Soot formation in a steady, nonpremixed, recirculating flame

An inverted step burner has been designed in which a steady ethylene, recirculating flame is established. The burner was housed within a vertical wind tunnel. Laser extinction was used to determine the soot volume fraction in the recirculation zone. Temperatures were determined by a thermocouple. One-dimensional laser-Doppler velocity (LDV) measurements were obtained with a frequency shift system to measure the flow field in the recirculating flame. All the measurements were obtained for a fixed ethylene flow rate; a low and a high velocity in the approach flow were investigated.

Variation in air velocity changed the structure of the flame. At low flow conditions, the soot loading has two distinct peaks at the lower and upper edge of the flame. At the higher air velocity, the upper part of the flame has a much lower relative soot loading as a result of the shorter residence time. The location of the peak values of the soot also changed with the residence time. The peak temperature was of the order of 1600°C. The soot loading was low in the regions of high temperature and relatively high in regions of low temperatures, reflecting the important role of thermal radiation in these luminous flames. The LDV measurements were used to reveal the nature of the flow field. The local soot loading in the flame increased as the approach flow velocity increased; this result suggests the possibility that soot may continue to grow when it is recirculated to regions of growth in a flame.     

Tomographic PIV measurements in a turbulent lifted jet flame

Measurements of instantaneous volumetric flow fields are required for an improved understanding of turbulent flames. In non-reacting flows, tomographic particle image velocimetry (TPIV) is an established method for three-dimensional (3D) flow measurements. In flames, the reconstruction of the particles location becomes challenging due to a locally varying index of refraction causing beam-steering. This work presents TPIV measurements within a turbulent lifted non-premixed methane jet flame. Solid seeding particles were used to provide the 3D flow field in the vicinity of the flame base, including unburned and burned regions. Four cameras were arranged in a horizontal plane around the jet flame. Following an iterative volumetric self-calibration procedure, the remaining disparity caused by the flame was less than 0.2 pixels. Comparisons with conventional two-component PIV in terms of mean and rms values provided additional confidence in the TPIV measurements.     

Experimental study of biogas combustion using a gas turbine configuration

The aim of the present work is to compare stability combustion domains, flame structures and dynamics between CH₄/air flames and a biogas/air flames (issued from waste methanisation) in a lean gas turbine premixed combustion conditions. Velocity profiles are obtained by Laser Doppler Anemometry measurements. CH* chemiluminescence measurements and temporal acquisition of chamber pressure are performed in order to describe flame structure and instabilities. Changes in flame structure and dynamics when fuel composition is varying are found to strongly depend on laminar flame speed. No clear correlation between the unstable flame and the reaction zone penetration in the corner recirculation can be found.     

Experimental investigation of dynamics of premixed acetylene–air flames in a micro-combustor

This paper describes a detailed experimental study performed to investigate the flame propagation behaviour of premixed flames in micro-channels. A novel, modular, stackable micro-combustor was developed for this purpose. For a chosen planar channel geometry, the flow condition and the mixture equivalence ratio of premixed acetylene–air were varied to investigate various modes of operation. Three different modes of operation were observed; they were (i) stable periodic operation – consisting of ignition, flame propagation, flame extinction, and re-ignition, (ii) a-periodic operation, and (iii) anchored flame condition. The present work also aims to provide quantitative information on the dynamics of premixed acetylene–air flames propagating inside micro-channels. A novel measurement approach based on OH^* chemiluminescence measurements employing a single photomultiplier unit was developed for this purpose. The data recorded were post processed using an in-house developed MATLAB code to evaluate the mean flame propagation speed measured between three different spatial locations along the length of the micro-channel. The results from the flame propagation speed measurements performed during ‘periodic’ mode of operation indicated that the flame travelled at higher propagation speed in the mid-length region of the channel compared to that at the initial entry point, suggesting flame acceleration. This flame acceleration could be attributed to a situation where the flame experienced different local equivalence ratio conditions at different upstream locations. The results suggest that after completion of a cycle of operation consisting of ignition, flame propagation and flame extinction, the fresh mixture that filled the channel was diluted with the exhaust gas from the previous cycle. This pocket of diluted mixture convected downstream with time, thus enabling the spatial variation in local equivalence ratio along the micro-channel.     

