Increase In Flame Temperature Research Articles

Analysis on premixed combustion of H2–CH4 mixed fuels for SiO2 particle preparation

The combustion of premixed H2 and CH4 fuels to prepare SiO2 nanoparticles was analyzed numerically. The commercial CFD-code Fluent was used to calculate the profiles of gas velocities, temperature, species concentrations and reaction rates for various process conditions in the premixed flame reactor. To understand the effect of CH4 addition to H2 fuel, the CH4 mole fraction was changed from 0% to 10%, while keeping the H2 concentration constant. With the increase of CH4 concentration, flame temperature increases, which will affect the properties of SiO2 particles prepared in premixed flame reactor. Using the data of gas temperatures and velocities extracted from Fluent, the trajectories and temperature histories of SiO2 particles were calculated inside the premixed flame reactor. This study shows that particles starting at different initial positions move inside the reactor with different particle trajectories and have different temperature histories, and the properties of SiO2 nanoparticles prepared in the premixed flame reactor will depend on those process variables significantly.

Characterization of a primary-swirled, high oxygen participation coal flame: Flame temperature, emissivity, NO, and burnout measurements

Measurements of flame temperature, emissivity, NO and carbon burnout were obtained for a high oxygen participation (47–70% O2 in CO2) oxy-coal burner in order to study the impact of oxygen location and CO2 flow rate in a tube-in-tube oxy-coal burner. Spatially averaged flame temperatures were measured in the range of 1930–2320K by two-color pyrometry. The addition of CO2 to the secondary oxidizer decreased flame temperature while small flow rates (up to 8.5kg/h) of O2 to the center tube increased flame temperature. Adding O2 to the center tube increased NO emission and initially improved burnout, but when center O2 flow rates increased above 8.54kg/h, burnout decreased. This was assumed to be caused by reduced residence time for entrained particles. Moderate swirl (15° swirl vane) improved most aspects of the burner by improving burnout and reducing NO, but high swirl (40° swirl vane) transported the coal radially outside of the secondary oxidizer flow and produced poor burnout. Increasing secondary CO2 or removing O2 from the center of the flame increased flame liftoff which decreased emissivity and soot formation but increased NO.

Simulation of nitrogen emissions in a premixed hydrogen flame stabilized on a low swirl burner

There is considerable interest in developing fuel-flexible, low emissions turbines for power generation. One approach is based on burning a variety of lean premixed fuels with relatively low flame temperatures. Such flames can be stabilized in a low swirl burner configuration, for example, using a variety of fuels such as pure hydrogen and hydrogen-seeded hydrocarbon mixtures. However, many hydrogen-rich fuels are thermodiffusively unstable and burn in cellular flame structures, which can have a significant impact on the local nitrogen chemistry. These cellular burning patterns are characterized by a local enhancement of fuel concentration and a corresponding increase in local flame temperature just downstream. In turn, these regions become sites for enhanced thermal NOx production. The structure of these cells, and their impact on the net emissions of a flame is influenced by the global flame stabilization mechanisms and by local turbulence properties. Here we investigate the role of thermodiffusive instabilities on NOx emissions in the context of a laboratory-scale low swirl burner fueled with a lean hydrogen–air mixture at atmospheric pressure. The simulations show how the cellular burning structures characteristic of lean premixed hydrogen combustion lead to local and global enhancements in the NOx emissions. We quantify the chemical pathways that lead to the formation of NO and N2O, and how they are enhanced within local regions of intense burning.

Radiation effects on flame stabilization on flat flame burners

This study investigates the impact of radiative heat transfer on the behavior of flat flame burners within the framework of a simplified one-dimensional model. Flat flame burners stabilize planar premixed flames downstream of a porous plug. Within this study, the porous plug is modeled as a thermally conducting, optically thick medium, allowing for both conductive and radiative heat transfer. Based on the simplified model, the impact of radiative heat exchange between the porous plug exit and the downstream environment is investigated. In “surface” combustion, flame stabilization occurs due to heat transfer between gas phase and porous solid. Results demonstrate that radiative heat transfer from a hot downstream environment to the porous plug significantly increases maximum attainable mass fluxes. For a cold downstream environment, plug properties do not affect the maximum supportable mass flux, although plug porosity and heat transfer between gas and solid have a significant impact on the “stand-off” distance between flame and plug exit. In addition, the model provides insight to a second “submerged” combustion mode, where the flame is stabilized within the porous plug of the burner. Here, increased flame temperatures lead to a dramatic increase of the maximum supportable mass flux. Overall, results show that radiative heat losses play a critical role in both combustion modes: in surface combustion, they are an important mode of heat dissipation, where they can prevent “flash-back” conditions with the flame moving into the porous matrix; in submerged combustion, they prevent flame stabilization close to inlet and exit faces and enable a “slow” solution branch that does not exist without radiative losses.

