Spectroscopic,calibration and RET issues for linear laser-induced fluorescence measurements of nitric oxide in high-pressure diffusion flames

Two different strategies are compared for linear laser-induced fluorescence (LIF) measurements of nitric oxide concentration ([NO]) in counter-flow diffusion flames at high pressures via the A-X(0,0) system. Excitation of NO via a rovibronic transition at 226.03 nm is found to be slightly better compared to a previously utilized excitation wavelength of 225.58 nm. An indirect approach based on the computed spectral overlap fraction is verified and applied to calibrate [NO] measurements in counter-flow diffusion flames at high pressures. A five-level model for NO molecular dynamics is presented and utilized to investigate the effects of rotational energy transfer (RET) on linear LIF measurements of [NO] at pressures up to 15 atm. The results indicate that rotational relaxation effects are essentially negligible under high-pressure conditions at low laser fluences, and thus they need not be accounted for when measuring [NO] using linear LIF. The calibration technique is validated by direct comparisons to [NO] measurements made at pressures up to 5 atm via another calibration method, based on doping NO in counter-flow premixed flames at the same pressure. Using this calibration technique, LIF measurements of [NO] are obtained in a series of counter-flow diffusion flames at pressures up to 15 atm. These measurements are found to be in excellent agreement with previously reported measurements of [NO] in similar flames. PACS 07.35.+k; 33.20.Sn; 42.62.Fi     

Response of non-premixed flames to bulk flow perturbations

This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to that of a premixed flame. The space–time dynamics of the non-premixed flame sheet in the fast chemistry limit is described by the stoichiometric mixture fraction surface, extracted from the solution of the

-equation. This procedure has some analogies to premixed flames, where the premixed flame sheet location is extracted from the G = 0 surface of the solution of the G-equation. A key difference between the premixed and non-premixed flame dynamics, however, is the fact that the non-premixed flame sheet dynamics are a function of the disturbance field everywhere, and not just at the reaction sheet, as in the premixed flame problem. A second key difference is that the non-premixed flame does not propagate and so flame wrinkles are convected downstream at the axial flow velocity, while wrinkles in premixed flames convect downstream at a vector sum of the flame speed and axial velocity. With the exception of the flame wrinkle propagation speed, however, we show that that the solutions for the space–time dynamics of the premixed and non-premixed reaction sheets in high velocity axial flows are quite similar. In contrast, there are important differences in their spatially integrated unsteady heat release dynamics. Premixed flame heat release fluctuations are dominated by area fluctuations, while non-premixed flames are dominated by mass burning rate fluctuations. At low Strouhal numbers, the resultant sensitivity of both flames to flow disturbances is the same, but the non-premixed flame response rolls off slower with frequency. Hence, this analysis suggests that non-premixed flames are more sensitive to flow perturbations than premixed flames at O(1) Strouhal numbers.     

Structure of partially premixed n-heptane–air counterflow flames

To avoid the complexities associated with the droplet/vapor transport and nonuniform evaporation processes, a fundamental investigation of liquid fuel combustion in idealized configurations is very useful. An experimental–computational investigation of prevaporized n-heptane nonpremixed and partially premixed flames established in a counterflow burner is described. There is a general agreement between various facets of our nonpremixed flame measurements and the literature data. The partially premixed flames are characterized by a double flame structure. This becomes more distinct as the strain rate decreases and partial premixing increases, which also increases the separation distance between the two reaction zones. The peak partially premixed flame temperature increases with increasing premixing of the fuel stream. The peak CO₂ and H₂O concentrations are relatively insensitive to partial premixing. The CO and H₂ peak concentrations on the premixed side increase as the fuel-side equivalence ratio decreases. These species are transported to the nonpremixed reaction zone where they oxidize. The C₂ species have peaks in the premixed reaction zone. The concentrations of olefins are ten times larger than those of the corresponding paraffins. The oxidizer is present in partially premixed flames throughout the combustion system and there are no regions characterized by simultaneous high temperature and high fuel concentration. As a result, pyrolysis reactions leading to soot formation are greatly diminished.     

The homogeneity of the ball inR 3 and special relativity

Einstein's velocity addition formula ofspecial relativity (SR) defines a transformation _v of the ballB _c of radiusc inR ³, representing all possible velocities in an inertial systemK, onto identical ballB _c , which represents the velocities in another systemK, moving with velocity v relative toK. Since _v maps the zero velocity ofB _c into arbitrary vector v ofB _c ,B _c is homogeneous under all possible _v.A similar homogeneity of the unit ballB inL(G, H) under a set of maps _a, a B, arises also in theLine Transmission Theory (TLT) for a lossless line. HereL(G, H) is the space of all linear operators between Hilbert spacesG,H, representing the signals on the line in the two directions. The explicit form of _a is obtained naturally in TLT.     

