Showing papers in "Combustion and Flame in 2003"

Two-stage ignition in HCCI combustion and HCCI control by fuels and additives

Abstract: A Rapid Compression Machine (RCM) has been used to study the effects of fuel structure and additives on the Homogeneous Charge Compression Ignition (HCCI) of pure hydrocarbon fuels and mixtures under well-determined conditions. Such information is needed for understanding ignition delays and burning rates in HCCI engines, and “knock” in spark-ignition engines. It is also valuable for validating basic chemical kinetic models of hydrocarbon oxidation. The pure fuels used in the study include: paraffins (n-heptane, iso-octane), cyclic paraffins (cyclohexane, methylcyclohexane), olefins (1-heptene, 2-heptene, 3-heptene), cyclic olefins (cyclohexene, 1,3-cyclohexadiene), and an aromatic hydrocarbon (toluene). The additives were 2-ethyl-hexyl-nitrate and di-tertiary-butyl-peroxide. It was found that fuels which contained the structure -CH2-CH2-CH2- showed two-stage ignition with relatively short ignition delays and that the ignition delay depended strongly on the energy released during the first-stage. For primary reference fuel mixtures (n-heptane + iso-octane), the ignition delay depended only on the molar ratio of n-heptane to oxygen and was independent of the octane number (percent iso-octane). On the other hand, the burn rate depended on both these parameters, which uniquely determine the equivalence ratio. When additives were included in the air/fuel mixtures, the ignition delay was reduced but the burn rate was not affected. These results indicate that for HCCI combustion, the ignition delay and the burn rate can be independently controlled using various fuel mixtures and additives.

Soot oxidation: dependence upon initial nanostructure

Abstract: Although the relation between carbon structure and reactivity is well-known from thermal and oxidative studies of coal, char, and graphite, the relation for soot remains unstudied. In this article, the dependence of the soot oxidation rate upon the length and curvature of the graphene segments, which depicts the nanostructure, is shown. Reflecting different ratios of edge to basal plane sites or amounts of ring strain imposed by curvature, burnout rates are found to differ by greater than 400% for the soots studied here. Surprisingly, the different soot nanostructures are readily produced by using different fuels and pyrolysis conditions.

A unified model for the prediction of laminar flame transfer functions: comparisons between conical and V-flame dynamics

Abstract: Transfer functions of premixed laminar flames submitted to incident flow perturbations are envisaged and a unified model is derived analytically. This model, based on a linearization of the G-equation for an inclined flame, includes convective effects of the flow modulations propagating upstream of the flame. It is shown that the flame dynamics is governed by two relevant parameters, a reduced frequency, ω∗, and the ratio of the flame burning velocity to the mean flow velocity, SL/ῡ, or equivalently the flame angle α with respect to the flow direction. In the limit of low driving frequencies, the flame motion is only controlled by ω∗ and the unified model reduces to previous kinematic formulations derived for rim stabilized conical flames and V-flames anchored on a central rod. Flame transfer functions for these flame geometries with the different velocity models proposed are derived and limiting cases are examined. In the conical flame case, the low-frequency model gives a good approximation of the gain, but only a fair approximation of the phase. Convective effects are shown to induce an increasing phase lag, while low-frequency models predict a saturation phenomenon. The convective model derived in this article improves results for the gain and the phase which agree with numerical simulations and experiments. It is shown in particular that 1) the correct transfer function phase trend is retrieved and depends on the flame angle α; 2) the reduced cut-off frequency corresponds to a situation where the convective wavelength along the flame front λ = (ῡ cos α)/f equals the flame length L; and 3) the flame response is weakly affected by the amplitude of these perturbations. In the V-flame case, the low-frequency model yields a good approximation of the phase but does not feature gain values in excess of one found in the simulations. This behavior is correctly predicted by the convective model and is shown to depend on the flame angle α. A V-flame behaves as an amplifier in a certain range of frequencies. It is shown that these types of flames are more susceptible to combustion instabilities than conical flames. The V-flame response is also shown to strongly depend on the amplitude of the fluctuations even for moderate perturbation levels.