Modeling of soot particle size distributions in premixed stagnation flow flames

Soot formation in five laminar premixed ethylene flames was studied computationally in order to investigate the ability of a sectional model to reproduce measured soot particle size distributions (PSDs). The flames were modelled using a burner-stabilised stagnation flow configuration due to the influence of the sampling probe. In the vicinity of the probe, the residence time is relatively long and the temperature reduced. It was found that under such conditions the coagulation efficiency for small (<10nm) particles becomes important due to the combination of a relatively low nucleation rate with a significant collision frequency. In previous studies, the impact of the probe was only accounted for by shifting the position of the measured PSDs. A model for the collision efficiency was introduced with a particle size dependent interpolation between a lower limit, based on the Lennard–Jones potential, and an upper limit, based on collision stabilisation caused by the surrounding gas. The developed model is consistent with a low collision efficiency for PAH coagulation and explains the measured high number densities of small particles. The size of particles affected by a reduced collision efficiency were found to decrease with increasing flame temperature. The effect is consistent with temperature dependent internal carbonisation processes changing the polarisability of particles leading to a strengthening of attractive forces and enhanced collisional stabilisation. The calculated PSDs were found to be in reasonable agreement with measurements.

A comparison of the emission and impingement heat transfer of LPG–H2 and CH4–H2 premixed flames

Experiments were performed to add hydrogen to liquefied petroleum gas (LPG) and methane (CH4) to compare the emission and impingement heat transfer behaviors of the resultant LPG–H2–air and CH4–H2–air flames. Results show that as the mole fraction of hydrogen in the fuel mixture was increased from 0% to 50% at equivalence ratio of 1 and Reynolds number of 1500 for both flames, there is an increase in the laminar burning speed, flame temperature and NOx emission as well as a decrease in the CO emission. Also, as a result of the hydrogen addition and increased flame temperature, impingement heat transfer is enhanced. Comparison shows a more significant change in the laminar burning speed, temperature and CO/NOx emissions in the CH4 flames, indicating a stronger effect of hydrogen addition on a lighter hydrocarbon fuel. Comparison also shows that the CH4 flame at α = 0% has even better heat transfer than the LPG flame at α = 50%, because the longer CH4 flame configures a wider wall jet layer, which significantly increases the integrated heat transfer rate.

Simulations of normal and inverse laminar diffusion flames under oxygen enhancement and gravity variation

Steady-state global chemistry calculations for 20 different flames were carried out using an axisymmetric Computational Fluid Dynamics (CFD) code. Computational results for 16 flames were compared with flame images obtained at the NASA Glenn Research Center. The experimental flame data for these 16 flames were taken from Sunderland et al. [4] which included normal and inverse diffusion flames of ethane with varying oxidiser compositions (21, 30, 50, 100% O2 mole fraction in N2) stabilised on a 5.5 mm diameter burner. The test conditions of this reference resulted in highly convective inverse diffusion flames (Froude numbers of the order of 10) and buoyant normal diffusion flames (Froude numbers ∼0.1). Additionally, six flames were simulated to study the effect of oxygen enhancement on normal diffusion flames. The enhancement in oxygen resulted in increased flame temperatures and the presence of gravity led to increased gas velocities. The effect of gravity-variation and oxygen enhancement on flame shape and size of normal diffusion flames was far more pronounced than for inverse diffusion flames. For normal-diffusion flames, their flame-lengths decreased (1 to 2 times) and flames-widths increased (2 to 3 times) when going from earth-gravity to microgravity, and flame height decreased by five times when going from air to a pure oxygen environment.