On transient behavior of non-premixed counter-flow diffusion flames within the REDIM based model reduction concept

In the present work non-stationary behavior of the counter-flow diffusion flame is examined in the context of the recently developed approach of model reduction called REaction–DIffusion Manifolds (REDIM) method. It is a natural extension of the ILDM approach which takes into account both the chemical reaction and the diffusion processes. It has been developed to treat both premixed and non-premixed regimes of combustion. In this work we investigate the ability of the concept to describe transient processes of extinction and re-ignition. A very simple flame configuration and transport model are considered in this current study for the sake of transparency because the main focus is on the transient and non-stationary behavior of flames. H₂/O₂/N₂ combustion system is considered in a non-premixed counter-flow diffusion 1D flame configuration. This study shows how the REDIM concept performs in the transient regimes; it interprets the effect of local extinction and reigniting phenomena using detailed and reduced models. It shows how the unstable/transient behavior of a detailed system can be successfully accounted with the help of the REDIM based reduced model.     

Production rate and decay lifetime measurements ofBs0 mesons at LEP usingDs and? mesons

The study of the properties of inclusive production ofD _s mesons and of events in which a and a muon are present in the same jet provides two independent measurements of the probability,f _s ^w , for a heavy quark to hadronize into a strangeB orD meson. The data sample analysed corresponds to 243,000 hadronicZ ⁰ decays. The combined value of these measurements isf _s ^w =0.19±0.06±0.08. From the flight distance distributions ofD _s and of (-lepton) secondary vertices, with the lepton emitted at high transverse momentum relative to the jet axis, two values are obtained for theB _s ⁰ meson lifetime. Combining these measurements with a previous result based on the study ofD _s- events, theB _s ⁰ meson lifetime is measured to be: 0.96±0.37 ps.     

Laser-induced fluorescence measurements and modeling of absolute CH concentrations in strained laminar methane/air diffusion flames

We have applied linear laser-induced fluorescence to obtain spatially resolved profiles of CH radicals in laminar methane/air and methane/nitric oxide/air counterflow diffusion flames at atmospheric pressure. Excitation and detection of transitions in the A–X band and calibrating the optical detection efficiency via Rayleigh scattering allowed the determination of absolute radical concentrations. Flames at strain rates from 59 to 269 s⁻¹ were studied to characterize the strain rate dependence of the CH concentration. The work shows that CH concentrations increase with increasing strain rate. Comparisons have been made with predicted CH levels obtained using two different chemical kinetic mechanisms (Lindstedt et al. and GRI-Mech. 3.0). Computed concentrations are shown to be in good agreement with experimental data. It was furthermore found that the addition of up to 600 ppm NO to the fuel did not have a measurable effect on the CH radical concentration. This is also in agreement with predictions from both mechanisms. The current work has shown that measurements of absolute CH radical concentrations are possible in non-premixed flames without the need for spatial temperature or quenching corrections.     

Intracavity laser absorption spectroscopy of sooting acetylene/air flames

Intracavity laser absorption spectroscopy (ICLAS) is used to measure the absolute concentration profiles of HCO and C₂ in low-pressure acetylene/oxygen/nitrogen flames with equivalence ratios ϕ=0.8, 1.0, 1.5, 2.0 and 2.5. The flames with ϕ=2.0 and 2.5 are soot-producing, with light extinction reaching 0.1% per pass in the flame with ϕ=2.5. This strong broadband extinction does not affect the sensitivity of ICLAS, however. The temperature profiles of the flames were measured using laser-induced fluorescence of the OH radicals. For C₂ concentration measurements, the (0–2) vibronic transition of the Swan band is used. The lines of this transition are located close to the HCO lines, making it possible to measure the two radical concentrations simultaneously. The C₂ concentration is highest in the ϕ=1.5 flame, and lower in the lean and heavily sooted ϕ=2.5 flames. PACS  33.20.Kf; 33.70.Fd; 42.60.Da     

The proper formula for relative entropy and its asymptotics in quantum probability

Umegaki's relative entropyS(,)=TrD (logD –logD ) (of states and with density operatorsD andD , respectively) is shown to be an asymptotic exponent considered from the quantum hypothesis testing viewpoint. It is also proved that some other versions of the relative entropy give rise to the same asymptotics as Umegaki's one. As a byproduct, the inequality TrA logAB TrA(logA+logB) is obtained for positive definite matricesA andB.     