Simultaneous measurements of OH,mixture fraction and velocity fields to investigate flame stabilization enhancement by electric field

Simultaneous stereoscopic PIV, OH and acetone planar laser-induced fluorescence measurements are performed to analyze the processes involved in the enhancement of flame stabilization by electric field. Instantaneous velocity and mixture fraction fields are measured simultaneously at the base of a lifted flame to analyze whether the flow properties in front of the flame when electric field is applied are compatible with a mechanism involving ionic wind. The measurements conditioned on the instantaneous flame bases with and without the electric field are compared. The velocity in front of the flame decreases with electric field what is in agreement with the assumption involving ionic wind. To analyze the mixture in front of the flame, a joined analysis of velocity and mixture fraction is required to show the mixture stays near stoichiometry when the electric field is applied. The need of a joined analysis illustrates the interest of performing the three laser diagnostics simultaneously.     

LES/CMC Simulations of Swirl-Stabilised Ethanol Spray Flames Approaching Blow-Off

Large Eddy Simulations (LES) with the Conditional Moment Closure (CMC) combustion model of swirling ethanol spray flames have been performed in conditions close to blow-off for which a wide database of experimental measurements is available for both flame and spray characterization. The solution of CMC equations exploits a three-dimensional unstructured code with a first order closure for chemical source terms. It is shown that LES/CMC is able to properly capture the flame structure at different conditions and agrees reasonably well with the measurements both in terms of mean flame shape and dynamic behaviour of the flame evaluated in terms of local extinctions and statistics of the lift-off height. Experimental measurements of the overall (liquid plus gaseous) mixture fraction, performed using the Laser-Induced Breakdown Spectroscopy technique, are also included allowing further assessment and validation of the numerical method. The sensitivity of the simulation results to the various boundary conditions is discussed.     

Flow characterization of flickering methane/air diffusion flames using particle image velocimetry

Phase-resolved measurements of the velocity field in acoustically forced, flickering laminar co-flowing methane/air diffusion flames were made. Identical flames have been studied extensively in the past in order to characterize the effects of the vortical structures responsible for the flicker on the flame structure, but the initial velocity perturbation and the velocity fields have not been reported previously. Phase-locked measurements of the instantaneous two-dimensional velocity field at ten phases within a full excitation cycle were made using particle image velocimetry. The velocity measurements were complemented by phase-resolved shadowgraphs recorded in the vicinity of the flame base. Measurements are reported for the two forcing conditions that have most often been studied for this burner. When integrated with the results of previous studies, these measurements provide a clearer picture of the interactions between the buoyancy-induced vortical structures and the flame sheets, as well as providing the initial conditions required for realistic modeling of these flames.     

Soot surface temperature measurements in pure and diluted flames at atmospheric and elevated pressures

Soot surface temperature was measured in laminar jet diffusion flames at atmospheric and elevated pressures. The soot surface temperature was measured in flames at one, two, four, and eight atmospheres with both pure and diluted (using helium, argon, nitrogen, or carbon dioxide individually) ethylene fuels with a calibrated two-color soot pyrometry technique. These two dimensional temperature profiles of the soot aid in the analysis and understanding of soot production, leading to possible methods for reducing soot emission. Each flame investigated was at its smoke point, i.e., at the fuel flow rate where the overall soot production and oxidation rates are equal. The smoke point was chosen because it was desirable to have similar soot loadings for each flame. A second set of measurements were also taken where the fuel flow rate was held constant to compare with earlier work. These measurements show that overall flame temperature decreases with increasing pressure, with increasing pressure the position of peak temperature shifts to the tip of the flame, and the temperatures measured were approximately 10% lower than those calculated assuming equilibrium and neglecting radiation.     

Development of 3D Pocket Tracking Algorithm from Volumetric Measured Turbulent Flames

The flame pocket formation, including reactant pocket, product pocket, soot pocket, and fluid parcel, is a common phenomenon in turbulent combustion occurred as a response of the flame to flow straining and shearing. Understanding pocket behavior is vital to study the flames in such a regime. This work addresses the research need to experimentally measure and track multiple flame pockets in 3D. For this purpose, volumetric measurements were performed to measure the high-speed turbulent flame structure at 15 kHz based on emission tomography. With the 3D flame structures, a new tracking algorithm was developed to identify and track the multiple flame pockets simultaneously in 3D. The instantaneously tracked 3D flame pockets enabled the extraction of key properties of pocket dynamics, including the favorable formation location, 3D3C movement speed, and pocket expanding/shrinking speed. The developed methods were evidently able to resolve the detailed behavior of flame pockets in highly turbulent flames.

