Hydrothermal flames: From phenomenological experimental demonstrations to quantitative understanding

Investigations of hydrothermal flames were reviewed with three primary objectives: (1) to classify and describe experiments performed to date, (2) to assess the state of the art of experimental instruments and measurement techniques, and (3) to characterize current understanding and outline future research needs of hydrothermal flame processes. For this review, hydrothermal flames are defined as rapid oxidation reactions occurring in a supercritical aqueous environment at sufficient temperature (∼1000 °C and higher) and rate to produce a luminous flame. Under these conditions, chemical kinetics are very fast, with reactions going to completion within 10–100 ms. Hydrothermal flame experiments were divided into two categories: laminar inverse diffusion flames in semi-batch reactor systems and turbulent diffusion flames in continuous reactor systems. Laminar diffusion flame studies have demonstrated the range of temperatures, pressures, and fuel and oxidant types and concentrations from which hydrothermal flames can be produced, while turbulent diffusion flame studies have focused on the application of hydrothermal flames to the remediation of aqueous waste streams. Experimental apparatuses, instrumentation, and techniques used in hydrothermal flame investigations are presented in some detail. Models developed to describe hydrothermal flame processes, especially flow fields in turbulent diffusion flame studies, are presented as well.     

A visual study on flame behavior in tone-excited non-premixed jet flames

A visualization study on the effect of forcing amplitude in tone-excited jet diffusion flames has been conducted. Visualization techniques are employed using optical schemes, which are a light scattering photography. Flame stability curve is attained according to Reynolds number and forcing amplitude at a fuel tube resonant frequency. Flame behavior is globally grouped into two from attached flame to blown-out flame according to forcing amplitude; one sticks the tradition flame behavior which has been observed in general jet diffusion flames and the other shows a variety of flame modes such as the flame of a feeble forcing amplitude where traditionally well-organized vortex motion evolves, a fat flame, an elongated flame, and an in-burning flame. Particular attention is focused on an elongation flame, which is associated with a turnabout phenomenon of vortex motion, and on a reversal of the direction of vortex roll-up. It is found that the flame length with forcing amplitude is the direct outcome of the evolution process of the formed inner flow structure. Especially the negative part of the acoustic cycle under the influence of a strong positive pressure gradient causes the shapes of the fuel stem and fuel branch part and even the direction of vortex roll-up to dramatically change.     

Growth characteristics of polycyclic aromatic hydrocarbons in dimethyl ether diffusion flame

The growth characteristics of polycyclic aromatic hydrocarbons (PAHs) in laminar dimethyl ether (DME) diffusion flame were investigated experimentally, and we assumed that the growth of PAHs within the flame was predominantly due to methyl addition/cyclization (MAC) mechanism. Methane and propane laminar diffusion flames were also investigated for comparison, and their PAHs growth characteristics had been explained by reactions concerning acetylene and propargyl radical. Laser-induced fluorescence (LIF) and laser-induced incandescence (LII) techniques were used to measure the relative concentration of soot and PAHs, respectively. Two-dimensional images of the OH-LIF, PAHs-LIF, and LII from soot were measured in the test flames. Furthermore, to investigate the growth characteristics of the PAHs in the flames, the fluorescence spectra of the PAHs were measured at several heights in the flames, using a spectrograph. The molecular size of the PAHs was estimated based on an emission wavelength region of the PAHs-LIF that varied along with the PAH size. The results show that although the PAHs were widely distributed within the unburned region similar to that of the methane and propane flames, the intensity and detection region of LII were much smaller than that of the methane and propane flames. The PAHs-LIF spectra indicated that the growth of the PAHs within the DME flame was much slower than the methane and propane flames, and thus a large number of small PAHs were discharged into the OH region distributed around the outer edge of the flame.     

Experimental study on self-excitations in nitrogen-diluted laminar lifted butane flames

Laminar lifted butane flames diluted with nitrogen have been investigated experimentally to determine distinctive self-excitation regimes in the flame stability maps and also to elucidate the individual self-excitation characteristics. Self-excitations of lift-off height are classified into five regimes in laminar free-jet lift-off butane flames diluted with nitrogen: a stationary lifted regime (regime I), a heat-loss-induced self-excitation (regime II), a buoyancy-induced self-excitation due to flame flicker as well as a heat-loss-induced self-excitation (III), a combined form of an oscillation prior to blow-out and a heat-loss-induced oscillation (regime IV), and a combined form of an buoyancy-induced self-excitation and a heat-loss-induced oscillation as well as an additional buoyancy-driven self-excitation due to flame flicker (regime V). Extremely low-frequency (<0.1 Hz) self-excitation is caused by conductive heat loss from the premixed wings to the trailing diffusion flame and can be explained by a proposed mechanism. It is also found that the flame oscillation prior to flame blow-out is also caused by buoyancy and also significantly affected by the conductive heat loss from the premixed wings to the trailing diffusion flame, thereby showing that the frequency with nozzle exit velocity increases in the triple-flame propagation mode and then decreases in the flame-front propagation mode. Characterization of the individual self-excitation mode is presented and also discussed with Strouhal numbers and its relevant parameters through the analysis of power spectrum for temporal variation of lift-off height.     

