The Formation of Nano—Size SiO2 Thin Film on an Aluminum Plate with Hexamethyldisilazane（HMDSA）and Hexamethyldisiloxane（HMDSO）

This study is using HMDSA(C6H19NSi2)or HMDSO(C6H18OSi2) vapor into C3H8/air premixed flames to form SiO2 thin film on the surface of an aluminum palte.With the addition of HMDSO or HMDS to premixed flames,an orange secondary flame or a flame brush appeared and was contributed to the formation of SiO2 particles.Based upon the EDS,XPS and FTIR analysis ,it is believed that the synthesized products consist of mainly SiO2 and a small amount of SiO,The pure SiO2 crystal structure,was proved by XRD analysis,which may form form the SiO2 amorphous structure after high temperature(1300℃) thermal treatment.The nano-size SIO2 particles,which ranged form 2.5-25nm,are proved by analysis of the BET and TEM.A 2-D CFD-RC code with 12 reduced chemical reaction mechanism.based upon the SIMPLER procedure,was successfully employed to predict the flame temperature and both of the SiO2 and SiO concentration profiles.Compared with the experimental results,the calculated temperature profiles in the post-flame region are in good agreement with the measured data and observation phenomena.     

Combustion Synthesis of SiO2 on the Aluminum Plate

IntnductionThere are a number of teChniques Presently available for the synthesis of nanopwhcles of oxides and nonokides, includingDndcrowave plasma synthesis['1, plasmaarc synthesisly, and pyrolysis and condensation ofchemical Precursors in the flame at ambient pressureal].The aPProach of uhlizing combushon synthesis toproduce oalde POwders, such as SiO2, A12O3 and TiO2, isalso a quite modem technology['"]. The synthesizedpndcles can be edictly deposited to form a thin film onthe surfac…     

Combustion synthesis of SiO2 on the aluminum plate

The approach of utilizing combustion synthesis to make fine particles of SiO₂, Al₂O₃ and TiO₂ is a quite modern technology. Through the chemical reaction in post-flame region, fine SiO₂ particles can be formed with high purity on plate surface. Therefore, the combustion synthesis of SiO₂ powders is an important area for further research and development, especially for the application of SiO₂ in the semiconductor industry. This investigation proposes an experimental approach (i.e., a gas-phase combustion synthesis) using two different kinds of organic compounds, Hexamethyldisilazane (HMDSA) and Hexamethyldisioxane (HMDSO), as the silicon precursors. A premixed gas burner is chosen with C₃H₈ as fuel, air as oxidant and part of the air was used as the carrying gas to entrain HMDSA/HMDSO vapor into the combustible mixture. Observations show that the C₃H₈/air flame changed color from a pale-blue flame to light yellow and then orange when different amounts of precursors were introduced. Through the chemical reaction in the post-flame region, fine SiO₂ particles were formed in the gas phase and then quenched and collected on an aluminum flat plate. The objective of this paper is to study the effects of HMDSO and HMDSA concentrations and flame temperatures on the synthesis of SiO₂ particles.     

Effect of ferrocene addition on sooting limits in laminar premixed ethylene-oxygen-argon flames

The critical sooting C/O ratio was measured for a series of atmospheric-pressure, laminar premixed ethylene-oxygen-argon flames doped with a small amount of ferrocene. Comparisons of the doped and undoped flames show marked increases in sooting tendency for flames doped with ferrocene. Under the same flame conditions, the critical C/O ratios in doped flames were uniformly lower than those of undoped flames. A five-step reaction mechanism of ferrocene decomposition, leading to the formation of the cyclopentadienyl, was proposed. Detailed kinetic modeling of the experimental flames showed little changes in the flame temperature and the aromatic concentration upon ferrocene doping. The experimental and computational results support the early suggestion that in premixed flames the increase in sooting tendency due to ferrocene addition is the result of induced nucleation by iron oxide nanoparticles. These particles provide a surface to initiate soot surface growth.     

