Flame Front Images Research Articles

Flame front visualization in highly turbulent jet flames using CH3 photofragmentation laser-induced fluorescence

Flame front visualization using methyl photofragmentation laser-induced fluorescence (CH3-PF-LIF) was demonstrated in turbulent premixed methane/air flames. A pump–probe method was used to detect CH3, where CH3 was first photolyzed to CH (X2П) fragments using a pump laser (212.8 nm), and the fragments were subsequently excited to CH in the C2Σ+ state by a probe laser at 314.4 nm. By detecting fluorescence from CH (C-X), instantaneous two-dimensional flame front imaging with a high signal-to-noise (SNR) ratio was achieved in fuel-lean and turbulent flames. A laser excitation scan was conducted to ensure that the detection of CH3 did not interfere with the other emissions. Visualization using CH3-PF-LIF was compared with visualization with conventional CH planar laser-induced fluorescence (CH-PLIF). The SNR of CH3-PF-LIF imaging is almost ten times higher than those of CH-PLIF, and flame front visualization using CH3-PF-LIF shows more topological structures than CH-PLIF in fuel-lean and highly turbulent flames.

Effect of ethanol addition on the laminar burning velocities of gasoline surrogates

Laminar burning velocities (LBV) and Markstein lengths of spherical flames of n-heptane (H), iso-octane (O), and ethanol (E) and their binary and ternary mixtures (in equal volume fractions) at elevated pressures and temperatures are experimentally investigated. Measurements are performed in a constant volume combustion vessel using two distinct techniques: flame front imaging and pressure rise rate. For pure components, ethanol shows a higher LBV compared to n-heptane and iso-octane at same conditions. A pressure dependence on LBV of each pure component is observed where ethanol is found to have the strongest pressure dependence. This is also indicated by the higher negative coefficients in three-body termination reactions through a sensitivity analysis. For binary mixtures, the EH mixture was found to have the highest LBVs compared to other blends. The pressure dependence on LBV of ethanol is also present in binary mixtures where at a higher pressures the EO blend is found to have a lower LBV compared to the OH blend due to competing reactions against the H radical pool. Non-linear behaviour of the EH mixture’s LBV was observed and verified through the chemical kinetics suggesting the widely used mixing rules for blends might need additional improvements, especially at higher temperatures. An equal volume EHO ternary mixture shows the LBV to be similar to the OH mixture at 380 K. At 450 K, the LBVs differ by 7% at atmospheric pressure, but due to ethanol’s pressure dependence, this difference decreases to zero as the pressure increases to 4 bar.

Combustion and Flame Front Morphology Characterization of H2–CO Syngas Blends in Constant Volume Combustion Bombs

The need to develop new, alternative, and bio-origin fuels for use in internal combustion engines has motivated the realization of this research, which aims to characterize the combustion process synthesis gas, represented by H2–CO blends, which are its main constituents. Syngas can be considered a biofuel because it is a mixture of carbon monoxide, hydrogen, and other hydrocarbons, and it is formed by partial combustion of biomass. Experimental tests have been developed in two constant volume combustion bombs with spherical and cylindrical geometries to analyze the combustion process and the influence of the blend composition on the burning velocity. In the first one, the pressure registered during the combustion has been used to obtain the mass burning rate, temperatures, and burning velocities. The cylindrical bomb has two optical accesses through which the combustion process can be visualized and recorded with the Schlieren technique, and it has been used to characterize the morphology of the flame, the evolution of the flame front, or the laminar burning velocities, among other parameters of interest in the combustion process. For initial conditions of 0.1 MPa and 300 K, blends with different compositions and equivalence ratios have been studied. The introduction of hydrogen enhances combustion velocity and pressure, introducing also instabilities visible on flame front images, similar effects to those produced by increasing the equivalence ratio. Regarding the morphology of the flames, note that the tend to wrinkle and the cellularity increases as the hydrogen content of the mixture increases and the equivalence ratio decreases. The dependence of the numerical values of burning velocity has been expressed as a correlation on pressure and temperature. Finally, comparing the results of the burning velocities obtained in the spherical bomb and in the cylindrical bomb with those of different authors of the bibliography has checked the consistency and validity of them. Results of syngas blends are essential for the validation, optimization, and development of kinetic models for combustion development.

