Inertial effects in nonstationary models of flame front evolution

The objective of the present study is to formulate flame front evolution models capturing the effects of flame extinction, ignition, and oscillations in addition to the conventional regime of flame propagation. The basic equations constituting the one-dimensional thermodiffusion model of combustion are reduced to a system of two ordinary differential equations for the flame front coordinate and flame temperature. These equations admit solutions that describe, for example, ignition, extinction, and nonlinear oscillations of the flame, which are observed during premixed gas combustion in microchannels with an elevated wall temperature or during gasless combustion of condensed systems. Similarity of the basic thermodiffusion models assuming an in-finitely small thickness of the chemical reaction zone allows application of a universal method to derive reduced equations in physically different systems. It is demonstrated that modeling of flame oscillations requires at least considering effects associated with flame acceleration (flame front “inertia”) and the rate of flame temperature variation. The accuracy of the proposed model with inertial effects is checked by results of direct numerical simulations of the original equations.     

A Newly Designed and Constructed Instrument for Coupled Infrared Extinction and Size Distribution Measurements of Aerosols

Although atmospheric particles are often non-spherical, Mie theory is commonly used to acquire aerosol optical depth, composition, and transport information from satellite retrievals. In the infrared (IR) region, the radiative effects of aerosols, usually modeled with Mie theory, are subtracted from satellite spectral data to determine key atmospheric and oceanic properties. To gain a better understanding of the infrared radiative effects of aerosols and the methods used to model them, an instrument has been designed to simultaneously measure infrared extinction spectra and particle size distributions obtained from a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS). Infrared extinction spectra are simulated with Mie theory using the measured particle size distributions and available literature optical constants. As a result, the errors associated with using Mie theory to model the infrared extinction of mineral dust aerosol can be quantitatively examined. Initial results for this instrument are presented here. For ammonium sulfate, the Mie theory simulation is in good agreement with our measured extinction spectrum. This is in accordance with the nearly spherical shape of ammonium sulfate particles. However, for illite, an abundant clay mineral, there is poor agreement between the experimental spectrum and the Mie simulation. This result is attributed to particle shape effects.     

Specific extinction coefficient of flame generated smoke

The experimental results for the mass specific extinction coefficient (σ_s) at λ=633 nm for flame generated smoke are summarized for seven studies involving 29 fuels. The measurements are for post‐flame smoke generated by overventilated burning. From an analysis of variance for the seven studies, it was found that between‐laboratory differences were the major source of variability. The estimated mean value of σ_s is 8.7 m²/g with an expanded uncertainty (95% confidence interval) of 1.1 m²/g. A major implication of this nearly universal value is that one can infer mass concentration of smoke by making light extinction measurements. Published in 2000 by John Wiley & Sons Ltd.     

Unsteady extinction mechanism of nonpremixed flame interacting with a vortex

The extinction mechanism of a CH₄/N₂-air counterflow nonpremixed flame interacting with a single vortex was numerically studied. An augmented reduced mechanism was used to treat the CH₄ oxidation reactions. The contribution of each term in the energy and the OH species equations were evaluated to investigate the unsteady extinction mechanism of nonpremixed flame. The flame temperature began to decrease due to the convection heat loss when the flame interacted with a vortex. The investigation of the radical behavior during the flame-vortex interaction process also provided useful information on the unsteady extinction mechanism. The OH radical concentration could be used as a good tracer of the state of the unsteady extinction of nonpremixed flame. The reduction mechanism of OH concentration was confirmed by analyzing the contribution of each term in the OH species equation. At initial stage of flame-vortex interaction, the OH production and consumption rates increased gradually, while the OH concentration was kept nearly constant. Near the extinction limit, the OH production rate decreased rapidly due to the low flame temperature, and the balance between the OH production and OH consumption by diffusion could not be maintained. The unsteady nonpremixed flame interacting with a vortex under the conditions of regime (V) shown in the spectral combustion diagram [Thévenin, D., Renard, P.H., Fiechtner, G.J., Gord, J.R., Rolon, J.C., 2000. Regimes of non-premixed flame-vortex interactions. Proceedings of the Combustion Institute 28, 2101-2108.] was finally extinguished due to low reactivity, which was induced by the low flame temperature.     

