Simultaneous imaging of Mie scattering,PAHs laser induced fluorescence and soot laser induced incandescence to a lab-scale turbulent jet pulverized coal flame

In this study, transient soot formation processes in a small-scale jet burner (CRIEPI burner) were investigated by simultaneous measurements of coal particles, polycyclic aromatic hydrocarbons (PAHs) and soot. Pairs of simultaneous measurements of “Mie scattering measurement for coal particles with laser induced fluorescence (LIF) for PAHs” and “LIF for PAHs with laser induced incandescence (LII) for soot” were performed to understand the transitive formation processes of soot formation in pulverized coal flame, whose signals were successfully separated. Findings in the present study are as follows. Coal particles, PAHs and soot were distributed in this order in radial direction from the central axis. Existing regions of coal particles, PAHs and soot were overlapped from the time averaged viewpoint while there were few overlapping areas of coal particles, PAHs and soot from the instantaneous viewpoint. This result indicates that a long time is required for the formation of soot from 2 to 3 rings PAHs through larger PAHs.     

激光诱导炽光法与消光法诊断烧蚀羽烟消光性能

为了研究流场中碳纤维增强环氧树脂复合材料在激光辐照时产生的烧蚀羽烟对入射激光的屏蔽效应,通过对朗伯-比尔定律进行分析,得到了评价羽烟消光性能的平均质量消光系数的表达式,其与羽烟场浓度和激光透过率相关。采用激光诱导炽光法(LII)和激光消光法,搭建了羽烟消光性能联合诊断实验平台,使待测激光落于LII的激发光平面上,通过同步采集待测激光的透过率和LII信号,获得激发光平面上羽烟浓度场和激光消光比,得到羽烟在不同气流速度下的平均质量消光系数。实验得到气流速度为7,10,20m/s时羽烟对1064nm激光的归一化质量消光系数分别为2.51,1.08,1.00。实验发现,质量消光系数受到气流速度影响,当气流速度较低时质量消光系数曲线波动幅度大,且曲线均值较大;当气流速度较高时质量消光系数趋于稳定且均值较小。     

激光诱导炽光法与消光法诊断烧蚀羽烟消光性能

为了研究流场中碳纤维增强环氧树脂复合材料在激光辐照时产生的烧蚀羽烟对入射激光的屏蔽效应,通过对朗伯-比尔定律进行分析,得到了评价羽烟消光性能的平均质量消光系数的表达式,其与羽烟场浓度和激光透过率相关。采用激光诱导炽光法（LII）和激光消光法,搭建了羽烟消光性能联合诊断实验平台,使待测激光落于LII的激发光平面上,通过同步采集待测激光的透过率和LII信号,获得激发光平面上羽烟浓度场和激光消光比,得到羽烟在不同气流速度下的平均质量消光系数。实验得到气流速度为7,10,20 m/s时羽烟对1064 nm激光的归一化质量消光系数分别为2.51,1.08,1.00。实验发现,质量消光系数受到气流速度影响,当气流速度较低时质量消光系数曲线波动幅度大,且曲线均值较大;当气流速度较高时质量消光系数趋于稳定且均值较小。     

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 particle size distribution measurements in a turbulent ethylene swirl flame

There is a need to better understand particle size distributions (PSDs) from turbulent flames from a theoretical, practical and even regulatory perspective. Experiments were conducted on a sooting turbulent non-premixed swirled ethylene flame with secondary (dilution) air injection to investigate exhaust and in-burner PSDs measured with a Scanning Mobility Particle Sizer (SMPS) and soot volume fractions (f_v) using extinction measurements. The focus was to understand the effect of systematically changing the amount and location of dilution air injection on the PSDs and f_v inside the burner and at the exhaust. The PSDs were also compared with planar Laser Induced Incandescence (LII) calibrated against the average f_v. LII provides some supplemental information on the relative soot amounts and spatial distribution among the various flow conditions that helps interpret the results. For the flame with no air dilution, f_v drops gradually along the centreline of the burner towards the exhaust and the PSD shows a shift from larger particles to smaller. However, with dilution air f_v reduces sharply where the dilution jets meet the burner axis. Downstream of the dilution jets f_v reduces gradually and the PSDs remain unchanged until the exhaust. At the exhaust, the flame with no air dilution shows significantly more particles with an f_v one to two orders of magnitude greater compared to the Cases with dilution. This dataset provides insights into soot spatial and particle size distributions within turbulent flames of relevance to gas turbine combustion with differing dilution parameters and the effect dilution has on the particle size. Additionally, this work measures f_v using both ex situ and in situ techniques, and highlights the difficulties associated with comparing results across the two. The results are useful for validating advanced models for turbulent combustion.     

