Relations Between Statistics of Three-Dimensional Flame Curvature and its Two-Dimensional Counterpart in Turbulent Premixed Flames

The relations between the actual flame curvature probability density function (PDF) evaluated in three-dimensions and its two-dimensional counterpart based on planar measurements have been analytically derived subject to the assumptions of isotropy and statistical independence of various angles and two-dimensional curvature. These relations have been assessed based on Direct Numerical Simulation (DNS) databases of turbulent premixed (a) statistically planar and (b) statistically axisymmetric Bunsen flames. It has been found that the analytically derived relation interlinking the PDFs of actual three-dimensional curvature and its two-dimensional counterpart holds reasonably well for a range of curvatures around the mean value defined by the inverse of the thermal flame thickness for different turbulence intensities across different combustion regimes. The flame surface is shown to exhibit predominantly two-dimensional cylindrical curvature but there is a significant probability of finding saddle type flame topologies and this probability increases with increasing turbulence intensity. The presence of saddle type flame topologies affects the ratios of second and third moments of two-dimensional and three-dimensional curvatures. It has been demonstrated that the ratios of second and third moments of two-dimensional and three-dimensional curvatures cannot be accurately predicted based on two-dimensional measurements. The ratio of the third moments of two-dimensional and three-dimensional curvatures remains positive and thus the qualitative nature of curvature skewness can still be obtained based on two-dimensional curvature measurements. As the curvature skewness is often taken to be a marker of the Darrius-Landau instability, the conclusion regarding the presence of this instability can potentially be taken from the two-dimensional curvature measurements.

     

Joint PDF Closure of Turbulent Premixed Flames

In this paper, a novel model for turbulent premixed combustion in the corrugated flamelet regime is presented, which is based on transporting a joint probability density function (PDF) of velocity, turbulence frequency and a scalar vector. Due to the high dimensionality of the corresponding sample space, the PDF equation is solved with a Monte-Carlo method, where individual fluid elements are represented by computational particles. Unlike in most other PDF methods, the source term not only describes reaction rates, but accounts for “ignition” of reactive unburnt fluid elements due to propagating embedded quasi laminar flames within a turbulent flame brush. Unperturbed embedded flame structures and a constant laminar flame speed (as expected in the corrugated flamelet regime) are assumed. The probability for an individual particle to “ignite” during a time step is calculated based on an estimate of the mean flame surface density (FSD), latter gets transported by the PDF method. Whereas this model concept has recently been published [21], here, a new model to account for local production and dissipation of the FSD is proposed. The following particle properties are introduced: a flag indicating whether a particle represents the unburnt mixture; a flame residence time, which allows to resolve the embedded quasi laminar flame structure; and a flag indicating whether the flame residence time lies within a specified range. Latter is used to transport the FSD, but to account for flame stretching, curvature effects, collapse and cusp formation, a mixing model for the residence time is employed. The same mixing model also accounts for molecular mixing of the products with a co-flow. To validate the proposed PDF model, simulation results of three piloted methane-air Bunsen flames are compared with experimental data and very good agreement is observed.     

Turbulence Intensity and Length Scale Effects on Premixed Turbulent Flame Propagation

The effects of varying turbulence intensity and turbulence length scale on premixed turbulent flame propagation are investigated using Direct Numerical Simulation (DNS). The DNS dataset contains the results of a set of turbulent flame simulations based on separate and systematic changes in either turbulence intensity or turbulence integral length scale while keeping all other parameters constant. All flames considered are in the thin reaction zones regime. Several aspects of flame behaviour are analysed and compared, either by varying the turbulence intensity at constant integral length scale, or by varying the integral length scale at constant turbulence intensity. The turbulent flame speed is found to increase with increasing turbulence intensity and also with increasing integral length scale. Changes in the turbulent flame speed are generally accounted for by changes in the flame surface area, but some deviation is observed at high values of turbulence intensity. The probability density functions (pdfs) of tangential strain rate and mean flame curvature are found to broaden with increasing turbulence intensity and also with decreasing integral length scale. The response of the correlation between tangential strain rate and mean flame curvature is also investigated. The statistics of displacement speed and its components are analysed, and the findings indicate that changes in response to decreasing integral length scale are broadly similar to those observed for increasing turbulence intensity, although there are some interesting differences. These findings serve to improve current understanding of the role of turbulence length scales in flame propagation.