A reduced chemical kinetic model for HCCI combustion of primary reference fuels in a rapid compression machine

Abstract: A model for the Homogeneous Charge Compression Ignition (HCCI) of Primary Reference Fuels (PRFs) in a Rapid Compression Machine (RCM) has been developed. A reduced chemical kinetic model that included 32 species and 55 reactions was used and the affect of wall heat transfer on the temperature of the adiabatic core gas was taken into account by adding the displacement volume of the laminar boundary layer to the cylinder volume. A simple interaction between n-heptane and iso-octane was also included. The results showed the well-known two-stage ignition characteristics of heavy hydrocarbons, which involve low and high temperature cycles followed by a branched chain explosion. The first stage energy release decreases and the ignition delay increases nonlinearly with increasing octane number and decreasing the initial pressure. The energy release rate and total energy released were determined primarily by the rate of CO oxidation during the explosive phase following the ignition delay. The model reproduced the pressure curves obtained in the RCM experiments over a wide range of conditions remarkably well and was very sensitive to the fuel structure, the mixture composition and the initial temperature and pressure. Thus, the model can be easily adapted for predicting “knock” in spark-ignition engines and ignition-delays and burning rates in HCCI engines.

Analysis of non-adiabatic heat-recirculating combustors

Abstract: A simple first-principles model of counter current heat-recirculating combustors is developed, including the effects of heat transfer from the product gas stream to the reactant stream, heat loss to ambient, and heat conduction in the streamwise direction through the dividing wall (and heat transfer surface) between the reactant and product streams. It is shown that streamwise conduction through the wall has a major effect on the operating limits of the combustor, especially at small dimensionless mass fluxes (M) or Reynolds numbers that would be characteristic of microscale devices. In particular, if this conduction is neglected, there is no small-M extinction limit because smaller M leads to larger heat recirculation and longer residence times that overcome heat loss if M is sufficiently small. In contrast, even a small effect of conduction along this surface leads to significantly higher minimum M. Comparison is made with an alternative configuration of a flame stabilized at the exit of a tube, where heat recirculation occurs via conduction through tube wall; it is found that the counter-current exchanger configuration provides superior performance under similar operating conditions. Implications for microscale combustion are discussed.

Uncertainty quantification in reacting-flow simulations through non-intrusive spectral projection

Abstract: A spectral formalism has been developed for the “non-intrusive” analysis of parametric uncertainty in reacting-flow systems. In comparison to conventional Monte Carlo analysis, this method quantifies the extent, dependence, and propagation of uncertainty through the model system and allows the correlation of uncertainties in specific parameters to the resulting uncertainty in detailed flame structure. For the homogeneous ignition chemistry of a hydrogen oxidation mechanism in supercritical water, spectral projection enhances existing Monte Carlo methods, adding detailed sensitivity information to uncertainty analysis and relating uncertainty propagation to reaction chemistry. For 1-D premixed flame calculations, the method quantifies the effect of each uncertain parameter on total uncertainty and flame structure, and localizes the effects of specific parameters within the flame itself. In both 0-D and 1-D examples, it is clear that known empirical uncertainties in model parameters may result in large uncertainties in the final output. This has important consequences for the development and evaluation of combustion models. This spectral formalism may be extended to multidimensional systems and can be used to develop more efficient “intrusive” reformulations of the governing equations to build uncertainty analysis directly into reacting flow simulations.

Modes of reaction front propagation from hot spots

Abstract: The results of computations with detailed chemical kinetic schemes for the autoignition of stoichiometric H2-CO-air and H2-air mixtures at high pressure and high temperature are reported, with and without a single hot spot. Autoignition delay and excitation times first are computed in zero-dimensional, homogeneous mixture, simulations. Spherical hot spots of three different radii are then studied, for a range of temperature differences between the centre of the hot spot and the surrounding mixture. The effects of the resulting localised initial temperature gradients on the propagation modes of the ensuing reaction waves are examined, with particular regard to possible transitions to a developing detonation. Five modes of reaction front propagation are identified and demonstrated. One mode involves normal flame deflagration, the other four involve different types of hot spot autoignition. These modes depend upon the value of the initial hot spot temperature gradient normalised by the critical temperature gradient for a developing detonation. The latter is conveniently obtained from the homogeneous computations. Upper and lower limits of this normalised temperature gradient, ξ, are observed for a developing detonation. The bounds for this also depend upon the ratio of the hot spot acoustic time to the heat release rate excitation time. A tentative first attempt is described to quantify the bounds for all the modes, in terms of the two dimensionless groups.

