Structure Of Turbulent Premixed Flames Research Articles

Effect of DC Electric Field on Turbulent Flame Structure and Turbulent Burning Velocity

ABSTRACT Effect of electric field on turbulent premixed flame structure and turbulent burning velocity is investigated through OH-PLIF technique. The ring-plate electrode configuration is used, which the ring electrode is high potential. Flame front structure and turbulent burning velocity are derived to estimate the effect of electric field on turbulent flame. Results show that electric field has similar effects compared with turbulence to some extent in this experiment, which could increase flame volume, turbulent burning velocity and decrease the flame surface density. But there are still substantial distinctions in detail. The perturbation induced by turbulence is the vortex, while that induced by electric field is the directional flow, and it can be verified through the PDF distribution of flame curvature with/without electric field. Effect of electric field is a local effect because it is dependent on the distribution of charged species. The research indicates electric field-assisted combustion is mitigated by the turbulence to some extent, and it may hinder the utilization of electric field at practical application for combustion enhancement, but other aspects need further researches. Two reasons are proposed to explain the mitigation of turbulence. Firstly, the response of turbulent burning velocity to the same perturbation decreases with turbulence intensity. Secondly, the wrinkled structure of turbulent flame mitigates the effect of electric field.

Effect of Ionic Wind Induced by DC Electric Field on Biogas/Air Turbulent Premixed Flame Structure

ABSTRACT Effect of DC electric field on lean premixed biogas turbulent flame structure at low and medium turbulence intensity was studied experimentally using OH-PLIF. OH signal distribution, turbulent burning velocity and flame structure geometric parameters were derived. Results show that downward electric field could broaden the downstream burned gas and push the flame tip down. The flame base is closer to the burner rim in downward electric field for the modification of thermal diffusion and flow modification caused by ionic wind. The effect on flame tip height reduction is more obvious for biogas flame with a higher CO2 ratio. According to the flame structure analysis, the turbulent burning velocity of biogas is increased slightly due to the modification of flow field by ionic wind generated by downward electric field, and the increment decreases with the increase of turbulence intensity. In addition, there’s a decrease in mean flame surface density and an increase in centerline flame brush thickness. The flame surface curvature PDF distribution is nearly not affected by the application of DC electric field. For turbulent flames, the effect of electric field is non-uniform and inconstant, which induces extra flow perturbation. This perturbation is different from turbulence, and the combined effects lead to the variation of flame structure.

Lean Blow-Off Scaling of Turbulent Premixed Bluff-Body Flames of Vaporized Liquid Fuels

The lean blow-off (LBO) limits and structure of turbulent premixed flames were investigated with vaporized liquid fuels stabilized by a bluff-body burner. Ethanol, heptane, and two kerosenes were used. To facilitate comparisons to gaseous-fueled flames, results were also obtained from methane flames. The measured LBO limits indicate that, for this burner, the ethanol and heptane flames are more resilient to blow-off than the kerosene fuels. Furthermore, a correlation based on a Damköhler number (), which is proportional to the laminar flame speed, does not lead to the successful collapse of the different fuels, indicating that the correlations based on laminar flame speed are not applicable. Average OH* chemiluminescence images of the ethanol and heptane flames are qualitatively similar to that from methane: the flame brushes of both exhibit an M shape when close to blow-off. In contrast, the distribution of OH* signal in the kerosene flames is primarily concentrated in regions further downstream of the bluff body. Ultimately, the results of this effort highlight the influence fuel type has on the LBO of bluff-body stabilized flames. Moreover, this work indicates the LBO behavior of flames produced with complex hydrocarbon fuels cannot be fully understood via high-temperature chemistry concepts such as the laminar flame speed.