     

Flame Induced Flow Features in the Presence of Darrieus-Landau Instability

The onset of hydrodynamic or Darrieus-Landau (DL) instability can largely impact on premixed flame morphology, turbulent flame speed and induced flow field. In this work, we focus on the latter induced flow by means of two dimensional direct numerical simulations (DNS) of slot burner flames performed in a parametric fashion. Results from linear stability analysis are used to select the adequate parameter range to be investigated. The presence of DL instability is initially assessed using a recently proposed statistical marker related to flame morphology. The differences between stable and unstable flames are then statistically investigated, utilizing a single, laminar, DL-induced corrugation as a reference state. Such DL-induced effects are investigated at various turbulence intensities, in terms of local propagation, induced strain rate patterns and flow field as well as vorticity production and transformation. Using displacement speed as a measure of local propagation, no noticeable statistical difference is observed between stable and unstable flames while strain rate and vorticity patterns are shown to be largely influenced by the DL induced morphology. From the modeling point view, an enhancement of counter gradient type transport for turbulent scalar fluxes is observed for hydrodynamically unstable flames.     

Effects of Turbulence on Self-sustained Combustion in Premixed Flame Kernels: A Direct Numerical Simulation (DNS) Study

The effects of mean flame radius and turbulence on self-sustained combustion of turbulent premixed spherical flames in decaying turbulence have been investigated using three-dimensional direct numerical simulations (DNS) with single step Arrhenius chemistry. Several flame kernels with different initial radius or initial turbulent field have been studied for identical conditions of thermo-chemistry. It has been found that for very small kernel radius the mean displacement speed may become negative leading ultimately to extinction of the flame kernel. A mean negative displacement speed is shown to signify a physical situation where heat transfer from the kernel overcomes the heat release due to combustion. This mechanism is further enhanced by turbulent transport and, based on simulations with different initial turbulent velocity fields, it has been found that self-sustained combustion is adversely affected by higher turbulent velocity fluctuation magnitude and integral length scale. A scaling analysis is performed to estimate the critical radius for self-sustained combustion in premixed flame kernels in a turbulent environment. The scaling analysis is found to be in good agreement with the results of the simulations.     

Heat Transfer Modeling in the Context of Large Eddy Simulation of Premixed Combustion with Tabulated Chemistry

Tabulated chemistry models like the Flamelet Generated Manifolds method are a good approach to include detailed information on the reaction kinetics in a turbulent flame at reasonable computational costs. However, so far, not all information on e.g. heat losses are contained in these models. As those often appear in typical technical applications with enclosed flames in combustion chambers, extensions to the standard FGM approach will be presented in this paper, allowing for the representation of non-adiabatic boundaries. The enthalpy as additional control variable for the table access is introduced, such that the chemistry database becomes three-dimensional with mixture fraction, reaction progress variable and enthalpy describing the thermo-chemical state. The model presented here is first validated with a two-dimensional enclosed Bunsen flame and then applied within the Large Eddy Simulations of a turbulent premixed swirl flame with a water-cooled bluff body and a turbulent stratified flame, where additional modeling for the flame structure using artificially thickened flames was included. The results are encouraging, as the temperature decrease towards the bluff body in the swirl flame and the cooling of the pilot flame exhaust gases in the stratified configuration can be observed in both experiments and simulation.     

Local curvature measurements of a lean,partially premixed swirl-stabilised flame

A swirl-stabilised, lean, partially premixed combustor operating at atmospheric conditions has been used to investigate the local curvature distributions in lifted, stable and thermoacoustically oscillating CH₄-air partially premixed flames for bulk cold-flow Reynolds numbers of 15,000 and 23,000. Single-shot OH planar laser-induced fluorescence has been used to capture instantaneous images of these three different flame types. Use of binary thresholding to identify the reactant and product regions in the OH planar laser-induced fluorescence images, in order to extract accurate flame-front locations, is shown to be unsatisfactory for the examined flames. The Canny-Deriche edge detection filter has also been examined and is seen to still leave an unacceptable quantity of artificial flame-fronts. A novel approach has been developed for image analysis where a combination of a non-linear diffusion filter, Sobel gradient and threshold-based curve elimination routines have been used to extract traces of the flame-front to obtain local curvature distributions. A visual comparison of the effectiveness of flame-front identification is made between the novel approach, the threshold binarisation filter and the Canny-Deriche filter. The novel approach appears to most accurately identify the flame-fronts. Example histograms of the curvature for six flame conditions and of the total image area are presented and are found to have a broader range of local flame curvatures for increasing bulk Reynolds numbers. Significantly positive values of mean curvature and marginally positive values of skewness of the histogram have been measured for one lifted flame case, but this is generally accounted for by the effect of flame brush curvature. The mean local flame-front curvature reduces with increasing axial distance from the burner exit plane for all flame types. These changes are more pronounced in the lifted flames but are marginal for the thermoacoustically oscillating flames. It is concluded that additional fuel mixture fraction and velocimetry studies are required to examine whether processes such as the degree of partial-premixedness close to the burner exit plane, the velocity field and the turbulence field have a strong correlation with the curvature characteristics of the investigated flames.     