The chemical effect of CO2 replacement of N2 in air on the burning velocity of CH4 and H2 premixed flames

Abstract: When CO2 is added to air or is used to replace N2 in air, it is anticipated that the burning velocity of fresh fuel mixtures may be affected through the following three mechanisms: i) the variation of the transport and thermal properties of the mixture, ii) the possible direct chemical effect of CO2, and iii) the enhanced radiation transfer by CO2. Experimental measurements of the burning velocity of CH4/O2/ CO2 mixtures at various equivalence ratios and pressures have been conducted by Zhu et al. [1] using double flames in the counterflow configuration. The radiative effect of CO2 on the burning velocity of CH4/O2/N2/CO2 mixtures has been recently studied by Ju et al. [2] and Ruan et al. [3]. Moreover, it has also been pointed out in several studies that CO2 is not inert but directly participates in chemical reactions primarily through CO OH 7 CO2 H [4-6]. The objective of this study is to numerically investigate the chemical effects of CO2 replacement of N2 in air on the burning velocity of lean to stoichiometric CH4/O2/N2/ CO2 and H2/O2/N2/CO2 mixtures at 1 atm.

Measurement and numerical simulation of soot particle size distribution functions in a laminar premixed ethylene-oxygen-argon flame

Abstract: Spatially resolved measurement of the soot particle size distribution function (PSDF) was made in a laminar premixed ethylene-argon-oxygen flame (φ = 2.07) using a scanning mobility particle sizer. The emphasis of the study was to follow the evolution of the PSDF from the onset of particle inception to particle mass growth. At the onset of soot inception, the PSDF was found to follow a power-law dependence on particle diameter. The PSDF becomes bimodal at larger height above the burner surface, and remains bimodal throughout the flame. Numerical simulation using a kinetic model proposed previously and a stochastic approach to solve aerosol dynamics equations again showed a bimodal PSDF. Further analysis revealed that bimodality is intrinsic to an aerosol process involving particle-particle coagulation and particle nucleation dominated by monomer dimerization.

Nanosecond gas discharge ignition of H2− and CH4− containing mixtures

Abstract: An analysis of the ignition of H2− and CH4− containing mixtures at high temperatures under the action of a nanosecond high-voltage discharge has been performed both numerically and experimentally for a wide range of parameters A comparison of the equilibrium and nonequilibrium excitation was performed The preliminary numerical analysis of ignition efficiency helped to plan experimental investigations of the initiation of the ignition by nanosecond discharge at high temperatures A novel experimental scheme for the investigation of ignition delay at high temperatures under the action of a high-voltage nanosecond discharge has been developed Electrical parameters on a nanosecond time scale and the ignition processes on a microsecond time scale were investigated Ignition delays for different mixtures, gas pressures and temperatures were obtained experimentally The dependence of the ignition delay upon temperature, high voltage amplitude, and the energy release into the discharge was determined experimentally for H2-O2, H2-air and CH4-air mixtures diluted with argon or helium Obtained data were compared with the results of numerical calculations

Kinetic modeling of the interactions between NO and hydrocarbons in the oxidation of hydrocarbons at low temperatures

Abstract: A general and detailed chemical kinetic model has been developed and tested to investigate the interaction between NO and hydrocarbons during the oxidation of a hydrocarbon at low temperatures. The model describes the influence of NO and was validated through comparison with several different experimental data sets for various temperatures, stoichiometries and hydrocarbon fuels. The good agreement observed across the whole investigation range confirms the validity of the kinetic assumptions and the reliability of the model. The effect of NO on the oxidation of hydrocarbons and the influence of hydrocarbons on the conversion of NO to NO2 are discussed. The kinetic scheme also refers to higher temperatures, as typical of reburning, but to reduce the breadth of the work the paper is focused on low temperature interactions. Nevertheless, the paper presents the complete set of reactions in the nitrogen submechanism.

Detailed Analysis of the Heat-Flux Method for Measuring Burning Velocities

Abstract: The heat flux method for determining the adiabatic burning velocity of gaseous mixtures of fuel and oxidizer and also producing well-defined reference flames is described in detail. Practical aspects of the heat flux method are discussed, especially the construction of the burner and attachment of thermocouples. An analysis is given of possible uncertainties and ways of correcting shortcomings. Conclusions from this analysis are applied to a typical measurement. The results are compared to other published results, including those from other often used methods, such as those with counter flow and closed vessels. For methane and air, a peak value of SL = 37.2 ± 0.5 cm/s was found.