二甲醚球形扩散火焰的振荡熄火动力学机理研究

采用详细的化学反应机理和组分输运模型，对二甲醚(DME)微重力球形扩散火焰在热焰与冷焰条件下的振荡熄火机理开展数值研究。结果表明，在微重力条件下可以建立稳定自持的冷焰，而且冷焰反应可以显著拓展熄火的可燃极限。在热焰与冷焰的稳态熄火点之前均观察到了振荡熄火过程。DME热焰的振荡熄火受单个振荡模式所控制，且振荡频率(约1 Hz)与环境氧含量无关。而冷焰的振荡熄灭受两个具有不同频率的双振荡模式所控制，在靠近熄火极限时高频振荡模式的振荡周期急剧增加。此外，高频与低频振荡模式之间存在着强烈耦合作用，导致冷焰的振荡熄火过程更加复杂。基于敏感性分析的结果表明，热焰的振荡熄火受小分子所参与的高温吸热/放热以及链分支/断裂反应之间的竞争关系所控制，而冷焰的振荡熄火受负温度系数机制下低温链分支与断裂反应之间的竞争关系所控制。     

Laminar flame speeds and extinction limits of conventional and alternative jet fuels

Experimental results of laminar flame speeds and extinction stretch rates for the conventional (Jet-A) and alternative (S-8) jet fuels are acquired and compared to the results from our earlier studies for neat hydrocarbon surrogate components, including n-decane and n-dodecane. Specifically, atmospheric pressure laminar flame speeds are measured using a counterflow twin-flame configuration for Jet-A/O₂/N₂ and S-8/O₂/N₂ mixtures at preheat temperatures of 400, 450, and 470 K and equivalence ratios ranging from 0.7 to 1.4. The flow field is recorded using digital particle image velocimetry. Linear extrapolation is then applied to determine the unstretched laminar flame speed. Experimental data for the extinction stretch rates of the nitrogen diluted jet fuel/oxidizer mixtures as a function of equivalence ratio are also obtained. In addition, the experimental data of Jet-A are compared to the computed values using a chemical kinetic mechanism for a kerosene surrogate reported in literature. A sensitivity analysis is further performed to identify the key reactions affecting the laminar flame speed and extinction stretch rate for this kerosene surrogate.     

基于化学爆炸模式分析方法的乙烯对冲扩散火焰熄火机理

重点探讨了化学爆炸模式分析(CEMA)在熄火机理研究中的具体方法理论及可行性,研究了化学反应/热质混合相互作用对熄火的影响,并利用爆炸因子和分叉因子的概念,确定出主导乙烯火焰熄火极限的关键反应动力学因素。结果表明：具有正特征值的CEM首次出现在熄火极限附近火焰化学当量等值面处,因此它可作为判断熄火的重要依据。火焰的熄火极限是放热与链分支、中断反应综合作用的结果,链分支反应R32(H+O₂ \stackrel{\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}}{?} O+OH)和放热反应R81(OH+CO \stackrel{\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}}{?} H+CO₂)对乙烯熄火极限的影响最显著,增大这两步反应速率能极大地扩宽燃烧稳定范围;而增大链中断反应R49(H+HCO \stackrel{\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}\mathrm{}}{?} H₂+CO)的速率会缩小燃烧稳定范围。基于CEMA方法与爆炸因子、分叉因子的概念,可系统地揭示详细反应动力学对熄火的影响机理。     

Effect of flame retardants on polyolefines

Phosphorus and Bromine flame retardants decrease the time of self-ignition delay and increase the mass rate of burning of polypropylene even though the limiting oxygen index (LOI) at ambient temperature is higher. The proposed model which combines ignition, steady burning and extinction shows that the phenomenon can be explained only by an increased rate of polymer volatilization caused by flame retardant.     

Ghosting flame mechanism under different extinctions modes in ceiling vent cabin

In this paper, large eddy simulation coupled with a low Mach number compressible thermal-drive flow model had been utilized to investigate the development of large-scale engine room fire, and the characteristics of engine room fire were analyzed through based upon the distinctive fire extinction behavior. Results indicated that extinction modes of engine room fire could be divided into oxygen control type and fuel control type. And the flame morphology could be divided into four stages: conical flame, pulsating flame, column flame, and ghosting flame. The appearance of the ghosting flame was affected by the extinction mode, but there were obvious differences in the causes of ghosting flame between the two modes. The cause of ghosting flame was related to the distribution of temperature field under the fuel control type, while the occurrence of ghosting flame in oxygen control type was mainly affected by oxygen concentration. Deflagration should be avoided when extinguishing fire by sealing the cabin.     