Statistics and Modelling of Turbulent Kinetic Energy Transport in Different Regimes of Premixed Combustion

The statistical behaviour of turbulent kinetic energy transport in turbulent premixed flames is analysed using data from three-dimensional Direct Numerical Simulation (DNS) of freely propagating turbulent premixed flames under decaying turbulence. For flames within the corrugated flamelets regime, it is observed that turbulent kinetic energy is generated within the flame brush. By contrast, for flames within the thin reaction zones regime it has been found that the turbulent kinetic energy decays monotonically through the flame brush. Similar trends are observed also for the dissipation rate of turbulent kinetic energy. Within the corrugated flamelets regime, it is demonstrated that the effects of the mean pressure gradient and pressure dilatation within the flame are sufficient to overcome the effects of viscous dissipation and are responsible for the observed augmentation of turbulent kinetic energy in the flame brush. In the thin reaction zones regime, the effects of the mean pressure gradient and pressure dilatation terms are relatively much weaker than those of viscous dissipation, resulting in a monotonic decay of turbulent kinetic energy across the flame brush. The modelling of the various unclosed terms of the turbulent kinetic energy transport equation has been analysed in detail. The predictions of existing models are compared with corresponding quantities extracted from DNS data. Based on this a-priori DNS assessment, either appropriate models are identified or new models are proposed where necessary. It is shown that the turbulent flux of turbulent kinetic energy exhibits counter-gradient (gradient) transport wherever the turbulent scalar flux is counter-gradient (gradient) in nature. A new model has been proposed for the turbulent flux of turbulent kinetic energy, and is found to capture the qualitative and quantitative behaviour obtained from DNS data for both the corrugated flamelets and thin reaction zones regimes without the need to adjust any of the model constants.     

The Scalar Gradient Alignment Statistics of Flame Kernels and its Modelling Implications for Turbulent Premixed Combustion

The effects of mean flame curvature on reaction progress variable gradient, $\nabla c$ , alignment with local turbulent strain rate are studied based on three-dimensional Direct Numerical Simulation (DNS) data of turbulent premixed flame kernels with different initial radii under decaying turbulence. A statistically planar flame is also considered in order to compare the results obtained from the kernels with a flame of zero mean curvature. It is found that the dilatation rate effects diminish with decreasing kernel radius due to defocusing of heat in the positively curved regions. This gives rise to a decrease in the extent of reaction progress variable gradient alignment with most extensive principal strain rate with decreasing kernel radius. The modelling implications of the statistics of the alignment of $\nabla c$ with local strain rate have been studied in terms of scalar dissipation rate transport. A new modelling methodology for the contribution of the scalar-turbulence interaction term in the transport equation for the mean scalar dissipation is suggested addressing the reduced effects of dilatation rate for flame kernels and the diminished value of turbulent straining at the small length scales at which turbulence interacts with small flame kernels. The performance of the new models is found to be satisfactory while comparing to DNS results. The existing models for the dilatation contribution and the combined chemical reaction and molecular dissipation contributions to the transport of mean scalar dissipation, which were originally proposed for statistically planar flames, are found to satisfactorily predict the corresponding quantities for turbulent flame kernels.     

Study of a Filtered Flamelet Formulation for Large Eddy Simulation of Premixed Turbulent Flames

Despite significant advances in the understanding and modelling of turbulent combustion, no general model has been proposed for simulating flames in industrial combustion devices. Recently, the increase in computational possibilities has raised the hope of directly solving the large turbulent scales using large eddy simulation (LES) and capturing the important time-dependant phenomena. However, the chemical reactions involved in combustion occur at very small scales and the modelling of turbulent combustion processes is still required within the LES framework. In the present paper, a recently presented model for the LES of turbulent premixed flames is presented, analysed and discussed. The flamelet hypothesis is used to derive a filtered source term for the filtered progress variable equation. The model ensures proper flame propagation. The effect of subgrid scale (SGS) turbulence on the flame is modelled through the flame-wrinkling factor. The present modelling of the source term is successfully tested against filtered direct numerical simulation (DNS) data of a V-shape flame. Further, a premixed turbulent flame, stabilised behind an expansion, is simulated. The predictions agree well with the available experimental data, showing the capabilities of the model for performing accurate simulations of unsteady premixed flames.     

