Showing papers in "Combustion and Flame in 2009"

A comprehensive detailed chemical kinetic reaction mechanism for combustion of n-alkane hydrocarbons from n-octane to n-hexadecane

Abstract: Detailed chemical kinetic reaction mechanisms have been developed to describe the pyrolysis and oxidation of nine n-alkanes larger than n-heptane, including n-octane (n-C8H18), n-nonane (n-C9H20), n-decane (n-C10H22), n-undecane (n-C11H24), n-dodecane (n-C12H26), n-tridecane (n-C13H28), n-tetradecane (n-C14H30), n-pentadecane (n-C15H32), and n-hexadecane (n-C16H34). These mechanisms include both high temperature and low temperature reaction pathways. The mechanisms are based on previous mechanisms for the primary reference fuels n-heptane and iso-octane, using the reaction classes first developed for n-heptane. Individual reaction class rules are as simple as possible in order to focus on the parallelism between all of the n-alkane fuels included in the mechanisms. These mechanisms are validated through extensive comparisons between computed and experimental data from a wide variety of different sources. In addition, numerical experiments are carried out to examine features of n-alkane combustion in which the detailed mechanisms can be used to compare reactivities of different n-alkane fuels. The mechanisms for these n-alkanes are presented as a single detailed mechanism, which can be edited to produce efficient mechanisms for any of the n-alkanes included, and the entire mechanism, with supporting thermochemical and transport data, together with an explanatory glossary explaining notations and structural details, is available for download from our web page.

Implementation of the NCN pathway of prompt-NO formation in the detailed reaction mechanism

Abstract: This work presents revised detailed reaction mechanism for small hydrocarbons combustion with possibly full implementation of available kinetic data related to the prompt NO route via NCN. It was demonstrated that model predictions with the rate constant of reaction CH + N{sub 2} = NCN + H measured by Vasudevan and co-workers are much higher than experimental concentrations of NO in rich premixed flames at atmospheric pressure. Analysis of the correlations of NO formation with calculated concentrations of C{sub 2}O radicals strongly supports the inclusion of reaction between C{sub 2}O and N{sub 2} and reduction of the rate constant of reaction between CH and N{sub 2}. Rate constants of the reactions of NCN consumption were mostly taken from the works of Lin and co-workers. Some of these reactions affect calculated profiles of NCN in flames. Proposed modifications allow accurate prediction of NO formation in lean and rich flames of methane, ethylene, ethane and propane. Agreement of the experiments and the modeling was much improved as compared to the previous Release 0.5 of the Konnov mechanism. Satisfactory agreement with available measurements of NCN radicals in low pressure flames was also demonstrated. (author)

Nonlinear effects in the extraction of laminar flame speeds from expanding spherical flames

Abstract: Various factors affecting the determination of laminar flames speeds from outwardly propagating spherical flames in a constant-pressure combustion chamber were considered, with emphasis on the nonlinear variation of the stretched flame speed to the flame stretch rate, and the associated need to nonlinearly extrapolate the stretched flame speed to yield an accurate determination of the laminar flame speed and Markstein length. Experiments were conducted for lean and rich n-butane/air flames at 1 atm initial pressure, demonstrating the complex and nonlinear nature of the dynamics of flame evolution, and the strong influences of the ignition transient and chamber confinement during the initial and final periods of the flame propagation, respectively. These experimental data were analyzed using the nonlinear relation between the stretched flame speed and stretch rate, yielding laminar flame speeds that agree well with data determined from alternate flame configurations. It is further suggested that the fidelity in the extraction of the laminar flame speed from expanding spherical flames can be facilitated by using small ignition energy and a large combustion chamber.

Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors

Abstract: This article presents a chemical mechanism for the high temperature combustion of a wide range of hydrocarbon fuels ranging from methane to iso-octane. The emphasis is placed on developing an accurate model for the formation of soot precursors for realistic fuel surrogates for premixed and diffusion flames. Species like acetylene (C_2H_2), propyne (C_3H_4), propene (C_3H_6), and butadiene (C_4H_6) play a major role in the formation of soot as their decomposition leads to the production of radicals involved in the formation of Polycyclic Aromatic Hydrocarbons (PAH) and the further growth of soot particles. A chemical kinetic mechanism is developed to represent the combustion of these molecules and is validated against a series of experimental data sets including laminar burning velocities and ignition delay times. To correctly predict the formation of soot precursors from the combustion of engine relevant fuels, additional species should be considered. One normal alkane (n-heptane), one ramified alkane (iso-octane), and two aromatics (benzene and toluene) were chosen as chemical species representative of the components typically found in these fuels. A sub-mechanism for the combustion of these four species has been added, and the full mechanism has been further validated. Finally, the mechanism is supplemented with a sub-mechanism for the formation of larger PAH molecules up to cyclo[cd]pyrene. Laminar premixed and counterflow diffusion flames are simulated to assess the ability of the mechanism to predict the formation of soot precursors in flames. The final mechanism contains 149 species and 1651 reactions (forward and backward reactions counted separately). The mechanism is available with thermodynamic and transport properties as supplemental material.

Effect of cylindrical confinement on the determination of laminar flame speeds using outwardly propagating flames

Abstract: Article history: The effect of nonspherical (i.e. cylindrical) bomb geometry on the evolution of outwardly propagating flames and the determination of laminar flame speeds using the conventional constant-pressure technique is investigated experimentally and theoretically. The cylindrical chamber boundary modifies the propagation rate through the interaction of the wall with the flow induced by thermal expansion across the flame (even with constant pressure), which leads to significant distortion of the flame surface for large flame radii. These departures from the unconfined case, especially the resulting nonzero burned gas velocities, can lead to significant errors in flame speeds calculated using the conventional assumptions, especially for large flame radii. For example, at a flame radius of 0.5 times the wall radius, the flame speed calculated neglecting confinement effects can be low by ∼15% (even with constant pressure). A methodology to estimate the effect of nonzero burned gas velocities on the measured flame speed in cylindrical chambers is presented. Modeling and experiments indicate that the effect of confinement can be neglected for flame radii less than 0.3 times the wall radius while still achieving acceptable accuracy (within 3%). The methodology is applied to correct the flame speed for nonzero burned gas speeds, in order to extend the range of flame radii useful for flame speed measurements. Under the proposed scaling, the burned gas speed can be well approximated as a function of only flame radius for a given chamber geometry - i.e. the correction function need only be determined once for an apparatus and then it can be used for any mixture. Results indicate that the flow correction can be used to extract flame speeds for flame radii up to 0.5 times the wall radius with somewhat larger, yet still acceptable uncertainties for the cases studied. Flow-corrected burning velocities are measured for hydrogen and syngas mixtures at atmospheric and elevated pressures. Flow-corrected flame speeds in the small cylindrical chamber used here agree well with previously reported flame speeds from large spherical chambers. Previous papers presenting burning velocities from cylindrical chambers report performing data analysis on flame radii less than 0.5 or 0.6 times the wall radius, where the flame speed calculated neglecting confinement effects may be low by ∼15 or 20%, respectively. For cylindrical chambers, data analysis should be restricted to flame radii less than 0.3 times the wall radius or a flow correction should be employed to account for the burned gas motions. With regard to the design of future vessels, larger vessels that minimize the flow aberrations for the same flame radius are preferred. Larger vessels maximize the relatively unaffected region of data allowing for a more straightforward approach to interpret the experimental data.

