Experimental Investigation of Buoyant Laminar Jet Diffusion Flames in an Inverted Configuration

Buoyant laminar jet diffusion flames are studied experimentally in an inverted configuration, where gaseous fuel-stream jets vertically downward into air. Flame shape, thermal structure, soot and stability behaviors are obtained until the blowoff limit is reached. By comparing with conventional jet flames, which are established when the fuel jets upward, the effects of buoyancy on laminar diffusion flames are analysed. Downward flame yields larger flame height, although the non-dimensional flame height increases linearly with the Reynolds number at nozzle exit, which is similar to upward flame. Possible reasons for the increased flame height include flow deceleration within downward buoyant flames and presence of more combustion products surrounded the jet stream, thus slowing mixing process between fuel and air. The different relative directions of buoyant flows and jet streams also result in different temperature distributions in downward and upward flames, and a stagnant interface produced by the balance between buoyant flow and jet stream is particularly observed downstream of downward flame. Downward flames contain more soot and the soot formation region is wider, which are mainly attributed to the modifications of flow field and soot path. In addition, downward and upward flames stabilize at different axial positions relative to the nozzle exit. Because of increased characteristic flame residence time, downward flames have higher blowoff limits. The downward jet flame provides an alternative configuration to upward jet flame in studying buoyant diffusion flames due to the different manifestations of buoyancy effects.     

Effect of Oxygen Enrichment in Propane Laminar Diffusion Flames under Microgravity and Earth Gravity Conditions

Diffusion flames are the most common type of flame which we see in our daily life such as candle flame and match-stick flame. Also, they are the most used flames in practical combustion system such as industrial burner (coal fired, gas fired or oil fired), diesel engines, gas turbines, and solid fuel rockets. In the present study, steady-state global chemistry calculations for 24 different flames were performed using an axisymmetric computational fluid dynamics code (UNICORN). Computation involved simulations of inverse and normal diffusion flames of propane in earth and microgravity condition with varying oxidizer compositions (21, 30, 50, 100 % O₂, by mole, in N₂). 2 cases were compared with the experimental result for validating the computational model. These flames were stabilized on a 5.5 mm diameter burner with 10 mm of burner length. The effect of oxygen enrichment and variation in gravity (earth gravity and microgravity) on shape and size of diffusion flames, flame temperature, flame velocity have been studied from the computational result obtained. Oxygen enrichment resulted in significant increase in flame temperature for both types of diffusion flames. Also, oxygen enrichment and gravity variation have significant effect on the flame configuration of normal diffusion flames in comparison with inverse diffusion flames. Microgravity normal diffusion flames are spherical in shape and much wider in comparison to earth gravity normal diffusion flames. In inverse diffusion flames, microgravity flames were wider than earth gravity flames. However, microgravity inverse flames were not spherical in shape.     

Soot formation and oxidation in oscillating methane-air diffusion flames at elevated pressure

Comparisons with respect to the sooting tendency are made between stationary diffusion flames and diffusion flames with pulsations induced by oscillating fuel flow. Time-resolved measurements of the soot particle properties in the flames are obtained by combining Rayleigh-scattering, laser-induced incandescence, and extinction measurements into the RAYLIX method. Furthermore, flame luminosity at 590 nm and OH*-chemoluminescence signals at 310 nm are monitored to obtain data regarding the flame structure. Mean soot volume fractions of oscillating flames are significantly different from those of stationary flames with the same mean fuel flow rate; oscillations of the total amount of soot are phase shifted and asymmetric compared with fuel flow oscillations.     

Two-dimensional imaging of soot volume fraction by the use of laser-induced incandescence

A recently developed laser-induced incandescence technique is used to make novel planar measurements of soot volume fraction within turbulent diffusion flames and droplet flames. The two-dimensional imaging technique is developed and assessed by systematic experiments in a coannular laminar diffusion flame, in which the soot characteristics have been well established. With a single point calibration procedure, agreement to within 10% was found between the values of soot volume fraction measured by this technique and those determined by conventional laser scattering-extinction methods in the flame. As a demonstration of the wide range of applicability of the technique, soot volume fraction images are also obtained from both turbulent ethene diffusion flames and from a freely falling droplet flame that burns the mixture of 75% benzene and 25% methanol. For the turbulent diffusion flames, approximately an 80% reduction in soot volume fraction was found when the Reynolds number of the fuel jet increased from 4000 to 8000. In the droplet flame case, the distribution of soot field was found to be similar to that observed in coannular laminar diffusion flames.     

