Sustained simultaneous high-speed imaging of scalar and velocity fields using a single laser

A high-speed technique that combines planar laser induced fluorescence (PLIF) detection of biacetyl and particle image velocimetry (PIV) for simultaneous imaging of scalar and velocity fields is demonstrated at a frame rate of 12 kHz for up to 32500 consecutive frames. A single diode-pumped, frequency-tripled Nd-YAG laser was used for excitation. Wavelength-separated recording was achieved for Mie scattering from silicone oil droplets with a CMOS camera and for the red-shifted fluorescence from biacetyl with an image-intensified CMOS camera. Interference between PIV and PLIF tracers was found to be negligible. Cross-talk between PIV and PLIF signals was low and a strategy to completely eliminate it was devised and is discussed. The signal-to-noise ratio is about 9 for single-shot scalar images. Example image sequences were recorded in an atmospheric pressure air jet at Re=2000. PACS 42.62.Fi; 33.50.Dq; 06.30.Gv; 06.60.Jn     

2D mixture fraction measurements in a high pressure and high temperature combustion system using NO tracer-LIF

Mixture fraction measurements in a jet-in-cross flow configuration at high pressures (15 bar) and temperatures (above 1000 K) were performed using planar laser induced fluorescence of nitric oxide (NO-PLIF) as trace species. The goal was the evaluation of this tracer LIF technique for the characterization of the mixing of fuel and hot exhaust gas in the mixing channel. The fuel (natural gas (NG) or H₂/N₂/NG mixture) along with the tracer were injected into the crossflow of the exhaust gas and PLIF measurements were performed in different planes. In order to relate the measured NO-LIF signal to fuel concentration and mixture fraction, effects of pressure, temperature and species concentration were taken into account. Numerical calculations and spectroscopic simulations that mimic the experimental conditions were performed to identify excitation schemes that give optimum correlations between the NO-LIF signal and the mixture fraction. The measured NO-PLIF images were transformed into mixture fraction plots using the computed correlations. The paper reports on the experimental challenges encountered during the measurements and the steps taken to overcome those difficulties. Examples of mixture fraction distributions are presented and discussed. The paper concludes with a detailed analysis on the accuracy of the measured mixture fraction values.     

Visualization of concentration field in a vortex ring using acetone PLIF

To improve the understanding of flame propagation through a nonpremixed vortex ring, the characteristics of fuel concentration in a vortex ring have been investigated experimentally. The vortex ring was generated by the ejection of propane with a single stroke motion of a speaker. Planar laser-induced fluorescence (PLIF) technique was adopted by seeding acetone as a tracer to fuel stream, in which the PLIF signal intensity is directly proportional to the concentration of acetone. This technique provides non-intrusive and instantaneous measurement of concentration field. Results showed that fuel concentration and its gradient decreased with the evolution of a vortex ring. When a nonpremixed flame propagated through a vortex ring, the flame location coincides with the inner most spiral mixing layer of fuel and air in a vortex ring.     

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。     

燃料射流与空气协流混合中的自点火预测研究

预混燃烧室燃料与空气混合过程中出现的自点火会引起回火与挂火,烧毁燃料喷嘴。针对这一问题,利用实验台模拟贫燃燃烧室预混过程,燃料射流与预热后的空气协流同向喷入石英管预混段中,研究自点火现象。本文结合机器学习和物理规律分析,开展湍流混合过程的自点火预测研究。基于二元逻辑回归建立了机器学习模型,模型的特征由分析影响自点火的物理规律得到,训练和校验模型所需的数据由燃料射流-空气协流的自点火实验获得。结果显示,机器学习方法能快速、准确地预测混合过程中自点火的发生和火焰类型,并揭示其关键影响因素。与传统的数值计算方法相比,机器学习方法预测自点火所需的时间仅为传统数值模拟方法的几千分之一。     

Coaxial turbulent jet flames: Scaling relations for measured stoichiometric mixing lengths

Previous studies have quantified the fuel–air mixing processes within jet flames having a central fuel jet surrounded by an oxidizer coflow, where the mixing primarily occurs in the far field. The present work instead quantifies mixing within coaxial jet flames having a central jet of oxygen and a surrounding finite thickness coflow of hydrogen. These flames are relatively short and the primary mixing occurs in the near field. The stoichiometric mixing length (L_S) was measured, which is the distance along the centerline at which the stoichiometric condition occurs. Values of L_S were measured for a wide range of velocity ratios for both reacting and nonreacting cases using Planar Laser Induced Fluorescence (PLIF). Acetone PLIF was utilized for nonreacting cases, while OH PLIF was used for reacting. In nonreacting cases the use of a nondimensional momentum flux ratio collapses the nonreacting coaxial jet values of L_S to a single curve, which confirms previous theory. It also was found that the reacting and nonreacting coaxial jet data collapses approximately to a single curve, if one uses both the nondimensional momentum flux ratio and an effective outer flow gas density which is predicted by the analysis of Tacina and Dahm.     

