Laser imaging system for determination of three-dimensional scalar gradients in turbulent flames

An imaging system for the measurement of three-dimensional (3D) scalar gradients in turbulent hydrocarbon flames is described. Combined line imaging of Raman scattering, Rayleigh scattering, and CO laser-induced fluorescence (LIF) allows for simultaneous single-shot line measurements of major species, temperature, mixture fraction, and a one-dimensional surrogate of scalar dissipation rate in hydrocarbon flames, while simultaneous use of two crossed, planar LIF measurements of OH allows for determination of instantaneous flame orientation. In this manner the full 3D scalar dissipation can be estimated in some regions of a turbulent flame on a single-shot basis.     

Method for absolute OH-concentration measurements in premixed flames by LIF and numerical simulations

We present and apply a methodology for the single-shot measurement of absolute concentrations of the OH-radical in a turbulent, premixed natural gas/air flame. The method is based on a combination of detailed numerical simulations of the turbulent flame and an experimental approach using planar laser-induced fluorescence (LIF). The numerical simulation is used to predict LIF intensities. It shows the existence of a sharp correlation between the LIF signal after excitation of the A–X(3,0) P₂(8) transition near 248.45 nm and OH concentrations for a wide range of conditions, including stationary and instationary laminar flames of different strain rates, with different models to treat molecular transport and different degrees of heat loss. This correlation allows the transformation of measured OH–LIF intensity images into absolute OH concentration maps. PACS 82.33.Vx; 82.20.Wt; 42.62.Fi An erratum to this article can be found at .     

Chirped-probe-pulse femtosecond CARS thermometry in turbulent spray flames

This paper presents temperature measurements in turbulent dilute and dense spray flames using single-laser-shot chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS). This ultrafast technique, with a repetition rate of 5 kHz, is applied to the piloted Sydney Needle Spray Burner (SYNSBURN^TM). The burner system features air-blast atomization of liquid injected from a needle that can be translated within a co-flowing air stream. The pilot-stabilized spray flames can range between the two extremes of dense and dilute by physically translating the needle tip relative to the burner's exit plane. The CPP-fs-CARS set-up has achieved integration times of 3 picoseconds (ps) as well as spatial resolution of approximately 800 µm along beam propagation and 60 µm in the transverse dimension. Brief details of the technique, calibration, correction of interferences, and spectral fitting processes are presented along with estimates of the associated error. The measurements are compared against well-established, line Raman–Rayleigh data for temperature collected in a turbulent CH₄/air jet diffusion flame, which is largely non-sooting. At peak gaseous flame temperatures of up to 2512 K, the relative accuracy and precision were 2.8% and ±3.4%, respectively. Measurements in turbulent spray flames are shown after applying the relevant corrections based on non-resonant background (NRB) behavior and camera saturation effects on the shape of the CARS signal spectrum. Preliminary mapping of the temperature fields demonstrates the wealth of information available in this dataset which will provide insights into the spatio-temporal structure of spray flames once relevant statistical analysis is applied.     

Measurements of flame orientation and scalar dissipation in turbulent partially premixed methane flames

Three turbulent flames were studied using a new experimental facility developed at Sandia National Laboratories. Line imaging of Raman and Rayleigh scattering and CO laser-induced fluorescence (LIF) yielded information on all major species, temperature, mixture fraction, and a 1D surrogate measure of scalar dissipation. Simultaneously, crossed planar OH LIF imaging provided information on the instantaneous flame orientation, allowing estimation of the full 3D (flame-normal) scalar dissipation rate. The three flames studied were methane–air piloted jet flames (Sandia flames C, D, and E), which cover a range in Reynolds number from 13,400 to 33,600. The statistics of the instantaneous flame orientation are examined in the different flames, with the purpose of studying the prevailing kinematics of isoscalar contours. The 1D and 3D results for scalar dissipation rate are examined in detail, both in the form of conditional averages and in the form of probability density functions. The effect of overall strain and Reynolds number on flame suppression and eventual extinction is also investigated, by examining the doubly conditional statistics of temperature in the form of S-shaped curves. This latter analysis reveals that double conditioning of temperature on both mixture fraction and scalar dissipation does not collapse the data from these flames onto the same curve at low scalar dissipation rates, as might be expected from simple flamelet concepts.     