Thermogravimetric analysis of soot emitted by a modern diesel engine run on catalyst-doped fuel

Abstract: Understanding the mechanisms that affect catalytic activity in porous ceramic diesel particulate filters (DPF) at the temperature range 200 to 400°C is important for the successful modeling of the initiation and evolution of catalytic regeneration by use of fuel additives This refers not only to the dry carbon particulate, but also to the volatile hydrocarbons adsorbed on it In this paper, a detailed analysis of the hydrocarbon adsorption-desorption and oxidation behavior of diesel particulate emitted by a modern diesel engine and collected on a SiC diesel filter is performed by use of thermogravimetric and differential scanning calorimetry analysis (TGA-DSC) Non-isothermal tests were performed with samples collected directly from a ceramic filter connected to the exhaust system of the diesel engine running under low and medium speed and load operating conditions with and without fuel additive Fuel additive concentration was varied to investigate its effect on the soot oxidation behavior Based on the TGA data, the kinetic parameters of the soot oxidation reaction were calculated The effect of volatile adsorbed hydrocarbons on the soot oxidation reaction was evaluated by comparing the calculated activation energies for samples collected from the center and the periphery of the filter at various exhaust temperatures prevailing at filter loading phase In particular it was seen that the catalytic activity of the fuel additive is enhanced by the presence of the volatile organic components

Direct numerical simulation of autoignition in non- homogeneous hydrogen-air mixtures

Abstract: The autoignition of spatially non-homogeneous hydrogen-air mixtures in 2-D random turbulence and mixture fraction fields is studied using the Direct Numerical Simulation (DNS) approach coupled with detailed kinetics. The coupling between chemistry and the unsteady scalar dissipation rate field is investigated over a wide range of different autoignition scenarios. The simulations show that autoignition is initiated at discrete spatially localized sites, referred to as kernels, by radical build-up in high-temperature, fuel-lean mixtures, and at relatively low dissipation rates. Detailed analysis of the dominant chemistry and the relative roles of reaction and diffusion is implemented by tracking the evolution of four representative kernels that characterize the range of ignition behaviors observed in the simulation. This evolution yields different autoignition delay scenarios as well as extinction at the different sites based on the local dissipation rates and their temporal histories. Where significant autoignition delay and extinction are observed, a shift in the relative roles of dominant reactions that contribute to radical production and consumption during this induction phase is observed. This shift is particularly characterized by an increased role of termination reactions during the intermediate stages of the induction period, which results in extinction in approximately two thirds of the ignition kernels in the computational domain. The fate of the different kernels is associated with: (1) the dissipation of heat that contributes to a slowdown in chemical reactions and a shift in the balance between chain-branching and chain-termination reactions; (2) the dissipation of mass that keeps the radical pool growth in check, and that is promoted by slower reaction rates; and (3) counter to the effects of dissipation of heat and intermediate species, the preferential diffusion of H2 relative to both heat and its diluent, N2, that promotes ignition. Ultimately, the balance between radical production and dissipation determines the success or failure of a given kernel to ignite. A new criterion for unsteady ignition is presented based on the instantaneous balance between radical production and dissipation. A Damkohler number, so defined, must remain above a critical value of unity at all times during the induction period if the kernel is to eventually ignite. Inherent in a multi-step kinetic description of ignition phenomena is the disparate time scales associated with different elementary reactions that, coupled with the characteristic scales of heat and mass dissipation, may yield different dominant chemistries at different stages of the induction process for a given kernel. To capture the strong history effects associated with radical build-up, new ignition progress variables based on key radical species are investigated.

A stochastic approach to calculate the particle size distribution function of soot particles in laminar premixed flames

Abstract: We introduce an efficient stochastic approach to solve the population balance equation that describes the formation and oxidation of soot particles in a laminar premixed flame The approach is based on a stochastic particle system representing the ensemble of soot particles The processes contributing to the formation and oxidation of soot particles are treated in a probabilistic manner The stochastic algorithm, which makes use of an efficient majorant kernel and the method of fictitious jumps, resolves the entire soot particle distribution (PSDF) without introducing additional closure assumptions A fuel-rich laminar premixed acetylene flame is computed using a detailed kinetic soot model Solutions are obtained for both, the stochastic approach and the method of moments combined with a modified version of the Premix, CHEMKIN code In this manner, the accuracy of the method of moments in a laminar premixed flame simulation is investigated It is found that the accuracy for the first moment is excellent (5% error), and mean error for rest of the moments is within 25% Also the effect of the oxidation of the smallest particles (burnout) has been quantified but was found not to be important in the flame investigated The time evolution of computed size distributions and their integral properties are compared to experimental measurements and the agreement was found to be satisfactory Finally, the efficiency of the stochastic method is studied

A study of spontaneous combustion characteristics of a turkish lignite: particle size, moisture of coal, humidity of air

Abstract: This study evaluated the spontaneous combustion characteristics of Askale lignite from Turkey. The effect of the gas flow rate, the moisture of the piles of coal, the humidity of the air and particle size on the spontaneous combustion characteristics of coal samples were examined using Crossing Point Methods adapted to our laboratories conditions. The amounts of three predominant oxygen functional groups (carboxyl, hydroxyl and carbonyl) in untreated and moist coal samples were also determined with wet chemical methods. The amounts of oxygen functional groups in moist coal samples do not differ significantly from that of untreated coal. The liability of spontaneous combustion of this lignite was increased with decreasing particle size, increasing moisture content of the coal and decreasing humidity of the air.

