Visualization of the unburned gas flow field ahead of an accelerating flame in an obstructed square channel

The effect of blockage ratio on the early phase of the flame acceleration process was investigated in an obstructed square cross-section channel. Flame acceleration was promoted by an array of top-and bottom-surface mounted obstacles that were distributed along the entire channel length at an equal spacing corresponding to one channel height. It was determined that flame acceleration is more pronounced for higher blockage obstacles during the initial stage of flame acceleration up to a flame velocity below the speed of sound of the reactants. The progression of the flame shape and flame area was determined by constructing a series of three-dimensional flame surface models using synchronized orthogonal schlieren images. A novel schlieren based photographic technique was used to visualize the unburned gas flow field ahead of the flame front. A small amount of helium gas is injected into the channel before ignition, and the evolution of the helium diluted unburned gas pocket is tracked simultaneously with the flame front. Using this technique the formation of a vortex downstream of each obstacle was observed. The size of the vortex increases with time until it reaches the channel wall and completely spans the distance between adjacent obstacles. A shear layer develops separating the core flow from the recirculation zone between the obstacles. The evolution of oscillations in centerline flame velocity is discussed in the context of the development of these flow structures in the unburned gas.     

Flame spread behaviour of blended fuel droplet array

Experimental investigation of the effect of blended fuel on flame spread along droplet array has been conducted. Flame spread rate is measured using high‐speed chemiluminescence images of an OH radical. The flame spread is observed with the initial droplet diameter, droplet spacing, and the mixing ratio of n‐heptane and n‐hexadecane. The mode of flame spread is categorized into two types: a continuous mode and an intermittent one. It is seen that flame spread rate is sensitively dependent on the relative flame position to droplet spacing. For a large droplet, the flame spread time is governed by a volatile fuel (heptane), but for a small droplet, it is controlled by a less volatile fuel (hexadecane). Copyright © 1999 John Wiley & Sons, Ltd.     

Measurements of propagation speeds and flame instabilities in biomass derived gas-air premixed flames

Three biomass derived gases (BDGs, named GG-H, GG-L and GG-V), which are derived from industry facilities and can be useful for reducing CO₂ and the application to combustors, are studied and examined for some basic flame characteristics such as unstretched laminar burning velocity, Markstein length, and cell formation over the entire flame surface. Experiments were conducted in a constant volume combustion chamber using a schlieren system. A better agreement between the measured and predicted unstretched laminar burning velocities is obtained using a suggested reaction mechanism modified from the GRI-Mech 3.0 mechanism. Additionally, cell formations on flame surfaces of the three mixtures were also analyzed and compared using high-speed schlieren images. It is shown that the GG-H-air flames and the GG-L-air flames have similar flame wrinkled surfaces, while the GG-V-air flames shows a stronger cellularity behavior. The effects of each fuel component in mixtures to cellularity are also evaluated by varying the concentration of each fuel in the reactant mixtures. The cellular instability is promoted (diminished) with hydrogen enrichment (methane addition); meanwhile the similar behavior is obtained for carbon monoxide addition.     

Numerical Simulation of Microgravity Flame Spread Over Solid Combustibles

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Combustion and flame spread on fuel-soaked porous solids

Fires caused by accidental spillage of flammable liquids have been a major safety concern in industries and urban areas. There has been a recent surge of interest in the research concerning the combustion and flame spread over an inert porous media soaked with flammable liquid. This interest has been driven by the need to better understand fire and its behaviour under these conditions and improve the relevant fire safety and prevention technologies. A review of key studies in this subject area has been conducted and summarised, focussing mainly on the theory plus a notable experimental findings about combustion and the flame spread phenomena of fuel-soaked porous media. The review covers topics such as flame spread behaviour, physical flame propagation aspects, heat transfer, temperature distribution; and fuel consumption over inert porous media. The review concludes with some practical safety and environmental considerations for decontamination of land soaked with flammable liquid.     

