Two-point time-series measurements of minor-species concentrations in a turbulent nonpremixed flame

We report a technique that is capable of making simultaneous two-point time-series measurements of minor-species concentrations in turbulent flames. The experimental setup, which incorporates picosecond time-resolved laser-induced fluorescence, has a spatial resolution of less than 250 microm and a temporal resolution of less than 100 micros, which spatially and temporally resolve microscales in many turbulent flows. Two-point time-series data are given for a standard turbulent nonpremixed flame at Re= 10,000, including a discussion of potential implications.     

A pulse-pileup correction procedure for rapid measurements of hydroxyl concentrations using picosecond time-resolved laser-induced fluorescence

The measurement of fluorescence lifetimes is important for determining minor-species concentrations in flames when using linear laser-induced fluorescence (LIF). Applications of LIF to turbulent flames require that the quenching rate coefficient be determined in less than ∼100 μs. Moreover, the measurement technique must be insensitive to the existence of relatively large backgrounds, such as occur from flame emission. To meet these goals, we have recently developed a rapid, gated photon-counting technique, termed LIFTIME. However, for ultimate application to turbulent time-series measurements, LIFTIME must be extended to photon count rates which unfortunately result in nonlinear discriminator operation. In this paper, a correction technique is derived to permit quantitative measurements of fluorescence lifetimes and concentrations at sampling rates up to 4 kHz. The technique was tested against liquid samples having a known lifetime and is further shown to reproduce previous hydroxyl concentration measurements in a series of laminar flames with total photon count rates of up to ∼35 million detected photoelectrons per second. The fluorescence lifetimes and hydroxyl concentrations are shown to be measured with ∼10% accuracy (68% confidence interval) for sampling times as low as 250 μs. Received: 9 October 1998 / Revised version: 30 December 1998 / Published online: 28 April 1999     

Compressible turbulent flame speeds of highly turbulent standing flames

In this paper we present the first measurement of turbulent burning velocities of a highly turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame–turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind the standing flame. The turbulent flame speed is directly measured for the high-Mach standing turbulent flame. From measurements of the flame turbulent speed and turbulent Mach number, transition into a non-linear compressibility regime at turbulent Mach numbers above 0.4 is confirmed, and a possible mechanism for flame generated turbulence and deflagration-to-detonation transition is established.     

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.     

Characterization of fluctuating hydroxyl concentrations in?a?turbulent nonpremixed hydrogen–nitrogen jet flame

Two-point OH time-series measurements using a high-speed, laser-induced fluorescence system have been performed in a turbulent nonpremixed jet flame to obtain both radial and axial space–time correlations. Turbulent OH structures in such flames are found to undergo convection both axially and radially, but OH convection does not satisfy the ‘frozen-turbulence’ hypothesis owing to various turbulent interactions and chemical reactions. While axial OH convection occurs at approximately the local mean bulk velocity, radial convection is largely compromised by strong turbulent mixing along the same direction. The hydroxyl integral length scale can be interpreted as the typical dimension of a convective OH structure, which is axially elongated and becomes more isotropic in the post-flame region. The hydroxyl integral time scale can be interpreted as approximately the ratio of an axial integral length scale to a corresponding local mean flow velocity. In general, macroscale fluctuations of OH are dominated by large-scale turbulence, with little contribution from small-scale turbulence and OH chemistry.     

Measurements of three-dimensional mean flame surface area ratio in turbulent premixed Bunsen flames

A data processing scheme with particular emphasis on proper flame contour smoothing is developed and applied to measure the three-dimensional mean flame surface area ratio in turbulent premixed flames. The scheme is based on the two-sheet imaging technique such that the mean flame surface area ratio is an average within a window covering a finite section of the turbulent flame brush. This is in contrast to the crossed-plane tomograph technique which applies only to a line. Two sets of Bunsen flames have been investigated in this work with the turbulent Reynolds number up to 4000 and the Damköhler number ranging from less than unity to close to 10. The results show that three-dimensional effects are substantial. The measured three-dimensional mean flame surface area ratio correlates well with a formula similar to the Zimont model for turbulent burning velocity but with different model constants. Also, the mean flame surface area ratio displays a weak dependency on turbulence intensity but a strong positive dependency on the turbulence integral length scale.     