Effect of burner diameter on structure and instability of turbulent premixed flames

To understand the potential role that the diameter of a burner plays in flame structure and flame instability, the influence of burner diameter on the turbulent premixed flame structure was investigated in two Bunsen-type burners. The nozzle inner diameters were 8 and 10 mm, Reynolds numbers were 2200, 3300, and 4400, equivalence ratios ranged from 0.8 to 1.2, and velocity fluctuations were varied by using two turbulence generation systems. The normalized flame edge length, local curvature of flame, and skewness of the flame curvatures were derived from Mie scatter images. Results showed that when the Reynolds number was small, the flames were smooth and the skewness of the flame curvatures was close to 0. When the Reynolds number was increasing to a turning point, the flames became wrinkled, and the skewness of the flame curvatures continued to decrease to negative. The turning point in the experiment with burner diameter of 10 mm was smaller than that in the other experiments. To measure the influence of burner diameter, a wavelet analysis was applied. An index was proposed to quantify the boundary conditions from the wavelet energy distribution. The index was observed to be negatively correlated with the skewness of the flame curvatures. In summary, the flame wrinkled easily with increasing burner diameter, and the wavelet energy distribution of inflow velocity may provide a way to judge the influence of the diameter.

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.

Conditional relationships for the layered brush structure of turbulent premixed flames in statistical steadiness

A turbulent premixed flame is decomposed into the three layers of internal region of fluctuating flamelets and leading and trailing edges with negligible density variation in the nonflamelet regime. Conditional averaging is performed in terms of the successively higher order differentials of c, (1−c), Σf′(=−∂c/∂n) and ∂2c/∂n2 to derive conditional relationships through the layered brush structure. The leading edge is defined as the region of negligible mean reaction rate to avoid the cold boundary difficulty for existence of a steadily propagating flame. The leading and trailing edges are identified in terms of the length scales of exponential decay, LLE and LTE, for c¯ and (1−c¯) becoming equal to that for Σf, as c¯ approaches zero and unity respectively. Analytical expressions are derived for LLE and LTE which are in good agreement with DNS results except for LTE interacting with the wall boundary layer in the stagnating flame. The turbulent burning velocity, ST, is given by the total diffusivity, (Dm+Dt), divided by LLE with its two limiting forms at strong and weak turbulence. The new c¯ transport equation is given in terms of the turbulent diffusivity, Dts, which is defined for the flux, 〈v′(Σf′)′〉, free from countergradient diffusion due to volume expansion. A rigorous expression is derived for the derivative, dΣf/dc¯, in terms of the mean orientation vectors and curvature, 〈n〉f, 〈n〉K and 〈∇ · n〉f. It is consistent with a familiar parabolic profile of Σf for approximately uniform 〈n〉f and 〈|∇ · n|〉f in the c¯ space. The conditional velocities show the asymptotic behavior of 〈v〉u approaching 〈v〉 and 〈v〉b approaching 〈v〉f at the leading edge and 〈v〉b approaching 〈v〉 and 〈v〉u approaching 〈v〉f at the trailing edge. Good agreement is shown for the analytical expressions of ST and the integrated profiles of Σf with DNS results of the two test flames in statistical steadiness.

The influence of large eddies on the structure of turbulent premixed flames characterized with stereo-PIV and multi-species PLIF at 20 kHz

Interactions between turbulent structures (eddies) and premixed flame fronts were characterized with simultaneous CH2O-/OH-PLIF and stereo-PIV at high framing rate (20 kHz). The high temporal resolution of these measurements permitted the tracking and subsequent characterization of the evolution of turbulence–flame interactions. Common events were identified and their effects on the flame structure were assessed, the first of these being that the flame front was locally broadened by large eddies (i.e. those larger than twice the laminar flame thickness). The results suggest that large eddies likely have a two-fold effect on the overall propagation rate, one linked to the increase in burning area (consistent with classical theories) and the other stemming from an enhancement in the local diffusivity experienced by the flame. While this latter point has not been predicted, it has been postulated that large eddies could convect preheated fluid upstream of the flame. To our knowledge the results presented here provide the first conclusive experimental evidence of this phenomenon. The second common event was the persistence of locally thin preheat layers through extremely turbulent flow fields (i.e. those possessing scales much smaller than the laminar flame thickness). Comparisons between sequences of such events and those where broadening occurred indicate that the latter is typically associated with elevated turbulent diffusivity, which supports arguments in favor of using a criterion based on a ratio of diffusivities rather than length scales to predict the onset of preheat-zone broadening.