Effects of hydrogen concentration and film thickness on the vented explosion in a small obstructed rectangular container

The explosion venting is an effective way to reduce hydrogen-air explosion hazards, but the explosion venting has been less touched in an obstructed container. The present study mainly focused on the effects of hydrogen concentration and film thickness on the explosion venting in a small obstructed rectangular container. High speed schlieren photography was employed to obtain the flame fine structure and velocity. Pressure transducers were used to measure the overpressure nearby the obstacle. The experimental results show that the obstacle has a significant effect on the flame shape, tip speed and overpressure. In the process of flame evolution, the flame surface becomes more wrinkled with time after the tulip flame. Compared with the cases without the obstacle, the flame surface becomes more distorted and wrinkled downstream of the obstacle under the influence of obstacle enhanced turbulence and flow instability. Upstream of the obstacle, the lower part of the flame surface becomes concave while the upper part shows convex. The pressure histories show that the maximum overpressure increases with the hydrogen concentration in the range of 11.8%–23.7%. Two main pressure peaks were observed for all hydrogen concentrations in the presence of the obstacle. The Helmholtz oscillations appear after the second pressure peak and its duration increases slightly when the hydrogen concentration increases. The combined effect of the obstacle and hydrogen concentration on the second peak overpressure is more significant than on the first peak overpressure. Moreover, the maximum overpressure shows a monotonic increase with the film thickness.     

Numerical study of fuel temperature influence on single gas jet combustion in highly preheated and oxygen deficient air

Combustion of a single jet of propane in a cross-flowing stream of preheated and oxygen deficient air is numerically analyzed with emphasis on influences of fuel temperature. Both Eddy-Break-Up and PDF/mixture fraction combustion models coupled with RNG k–ε turbulent model were applied and the predicted results were compared. Thermal and prompt NO models were employed to calculate NO emissions. Results show that the Eddy-Break-Up model is more suitable for predicting temperature field and flame shape. It was showed that flame during high temperature air combustion condition is spread over a much larger volume. Flame volume increases with a reduction of oxygen concentration, and this trend is clearer if oxygen concentration in the preheated air is below 10%. Additionally, it is almost constant at fixed oxygen concentration and fuel inlet temperature for the temperature of the preheated and oxygen deficient air equal to 1041–1273 K. Increase of the fuel inlet temperature results in smaller flame, shorter mean residence time, smaller temperature peaks, and lower emission of NO.     

Transient burning of a convective fuel droplet

The transient burning of an n-octane fuel droplet in a hot gas stream at 20 atmosphere pressure is numerically studied, with considerations of droplet regression, deceleration due to the drag of the droplet, internal circulation inside the droplet, variable properties, non-uniform surface temperature, and the effect of surface tension. An initial envelope flame is found to remain envelope in time, and an initial wake flame is always transitioned into an envelope flame at a later time, with the normalized transition delay controlled by the initial Reynolds number and the initial Damkohler number. The initial flame shape is primarily determined by the initial Damkohler number, which has a critical value of Da₀=1.02. The burning rates are modified by the transition, and are influenced by the intensity of forced convection which is determined by initial Reynolds number. The influence of surface tension is also studied as the surface temperature is non-uniform. Surface tension affects the liquid motion at the droplet surface significantly and affects the change of surface temperature and burning rate modestly. The influence of surface tension generally increases with increasing initial Reynolds number within the range without droplet breakup. We also studied cases with constant relative velocity between the air stream and the droplet. The results show that in these cases the initial envelope flame still remains envelope, but the evolution from an initial wake flame to an envelope flame is inhibited. Validation of our analysis is made by comparing with a published porous-sphere experiment Raghavan et al. (2005) [6] which used methanol fuel.     

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.     

应用高速纹影法对汽油机燃烧过程的研究

根据光学纹影法的基本原理和高速摄影技术,在可视化发动机台架上搭建了一套反射式纹影光路系统,使其能够较清晰准确地记录燃料初期蓝焰的扩散过程。通过对比,纹影图像比直接摄影图像能更早地观察到火焰的产生,火焰轮廓更大,能够反应火焰的变化形态。试验将此系统应用在汽油机燃烧过程的研究中,并分析了空燃比和负荷时汽油机火焰传播速度及火焰形态的影响。结果表明:在理论空燃比附近,随着混合气变浓,着火始点靠前,火焰传播速度加快,火焰的表面皱折变大;负荷的提高使得燃烧更加充分,着火提前,火焰传播速度提高,火焰皱折增大。     

Pool fires – An empirical correlation

This study presents the experimental procedure and results of highly controlled pool fire tests, in a quiescent environment, designed to accurately measure the fuel burning rate and, consequently for sufficiently large pools, the thermal radiation flux back to the fuel surface. Steps were taken to minimize the effects of in-depth absorption of flame radiation, circulation within the liquid, and changes in fuel composition due to distillation of more volatile fuel components. With these precautions, focus is placed on gas phase phenomena controlling the heat release rate per unit pool area. The primary variables considered are: pool diameter, heat of gasification, flame sootiness as characterized by the inverse of the fuel smoke-point flame height, and, to a lesser extent, absorption of flame radiation by the fuel vapors just above the liquid surface. Results reported herein agree well with literature values for experiments conducted under similarly controlled conditions. A simple empirical formula is developed based primarily on heat of gasification and smoke point and is shown to correlate the mass burning rate within 9%, on average, of the experimental data.     