High repetition rate planar laser induced fluorescence of OH in a turbulent non-premixed flame

Received: 1 February 1999 / Revised version: 17 February 1999     

2D imaging of laser wing effects and of soot sublimation in laser-induced incandescence measurements

The distribution of Laser-Induced Incandescence (LII) signal in sooting flames along the laser beam is imaged using two directions of observation: one counter to the propagation direction of the incident laser (backward LII) and one at right angles. It is shown that the effective probe volume, in which the LII signal is observed, is highly dependent on the laser irradiance profile. At high fluence, the LII from the central part of the beam decreases because of soot sublimation. This decrease can be compensated by an increase in the LII from the wings of the laser beam. This interaction is particularly important in the extraction of quantitative information in the backward LII case, which is the configuration best suited to the practical application of LII for in-situ particle concentration measurements in the exhaust of aero-engines.     

Soot sublimation studies in a premixed flat flame using laser-induced incandescence (LII) and elastic light scattering (ELS)

Laser-induced incandescence (LII) as a diagnostic technique is based on rapid heating of soot particles to temperatures of several thousand Kelvin and subsequent detection of the thermal radiation from the laser-heated particles. At such high temperatures, soot sublimation effects must be considered when estimating uncertainties in LII measurements. In this work we have investigated the use of various laser fluences in LII using a Nd:YAG laser at 1,064 nm. Using another Nd:YAG laser at 532 nm, the elastic light scattering (ELS) signal from soot particles heated by the 1,064-nm laser was monitored. This approach makes it possible to determine at which fluence of the LII laser soot sublimation starts to become visible as a decrease in the ELS signal. By performing the measurements at various heights in a premixed sooting flat ethylene/air flame, the fluence threshold above which the ELS signal decreased was found to be higher at the lower flame heights corresponding to younger, smaller and less aggregated particles. The results from this work indicate that the different fluence thresholds for sublimation may be explained by a lower absorption function E(m) for the younger soot particles.     

Detailed analysis of early-stage NOx formation in turbulent pulverized coal combustion with fuel-bound nitrogen

A carrier-phase direct numerical simulation (CP-DNS) of pulverized coal combustion in a mixing layer is performed, considering three NO_x formation mechanisms (fuel-NO_x, thermal-NO_x and prompt-NO_x). Detailed analyses, including reaction path analysis, chemical timescale analysis, and a priori and budget analyses are conducted to investigate the NO_x production mechanisms and the performance of the flamelet model. Considering the high computational cost of CP-DNS, this work focuses on the early phase governed by devolatilization, where char reactions are less important. The reaction path analyses show that the principal thermal-NO reaction contributes to the net consumption of NO in fuel-bound nitrogen pulverized coal flames, which is essentially different from fuel-nitrogen-free flames. The chemical timescale analyses show that the production rates of NO_x species are faster than those of major species, which confirms the suitability of the flamelet tables. The a priori analyses show that the gas temperature and major/intermediate species can be predicted well by the flamelet model, while the NO_x species show significant discrepancies in certain regions. Finally, the budget analyses explain why the flamelet model performs differently for major/intermediate and NO_x species.     

Laser induced incandescence measurements of soot volume fraction and effective particle size in a laminar co-annular non-premixed methane/air flame at pressures between 0.5–4.0 MPa

An auto-compensating laser-induced incandescence (AC-LII) technique was applied for the first time to measure soot volume fraction (SVF) and effective primary particle diameter (dp_eff) in a high pressure methane/air non-premixed flame. The measured dp_eff profiles had annular structures and radial symmetry, and the particle size increased with increasing pressure. LII-determined SVFs were lower than those measured by a line of sight attenuation (LOSA) technique. The LOSA measured soot volume fractions were corrected for light scattering using the Rayleigh–Debye–Gans polydisperse fractal aggregate (RDG-PFA) theory, the dp_eff data, and assumptions regarding the soot aggregate size distribution. The correction dramatically improved agreement between data obtained using these two measurement techniques. Qualitatively, soot volume distributions obtained using LII had more annular shapes than those obtained using LOSA. Nonetheless, it has been demonstrated that the AC-LII technique is very well suited for application in media where attenuation of the excitation laser pulse energy can exceed 45%. This paper also underlines the importance of correcting LOSA SVF measurements for light scattering in high pressure flames. PACS 07-60.-j; 47.70.Pq; 65.80.+n; 78.67.-n     

利用平面激光诱导荧光技术测量燃烧场内NO的浓度分布

介绍了平面激光诱导荧光的原理及实验装置,利用可调谐OPO激光器,在甲烷 空气火焰及一些高能燃剂燃烧火焰中测得了NO分子在不同压力、不同燃烧时刻的系列荧光谱线及二维浓度分布,并给出实验结果分析。     

利用平面激光诱导荧光技术测量燃烧场内NO的浓度分布

 介绍了平面激光诱导荧光的原理及实验装置，利用可调谐OPO激光器，在甲烷 空气火焰及一些高能燃剂燃烧火焰中测得了NO分子在不同压力、不同燃烧时刻的系列荧光谱线及二维浓度分布，并给出实验结果分析。     