A study of the transition flow region in free gas jets

Data are presented here which have been obtained in a study of transitions in gas jets and in flames. It is shown that the flow characteristics of gas jets and of hot flames remain the same as of jets of an incompressible fluid.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 21, No. 1, pp. 58–62, July, 1971.     

Plasma-assisted ignition and deflagration-to-detonation transition

Non-equilibrium plasma demonstrates great potential to control ultra-lean, ultra-fast, low-temperature flames and to become an extremely promising technology for a wide range of applications, including aviation gas turbine engines, piston engines, RAMjets, SCRAMjets and detonation initiation for pulsed detonation engines. The analysis of discharge processes shows that the discharge energy can be deposited into the desired internal degrees of freedom of molecules when varying the reduced electric field, E/n, at which the discharge is maintained. The amount of deposited energy is controlled by other discharge and gas parameters, including electric pulse duration, discharge current, gas number density, gas temperature, etc. As a rule, the dominant mechanism of the effect of non-equilibrium plasma on ignition and combustion is associated with the generation of active particles in the discharge plasma. For plasma-assisted ignition and combustion in mixtures containing air, the most promising active species are O atoms and, to a smaller extent, some other neutral atoms and radicals. These active particles are efficiently produced in high-voltage, nanosecond, pulse discharges owing to electron-impact dissociation of molecules and electron-impact excitation of N(2) electronic states, followed by collisional quenching of these states to dissociate the molecules. Mechanisms of deflagration-to-detonation transition (DDT) initiation by non-equilibrium plasma were analysed. For longitudinal discharges with a high power density in a plasma channel, two fast DDT mechanisms have been observed. When initiated by a spark or a transient discharge, the mixture ignited simultaneously over the volume of the discharge channel, producing a shock wave with a Mach number greater than 2 and a flame. A gradient mechanism of DDT similar to that proposed by Zeldovich has been observed experimentally under streamer initiation.     

Jet flames from noncircular burners

Gas jets from noncircular exits entrain more air from surroundings than jets from circular exits of equivalent area. Because the mixing rate of fuel and air governs the combustion and pollutant emission of diffusion flames and partially premixed flames, noncircular geometries offer a passive control of the flame characteristics. In this paper, the literature on nonburning, noncircular jets is reviewed and recent studies on noncircular jet flames are discussed with focus on the work conducted in the author’s laboratory. The funding for the studies from which the material for this paper is extracted was provided by the Gas Research Institute, Chicago, Illinois and the Oklahoma Center for Advancing Science and Technology, Oklahoma City, Oklahoma in USA.     

Effect of Gravity on Premixed Methane–Air Flames

Premixed flames under different levels of gravity were studied experimentally and numerically. The experiments were carried out in the Bremen Drop tower. The object of investigation were conical premixed rich, lean, and stoichiometric CH₄–air flames with wide range of flow regimes, at Reynolds numbers of 600–2000, which were generated on specially designed cone nozzle and premixed cylinder chamber with grids and beads. Planar laser-induced fluorescence OH radicals and high-speed video recording was performed under microgravity, terrestrial conditions, and inverted gravity. All the experiments were performed at atmospheric pressure. Our experiments confirm that gravity has a complex influence on laminar and weakly turbulent premixed flames. Gravity causes flame flickering, while under reduced gravity flames are stable and almost not flicker. Based on the experimental data and numerical simulation, a correlation of flickering frequency with different mixture equivalence ratio and gravity is formed.     

Comparison of nanosecond and picosecond excitation for two-photon laser-induced fluorescence imaging of atomic oxygen in flames

Two-photon laser-induced fluorescence (LIF) imaging of atomic oxygen is investigated in premixed hydrogen and methane flames with nanosecond and picosecond pulsed lasers at 226 nm. In the hydrogen flame, the interference from photolysis is negligible compared with the LIF signal from native atomic oxygen, and the major limitations on quantitative measurements are stimulated emission and photoionization. Excitation with a nanosecond laser is advantageous in the hydrogen flames, because it reduces the effects of stimulated emission and photoionization. In the methane flames, however, photolytic interference is the major complication for quantitative O-atom measurements. A comparison of methane and hydrogen flames indicates that vibrationally excited CO2 is the dominant precursor for laser-generated atomic oxygen. In the methane flames, picosecond excitation offers a significant advantage by dramatically reducing the photolytic interference. The prospects for improved O-atom imaging in hydrogen and hydrocarbon flames are presented.     