Effect of particle size on combustion of aluminum particle dust in air

Abstract: The combustion of aluminum particle dust in a laminar air flow is theoretically studied under fuel-lean conditions. A wide range of particle sizes at nano and micron scales is explored. The flame speed and temperature distribution are obtained by numerically solving the energy equation in the flame zone, with the particle burning rate modeled as a function of particle diameter and ambient temperature. The model allows for investigation into the effects of particle size, equivalence ratio, and chemical kinetics on the burning characteristics and flame structures of aluminum-particle/air mixtures. In addition, the flame behavior with ultra-fine particles in the sub-nanometer range is examined by asymptotically treating particles as large molecules. Calculated flame speeds show reasonable agreement with experimental data. As the particle diameter decreases from the micron to the nano range, the flame speed increases and the combustion transits from a diffusion-controlled to a kinetically controlled mode. For micron-sized and larger particles, the flame speed can be correlated with the particle size according to a d − m relationship, with m being 0.92. For nano-particles, a d −0.52 or d −0.13 dependence is obtained, depending on whether the d 1.0 - or d 0.3 -law of particle burning time is implemented in the flame model, respectively. No universal law of flame speed exists for the entire range of particle sizes.

An experimental and kinetic modeling study of premixed NH3/CH4/O2/Ar flames at low pressure

Abstract: An experimental and modeling study of 11 premixed NH 3 /CH 4 /O 2 /Ar flames at low pressure (4.0 kPa) with the same equivalence ratio of 1.0 is reported. Combustion intermediates and products are identified using tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. Mole fraction profiles of the flame species including reactants, intermediates and products are determined by scanning burner position at some selected photon energies near ionization thresholds. Temperature profiles are measured by a Pt/Pt–13%Rh thermocouple. A comprehensive kinetic mechanism has been proposed. On the basis of the new observations, some intermediates are introduced. The flames with different mole ratios ( R ) of NH 3 /CH 4 ( R 0.0, R 0.1, R 0.5, R 0.9 and R 1.0) are modeled using an updated detailed reaction mechanism for oxidation of CH 4 /NH 3 mixtures. With R increasing, the reaction zone is widened, and the mole fractions of H 2 O, NO and N 2 increase while those of H 2 , CO, CO 2 and NO 2 have reverse tendencies. The structural features by the modeling results are in good agreement with experimental measurements. Sensitivity and flow rate analyses have been performed to determine the main reaction pathways of CH 4 and NH 3 oxidation and their mutual interaction.

Ammonia chemistry in oxy-fuel combustion of methane

Abstract: The oxidation of NH 3 during oxy-fuel combustion of methane, i.e., at high [ CO 2 ] , has been studied in a flow reactor. The experiments covered stoichiometries ranging from fuel rich to very fuel lean and temperatures from 973 to 1773 K. The results have been interpreted in terms of an updated detailed chemical kinetic model. A high CO 2 level enhanced formation of NO under reducing conditions while it inhibited NO under stoichiometric and lean conditions. The detailed chemical kinetic model captured fairly well all the experimental trends. According to the present study, the enhanced CO concentrations and alteration in the amount and partitioning of O/H radicals, rather than direct reactions between N-radicals and CO 2 , are responsible for the effect of a high CO 2 concentration on ammonia conversion. When CO 2 is present as a bulk gas, formation of NO is facilitated by the increased OH/H ratio. Besides, the high CO levels enhance HNCO formation through NH 2 + CO . However, reactions NH 2 + O to form HNO and NH 2 + H to form NH are inhibited due to the reduced concentration of O and H radicals. Instead reactions of NH 2 with species from the hydrocarbon/methylamine pool preserve reactive nitrogen as reduced species. These reactions reduce the NH 2 availability to form NO by other pathways like via HNO or NH and increase the probability of forming N 2 instead of NO.