LES of Low to High Turbulent Combustion in an Elevated Pressure Environment

A subgrid scale flame surface density combustion model for the Large Eddy Simulation (LES) of premixed combustion is derived and validated. The model is based on fractal characteristics of the flame surface, assuming a self similar wrinkling of the flame between smallest and largest wrinkling length scales. Experimental and direct numerical simulation databases as well as theoretical models are used to derive a model for the fractal parameters, namely the cut-off lengths and the fractal dimension suitable in the LES context. The combustion model is designed with the intent to simulate low as well as high Reynolds number premixed turbulent flame propagation and with a focus on correct scaling with pressure. The combustion model is validated by simulations of turbulent Bunsen flames with methane and propane fuel at pressure levels between 0.1 MPa and 2 MPa and at turbulence levels of $0 < u^{\prime }/s_{L}^{0} < 11$ , conditions typical for spark ignition engines. The predicted turbulent flame speed is in a very good agreement with the experimental data and a smooth transition from resolved flame wrinkling to fully modelled, nearly subgrid-only wrinkling is realized. Evaluating the influence of mesh resolution shows a predicted mean flame surface and turbulent flame speed independent of mesh resolution for cases with 9–86 % resolved flame surface. Additional simulations of a highly turbulent jet flame at 0.1 MPa and 0.5 MPa and the comparison with experimental data in terms of flame shape, velocity field and turbulent fluctuations validates the model also at conditions typical for gas turbines.     

Topology and Brush Thickness of Turbulent Premixed V-shaped Flames

Topology and brush thickness of turbulent premixed V-shaped flames were investigated using Mie scattering and Particle Image Velocimetry techniques. Mean bulk flow velocities of 4.0, 6.2, and 8.3 m/s along with two fuel-air equivalence ratios of 0.6 and 0.7 were tested in the experiments. Using a novel experimental turbulence generating apparatus, three turbulence intensities of approximately 2 %, 6 %, and 17 % were tested in the experiments. The results show that topology of the flame front is significantly altered by changing the turbulence intensity. Specifically, at relatively small turbulence intensities, the flame fronts feature wrinkles which are symmetric with respect to the vertical axis. At moderate values of turbulence intensities, the flame fronts form cusps. The formation of cusps is more pronounced at large mean bulk flow velocities. The results associated with relatively large turbulence intensity show that flame surfaces feature: mushroom-shaped structures, freely propagating sub-flames, pocket formation, localized extinction, and horn-shaped structures. Analysis of the results show that the flame brush thickness follows a linear correlation with the root-mean-square of the flame front position. The correlation is in agreement with the results of past experimental investigations associated with moderately turbulent premixed V-shaped flames, and holds for the range of turbulence conditions tested. This suggests that the underlying mechanism associated with the dynamics of moderately turbulent premixed V-shaped flames proposed in past studies can potentially be valid for the the wide range of turbulence conditions examined in the present investigation.     

Subgrid Scale Modeling in Large-Eddy Simulation of Turbulent Combustion Using Premixed Flamelet Chemistry

Large-eddy simulation (LES) of turbulent combustion with premixed flamelets is investigated in this paper. The approach solves the filtered Navier–Stokes equations supplemented with two transport equations, one for the mixture fraction and another for a progress variable. The LES premixed flamelet approach is tested for two flows: a premixed preheated Bunsen flame and a partially premixed diffusion flame (Sandia Flame D). In the first case, we compare the LES with a direct numerical simulation (DNS). Four non-trivial models for the chemical source term are considered for the Bunsen flame: the standard presumed beta-pdf model, and three new propositions (simpler than the beta-pdf model): the filtered flamelet model, the shift-filter model and the shift-inversion model. A priori and a posteriori tests are performed for these subgrid reaction models. In the present preheated Bunsen flame, the filtered flamelet model gives the best results in a priori tests. The LES tests for the Bunsen flame are limited to a case in which the filter width is only slightly larger than the flame thickness. According to the a posteriori tests the three models (beta-pdf, filtered flamelet and shift-inversion) show more or less the same results as the trivial model, in which subgrid reaction effects are ignored, while the shift-filter model leads to worse results. Since LES needs to resolve the large turbulent eddies, the LES filter width is bounded by a maximum. For the present Bunsen flame this means that the filter width should be of the order of the flame thickness or smaller. In this regime, the effects of subgrid reaction and subgrid flame wrinkling turn out to be quite modest. The LES-results of the second case (Sandia Flame D) are compared to experimental data. Satisfactory agreement is obtained for the main species. Comparison is made between different eddy-viscosity models for the subgrid turbulence, and the Smagorinsky eddy-viscosity is found to give worse results than eddy-viscosities that are not dominated by the mean shear. Paper presented on the Eccomas Thematic Conference Computational Combustion 2007, submitted for a special issue of Flow, Turbulence and Combustion.     