The stabilization mechanism and structure of turbulent hydrocarbon lifted flames

The structure and stabilization mechanism of turbulent lifted non-premixed hydrocarbon flames have been investigated using combined laser imaging techniques. The techniques include Rayleigh scattering, laser induced predissociation fluorescence of OH, LIF of PAH, LIF of CH₂O, and planar imaging velocimetry. The geometrical structure of multi-reaction zones and flow field at the stabilization region have been simultaneously measured in 16 hydrocarbon flames. The data reveal the existence of triple flame structure at the stabilization region of turbulent lifted flames. Increasing the jet velocity leads to an increase of the lift-off height and to a broadening of the lift-off region. Further analysis of the stabilization criterion at the lift-off height based on the premixed nature of triple-flame propagation and flow field data has been presented and discussed.     

Simultaneous measurement of localized heat release with OH/CH2O-LIF imaging and spatially integrated OH∗ chemiluminescence in turbulent swirl flames

The in-situ and localized observation of heat release in turbulent flames is important for the validation of computational modeling of turbulent flows with combustion. In the present work we obtain localized information on heat release rate (HRR) by the commonly accepted technique of the simultaneous and single-shot planar imaging of OH and CH₂O concentrations by laser-induced fluorescence (LIF). Additionally, we combine this with the simultaneous line-of-sight and temporally resolved chemiluminescence detection of OH^?, spatially integrated within the flame volume, interrogated by the laser sheets used for the HRR imaging technique. The combined diagnostic methods are demonstrated for a swirl-stabilized, premixed turbulent methane/air flame of 30-kW thermal power, and they show the existence of correlations between both HRR-sensitive diagnostic techniques.     

Blowout limits of inclined nonpremixed turbulent jet flames

The blowout behavior of inclined nonpremixed turbulent jet flames is investigated by varying the jet inclined angle in the range of -90° to 90° The critical jet velocity at blow-out limit is quantified experimentally for various nozzle diameters, different fuels and inclined angles. Numerical simulations are performed to emphasize the flow field difference for the positive and negative inclined angles. Physical modeling is conducted to incorporate the effect of the inclined angle on blow-out behavior. Major findings include: (1) The negatively inclined jet flames show more intense yellow luminosity with larger sooting zones than the positively inclined jet flames; (2) The blowout limit decreases appreciably with the jet inclined angle for the negatively inclined flames, while for the positively inclined jet flames, this decrease is relatively small; (3) Physical analysis of the flow development of inclined jets is conducted, indicating the centerline velocity along the jet trajectory decreases faster for the flame with smaller inclined angle. And the decrease rate is relatively larger for the negatively inclined jet flames; (4) Based on the analysis of the flow development as well as the characteristic velocity with the inclined angle variation, a model based on the Damköhler number (Da) accounting for the effect of jet inclined angle is developed to characterize the blowout limits of inclined jet flames. The proposed model successfully correlates the experimental data. The present findings provide new data and a basic scaling law for the blowout limit of nonpremixed inclined turbulent jet flames, revealing the effect of the relative angle between the jet momentum and buoyancy.     

Raman-Rayleigh-LIF measurements of temperature and species concentrations in the Delft piloted turbulent jet diffusion flame

We report a series of Raman-Rayleigh-LIF measurements in two turbulent natural-gas jet diffusion flames produced by the Delft piloted jet diffusion flame burner. The main objective of the Raman-Rayleigh-LIF measurements was to obtain detailed information on the major species concentrations in the flames. The measurements provide simultaneous data on temperature, the concentrations of the major species and the radicals OH and NO and mixture fraction. The application of the Raman technique in the undiluted natural-gas flames proves to be very challenging because of the high fluorescence interference levels. The interference contributions to the recorded Raman signals are identified and subtracted using empirical correlations between the Raman signals and the signals on fluorescence interference monitor channels. The calibration and data reduction of the Raman-Rayleigh and LIF signals are discussed in detail. The resulting dataset compares excellently with data from previous experiments. Because the Raman-Rayleigh-LIF data provide quantitative concentrations and accordingly quantitative mixture fractions, they form a valuable and useful extension of the existing database for the Delft piloted jet diffusion flame burner. Received: 19 October 1999 / Revised version: 31 January 2000 / Published online: 7 June 2000     

Gas-phase temperature imaging in spray systems using multi-line NO-LIF thermometry

