Flame-vortex Interaction Research Articles

Local effects in vortex-flame interactions: Implications for turbulent premixed flame scaling and observables

A computational investigation was carried out to assess the effects of highly energetic vortices on the local structure of laminar premixed flames. The study involved a lean atmospheric methane-air flame interacting with vortex sizes ranging from five times to half of the laminar flame thickness. The characteristic velocities of the vortices were chosen to correspond to extreme turbulence intensities. Vortices smaller than the flame thickness were found to have a minimal effect on the flame structure due to the high rate of dissipation. Vortices with sizes greater than or equal to the flame thickness were found to induce flame siphoning and/or product-side local flame annihilation and as a result transport considerable amounts of formaldehyde and thermal energy to the unburned side thereby modifying the state of the reactants. The results of the present study for high activation energy reacting flows raise questions regarding the general validity of: (1) conjectured mechanisms of the so-called flame broadening under intense turbulence conditions; and (2) qualitative arguments regarding sub-flame-thickness vortex effects on the preheat zone in the context of the Borghi–Peters regime diagram, as originally postulated by Damköhler.

Effect of inlet flow turbulence on flame–vortex dynamics during thermo-acoustically induced flame flashback in a premixed dump combustor

The present article reports the investigation on the effect of inlet flow turbulence on flame–vortex dynamics during the thermo-acoustically induced flame flashback in a premixed dump combustor. The inlet turbulence intensity can be varied independently of the Reynolds number by using plates of different slot widths and is placed significantly upstream of the nominal flame-stabilization zone. The study aims to experimentally address the flame flashback behavior and the conventional combustion dynamics mechanisms involving flame–vortex interaction. The reacting flow-field is characterized using high-speed simultaneous measurement of dynamic pressure fluctuation with the time-resolved/phase-resolved visualization of flame and flow-field utilizing CH*/OH* chemiluminescence and particle image velocimetry. We note that higher turbulence intensity delays the onset of combustion dynamics and significantly influence the flame flashback behavior. In the present work, we observed two types of flame flashback behavior with large amplitude oscillations — (a) intermittent flame flashback, which relates to upstream flame displacement during certain phases of the acoustic pressure cycle and (b) permanent flame flashback, which results in flame quenching. The results reveals that the recirculation zone length plays a key role in determining the flame flashback. The role of enhanced turbulence is seen in two effects- (1) Dictating the length of the recirculation zone and (2) The modulation of the flame by the wake vortex, which correlates with the tendency of flame to flashback directly.

Experimental investigation of the helical mode in a stratified swirling flame

In a centrally-staged combustor, the stratified swirling flame often manifests a helical mode, which is characterized by the emergence of helical vortices, including the precessing vortex core (PVC) and the outer helical vortex (OHV). To understand the flame-vortex interaction mechanism responsible for the coherent heat release rate (HRR) fluctuation of such a helical mode, this work employs 20 kHz simultaneous measurements of PIV/CH2O PLIF and PIV/CH* chemiluminescence to resolve the unsteady flow field and flame motion. Through a combination of time-resolved and phase-averaged analysis, together with the three-dimensional reconstruction of the coherent flow and flame structures, we arrive at the qualitative understanding that in the helical mode the pilot flame dominated by PVC is a major source of coherent HRR fluctuation, while the main flame only slightly modifies the fluctuation intensity. Quantitative analyses further demonstrate the different effects of PVC and OHV on the coherent HRR fluctuation. Specifically, the PVC affects the primary heat-release process by dictating the convective transport of the reactants upstream of the flame front. And the OHV contributes to additional heat release by inducing stretching and roll-up of the pilot flame tip; it happens during the OHV's second growth, which is likely caused by the thermal expansion resulting from the primary heat release. This explains our finding that the OHV-induced heat release occurs at a slightly delayed phase angle and a downstream location compared with the primary PVC-induced heat release.

