The results obtained show that there exists another type of vortex–flame interaction in the plane shear layer in addition to the vortex bursting, and that it is caused by the rolling-up motion particular to the coherent structure in the plane shear layer and is simply named the vortex boosting. It is qualitatively concluded therefore that, in the ordinary turbulent premixed flames formed in the plane premixed shear layer, these two fundamental vortex-flame interactions get tangled with each other to augment the propagation velocity. An empirical expression which qualitatively takes into account of the effects of both vortex and chemical properties is finally proposed.

Soot topography in a planar diffusion flame wrapped by a line vortex

An experimental study of the interaction of a planar diffusion flame with a line vortex is presented. A planar diffusion flame is established between two coflowing, equal velocity streams of acetylene diluted with nitrogen and air. A line vortex is generated on demand by momentarily pulsing one of the flow streams by way of electromagnetic actuation of a piston in the flow apparatus. The flame–vortex interactions are diagnosed by planar laser-induced incandescence for soot yield and by particle image velocimetry for vortex flow characterization. The results show that soot formation and distribution are influenced by the reactant streams from which vortices are initiated. The vortices interacting with the flame from the air side produce more soot and soot is distributed in and around the vortex core in diffuse layers. In contrast, topography of soot in vortices interacting from the fuel side is such that soot is confined to thinner layers around the vortex core which does not contain any soot. The flame curvature is found to influence the local soot production with the flame regions convex to the fuel side containing more soot locally. It is also found that the overall soot yield is less sensitive to the vortex strength and is of lower magnitude when vortex is spun from the fuel side. The knowledge of this type of asymmetry in soot yield in flame–vortex interactions is useful for combustion engineering and design of practical devices.

모형 가스터빈 연소기에서 선회 예혼합화염의 대와동모사(LES)

본 논문에서는 대와동모사를 이용하여 모형 가스터빈 연소기에서 난류 예혼합연소의 선회 유동구조와 화염특성이 검토되었다. 비정상 화염 거동을 모사하기 위하여 G-방정식 화염편 모델이 적용되었다. 결과로서, 입구 선회수 증가에 따른 코너 및 중앙 재순환 유동이 뚜렷한 차이를 보이며, 화염의 길이도 점차 감소됨을 확인 할 수 있었다. 또한 강선회 조건에서 역화현상의 원인이 확인되었다. 정확한 비정상 화염거동의 모사를 위하여, 연소실 내 음향파 거동의 예측성능이 우선적으로 검토되었으며, 스텝 모서리 근처에서 생성된 와동이 화염면 변동에 가장 큰 영향을 주고 있음을 알 수 있었다. 마지막으로 비정상 화염-와동 상호작용에 대한 해석을 통해 선회와 음향파의 전개로부터 생성된 와동의 진동이 화염면 및 열발생의 변동과 밀접하게 관련되어짐을 체계적으로 규명하였다. In the present paper, the swirl flow structure and flame characteristics of turbulent premixed combustion in a model gas turbine combustor are investigated using large eddy simulation(LES). A G-equation flamelet model is employed to simulate the unsteady flame behavior. When inlet swirl number is increased, the distinct flow structures, such as the shapes of corner recirculation and center toroidal recirculation zone, are observed and the flame length is shorted gradually. Also, the phenomena of flashback are identified at strong swirl intensity. In order to get the accurate description of unsteady flame behavior, the predictive ability of the acoustic wave in a combustor is primarily evaluated. It is found that the vortex generated near the edge of step plays an important role in the flame fluctuation. Finally it is examined systematically that the flame and heat release fluctuation are coupled strongly to the vortex shedding generated by swirl flow and acoustic wave propagation from the analysis of flame-vortex interaction.

Unsteady extinction mechanism of nonpremixed flame interacting with a vortex

The extinction mechanism of a CH 4/N 2–air counterflow nonpremixed flame interacting with a single vortex was numerically studied. An augmented reduced mechanism was used to treat the CH 4 oxidation reactions. The contribution of each term in the energy and the OH species equations were evaluated to investigate the unsteady extinction mechanism of nonpremixed flame. The flame temperature began to decrease due to the convection heat loss when the flame interacted with a vortex. The investigation of the radical behavior during the flame–vortex interaction process also provided useful information on the unsteady extinction mechanism. The OH radical concentration could be used as a good tracer of the state of the unsteady extinction of nonpremixed flame. The reduction mechanism of OH concentration was confirmed by analyzing the contribution of each term in the OH species equation. At initial stage of flame–vortex interaction, the OH production and consumption rates increased gradually, while the OH concentration was kept nearly constant. Near the extinction limit, the OH production rate decreased rapidly due to the low flame temperature, and the balance between the OH production and OH consumption by diffusion could not be maintained. The unsteady nonpremixed flame interacting with a vortex under the conditions of regime ( V ) shown in the spectral combustion diagram [Thévenin, D., Renard, P.H., Fiechtner, G.J., Gord, J.R., Rolon, J.C., 2000. Regimes of non-premixed flame–vortex interactions. Proceedings of the Combustion Institute 28, 2101–2108.] was finally extinguished due to low reactivity, which was induced by the low flame temperature.

A temporal PIV study of flame/obstacle generated vortex interactions within a semi-confined combustion chamber

Experimental data obtained using a new multiple-camera digital particle image velocimetry (PIV) technique are presented for the interaction between a propagating flame and the turbulent recirculating velocity field generated during flame–solid obstacle interaction. The interaction between the gas movement and the obstacle creates turbulence by vortex shedding and local wake recirculations. The presence of turbulence in a flammable gas mixture can wrinkle a flame front, increasing the flame surface area and enhancing the burning rate. To investigate propagating flame/turbulence interaction, a novel multiple-camera digital PIV technique was used to provide high spatial and temporal characterization of the phenomenon for the turbulent flow field in the wake of three sequential obstacles. The technique allowed the quantification of the local flame speed and local flow velocity. Due to the accelerating nature of the explosion flow field, the wake flows develop ‘transient’ turbulent fields. Multiple-camera PIV provides data to define the spatial and temporal variation of both the velocity field ahead of the propagating flame and the flame front to aid the understanding of flame–vortex interaction. Experimentally obtained values for flame displacement speed and flame stretch are presented for increasing vortex complexity.

05/02228 An experimental study on vortex-flame interaction of a spark-ignited flame with coherent structure in a plane premixed shear flow

05/02228 An experimental study on vortex-flame interaction of a spark-ignited flame with coherent structure in a plane premixed shear flow

Time-accurate calculation of variable density flows with strong temperature gradients and combustion

A time-accurate algorithm is proposed for low Mach number, variable density flows with or without chemical reactions. The algorithm is based on a predictor–corrector time integration scheme that employs a projection method for the momentum equation. A constant-coefficient Poisson equation is solved for the pressure following both the predictor and corrector steps to fully satisfy the continuity equation at each time step. Spatial discretization is performed on a collocated grid system that offers computational simplicity and straightforward extension to curvilinear coordinate systems. To avoid the pressure odd–even decoupling that is typically encountered in such grids, a flux interpolation technique is introduced for the equations governing variable density flows. An important characteristic of the proposed algorithm is that it can be applied to flows in both open and closed domains. Its robustness and accuracy are illustrated with a series of numerical experiments. In particular, we present simulations of non-isothermal, turbulent channel flow as well as simulations of a premixed flame–vortex interaction.