Flame temperature measurements during electrically assisted aerosol synthesis of nanoparticles

Fourier transform infrared (FTIR) emission/transmission (E/T) spectroscopy is used to measure the temperature during TiO₂ formation by titanium tetraisopropoxide (TTIP) oxidation in a premixed flat flame. In the absence of particles, FTIR is systematically compared to coherent anti-Stokes Raman scattering (CARS) in premixed flames in the presence and absence of external electric fields including line-of-sight as well as tomographic reconstructed FTIR measurements. Time resolved CARS flame temperature measurements probe the effect of electric fields on the premixed flat flame. Furthermore, FTIR is used to measure axial and radial flame temperature profiles in TiO₂ particle-laden flames at various electric field strengths. Along with the visible reduction in flame height when applying the electric field, the external electric fields reduce the high temperature region of the flame and lead to a steeper temperature gradient further downstream. The maximum flame temperature, however, remains constant. The precision control of particle crystallinity and the specific surface area by external electric fields is confirmed as well as their effect on the anatase to rutile phase transformation is discussed.     

A mechanistic study of Soret diffusion in hydrogen–air flames

The separate and combined effects of Soret diffusion of the hydrogen molecule (H₂) and radical (H) on the structure and propagation speed of the freely-propagating planar premixed flames, and the strain-induced extinction response of premixed and nonpremixed counterflow flames, were computationally studied for hydrogen–air mixtures using a detailed reaction mechanism and transport properties. Results show that, except for the conservative freely-propagating planar flame, Soret diffusion of H₂ increases the fuel concentration entering the flame structure and as such modifies the mixture stoichiometry and flame temperature, which could lead to substantial increase (decrease) of the flame speed for the lean (rich) mixtures respectively. On the other hand, Soret diffusion of H actively modifies its concentration and distribution in the reaction zone, which in turn affects the individual reaction rates. In particular, the reaction rates of the symmetric, twin, counterflow premixed flames, especially at near-extinction states, can be increased for lean flames but decreased for rich flames, whose active reaction regions are respectively located at, and away from, the stagnation surface. However, such a difference is eliminated for the single counterflow flame stabilized by an opposing cold nitrogen stream, as the active reaction zone up to the state of extinction is always located away from the stagnation surface. Finally, the reaction rate is increased in general for diffusion flames because the bell-shaped temperature distribution localizes the H concentration to the reaction region which has the maximum temperature.     

Tubular premixed and diffusion flames: Effect of stretch and curvature

Tubular flames are ideal for the study of stretch and curvature effects on flame structure, extinction, and instabilities. Tubular flames have uniform stretch and curvature and each parameter can be varied independently. Curvature strengthens or weakens preferential diffusion effects on the tubular flame and the strengthening or weakening is proportional to the ratio of the flame thickness to the flame radius. Premixed flames can be studied in the standard tubular burner where a single premixed gas stream flows radially inward to the cylindrical flame surface and products exit as opposed jets. Premixed, diffusion and partially premixed flames can be studied in the opposed tubular flame where opposed radial flows meet at a cylindrical stagnation surface and products exit as opposed jets. The tubular flame flow configurations can be mathematically reduced to a two-point boundary value solution along the single radial coordinate. Non-intrusive measurements of temperature and major species concentrations have been made with laser-induced Raman scattering in an optically accessible tubular burner for both premixed and diffusion flames. The laser measurements of the flame structure are in good agreement with numerical simulations of the tubular flame. Due to the strong enhancement of preferential diffusion effects in tubular flames, the theory-data comparison can be very sensitive to the molecular transport model and the chemical kinetic mechanism. The strengthening or weakening of the tubular flame with curvature can increase or decrease the extinction strain rate of tubular flames. For lean H₂-air mixtures, the tubular flame can have an extinction strain rate many times higher than the corresponding opposed jet flame. More complex cellular tubular flames with highly curved flame cells surrounded by local extinction can be formed under both premixed and non-premixed conditions. In the hydrogen fueled premixed tubular flames, thermal-diffusive flame instabilities result in the formation of a uniform symmetric petal flames far from extinction. In opposed-flow tubular diffusion flames, thermal-diffusive flame instabilities result in cellular flames very close to extinction. Both of these flames are candidates for further study of flame curvature and extinction.     