A study of flame spread along a droplet array at elevated pressures up to a supercritical pressure

Experimental investigations on flame spread along a droplet array have been conducted at elevated pressures up to supercritical pressures of the fuel droplet under normal gravity and microgravity. The flame spread rate is measured using high‐speed chemiluminescence images of OH radicals and direct visualization is employed to observe the images of the vaporizing fuel around the unburnt droplet. The mode of flame spread is categorized into two: a continuous mode and an intermittent one. There exist a limit droplet spacing and a limit ambient pressure in normal gravity, above which flame spread does not occur. It is seen that flame spread rate is dependent upon the relative position of flame to droplet spacing. In microgravity, the limit droplet spacing of flame spread and the droplet spacing of maximum flame spread rate are larger than those in normal gravity. In microgravity, the flame spread rate with ambient pressure decreases initially, shows a minimum, and then decreases again after taking a maximum. Flame spread time is determined by competing effects between the increased transfer time of the thermal boundary layer due to reduced flame diameter and the decreased ignition delay time in terms of the increase of ambient pressure. In normal gravity, the flame spread rate with ambient pressure decreases monotonically and there exists a limit ambient pressure, except at small droplet spacing, under which flame spread extends to the range of supercritical pressures of fuel. This is because natural convection induces the upward flow of hot gases into a plume above the burning droplets and limits the lateral transfer of thermal boundary layer. Consequently, it is found that flame spread behaviour under microgravity is considerably different from that under normal gravity due to the absence of natural convection. Copyright © 1999 John Wiley & Sons, Ltd.     

Turbulent diffusion flame sooting—Relationship to smoke-point tests

Soot volume fractions have been measured in turbulent diffusion flames issuing into still air for a range of gaseous and liquid fuels. The liquid fuels were prevaporized and all fuels had a common inlet temperature. It was found that flame widths were independent of fuel stoichiometry and were only a function of fluid mechanics. Soot quantities peaked at a height of 0.25–0.55 of flame length. Normalized radial profiles of soot volume fraction were approximately self-similar for all fuels.A characteristic soot volume fraction was defined for each fuel and found to be largely independent of residence time in the flame. The turbulent flame characteristic soot volume fraction was related to its laminar flame counterpart and was found to be predictable from the laminar flame smoke-point flow rate.     

Characteristics of liquid ethanol diffusion flames from mini tube nozzles

A series of experiments was conducted to explore the combustion characteristics of a diffusion flames from mini tubes fueled by liquid ethanol with visual observations of the flame shape, the dynamic liquid-vapor interface during phase change inside the capillary tubes and the tube outer surface temperature using CCD and IR cameras. As the fuel supply rate increased, the interface location rose to the tube exit and the temperature gradient on the outer tube surface increased, consequently the evaporating became much stronger and the interface tended to be unstable. The combustion characteristics are closely related to the rapid phase change and violent evaporation and interfacial dynamics, with the violent evaporation, actually explosive boiling, inducing an explosive flame. The intensity of the explosive flame became stronger as the flowrate increased with the maximum flame height, interface location movement, and sound intensity all significantly increasing. The periodicity of the explosive flame was directly proportional to the interface moving distance and inversely proportional to the fuel flow rate.     

乳化燃料滴的蒸发与着火

本文研究了乳化燃料滴的着火现象，探讨了液相输运过程处于两种极限情况下着火延迟的变化规律。结果表明液相输运过程对乳化油的着火有决定作用。挂滴法的实验结果与凝固模型较接近，水分的加入使液滴表面温度，燃料浓度明显下降，从而使着火延迟时间变长。水分的加入对高沸点燃料着火的影响更为显著。     

Temperature distribution on inclined plate caused by interaction with supersonic jet