     

Measurement of Three-Dimensional Temperature Field of Flickering Premixed Flame with and without Coflow

The three-dimensional (3D) temperature field of the flickering flame with and without coflow can be measured using the flame reaction technique combined with tomographic reconstruction. This combined experimental technique facilitates the non-intrusive measurement of the unsteady 3D temperature field of a premixed methane/air flame. The target flame visualization, which was achieved by the flame reaction of sodium in the supplied mists of sodium chloride solution and line-of-sight intensity images of the flame, was transformed into the temperature field using calibration with the sodium D-line reversal method combined with imaging from six CCD cameras located around the flame. The uncertainty in tomographic temperature measurement was confirmed for the steady axisymmetric flame under the influence of strong coflow. Tomographic temperature measurements were applied to the flickering flame with and without coflow, and the results were analyzed using proper orthogonal decomposition (POD) to understand the unsteady behavior of the temperature field of the flickering flame. The flickering energy was found to be dominant in the first two POD modes. Flame flickering with and without coflow was found to be dominant in the axisymmetric and non-axisymmetric modes, respectively. The characteristics of the flickering flame with and without coflow are discussed in this paper, based on spectrum analysis. The results suggest that the structure of the flickering flame is highly modified by the presence of even a small magnitude of coflow.     

Experimental investigation of flame/solid interactions in turbulent premixed combustion

An experimental study has been carried out to investigate the interaction between propagating turbulent premixed flames and solid obstacles. The experimental rig was configured specifically to allow detailed measurements with laser-based optical diagnostics. A wall-type solid obstacle was mounted inside a laboratory-scale combustion chamber with rectangular cross-section. The flame was initiated, by igniting a combustible mixture of methane in air at the center of the closed end of the combustion chamber. The flame front development was visualized by a high-speed (9000 frame/s) digital video camera and flame images were synchronized with ignition timing and chamber pressure data. The tests were carried out with lean, stoichiometric and rich mixtures of methane in air. The images were used to calculate highly resolved temporal and spatial data for the changes in flame shape, speed, and the length of the flame front. The results are discussed in terms of the influence of mixture equivalence ratio on the flame structure and resulting overpressure. The reported data revealed significant changes in flame structure as a result of the interaction between the propagating flame front and the transient recirculating flow formed behind the solid obstacle. Combustion images show that the flame accelerates and decelerates as it impinges on the obstacle wall boundaries. It is also found that the mixture concentrations have a significant influence on the nature of the flame/solid interactions and the resulting overpressure. The highest flame speed of 40 m/s was obtained with the unity fuel–air equivalence ratio. Burning of trapped mixture behind the solid obstruction was found to be highly correlated with the flame front length and the rate of pressure rise.     

Feasibility of particle image velocimetry in vegetative fire spread experiments

This study is part of an ongoing effort to improve the understanding of mechanisms that control the spread of fires with a focus on the turbulent flow modified by the flame front. A large-scale PIV system was used to measure the flow field inside and in the vicinity of a flame front spreading across a bed of fuel in an open environment. The vegetative fuel consisted of a 10-m-long and 5-m-wide bed of excelsior (1?kg/m2 fuel load) leading to a nearly 1.5-m-high flame front. The velocity field was investigated in a measurement region about 1.5?m high and 2?m long. In such a configuration, a 450-mJ laser source was used to generate the light sheet, and the flow was seeded using zirconium oxide particles (ZrO₂). The PIV measurements in the presence of flame were improved by the use of a liquid crystal shutter in front of the PIV camera, allowing very short exposure times and eliminating the flame trace in the tomographic pictures. Despite the variability of the external conditions, leading to a difficult seeding over the whole PIV area, the present study shows the feasibility of the optical method of fluid visualization in the field. The measurements of the velocity fields show some features of the dynamics of fire plumes. This preliminary study demonstrates the feasibility of the method in the open, but some strong efforts to improve the seeding of the flow must be made.     

Non-linear Response of Turbulent Premixed Flames to Imposed Inlet Velocity Oscillations of Two Frequencies

This paper describes an experimental study investigating the non-linear response of lean premixed air/ethylene flames to strong inlet velocity perturbations of two frequencies. The combustor has a centrally-placed bluff body and a short quartz section. The annulus between the bluff body and the flow tube, which also housed the acoustic pressure transducers, allowed the reactants into the combustor. The inlet flow was perturbed using loudspeakers. High speed laser tomography, OH* chemiluminescence and OH Planar Laser Induced Fluorescence (PLIF) have been used for flow visualization, heat release and flame surface density (FSD) measurements respectively. The heat release fluctuations increased initially linearly with inlet velocity amplitude for a single frequency forcing, with saturation occurring after forcing amplitudes of around 15% of the bulk velocity, which was found to occur due to vortex roll up and subsequent flame annihilation. The introduction of energy at the second frequency (i.e, the harmonic) was found to change the vortex formation and shedding frequency, depending on the level of forcing. This resulted in a non-linear flame response transfer function (defined as the amplitude of unsteady heat release divided by the amplitude of velocity perturbation at the fundamental) whose amplitude depended greatly on the amount of harmonic content present in the perturbations. The introduction of higher harmonics reduced the flame annihilation events, which are responsible for saturation, thus reducing non-linearity in the amplitude dependence of the flame response. These results were further verified using sequential time-resolved OH PLIF measurements. The findings from this study suggest that the acoustic response of the flame was mostly due to flame area variation effected by modulation of the annular jet and evolution of the shear layers.     