Measurement and analysis of soot inception limits of oxygen-enriched coflow flames

This study explores the criteria for soot inception in oxygen-enriched laminar coflow flames. In these experiments we select an axial height in the coflow flame at which to identify the sooting limit. The sooting limit is obtained by varying the amount of inert until luminous soot first appears at this predefined height. The sooting limit flame temperature is found to increase linearly with stoichiometric mixture fraction, regardless of fuel type. To understand these results, the relationships between flame structure, temperature, and local C/O ratio is explored through the use of conserved scalar relationships. Comparison of these relationships with the experimental data indicates that the local C/O ratio is a controlling parameter for soot inception in diffusion flames (analogous to the global C/O ratio in premixed flames). Analysis of experimental results suggests that soot inception occurs when the local C/O ratio is above a critical value. The values for critical C/O ratios obtained from the analysis of experiments using several fuels are similar in magnitude to the corresponding C/O ratios for premixed flames. In addition, temperatures and PAH fluorescence were measured to identify regions in these flames most conducive to particle inception. Results indicate that the peak PAH concentration lies along a critical iso-C/O contour, which supports a theory that soot particles first appear along this critical contour, given sufficient temperature.     

甲烷预混多喷嘴阵列燃烧器热声振荡模态实验研究

为研究燃气轮机模型燃烧室的非预混燃烧流场,采用大涡模拟方法分别结合火焰面生成流形模型（FGM）和部分预混稳态火焰面模型（PSFM）对甲烷/空气同轴射流非预混燃烧室开展了数值模拟研究,并与试验结果进行对比。结果表明：FGM所预测的速度分布、混合分数分布、燃烧产物及CO分布与试验结果更符合;两种模型均能捕捉到燃烧室中的火焰抬举现象;燃烧过程中的火焰结构较为复杂,同时存在预混燃烧区域和扩散燃烧区域,扩散燃烧主要分布在化学恰当比等值线附近,预混燃烧区域主要分布在贫油区。     

Charge fraction distribution of nucleation mode particles: New insight on the particle formation mechanism

We measured particle size distributions of total and singly charged nanoparticles in premixed flames with different flame stoichiometry and temperature to investigate particle inception.Particle charging in flames occurs by diffusion charging involving ions formed by chemi-ionization reactions in the flame front. It can be described by a Boltzmann charge fraction distribution evaluated at the local flame temperature where the particles interact with the chemi-ions. As the particles coagulate in the post flame zone, their charge fraction is reduced. The charge distribution of the coagulated aerosol again results in a Boltzmann curve, this time evaluated at the local post flame gas temperature where the particles had their last coagulation event. Particle nucleation in the post flame zone, where chemi-ions are drastically reduced, produces uncharged particles.Considering the above charging processes, the charge fraction of the nucleation mode contains information on the location within the flame these particles were formed. The results show that in flames near the particle inception threshold, particles are charged close to the flame front and remain charged even late in the post flame zone. Furthermore, smaller particles undergo less charge neutralization by coagulation as they travel through the post flame zone than larger particles. A different scenario is observed in richer flames; the smaller particles eventually become uncharged, indicating that significant amounts of freshly nucleated particles in these flames are formed in the post flame zone. Whether nucleation preferentially occurs close to the flame front or persists into the post flame zone also depends on flame temperature.     

Convective heat transfer from a partially premixed impinging flame jet. Part I: Time-averaged results

Axial and radial profiles of time-averaged local heat fluxes of methane-air jet flames impinging normal to a cooled plate are reported, as functions of equivalence ratio, Reynolds number, and nozzle-plate spacing. Time-resolved behavior for these conditions is examined in the companion paper, Part II. Flame structure was studied visually and photographed. Both premixed and diffusion flame behavior was observed. Nozzle-stabilized flames revealed a stable, axisymmetric flame structure at nozzle-plate spacings less than 14 diameters. At greater nozzle-plate spacings, buoyancy-induced instabilities caused the flame to oscillate visibly. Lifted flames exhibited varied flame structures dependent upon the Reynolds number, equivalence ratio, and nozzle-plate spacing, stabilizing in the free jet, at the stagnation zone, or downstream in the wall jet. Local heat flux measurements made in the stagnation zone and along the plate adjacent to the wall jet flame revealed correlation of the local heat flux to the flame structure. Negative heat fluxes resulted from cool gases impinging on the hotter plate. The magnitude of positive heat fluxes depended on the proximity of the flame to the sensor surface, the rate of heat release, and the local molecular and turbulent transport.     