Combustion characteristics and flame stability at the microscale: a CFD study of premixed methane/air mixtures

A two-dimensional elliptic, computational fluid dynamics (CFD) model of a microburner is solved to study the effects of microburner dimensions, conductivity and thickness of wall materials, external heat losses, and operating conditions on combustion characteristics and flame stability. We have found that the wall conductivity and thickness are very important as they determine the upstream heat transfer, which is necessary for flame ignition and stability, and the material's integrity by controlling the existence of hot spots. Two modes of flame extinction occur: a spatially global type for large wall thermal conductivities and/or low flow velocities and blowout. It is shown that there exists a narrow range of flow velocities that permit sustained combustion within a microburner. Large transverse and axial gradients are observed even at these small scales under certain conditions. Periodic oscillations are observed near extinction in cases of high heat loss. Engineering maps that delineate flame stability, extinction, and blowout are constructed. Design recommendations are finally made.     

Solid flame combustion of cylindrical samples with stepwise varying diameter

Unsteady propagation of a gasless combustion wave in a cylindrical sample with a stepwise varying diameter was studied numerically. For the diameter of the smaller cylinder which ignites the larger cylinder, the critical values for combustion extinction were calculated as functions of the Zel’dovich and Arrhenius parameters. At a diameter larger than the critical value, the combustion wave entered a semibounded volume of the reaction medium. The critical diameter was shown to be a characteristic of the reaction medium.     

Stabilization of an inverted propane-air flame on a string stretched along the flow

The stabilization conditions for an inverted flame on a long thin string stretched along the cylindrical burner axis were studied experimentally. Gas temperature distributions in the inverted flame are obtained. The boundaries of stable combustion that are simultaneously the condition of excitation of acoustic self-oscillations of the flame are found. Inverted flame blow-off velocities are measured, and the variation of its geometrical characteristics are studied. It is found that during overturning of the flame relative to the vector of the acceleration due to gravity, stabilization of the inverted flame in the open atmosphere is impossible. The physical mechanism involved in the formation of the inverted flame in flow parallel to the stabilizer surface is considered. The role of the hydrodynamic stretching of the flame in flame blow-off and extinction is determined. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 2, pp. 3–11, March–April, 2008.     

Screening flame retardants for plastics using microscale combustion calorimetry

Microscale combustion calorimetry (MCC) was evaluated as a screening test for efficacy of flame-retardant additives in polymers. The MCC method separately reproduces the gas and condensed phase processes of flaming combustion in a nonflaming laboratory test and forces them to completion to obtain intrinsic/material combustion properties. At flame extinction, these MCC combustion properties are comparable in magnitude and effect to the extrinsic factors (sample size and orientation), physical behavior (dripping, swelling), and chemical processes (flame inhibition, charring) associated with flame retardancy. Consequently, MCC properties by themselves cannot correlate flame resistance of plastics over a broad range of flame-retardant chemical composition. POLYM. ENG. SCI., 47:1501–1510, 2007. © 2007 Society of Plastics Engineers     

Combustion of residual steel gases: laminar flame analysis and turbulent flamelet modeling

In recovery combustion systems operating in the steel industry, energy is provided by boilers burning residual gases of blast furnace and coke oven. To help understand combustion of this particular type of fuels, a numerical study is conducted where the major chemical properties of steel gas flames are collected. The chemical composition of representative fuel and oxidizer steel gas is varied over a large range in calculations using detailed chemistry and complex transport properties. The chemical equilibrium compositions, premixed flame speeds and diffusion flame extinction strain rates are determined. The advantages and shortcomings of the use of vitiated air emerge, and its introduction into the boiler appears as an interesting alternative to reduce NO_x emission. The detailed information obtained with laminar flame calculations is also introduced in flamelet turbulent combustion modeling. Reynolds Averaged Navier Stokes (RANS) simulations of a test case burner are performed and some comparisons between numerical predictions and experimental results are presented.     

Some basic aspects of flame resistance of polymeric materials

For a better insight into flame resistance and flame extinction it is useful to split the combustion process into its constituent elements. It then appears that the thermal decomposition is the first link in a series of reactions and therefore a better understanding of this decomposition is a prerequisite. Further, it is found that the amount of char and the amount of incombustible gases that may be formed in thermal decomposition are very important quantitative measures of flame resistance. For a large number of model substances the char residue upon pyrolysis has been determined. The residue is found to be very clearly related to the chemical structure of the polymer, so much so that the amount of char can be predicted from the structure. Finally, it has been demonstrated that there is a very significant relation between the pyrolysis residue (%) and the (limiting) oxygen index.     