平板狭缝间C1~C4烷烃/空气预混射流火焰的燃烧特性

对平行平板狭缝间C₁~C₄烷烃预混射流火焰进行了实验研究,考察了壁面温度、狭缝间距、当量比、燃料种类等对火焰形态和稳定性的影响,并利用高速相机获得了狭缝间的火焰图像。结果表明,随着狭缝间距的减小,火焰经历了稳定、脉动和熄火3个阶段。其中,火焰脉动发生在大于熄火间距的狭小范围内,其脉动频率随着壁面温度的升高而增加。对同一种燃料,当预混气当量比和壁面温度保持恒定时,火焰的脉动频率在脉动发生的区域内保持不变。对比C₁~C₄烷烃预混火焰的脉动频率及脉动火焰持续距离范围,发现甲烷预混火焰的均最小,而其他3种燃料则比较接近。     

Combustion of Ytterbium Metal

Ytterbium metal powder burns with a luminous vapor phase diffusion flame in oxygen. Consolidated stoichiometric mixtures with both polytetrafluoroethylene and hexachloroethane upon ignition yield intense luminous flames. With UV/Vis spectroscopy of the combustion flames Yb, YbO, YbCl, and YbF species have been identified contributing to the selective emission in the green spectral range. The flame temperatures of ytterbium combustion in oxygen and with halocarbon based oxidizers are in the same range as for comparable magnesium based systems.     

Numerical calculation of the quenching of stretched laminar flames in H2 + and CH4 + air mixtures

We have calculated numerically the equations of hydrodynamics and heat and mass transfer, describing the structure of normal and stretched flames, with consideration given to the detailed kinetics of the chemical conversion, multicomponent diffusion, and thermal diffusion. We have found the limit stretching intensities of double and single hydrogen-and methane-air flames, determining flame separation over a broad range of variations in the original mixture. We have calculated the velocities of the corresponding normal flames and we have determined the possibilities of simplifying the original model of molecular transfer.Chernogolovka. Translated from Fizika Goreniya i Vzryva, No. 2, pp. 58–62, March–April, 1991.     

Deposit morphology on SiC fibers in methane-acetylene/air laminar diffusion flames

The morphologies of soot deposit on 15 μm diameter silicon carbide (SiC) fibers have been investigated with a scanning electron microscope (SEM) in methane-acetylene/air laminar diffusion flames with co-flowing air. The morphologies are shown to be strongly dependent on the fuels ratio. Two kinds of processes by which mature soot particles are produced were proved to exist in a sooting flame: one is the transition from the condensed-phase deposits; the other is the aggregation of the smaller soot particles (or chains of them) carried along the particle path line. Different transition processes are compared between the present work and previous work done by other researchers that used propane/air laminar diffusion flames. It seems the presence of C=C in methane-acetylene laminar diffusion flames is the key factor that causes the difference of transition processes in those two kinds of flames.     

甲烷/富氧射流扩散火焰NOx的排放特性

对甲烷/富氧同轴射流扩散火焰燃烧条件下氧化剂流速对NOx排放的影响进行了实验研究. 通过对火焰径向温度分布、火焰形态以及喷嘴出口附近扩散燃烧的流场的观测,分析了不同条件下NOx的生成特性. 结果显示,在保持氧化剂流量不变的条件下,NOx排放指数EINOx随氧化剂流速的增加而减小,在保持氧浓度及过量空气系数不变的条件下,小火焰有利于保持较低的EINOx.     

On the Nature of Superadiabatic Temperatures in Premixed Rich Hydrocarbon Flames

The propagation of flat laminar flames is studied numerically. It is shown that in rich homogeneous propane–air and methane–air mixtures, the maximum flame temperature exceeds the flame temperature in thermodynamic equilibrium. The degree of superadiabaticity depends on the concentration of the fuel mixed with air. It is shown that the superadiabaticity of rich flames is due to the diffusion of hydrogen from the reaction zone to the preheating zone and its preferential (compared to hydrocarbon) oxidation. The total enthalpy of the gas varies along the coordinate at the flame front and has a maximum.     