Turbulent burning velocity, burned gas distribution, and associated flame surface definition

Abstract: Experimental studies of premixed, turbulent, gaseous explosion flames in a fan-stirred bomb are reported. The turbulence was uniform and isotropic, while changes in the rms turbulent velocity were achieved by changes in the speed of the fans. Central spark ignitions created mean spherical flame propagation. The spatial distributions of burned and unburned gases during the propagation were measured from the Mie scattering of tobacco smoke in a thin planar laser sheet. The plane was located just in front of the central spark gap and was generated by a copper vapor laser operating at a pulse rate of 4.5 kHz. High-speed schlieren images also were captured simultaneously. The distributions of the proportions of burned and unburned gases around circumferences were found for all radii at all stages of the explosion, and mean values of these proportions were derived as a function of the mean flame radius. The flame brush thickness increased with flame radius. The way the turbulent burning velocity is defined depends on the chosen associated flame radius. Various definitions are scrutinized and different flame radii presented, along with the associated turbulent burning velocities. Engulfment and mass turbulent burning velocities are compared. It is shown how the latter might conveniently be obtained from schlieren cine images. In a given explosion, the burning velocity increased with time and radius, as a consequence of the continual broadening of the effective spectrum of turbulence to which the flame was subjected. A decrease in the Markstein number of the mixture increased the turbulent burning velocity.

Influence of H2 on the response of lean premixed CH4 flames to high strained flows

Abstract: A combined experimental and numerical investigation on the effects of H2 addition to lean-premixed CH4 flames in highly strained counterflow fields (with strain rates up to 8000 s 1 ) using preheated flows indicate significant enhancement of lean flammability limits and extinction strain rates for relatively small amounts of H 2 addition. Numerical modeling of the counterflow opposed jet configuration used in this study indicated extinction strain rates which were within 5% of experimentally measured values for equivalence ratios ranging from 0.75 to less than 0.4. Both experimental and numerical results indicate that increasing H 2 in the fuel significantly increases flame speeds and thus extinction strain rates. Furthermore, increasing H 2 decreases the dependency of extinction equivalence ratio on the strain rate of the flow. For all of the mixtures investigated, extinction temperatures depend primarily on equivalence ratio and not fuel composition for the range of H2 content studied, which suggests that extinction can be correlated to flame temperature and O2 concentration. Nonetheless, H2 addition greatly increases the maximum allowable strain rate before extinction temperatures are reached. Inspection of the model-predicted species profiles suggest that the enhancement of CH4 burning rates with H2 addition is driven by early H2 breakdown increasing radical production rates early in the flame zone to enhance CH4 ignition under conditions where otherwise CH4 combustion might be prone to undergo extinction. © 2003 The Combustion Institute. All rights reserved.

Optimally-reduced kinetic models: reaction elimination in large-scale kinetic mechanisms

Abstract: A new optimization-based approach to kinetic model reduction is presented. The reaction-elimination problem is formulated as a linear integer program which can be solved to guaranteed global optimality. This formulation ensures that the solution to the integer program is the smallest possible reduced model consistent with the user-set tolerances. The method is applied to generate optimally-reduced models for isobaric, adiabatic homogeneous combustion. The computational cost and accuracy of the reduced models are compared to those of the full mechanism. Results are shown for GRImech 3.0 and the Lawrence Livermore n-heptane combustion mechanism. The accuracy of the integer programming approach is compared to existing reaction elimination methods. The method is also applied to generate a library of reduced kinetic models for an adaptive chemistry simulation of a 2-D laminar, partially-premixed methane burner flame. Preliminary results are presented comparing the computational cost of the full GRImech 3.0 chemistry to that of the reduced model library.