Stabilization mechanism revelation of a novel vortex-tube combustion technique: LES with sgs-pdf approach

The paper revealed the in-depth stabilization mechanisms of a novel vortex-tube combustion technique by using ethanol as fuel, which is implemented by a stratified vortex-tube combustor (SVC). The stability properties of the SVC are investigated, showing that the SVC has a wide stability limit and low-pressure fluctuation amplitudes with a uniform flame front. The equivalence ratio at the lean flammability limit is always below 0.2, and the amplitude of pressure fluctuation is less than 2000 Pa, indicating a highly steady combustion process. The non-premixed flame structure guarantees high mass concentrations near the reaction zone, while the vortex flow also decreases the local flow velocity, inhibiting flame blow-out, thus providing good self-adjusting capacity under various global equivalence ratios. The vortex–flame interaction transports the interior high-enthalpy burnt gas to the exterior unburnt gas region, thereby promoting ignition. The exterior unburnt gas is also transported to the flame front where it promotes reaction and yields an intensified combustion. The large tangential velocity and density gradient result in the large values of Richardson number, which suggests that laminarization of the flow occurs and results in good aero-dynamic and thermo-dynamic stabilities. The small values of the Rayleigh number indicate good flame-dynamic stability. Therefore, the resultant good self-adjusting capacity and three types of dynamic stabilities are the intrinsic causes of the ultra-steady combustion process in this combustor.

Dynamics of stratified swirl flame near lean blow out

This paper studies the flame dynamics near lean blowout (LBO) conditions in a stratified swirl burner. The novel BASIS burner is used and fueled with premixed methane/air mixture at atmospheric conditions. In the experiments, only the pilot flame works with four different equivalent ratios. It is found that as the flame changes from stable combustion to near LBO, the flame macrostructure changes from V-shape to M-shape. Meanwhile, a characteristic frequency of 405 Hz is found in the spectrum of heat release signal. The closer to LBO, the higher amplitude is found at the frequency peak. Post-processing of flame images illustrates that the 405 Hz comes from the circumferential movement of the flame. Large-eddy simulations (LESs) via OpenFOAM reproduce the flame dynamics near LBO, which agrees with the experimental results. LESs provide more insights into the flow fields, showing that the characteristic frequency of 405 Hz comes from the flame-vortex interaction. This study reveals the mechanism of flame and vortex interactions in a stratified swirl burner near LBO. The heat release signal is also found can be used as a candidate precursor for LBO.

Bayesian calibration of a methane-air global scheme and uncertainty propagation to flame-vortex interactions

Simplified chemistry models are commonly used in reactive computational fluid dynamics (CFD) simulations to alleviate the computational cost. Uncertainties associated with the calibration of such simplified models have been characterized in some works, but to our knowledge, there is a lack of studies analyzing the subsequent propagation through CFD simulation of combustion processes.This work propagates the uncertainties - arising in the calibration of a global chemistry model - through direct numerical simulations (DNS) of flame-vortex interactions. Calibration uncertainties are derived by inferring the parameters of a two-step reaction mechanism for methane, using synthetic observations of one-dimensional laminar premixed flames based on a detailed mechanism. To assist the inference, independent surrogate models for estimating flame speed and thermal thickness are built taking advantage of the Principal Component Analysis (PCA) and the Polynomial Chaos (PC) expansion. Using the Markov Chain Monte Carlo (MCMC) sampling method, a discussion on how push-forward posterior densities behave with respect to the detailed mechanism is provided based on three different calibrations relying (i) only on flame speed, (ii) only on thermal thickness, and (iii) on both quantities simultaneously.The model parameter uncertainties characterized in the latter calibration are propagated to two-dimensional flame-vortex interactions using 60 independent samples. Posterior predictive densities for the time evolution of the heat release and flame surface are consistent, being that the confidence intervals contain the reference simulation. However, the two-step mechanism fails to reproduce the flame response to stretch as it was not considered in the calibration. This study highlights the capabilities and limitations of propagating chemistry-models uncertainties to CFD applications to fully quantify posterior uncertainties on target quantities.