Investigation of the fuel effects on burning velocity and flame structure of turbulent premixed flames based on leading points concept

ABSTRACTThe measurements on flame structure and burning velocities of C3H8/air, CH4/air and CO/H2/air turbulent premixed flames were conducted. Various experimental conditions for these mixtures were considered to understand the molecular and thermal diffusion process in turbulent flame, which was referred to the fuel effects. The fuel effects are interpreted as local burning velocity variation caused by molecular and thermal diffusivity with stretch effect. Leading points concept is a promising scaling approach to incorporate the fuel effects in turbulent flames recently. In this article, data across a variety of equivalence ratio and hydrogen fractions were obtained using Bunsen burner. Flame front was detected with OH-PLIF technique and turbulent burning velocity ST referred to leading edge was derived. A new leading points characteristic speed SL,LP was presented invoking both negative and positive Markstein number mixtures, revising the previous model in literature which is only suitable for negative Markstein number fuels. The turbulent burning velocity ST under relatively weak turbulence intensity in this study and data of high turbulence intensity, high pressure from literature were renormalized with SL,LP instead of SL,0 based on leading points concept, where SL,0 is the unstretched laminar flame speed. The results show that all the data sets were collapsed well which validated the significance of SL,LP in normalizing ST of different mixtures. An empirical formula for turbulent burning velocity correlation was obtained as . This formula is similar to that of Kobayashi; however, it shows superiority in incorporating the fuel effects.

Structures of turbulent premixed flames in the high Karlovitz number regime – DNS analysis

Lean premixed turbulent methane-air flames have been investigated using direct numerical simulations (DNS) for different Karlovitz numbers (Ka), ranging from 65 to 3350. The flames are imposed to a high intensity small-scale turbulent environment, corresponding to high Ka conditions, and the effect on the flame structure is investigated during the transition from initial laminar flame to highly distorted turbulent flame. The focus is on the internal structure of different sub-layers of these flames. The preheat layer, fuel consumption layer and oxidation layer are characterized by the distribution of formaldehyde, the fuel consumption rate and the CO consumption rate, respectively. Different measures that quantify sub-layer thickness for turbulent flames have been defined and analyzed. The flame brush is broadened with time while the local thickness (excluding large scale wrinkling) of all three layers initially show thinning due to the interaction of the flame with the turbulent flow field. As time passes, the local thickness of the preheat layer and fuel consumption layer are restored while the oxidation layer remains thinned due to suppression of CO consuming reactions. As Ka increases there is an increasing probability of finding thinned, large gradient regions in each of these sub-layers. The contribution to the evolution of flame thickness from normal strain rate, chemical reaction and normal and tangential diffusion is analyzed in terms of a gradient transport equation. The relative size of the terms changes as Ka increase and, in particular, the term due to chemical reactions loses its relative significance. The observed thinning of the local flame structure is attributed to the preferential alignment of the flame normal with the compressive strain rate eigenvectors. Such alignment provides a mechanism for the flame thinning, consistent with the behavior of non-reacting scalars. A preferred angle of about 20 degrees is observed between the flame normal and the compressive strain rate eigenvector.