Two-dimensional gas-phase temperature fields were measured in spray flames and evaporating spray systems using laser-induced fluorescence (LIF) of nitric oxide (NO). The recently developed multi-line technique yields absolute temperature without calibration. It is successfully applied to temperature measurements in the presence of droplets. The method is based on the temperature dependence of the NO-LIF signal. Measurements have been carried out in heated nitrogen flows at room temperature to validate the accuracy (<±1%) and precision (1%) of the technique by comparing results to thermocouple readings. Temperature measurements in a dilute evaporating acetone spray at room temperature showed cooling of the entrained air of 15±6 K. Temperature imaging in an ethanol spray flame at various conditions yields the entire temperature range from the coflow temperature at 300±4 K (1%) to the flame temperature at 1900±40 K (2%). PACS 07.20.Dt; 32.50.+d; 42.62.Fi     

The compositional structure of highly turbulent piloted premixed flames issuing into a hot coflow

Simultaneous line measurements of major species and temperature by the Raman–Rayleigh technique, combined with CO two-photon laser-induced fluorescence and crossed-plane OH planar laser-induced fluorescence have been applied to a series of flames in the Piloted Premixed Jet Burner (PPJB). The PPJB is capable of stabilizing highly turbulent premixed jet flames through the use of a stoichiometric pilot and a large coflow of hot combustion products. Four flames with increasing jet velocities and constant jet equivalence ratios are examined in this paper. The characteristics of these four flames range from stable flame brushes with reaction zones that can be described as thin and “flamelet-like” to flames that have thickened reaction zones and exhibit extinction re-ignition behaviour. Radial profiles of the mean temperature are reported, indicating the mean thermal extent of the pilot and spatial location of the mean flame brush. Measurements of carbon monoxide (CO) and the hydroxyl radical (OH) reveal a gradual decrease in the conditional mean as the jet velocity is increased and the flame approaches extinction. Experimental results for the conditional mean temperature gradient show a progressive trend of reaction zone thickening with increasing jet velocities, indicating the increased interaction of turbulence with the reaction zone at higher turbulence levels. For the compositions examined, the product of CO and OH mole fractions ([CO][OH]) is shown to be a good qualitative indicator for the net rate of production of carbon dioxide. The axial variation of [CO][OH] is shown to correlate well with the mean chemi-luminescence of the flames including the extinction re-ignition regions. The experimental findings reported in this paper further support the hypothesis of an initial ignition region followed by extinction and re-ignition regions for certain PPJB flames.     

Studies of autoignition-assisted nonpremixed cool flames

Autoignition-assisted nonpremixed cool flames of diethyl ether (DEE) are investigated in both laminar counterflow and turbulent jet flame configurations. First, the ignition and extinction limits of laminar nonpremixed cool flames of diluted DEE are measured and simulated using detailed kinetic models. The laminar flame measurements are used to validate the kinetic models and guide the turbulent flame measurements. The results show that, below a critical mixture condition, for elevated temperature and dilute mixtures, the cool flame extinction limit and the low-temperature ignition limit merge, leading to autoignition-assisted cool flame stabilization without hysteresis. Based on the findings from the laminar flame experiments, autoignition-assisted turbulent lifted cool flames are established using a Co-flow Axisymmetric Reactor-Assisted Turbulent (CARAT) burner. The lift-off heights of the turbulent cool flames are quantified using formaldehyde planar laser-induced fluorescence. Based on an analogy with autoignition-assisted lifted hot flames, a correlation is proposed such that the autoignition-assisted cool flame lift-off height scales with the product of the flow velocity and the square of the first-stage ignition delay time. Using this scaling, we demonstrate that the kinetic mechanism that most accurately predicts the laminar flame ignition and extinction limits also best predicts the turbulent cool flame lift-off height.     

Single-shot,spatially-resolved stand-off detection of atomic hydrogen via backward lasing in flames

We report on an experimental demonstration of spatially-resolved detection of atomic hydrogen in flames using a single-ended configuration yielding 656-nm lasing in the backward direction upon 2-photon pumping with 205-nm femtosecond laser pulses. Spatial resolution is achieved by temporally-resolved detection of the backward lasing using a streak camera. The method is demonstrated in CH₄/O₂ flames; both in a setup consisting of two flames, with variable spacing between the flames, and in a single flame. Results from the two-flame experiment show that the backward lasing technique is able to determine changes in the separation between the flames as this distance was altered. By maximizing the temporal resolution of the streak camera, obtaining a highest spatial resolution of 1.65 mm, it is possible to resolve the hydrogen signal present in the two reaction zones in the single flame, where the separation between the reaction zones is ∼2 mm. The lasing signal is strong enough to allow single-shot measurements. Results obtained by backward lasing are compared with 2-photon planar laser-induced fluorescence (LIF) images recorded with detection perpendicular to the laser beam path and the results from the two methods qualitatively agree. Although further studies are needed in order to extract quantitative hydrogen concentrations, the present results indicate great potential for spatially resolved single-ended measurements, which would constitute a very valuable asset for combustion diagnostics in intractable geometries with limited optical access. It appears feasible to extend the technique to detection of any species for which resonant two-photon-excited lasing effect has been observed, such as O, N, C, CO and NH₃.     

