Equilibrium Flame Temperature Research Articles

Characterization of a Nitrogen Diluted Hydrogen Diffusion Flame for Model Validation

Dilute hydrogen diffusion flames have been considered as a gas turbine combustion strategy that provides relatively low levels of NOx emissions for application in integrated gasification combined cycle power generation. These flames also represent a challenging environment for computational modeling efforts due to the complexity of molecular transport effects, turbulence-chemistry interaction, and near extinction flame conditions. In order to provide data for validation of computational modeling efforts, measurements of major species concentration and flame temperature were made in such a flame using spontaneous Raman scattering. Experimental results demonstrate the importance of differential species diffusion, which occurs due to the disparity between diffusion characteristics of hydrogen and nitrogen. Additionally, the flame temperatures observed were quite low relative to the equilibrium flame temperature, due to flame strain. This confirms the fact that suppression of the thermal mechanism of NOx formation plays a significant role in reducing NOx emissions from this type of flame.

Exploring the Effects of Nanostructured Particles on Liquid Nitromethane Combustion

Monopropellants consisting of liquid nitromethane and high-surface-area particles of silicon- and aluminum-based oxides were examined to determine the effects of low concentrations of nanostructured materials on deflagration processes. The combustion rates were characterized by measuring linear burning rates in a large pressure vessel filled with argon. Results showed that nitromethane burning rates may be increased at lower pressures with dilute additions of particles. Increases in nitromethane burning rates of greater than 50 % were found with less than 1.0 wt% of particle addition at a nominal pressure of 5.24 MPa. The particle additions were estimated to have only small effects on the equilibrium flame temperatures, density, viscosity, and specific heat. Nitromethane with particle additives displayed a lower burning-rate equation pressure exponent (i.e., reduced pressure sensitivity), which was inversely proportional to particle concentration. Above a pressure of approximately 9 MPa, up to the maximum test pressure (∼14 MPa), the particle additives did not affect the nitromethane burning rate. Condensed-phase temperature profiles of the deflagrating fluid, surface tension, and fluid thermal conductivities were measured in order to elucidate some of the mechanisms causing enhanced burning rates at lower pressures.

Study of Extinction Properties in Dimethyl Ether Diffusion Flames (1st Report, Effects of Equilibrium Flame Temperature and Flame Structure on Extinction Properties)

Recently many issues related to automotive engineering, such as the environment and energy problems have gained attention. These social-related issues are giving impetus to the adoption of urgent measures such as alternative fuels and new combustion techniques for the internal combustion engines. In this context, dimethyl ether is thought to be a potential alternative to diesel fuel, as it has lower overall pollutant emissions and better economy. On the other hand, oxygen-enriched combustion has been gaining acceptance as an environmental friendly combustion, because this can reduce soot and NOx emission. The purposes of this paper are to examine the extinction properties of DME flames and the feasibility concerning the application of oxygen-enriched combustion to DME flames using a counterflow burner. Firstly, the strain rate at extinction is measured for dimethyl ether and propane flames as a function of equilibrium flame temperature under the same flame structure. Secondly, the strain rate at extinction is also measured under the condition that the flame structures are changed, but the equilibrium flame temperatures are not varied. In addition, numerical calculations are also performed using detailed chemical-kinetic mechanism to obtain value for extinction and compared with measurements.

2214 DME拡散火炎の消炎特性に及ぼす不活性ガス種の影響(J16-1 燃料多様化と燃焼・化学反応制御(1),ジョイントセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Experimental studies are conducted on extinction of non-premixed dimethyl ether (DME) flames stabilized in the counterflow configuration. Studies are carried out by injecting a fuel stream made up of fuel and inert gas (N_2 or CO_2 or Ar or He) from one duct an oxidizer stream made up ofO_2 and inert gas (N_2 or CO_2 Ar or He) from the other duct. Critical conditions of extinction are measured by increasing the flow rate of the counterdowing streams until the flame extinguishes. Numerical studies are also performed using detailed chemistry at conditions corresponding to those used in the experiments and compared with measurements. The present study highlights the examination of the influence of equilibrium flame temperature, flame structure and kind of mert gas on extinction properties of DME. Results are compared and discussed in terms of Lewis number effect.