Effects of stratification on flame structure and pollutants of a swirl stabilized premixed combustor

As mixing of fuel and air before combustion is not perfect in many practical premixed applications, the mixture is not spatially and temporally uniform and burns in partially premixed regime. Depending on the degree of mixture non-uniformity, the structure, amount of emissions and even flammability limits of a flame in this intermediate regime could be totally different from a premixed one. To investigate this effect on lean premixed combustion (LPM), a reduced scale swirl-stabilized burner is built and analyzed experimentally and numerically in premixed and two different partially premixed configurations. Experiments are initially done visually using a digital camera for a confined flame over a wide range of swirl numbers and all configurations from stoichiometry to lean blow out. Measurements of CO, NO/NOx are performed for the confined cases at the exit of the combustor using a Nondispersive Infrared (NDIR) and a Chemiluminescence analyzer (CLA) respectively. Numerical simulation is done by solving a system of partial differential equations including mass, momentum, species for confined cases under iso-thermal conditions using large eddy simulation. Results show that spatial and temporal non-uniformity of mixture, stratification not only affect the structure of turbulent premixed flames but also alternate the stability and pollutant emissions of the swirl burner at the same global operating conditions. In addition, NOx emission for axial stratified flames is more pronounced at medium to high swirl numbers.

Thin reaction zone and distributed reaction zone regimes in turbulent premixed methane/air flames: Scalar distributions and correlations

A series of premixed turbulent methane/air jet flames in the thin reaction zone (TRZ) and distributed reaction zone (DRZ) regimes were studied using simultaneous three-scalar high-resolution imaging measurements, including HCO/OH/CH2O, CH/OH/CH2O, T/OH/CH2O and T/CH/OH/. These scalar fields offer a possibility of revisiting the structures of turbulent premixed flames in different combustion regimes. In particular, CH2O provides a measure of the preheat zone, CH/HCO a measure of the inner layer of the reaction zone, and OH a measure of the oxidation zone. Scalar correlations are analyzed on both single-shot and statistical basis, and resolvable correlated structures of ∼100µm between scalars are captured. With increasing turbulence intensity, it is shown that the preheat zone and the inner layer of the reaction zone become gradually broadened/distributed, and the correlation between HCO and [OH]LIF×[CH2O]LIF decreases. A transition from the TRZ regime to the DRZ regime is found around Karlovitz number of 70–100. The physical and chemical effects on the broadening of the flame are investigated. In the TRZ regime the inner layer marker CH and HCO remains thin in general although occasional local broadening of CH/HCO could be observed. Furthermore, there is a significant probability of finding CH and HCO at rather low temperatures even in the TRZ regime. In the DRZ regime, the broadening of CH and HCO are shown to be mainly a result of local reactions facilitated by rapid turbulent transport of radicals and intermediate reactants in the upstream of the reaction paths. Differential diffusion is expected to have an important effect in the DRZ regime, as H radicals seemingly play a more important role than OH radicals.

Interaction of turbulent premixed flames with combustion products: Role of stoichiometry

Stabilization methods of turbulent flames often involve mixing of reactants with hot products of combustion. The stabilizing effect of combustion product enthalpy has been long recognized, but the role played by the chemical composition of the product gases is typically overlooked. We employ a counterflow system to pinpoint the effects of the combustion product stoichiometry on the structure of turbulent premixed flames under conditions of both stable burning and local extinction. To that end, a turbulent jet of lean-to-rich, CH4/O2/N2-premixed reactants at a turbulent Reynolds number of 1050 was opposed to a stream of hot products of combustion that were generated in a preburner. While the combustion product stream temperature was kept constant, its stoichiometry was varied independently from that of the reactant stream, leading to reactant-to-product stratification of relevance to practical combustion systems. The detailed structure of the turbulent flame front was analyzed in two series of experiments using laser-induced fluorescence (LIF): joint CH2O LIF and OH LIF measurements and joint CO LIF and OH LIF measurements. Results revealed that a decrease in local CH2O+OH and CO+OH reaction rates coincide with the depletion of OH radicals in the vicinity of the combustion product stream. These critical combustion reaction rates were more readily quenched in the presence of products of combustion from a stoichiometric flame, whereas they were favored by lean combustion products. As a result, stoichiometric combustion products contributed to a greater occurrence of local extinction. Furthermore, they limited the capacity of premixed reactants to ignite and of the turbulent premixed flames to stabilize. In contrast, lean and rich combustion products facilitated flame ignition and stability and reduced the rate of local extinction. The influence of the combustion product stream on the turbulent flame front was limited to a zone of approximately two millimeters from the gas mixing layer interface (GMLI) of the product stream. Flame fronts that were separated from the GMLI by larger distances were unaffected by the product stream stoichiometry.