Self-induced combustion oscillations of laminar premixed flames stabilized on annular burners

Abstract: Self-induced instabilities of laminar premixed flames stabilized over an annular burner have been studied in a set of experiments. A method was developed to determine the stability map of these systems using the response of both the burner and the flame to forced oscillations of the flow. This method is detailed for a well controlled example. The natural unstable motion of the flame is analyzed by measuring velocity fluctuations at the burner outlet, pressure fluctuations inside the burner and variations of the spontaneous light emitted by the flame. The burner's response to external pressure modulations is first characterized without flow and combustion. The flame's response to forced oscillations of the flow at the burner outlet is then used to determine the flame transfer function over the range of frequencies of interest. Using these elements, a mechanism is proposed for the onset of instability, which is shown to result from a coupling of the flame's response to flow oscillations with the bulk resonance mode of the burner. The driving mechanism leading to self-sustained oscillations of the flame front is produced by strong variations of the flame's surface area due to cyclic annihilations of neighboring elements in the flame front. During the collapse of large portions of the flame, a pressure pulse is released, which when properly phased with the burner acoustics, leads to resonance. A theoretical model for this instability is proposed and it is shown that at resonance, pressure fluctuations inside the burner and heat release fluctuations outside the burner must be in phase, in agreement with the Rayleigh criterion. Modeling predictions are compared to measurements using a diagram combining the acoustical response of the burner and measurements of the flame transfer functions. Predictions of the potentially unstable flow operating conditions are in good agreement with measurements. A criterion for the onset of instability is derived based on a balance of energy provided to and dissipated by the system. It is found that the gain of the flame transfer function at the resonant frequency of the burner must be sufficient to compensate for the losses of the system. The combined analysis of the burner acoustics and of the flame response to forced modulations of the flow provides a suitable description of instability modes observed experimentally.

Numerical study of the effects of material properties on flame stabilization in a porous burner

Abstract: Development of porous burners has been encouraged by lower emission standards as well as the advantages these burners offer; such as fuel flexibility, the ability to operate at low equivalence ratios, and effective flame speeds greater than the laminar flame speed. Although a burner may be constructed from a single section of porous media, a burner consisting of two sections with different characteristics has received significant attention in the last decade. Through proper selection of the properties of the two sections, the interface between the two sections serves as a flame holder preventing flashback for a range of conditions. In this paper, we present the results from a one-dimensional computational study on flame stabilization in a two section porous burner. The stable operating limits are predicted for a range of equivalence ratios and are compared to experimental values. A parametric study, in which the properties of the two sections are varied independently, is presented. The results indicate that matrix properties significantly affect the stable operating range. In addition, the upstream section acts primarily as a flashback arrestor and for the widest operating range, it should have a low conductivity, low volumetric heat transfer coefficient, and high radiative extinction coefficient. The downstream section acts primarily to recirculate heat through the matrix; it should have a high conductivity, high volumetric heat transfer coefficient, and an intermediate radiative extinction coefficient.

Analysis of the mechanism of the low-temperature oxidation of coal

Abstract: The mechanism of the oxidation of coal at low temperatures, i.e., below 100°C, was examined using measurements of the gases emitted from a bed of coal in an isothermal flow reactor. Employing an online two-column micro gas chromatograph, transient rates of production of CO2 and CO were monitored during desorption and oxidation experiments. A bituminous coal was milled into three nominal top size classes: 0-0.5 mm, 0-1 mm, and 0-2 mm. Desorption experiments with unoxidized coal samples at 20-70°C indicated that even an unoxidized coal incorporates oxygenated complexes in its structure. The threshold for thermal decomposition of these oxygenated species was found to be between 50 and 70°C. Carbon oxides liberated from oxidizing coal were compared with those from the thermal decomposition of coal oxidized at the same temperature, suggesting that two parallel reaction sequences contribute to the emission of carbon oxides during oxidation. A multi-step reaction mechanism was also proposed to describe low-temperature oxidation of coal and to explain the phenomena observed during the desorption and oxidation experiments.

Direct observations of reaction zone structure in propagating detonations

Abstract: We report experimental observations of the reaction zone structure of self-sustaining, cellular detonations propagating near the Chapman-Jouguet state in hydrogen-oxygen-argon/nitrogen mixtures. Two-dimensional cross sections perpendicular to the propagation direction were imaged using the technique of planar laser induced fluorescence (PLIF) and, in some cases, compared to simultaneously acquired schlieren images. Images are obtained which clearly show the nature of the disturbances in an intermediate chemical species (OH) created by the variations in the strength of the leading shock front associated with the transverse wave instability of a propagating detonation. The images are compared to 2-D, unsteady simulations with a reduced model of the chemical reaction processes in the hydrogen-oxygen-argon system. We interpret the experimental and numerical images using simple models of the detonation front structure based on the “weak” version of the flow near the triple point or intersection of three shock waves, two of which make up the shock front and the third corresponding to the wave propagating transversely to the front. Both the unsteady simulations and the triple point calculations are consistent with the creation of keystone-shaped regions of low reactivity behind the incident shock near the end of the oscillation cycle within the “cell.”

