Self-sustained Flame Research Articles

A CFD study of propane/air microflame stability

A two-dimensional elliptic computational fluid dynamics model of a microburner is solved to study the effects of microburner wall conductivity, external heat losses, burner dimensions, and operating conditions on combustion characteristics and the steady-state, self-sustained flame stability of propane/air mixtures. Large gradients are observed, despite the small scales of the microburners. It is found that the wall thermal conductivity is vital in determining the flame stability of the system, as the walls are responsible for the majority of the upstream heat transfer as well as the external heat losses. Furthermore, there exists a range of flow velocities that allow stabilized combustion in microburners. It is found that the microburner dimensions strongly affect thermal stability. Engineering maps denoting flame stability are constructed and design recommendations are made. Finally, comparisons with methane/air systems are made.

Application of laser ignition on laminar flame front investigation

The first stages of laser-induced spark ignition were investigated as a function of time. Experiments were conducted using a premixed laminar CH4/air burner. Laser-induced breakdown was achieved by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. An anti-reflection coated lens with a focal length of 100 mm was used. The results obtained from an intensified high-speed and PIV CCD camera and a Cassegrain optics system coupled to an ICCD spectrometer provided information about the formation of laser-induced plasma and its transition to a flame kernel and a self-sustaining flame. The localization of the kernel and its time development were reproducible. Two types of flame fronts develop: one that expands against the flow direction, and one that moves with the flow. The initial flame expansion along the laser axis is asymmetric because of the shape of the plasma, different ionization levels inside the plasma, and the shock-wave expansion. Development of the fast flame occurs behind the shock wave induced by the plasma. This is important when laser ignition is used as a flame holder. An ICCD spectrometer coupled to an optical fiber permitted chemiluminescence visualization. The spectrum obtained during the plasma and flame kernel formation defined different stages in flame formation. The results obtained with these two optical techniques were synchronized to obtain the temporal resolution of the flame kernel evolution. Laser-induced ignition of a very lean mixture can be controlled to provide local heat release and extinction in a flame.

입자상 활성탄의 흡착과 재생에 의한 PCBs 오염제거

활성탄에 흡착되어 있는 유기질을 처리하기 위하여 역류산화반응이 개발되었고 그것에 대한 평가가 이루어 졌다. 역류산화반응이란 산소의 흐름과 반대로 이동하는 자발적인 불꽃을 이용하여 유기질을 파괴 및 제거함과 동시에 폐활성탄을 재생하는 방법이다. 본 연구를 통하여 얻어진 실험결과를 고찰해 보면, 활성탄의 질량손실과 불꽃의 온도가 산소의 흐름속도에 절대적으로 의존하였고, 재생탄의 비표면적이 거의 완전히 회복되었으며, 특히 활성탄에 흡착되어 있는 열에 안정한 PCBs를 거의 완전히 파괴 및 제거(99.99%이상)할 수 있었다. Counterflow oxidation was developed and evaluated for treatment of organics that adsorbed in/on granular activated carbon(GAC). This reaction is a method that destructs and removes organics adsorbed, at the same time, regenerates waste carbon, uti lizing a self-sustained flame which propagates itself ina direction counter to the oxygen flow. The results showed that the mass loss of carbon and flam temperature were strongly dependent on the flow rate of oxyen, adsorptive capability of regenerated carbon completely was recovered, as well as destrution and removal efficiency of thermally stable PCBs was achieved with the value of better than 99.99%.

Steam boiler parameters and nitrogen oxide emissions during three-stage coal combustion

This paper describes the influence of burner operating conditions, burner geometry and fuel parameters on the formation of nitrogen oxide during combustion of pulverized coal. Main attention has been paid to combustion test facilities with self-sustaining flames, while extensions have been made to full-scale boilers and furnace modeling. Since coal combustion and flame aerodynamics have been reviewed earlier, these phenomena are only treated briefly.