Stretch rate effects and flame surface densities in premixed turbulent combustion up to 1.25 MPa

Independent research at two centres using a burner and an explosion bomb has revealed important aspects of turbulent premixed flame structure. Measurements at pressures and temperatures up to 1.25 MPa and 673 K in the two rigs were aimed at quantifying the influences of flame stretch rate and strain rate Markstein number, Masr, on both turbulent burning velocity and flame surface density. That on burning velocity is expressed through the stretch rate factor, Io, or probability of burning, Pb0.5. These depend on Masr, but they grow in importance as the Karlovitz stretch factor, K, increases, and are evaluated from the associated burning velocity data. Planar laser tomography was employed to identify contours of reaction progress variable in both rigs. These enabled both an appropriate flame front for the measurement of the turbulent burning velocity to be identified, and flame surface densities, with the associated factors, to be evaluated. In the explosion measurements, these parameters were derived also from the flame surface area, the derived Pb0.5 factor and the measured turbulent burning velocities. In the burner measurement they were calculated directly from the flame surface density, which was derived from the flame contours.A new overall correlation is derived for the Pb0.5 factor, in terms of Masr at different K and this is discussed in the light of previous theoretical studies. The wrinkled flame surface area normalised by the area associated with the turbulent burning velocity measurement, and the ratio of turbulent to laminar burning velocity, ut/ul, are also evaluated. The higher the value of Pb0.5, the more effective is an increased flame wrinkling in increasing ut/ul. A correlation of the product of k and the laminar flame thickness with Karlovitz stretch factor and Markstein number is explored using the present data and those of other workers. Some generality is revealed, enabling the wave length associated with the spatial change in mean reaction progress variable to be expressed by the number of laminar flame thicknesses, and the flame volume to be found.

Burning velocity and statistical flame front structure of turbulent premixed flames at high pressure up to 1.0 MPa

Statistical flame front structure of turbulent premixed flames at high pressure up to 1.0MPa was measured on a nozzle-type Bunsen burner with OH-PLIF technique. Turbulent burning velocity, flame surface density and flame brush thickness, as well as the local curvature and radius of curvature were derived from the experimental OH-PLIF images. Turbulence–flame interaction was analyzed based on the geometric parameters combined with laminar flame properties and turbulence length scales. Results show that the flame wrinkles at high pressure are dominated by small scale cusps superimposed with large scale flame branches which is a general characteristic of the turbulent premixed flames at high pressure. ST/SL increases remarkably with u′/SL and the influence of elevated pressure on ST/SL is significant. This is mainly due to the increase of flame front area caused by the turbulence wrinkling. Flame surface density significantly increases with the increase of pressure indicating that there is a large amount of fine cusps and small wrinkles in the flame front at high pressure. This would be due to the enhancement of the flame instability represented by effective Lewis number Leeff and flame intrinsic instability scale li. With the increase of turbulence intensity, the Σ at high pressure increases while slightly decreases at normal pressure. The most frequent length scale of the flame front moves to smaller value and the possibility increases with the increase of u′/SL for all pressures. The effect of flame intrinsic instability on finer flame front at high pressure is mainly on the formation of a large number of convex structures which enlarge the effective contact surface between flame front and unburned reactants, resulting in the increase of ST/SL.