Study of Extinction Properties in Dimethyl Ether Diffusion Flames (2nd Report, Effects of Kind of Inert Gases on Extinction Properties)

Experimental studies are conducted on extinction of non-premixed dimethyl ether (DME) flames stabilized in the counterflow configuration. Studies are carried out by injecting a fuel stream made up of fuel and inert gas (N2 or CO2 or Ar or He) from one duct an oxidizer stream made up of O2 and inert gas (N2 or CO2 or Ar or He) from the other duct. Critical conditions of extinction are measured by increasing the flow rate of the counterflowing streams until the flame extinguishes. Numerical studies are also performed using detailed chemistry at conditions corresponding to those used in the experiments and compared with measurements. The present study highlights the examination of the influence of equilibrium flame temperature, flame structure and kind of inert gas on extinction properties of DME. Results are compared and discussed in terms of Lewis number effect.

Uniform Magnetic Fields and Equilibrium Flame Temperatures

The impact of a constant uniform magnetic field on equilibrium flame temperature under constant volume conditions was examined. The equilibrium combustion compositions were found using an expression for Helmholtz free energy that included a magnetic-field contribution. Changes in the Helmholtz free energy for a mixture of paramagnetic and diamagnetic ideal gases were minimized using the method of Lagrange multipliers. The magnetic susceptibility for the paramagnetic gases was calculated using the Curie law. Equilibrium flame temperatures were obtained from the first law of thermodynamics for a closed adiabatic system with no work interactions. A model reaction of methane in air was used to quantitatively examine changes in the equilibrium flame temperature and product mole fractions as a function of volume and magnetic induction. Equilibrium flame temperature and product mole fractions were plotted as a function of magnetic-field strength and volume for all product species. The results indicate that with the increase in magnetic-field strength, the equilibrium flame temperature increases for all volumes considered.

Emission spectroscopy of flame fronts in aluminum suspensions

Spatially resolved emission spectra from Bunsen-type flames stabilized in aluminum suspensions in air and oxygen–argon/helium mixtures were obtained using a mechanical-optical scanning system. A low resolution (1.5 nm) spectrometer was used to acquire the broad spectra over the 350–1000 nm range, and a high-resolution (0.04 nm) instrument was used for observation of AlO molecular bands and non-ionized atomic aluminum. The temperature of condensed phase emitters in the flame was derived using polychromatic fitting of the continuum spectra to Planck’s law. AlO temperature was found by fitting of the theoretically calculated shape of the band to experimental data. Peak temperatures of the condensed emitters were found to be approximately 3250 K in aluminum-air flames and approximately 3350 K for oxygen–argon/helium flames. Temperatures derived from AlO spectra coincide with the temperature of the condensed emitters with measurement accuracy and are only 100–200 °C lower than the computed equilibrium flame temperatures. The radial distribution of the temperature profile of the continuous emitters was found via Abel deconvolution and recovered the double-front structure of the Bunsen flame cone, with the outer flame being attributed to a diffusion flame of the fuel-rich products with ambient air. The observation of atomic aluminum lines seen in emission from the outer flame edge and partial self-absorption from the inner flame confirms the structure associated with the double-front structure. The implications of these results for the regime of particle combustion in a dust flame are discussed.

Aspects of laminar premixed flame extinction limits

Numerical methods have been used to examine (1) the combined effects of stretch and upstream heatloss on the properties of a stoichiometric, laminar, pre-mixed methane-air flame, and (2) the combined effects of stretch and an extended, radiative heat loss on the properties of a fuel-lean methane-air flame, equivalence ratio Σ=0.57. Appropriate opposed flow configurations were investigated in both cases. In the first case, fresh gas was discharged towards the stagnation plane from identical, coaxial, plug flow nozzles maintained at constant temperature and separated by a fixed distance. Continuation methods were used in tandem with Newton solutions to study the flame behaviour as the stretch rate (or nozzle inlet velocity) was varied. High and low stretch extinction limits were observed as positions of vertical tangency on the steady-state solution curves of flame property vs stretch rate. The complete series of solutions between these positions formed a limit cycle with both stable and unstable branches. The chemical behaviour at the limits and along the unstable branch is discussed. The effect of radiative losses on the fuel-lean flame was studied in the unburnt-to-unburnt (UTU)configuration as above, but also in the unburnt-to-burnt (UTB) configuration where the ignited unburnt stream is opposed by a stream of its own equilibrium combustion products. If the flame is adiabatic, the product stream is input at the adiabatic equilibrium flame temperature. Such a system shows no abrupt extinction limits. On the other hand, if the radiative “sink” temperature is below some critical value, then, irrespective of the magnitude of the heat loss rate on a volumetric basis, the unburnt-to-burnt system will show abrupt extinctions if the stretch rate is increased sufficiently. Because of extremely slow numerical convergence, the numerical investigation of these limits for the methane-air system is not yet complete. However, such an investigation is in progress, and this combination of stretch and radiative loss effects is clearly of importance in relation to behaviour at composition limits of flammability.