NONLINEARITY EFFECT OF TRANSPORT PROPERTIES ON COMBUSTION OF VOLATILE ORGANIC PARTICLES

This paper presents the structure of laminar, one-dimensional, and steady-state flame propagation in a uniform cloud of volatile organic particles. In this model, it is assumed that particles vaporize to yield a gaseous fuel. In order to study the effects of thermal conductivity and thermal diffusion on combustion of organic particles, an asymptotic analysis, based on the large Zeldovich number or high rate of reaction, is used. In the preheat zone, the rate of chemical reaction is small, and transfer phenomena play a significant role in temperature and mass distributions. Therefore, according to the kinetic theory of gases, thermal conductivity (λ) and diffusion coefficient (D) are considered to be proportional to T1/2 and T3/2, respectively. Finally, flame characteristics for different equivalence ratios of solid mixture for different Lewis numbers are reported. Results show that both flame temperature and burning velocity increase with rise in the Lewis number.

Experimental correction of combustion gas properties of AN-based composite solid propellants used for turbo-pump starter

Solid propellant gas generators play a role as a turbo-pump starter in liquid propellant propulsion systems by supplying pressurized gas to power turbines for engine start. Among the required combustion gas properties provided by solid propellant gas generators, the combustion gas temperature should not exceed a certain temperature which may damage the turbine blades. For such purposes, phase stabilized ammonium nitrate (AN)-based propellants have been widely used with a low combustion temperature. However, gas generator propellants with ammonium nitrate have historically exhibited incomplete combustion resulting in increased flame temperatures differing significantly from equilibrium values. In consideration of design requirements, an engineering model of solid propellant gas generator was manufactured using the combustion gas properties calculated by a chemical equilibrium code and then hot-fire tests were performed. Procedures for the correction of T 0 , k and M w of the combustion gas from the experimental results are introduced and the following effects on the design of the solid propellant gas generator are presented. From the experimental correction of the combustion gas properties, it is found that the amount of the propellant could be reduced while providing the same amount of available power to the turbines and consequently, the size of the gas generator could also be decreased.

Nitrogen-Doped Carbon Nanotubes from Amine Flames

A novel approach has been developed for synthesizing nitrogen-doped carbon nanotubes (N-CNTs) from flames using liquid amines as fuels, such as isopropylamine, n-propylamine and n-butylamine, which not only created a high reaction temperature but also provided a source of C and N. The microstructure and morphologies of the N-CNTs were characterized by scanning and transmission electron microscopy, laser Raman spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. These N-CNTs were different from the conventional N-CNTs from a chemical vapor deposition process with primarily a 'pyridine-like' structure at N substitutions (one N atom only bonded to two C atoms). They were dominantly 'graphite-like' (one N atom substituted for C in graphite layers and bonded to three C atoms) with few C congruent N bonds due to the special formation conditions of high temperature and oxidative environment. It was found that: (1) amine fuel with a side chain structure was less suitable for preparation of orderly grown N-CNTs; (2) amine fuels with a higher N content generally introduced a larger quantity of N dopant; (3) the proportion of the 'graphite-like' N dopant decreased with increasing flame temperature, which provided a possibility for controllable N doping in N-CNTs from amine flames. The growth mechanism of the N-CNTs was also simulated and discussed.

Effect of O2/CO2 Ratio on Fuel-NOx Formation in Oxy-Coal Combustion

The purpose of this study is to clarify the fundamental features of NOx formation in oxy-coal combustion. The pure effects of various factors related to oxygen concentration, flame temperature and flame shape on NOx formation are investigated by experimental and theoretical analysis, resulting in the following conclusions: (1) In oxy-coal combustion (i.e., CO2/O2 atmosphere), the emission of fuel-NOx increases with O2 concentration. (2) The flame shape changes from a detached flame to an attached flame with the increase in flame temperature. (3) During oxy-coal combustion, the gasification reaction between CO2 and carbon on the surface of char particles occurs more vigorously than during combustion in air. (4) The single particle model for estimating NOx emission is applicable to the detached flame. This implies that in the case of the detached flame the pulverized coal burns with a structure of premixed flame.

Statistical Analysis of Effects of Nanoparticles on Hydrocarbon Pool Fire

The effects of pool diameters, height, fuel type, and presence of nanoparticles on flame temperature are tested. Nanoparticles were added and mixed with the fuel and its effect on flame spread time over the liquid fuel surface is studied. The results showed that pool diameter has the maximum effect on the increase of flame temperature. Nanoparticles reduce the flame temperature from 10 to 20% while flame spread time is increased 15% to 30% for kerosene and 6% to 26% for diesel oil. Experimental measurement of flame temperature distribution in a pool fire is carried out. Statistical analysis of flame temperature, flame spread time was made using Yates and ANOVA methods.