Combined CARS/LDA instrument for simultaneous temperature and velocity measurements

The performance of a combined CARS/LDA instrument capable of measuring temperature and two velocity components with a time coincidence of about 4 s is evaluated in a turbulent premixed propane-air Bunsen-burner flame. Measurements near the base of the flame exhibit negative axial correlations, indicative of normal gradient transport; those near the flame tip show strong positive axial correlations, indicative of transport counter to the temperature gradient. The radial correlations are positive both in the reaction zone and in the plume. An analysis of temperature data from measurements made (1) independent of and (2) coincidental with LDA measurements indicates that the CARS/LDA instrument provides a density-weighted velocity, temperature, and velocity temperature correlation due to the density variations in the flame.     

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.

     

Temperature characteristics of turbulent,premixed flames stabilised on a step

The paper presents fine-wire thermocouple measurements of temperature and temperature fluctuations in a confined, two-dimensional, premixed methane-air flame of 0.8 equivalence ratio, stabilised by a backward-facing step, and with bulk flow Reynolds numbers ranging from 2800 to 13000. The results indicate a narrowing of the flame front with decrease in Reynolds number. The temperature fluctuation measurements were compensated for thermal inertia and the probability density functions were bimodal with the probability density at the two extremes of the temperature range increasing with fall in Reynolds number. Temperature power spectra indicated that the preferred frequencies corresponded to the acoustic frequencies of the combustor system, although the influence of vortex shedding was evident at the lowest Reynolds number.     

Extended Proper Orthogonal Decomposition for Analysis of Unsteady Flames

The aim of this work is to present the use of proper orthogonal decomposition (POD) and extended proper orthogonal decomposition (EPOD) for revealing flame dynamics as a set of statistical quantities referred as modes. The flame fluctuations are used to derive an empirical functions base representing the most important features of the flame. The capabilities of the technique are exemplified in the case of an unsteady laminar flame. The flame is naturally unsteady and can be excited to amplify the fluctuations. The data base consists of synchronous Particle Image Velocimetry (PIV) and OH-Planar Laser Induced Fluorescence (OH-PLIF) measurements. It was found that the POD based upon the PIV vectors only reveals flow features when the excitation is added. On contrary, the OH-PLIF based POD performs well in any case and constitutes a suitable base for the EPOD analysis.     

Development of a high-speed spectrophotometer for transient measurement of pulverized-coal flame radiation emission

A high-speed spectrophotometer system was developed for the transient measurement of pulverized-coal flame radiation. The system is a grating spectrograph with a single detector instead of a detector array. The position of the four-sided rotatable mirror determines the wavelength of the measured spectral radiation intensity, and the plane grating is mounted in a new arrangement. The system allows repeated measurement of radiation intensity spectra at wavelengths of 2.4–4.0 μm within a period of 1 or less. A desktop combustion system with the ability to vary parameters influencing the combustion of pulverized coal has been designed and assembled.

Experiments performed include steady-state measurement of radiation from a constant-intensity light source and transient measurement of radiation from a pulverized-coal flame.

Preliminary measurements showed that this spectrophotometer is capable of obtaining the desired measurements. Transient measurement of the flame radiation of pulverized coal performed on samples of Yugoslav pulverized coals produced data that comply well with results of other authors.     

Measurement of velocity-temperature correlations in a turbulent diffusion flame

The paper reports on the nonintrusive, simultaneous measurement of velocity and temperature fluctuations in a turbulent jet diffusion flame. Velocity fluctuations were measured using laser Doppler anemometry (LDA), whereas coherent anti-Stokes Raman spectroscopy (CARS) was used for temperature measurements. The simultaneous measurements were affected by both density bias and velocity bias because the LDA imposed a form of biased sampling on the CARS. The measured velocity-temperature correlation coefficients indicated that the gradient-diffusion hypothesis is reasonably accurate for the radial direction. However, for the axial direction the gradient diffusion hypothesis is accurate only in the central region of the flame, while countergradient diffusion is found in the outer region.