Simultaneous planar measurements of soot structure and velocity fields in a turbulent lifted jet flame at 3 kHz

We describe a newly developed combustion diagnostic for the simultaneous planar imaging of soot structure and velocity fields in a highly sooting, lifted turbulent jet flame at 3000 frames per second, or two orders of magnitude faster than “conventional” laser imaging systems. This diagnostic uses short pulse duration (8 ns), frequency-doubled, diode-pumped solid state (DPSS) lasers to excite laser-induced incandescence (LII) at 3 kHz, which is then imaged onto a high framerate CMOS camera. A second (dual-cavity) DPSS laser and CMOS camera form the basis of a particle image velocity (PIV) system used to acquire 2-component velocity field in the flame. The LII response curve (measured in a laminar propane diffusion flame) is presented and the combined diagnostics then applied in a heavily sooting lifted turbulent jet flame. The potential challenges and rewards of application of this combined imaging technique at high speeds are discussed.     

Soot studies of laminar diffusion flames with recirculation zones

This paper describes the unusual sooting structure of three flames established by the laminar recirculation zones of a centerbody burner. The vertically mounted burner consists of an annular air jet and a central fuel jet separated by a bluff-body. The three ethylene fueled flames are identified as: fully sooting, donut-shape, and ring-shape sooting flames. Different shapes of the soot structures are obtained by varying the N₂ dilution in the fuel and air jets while maintaining a constant air and fuel velocity of 1.2 m/s. All three flames have the unusual characteristic that the soot, entrained into the recirculation zone, follows discrete spiral trajectories that terminate at the center of the vortex. The questions are what cause: (1) the unusual sooting structures and (2) the spiral trajectories of the soot? Flame photographs, laser sheet visualizations, and calculations with a 2D CFD-based code (UNICORN) are used to answer these questions. The different sooting structures are related to the spiral transport of the soot, the spatial location of the stoichiometric flame surface with respect to the vortex center, and the burnout of the soot particles. Computations indicate that the spiral trajectories of the soot particles are due to thermophoresis.     

Simultaneous Raman/LIF measurements of major species and NO in turbulent H2/air diffusion flames

A single-pulse spontaneous Raman scattering apparatus, based on a flashlamp-pumped dye laser, was used to determine the concentrations of the major species and the temperature in turbulent H₂/N₂/air jet diffusion flames. The concentrations of nitric oxide were simultaneously measured by Laser-Induced Fluorescence (LIF) after excitation of theA ^{2 +}–X ² transition with a Nd: YAG-pumped dye laser. Some fundamentals of the employed methods, including the calibration procedure, quenching corrections, and accuracy are discussed. Besides a detailed study of the experimental technique, a main goal of the presented investigations was the generation of comprehensive data sets of high accuracy from well-defined turbulent flames which allow for a quantitative comparison with model calculations. Two flames with different fuel dilution and Reynolds numbers were investigated in a pattern of typically 100 measuring locations each comprising 300 single shots. In addition, four flames with different flow velocities but same fuel composition were compared with respect to their temperature and NO concentration profiles. The results show that differential diffusion plays an important role in these flames, especially near the flame base, where the temperature is increased above the adiabatic flame temperature and deviations from adiabatic equilibrium are large. The correlations between NO and mixture fraction and NO and temperature reveal characteristic features of the different flames.     