Effects of pressure on structure and extinction of tubular flame

A numerical study on effects of pressure on structure and extinction of a tubular flame was made. The adopted approach was the detailed kinetics theory combined with the exact similarity solution for the flow field. The calculation was made for stoichiometric methane-air mixture, and the structure and the extinction limits were determined for pressures ranging from 1 to 8 atm by using the so-called C1 chemistry. It was found that at elevated pressures the flame radius is decreased because of the decrease in the burning velocity, while the reaction zone width is decreased due to the decelerated transport properties. The flame temperature is increased, approaching the equilibrium flame temperature due to the accelerated reaction rates. The extinction flame temperature increases, whereas the extinction flame radius decreases with pressure, with a minimum radius of around 0.2 mm at the highest pressure of 8 atm. The critical strain rate at extinction limit increases with pressure initially, reaching a maximum at around 2.7 atm and then decreasing for higher pressures. It appears that this behavior is closely related to the effects of curvature on extinction, which can be explained in terms of the simplified asymptotic analysis.

Correlation of lean blowoff in an annular combustor

Introduction F UEL effects on limits of lean blowoff and spark ignition and gas turbine engine emissions or combustor efficiency can be examined with characteristic time models. As these models continue to be validated for engines burning alternative fuels, their potential as interim design tools is demonstrated. Combustor geometry, inlet conditions, and fuel injector properties are also model parameters. The lean blowoff model was developed by Plee and Mellor for simple geometries and was extended to can-type combustors by Plee and Leonard and Mellor. This Note extends the model further to lean blowoff limits in the GE J85 annular combustor. The spark ignition and lean blowoff data for the J85 were taken by Oiler et al. The ignition data have already been correlated by Naegeli et al. The blowoff data examined here were taken at the same inlet conditions as the ignition data. The fuels, operating conditions, and blowoff equivalence ratios are listed in Table 1. The stoichiometric adiabatic flame temperatures 7^=1 were calculated at Southwest Research Institute using an in-house constant pressure equilibrium flame temperature program.

An Experimentally Verified NOx Prediction Algorithm Incorporating the Effects of Steam Injection

A Perfectly Stirred Reactor model, kinetic rate data, and equilibrium adiabatic flame temperatures have been incorporated into a closed form NOx prediction algorithm. The algorithm accounts for combustor inlet parameters and steam injection. Combining this model with the operating map for the gas turbine allows prediction of NOx with variation in parameters such as inlet guide vane angle, ambient temperature, and load. If the actual values of the model input variables are measured, real time prediction of NOx emissions may be generated using a microcomputer. This signal may then be used as an input to NOx abatement systems such as Selective Catalytic Reduction. The semitheoretical technique is based upon the extended Zeldovich chain reaction kinetics for the production of NOx in the post-flame zone. The temperature and concentration of major product species in the post-flame zone are taken to be those appropriate to equilibrium. Steam injection and inlet effects enter the model through their influence on the abiabatic equilibrium flame temperature. Mixing effects are accounted for empirically.

A study of ClO 2C 2H 2 flames at low pressures

Flammability limits of ClO 2C 2H 2 mixtures initiated by an electric spark have been determined in a spherical vessel. Premixed ClO 2C 2H 2 flames stabilized on a low pressure burner exhibited a complex structure. The maximum burning velocity observed for the equimolecular mixture was S u = 900 cm/sec at P = 50 Torr. Many bands or lines have been recorded in the emission flame spectrum: OH, CH, C 2, C Cl, Cl 2, C and cool flame bands near the upper limit. Rotational temperatures of CH have been determined and compared with thermocouple measurements and theoretical equilibrium flame temperatures. Composition profiles of stable species have been obtained through the flame front by means of a quartz probe and mass spectrometric analysis. Reaction mechanism and elementary reactions, which could be important in the propagation of ClO 2C 2H 2 flames, are discussed.