Fourier–physical space coherent structure in flame–vortex interactions relevant to flame–turbulence interactions using a new signal periodization procedure

The aim of the current study is to characterize key multidimensional relationships between coherent structures in physical vs Fourier/scale space representations of flame–turbulence interactions, as a basis for future analysis of the nonlinear couplings between key resolved scale (RS) and subfilter scale (SFS) motions in large-eddy simulation (LES) of premixed turbulent combustion. However, applying the bounded Fourier transform (FTF) in the nonperiodic flame-normal direction requires the removal of nonphysical Fourier content from the boundary discontinuities. To this end, we have developed a broadly applicable “discontinuity pollution removal” procedure for application to the FTF of multidimensional signals with a single nonperiodic direction. The procedure balances periodization of the signal near the boundaries with minimization of signal modification away from the boundaries. We applied the procedure in a physical–Fourier space analysis of the interactions between a flame and single-scale eddies modeled as the impact of a train of two-dimensional (2D) vortices on an initially planar premixed flame. We find that a specific spectrally broad localized coherent structure in Fourier space connects RS to SFS fluctuations in thermal energy and species concentration that, in physical space, are localized to the corrugations in the flame front in response to eddy–flame interactions. Within the RS fluctuations of energy and species concentration, the flame corrugation structure in physical space is found to be localized to sub-volumes within the RS region of 2D Fourier space. This new understanding of physical–Fourier space relationships categorizes classes of RS–SFS interactions relevant to SFS modeling in LES of premixed turbulent combustion.

Instability and mode transition analysis of a hydrogen-rich combustion in a model afterburner

Combustion instabilities were investigated experimentally for a hydrogen-rich combustion in a model afterburner installed at the end of a high-enthalpy wind tunnel. Air was supplied at 0.3 MPa and 950 K. The combustion instabilities were studied with the time-resolved measurements of a near-infrared (NIR) emission from water molecules over 780 nm using a high-speed video camera. Pressure was also measured in the combustor. The pressure and the NIR images were analyzed by data-driven approach, which include the fast Fourier transform (FFT), the wavelet transform, the dynamic mode decomposition (DMD) and the Gaussian process latent variable methods (GP-LVM). Thermoacoustic instability was observed under a rich condition, and the amplitude of the pressure oscillation was the maximum at the overall equivalence ratio of approximately 2.4 or 2.7 as a result of the FFT. The combustion dynamics were investigated in detail for an experimental run at the equivalence ratio of 2.4. A pressure spectrogram indicated a flame–vortex interaction with a Strouhal number of 0.5 (2300 Hz), thermoacoustic instability (560 Hz), and their transitions with the wavelet transform. For NIR images, the same tendency was also observed in the spectrogram of the modes obtained by the Gabor-filtered DMD, which could clearly resolve the high-order harmonic modes of the flame–vortex interaction and the thermoacoustic instability. Furthermore, NIR images were analyzed with GP-LVM to study the evolution of the combustion dynamics in a three-dimensional latent space. Recurrence plots with the Euclidean distance function were used to visualize the evolutions of the combustion dynamics. A limit cycle behavior of the flame–vortex interaction was clearly observed, whereas the limit cycle of the thermoacoustic instability showed more complicated behaviors. The transition behaviors of the instabilities were observed in the recurrence plots in detail, indicating that the flame–vortex interaction excited the fourth harmonic mode of the thermoacoustic instability, followed by the basic mode.