Detection of trace nitric oxide concentrations using 1-D laser-induced fluorescence imaging

Spectrally resolved laser-induced fluorescence (LIF) with one-dimensional spatial imaging was investigated as a technique for detection of trace concentrations of nitric oxide (NO) in high-pressure flames. Experiments were performed in the burnt gases of premixed methane/argon/oxygen flames with seeded NO (15 to 50 ppm), pressures of 10 to 60 bar, and an equivalence ratio of 0.9. LIF signals were dispersed with a spectrometer and recorded on a 2-D intensified CCD array yielding both spectral resolution and 1-D spatial resolution. This method allows isolation of NO-LIF from interference signals due to alternative species (mainly hot O₂ and CO₂) while providing spatial resolution along the line of the excitation laser. A fast data analysis strategy was developed to enable pulse-by-pulse NO concentration measurements from these images. Statistical analyses as a function of laser energy of these single-shot data were used to determine the detection limits for NO concentration as well as the measurement precision. Extrapolating these results to pulse energies of ～?16 mJ/pulse yielded a predicted detection limit of ～?10 ppm for pressures up to 60 bar. Quantitative 1-D LIF measurements were performed in CH₄/air flames to validate capability for detection of nascent NO in flames at 10–60 bar.     

Simultaneous multiple-line Raman/Rayleigh/LIF measurements in combustion

It is demonstrated that multiple 1D Raman scattering, Rayleigh scattering, and laser-induced fluorescence (LIF) measurements can be performed simultaneously. This can be used for quasi-2D (or quasi-3D) single-shot measurements of multiple species and the temperature in turbulent reacting and non-reacting flows. The technique has the potential to yield more precise information than most competitive planar imaging approaches in combustion. For example, it can be used to overcome Raman/LIF interference problems in technical flames. This is achieved by a new optical set-up that makes use of an imaging spectrograph combined with fiber optics. Received: 13 October 1999 / Revised version: 26 November 1999 / Published online: 27 January 2000     

The influence of wavelength in extinction measurements and beam steering in laser-induced incandescence measurements in sooting flames

The accuracy of laser-induced incandescence (LII) measurements is significantly influenced by the calibration process and the laser profile degradation due to beam steering. Additionally, the wavelength used for extinction measurements, needed for LII calibration, is critical and should be kept as high as possible in order to avoid light absorption by molecular species in the flame. The influence of beam steering on the LII measurement was studied in turbulent sooting C₂H₄/air flames at different pressures. While inhomogeneities in the laser profile become smoothed out in time-averaged measurements, especially at higher pressure, the corresponding single-shot beam profiles reveal an increasing effect of beam steering. In the current configuration it was observed that the resulting local laser fluence remains within certain limits (30% to 200%) of the original value. A sufficiently high incident laser fluence can thus prevent the local fluence from dropping below the LII threshold value of approximately 0.3 J/cm² at the cost of increased soot surface vaporization. A spatial resolution in the dimension of the sheet thickness of below 1 mm cannot be guaranteed at increased pressure of 9 bars due to beam steering. A feasibility study in a combustor at technical conditions demonstrates the influence of both effects beam steering and choice of calibration wavelength and led to the conclusion that, however, a shot-to-shot calibration of LII with simultaneously measured extinction can be realized.     

Turbulent propagation of premixed flames in the presence of Darrieus–Landau instability

We investigate the role played by hydrodynamic instability in the wrinkled flamelet regime of turbulent combustion, where the intensity of turbulence is small compared to the laminar flame speed and the scale large compared to the flame thickness. To this end the Michelson–Sivashinsky (MS) equation for flame front propagation in one and two spatial dimensions is studied in the presence of uncorrelated and correlated noise representing a turbulent flow field. The combined effect of turbulence intensity, integral scale, and an instability parameter related to the Markstein length are examined and turbulent propagation speed monitored for both stable planar flames and corrugated flames for which the planar conformation is unstable. For planar flames a particularly simple scaling law emerges, involving quadratic dependence on intensity and a linear dependence on the degree of instability. For corrugated flames we find the dependence on intensity to be substantially weaker than quadratic, revealing that corrugated flames are more resilient to turbulence than planar flames. The existence of a threshold turbulence intensity is also observed, below which the corrugated flame in the presence of turbulence behaves like a laminar flame. We also analyze the conformation of the flame surface in the presence of turbulence, revealing primary, large-scale wrinkles of a size comparable to the main corrugation. When the integral scale is much smaller than the characteristic corrugation length we observe, in addition to primary wrinkles, secondary small-scale wrinkles contaminating the surface. The flame then acquires a multi-scale, self-similar conformation, with a fractal dimension, for one-dimensional flames, plateauing at 1.23 for large intensities. The existence of an intermediate integral scale is also found at which the turbulent speed is maximized. When two-dimensional flames are subject to turbulence, the primary wrinkling patterns give rise to polyhedral-cellular structures which bear a very close resemblance to those observed in experiments on hydrodynamically unstable expanding spherical flames.