Fluorocarbon combustion studies V—trifluorochloromethane- and trifluorobromomethane-fluorine flames

Burning velocities in a flat flame deflagration tube were measured, and the behaviour of flames was observed in CF 3Cl F 2 mixtures and in CF 3Br F 2 mixtures diluted with Ar and He. Rich CF 3Cl F 2 flames tend to become cellular. The cellular flames can propagate through a tube which quenches flat flames. The rich limit is near the region of maximum adiabatic equilibrium flame temperature. CF 3Br F 2 mixtures are highly reactive at room temperature. Lean mixtures explode spontaneously. Very rich mixtures propagate unusual dendritic flames. The burning velocities observed with highly diluted CF 3Br F 2 flames are proportional to the square roots of the thermal conductivities of the diluents.

Thermochemical equilibrium in hydrocarbon-oxygen reactions involving polyatomic forms of carbon

Equilibrium composition and flame temperatures have been calculated for the reaction of acetylene and ethylene with oxygen over a range of mixture ratios. Emphasis has been placed on the determination of conditions necessary for the formation of solid carbon in the products. The influence of the presence of long-chain carbon molecules on the mixture ratio and temperature required for carbon deposition has been investigated. Premixed flames of acetylene and ethylene with oxygen have been studied experimentally to determine the correlation between theoretical calculations and actual combustion reactions. Calculations indicate that, for acetylene, carbon begins to appear in the condensed form at an equivalence ratio (actual fuel oxygen ratio divided by the stoichiometry ratio) of approximately 2·7 when the only forms of carbon vapour considered are monatomic and diatomic. The equilibrium flame temperature is 3 275°K. If longer chain forms of carbon vapour are considered (consistent with a triple point pressure of about 100 atm 4 000°K) then carbon formation begins at an equivalence ratio of 3·37 and the flame temperature would be 3095°K. This difference is not so pronounced with the other hydrocarbons considered. Experimentally, conclusive evidence of carbon formation was apparent at an equivalence ratio of 2·9. Based upon the property data used, the experimental evidence would indicate the presence of carbon molecules containing up to five atoms of carbon. Of these forms the odd number species probably redominate.

Absorption Spectra of Flat Flames using a Multiple-Reflexion Technique

MANY previous attempts to detect absorption bands of intermediate oxidation or pyrolysis products in premixed (Bunsen-type) flames have failed. There are several difficulties in studying absorption spectra of flames; the reaction zone where intermediate products may be expected is usually extremely thin and often curved; it is necessary to have a background source the brightness of which exceeds that corresponding to the effective excitation temperature in the flame (which may sometimes be higher than the equilibrium flame temperature); the light beam may be thrown out of the flame by refraction effects; and the concentration of the absorbing species may be low because of their instability or reactivity at high temperatures, thus necessitating long path-length for their detection. To study a thin reaction zone, even if flat, it is necessary to restrict the light to a narrow beam of very small angular aperture.

The Application of Radiation Measurement Techniques to the Determination of Gas Temperatures in Liquid Propellant Flames

Radiation temperature measurements were made throughout the flame developed within an open-tube combustor using liquid oxygen and a heptane-turpentine mixture as the reactants. The temperature measurement technique applied was the two-color radiation method. Observed absolute temperature measurements did not agree with theoretical equilibrium flame temperatures in most cases. The results can be explained by the existence of one, or a combination of three conditions within the chamber: (a) gas temperature stratification; (b) lack of thermodynamic equilibrium; and (c) optical effects such as selective absorption and scattering of light. These conditions limit measurements to qualitative observations concerning absolute temperatures as a function of flame position and time. In general, gas stratification effects were found to become more pronounced as the flame was made increasingly fuel rich. Flame temperatures were found to vary inversely with the radiation intensity, and fluctuating temperatures were found to exist at a frequency compatible with an acoustical mode of the chamber, even when sound oscillations of the same frequency were not apparent. Very high frequency oscillations of tempera­ ture, emissivity, and radiation were also found.