Two-dimensional imaging of glyoxal (C2H2O2) in acetylene flames using laser-induced fluorescence

2 H₂O₂). Laser-induced fluorescence spectra from glyoxal vapor using the same excitation wavelength of 428 nm showed the same strongest lines as the signal from the flame. Glyoxal was visualized in two different modes; two-dimensional imaging and a spatial-spectral mode where spectra were obtained at different spatial positions in the flame simultaneously. For the premixed laminar rich flame it is shown that glyoxal is produced early in the flame, before the signals for C₂ and CH appear. For the turbulent non-premixed flames it is shown that glyoxal is produced in a layer on the fuel rich side of the flames. Here the fuel is premixed with ambient air. This layer is thin and has a high spatial resolution. The general trend was that the glyoxal signal appeared in regions with a lower temperature compared with the emission from C₂ and CH. The imaging of glyoxal in turbulent acetylene flames is a promising tool for achieving new insight into flame phenomena, as it gives very good structural information on the flame front. Tests so far do not indicate that the detected glyoxal is a result of photo-production. To our knowledge, this is the first detection of glyoxal in flames using laser-induced fluorescence. Received: 19 December 1996/Revised version: 26 May 1997     

稀释剂对H2/CH4预混火焰熄灭特性影响

掺氢天然气在稀释气体作用下的熄灭特性研究对实际燃烧设备的设计和优化具有重要的指导意义。本文利用对冲火焰法测量了掺氢天然气层流火焰在N₂和CO₂作用下的熄灭拉伸率,并采用数值模拟耦合详细化学反应机理对N₂,CO₂和He的稀释剂效应展开研究。结果表明,Li、GRI Mech 3.0和FFCM-1机理均能定性反映燃料熄灭拉伸率随当量比的变化规律,且FFCM-1机理综合预测精度最高。实验和模拟发现,不同稀释剂气体对掺氢天然气熄灭拉伸率降低幅度满足:He22。进一步研究发现,CO₂由于热容大,在反应体系中会降低火焰温度,同时增强了链终止反应强度,通过热效应和化学效应两方面对火焰熄灭特性起作用。He则能显著改变燃料混合物的平均摩尔质量,从而改变体系中重要反应物和自由基的扩散特性,从扩散效应方面影响火焰的熄灭特性。     

An experimental and numerical study on the adequacy of CH as a flame marker in premixed methane flames

The CH radical is frequently used as a flame marker because it is relatively short-lived and is present over a narrow region in flames. Discontinuities in the CH field are thus often interpreted as localized extinction of the flame. Recently, however, the adequacy of CH laser-induced fluorescence (LIF) as a flame marker was questioned by an experimental study of flame–vortex interactions in highly N₂-diluted premixed methane flames. We demonstrate both experimentally and numerically that anomalies in the transient response of CH in this earlier study were due to reactant composition variations in the vortex. In addition, we evaluate the adequacy of CH LIF as a flame marker over a much broader range of conditions. Previous numerical studies showed that heat release rate correlates reasonably well with peak [HCO] and the concentration product [OH][CH₂O], but poorly with [CH], in highly N₂-diluted premixed methane flames. Here, the correlation between heat release rate and CH is investigated both experimentally, by performing simultaneous measurements of CH, OH, and CH₂O LIF, and numerically. We consider undiluted and N₂-diluted premixed methane flames over a range of strain rates and stoichiometries. Results are reported for flames subjected to unsteady stretch and reactant composition variations. For all N₂-dilution levels considered, the peak CH LIF signal correlates poorly with heat release rate when the stoichiometry of the reactant mixture changes from rich to lean. However, when flames are subjected to stretch, the correlation between CH and heat release rate improves as the N₂-dilution level decreases. The correlation is reasonably good for undiluted flames with equivalence ratios of 0.8 < Φ < 1.2. This result is particularly encouraging, given the relevance of undiluted flames to practical applications, and it motivates further investigation of the parameter space for which difficulties may exist in using CH as a flame marker.     

A review of chemical diffusion: Criticism and limits of simplified methods for diffusion coefficient calculation