Photoluminescence of cubic and monoclinic Gd 2O 3:Eu phosphors prepared by flame spray pyrolysis

In this paper, europium-doped gadolinium phosphor, which is a potentially bifunctional material with both fluorescent and magnetic properties, has been prepared in a one-step procedure via flame spray pyrolysis, and its crystal structure, morphology, and PL intensity were investigated. All the prepared phosphors were submicron-sized with spherical shapes and either a pure cubic or pure monoclinic phase. In order to observe the effects of temperature on the crystal phases of the prepared phosphors, we applied a H 2 vs. N 2/O 2 diffusion flame, with the maximum flame temperature ranging from T max=1375 to 2050 K. The temperature profiles under various flame conditions are also reported herein to further elucidate the rapid synthesis process. The PL intensity in the cubic phase improved linearly with increasing flame temperature until the transition to a monoclinic phase. The peak of the photoluminescence(PL) spectrum from the phosphors prepared at T max=1733 K in the cubic phase was narrower and twice as strong as the peak of the PL spectrum from the phosphors prepared at T max=2050 K in the monoclinic phase. This paper provides important data showing the relationship between the synthesis temperature and the phase transition in Gd 2O 3:Eu in the continuous one-step use of flame spray pyrolysis.

Effect of O2/CO2 Ratio on Fuel-NOx Formation in Oxy-coal Combustion

The purpose of this study is to clarify the fundamental features of NOx formation in oxy-coal combustion. The pure effects of various factors related to oxygen concentration, flame temperature and flame pattern on NOx formation are investigated by experimental and theoretical analysis, resulting in the following conclusions: (1) In oxy-coal combustion (CO2/O2 atmosphere), the emission of fuel-NOx increases with O2 concentration. (2) The flame pattern changes from a detached flame to an attached flame with the increase in flame temperature. (3) During oxy-coal combustion, the gasification reaction between CO2 and carbon on the surface of char particles occurs more vigorously than during combustion in air. (4) The single particle model for estimating NOx emission is applicable to the detached flame pattern. This implies that in the case of the detached flame the pulverized coal burns with a structure of premixed flame.

化学発光分光法による乱流火炎構造の計測に関する基礎的研究(熱工学,内燃機関,動力など)

To examine temperature of the local reaction zone of turbulent premixed flames, dependence of the emission by OH, CH and C_2 radicals obtained from hydrocarbon-air premixed flames on flame temperature has been examined. To detect weak emissions of C_2 radical, both spectrometer and amplifies were newly developed to achieve the signal-noise ratio of the newly developed system is two-digit higher than that of the conventional system. Measurements were made at various equivalence ratios in laminar premixed methane/air and propane/air flames, which have different flame temperatures. The chemiluminescence intensity ratio of 515nm/470nm bans of C_2, C_2^*(0,0)/C_2^*(1,0), in both methane/air and propane/air flames has a high correlation with flame temperature. The chemiluminescence intensity ratio of C_2^*(0,0)/C_2^*(1,0) has two branches for the flame temperature. i.e., as it increases, flame temperature increases, has maximum near the stoichiometry and decreases. The error in C_2^*(0,0)/C_2^*(1,0) measurement was within 3% for propane flames, within 6% for methane flames. It has been shown in the present study that temperature of the local reaction zone of premixed flames can be estimated by measuring the chemiluminescence intensity ratio of C_2^*(0,0)/C_2^*(1,0).

In-Depth Experimental Study of Solid-Waste Destruction by High Temperature Air Combustion

The objective of this article is to investigate the application of high temperature air combustion in thermal destruction of solid waste. The secondary air was preheated at different temperatures and burned with low calorific value and high content of diluted inert gas released from solid waste. It was discovered that the combustion with the higher preheated secondary air temperature had affected an increase in average flame temperature, and reduction in supplementary fuel consumption, but caused higher NOX emission. The maximum percentage of energy saving and NOX emission increasing occurred in the case of preheated secondary air at 900°C with a value of 56% and 40%, respectively.