Turbulent Flame Speeds of Premixed Supersonic Flame Kernels

It is unclear whether turbulent flame speed scalings established in low speed regimes are applicable to supersonic flames. To investigate this question, the canonical flame kernel is investigated in a scramjet-like channel having a one degree wall divergence. The growth, shape and internal kernel dynamics are investigated. Results are presented for three Mach numbers, four equivalence ratios, and three turbulence generators. Schlieren photography provides flame images for growth rate statistics and particle image velocimetry (PIV) provides turbulence statistics and investigation of internal kernel dynamics. Supersonic flame kernels are self-propagating and respond to the equivalence ratio in a fashion that is similar to low speed flames. However, supersonic flame kernels have features that are not present in subsonic flame kernels. Baroclinicity, resulting from pressure-density misalignment, creates a reacting vortex ring structure. Further, the mean kernel shape has a Mach number dependence and the vortex ring enhances the turbulent flame speed through entrainment of reactants and augmented flame surface growth. Hence, the previously established (low speed) flame speed scalings are inappropriate for supersonic flame kernels. Drawing motivation from vortex ring literature, the ring propagation velocity is used as the characteristic velocity and a new flame speed scaling is proposed.     

Flame Stabilization Mechanisms in Lifted Flames

Flame stabilization and the mechanisms that govern the dynamics at the flame base of lifted flames have been subject to numerous studies in recent years. A combined Large Eddy Simulation-Conditional Moment Closure (LES-CMC) approach has been successful in predicting flame ignition and stabilization by auto-ignition, but accurate modelling of the competition between turbulent quenching and laminar and turbulent flame propagation at the anchor point had not been demonstrated. This paper will consolidate LES-CMC results by analysing a wide range of lifted flame geometries with different prevailing stabilization mechanisms. The simulations allow a clear distinction of these mechanisms. It is corroborated that LES-CMC accurately predicts the competition between turbulence and chemistry during the auto-ignition process, the dynamics of turbulent flame propagation can be captured, however, the dynamics of the extinction process are not approximated well under certain conditions. The averaging process inherent in the CMC methods does not allow for an instant response of the transported conditionally averaged reactive species to the changes in the flow conditions and any response of the scalars will therefore be delayed. The dimensionality of the CMC implementation affects the solution and higher dimensionality does no necessarily improve results. Stationary or quasi-stationary conditions, however, can be well predicted for all flame configurations.     

A Comparison of Strategies for Direct Numerical Simulation of Turbulence Chemistry Interaction in Generic Planar Turbulent Premixed Flames

Three different methods to introduce turbulence in the computational domain of Direct Numerical Simulations (DNS) of statistically planar turbulent premixed flame configurations have been reviewed and their advantages and disadvantages in terms of run time, natural flame development, control of turbulence parameters and convergence of statistics extracted from the simulations have been discussed in detail. It has been found that there is no method, which is clearly superior to the other two alternative methods. An analysis has been performed to explain why Lundgren’s physical space linear forcing results in an integral length scale which is, independent of the Reynolds number, a constant fraction of the domain size. Furthermore, an evolution equation for the integral length scale has been derived, and a scaling analysis of its terms has been performed to explain the evolution of the integral length scale in the context of Lundgren’s physical space linear forcing. Finally, a modification to Lundgren’s forcing approach has been suggested which ensures that the integral length scale settles to a predetermined value so that DNS of statistically planar turbulent premixed flames with physical space forcing can be conducted for prescribed values of Damköhler and Karlovitz numbers.     