Turbulent flame-vortex dynamics of bluff-body premixed flames

Abstract To better understand the coupled dynamics of turbulence and premixed flames, this study experimentally investigates the effects of turbulence on reacting vorticity dynamics. In this investigation, a bluff-body stabilized, turbulent, premixed flame is analyzed in a high-speed combustor. A custom-designed turbulence generator is used to vary the inflow turbulence conditions and understand the subsequent impacts on the flame-flow field. The turbulent reacting flow-field is measured using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. A Lagrangian tracking method is implemented to study flame-vortex dynamics. Lagrangian fluid elements are tagged from the experimental data and are tracked as they convect through the flame, from reactants to products. The components of the vorticity transport equation are decomposed along the Lagrangian trajectories to determine their relative balance under various turbulence conditions. The results confirm that altering the turbulence intensity influences flame-vortex interactions and the relative balance of the vorticity components. At high turbulence intensities, the baroclinic torque and dilatation decrease, while the magnitude of viscous diffusion increases, and vortex stretching dominates the reacting flow field. These results suggest that increasing the turbulence intensity alters the dominant physical processes in the reacting domain as the combustion-driven phenomena become less significant.

Experimental investigation on the propagation of flow and flame in a confined combustion chamber equipped with a single-hole perforated plate

The effects of the hole size and perforated plate position on the propagation of flame and flow and pressure oscillation were explored in a constant volume combustion chamber (CVCB) with a single-hole perforated plate. Stoichiometric hydrogen-air mixture was used in the experiments, and the propagation of the flame and jet flow were recorded by high-speed schlieren photography. The results demonstrated that the flame velocity firstly increases and then decreases with the increasing hole size. With the hole size of 13 mm, the flame velocity, peak pressure, and pressure oscillation reached the maximum under the current experimental conditions. Moreover, the influence of the position of the perforated plate was investigated. It was found that the jet flow before the flame front is prominently distinct when the perforated plate is in positions A and B, resulting in an apparent difference in the flame shape behind the perforated plate, which means that the movement of the jet flow plays a leading role in the development of the flame. In Position B, the flame front overlapped with the jet flow and has the same shape. Besides, an interesting phenomenon was captured: with perforated plate in Position B, a secondary flame front was generated with higher flame tip velocity before the primary flame front under the effect of the flame-vortex interaction.

Dynamics of periodically-excited vortices in swirling flames

This work investigates the dynamics of periodically-excited vortices in swirling cold flow and hot flame using high-speed particle image velocimetry, for three different Strouhal numbers, 0.68, 1.02, and 1.70. The periodic upstream perturbation induces coherent vortices in the shear layers both inside and outside of the flame surface. We find that the outer vortex rings (OVRs) play a dominant role in tuning the flame dynamics and heat release, whereas the inner vortex rings (IVRs) are related to the formation of the center recirculation zone (CRZ) and suffer from much stronger dissipation. The evolutions of core vorticity, circulation, trajectory, and convective velocity are quantitatively analyzed for the OVRs to understand two key mechanisms: vortex formation and vortex detachment. The results show that the growing and shedding of the OVRs are governed by the inlet shear layer as well as the variations in inlet velocity and acceleration. Furthermore, both the circulation increment during vortex formation and the vortex convection after detachment are qualitatively captured by existing vortex models, demonstrating the prospect of vortex dynamics in understanding general flame-vortex interaction problems.

Direct numerical simulations of nanoparticle formation in premixed and non-premixed flame–vortex interactions