Measurements of minimum ignition energy in premixed laminar methane/air flow by using laser induced spark

Abstract: Reducing engine pollutant emissions and fuel consumption is an important challenge. Lean-burning engines are a promising development; however, such engines require high-energy ignition systems for typical working conditions (equivalence ratio, Φ < 0.7). Laser-induced ignition is envisaged as a way to obtain high-energy ignition as a result of progress that has been made in laser beam technology in terms of stability, size, and energy. This study investigated the minimum energy necessary to ignite a laminar premixed methane air mixture experimentally. A parametrical study was performed to characterize the effects of the flow velocity, equivalence ratio, and lens focal length on the minimum energy required for ignition. Experiments were conducted using a premixed laminar CH4/air burner. Laser-induced breakdown was achieved by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser with an anti-reflection-coated lens. Mixture ignition and the early stages of flame propagation were studied using a high speed Schlieren technique. Despite the stochastic characteristic of the laser breakdown phenomena, good reproducibility in the minimum energy required for the ignition measurements was observed. The cases in which the CH4/Air mixture flow ignites are defined as those with a laminar flame front propagation visible in the Schlieren images 10 ms after the energy deposition. The same minimum ignition energy (MIE) versus equivalence ratio (Φ) type of curves were obtained with a laser-induced spark and with a spark plug. Due to the threshold of energy required to obtain breakdown and the stochastic character of the energy absorption by the spark, a constant value was obtained (corresponding to the breakdown threshold) when the minimum ignition energy was lower than the breakdown threshold. As already noticed by several authors, MIE values higher than those observed using spark plugs were obtained. However, these differences tended to disappear at the lean and rich fuel limits.

Spectral radiative effects and turbulence/radiation interaction in a non-luminous turbulent jet diffusion flame

Abstract: A non-luminous turbulent jet diffusion flame is numerically simulated using a Reynolds stress second-order closure, the steady laminar flamelet model, and different approaches for radiative transfer. The commonly used optically thin approximation is compared with the discrete ordinates method. Calculations using the Planck mean absorption coefficient are compared with computations performed using the spectral line-based weighted-sum-of-gray-gases model. The interaction between turbulence and radiation is simulated, and its influence on the predicted results is investigated. It is shown that the discrete ordinates method and the optically thin approximation yield relatively close results for the present flame if the medium is modelled as gray using the Planck mean absorption coefficient. In both cases, the predicted fraction of radiative heat loss is significantly overestimated. However, if the spectral nature of gaseous radiation is accounted for, the computed radiation loss is closer to the experimental data. The fluctuations of the species have a minor role in the interaction between turbulence and radiation, which is mainly due to the temperature fluctuations.

Kinetic modeling of the interactions between NO and hydrocarbons at high temperature

Abstract: This paper presents a general and detailed chemical kinetic scheme developed and validated to investigate the interactions between NO and simple hydrocarbons during thermal oxidation and reburning. In a previous paper [1] the low temperature mechanism was presented. In this work the attention is drawn on the high-temperature conditions, referring typically to the reburning process where the hydrocarbon fragments reduce NO to HCN and N2. The goal is to obtain a better understanding of the interactions between NO and hydrocarbons, through the development of a general detailed kinetic model, which describes accurately the influence of NO in a wide temperature range, for different fuels and stoichiometry conditions. The model has been validated through the comparison with experimental measurements coming from different research groups, referring to several hydrocarbon fuels in different operative conditions. Even though the characteristic mechanisms are quite different from the low temperature conditions, the observed agreement in the whole investigation range confirms the correctness of the kinetic assumptions and extends the reliability of the model.