TIME-DEPENDENT MODEL OF A BUOYANT DOWNWARD FLAME SPREAD OVER A THERMALLY THIN SOLID FUEL

A time-dependent model was developed and solved numerically to study a purely buoyant downward flame spread over a thermally thin solid fuel in various gravity environments. According to the specified burn-out solid density and no retained ash, the flame propagation behavior over a thin solid fuel surface could be simulated. At ignition, the flame is premixed. After several transition burnouts the flame transitions into a self-sustained steady spread diffusion flame. When the gravity level was varied, the Damkohler number effects were verified. An unstable flame spread was noted near the extinction limit at which the flame spread rate decelerated. Blow-off extinction was predicted after the flame spread a short distance. The ignition delay time increased with increasing gravity level. Compared with the experimental measurements, the predicted blow-off extinction limit was closer than that predicted by the steady combustion model.

98/00500 Low-NO x-emission system for boilers in the Jaworzno Power Plant

This paper describes the influence of burner operating conditions, burner geometry and fuel parameters on the formation of nitrogen oxide during combustion of pulverized coal. Main attention has been paid to combustion test facilities with self-sustaining flames, while extensions have been made to full-scale boilers and furnace modeling. Since coal combustion and flame aerodynamics have been reviewed earlier, these phenomena are only treated briefly.

97/04902 Secondary effects in sampling ammonia during measurements in a circulating fluidised-bed combustor: Kassman, H. et al. J. Institute of Energy, September 1997, 70, 95–101

Ammonia often occurs in combustion gases as a product of fuel-nitrogen. Since either NO or N2 may predominate as the product of ammonia oxidation, there is considerable interest in understanding the factors responsible for selecting the final products. In flames, the selectivity is known to depend on whether the reaction zones are premixed or nonpremixed. This paper reports on a combined experimental and modeling investigation of ammonia chemistry in a hot combustion environment that is below flame temperatures, such as in post-combustion gases. Experiments that mix highly diluted ammonia–methane and oxygen–water streams are interpreted in terms of a plug-flow model, a simplified mixing reactor model, and a two-dimensional direct numerical simulation. The study finds that the final products of ammonia oxidation remain sensitive to mixing even at temperatures below those of self-sustaining flames. At low temperatures, ammonia oxidation occurs in a premixed reaction zone, but at sufficiently high temperatures a nonpremixed reaction zone may develop that produces significantly less NO than the equivalent premixed system. A direct numerical simulation is required to predict the behavior over the full range of conditions investigated experimentally, while a simplified mixing reactor model captures the essential features as long as the radial gradients are not too steep.

Metal-sulfur combustion

A number of metal-sulfur compounds have been synthesized in the present investigation via self-propagating, high-temperature synthesis (SHS) process. Equilibrium calculations indicate that a large number of metals react with sulfur energetically, yielding adiabatic flame temperatures sufficiently high to support self-sustained flame propagation. To permit the combustion process to proceed entirely in the condensed phase, the pressure of the environment has to be kept at elevated levels (1–20 M Pa) for most of the metal-sulfur mixtures studied. Computed adiabatic flame temperatures of some of the metal-sulfur reactions exceed 5000 K (Ca−S, Sr−S, Ba−S). The present experimental investigation was carried out to verify the equilibrium calculations. Seven metals have been studied (i.e., Mg, Si, Ti, Mn, Cr, n, and Mo). Samples are prepared by melting the metal-sulfur precursor mixture, which eliminates any trapped gas so that the density of the sample approaches the theoretical value. X-ray diffraction analysis of the products shows that the synthesized samples have a high purity and that the reactions are close to completion. Ignition temperatures for the metal-sulfur mixtures were found to be in the range of 350–600 °C. Flame speeds measured for several of the metal-sulfur mixtures range from 5 to 10 mm/s. The large-energy-release, low-ignition temperatures and high adiabatic flame temperatures associated with metal-sulfur combustion make this process also promissing for a variety of “energetic” applications.