Direct numerical simulations (DNSs) of nanoparticle formation in reactive flows are challenging, and only greatly simplified DNS test-cases are possible, which help clarify the turbulence–particle–dynamics interaction and guide the necessary modeling efforts. As a basis for such studies, a new DNS database is introduced, which resolves the smallest relevant scales of the nanoparticle concentration field to obtain insights into the statistics of nanoparticle formation in reactive flows. Formation and evolution of iron oxide nanoparticles in premixed and non-premixed flames wrapped-up by a vortex have been investigated using the sectional model and direct chemistry. The DNSs capture the “engulfing” and local dilution of the particle fields. Different zones of high particle number concentration have been found in every flame, and it was shown that the thickness of these zones decreases with increasing Schmidt number, which confirms that in simulations of nanoparticle-forming turbulent reacting flows, the grid resolution has to be very fine to resolve the characteristic scale for high sections. The contributions to the change in particle concentration due to diffusion, coagulation, and nucleation have been analyzed in detail, and dominant contributions across the particle number concentration layers and across the flames have been identified. This analysis has also been carried out in terms of flat, concave, and convex iso-surface geometries, induced by the flame–vortex interaction and characterized by the curvature of the particle number concentration fields and also by the flame curvature. The results demonstrate that the flame curvature effects cannot be ignored in modeling strategies. The probability density functions for the particle number concentrations have been analyzed and quantified in terms of Shannon information entropy, which illustrates the effect of fast diffusion (and entropy production) of the smaller particles and slow diffusion (and entropy production) of the largest particles with high Schmidt numbers. In addition, the unclosed filtered or averaged agglomeration term was evaluated as a basis for future modeling efforts, showing that agglomeration rates will be underestimated by orders of magnitude unless suitable models are developed.

Direct Numerical Simulation of Flame Propagation and Deflagration to Detonation Transition in Confined Space with Different Perforated Plate Positions

ABSTRACT Flame acceleration and the deflagration to detonation transition (DDT) are important physical phenomena in combustion theories. The main objective of this work is to provide a comprehensive understanding of the effect of the positions of obstacles when a flame propagates in obstructed confined channels, using two-dimensional direct numerical simulations (DNSs) with the detailed chemical kinetics mechanism of H2. The underlying mechanism for the flame propagation and detonation, affected by the obstacles with the flame-shockwave and flame–vortex interactions, is investigated. In 1000 K cases, two different detonation modes associated with flame propagation are observed, where for the case with obstacles near the ignition kernel, autoignition takes place in the end-gas region as predicted, while for the case with obstacles far away from the ignition kernel, autoignition occurs, instead, in front of the flame and triggers a DDT immediately. Both of two modes consist three processes: flame propagation stages, flame-pressure wave/shockwave interaction, and detonation development. The evolutions of centerline temperature and pressure are employed to explicitly demonstrate the detonation process. The differences in the shockwave formation, peak temperature, and pressure values can be clearly observed. The mechanisms of the different detonation modes induced by different obstacle positions are analyzed in detail for the first time. The results indicate that the compressed and preheated temperature of the unburned mixture determines the detonation combustion mode. The conclusion is further proved in the flame-shockwave number variation and detonation modes transition when initial temperature is changed. This study will provide new insight into the DDT and detonation phenomena in engines.

An exploration of stabilization mechanisms in a novel vortex‐tube combustor with localized stratified peculiarity

Experimental and numerical (via ANSYS FLUENT) studies have been conducted on the combustion stability and stabilization mechanisms in a localized stratified vortex-tube combustor (LSVC) under lean conditions. The stability limit and flame configuration were obtained under different combustion conditions. Combined with the flow field distribution, the formation mechanisms of the local stratification of species and the resultant flame configuration were analyzed. Results show that the local stratification peculiarity is responsible for the dual flame appearance. On the basis of the local stratification of species, the local equivalence ratio is close to stoichiometry in the vicinity of the flame front, while it is above 1.0 in the interior, enabling the achievement of stable combustion at a global equivalence ratio as low as 0.12 in the LSVC. The flow field can help the transport of the reactive species and yields an intensified combustion and a large density gradient. The peak heat release rate (HRR) of 0.5 W/mm3 in the LSVC is much higher than that of 0.1 W/mm3 in the rapidly mixed vortex-tube combustor (RMVC) at the global equivalence ratio of 0.6 and the maximum tangential velocity of 26.44 m/s. The flame–vortex interaction theory provides a new perspective to interpret the rapid flame propagation in vortex-tube combustors. Based on the pressure jump theory, the flame speed was obtained via a specific formula closely related to the density gradient and the injection velocity. It turns out that the flame speed in the LSVC is remarkably higher than that in the RMVC at a certain same combustion condition. Moreover, the decrease of local flow velocity resulted from the strong swirl provides a favorable guarantee for the balance with the local flame speed.