Imidazolium-Based Stabilization of Aqueous Multiwall Carbon Nanotube Dispersions

Imidazolium bromide, an ionic liquid surfactant acrylate, as well as its homopolymer and various copolymers are demonstrated to be superior dispersing aids for preparing aqueous multiwall carbon nanotube (MWCNT) dispersions. We demonstrate apparent complete exfoliation in water with extinction coefficients at 500 nm of about 60 cm² mg⁻¹ of MWCNT dispersed, a new lower bound and the highest reported extinction to date. The efficacy or efficiency of dispersion (activated by ultrasonication) is examined in terms of a quotient of extinction and weight ratio of the active monomer and MWCNT. A rank ordering of the results obtained for seven stabilizers based on the same imidazolium bromide monomer provides insights into roles of π-overlap adsorption onto MWCNT surfaces and how hydrogel properties of some of these polymers provide stability in water at higher concentrations than previously considered feasible for surfactant-stabilized and polymer-stabilized MWCNT dispersions. Simple nanocomposite film formation is demonstrated by casting and thermal diffusivity and electrical conductivity properties are examined.     

Laminar flame theory. Part II

The laminar flame theory under the condition of minimum entropy production is presented. The equation of the temperature boundary layer is obtained for Le=0, and its general solution is found. It is shown that the mechanism of laminar flame propagation can be treated from the viewpoint of stationary thermal explosion in a cylindrical vessel of radius 2 as the critical regime in which the burnup is taken into account in an unusual form. Tomsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 5, pp. 22–30, September–October, 1993     

Lean catalytic combustion of alkanes at short contact times

The catalytic combustion of methane, ethane, propane, andn-butane in air over platinum coated ceramic foam monoliths at contact times ranging from 3 to 25 ms and catalyst temperatures between 800 and 1100°C has been examined for compositions leaner than the homogeneous flammability limits. The extinction limits for heterogeneous reaction for these fuels has been determined as a function of preheat temperature up to 700°C in order to simulate the performance of a catalytic combustor or incinerator with exhaust gas preheat. The effect of the superficial velocity on the extinction limits has also been examined. We observe that the extinction composition decreases with increasing preheat temperature and increases with increasing flow rate for all fuels, and the reactivity of the fuels increases with the length of the carbon chain. Methane can be made to react completely as lean as 2.3% in air, while ethane, propane, and butane can be made to react completely as lean as 0.6, 0.4 and 0.2% respectively. For butane we observe only a single steady state for preheat temperatures above 200°C and a bifurcation diagram showing the extinction behavior for this system was constructed.This research is partially supported by GRI under grant No. 5095-260-3404 and by NSF under grant No. CTS-9311295-02.     

The domain model for heterogeneous catalysis

A domain theory inspired by non-uniformity of catalytic surfaces is introduced and a rigorous mathematical formulation is presented. This new analysis allows the classification of hysteresis systems into two distinct classes with qualitatively different behaviors for the first time. Heretofore unexplainable behaviors such as multistage extinction process, “memory” effects during induction period and discontinuous hysteresis can now be accounted for by the theory. Predicted hysteresis behaviors in an oscillating scan experiment, which has never been carried out, are also delineated by the theory. The classifications, verifications and predictions facilitated by the present formulation enhance the understanding of underlying system nonlinearities which are the origins of hysteresis as well as oscillatory and aperiodic dynamic behaviors.     

Dual-cavity spectrometer for monitoring broadband light extinction by atmospheric aerosols

Abstract

Atmospheric Aerosols affect Earth’s climate directly by scattering and absorbing solar radiation. In order to study the optical properties of aerosols, we developed a broadband cavity-enhanced spectrometer that uses a supercontinuum laser source and a compact spectrometer, to measure simultaneously the extinction coefficient of aerosols over a broad wavelength region from 420 to 540?nm. The system employs a dual cavity approach with a reference and a sample cavity, accounting for changes in gases background and for laser spectral and intensity fluctuations. We tested the system with aerosolized salt particles and polystyrene latex spheres. We performed calculations using Mie theory and found good agreement with the measured extinction. We also found that the extinction coefficient of non-absorbing aerosol favorably compares with the scattering coefficient measured by a nephelometer. Finally, we generated soot particles and found an extinction Ångström exponent in good agreement with values reported in the literature. Wavelength dependent detection limits (1σ) for the instrument at 5?nm wavelength resolution and for an integration time of ～10?min were found to be in the range ～5?Mm^?1 to 13?Mm^?1. The broadband dual-cavity extinction spectrometer is simple and robust and might be particularly useful for laboratory measurements of the extinction coefficient of brown carbon aerosol. The laboratory tests suggest that the prototype is promising for future developments of a field-deployable instrument.

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