Combustion tube studies of dust flame acceleration

A new pneumatic dispersion system for obtaining a good quality uniform dust suspension in a horizontal dust combustion tube was developed. The effect of three different dispersion techniques on self-sustained dust flame acceleration in such a combustion tube was examined. The importance of the dispersion quality in the test tube for maintaining a self-sustained dust flame acceleration was demonstrated. A combustion tube for studies of flame acceleration in fine aluminum dust-air mixture and its transition to detonation under industrial ignition conditions was constructed in the course of the present study. It consists mainly of an initiation section and a test section. The initiation section must be equipped in a well-developed dispersion system for creating a good dispersion condition in the test tube. The length of this section is 3 meters. The test tube requires only to distribute uniformly the dust over the bottom of the tube prior to the experiment. The aluminum dust spherical in shape with 6 μm in diameter was used for tests. Experimental results demonstrated that the increase in flame velocity is roughly linear through the entire length of the test tube. The highest flame propagation velocity in fine aluminum dust-air mixture approaches some 1200m/s at a distance of 4.8m from the ignition plane.

Numerical modeling of spark ignition and flame initiation in a quiescent methane-air mixture

The initiation of a spark kernel and the subsequent propagation of a self-sustained flame in an internal combustion engine have been investigated numerically. A theoretical model which employs a two-dimensional cylindrical coordinate system and assumes axial symmetry has been developed. It considers the various physical and chemical phenomena associated with the ignition process and employs a detailed chemical reaction scheme for a methane-air mixture which contains 29 chemical species and 97 reaction steps. The thermodynamics and transport properties of the plasma at high temperatures are evaluated by a statistical thermodynamics approach, while assuming local thermodynamic equilibrium. Using the PHOENICS and the CHEMKIN codes, the appropriate conservation equations are solved in the domain of solution. It was concluded that the kernel growth can be described as a two-step process. In the early short stage (1–5 μs) the mass and energy transfer processes are very much dominated by the pressure wave and the violently expanding plasma kernel, while the contribution of the chemical reactions is negligible. This stage is followed by a much longer period in which diffusion and thermal conduction control mass and energy transfer as the flame becomes gradually self-sustained. Owing to the heat release by chemical reactions, the expansion of the combustible mixture is accelerated at the beginning of the diffusive stage.

Polymers and Composites- An Examination of Fire Spread and Generation of Heat and Fire Products

Fiber reinforced composite (FRC) materials are used extensively because of their favorable physico-chemical properties and high strength- to-weight ratio. The use of composites in Army vehicles as a means of decreas ing weight and enhancing survivability, without reducing personnel safety, has been under study for some time. Although FRC materials are very attractive in terms of their physico-chemical properties, concern for possible fire hazard is understandable as organic polymers are one of the major constituents of the materials. A joint study thus was undertaken by the U.S. Army Materials Tech nology Laboratory (MTL) and the Factory Mutual Research Corporation (FMRC) to quantify flammability behavior of selected composite materials for the assessment of fire hazard. In the study, eight FRC materials, identified as MTL #1 to #8, were used. The FRC materials were 3 to 45 mm in thickness. The flammability behavior was examined by using the FMRC Flammability Apparatus (50 kW-Scale) and Ox ygen Index (OI) apparatus, Thermogravimetric Analysis (TGA) instrument, NBS smoke chamber (ASTM E 662), and Gas Chromatograph-Mass Spec trometer (GC-MS) instrument at MTL. This article presents results for ignition, flame spread, heat release rate, gen eration rates of products, light obscuration by smoke and flame extinction by Halon. In comparison to ordinary combustibles, such as cellulosics and most non fire retarded plastics, the eight FRC materials have higher resistance to ig nition (as indicated by the Thermal Response Parameter, TRP) and flame spread (as indicated by higher values of the Fire Propagation Index, FPI). The FPI values for the FRC materials, examined in this study, ranged from 3 to 13, indicating that for Group 1 FRC materials (FPI < 10), self-sustained flame spread beyond the ignition zone would be difficult, whereas for other Group 2 materials ( FPI ≥ 10), flame spread beyond the ignition zone would be ex pected, although at a slower rate. For Group 2 materials fire protection is re quired, which could be provided by techniques such as surface coating, surface lamination using highly fire resistant FRC materials, and others. Generation of heat, smoke, toxic and corrosive products is closely related to FPI. Within the FRC materials, examined in this study, differences were found between the generation rates of heat, smoke, and other products. Results for flame extinction by Halon 1301 are also discussed. The flame ex tinction data are consistent with the design of the current suppression system for the crew compartment of Army combat vehicles. The study suggests that the FPI concept and associated parameters related to generation of heat, smoke, toxic, and corrosive products, is a useful concept for realistic flammability quantification and screening of FRC materials and for use in the hazard assessment. This, however, needs to be validated by perform ing large-scale fire tests.