Modeling of homogeneous mercury speciation using detailed chemical kinetics

Abstract: Homogeneous mercury speciation in combustion-generated flue gases was modeled by a detailed kinetic model consisting of 107 reactions and 30 species. This kinetic model includes the oxidation and chlorination of key flue-gas components, as well as six mercury reactions involving HgO with new reaction rate constants calculated neither from experimental data nor by estimated, which was commonly used by other investigators before, but directly from transition state theory (TST). An important and previously unrecognized pathway of homogeneous Hg oxidation mechanism including Hg reactions involving HgO was proposed. Among those reactions involving HgO, the progress of reaction HgO + HCl → HgCl + OH is HgO + HCl → TS 1( HgClOH )→ M ( HgClOH )→ TS 2( HgClOH )→ HgCl + OH , in which the controlling step is HgO + HCl → TS 1( HgClOH )→ M ( HgClOH ). The progress of reaction HgO + HOCl → HgCl + HO 2 is HgO + HOCl → M ( HgClOOH )→ TS ( HgClOOH )→ HgCl + HO 2 , in which the controlling step is M ( HgClOOH )→ TS ( HgClOOH )→ HgCl + HO 2 . Four other reactions are one-step, with no intermediates formed. The performance of the model was assessed through comparisons with experimental data conducted by three different groups. The comparison shows that model calculations were in agreement with only one set of all the three groups experimental data. The deviation occurs due to the absence of accurate rate constants of existing mechanism, the adding of reactions involving HgO, as well as the exclusion of heterogeneous Hg oxidation mechanism. Analyses by quantum chemistry and sensitivity simulations illustrated that the pathway Hg + ClO = HgO + Cl is more significant than some of the key reactions in the kinetic mechanism proposed in the literature, which indicates the necessity of including reactions involving HgO in the mercury speciation kinetic mechanism. Studies on the effects of oxygen show that O 2 weakly promotes homogeneous Hg oxidation, especially under the condition of low Cl 2 concentration. In all cases, 1.5–6.0% of the mercury is predicted to be present as HgO.Keywords: Mercury speciation; Reaction mechanism; Quantum chemistry; Kinetic modeling

The dynamics of in-situ combustion fronts in porous media

Abstract: The sustained propagation of combustion fronts in porous media is a necessary condition for the success of in situ combustion for oil recovery. Compared to other recovery methods, in situ combustion involves the complexity of exothermic reactions and temperature-dependent chemical kinetics. In the presence of heat losses, the possibility of ignition and extinction also exists. In this paper, we address some of these issues by studying the properties of forward combustion fronts propagating at a constant velocity in the presence of heat losses. We extend the analytical method used in smoldering combustion [7], to derive expressions for temperature and concentration profiles and the velocity of the combustion front, under both adiabatic and non-adiabatic conditions. Heat losses are assumed to be relatively weak and they are expressed using two modes: 1) a convective type, using an overall heat transfer coefficient; and, 2) a conductive type, for heat transfer by transverse conduction to infinitely large surrounding formations. In their presence we derive multiple steady-state solutions with stable low and high temperature branches, and an unstable intermediate branch. Conditions for self-sustaining front propagation are investigated as a function of injection and reservoir properties. The extinction threshold is expressed in terms of the system properties. An explicit expression is also obtained for the effective heat transfer coefficient in terms of the reservoir thickness and the front propagation speed. This coefficient is not only dependent on the thermal properties of the porous medium but also on the front dynamics.

Soot surface oxidation in hydrocarbon/air diffusion flames at atmospheric pressure

Abstract: Soot oxidation was studied experimentally in laminar hydrocarbon/air diffusion flames at atmospheric pressure. Measurements were carried out along the axes of round jets burning in coflowing air considering acetylene, ethylene, propylene and propane as fuels. Measurements were limited to the initial stages of soot oxidation (carbon consumption less than 70%) where soot oxidation mainly occurs at the surface of primary soot particles. The following properties were measured as a function of distance above the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using Transmission Electron Microscopy (TEM), concentrations of stable major gas species (N2, H2O, H2, O2, CO, CO2, CH4, C2H2,C2H4, C2H6, C3H6, and C3H8) by sampling and gas chromatography, concentrations of some radical species (H, OH, O) by the deconvoluted Li/LiOH atomic absorption technique and flow velocities by laser velocimetry. It was found that soot surface oxidation rates are not particularly affected by fuel type for laminar diffusion flames and are described reasonably well by the OH surface oxidation mechanism with a collision efficiency of 0.10, (standard deviation of 0.07) with no significant effect of fuel type in this behavior; these findings are in good agreement with the classical laminar premixed flame measurements of Neoh et al. Finally, direct rates of surface oxidation by O2 were small compared to OH oxidation for present conditions, based on estimated O2 oxidation rates due to Nagle and Strickland-Constable (1962), because soot oxidation was completed near the flame sheet where O2 concentrations were less than 1.2% by volume.