Tubulent Methane/Air Premixed Flame Structure at High Karlovitz Numbers

2D Direct Numerical Simulations of methane/air turbulent premixed flames at initial Karlovitz numbers ranging from 600 to 9500 are performed. Instantaneous results are then extracted and analyzed with a focus on the inner flame structure. Snapshots reveal that the distributed reaction zone regime, theoretically reached around Ka?≈?100, is not attained before Ka?≈?2000. A correction of the definition of Ka is proposed in order to account for gas expansion across the flame, and is found to be consistent with the previous observations. The fuel-consumption zone is shown to be highly affected by turbulence and the characteristics of flames developing at lower Ka cannot be seen: the reaction zone is indeed strongly stretched and curved by intense turbulence leading to the formation of large protruding structures. In addition, the heat release rate layer is found to be broader and more distributed than at lower Ka as small turbulent eddies are able to survive inside it. No local flame quenching is however noticed. A statistical analysis of the distributed flame highlighted three major features characterizing this regime: significant broadening of the whole flame results from the presence of small eddies inside the reaction zone, temperature evolves linearly with respect to the progress variable and minor species peak mass fractions are lower than in a laminar flame. These results have important consequences for turbulent combustion modelling of flames in the distributed combustion regime.     

An Experimental Investigation of the Turbulence Structure of a Lifted H2/N2 Jet Flame in a Vitiated Co-Flow

Measurements of mean velocity components, turbulent intensity, and Reynolds shear stress are presented in a turbulent lifted H₂/N₂ jet flame as well as non-reacting air jet issuing into a vitiated co-flow by laser doppler velocimetry (LDV) technique. The objectives of this paper are to obtain a velocity data base missing in the previous experiment data of the Dibble burner and so provide initial and flow field data for evaluating the validity of various numerical codes describing the turbulent partially premixed flames on this burner. It is found that the potential core is shortened due to the high ratio of jet density to co-flow density in the non-reacting cases. However, the existence of flame suppressed turbulence in the upstream region of the jet dominates the length of potential core in the reacting cases. At the centreline, the normalized axial velocities in the reacting cases are higher than the non-reacting cases, and the relative turbulent intensities of the reacting flow are smaller than in the non-reacting flow, where a self-preserving behaviour for the relative turbulent intensities exists at the downstream region. The profiles of mean axial velocity in the lifted flame distribute between the non-reacting jet and non-premixed flame both in the axial and radial distributions. The radial distributions of turbulent kinetic energy in the lifted flames exhibit a change in distributions indicating the difference of stabilisation mechanisms of the two lifted flame. The experimental results presented will guide the development of an improved modelling for such flames.     

Correlation coefficients of the fluctuating density in turbulent premixed flames

 Two-point density measurements by laser induced Rayleigh scattering are used in this study to fully characterise the scalar field in a Bunsen type turbulent premixed flame. The two points are separated within the flame brush in the axial or radial directions. Correlation coefficients are obtained by comparing the evolution of one-point density fluctuations in time or the two-point density fluctuations in both space and time. Time and length scales of the scalar field, and the mean convection velocity of the turbulent scalar structures are deduced from these correlation coefficients. Time scales are calculated from the auto-correlation coefficients, length scales are determined from the space correlation coefficients and the mean convection velocity of the scalar structures in the axial direction is deduced from the space–time correlation coefficients. The relevance of these results for analysing and modelling the structure of turbulent premixed flames is discussed. Received: 30 April 1996 / Accepted: 2 September 1997     

Influences of acoustic instabilities on turbulent-flame propagation

Influences of acoustic instabilities on premixed turbulent-flames have been studied experimentally in a Taylor–Couette (TC) combustor for downward flame propagation in a turbulent flow-field generated in the annulus between two cylinders. Flow-field velocities were measured at a fixed location upstream of the propagating flame using laser-doppler velocimetry, while flame speeds were determined from video-recorded images. It is found that the existence of pre-ignition turbulence in the combustor (generated by rotation of the combustor-cylinder walls) does not eliminate acoustic instabilities, however as the level of pre-ignition turbulence is increased the influence of the secondary acoustic instability on the turbulent-flame speed becomes insignificant. For low intensities of pre-ignition turbulence the flame is found to accelerate during the latter stages of flame propagation, while for high levels of pre-ignition turbulence the flame propagates at a statistically constant speed, even though velocity fluctuations have been substantially amplified by the time the flame reaches the bottom end of the combustor as a result of acoustic instability.