Flame configuration associated with localized energy addition to a flowing combustible mixture

An approximate theoretical analysis is used to consider the configuration of a laminar axisymmetric flame in a combustible gaseous mixture, subject to large-activation-energy kinetics and flowing at a velocity in excess of the normal flame velocity (i.e., flowing supercritically). Attention is centered on nonintrmive energy addition from a line or point source by use of a laser. For a continuous-ignition point source, the flame appears as a semi-finite bluff body, with its nose directed upwind. Away from the axis of symmetry, the flame approaches the conical shape that is observed when an adiabatic flame is stabilized in supercritical flows by different types of localized intrusive flameholders. In general, the temperature varies along the flame locus; the temperature is a maximum at the point of the locus that is on the axis of symmetry, and the temperature approaches the adiabatic flame temperature at large lateral distance from the axis. The corresponding, planarsymmetric flame locus that arises from a line source in a supercritically flowing mixture is also analyzed. The solutions presented here are based on very simplified dynamics and hold only for negligible chemical exothermicity and negligible energy addition by the ignition source; they are intended only as starting points for needed further investigations. In concluding remarks, qualitative expectations for a long train of intermittent or periodic, but rapidly repeated, pulses are discussed briefly. For flame stabilization, such an energy source need only to initiate a train of downwind-conv ected flame kernels, which will result in a self-sustaining flame. Clearly, the continuous source is highly energy demanding relative to the intermittent source, which therefore seems of greater practical interest; the continuous source, however, would be of laboratory interest.

Upward Fire Spread: Key Flammability Properties, Similarity Solutions And Flammability Indices

Measurement methods and their interpretation are discussed for determining key flammability properties of charring and non-charring materials that are required as an input to a comprehensive Upward Flame Spread and Growth computer model. In addition, desirable similarity situations for upward flame spread are outlined which provide simple analytic solutions for upward flame spread rates. These solutions can be used to provide insight and illustrate how the material properties affect upward flame spread. Among other material parameters (critical heat flux, ignition parameter, xR, xA), an important parameter is the material flammability number, MFN = (xR+ xC). ?Hc /?Hv, (xR radiant fraction and xC convective fraction of the heat release rage applies back to the surface, ?HC, = heat of combustion, ?Hv = effective heat of gasification), which affects monotonically the ratio of flame height to pyrolysis length. For low values of this parameter (so that Zf/Zp < 1), self-sustained flame spread will even stop (except if the material is heated uniformly throughout up to the pyrolysis temperature by an external heat flux).

Nonadiabatic propagation of a planar premixed flame Constant-volume enclosure

The unsteady one-dimensional laminar nonisobaric propagation of a flame through a gaseous fuel-lean combustible premixture, characterized by large Arrhenius activation temperature, is examined under ShvabZeldovich-type formulation, via numerical integration by the method of lines with spline interpolation. The premixture is confined between plane parallel impervious noncatalytic isothermal walls, although adiabatic walls are also considered. The straightforward extension of the Shvab-Zeldovich approximation adopted is suppression of acoustic phenomena, nonessential for present purposes, by taking the pressure to be temporally variant, but spatially uniform. The relative importance of three phenomena not present in isobaric flame propagation (the variation of the diffusion coefficient with pressure, the change of the reaction rate with pressure, and the explicit temporal-pressure-derivative contribution in the energy equation) is studied. Initiation of combustion is effected by two models of ignition: 1) simulation of spark ignition (what minimum enthalpy-augmentation of a given duration at one wall leads to self-sustained propagation of flame across the charge), and 2) simulation of hot-kernel ignition (what minimum enthalpy-augmentation of a given volume at one specific internal site leads to self-sustained flame propagation).