Laminar burning velocity measurements in constant volume vessels – Reconciliation of flame front imaging and pressure rise methods

Laminar burning velocity measurements have been made in a constant volume vessel using both flame front imaging and the pressure rise methods. Results from the two different methods are shown to be the same, so long as appropriate techniques are used for analysing the data. Comparisons are presented for the laminar burning velocity of mixtures with air of methane, ethanol and biogas (60% methane, 40% carbon dioxide) for a wide range of flammable mixtures at pressures of 2 and 4bar and temperatures of 380 and 450K.Methods for measuring the laminar burning velocity are still the subject of controversy, with different researchers favouring different approaches. Open flame techniques are very popular and the so-called heat flux method is now well established. The alternative technique of using a constant volume combustion vessel is also in common use, and has two distinct methods of use: either the imaging of flame front propagation at conditions of constant pressure, or the measurement of the pressure rise combined with a constant volume combustion model. The pressure rise method requires a more complex analysis, but has the advantage that a single experiment generates data across a range of linked temperatures and pressures, and the pressure and temperature rise also mean that data can be obtained for engine-like conditions.

Measurement of Laminar Flame Speed and Flammability Limits of a Biodiesel Surrogate

Laminar flame speed of a newly developed biodiesel surrogate, 1,3-dimethoxyoctane, has been experimentally determined at standard atmospheric pressure and 433 K using a spherical combustion chamber. The 29 L combustion chamber has two pairs of fused silica windows for optical access and is also equipped with 4 pairs of axial fans for generating isotropic turbulence, which is used here to prepare homogeneous combustible mixtures of fuel and air. Schlieren technique was used to visualize and document the temporal evolution of the outwardly propagated spherical 1,3-dimethoxyoctane/air flame. An in-house developed Matlab code was employed for postprocessing of the flame front images and determining the time histories of its radius. Validation of the experimental setup and methodology was achieved by measuring the laminar flame speed of n-heptane/air mixtures at atmospheric pressure and 353 K, and compared with published reports. The results revealed that the proposed biodiesel surrogate, 1,3-dimethoxyoctane, ...

High-speed flamefront imaging in premixed turbulent flames using planar laser-induced fluorescence of the CH C−X band

We describe efforts to develop kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the CH radical for application to premixed flames. The basic approach used here involves excitation and detection of the CH radial via the C2Σ+−X2Π (v′=0, v″=0) band, which has transitions in the wavelength range λ≈310–320nm. Transitions in this band are generally stronger than those in the A−X and B−X bands of CH and the radiative lifetimes are shorter too. Thus, the C−X band should have advantages with regard to CH detectability in atmospheric flames, and we show that good CH-PLIF signal-to-noise and signal-to-background ratios can be attained at a 10-kHz interrogation rate and that the spatial resolution (of the CH layer) is reasonably good as well. Of course, strong OH lines, from the A2Σ+−X2Π (0,0) and (1,1) bands, lie nearby the CH C−X lines. While this can create some interference in the detection of CH, we demonstrate that OH lines can be avoided, if desired, or excited, if desired. Indeed, easy access to either CH or OH is a substantial benefit of the method outlined herein. Furthermore, we show that simultaneous imaging of CH and OH—using a single laser system and camera—is possible too. We demonstrate the utility of this approach for resolving the flamefront dynamics with 10-kHz measurements in a turbulent, premixed methane–air Bunsen flame: we see intrusion of flame and products into the reactant-zone, which appears to accelerate the consumption of the reactant core, and fingers of flame and reactants that extend into the product-zone and then pinch off and burn out.

Spectral interferences from formaldehyde in CH PLIF flame front imaging with broadband B-X excitation

In this letter we show that the use of slightly broadband laser sources like multimode alexandrite lasers for CH PLIF bears the danger of interferences from formaldehyde. Such interferences occur when the laser frequency is not carefully tuned to the B-X (0,0) R-branch band head at around 387.2 nm and manifest either in the appearance of an additional layer in the PLIF image or in a slight broadening of the actual CH layer. With careful frequency selection the formaldehyde signals were not observed. Suppression of the interferences is feasible by employing appropriate spectral filters.

Dynamics of Lean-Premixed Turbulent Combustion at High Turbulence Intensities

Premixed turbulent flames of methane-air stabilized on a Bunsen-type burner were studied to investigate the structure of the flame front at a wide range of turbulence intensities. The nondimensional turbulence rms velocity, rms velocity divided by the laminar flame speed, covered the range from about 3 to 24. The equivalence ratio was varied from 0.6 to stoichiometric. The flame front data were obtained using planar Rayleigh imaging, and particle image velocimetry was used to measure instantaneous velocity field for the experimental conditions studied. Flame front thickness increased slightly with increasing nondimensional turbulence rms velocity. There was no significant difference in flame thickening whether the flame thickness was evaluated at progress variable 0.5, corresponding to the reaction zone, or 0.3, corresponding to the preheat zone. Flame front curvature decreased with increasing turbulence rms velocity. Flame front curvature statistics displayed a Gaussian-like distribution, which centered about zero for all the flame conditions studied during the investigation. Flame surface densities evaluated from flame front images showed almost no dependence on the nondimensional turbulence intensity. Flame surface densities integrated over the flame brush volume also did not show any sensitivity to the nondimensional turbulence rms velocity. This was discussed in the framework of a flame surface density-based turbulent premixed flame propagation closure model. The implication is that the conceptual increase in flame surface density with turbulence may not be the dominant mechanism for flame velocity enhancement in turbulent combustion in the region specified as the flamelet combustion regime by the current turbulent premixed combustion diagrams. Further, the applicability of the flamelet approach may be limited to a much smaller range of conditions than presently believed.