Effect of an inlet temperature disturbance on the propagation of methane-air premixed flames in small tubes

A flame stabilized in a tube is affected by the temperature disturbance and velocity profile at the inlet boundary. Thus, a multi-dimensional analysis is necessary near the flame. The deviation between one-dimensional and two-dimensional analyses near the flame was investigated quantitatively. The temperature profile in the radial direction was varied to investigate its effects on the propagation of methane-air premixed flames in small tubes. A numerical experiment with Navier-Stokes equations, an energy equation and species equations was conducted coupled with a single-step global-reaction model. Three different temperature profiles were examined for slip and no-slip wall boundary conditions. The effect of temperature profiles on the flame propagation velocity and flame shapes was not negligible depending on the magnitude of the temperature deviation and the tube diameter. This study evaluated a critical length scale of a computational domain or a thermal entrance length of a premixed flame over which the inlet temperature disturbance does not affect the flame characteristics.     

A study on flame structure and extinction in downstream interaction between lean premixed CH4-air and (50% H2 + 50% CO) syngas-air flames

Downstream interactions between lean premixed flames with mutually different fuels of (50% H₂ + 50% CO) and CH₄ are numerically investigated particularly on and near lean extinction limits in order to provide fundamental database for the design of cofiring burners with hydrocarbon and syngas under a retrofit concept. In the current study the anomalous combination of lean premixed flames is provided such that even a weaker CH₄-air flame temperature is higher than a stronger syngas-air flame temperature, and, based on a deficient reactant concept, the effective Lewis numbers Le_eff ≈ 1 for lean premixed (50% H₂ + 50% CO)-air mixture and Le_D < 1 for CH₄-air mixture. It is found that the interaction characteristics between lean premixed (50% H₂ + 50% CO)-air and CH₄-air flames are quite different from those between the same hydrocarbon flames. The lean extinction boundaries are of slanted shape, thereby indicating strong interactions. The upper extinction boundaries have negative flame speeds while the lower extinction boundaries have both negative and positive flame speeds. The results also show that the flame interaction characteristics do not follow the general tendency of Lewis number, which has been well described in interactions between the same hydrocarbon flames, but have the strong dependency of direct interaction factors such as flame temperature, the distance between two flames, and radical-sharing. Importance of chain carrier radicals such as H is also addressed in the downstream interactions between lean premixed (50% H₂ + 50% CO)-air and CH₄-air flames.     

A comparison of the heat transfer behaviors of biogas–H2 diffusion and premixed flames

An experimental study was conducted to investigate the influence of hydrogen addition on the heat transfer characteristics of a biogas (60%CH₄–40%CO₂) flame. Results show improved flame stability and higher flame temperature in the premixed flame upon hydrogen addition. Both temperature and burning speed are increased in 1.0 ≤ Ф ≤ 1.5. Comparison of the premixed and diffusion flames reveals that the former yields higher heat transfer than the latter, due to higher flame temperature and larger volume of hot gas in the premixed flame. The total heat transfer rates of the two flames show opposite trends with increasing level of hydrogen addition, which is explained by the different structures. In the premixed flame, the contact of large cool core with target plate configures the high-temperature flame zone to a radial location with larger distance from the stagnation point than that of the diffusion flame, contributing to its higher heat transfer rate.     