Simultaneous multi-species and temperature visualization of premixed flames in the distributed reaction zone regime

Structures of turbulent premixed flames, operating in the thin and distributed reaction zone regimes, were investigated for stoichiometric premixed methane/air jet flames with jet Reynolds number up to 40,000 and corresponding Karlovitz number up to 286. Multi-species planar laser-induced fluorescence with high spatial resolution was applied to simultaneously image combinations of CH/OH/CH2O and HCO/OH/CH2O. In addition, OH/CH2O imaging was performed in combination with simultaneous Rayleigh scattering thermometry. The CH and HCO layers showed progressive broadening along the axial distance for flames with Reynolds number above 21,000 and the corresponding Karlovitz number above 126. At Reynolds number 40,000 and the corresponding Karlovitz number of 286, a mean CH layer thickness more than 10 times larger than that under laminar condition was observed, providing a clear experimental evidence of distributed reaction zone owing to turbulence/flame interaction. Additionally, spatial correlations between species show that OH and CH2O locate at mutually exclusive regions. In contrast, both CH and HCO can overlap substantially with CH2O. The regions of strong CH/HCO signals correspond to regions with weak CH2O signals. Moreover, CH and HCO are shown to be able to penetrate deeper into the OH layer than CH2O. Regions where CH and HCO appear distributed show a rather homogeneous temperature distribution with reduced maximum temperature compared with non-distributed conditions.

Heat release imaging in turbulent premixed methane–air flames close to blow-off

Estimates of the heat release (HR) of unconfined lean premixed methane–air flames stabilized on an axisymmetric bluff body have been measured for conditions increasingly closer to blow-off. Simultaneous imaging of OH- and CH2O-PLIF was performed, and HR measurements obtained using the pixel-by-pixel multiplication of the OH- and CH2O-PLIF images. Blow-off was approached by slowly reducing the fuel flow rate. At conditions far from blow-off, HR occurred along the shear layer, whereas at conditions near blow-off, HR was also observed inside the recirculation zone (RZ). Localised extinctions along the flame front were seen at conditions away from blow-off, and increased in frequency and size as blow-off was approached. At conditions near blow-off, HR was detected on the boundary of flame pockets inside the RZ which had detached from the fragmented flame at the attachment point. Regions void of OH in the RZ near blow-off were often seen to be filled with CH2O. Regions void of both OH and CH2O were also observed, but less often, indicating the presence of both preheated gases and fresh reactants inside the RZ. Such images do not show a connection with the annular air jet, implying the cold reactants entered the RZ from the top. HR was observed to increase as a function of the absolute value of flame front curvature for the near unity Lewis number flames investigated. The measurements reported here are useful for model validation and for exploring the changes in turbulent premixed flame structure as extinction is approached.

Flame front structure of turbulent premixed flames of syngas oxyfuel mixtures

In order to investigate oxyfuel combustion characteristics of typical composition of coal gasification syngas connected to CCS systems. Instantaneous flame front structure of turbulent premixed flames of CO/H2/O2/CO2 mixtures which represent syngas oxyfuel combustion was quantitatively studied comparing with CH4/air and syngas/air flames by using a nozzle-type Bunsen burner. Hot-wire anemometer and OH-PLIF were used to measure the turbulent flow and detect the instantaneous flame front structure, respectively. Image processing and statistical analyzing were performed using the Matlab Software. Flame surface density, mean progress variable, local curvature radius, mean flame volume, and flame thickness, were obtained. Results show that turbulent premixed flames of syngas possess wrinkled flame front structure which is a general feature of turbulent premixed flames. Flame surface density for the CO/H2/O2/CO2 flame is much larger than that of CO/H2/O2/air and CH4/air flames. This is mainly caused by the smaller flame intrinsic instability scale, which would lead to smaller scales and less flame passivity response to turbulence presented by Markstain length, which reduce the local flame stretch against turbulence vortex. Peak value of Possibility Density Function (PDF) distribution of local curvature radius, R, for CO/H2/O2/CO2 flames is larger than those of CO/H2/O2/air and CH4/air flames at both positive and negative side and the corresponding R of absolute peak PDF is the smallest. This demonstrates that the most frequent scale is the smallest for CO/H2/O2/CO2 flames. Mean flame volume of CO/H2/O2/CO2 flame is smaller than that of CH4/air flame even smaller than that of CO/H2/O2/air flame. This would be due to the lower flame height and smaller flame wrinkles.