Planar laser-induced fluorescence of HCO for instantaneous flame front imaging in hydrocarbon flames

The detection of the formyl radical (HCO) is of great interest in the field of combustion research since it can provide information about the local heat release (HR) rate which is a key parameter in the understanding of combustion processes. Unfortunately, due to the low signal level, HCO planar laser-induced fluorescence (PLIF) has so far not been applicable for single-shot imaging, which is necessary in particular for studying turbulent flames. In the present paper for the first time, to the best of our knowledge, single-shot PLIF imaging of HCO in flames is demonstrated by employing a multimode frequency-tripled alexandrite laser. On the basis of long pulse duration along with a spectrally broad bandwidth enabling multi-line excitation a sufficient signal-to-noise ratio is achieved. In detail, excitation in the B–X system around 258.69nm is applied and subsequent fluorescence detection in the spectral range 300–400nm is performed. A series of experiments concerning spectral interferences, saturation behavior and the influence of flame stoichiometry (in the range ϕ=0.6–2) have been conducted. Two typical fuels have been employed: methane (CH4) as conventional hydrocarbon as well as dimethyl ether (DME) as modern bio fuel. Single-shot HCO imaging is finally demonstrated in a laminar DME/air Bunsen flame and in a slightly turbulent methane/oxygen welding torch flame. The results indicate that the developed HCO PLIF technique offers a strong potential for improved flame studies particularly in turbulent flames. In general, the approach to employ laser sources with reasonably broader bandwidth and longer pulse duration compared to conventional Nd:YAG pumped systems for PLIF will open up new possibilities in the field of combustion diagnostics.

Experimental studies on interactions between a freely propagating flame and single obstacles in a rectangular confinement

Experimental investigations were performed to assess the effects of different shaped obstructions on flame propagation in a rectangular confinement, 235 mm in height, with a 1000 × 950 mm cross section and a large top-venting area of 1000 × 320 mm . Four different single obstacles were used: rectangular, cylindrical, triangular, and square cross-sections with blockage ratios of 5 and 10%. Temporally resolved flame front images were recorded by a high-speed video camera to investigate the interaction between a propagating flame and the obstacle. The local flame displacement speeds and their probability density functions (pdfs) were obtained for the different obstacles. Before the freely propagating flame impinges on the obstacle, the flame propagation speed remains close to the laminar burning velocity, regardless of the obstacles used. As the propagating flame impinges on the obstacle, the local propagation speed increases due to the expansion of the burnt gas and the blockage of the obstacle. This local speed increase becomes larger in going from a circular to a triangular and to a square obstacle. The averaged flame displacement speeds were not significantly different with different blockage ratios for the same obstacle investigated in this work, nor were they significantly different for different shapes at the same blockage ratio investigated in this work. However, the fastest increase in the averaged flame speed with time was observed for the rectangular plate. In order to explain why the results obtained from this work were different from those published in the literature for large L / D , a discussion of both the flame speed and pressure was given.

Fractal characterisation of high-pressure and hydrogen-enriched CH 4–air turbulent premixed flames

Flame surface characteristics were measured in turbulent premixed flames of hydrogen-enriched methane–air mixtures at elevated pressures. The equivalence ratio range was from 0.6 to 0.8 and the hydrogen mole fraction in the fuel was varied from 0 to 0.2. The pressure range covered from atmospheric to 0.9 MPa. Planar Mie scattering measurements were performed to obtain the flame front images, which were further analyzed for fractal parameters, flame front curvature statistics, and flame surface density. Non-dimensional turbulence intensity, u′/ S L, changed in proportion to the pressure and was 1.53 at atmospheric pressure and 6.51 at 0.9 MPa. With increasing pressure the flame images displayed finer structures indicating that the flame surface area was increasing with pressure. This was also indicated by the probability density function of the flame front curvature as a function of the pressure. Fractal dimension showed a strong dependence on pressure and increased from about 2.1 at atmospheric pressure to about 2.25 at 0.9 MPa. Fractal parameters, including inner and outer cut-offs, agree to a certain extend with the previous experimental data obtained at atmospheric and elevated pressures. The sensitivity of the fractal dimension to u′/ S L was found to depend on the way u′/ S L was varied. A strong correlation between the inner cut-off scale and the average flame radius of curvature was demonstrated. The implications of these results for flame surface density estimations are discussed.