Transported PDF modeling of high-Reynolds-number premixed turbulent flames

The transported PDF approach, closed at the joint composition-enthalpy level, is applied to model premixed turbulent flames at a wide range of Reynolds numbers. The initial aim of the study is to establish the impact of closure approximations for the scalar dissipation rate upon the relationship between turbulence fluctuations and predicted turbulent burning velocities. The cases considered feature stoichiometric methane-air flames with the chemical source term extracted from a detailed chemistry simulation of the corresponding unstrained laminar flame. The transported PDF approach is subsequently combined with a systematically reduced C/H/O mechanism featuring 142 reactions and 14 solved and 15 steady-state species and applied to piloted premixed stoichiometric methane-air flames investigated experimentally by Chen et al. [Combust. Flame 107 (1996) 223-226]. The cases considered here feature Re=24,200 (flame F3) and 52,500 (flame F1) and Damköhler numbers approaching unity. The effects of variations in the time-scale ratio (2?C??8) and heat losses to the burner were investigated, along with the impact of an extended algebraic relationship for the scalar dissipation rate that accounts for small-scale properties. Comparisons with experimental data show that the modified Curl's model and the extended scalar dissipation-rate closure produce turbulent burning velocities in close agreement with measurements. The study further indicates that a closure at the joint scalar level combined with comprehensive chemistry has the potential to reproduce the detailed chemical structure of premixed turbulent flames. The importance of boundary conditions and comprehensive scalar statistics, including the scalar dissipation rate, is also emphasized by the study.     

Response to Ahmad R. Shouman and Walter Gill,equivalence of the criticality condition for both the semenov and the frank-kamentskii problems,Combust. Flame 58:81–86 (1984)

Recent advances in the mathematical study of unsteady laminar premixed flames have required the imposition of nearly constant flame temperature. This condition disallows investigations into such fundamentally important problems as flame propagation into thermal gradients and regions of varying equivalence ratio. Here a rational method of analysis is presented to derive asymptotic solutions for one-dimensional unsteady propagation of deflagration waves with O(1) flame temperature variations. The propagations of premixed flames down thermal and concentration gradients are studied as example problems.     

Kinetic modeling study of ethanol and dimethyl ether addition to premixed low-pressure propene–oxygen–argon flames

The chemical composition of flames of mixed hydrocarbon–oxygenate fuels was examined systematically for a series of laminar, premixed low-pressure propene–oxygen–argon flames blended with ethanol or dimethyl ether (DME). All flames were established at a carbon-to-oxygen ratio of C/O = 0.5 at 40 mbar. Propene was replaced incrementally by either additive, so that the entire range from pure propene to pure ethanol or pure DME was accessible. Experimental results have been reported previously (J. Wang et al., J. Chem. Phys. A 112 (2008) 9255–9265), including temperature profiles measured with laser-induced fluorescence (LIF) and quantitative mole fraction profiles for a large number of species obtained from molecular-beam mass spectrometry (MBMS), using electron ionization (EI) and vacuum-ultraviolet (VUV) photoionization (PI). The effects of oxygenate addition to the propene base flame were seen to result in interesting differences, especially regarding trends to form aldehydes. The entire flame series is now analyzed with a comprehensive kinetic model that combines the chemistries of propene, ethanol, and DME combustion. The flames of pure fuels are also compared with the predictions of different detailed mechanisms taken from the literature. Quantitative comparison of C₁- to C₆-species from this model with the measurements is provided. Major trends of propene replacement by the oxygenates are reproduced in quantitative agreement with the experiments, enabling a more detailed understanding of the combined reaction sequences in such fuel blends.     

A mechanistic evaluation of Soret diffusion in heptane/air flames

The influence of Soret diffusion on the structure and response of n-heptane/air flames is investigated numerically with detailed reaction mechanism and transport. Unstretched freely-propagating planar premixed flames as well as stretched counterflow premixed and diffusion flames are studied, with emphasis on the separate and combined Soret effects of heptane and its oppositely-directed decomposition species H₂ and H, as well as those of the major species O₂, N₂, CO₂ and H₂O. Results show that, in the unstretched case for which the flame temperature remains at its adiabatic value, Soret diffusion primarily affects the chemical kinetics embedded in the flame structure and the net effect is small; while in the stretched cases, its impact is mainly through those of heptane and the secondary fuel H₂ in modifying the flame temperature, with substantial effects, while H₂ and H also affect the chemical kinetics especially when the active reaction zone is localized. The dilution/enrichment of the reactant concentrations in the flame through the Soret diffusion of the major species O₂, N₂, CO₂ and H₂O further exert finite effects on the flame burning intensity.