Simultaneous 36 kHz PLIF/chemiluminescence imaging of fuel,CH2O and combustion in a PPC engine

The requirements on high efficiency and low emissions of internal combustion engines (ICEs) raise the research focus on advanced combustion concepts, e.g., premixed-charge compression ignition (PCCI), partially premixed compression ignition (PPCI), reactivity controlled compression ignition (RCCI), partially premixed combustion (PPC), gasoline compression ignition (GCI) etc. In the present study, an optically accessible engine is operated in PPC mode, featuring compression ignition of a diluted, stratified charge of gasoline-like fuel injected directly into the cylinder. A high-speed, high-power burst-mode laser system in combination with a high-speed CMOS camera is employed for diagnostics of the autoignition process which is critical for the combustion phasing and efficiency of the engine. To the authors’ best knowledge, this work demonstrates for the first time the application of the burst-system for simultaneous fuel tracer planar laser induced fluorescence (PLIF) and chemiluminescence imaging in an optical engine, at 36?kHz repetition rate. In addition, high-speed formaldehyde PLIF and chemiluminescence imaging are employed for investigation of autoignition events with a high temporal resolution (5 frames/CAD). The development of autoignition together with fuel or CH₂O distribution are simultaneously visualized using a large number of consecutive images. Prior to the onset of combustion the majority of both fuel and CH₂O are located in the recirculation zone, where the first autoignition also occurs. The ability to record, in excess of 100 PLIF images, in a single cycle brings unique possibilities to follow the in-cylinder processes without the averaging effects caused by cycle-to-cycle variations.     

氢气对薄反应区湍流火焰详细火焰结构影响研究

氢是一种非常有前景的清洁可再生能源载体.掺氢燃料预混稀燃是当前开发清洁高效的低排放燃气轮机最重要的能源转化方式之一。本文基于预混CH₄/H₂/air本生灯火焰,对氢气掺混影响下的湍流火焰详细火焰结构进行了测量和表征。实验采用CH₂O和OH基平面激光诱导荧光(PLIF,Planar Laser Induced Fluorescence)同步测量技术,获得了火焰预热区、反应区以及已燃区的详细火焰结构信息。本文对反应区和预热区火焰厚度进行了提取和统计。研究表明,氢气对火焰反应区、预热区均有明显作用。结果表明,掺氢小幅度增厚反应区厚度,但能够比较明显地降低预热区厚度。     

High-speed CH planar laser-induced fluorescence imaging using a multimode-pumped optical parametric oscillator

We report on high-speed CH planar laser-induced fluorescence (PLIF) imaging in turbulent diffusion flames using a multimode-pumped optical parametric oscillator (OPO). The OPO is pumped by the third-harmonic output of a multimode Nd:YAG cluster for direct signal excitation in the A-X (0,0) band of the CH radical. The lasing threshold, conversion efficiency, and linewidth are shown to depend on the number of pump passes in the ring cavity of the OPO. Single-shot CH PLIF images are acquired at 10 kHz with excitation energy up to 6 mJ/pulse at 431.1 nm. Signal-to-noise ratios of ~25-35 are the highest yet reported for high-speed CH PLIF.     

Reaction zone structures and mixing characteristics of partially premixed swirling CH4/air flames in a gas turbine model combustor

The mixing, reaction progress, and flame front structures of partially premixed flames have been investigated in a gas turbine model combustor using different laser techniques comprising laser Doppler velocimetry for the characterization of the flow field, Raman scattering for simultaneous multi-species and temperature measurements, and planar laser-induced fluorescence of CH for the visualization of the reaction zones. Swirling CH₄/air flames with Re numbers between 7500 and 60,000 have been studied to identify the influence of the turbulent flow field on the thermochemical state of the flames and the structures of the CH layers. Turbulence intensities and length scales, as well as the classification of these flames in regime diagrams of turbulent combustion, are addressed. The results indicate that the flames exhibit more characteristics of a diffusion flame (with connected flame zones) than of a uniformly premixed flame.     

Temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence for PLIF applications

The temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence were studied in support of planar laser-induced fluorescence (PLIF) imaging of temperature and mixture fraction in flows of practical interest. The temperature dependencies (300–875 K) of absorption and fluorescence were measured for gaseous 3-pentanoneat atmospheric pressure in a nitrogen bath gas using 248, 266, and 308 nm excitation. The results indicate that the fluorescence signal per unit mole fraction using 248 nm excitation is highly temperature-sensitive below 600 K, while the signal from 308 nm excitation is not temperature sensitive below 500 K. For quantitative measurements over a broad range of temperatures, one must choose excitation schemes carefully to balance the trade-off between measurement sensitivity and the amount of signal at the expected conditions. As an example of such a choice and to show the capabilities of ketone PLIF techniques, we include temperature and mixture fraction images of a 300–650 K heated air jet using near-simultaneous 308 and 266 nm excitation. Received: 29 May 2002 / Revised version: 5 November 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +1-650/723-1748, E-mail: jkoch@stanford.edu RID="**" ID="**"E-mail: hanson@me.stanford.edu     

Effect of injection dynamics on detonation wave propagation in a linear detonation combustor

The effect of reactant injection and mixing on detonation wave propagation is studied in a self-excited, optically-accessible linear detonation combustor operated with natural gas and oxygen. Fuel injection and mixing processes are captured with 100 kHz planar laser induced fluorescence (PLIF) measurements of acetone tracer injected into the fuel stream. Measurements are acquired at multiple orthogonal planes downstream of the reactant injection site to investigate the three-dimensional mixing field in the chamber. The fuel distribution field is correlated with simultaneously acquired OH${}^{*}$ chemiluminescence measurements that provide a qualitative indication of heat release in the combustor. These measurements are used to provide quantitative information of the fuel injector recovery process and its impact on detonation wave structure across a range of equivalence ratios. While significant differences in the detonation wavefront are observed with change in equivalence ratio, the characterization of the fuel refill process into the chamber after the passage of the detonation wave highlights some key generalizable features. The time available for fuel recovery is consistently between 12 – 19% of the detonation wave period across an equivalence ratio range of 0.83 – 1.48. A linear correlation between injector recovery times and the ratio of the average detonation wave pressure amplitude relative to the pressure drop across the fuel injector is observed. Instantaneous and phase-averaged measurements of acetone-PLIF with the time-coincident OH${}^{*}$ chemiluminescence images provide qualitative information of wave structure and injection dynamics along with quantification of fuel injector recovery, a key metric that drives combustor operation and performance. These measurements significantly enhance the ability to obtain detailed information on the intra- and inter-cycle spatiotemporal evolution of the reactant refill process and its coupled effects on the detonation wave structure and propagation.     

Experimentally based correction procedures for planar laser-induced fluorescence measurements of mean species concentration

Experimentally-based correction procedures are demonstrated which enhance the quantitative nature of planar laser-induced fluorescence (PLIF) images for mean species concentration by correcting for the influence of the electronic quenching rate coefficient. Implementation of these methods requires only the ability to make PLIF and laser-saturated fluorescence (LSF) measurements. Though applied herein to NO, these procedures are broadly applicable both in terms of species and users. Moreover, they are generally effective regardless of the error gradients associated with spatial variations in the electronic quenching rate coefficient. In such general environments, these methods produce quenching-corrected, spatially resolved PLIF images of mean species concentration with a total uncertainty equivalent to that of a single LSF measurement. Received: 22 November 1999 / Revised version: 3 March 2000 / Published online: 16 June 2000     

Measurements of self-excited instabilities and nitrogen oxides emissions in a multi-element lean-premixed hydrogen/methane/air flame ensemble

Understanding the distinguishing physical properties of multi-element lean-premixed high hydrogen content flames is expected to be integral to the development of carbon-neutral, and ultimately carbon-free, gas turbine combustion systems. Despite their fundamental importance, the thermoacoustic and emission-related characteristics of such small-scale flame ensembles are not thoroughly understood, particularly for the full range of 0 to 100% hydrogen content blended with methane fuel. Here we investigate the structure and collective behavior of a multi-element lean-premixed hydrogen/methane/air flame ensemble using measurements of nitrogen oxides emissions and self-excited instability, combined with OH* and OH PLIF flame visualizations. Our results indicate that the system's responses can be classified into several distinctive stages according to their static and dynamic stability, including flame blowoff and thermoacoustically stable regions under relatively low hydrogen concentration conditions, low-frequency self-excited instabilities in intermediate hydrogen concentration, and triggering of intense pressure perturbations at about 1.7 kHz under high- or pure hydrogen combustion conditions. While the low-frequency combustion dynamics are dominated by axisymmetric translational movements of parallel flame fronts, the higher frequency response originates from significant lateral modulations accompanied by small-scale vortical rollup and flame surface annihilation due to front merging and pinch-off. Longitudinal-to-transverse dynamic transition is observed to play a mechanistic role in kinematically accommodating higher-frequency heat release rate fluctuations, and this newly identified mechanism suggests the possibility of high-frequency transverse modes, if such lateral motions are strong enough to induce inter-element flame interactions. In contrast to the substantial differences in thermoacoustic properties for different fuel compositions, the total nitrogen oxides emissions are found to depend primarily on adiabatic flame temperature; the influence of fuel composition is limited to approximately 20% under the inlet conditions considered.     