Numerical Simulation of Hot Jet Detonation with Different Ignition Positions

Ignition position is an important factor affecting flame propagation and deflagration-to-detonation transition (DDT). In this study, 2D reactive Navier–Stokes numerical studies have been performed to investigate the effects of ignition position on hot jet detonation initiation. Through the stages of hot jet formation, vortex-flame interaction and detonation wave formation, the mechanism of the hot jet detonation initiation is analyzed in detail. The results indicate that the vortexes formed by hot jet entrain flame to increase the flame area rapidly, thus accelerating energy release and the formation of the detonation wave. With changing the ignition position from top to wall inside the hot jet tube, the faster velocity of hot jet will promote the vortex to entrain jet flame earlier, and the DDT time and distance will decrease. In addition, the effect of different wall ignition positions (from 0 mm to 150 mm away from top of hot jet tube) on DDT is also studied. When the ignition source is 30 mm away from the top of hot jet tube, the distance to initiate detonation wave is the shortest due to the highest jet intensity, the DDT time and distance are about 41.45% and 30.77% less than the top ignition.

POD Scale Analysis of Turbulent Premixed Flame Structure at Elevated Pressures

ABSTRACT Turbulent combustion is usually operated at elevated pressures for practical applications to achieve high power density. There will be more wrinkles on the turbulent flame surface with pressure rising, resulting in a higher turbulent burning velocity. Wrinkling process of the turbulent flame at elevated pressure is still not well understood. One reason is that the large and small-scale wrinkles can not be distinguished because of their multi-scale nature. In this paper, the Proper Orthogonal Decomposition (POD) scale analysis is used to describe the multi-scale nature of turbulent premixed flames at elevated pressure. Turbulent premixed CH4/air flames are conducted in a high-pressure combustion chamber under 0.1 to 0.3 MPa. Turbulence is generated and controlled by multi-grid perforated plates. Turbulent flame front is detected by OH-PLIF technique. Results show that the POD scale analysis can decompose the multi-scale of turbulent flame surface to different modes. Reconstruction of turbulent flame front shows that there is an inverse relationship between POD modes and physical length of scales. Small mode number represents large scale of flame wrinkles, while large mode number stands for small-scale wrinkles. The large and small scale of the flame wrinkles both increase with pressure rising, leading to higher flame surface density and turbulent burning velocity. This indicates that the increased turbulent flame wrinkling at elevated pressure is not only caused by flame instability, which mainly acts on the small wrinkles. Spectral analysis of the turbulent premixed flame shows that the decreased flame thickness, which enlarges the turbulence–flame interaction range, and the decreased laminar burning velocity, which extends the vortex–flame interaction time, are two dominant factors promoting the flame wrinkling at elevated pressure. Turbulent burning velocity correlation shows a linear relationship with the analytical formulation when it takes laminar burning velocity and flame thickness into account.

A scaling analysis for the evolution of small-scale turbulence eddies across premixed flames with implications on distributed combustion