Cinematographic imaging of hydroxyl radicals in turbulent flames by planar laser-induced fluorescence up to 5 kHz repetition rate

Temporally resolved measurements of transient phenomena in turbulent flames, such as extinction, ignition or flashback, require cinematographic sampling of two-dimensional scalar fields. Hereby, repetition rates must exceed typical flame-inherent frequencies. The high sensitivity planar laser-induced fluorescence (PLIF) has already proved to be a practical method for scalar imaging. The present study demonstrates the feasibility of generating tuneable narrowband radiation in the ultraviolet (UV) spectral range at repetition rates up to 5 kHz. Pulse energies were sufficiently high to electronically excite hydroxyl radicals (OH) produced in a partially-premixed turbulent opposed jet (TOJ) flame. Red-shifted fluorescence was detected two-dimensionally by means of an image-intensified CMOS camera. Sequences comprising up to 4000 frames per run were recorded. Besides statistically stationary conditions, extinction of a turbulent flame due to small Damköhler numbers is presented showing the potential of the technique.     

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     

Simultaneous Rayleigh scattering and laser-induced CH fluorescence for reaction zone imaging in high-speed premixed hydrocarbon flames

4 /air flames where CH concentration is on the order of 1 ppm based on flamelet calculations. The present experimental conditions are also examined and shown to be suitable for quantitative measurements of CH radical based on the two-level model analysis. A linear relationship can be found between the measured CH signal intensity and the calculated CH concentration within a maximum 30% uncertainty range. The FWHM thickness of the CH profile in a stoichiometric laminar methane flame was shown to be less than 0.3 mm, which is the smallest ever achieved. Simultaneous image pairs of flame temperature and concentration of CH radicals from a premixed turbulent Bunsen flame at an exit velocity of 65 m/sec are obtained to demonstrate the system superiority of application on high-speed reacting flows. Received: 29 January 1996/Revised Version: 3 May 1996     

Visualization of turbulent spots and unsteady wake-induced boundary-layer transition with thermochromic liquid crystals

A method of using thermochromic liquid crystals has been developed to visualize the thermal footprints of turbulent spots convecting downstream in an otherwise laminar boundary layer over a heated surface. This technique has been employed to visualize the development of turbulent spots under the influence of adverse pressure gradients. It has also been used to visualize the transitional events that occur during unsteady wake-induced boundary layer transition typically of those occurring in multi-stage turbomachines. The results show that liquid crystal is not only capable of providing quantitative information about the growth and development of individual spots but also allows a detailed study of formation of turbulent spots occurring naturally during a complicated transition process.     

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.     

Spatial mapping of flame radical emission using a spectroscopic multi-colour imaging system

An intensified multi-colour digital imaging system allowing simultaneous monitoring of light from an object in four wavelength bands was used for flame emission studies. The spatial distribution of the molecular emission from different flame radicals, such as OH, C₂, CH, and CN was recorded, also in the presence of a heavy background due to Planck-radiating soot particles. Exposure times down to 8 s could be reached allowing studies of turbulent flames. The imaging spectroscopic recordings were supported by simultaneous point monitoring of the full emission spectrum. A technique for imaging flow measurements using a spectroscopic gas correlation technique is proposed.     

用于湍流燃烧温度测量的激光诊断技术

在湍流燃烧模型及CFD仿真软件的实验验证以及实际燃烧装置性能改进中,准确的温度测量以及温度梯度分布测量十分重要.Rayleigh散射、过滤Rayleigh散射和双线平面激光诱导荧光等基于激光的测温技术已在湍流燃烧实验研究中广泛应用,但每种测温技术都不能满足所有的测量环境.因此,须根据具体的探测对象和测量需求,对测温方法和实验方案进行合理选择.文章主要对这3种测温技术的工作原理、适用条件、研究现状和实际应用中需注意的问题进行综述.      