Comparison of laser-induced and planar laser-induced fluorescence measurements of nitric oxide in a high-pressure, swirl-stabilized, spray flame

We report spatially resolved linear laser-induced fluorescence (LIF) and planar laser-induced fluorescence (PLIF) measurements of nitric oxide (NO) in a pre-heated, high-pressure (4.27 atm), lean direct-injection (LDI) spray flame. The feasibility of using PLIF in lieu of LIF is assessed with respect to measuring NO concentrations in high-pressure LDI spray flames. NO is excited via the resonant Q₂(26.5) transition of the γ(0,0) band while a non-resonant wavelength is employed to subtract background interferences. LIF detection is performed in a 2-nm region centered on the γ(0,1) band. PLIF detection is performed in a 68-nm window that captures fluorescence from several vibrational bands. An in situ NO doping scheme for fluorescence calibration is successfully employed to quantify the LIF signals. However, a similar calibration scheme for the reduction of PLIF images to quantitative field measurements is plagued by the laser-excited background. Excitation scans and calibration comparisons have been performed to assess the background contribution for PLIF detection. Quantitative radial NO profiles measured by LIF are presented and analyzed so as to correct the PLIF measurements to within the accuracy bars of the LIF measurements via a single-point scaling of the PLIF image. Received: 23 November 1999 / Revised version: 17 January 2000 / Published online: 27 April 2000     

Improvement of LIEF by wavelength-resolved acquisition of multiple images using a single CCD detector – Simultaneous 2D measurement of air/fuel ratio, temperature distribution of the liquid phase and qualitative distribution of the liquid phase with the Multi-2D technique

The exciplex tracer system fluorobenzene (FB) and diethyl-methyl-amine (DEMA) in a solution of n-hexane and methyl-tert.-butylether (MTBE) was used to investigate the mixture formation in a fired direct injection spark ignition engine. The scope of this paper is the recently developed Multi-2D technique, which allows for the simultaneous measurement of the local air/fuel ratio (λ-distribution of the vapor phase), the qualitative distribution of the liquid phase, the temperature distribution of the liquid phase, and the detection of Mie scattering in this application. Basically, the Multi-2D technique consists of a new optical setup, which images the same field of view four times onto one camera, thus combining spatial and spectral resolution based on interference filters. The liquid temperature is derived via “two-line” thermometry. Using the liquid phase temperature the crosstalk from the liquid into the spectral detection range of the vapor phase is corrected. Quantitative results of the crosstalk-corrected vapor phase signals are achieved by an in-situ calibration. PACS 42.62.Fi; 44.35.+c     

Time-resolved PLIF imaging of Cu in a laser-ablated copper plasmaplume

Planar laser-induced fluorescence (PLIF) has been used to determine the relative number density of ground state copper atoms in laser-ablated plasma plumes. An ablation laser power flux of ~1.5 GW/cm ² is applied to a solid copper target in a background gas, producing a plasma plume suitable for studying homogeneous copper vapor condensation. Density is measured at postablation time delays ranging from 5 μs to 10 ms with 1-100 torr of either argon or helium as the background gas. Planar laser-induced fluorescence images are used to spatially resolve the relative density within the plume, The decrease in density is due to the homogeneous condensation of copper vapor to form particulate     

Measurement of temperature, fuel concentration and equivalence ratio fields using tracer LIF in IC engine combustion

A technique based on planar laser-induced fluorescence of 3-pentanone, for measurements of absolute concentration, temperature and fuel/air equivalence ratios in turbulent, high-pressure combustion systems such as an internal combustion engine is presented. Quasi-simultaneous excitation with 248 nm and 308 nm of 3-pentanone that is used as a fluorescence tracer doped to iso-octane, yields pairs of strongly temperature-dependent fluorescence images. Previous investigations have resulted in information on temperature and pressure dependence of absorption cross-sections and fluorescence quantum yields. Using these data the ratio of corresponding fluorescence images can be converted to temperature images. Instantaneous temperature distribution fields in the compression stroke and in the unburned end-gas of an SI engine were measured. The temperature fields obtained from the two-line technique are used to correct the original tracer-LIF images in order to evaluate quantitative fuel distributions in terms of number densities and fuel/air equivalence ratio. Received: 10 March 2000 / Revised version: 19 April 2000 / Published online: 16 August 2000     

基于三种气化炉的整体煤气化湿化燃气轮机循环热力性能分析

将燃料湿化和空气湿化应用到整体煤气化燃气轮机循环中,以降低NOx排放并有效利用系统低品位热。基于水煤浆、干煤粉及输运床三种气化炉,构建了多种整体煤气化湿化燃气轮机循环,分析了其热力性能并给出了湿化方式建议。研究表明：燃料湿化循环系统效率较高;空气湿化循环燃气轮机比功较大;整体煤气化湿化燃气轮机循环具有利用系统外部中低品...