Effect of fuel structure on synthesis of carbon nanotubes in diffusion flames

Carbon nanotubes (CNTs) were synthesized on nickel nitrate coated nickel foam in co-flow diffusion flames. Two different fuel structures, methane and ethylene, were used to synthesize CNTs. The effect of fuel structure on CNTs was systematically studied. Results showed that carbon nanomaterials, including nanotubes and nanofibers, were successfully synthesized in all conditions in methane flames, while, in ethylene flames, bamboo-like CNTs were synthesized in limited conditions. It was also found that carbon nanomaterials synthesized in ethylene flames had more defects than that of in methane flames. In addition, metal nickel nanoparticles acted as catalysts in the synthesis of CNTs, and carbon nanomaterials diameter was dependent on the catalyst particle size. Flame-synthesized CNTs were based on the vapor-liquid-solid mechanism, and a “recursive growth mechanism” for CNTs growth was proposed, which would be more conducive to the understanding of CNTs growth mechanism. This method offers another possibility for low-cost, large-scale synthesis of carbon nanomaterials.     

Quantitative hydroxyl concentration time-series measurements in turbulent nonpremixed flames

Quantitative hydroxyl concentration time-series measurements have been obtained by picosecond time-resolved laser-induced fluorescence in a series of methane-air and hydrogen-argon-air nonpremixed flames. The recovery of a quantitative time series is complicated by the need to account for fluctuations in the fluorescence lifetime. We have recently developed instrumentation that enables the simultaneous measurement of fluorescence signal and lifetime. The present research represents the first application of this technique to turbulent flames. The correction for hydroxyl lifetime fluctuations is shown to be significant for mean concentrations and thus probability density functions but negligible for power spectral densities (PSD's). The hydroxyl PSD's were found to vary slightly with radial and axial location in the flames and to vary significantly with Reynolds number. However, the PSD's in the H(2)-Ar-air flames are nearly identical to those in the CH(4)-air flames.     

Measurements of hydroxyl concentrations and lifetimes in laminar flames using picosecond time-resolved laser-induced fluorescence

Picosecond time-resolved laser-induced fluorescence (PITLIF) can potentially be used to obtain measurements of minor species concentrations in rapidly fluctuating flames. Previous studies demonstrated this potential for atomic sodium by monitoring the temporal fluorescence signal with both an equivalent-time and a real-time sampling method. In this developmental study, PITLIF is used to determine hydroxyl concentrations in laminar CH(4)-O(2)-N(2) flames by the measurement of both the integrated fluorescence signal and the fluorescence lifetime. The quenching environment can be monitored with real-time sampling, and thus the necessary quenching rate coefficient is obtained in 348 us, which is fast enough for use in many turbulent flows. Fluorescence lifetimes of OH are also measured at different equivalence ratios in laminar flames by the use of the equivalent-time sampling technique. These results compare favorably with predicted lifetimes based on relevant quenching cross sections and calculated species concentrations.     

商品广告摄影色彩的控制与运用

当代商品广告的设计与制作离不开摄影,广告摄影已经成为商品广告最重要的表现手段之一。在商品广告摄影的创作与表现过程中,对色彩的控制与运用是最重要的,能够正确再现商品的色彩,能够创造性地营造图面色彩氛围,是有效表现和宣传商品的关键,是决定商品广告摄影成败的关键。精确控制色彩是商品广告摄影的核心,巧妙运用色彩是商品广告摄影的灵魂。     

Optical diagnostics on sooting laminar diffusion flames in microgravity

Laser-induced incandescence was successfully applied to the investigation of soot formation in both buoyant and non-buoyant laminar jet diffusion flames. Microgravity experiments were conducted in the Drop Tower Bremen, Germany. By the use of imaging laser-induced incandescence (LII) it was possible for the first time to obtain simultaneously two-dimensional information on soot concentration and primary particle size under microgravity. Additionally, temperature fields were measured by 2-color emission pyrometry. Results for the fuels propane and ethene show that soot formation and oxidation is drastically altered under microgravity. Maximum temperatures are reduced by roughly 220 K and 120 K, respectively, which in the case of ethene results in a termination of oxidation processes and the emission of soot. The distribution of soot within the non-buoyant flames is always concentrated in relatively small bands. For all non-buoyant flames investigated the maximum primary particle size roughly doubles compared to the buoyant ones.     