An experimental study of flammability limits using counterflow flames

Measurements of the lean and rich limits of flammability of methane-air mixtures and detailed experimental studies of the structure of premixed flames near the lean limit of flammability were made using a counterflow flame (a double flame) stabilized in the forward stagnation region of a porous cylinder. The flammability limits were determined by measuring a discernible break in the distance between the two flame zones with change in composition of the test mixture. The equivalence ratios of the lean and rich limits were found to be 0.47 and 1.72, respectively, which are somewhat broader than those obtaained by the standard technique. From previous results of structure analysis of rich fuel-air flames and the present results of lean fuel-air flames, it becomes clear that, in a double flame inside the flammability limits, the two reaction zones are completely separated, the reactions in the inner flame zone are completed, and the inner flame has the characteristics of a self-sustaining flame. On the other hand, in a double flame outside the flammability limits, the two reaction zones are not completely separated and the inner flame is closely correlated with the outer diffusion flame, so that the inner flame is not a self-sustaining flame. The flammability limit, defined as the limit of flame propagation, can be considered as the limit at which the supplies of reactants from the burned side must be necessary for a premixed flame to be established. The flammability limits determined in the present study depend only on the physicochemical properties of the mixture. The propagation velocity at the flammability limits was found to have a finite value.

Structure analysis of righ fuel-air flames in the forward stagnation region of a porous cylinder

An experimental study was made of the structure of rich fuel-air flames established in the forward stagnation region of a porous cylinder, from the surface of which premixed methane and air were ejected into a uniform air stream (the secondary air). Aerodynamic, temperature, and stable species concentration profiles were measured for flames at atmospheric pressure. These distributions were analyzed to yield the reaction-rate profiles of stable species throughout the flame zone. The stabilized flame is really a double flame. For the equivalence ratioof the ejected mixture >2.7, the stagnation point lies between the inner flame and the cylinder surface, and net reaction-rate profiles of various species are similar to those of a pure diffusion flame. Therefore, for >2.7, the flame can be treated essentially as a diffusion flame. For mixtures inside the rich limit of flammability (<1.64), two reaction zones are completely separated, and the inner flame plays the role of fuel (hydrogen and carbon monoxide) supply to the outer diffusion flame. For <2.7, the stagnation point lies on the air side of the inner flame. Even for mixtures outside the rich limit of flammability (1.64<<2.7), the inner flame maintains the characteristics of a self-sustaining flame and has a propagation velocity, although the inner flame is affected thermally by the outer flame. A flame can propagate in the fuel-air mixture outside the rich limit of flammability if the adiabatic state is retained or if heat is added from the burned side of the flame. Besides the rich limit of flammability usually defined, there is another limit of flame propagation. In the mixture beyond this limit a flame will never propagate. This limit can be regarded as fundamental in the sense that it depends only on the internal properties of the mixture. The equivalence ratio of this limit is 2.7 for methane-air mixture.

Ignition phenomena in hydrogen-air mixtures

This paper presents an experimental investigation into the effect of spark gap and spark duration on the minimum ignition energy (MIE) of partially dissociated NH3 in air at 295 K and 1 atm. The measurements were carried out using a Hartmann bomb apparatus with the degree of NH3 dissociation varying from 0 to 10%, corresponding to 0 to 7.5%v/v H2 in the fuel mixture with a fixed H2/N2 ratio of 3, and equivalence ratio varying from 0.7 to 1.2, assuming complete oxidation. Axisymmetric two-dimensional CFD modelling was performed using Ansys Fluent incorporating the Okafor mechanism to predict MIE. At a given equivalence ratio, both measured and predicted MIE values decreased significantly with increasing the degree of NH3 dissociation, due to increased presence of H2 in the mixture. There were optimum spark gap and spark duration for a given degree of NH3 dissociation and equivalence ratio at which the MIE was the lowest. The increased presence of H2 due to rising degree of NH3 dissociation enhances the mixture's oxidation reactivity and combustion intensity, which in turn increases the flame kernel temperature and size, the rate of production of key radicals (OH, NH2, H, and O) and rate of heat generation, leading to a more self-sustaining flame and significantly reduced MIE.