DNS Analysis on the Indirect Relationship between the Local Burning Velocity and the Flame Displacement Speed of Turbulent Premixed Flames

The local burning velocity and the flame displacement speed are the dominant properties in the mechanism of turbulent premixed combustion. The flame displacement speed and the local burning velocity have been investigated separately, because the flame displacement speed can be used for the discussion of flame-turbulence interactions and the local burning velocity can be used for the discussion of the inner structure of turbulent premixed flames. In this study, to establish the basis for the discussion on the effects of turbulence on the inner structure of turbulent premixed flames, the indirect relationship between the flame displacement speed and the local burning velocity was investigated by the flame stretch, the flame curvature, and the tangential strain rate using DNS database with different density ratios. It was found that for the local tangential strain rate and the local flame curvature, the local burning velocity and the flame displacement speed had the opposite correlations in each density ratio case. Therefore, it is considered that the local burning velocity and the flame displacement speed have a negative correlation.

Flame front characteristics of turbulent premixed flames diluted with CO2 and H2O at high pressure and high temperature

Flame front structure characteristics of turbulent premixed flames including the local radius of curvature, fractal inner cutoff scale and local flame angle were calculated from the experimental OH-PLIF images of CH4/air flames and those diluted with CO2 and superheated H2O at 0.5MPa and 573K. The convex and concave structures of the flame front were detected and statistical analysis including PDF and ADF of the local radius of curvature and the local flame angle was conducted. Results showed that the flame front of turbulent premixed flames at high pressure and high temperature is a wrinkled flame front with small scale convex and concave cusps superimposed on large scale branches, while the whole flame front tends to be convex to the unburned mixture. The effect of EGR gas dilution on the flame front structure of turbulent premixed flames is dominated by CO2 rather than H2O dilution. The quantitative flame front characteristics reveal the complicated effect of CO2 dilution on turbulent premixed flames characteristics observed in our previous research. In the case of CO2 dilution, some local wrinkled structures become sharp and propagate deep into the burned mixture, resulting in a decrease of the fractal inner cutoff scale, the formation of a thick flame brush and the enlargement of the mean flame volume, while the overall wrinkled scale increases significantly due to the suppression of the concave structure with CO2 dilution, leading to a decrease of the turbulent flame front area, and subsequently to a decrease of ST/SL.

Measurements in turbulent premixed bluff body flames close to blow-off

The structure of unconfined lean premixed methane–air flames stabilized on an axisymmetric bluff body has been examined for conditions increasingly closer to blow-off and during the blow-off event. Fast imaging (5kHz) of OH∗ chemiluminescence and OH-PLIF and PIV (at 1kHz) were used to obtain instantaneous and time-averaged images, temporal sequences, spectra of OH, 2-D estimates of flame surface density, curvature, turbulence statistics, and measurements of the duration of the blow-off transient. Blow-off was approached by slowly reducing the fuel flow rate, and the flame shape was seen to change from a cylindrical shape at stable burning conditions, with the flame brush closing across the flow at conditions close to the blow-off condition. This was followed by entrainment of fresh reactants from the downstream end of the recirculation zone (RZ), and fragmentation of the downstream flame parts. Just before the blow-off event, reaction fronts were observed inside the RZ, with progressive fragmentation occurring, leading to a shorter flame brush. Complete extinction occurred once the flame at the attachment point had been destroyed, and stabilization at the shear layers was no longer possible. Measurements showed a gradual reduction in FSD during the approach to blow-off and during the extinction event itself, and higher values of flame front curvature at conditions approaching extinction. The local Karlovitz number was estimated based on the local turbulence velocity and lengthscale characteristics and it reached a maximum value of about 10 at the location where the flame bends towards the axis. Quantification of the duration of the blow-off event showed that it was an order of magnitude longer than the characteristic timescale of the burner d/Ub. The measurements reported here are useful for model validation and for exploring the changes in turbulent premixed flame structure as extinction is approached.