UV planar laser induced fluorescence imaging of hot carbon dioxide in a high-pressure flame

UV planar laser-induced fluorescence (PLIF) images of hot carbon dioxide (CO₂) are obtained in a laminar flame (CH₄/air) at high pressure (20 bar) with excitation wavelengths at 239.34 nm and 242.14 nm. Excitation wavelengths are chosen to minimize the contribution of nitric oxide and molecular oxygen LIF signals. Spectrally resolved single point measurements are used for correction of the remaining oxygen LIF interference. The continuum LIF signal from electronically excited CO₂ is detected in a broad (280–400 nm) emission region. The UV PLIF of hot CO₂ has the potential for application to a wide variety of diagnostic needs in high-pressure flames, combustors, and engines. PACS 42.62.Fi; 42.30.Va; 07.25+k; 39.30+w     

Experimental analysis of local flame extinction in a turbulent jet diffusion flame by high repetition 2-D laser techniques and multi-scalar measurements

In this paper, we present a detailed experimental study of turbulence chemistry interactions in the “DLR_B” turbulent jet diffusion flame. The flame operates on mixtures of CH₄, H₂, and N₂ in the fuel stream at Re = 22,800 and is a target flame within the TNF workshop. Extinction and re-ignition events can be tracked in real time and related to the underlying flow field phenomena and temperature fields. Time resolved measurements of OH radical concentration fields are performed in combination with temperature and velocity field measurements. For this purpose, we combined high repetition rate (33 kHz) PLIF imaging with stereoscopic PIV and double pulse Rayleigh imaging techniques. Comparisons are made with results from multi-scalar Raman/Rayleigh/LIF point measurements that reveal the thermochemical state of the flame. The large deviations from equilibrium observed on resulting OH/temperature joint pdfs could be related to strain rate and Damköhler number variations caused by turbulent flow structures leading to frequent extinctions. The 2D measurement series uniquely reveal the underlying mechanism that can lead to such events. Finally, comparisons are made to strained laminar flame calculations, which are generally found to be in good agreement with the measured data.     

Modelling soot formation in a premixed flame using an aromatic-site soot model and an improved oxidation rate

An updated rate of O₂ oxidation of one to four ring polyaromatic hydrocarbons in premixed flames is presented based on density function theory simulations of oxygen attack at different radical sites on various PAHs. The rate is in agreement with other rates found in the literature; however, it is several orders of magnitude lower than the currently accepted oxidation rate of multi-ring aromatic species, including soot. Simulations are presented of a premixed flame using this improved rate and a new advanced soot particle model, which is developed in this paper. This model includes unprecedented detail of the particles in the ensemble, including the aromatic content, C/H composition and primary-particle aggregate structure. The O₂ oxidation rate calculated in this paper is shown to give a better prediction of particle number density and soot volume fraction for a premixed flame. The predicted particle size distributions are shown also to describe better the experimental data. Predicted C/H ratio and PAH size distributions are shown for the flame. Computed TEM-style images are compared to experimental TEM images, which show that the aggregate structure of the particles is well predicted.     

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.     

Optical diagnostics on coal ignition and gas-phase combustion in co-firing ammonia with pulverized coal on a two-stage flat flame burner

Gradual substitution of coal with green ammonia is a practical approach for the coal power phasedown at a minimal cost of modification, but the ignition and gas-phase reaction during co-firing NH₃ with coal remain largely unclear. In this work, we investigate the co-combustion behaviors of NH₃ and a high-volatile coal on a two-stage flat flame burner. Remarkably, the post-flame oxygen mole fraction X_i,O2 of the inner stage can be manipulated to reproduce a proper reducing-to-oxidizing environment that coal particles experience in the practical combustor. We first reveal that, under certain values of X_i,O2 and NH₃ co-firing energy ratios E_NH3, the reaction intensity (manifested by OH-PLIF signals) in the NH₃-coal flame is stronger than burning either pure coal or NH₃. This synergetic effect originates from an NH₃-combustion-induced enhancement of volatile release. We then propose a characteristic time scale τ_OH from the OH signals for the initiation of overall reactions in the system. In the case of X_i,O2=0, τ_OH monotonically increases with E_NH3, while for X_i,O2=0.2, the trend transitions to a decreasing one. It can be interpreted by comparing τ_OH with the characteristic O₂ diffusion time, coal particle heating time, and the coal pyrolysis time under different X_i,O2. Furthermore, the coal particle ignition in coal-NH₃ flames can no longer be determined by visual images. Instead, we apply CH* chemiluminescence to identify the stages of coal particle ignition and volatile combustion in the NH₃-coal flame. While NH₃ addition has both positive (elevating temperatures & diluting coal particles) and negative (consuming O₂) effects on coal ignition, the combined influence of E_NH3 is marginal on coal ignition delay time. On the other hand, the volatile combustion time decreases linearly with E_NH3, suggesting a pure effect of reduced coal feed rate.