Laminar burning velocities at high pressure for primary reference fuels and gasoline: Experimental and numerical investigation

Abstract: Spherical flames of n -heptane, iso-octane, PRF 87 and gasoline/air mixtures are experimentally investigated to determine laminar burning velocities and Markstein lengths under engine-relevant conditions by using the constant volume bomb method. Data are obtained for an initial temperature of 373 K, equivalence ratios varying from ϕ = 0.7 to ϕ = 1.2 , and initial pressures from 10 to 25 bar. To track the flame front in the vessel a dark field He–Ne laser Schlieren measurement technique and digital image processing were used. The propagating speed with respect to the burned gases and the stretch rate are determined from the rate of change of the flame radius. The laminar burning velocities are obtained through a linear extrapolation to zero stretch. The experimentally determined Markstein numbers are compared to theoretical predictions. A reduced chemical kinetic mechanism for n -heptane and iso-octane was derived from the Lawrence Livermore comprehensive mechanisms. This mechanism was validated for ignition delay times and flame propagation at low and high pressures. In summary an overall good agreement with the various experimental data sets used in the validation was obtained.

An experimental and kinetic modeling study of n-butanol combustion

Abstract: n-Butanol is a fuel that has been proposed as an alternative to conventional gasoline and diesel fuels. In order to better understand the combustion characteristics of n-butanol, this study presents new experimental data for n-butanol in three experimental configurations. Species concentration profiles are presented in jet stirred reactor (JSR) at atmospheric conditions and a range of equivalence ratios. The laminar flame speed obtained in an n-butanol premixed laminar flame is also provided. In addition, species concentration profiles for n-butanol and n-butane in an opposed-flow diffusion flame are presented. The oxidation of n-butanol in the aforementioned experimental configurations has been modeled using an improved detailed chemical kinetic mechanism (878 reactions involving 118 species) derived from a previously proposed scheme in the literature. The proposed mechanism shows good qualitative agreement with the various experimental data. Sensitivity analyses and reaction path analyses have been conducted to interpret the results from the JSR and opposed-flow diffusion flame. It is shown that the main reaction pathway in both configurations is via H-atom abstraction from the fuel followed by β-scission of the resulting fuel radicals. Several unimolecular decomposition reactions are important as well. This study gives a better understanding of n-butanol combustion and the product species distribution.

Towards the understanding of cyclic variability in a spark ignited engine using multi-cycle LES

Abstract: Large-Eddy Simulation (LES) has been used to analyze the occurrence and the causes of cycle-to-cycle combustion variations in a spark ignited four-valve single cylinder engine fueled with a homogeneous propane–air mixture. The combustion modeling combines an Eulerian model derived from the RANS AKTIM model that mimics the spark ignition and the Extended Coherent Flame Model (ECFM-LES) that describes the flame propagation. The motion of piston and valves is accounted for using an Arbitrary Eulerian Lagrangian (ALE) technique with body-fitted meshes. The computation covers nine consecutive complete four-stroke cycles following an initialization cycle. The obtained LES results are compared with experimental measurements. Although the number of computed cycles is fairly low, LES is shown to be able to reproduce both quantitatively and qualitatively the cyclic variability observed experimentally. The investigation of the possible causes of variability illustrates the unprecedented possibility LES offers for understanding cycle-to-cycle variations.

A modelling study of aromatic soot precursors formation in laminar methane and ethene flames

Abstract: A relatively short kinetic mechanism (93 species and 729 reactions) was developed to predict the formation of poly-aromatic hydrocarbons (PAH) and their growth of up to five aromatic rings in methane and ethane-fueled flames. The model is based on the C{sub 0}-C{sub 2} chemistry with recent well-established chemical kinetic data. Reaction paths for mostly stable and well studied PAH molecules were delineated and the reaction rate constants for PAH growth were collected. These were obtained by analysing the data reported in the literature during the last 30 years, or by using the estimates and optimisations of experimentally measured concentration profiles for small and PAH molecules. These profiles were collected by 12 independent work groups in laminar premixed CH{sub 4} and C{sub 2}H{sub 4} flames under atmospheric pressure or in shock tube experiments under elevated pressure. The simulated flame speeds, temporal profiles of small and large aromatics and also soot particles volume fraction data are in good agreement with the experimental data received for different temperatures, mixing ratios and diluents. The extensive analysis of PAH reaction steps showed that the main reaction routes can be conditionally divided into ''low temperature'' reaction routes, dominating at T 1550 K. The presented mechanism can be used as the basis for further extensions or reductions applied in kinetic schemes involving PAH and soot production in practical fuel combustion. (author)« less