Combustion synthesis of fullerenes and fullerenic nanostructures

Samples of condensable material collected from low-pressure premixed and diffusion benzene/oxygen/argon flames were analyzed chemically to determine fullerene yield and by high-resolution transmission electron microscopy to characterize the fullerenic material on and within the soot particles. Results show that fullerene formation is sensitive to changes in operating conditions, such as fuel/oxygen ratio, chamber pressure, and inert gas dilution, and that the formation of amorphous and fullerenic carbon occurs early in the flame, with the structures becoming more curved with greater residence time. All flames exhibit a fullerene maximum with the premixed flame showing two distinct regions of formation. Additionally, the fullerene maximum in the diffusion flames is always just above the stoichiometric flame surface and a maximum is observed with increasing dilution due to competing dilution effects. Image analysis data show that the curvatures and diameters of the structures are consistent with the chemical analysis and that nanostructures, found at greater residence times than fullerenes, are formed directly from curved structures in the soot. These data complement previous fullerene studies and shed light on several proposed mechanisms for fullerene formation in combustion.     

二氧化碳和富氧空气对甲烷与乙烯燃烧的影响

利用碳氢燃烧实验台,研究了二氧化碳和富氧空气对甲烷、乙烯层流燃烧火焰的特性影响,分析了不同气氛下火焰结构特性和温度分布规律. 结果表明,随氧浓度从21%增加到50%(j),甲烷、乙烯火焰高度下降70%,火焰温度和亮度同时增加,且发光区域呈向下收缩趋势;相同氧浓度下,乙烯火焰亮度高于甲烷. 随CO2浓度由0增加到20%(j),火焰高度增加28%,各高度处火焰边缘温度平均下降290℃,中心温度平均下降132℃. 火焰亮度降低,由黄色变为暗黄色,底部亮度更低,CO2浓度超过20%(j)后,火焰出现悬浮状态,最终被吹熄.     

Conversion of fuels containing nitrogen to oxides of nitrogen in hydrogen and methane flames

Nitrogen-containing compounds such as hydrogen cyanide, acetonitrile, acrylonitrile, pyridine, benzonitrile, ammonia and methylamine, which are typical of the products likely to be encountered during the decomposition of nitrogen-containing polymers in fires, have been introduced into hydrogen and methane flames burning in oxygen-argon atmospheres. There is a complete conversion of fuel nitrogen in all cases to oxides of nitrogen and molecular nitrogen. The relative conversion to oxides of nitrogen (as NO_x/N₂) increases as the injection rate of nitrogen-containing fuels is decreased. The relative yields of oxides of nitrogen tend to be similar with methane and hydrogen premixed flames and markedly greater than observed with hydrogen diffusion flame. In all cases the yield of oxides of nitrogen-containing products such as hydrogen cyanide can also present a toxic risk during the burning of nitrogen-containing polymers, particularly when high temperature are involved. The combustion of these products in flame zones cannot be assumed to alleviate the additional toxic risk because of their conversion to oxides of nitrogen.     

Calculation of the effective height of a diffusion flame

Experimental studies of coaxial burner flames make it possible to decide upon a Burke — Schumann model for short flames. Qualitative and scale conformity of diffusion flame shape is obtained for this solution. A satisfactory approximation of a flame by a plane model is given which considers changes in diffusion conditions in the gas phase and the mass combustion rate, and changes in the local ratios of the main ingredients of mixed solid fuels and their stoichiometric ratio.Polytechnic Institute, 610601 Kirov. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 3, pp. 35–41, May–June, 1994.     

Hydrocarbon Nanoparticles Formed in Flames and Diesel Engines

The formation of nanoparticles in laboratory hydrocarbon flames is reviewed in terms of particle morphology, chemical composition, and health hazards. The nascent nanoparticles in the nucleation mode have been widely reported in diverse laboratory flames, and are distinguished by their occurrence as singlet particles that form translucent images in transmission electron microscopy (TEM). Their sizes range from about 10 nm or more down to 2 to 3 nm, the limit of resolution of the TEM, and they possess a liquid-like quality. These particles are widely considered to be the precursor stage to the more readily observed carbonaceous aggregates consisting of chained primary particles that are opaque to the electron beam of the TEM. Nanoparticles sampled from the inverse diffusion flame and the particle effluent from diesel engines show a strong resemblance by GCMS analysis, and they contain many of the stabilomer PAHs and their isomers in the 200 to 302 atomic mass range. Many of these chemical species have high relative mutagenicities. Distinctive bimodal particle size distributions can be observed in both flame and engine samples. Recent TEM micrographs of diesel particulates show images of precursor-like nanoparticles, of as yet unknown chemical composition, that are formed in a diesel engine at many operating conditions.     