IntroductionThe phenomena of the impingement of a supersonicjet on a solid obstacle is very interesting and importantin relation to the aeronautical and other industrialengineerings[']-l']. A large number of papers hadconcerned with the impingement on a perpendicularplate and evidenced the flow phenomena and thepressure distribuhon on a plate and so on. However, theinvestigation of the impingement on an inclined platecan only be found in merely several papers['-']. Ih thesestudies, it is menti…     

Comparison of burning characteristics of live and dead chaparral fuels

Wildfire spread in living vegetation, such as chaparral in southern California, often causes significant damage to infrastructure and ecosystems. The effects of physical characteristics of fuels and fuel beds on live fuel burning and whether live fuels differ fundamentally from dead woody fuels in their burning characteristics are not well understood. Toward this end, three common chaparral fuels prevalent in southern California, chamise, manzanita, and ceanothus, were investigated by burning them in a cylindrical container. The observed fire behavior included mass loss rate, flame height, and temperature structure above the burning fuel bed. By using successive images of the temperature field, a recently developed thermal particle image velocity (TPIV) algorithm was applied to estimate flow velocities in the vicinity of the flame. A linear regression fit was used to explain the observed time difference between when maximum flame height and maximum mass loss rate occur, as a function of fuel moisture content. Two different methods were used to extract power laws for flame heights of live and dead fuels. It was observed that the parameters defined in the well-known two-fifths power law for flame height as a function of heat release rate were inadequate for live fuels. As the moisture content increases, the heat release rate in the power law needs to be calculated at the time when the maximum flame height is achieved, as opposed to the maximum mass loss rate. Dimensionless parameters were used to express local temperature and velocity structure of live and dead chaparral fuels in the form of a Gaussian profile over different regimes in a fire plume.     

Postulations of flame spread mechanisms

An experimental study was conducted to explore the flame spread mechanism over thin solid fuel sheets. Flame spread rates over paper at various inclined angles were measured, and Schlieren photography was used to qualitatively assess heat transfer to the unburnt material in front of the pyrolysis zone. Two different types of flame spread were observed. One is te downward flame spread observed in the range of ?90 to ?30 deg from the horizontal. In this region, flame spread rate was almost constant with time, although it increased slightly with increasing angle. The other type of spread observed was the accelerative upward flame spread at angles of zero to 90 deg. Flame spread at angles from ?30 deg to zero seemed unsatable and increased by repetitive acceleration and deceleration In the range of inclined angles from ?90 to ?30 deg, heat transfer from the flame zone to the unburnt material seemed to take place mainly through the gas phase in the region 0.2 ～ 0.4 cm in front of the pyrolysis zone. In this case, the direction of the gas stream could be considered to oppose that of the flame spread. In the case of upward flame spread, the unburnt material in front of the pyrolysis zone seemed to be heated by convection of the bottom side, where the direction of the gas stream was obviously parallel with that of flame spread.     

用光学纹影摄影术观察分析定容燃烧室内氢燃料的燃烧过程

论述了采用纹影摄影术和高速摄影法观察分析氢气和空气预混合燃料在定容燃烧室内的火花点火燃烧过程,定性地分析了预混合氢气燃料的火焰形态和变化过程,以及燃烧室内的初始压力和空燃比对火焰传播速度及其燃烧压力的影响,通过采用纹影摄影术方法,初步揭示了预混合氢气燃料在定容燃烧室内燃烧时火焰初期紊流产生的机理,以及由开始的层流状火焰发展到湍流状火焰的过程,研究结果表明,预混合氢气燃料燃烧的火焰传播速度及燃烧压力明显地受燃烧室内的初始压力和空燃比的影响。     

Experimental investigation of burning rates of pure ethanol and ethanol blended fuels

A fundamental experimental study to determine the burning rates of ethanol and ethanol-blended fossil fuels is presented. Pure liquid ethanol or its blends with liquid fossil fuels such as gasoline or diesel, has been transpired to the surface a porous sphere using an infusion pump. Burning of the fuel takes place on the surface of the porous sphere, which is placed in an air stream blowing upwards with a uniform velocity at atmospheric pressure and temperature under normal gravity conditions. At low air velocities, when ignited, a flame envelopes the sphere. For each sphere size, air stream velocity and fuel type, the fuel feed rate will vary and the same is recorded as the burning rate for that configuration. The flame stand-off distances from the sphere surface are measured by post-processing the digital image of the flame photograph using suitable imaging software. The transition velocity at which the flame moves and establishes itself at the wake region of the sphere has been determined for different diameters and fuel types. Correlations of these parameters are also presented.