Assessment of interferences to nonlinear two-line atomic fluorescence (NTLAF) in sooty flames

Nonlinear excitation regime two-line atomic fluorescence (NTLAF) is a laser-based thermometry technique that has application in turbulent flames with soot. However, no assessment of the various interferences from soot or its precursors in flames with high soot loadings on the technique is available. To examine these issues, both on- and off-wavelength NTLAF measurements are presented and compared for laminar nonpremixed ethylene-air flames. Laser-induced incandescence (LII) measurements were used to determine the corresponding soot concentration and location in the investigated flames. The measurements indicate that interferences, such as spurious scattering and laser-induced incandescence from soot, are not significant for the present set of flame conditions. However, interferences from soot precursors, predominantly condensed species (CS) and perhaps polycyclic aromatic hydrocarbons (PAH), can be significant. Potential detection schemes to correct or circumvent these interference issues are also presented.     

Simultaneously calibrated two-line atomic fluorescence for high-precision temperature imaging in sooting flames

Simultaneously calibrated, non-linear two-line atomic fluorescence (SC-nTLAF) thermometry for application in turbulent sooting flames has been developed to increase the precision of single-shot, planar measurements of gas temperature. The technique has been demonstrated in both steady and turbulent sooting flames, showing good agreements with previous optical measurements. The SC-nTLAF involves imaging simultaneously laser-induced fluorescence (LIF) of atomic indium in both the target flame and a non-sooting calibration flame for which the temperature distribution is known. The LIF intensities from the reference flame enable correction for fluctuations, not only in the laser power, but also in the laser mode. The resulting precision was found to be ±67 K and ±75 K (based on one standard deviation) in the rich and oxidizing regions of a steady sooting flame for which the measured temperature was 1610 K and 1854 K, respectively, with a spatial resolution of 550 × 550 µm². This corresponds to a relative precision of ∼ 4.1%. The resulting precision in the single-shot temperature images for a well-characterized, lifted ethylene jet diffusion flame (fuel jet Reynolds number = 10,000) compares favorably with previously reported data obtained with shifted-vibrational coherent anti-Stokes Raman spectroscopy (CARS), together with increased spatial resolution. The planar imaging also provides more details of the temperature distribution, particularly in the flame brush region, which offers potential for measurement of more parameters, such as gradients and spatial corrections. The new calibration method has also achieved a significant time-saving in both data collection and processing, which is an estimated total of ∼ 60%–70% compared with conventional nTLAF.     

空气CH4浓度变化及其与CO的相关性

CH4是一种重要的温室气体,在空气中的含量仅次于CO2.化合物之间的相关性在化合物的浓度测量和估算等方面都有重要的意义.通过分辨率为1 cm-1的长开放光路傅里叶变换红外光谱仪测量采样路径内的北京西四环高速公路附近空气CH4和CO透过率光谱,进行非线性最小二乘光谱拟合,计算出待测组分浓度.北京秋季空气CH4浓度变化趋势几乎一样.白天的浓度变化趋势表明城市中人为活动对CH4的排放影响极大,尤其是机动车尾气的排放,而晚上浓度主要是近地面的积累.2005年9月4日到2005年9月10日的连续浓度变化表明北京秋季每天CH4和CO浓度变化趋势相同,它们的浓度变化具有一定的相关性.     