Measurements of Fractal Properties of Premixed Turbulent Flames and Their Relation to Turbulent Burning Velocities

ABSTRACTThe fractal properties of premixed transient flames propagating downwards through a near-isotropic turbulent flow field in a fan-stirred cruciform burner were investigated. The long vertical section of the cruciform burner was used to provide a downward propagating premixed flame at 1 atm. The large horizontal vessel equipped with a pair of counter-rotating fans and perforated plates at each end was used to generate near-isotropic turbulence. Turbulent flame front images were obtained using high-speed laser sheet imaging for both methane-air and propane-air mixtures. The nondimensional turbulent intensity (u′/SL), Reynolds number based on the integral length scale, and turbulent Karlovitz number were varied from 1 to 10, from 698 to 6032, and from 0.05 to 1.43, respectively. Hundreds of runs for each experimental condition were carried out to obtain sufficient images of these turbulent transient flame fronts just in the central uniform region. These images were then processed to extract fractal dimension, inner and outer cutoffs using both the circle and the caliper methods. It was found that the mean fractal dimension is only 2.18, nearly independent of u′/SL, in support of recent Bunsen-flame results found by Gülder and his co-workers. This contradicts the findings of many previous studies in which the fractal dimension may approach asymptotically to a value of 2.33 when u′/SL > 3. The inner (εi) and outer (ε0) cutoffs are found to be nearly constant for all flames studied, where ε0 is an order of magnitude greater than εi and it is smaller than the integral length scale of unreacted turbulence. Finally, the present fractal characteristics cannot predict turbulent burning velocities correctly when the available fractal closure model was used, indicating a limit of the fractal analysis on prediction of turbulent burning velocities.

Flame front surface characteristics in turbulent premixed propane/air combustion

The characteristics of the flame front surfaces in turbulent premixed propane/air flames were investigated. Flame front images were obtained using laser-induced fluorescence (LIF) of OH and Mie scattering on two Bunsen–type burners of 11.2-mm and 22.4-mm diameters. Nondimensional turbulence intensity, u′/ S L , was varied from 0.9 to 15, and the Reynolds number, based on the integral length scale, varied from 40 to 467. Approximately 100 images were recorded for each experimental condition. Fractal parameters (fractal dimension, inner and outer cutoffs) and corresponding standard deviations were determined by analysis of the flame front images using the caliper technique. The fractal dimensions derived from OH and Mie scattering images are almost identical. However, inner and outer cutoffs from OH images are consistently higher than those obtained from Mie scattering. The self-similar region of the flame front wrinkling is about a decade for all flames studied. In the nondimensional turbulence intensity range from 1 to 15, it was found that the mean fractal dimension is about 2.2 and it does not show any dependence on turbulence intensity. This contradicts the findings of the previous studies that showed that the fractal dimension asymptotically reaches to 2.35–2.37 when the nondimensional turbulence intensity u′/ S L exceeds 3. It is shown that the reason for this discrepancy is the image analysis method used in the previous studies. Examples are given to show the inadequacy of the circle method used in previous studies for extraction of fractal parameters from flame front images. The fractal parameters obtained so far, in this and previous studies, are not capable of correctly predicting the turbulent burning velocity using the available fractal area closure model.

画像処理法を用いた火花点火機関における火炎伝播の解析

To obtain a better understanding of the combustion process in a spark ignition engine with or without an unscavenged horizontal prechamber, the flame propagations in both the pre-and main chambers are simultaneously visualized with a high speed camera. By using a CCD video camera and a A/D converter, the flame photographs are converted into digital data and are image-processed with a personal computer to make more clear the flame front and to clarify its brightness level in the burnt gas region.Consequently, the images of flame front for each crank angle enhanced by thresholding and edge-detecting techniques are used to make flame propagation maps, which are able to examine in detail the effect of torch jet on the combustion process. In addition, the brightness distributions of burnt gas obtained from the flame image data in the main combustion chamber show a resonable agreement with the gas temperature diagrams measured by an infrared pyrometer.

Flame front imaging in an internal-combustion engine simulator by laser-induced fluorescence of acetaldehyde

Acetaldehyde has been used as a fluorescent dopant for two-dimensional imaging of the flame front in an internalcombustion-engine simulator. The molecule was excited with a XeCl-laser-light sheet at 308 nm, and broadband fluorescence centered at 400 nm was detected. In this way, the flame front could be marked by mapping regions of unburned gas. Also, the intake process into the engine could be followed.