In this work, we propose a physics-based scaling theory for the evolution of small-scale turbulence eddies in the unburnt mixture when they travel across the preheat zone of a premixed flame. This scaling theory is developed to explicitly account for the effects of viscous diffusion on the length and velocity scales of turbulence eddies that are smaller than the inner reaction zone thickness of a premixed flame. This scaling suggests that both the length and velocity scales of the turbulence eddies are significantly altered as they pass through the preheat zone. The applicability of the scaling is assessed using a series of detailed numerical simulations of flame-vortex interactions with finite-rate chemistry and detailed molecular transport. The scaling estimates of eddy evolution are compared to simulation predictions with reasonable agreement. The proposed scaling is then applied to estimate the required length and velocity scales of turbulence eddies in the unburnt mixture of a turbulent premixed flame to potentially create significant perturbation of the reaction zone by local mixing after passing through the preheat zone. These requirements are translated into a minimum estimated Karlovitz number (Ka) of approximately in the unburnt mixture of a turbulent premixed flame to potentially achieve distributed combustion. This estimated minimum Ka is significantly higher than the classical scaling estimate () for turbulent premixed flames, which supports the observations in recent experimental and Direct Numerical Simulation studies. The proposed scaling analysis implies that it is unlikely that turbulence eddies on the unburnt side of realistic turbulent premixed flames can be simultaneously sufficiently small and sufficiently strong to significantly alter the structure of the inner reaction zone.

Hybrid regularized Lattice-Boltzmann modelling of premixed and non-premixed combustion processes

A Lattice-Boltzmann model for low-Mach reactive flows is presented, built upon our recently published model (Comb & Flame, 196, 2018). The approach is hybrid and couples a Lattice-Boltzmann solver for the resolution of mass and momentum conservation and a finite difference solver for the energy and species conservation. Having lifted the constant thermodynamic and transport properties assumptions, the model presented now fully accounts for the classical reactive flow thermodynamic closure: each component is assigned NASA coefficients for calculating its thermodynamic properties. A temperature-dependent viscosity is considered, from which are deduced thermo-diffusive properties via specification of Prandtl and component-specific Schmidt numbers. Another major improvement from our previous contribution is the derivation of an advanced collision kernel compatible of multi-component reactive flows stable in high shear flows. Validation is carried out first on premixed configurations, through simulation of the planar freely propagating flame, the growth of the associated Darrieus-Landau instability and three regimes of flame-vortex interaction. A double shear layer test case including a flow-stabilized diffusion flame is then presented and results are compared with DNS simulations, showing excellent agreement.

Numerical Investigation of a New Single Vortex Generation Technique

In this work a new single vortex generation method based on the flow past a rotating cylinder is proposed. Two dimensional numerical simulation for a laminar flow at Re = 100 is conducted to investigate this method. An impulsive drop of the rotation speed appeared to trigger a vortex roll up process; one single vortex is growing behind the cylinder and detached from it. The process is studied for different rotation drop rates and several final rotation rates. Vortex characteristics are computed and compared for different cases. Increasing rotation drop rate generates a more rapidly growing vortex. Its trajectory is closer to the geometric centreline and its size and intensity are higher. This new vortex generation method allows the investigation of new experimental configurations for studying vortex flame interaction.

Investigation on bluff-body and swirl stabilized flames near lean blowoff with PIV/PLIF measurements and LES modelling

Lean premixed combustion (LPC) is a promising technology for low-NOx emission, while it increases the risk of blowoff at the same time. Experiments and Large Eddy Simulations (LES) on swirl stratified lean-premixed CH4/air flames were performed to study the differences between the stable and near blowoff flame. The flow fields and instantaneous flame structures were measured by simultaneous Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (OH-PLIF). The Thickened Flame (TF) model coupled with a two-step reduced chemical mechanism was used in LES modelling. The non-dynamic formulation of sub-grid flame wrinkling model is performed well for stable condition while it cannot predict the near lean blowoff flame reasonable. Compared with the stable flame, several significant differences can be observed in the near lean blowoff flame. The height of high-temperature-zone is relatively low and the heat loss of flame attachment can be easily enhanced by the low temperature spot induced by flame-vortex interaction. The flame attachment is subject to higher excess strain rate and turbulence fluctuation. Meanwhile, a Processing Vortex Core (PVC) appears downstream of the centerline. It is concluded that lean blowoff is the result of interactions between the fuel/air mixture ignition, PVC instability and flame attachment lift-off.