Influence of Electrical Discharges on Combustion (The Generalized Borghi Diagram)

There are several mechanisms associated with the thermal, kinetic, and electromechanical influence of electrical fields on burning of mixes. Here, it is shown that the burning processes in the presence of the electrical fields involve, along with the known criteria, an additional criterion that is describing the processes arisen from affecting of the electrical discharges on mix, ignition and flame propagation. The role of D or A electrical fields and/or their influence on the induction time of combustion, temperature of ignition, the borders of a steady burning area, the laminar and turbulent flames propagation, and the flame flat front stability are shortly examined. Specially, affecting the combustion by an external electric field through the overheating turbulence and streamer branching is discussed. Problems of low-temperature plasma instabilities and plasma turbulence are shortly studied. The effects of the overheating instability development and generating a specific kind of turbulence during the electrical discharge burning are discussed. The modified Karlovitz Λ_K, Damköler Λ_D criteria and the generalized Borghi diagram that divides the space of non-dimensional parameters into branches of different flame behavior are presented.     

Numerical investigations on premixed spherical flames for Lewis numbers larger than unity

We present numerical simulations of premixed spherical flames under μg conditions using the thermo-diffusive approximation. The employed numerical method is based on a finite volume discretization with explicit Runge-Kutta time integration, both of second order. A multiresolution technique is used to represent the solution on an adaptive, locally refined grid, which allows efficient and accurate computations at a reduced computational cost. We study the ignition limit, i.e. the critical radius for which the flame extinguishes, for varying Lewis numbers larger than unity. We also present fully three-dimensional simulations of initially stretched spherical flames and show their relaxation towards spherical flames, which justifies the one-dimensional spherically symmetric simulations.     

Quantitative technique for imaging mixture fraction, temperature, and the hydroxyl radical in turbulent diffusion flames

A technique for obtaining simultaneous quantitative images of the hydroxyl radical, OH, temperature, mixture fraction, and scalar dissipation rates in turbulent diffusion flames is described. Mixture fraction is obtained from images of Rayleigh and fuel Raman scattering. We quantified the OH laser-induced fluorescence (LIF) images using detailed calibration and a correction for quenching and population distribution effects based on the simultaneous mixture fraction and temperature images. This correction was derived from calculations of laminar counterflow diffusion flames for identical fuel mixtures. These laminar flame computations are further used to estimate the errors in the measured OH concentrations. The technique is applied to piloted, nonpremixed flames over a range of jet velocities. The measured mixture fraction, temperature, and OH concentrations are in good agreement with those obtained earlier in similar flames using the single-point Raman/Rayleigh/LIF technique.     

N2 and CO vibrational CARS and H2 rotational CARS spectroscopy of CH4/N2O flames

Broadband CARS spectra of N2 and CO have been obtained from the postflame gases of rich CH4/N2O flames using the nonplanar BOXCARS technique. The temperature and concentration of both N2 and CO in these flames were estimated from CARS spectra with the aid of model calculations and agreed with standard thermochemical predictions. In addition, several pure rotational H2 CARS transitions, certain of which had been previously unobserved, were seen in several spectral regions, most notably in both the CO and NO CARS regions. These observations are important in future modeling of CARS data.     

Coloration caused by mechanical stress on rare earth oxysulfides

Oxysulfide powders of yttrium, gadolinium, terbium and lanthanum display distinctive colors when they are either heavily ball-milled or pressed in a steel die. These colors are due to optical absorption bands in the visible region, peak positions of which obey an empirical rule like the Mollowo-Ivey relation of color centers in alkali halides. The c-axis shrinks in the colored samples. The colors are bleached and the cell constants are restored to the original values by heat treatment up to 450°C. Some distortions, however, still remain in the annealed samples.     

基于背景代表色提取的迷彩伪装颜色选取算法

为了达到好的伪装效果,迷彩颜色应与背景色调相融合,以使人眼及光学仪器难以探测和分辨.因此,背景主色的准确提取是确定迷彩颜色的前提.利用灰度直方图可以确定背景图像中的主要灰度,但无法区分不同色调.而基于颜色直方图的背景主色提取方法的运算量太大.本文提出了一种基于HSI模型和量化颜色直方图的迷彩颜色选取算法.利用HSI颜色模型描述背景颜色特性,通过特殊量化方式对背景的颜色直方图进行量化,接着借助阈值方法选取背景主色作为迷彩颜色.结合迷彩伪装图案设计方法对上述迷彩颜色选取算法进行了实验分析,并通过边缘检测和相关跟踪方法对不同背景下的目标迷彩伪装效果进行了验证.