火焰层析成像的最新进展(英文)

To reduce greenhouse gas emissions from fossil fuel fired power plants,a range of new combustion technologies are being developed or refined,including oxy-fuel combustion,co-firing biomass with coal and fluidized bed combustion.Flame characteristics under such combustion conditions are expected to be different from those in normal air fired combustion processes.Quantified flame characteristics such as temperature distribution,oscillation frequency,and ignition volume play an important part in the optimized design and operation of the environmentally friendly power generation systems.However,it is challenging to obtain such flame characteristics particularly through a three-dimensional and non-intrusive means.Various tomography methods have been proposed to visualize and characterize flames,including passive optical tomography,laser based tomography,and electrical tomography.This paper identifies the challenges in flame tomography and reviews existing techniques for the quantitative characterization of flames.Future trends in flame tomography for industrial applications are discussed.     

A Review of Research and an Experimental Study on the Pulsation of Buoyant Diffusion Flames and Pool Fires

This paper reviews the past research, experimental techniques and scaling relationships used in the studies of oscillatory buoyant diffusion flames and reports an experimental investigation conducted to determine the pulsating characteristics of such flames. The experimental data were obtained by using three techniques, namely, pressure fluctuation measurements, thermal imaging and high-speed video photography. Present findings are compared with data sets reported in the literature and correlations for pulsation frequency suggested by previous studies are independently verified. Analysis of the experimental data on frequency of pulsations in different burners shows that for a fixed-diameter flame the pulsation frequency is almost independent of fuel flow rate. The equation f=1.68D^−0.5 gives the best approximation for the relationship between pulsating frequency and diameter over a wide range of data. An alternative way of expressing the relationship between the key variables is St=0.52*(1/Fr)^0.505. This proves to be a better way of expressing the relationship since it can include the effect of the fuel flow rate. Slight modifications to this expression allows prediction of flame oscillations under elevated/reduced gravity and isothermal buoyant plumes. This relationship and the observations of the present study confirm the hydrodynamic nature of flame puffing: interplay of buoyancy and fluid motion. © 1996 by John Wiley & Sons, Ltd.     

Stability of Refractories in Hydrogen-Fluorine Flames

The flame resistance of six refractories of commercial purity was measured in the well-mixed stoichiometric portion and in the unmixed fluorine-rich portion of a hydrogen-fluorine diffusion flame. Graphite, silicon carbide, zircon, alumina, zirconia, and magnesia are decreasingly resistant to the test flame in the order indicated. The rate of ablation is about three times more rapid and the average surface temperatures are 130° to 745° C. higher where the flame is fluorine-rich. Graphite and silicon carbide are suggested for use in HF flames because of their good thermal-shock resistance and relative chemical inertness to the flame. Consideration of the ablation chemistry indicates that zircon dissociates and then, like graphite, silicon carbide, and zirconia, ablates by flame-solid reaction, whereas alumina and magnesia form liquids which flow from the impingement area.     

Pulverized coal flame structures at elevated pressures. Part 1. Detailed operating conditions

The tests and simulations in this study characterize the chemical structure of pressurized pulverized coal flames, particularly (1) how the O₂ in simulated near-burner flame zones is apportioned among the various fuel components; and (2) the burner operating conditions and mechanisms that most strongly affect flame structure. CFD simulations resolved the structures of flames of a subbituminous and two hv bituminous coals for stoichiometric ratios (SR) from 0 to 1.8 for pressures from 1.0 to 3.0 MPa. The structures of all flames were largely determined by the accumulation of particles in the turbulent boundary layer on the flow tube wall. Gaseous fuel compounds always ignited first on the wall at the burner inlet, and this flame propagated toward the flow axis to form a 2D parabolic flame surface. Within the core, residual gaseous fuels, soot, and char may have eventually reached their ignition threshold and burned in a premixed mode. Residual CO, H₂, and char burned in the near-wall region after the volatiles flame had propagated deeper into the core.Whether or not the flame closed on the centerline was mainly determined by pressure and SR. Inlet conditions that formed closed flames at a lower test pressure eventually sustained open flames at progressively higher pressures. The impact of decreasing SR was qualitatively similar, due to the lower heat release rates for progressively lower SR. As the pressure is increased, flame ignition and, by association, flame stability will become more problematic due to the greater thermal capacitance of air streams at progressively higher pressures.