Novel Preparation and Physicochemical Characterization of a Nanocrystalline Cobalt Ferrite System

Well-defined CoFe2O4 nanoparticles with an average grain size of about 50 nm were successfully synthesized by a combustion method employing different ratios of fuel to cations within the range 0–2.67. The as-prepared powders were characterized and investigated by TG, XRD, IR, SEM, TEM and VSM methods, and via isopropanol conversion at 250–450 °C. The results showed that increasing the ratio between fuel and cations stimulated the formation of cobalt ferrite due to the increasing flame temperature. Changing the fuel/cation ratio brought about modifications in the structure, surface activity, selectivity and magnetic properties of the investigated solids. A fuel/cation ratio of 2 led to the formation of catalyst having a high activity (49%) and magnetization (77.54 emu/g). All the solids investigated behaved as dehydrogenation catalysts leading to the formation of acetone as the major product. The fuel/cation ratio did not alter the mechanism of dehydrogenation of isopropanol, but increased the concentration of active sites involved in the catalyzed reaction.

Experimental and Numerical Investigation of Fe(CO)5 Addition to a Laminar Premixed Hydrogen/Oxygen/Argon Flame

Abstract Low-pressure, lean, laminar, premixed hydrogen/oxygen/argon flames seeded with iron pentacarbonyl (35−170 ppm Fe(CO)5) were modeled with detailed chemistry and the results were compared to laser-induced fluorescence imaging measurements of iron atom concentration and gas-phase temperature. The model includes recent rate coefficients for the decomposition of iron pentacarbonyl and thermodynamic data. The simulated iron concentrations correspond well with the measurements with only minor discrepancies in the rise of the iron profiles at low Fe(CO)5 concentrations. In addition, it was shown that the mechanism is able to predict the effect of Fe(CO)5 on the flame speed also for lean conditions, where the model was not established yet. The major iron species, aside from atomic iron, in this flame are predicted to be FeOH and Fe(OH)2 with some FeO2 early in the flame. The observed increased flame temperatures in the presence of Fe(CO)5 are attributed to catalytic hydrogen recombination.

Effects of Lewis number and preferential diffusion on flame characteristics in 80%H 2/20%CO syngas counterflow diffusion flames diluted with He and Ar

Numerical study is conducted to grasp flame characteristics in H 2/CO syngas counterflow diffusion flames diluted with He and Ar. An effective fuel Lewis number, applicable to premixed burning regime and even to moderately stretched diffusion flames, is suggested through the comparison among fuel Lewis number, effective Lewis number, and effective fuel Lewis number. Flame characteristics with and without the suppression of the diffusivities of H, H 2, and He are compared in order to clarify the important role of preferential diffusion effects through them. It is found that the scarcity of H and He in reaction zone increases flame temperature whereas that of H 2 deteriorates flame temperature. Impact of preferential diffusion of H, H 2, and He in flame characteristics is also addressed to reaction pathways for the purpose of displaying chemical effects.

Chemical speciation of premixed ethylbenzene flames at the soot onset limit at various ( [formula omitted]) pairs

This work investigates the similarities/differences of one-dimensional, laminar, atmospheric pressure premixed ethylbenzene flames at their soot onset threshold ( ϕ critical ). The goals of this investigation are to contrast the entire temperature profiles of the flames at various critical equivalence ratios, ϕ critical , and to report on hydrocarbon specie profiles. Liquid ethylbenzene was pre-vaporized in nitrogen, blended with an oxygen–nitrogen mixture and, upon ignition, premixed flat flames were stabilized over a burner at atmospheric pressure. The C/O ratio was regulated and simultaneously the temperature profile was adjusted by varying the heat lost from the flame to the burner, to obtain visually similar flames, all at their apparent soot onset limit. Temperature profiles of five such flames were determined with thermocouples. The results indicate that as the axial temperature profiles in the flames rose, ϕ critical ((C/O) critical) also increased. Sampling was performed at various heights along the axes of three of these flames ( ϕ c 1 = 1.68 , ϕ c 2 = 1.74 and ϕ c 3 = 1.83 ) to monitor changes in chemical speciation. The mole fractions of CO increased in the order of increasing equivalence ratio and hence increasing flame temperature. The CO 2 mole fractions of the three flames were similar. However, the mole fractions of light hydrocarbons and polycyclic aromatic hydrocarbons (PAH), which are suspected soot precursors, decreased in the order of increasing equivalence ratio and hence increasing flame temperature. Whereas direct measurements of particulate matter in the flame were not made in this work, these observations on soot precursors along with theoretical flame emissivity considerations suggest that particulate loadings at the sooting limit also decreased in the order of increasing equivalence ratio and hence increasing flame temperature.