Flame speed and self-similar propagation of expanding turbulent premixed flames

In this Letter we present turbulent flame speeds and their scaling from experimental measurements on constant-pressure, unity Lewis number expanding turbulent flames, propagating in nearly homogeneous isotropic turbulence in a dual-chamber, fan-stirred vessel. It is found that the normalized turbulent flame speed as a function of the average radius scales as a turbulent Reynolds number to the one-half power, where the average radius is the length scale and the thermal diffusivity is the transport property, thus showing self-similar propagation. Utilizing this dependence it is found that the turbulent flame speeds from the present expanding flames and those from the Bunsen geometry in the literature can be unified by a turbulent Reynolds number based on flame length scales using recent theoretical results obtained by spectral closure of the transformed G equation.     

How fast can we burn, 2.0

We review the state of the art in measurements and simulations of the behavior of premixed laminar and turbulent flames, subject to differential diffusion, stretch and curvature. The first part of the paper reviews the behavior of premixed laminar flames subject to flow stretch, and how it affects the accuracy of measurements of unstrained laminar flame speeds in stretched and spherically propagating flames. We then examine how flow field stretch and differential diffusion interact with flame propagation, promoting or suppressing the onset of thermodiffusive instabilities. Secondly, we survey the methodology for and results of measurements of turbulent flame speeds in the light of theory, and identify issues of consistency in the definition of mean flame speeds, and their corresponding mean areas. Data for methane at a single operating condition are compared for a range of turbulent conditions, showing that fundamental issues that have yet to be resolved for Bunsen and spherically propagating flames. Finally, we consider how the laminar flame scale response of flames to flow perturbations interacting with differential diffusion leads to very different outcomes to the overall sensitivity of the burning rate to turbulence, according to numerical simulations (DNS). The paper concludes with opportunities for future measurements and model development, including the perennial recommendation for robust archival databases of experimental and DNS results for future testing of models.     

Instantaneous three-dimensional visualization of concentration distributions in turbulent flows with crossed-plane laser-induced fluorescence imaging

A laser-based technique for measuring instantaneous three-dimensional species concentration distributions in turbulent flows is presented. The laser beam from a single laser is formed into two crossed light sheets that illuminate the area of interest. The laser-induced fluorescence (LIF) signal emitted from excited species within both planes is detected with a single camera via a mirror arrangement. Image processing enables the reconstruction of the three-dimensional data set in close proximity to the cutting line of the two light sheets. Three-dimensional intensity gradients are computed and compared to the two-dimensional projections obtained from the two directly observed planes. Volume visualization by digital image processing gives unique insight into the three-dimensional structures within the turbulent processes. We apply this technique to measurements of toluene-LIF in a turbulent, non-reactive mixing process of toluene and air and to hydroxyl (OH) LIF in a turbulent methane-air flame upon excitation at 248 nm with a tunable KrF excimer laser. PACS 42.30.Va; 32.50.+d; 42.62.Cf     

LES/CMC modelling of ignition and flame propagation in a non-premixed methane jet

The Large Eddy Simulation (LES) / Conditional Moment Closure (CMC) model with detailed chemistry is used for modelling spark ignition and flame propagation in a turbulent methane jet in ambient air. Two centerline and one off-axis ignition locations are simulated. We focus on predicting the flame kernel formation, flame edge propagation and stabilization. The current LES/CMC computations capture the three stages reasonably well compared to available experimental data. Regarding the formation of flame kernel, it is found that the convection dominates the propagation of its downstream edge. The simulated initial downstream and radial flame propagation compare well with OH-PLIF images from the experiment. Additionally, when the spark is deposited at off-centerline locations, the flame first propagates downstream and then back upstream from the other side of the stoichiometric iso-surface. At the leading edge location, the chemical source term is larger than others in magnitude, indicating its role in the flame propagation. The time evolution of flame edge position and the final lift-off height are compared with measurements and generally good agreement is observed. The conditional quantities at the stabilization point reflect a balance between chemistry and micro-mixing. This investigation, which focused on model validation for various stages of spark ignition of a turbulent lifted jet flame through comparison with measurements, demonstrates that turbulent edge flame propagation in non-premixed systems can be reasonably well captured by LES/CMC.