A review of the physics and modelling of mass diffusion involving different gaseous chemical species is firstly proposed. Both accurate and simplified models for mass diffusion involve the calculation of individual species diffusion coefficients. Since these are computationally expensive, in CFD they are commonly estimated by assuming constant Lewis or Schmidt numbers for each chemical species. The constant Lewis number assumption is particularly used. As a matter of fact, these assumptions have never been theoretically justified nor verified in practical flames. The only published information are the first observations by Smooke and Giovangigli about the Lewis number against temperature distributions in methane–air premixed and counterflow diffusion one-dimensional flames. The aim of this work is to verify these assumptions. Functional dependences of molecular properties appearing in these numbers are made explicit to show that while Sc _i depends only on composition, Le _i depends also on temperature and therefore it certainly cannot be assumed constant in a flame. Then, accurately calculating molecular properties, distributions of these characteristic numbers against temperature are obtained a posteriori from numerical simulations of different flames, premixed and non-premixed, and burning different fuels. For non-premixed flames, individual species Lewis number distributions are broad for most of the species considered in this article, whilst they are tight for premixed flames. Some attention is focused on the particular shape of Lewis distributions in non-premixed flames: they are characterized by four or five (when extinction is experienced) branches associated to precise regions in the flame (basically, lean, rich and stoichiometric combusting zones). Instead, the Schmidt distributions are always tighter, also when extinctions take place: for many species they can be approximatively assumed constant. Finally, a simplified procedure to estimate individual species diffusion coefficients is suggested, assuming the median of non-premixed flame Schmidt distributions has a constant value for each chemical species.     

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.     

The Lipatov argument for φ 3 4 perturbation theory

We extend to ₃ ⁴ the work of S. Breen on the leading behavior at large order of ₂ ⁴ perturbation theory. Using a phase space expansion to obtain new estimates on the high energy behavior of ₃ ⁴ Feynman graphs, and a rigorous semiclassical expansion, we prove that the radius of convergence of the Borel transform of the pertubative series for ₃ ⁴ Euclidean field theory is the one computed by the Lipatov method.     

Flame structure analysis and flamelet progress variable modelling of strained coal flames

Strained two-phase pulverised coal flames in a counterflow configuration are investigated numerically. Three operating conditions with different coal-to-primary-air ratios and inlet velocities were evaluated in order to establish different flame regimes. At first, the two-phase flow of the fully resolved reference cases is calculated solving the transport equation for the species and directly evaluating the reaction rates. Different flame structures are identified using the heat release rate and the chemical explosive mode as markers, showing that complex structures with a combination of lean premixed and non-premixed flames can be observed in strained counterflow coal flames. In addition to the fully resolved simulation, the suitability of the Flamelet-Progress Variable (FPV) model is investigated. Both premixed and non-premixed tables are employed. At first, the suitability of the look-up tables is evaluated by means of an a priori analysis, using the fully resolved simulations as reference solutions, showing that the non-premixed flamelet table correctly predicts the structure of the strained coal flames, while the premixed table shows sensible deviations in terms of temperature and species, especially at rich conditions. Finally, the a posteriori analysis shows that the fully coupled FPV model with a non-premixed flamelet look-up table can accurately predict strained coal flames.     

On the Nature of the “Low-Temperature Anomaly” in Metal–Semiconductor Schottky-Barrier Contacts

It is shown that the low-temperature anomaly of the I-V characteristics of homogeneous metal–semiconductor Schottky-barrier contacts (an increase in the ideality factor n and a decrease in the barrier height _bm, measured using the saturation current, with decreasing temperature, as well as the fact that their product _bn n_bm coincides very closely with the actual barrier height _b(V)) is a consequence of two factors: (1) the nonlinear dependence of the actual barrier height on the bias, resulting in an increase in the ideality factor with increasing bias voltage (current) and (2) measurement of the parameters n and _bm for the same current for all temperatures. A new expression for the flat-band barrier height _bf is derived.     

Diffraction of a plane wave in air by a cylindrical shell with two local masses

The diffraction of a plane wave by a cylindrical shell with two inertial masses attached symmetrically (at = ±₁) to its surface is investigated. The frequency response characteristics of the scattered field are calculated for a shell equipped with diametrically opposite masses (₁ = /2) and driven only by symmetric (about the plane =0, =) modes of vibration. It is established that the level of the scattered field in the interval of frequencies corresponding to even symmetric mode orders increases upon transition from the model with a single local mass (at =) to the model with two masses (at =±/2) (i.e., with a split inhomogeneity). At the frequencies of odd modes the scattered field coincides with the field of the homogeneous shell. The directivity patterns of the scattered field of a shell with two masses (at =±/2) are determined.Architecture and Construction Institute, Nizhegorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 3, pp. 289–299, March, 1994.     

Control and precise measurement of carrier-envelope phase dynamics

The evolution of the carrier-envelope offset phase _CEO of a 10-fs Ti:Sapphire laser has been traced on time scales from microseconds to seconds using various techniques. Precise locking of this phase has been achieved down to an rms deviation of 1/40 of an optical cycle. Stability measurements have been performed independently of the feedback loop, focusing on the phase jitter introduced by the feedback loop itself, the pump laser, and a prism compressor. It is shown that a multi-mode pump laser introduces more phase noise on _CEO than a single-mode pump laser. PACS 42.65.Re; 42.60.Mi; 42.62.Eh