The influence of molecular structure of fatty acid monoalkyl esters on diesel combustion

Abstract: The subject of this paper is a series of experiments conducted on a single-cylinder research engine investigating the influence of molecular structure on the combustion behaviour of fatty acid alcohol ester (biodiesel) molecules under diesel engine conditions. The fuels employed in these experiments comprised various samples of pure individual fatty acid alcohol ester molecules of different structure, as well as several mixtures of such molecules. The latter consisted in biodiesel fuels produced by the transesterification of naturally occurring plant oils or animal fat with a monohydric alcohol. It was observed that the molecular structure of the fuel significantly influenced the formation of NOx and particulate matter and their respective concentration in the exhaust gas. The influence on the formation of NOx in particular, appeared to be exerted first through the effect which the molecular structure had on the auto-ignition delay occurring after the fuel was injected into the combustion chamber, and second through the flame temperature at which the various molecules burned. The emission of particulates on the other hand showed correlation with the number of double bonds in the fuel molecules for the case of larger accumulation mode particles, and with the boiling point of the fuel samples for the case of the smaller, nucleation mode particles. The effect of ignition delay on the exhaust emissions of these pollutants was isolated by adding the ignition promoting molecule 2-ethylhexyl nitrate to some of the fuel samples in closely specified concentrations, so as to equalise the ignition delay for the relevant fuel samples. The removal of the ignition delay as a main influence on the combustion process enabled the observation of the lesser effects of adiabatic flame temperature.

Numerical simulation and experiments of burning douglas fir trees

Abstract: Fires spreading in elevated vegetation, such as chaparral or pine forest canopies, are often more intense than fires spreading through surface vegetation such as grasslands. As a result, they are more difficult to suppress, produce higher heat fluxes, more firebrands and smoke, and can interact with, or create, local weather conditions that lead to dangerous fire behavior. Such wildland fires can pose a serious threat to wildland–urban interface communities. A basic building block of such fires is a single tree. In the work presented here, a number of individual trees, of various characteristics, were burned without an imposed wind in the Large Fire Laboratory of the National Institute of Standards of Technology. A numerical model capable of representing the spatial distribution of vegetation in a tree crown is presented and evaluated against tree burning experiments. For simplicity, the vegetation was assumed to be uniformly distributed in a tree crown represented by a well defined geometric shape (cone or cylinder). Predictions of the time histories of the radiant heat flux and mass loss rates for different fuel moisture contents and tree heights compared favorably to measured values and trends. This work is a first step toward the development and application of a physics-based computer model to spatially complex, elevated, vegetation present in forest stands and in the wildland–urban interface.

Numerical evaluation of equivalence ratio measurement using OH∗ and CH∗ chemiluminescence in premixed and non-premixed methane–air flames

Abstract: This work presents results from detailed chemical kinetics calculations of electronically excited OH (A{sup 2}{sigma}, denoted as OH{sup *}) and CH (A{sup 2}{delta}, denoted as CH{sup *}) chemiluminescent species in laminar premixed and non-premixed counterflow methane-air flames, at atmospheric pressure. Eight different detailed chemistry mechanisms, with added elementary reactions that account for the formation and destruction of the chemiluminescent species OH{sup *} and CH{sup *}, are studied. The effects of flow strain rate and equivalence ratio on the chemiluminescent intensities of OH{sup *}, CH{sup *} and their ratio are studied and the results are compared to chemiluminescent intensity ratio measurements from premixed laminar counterflow natural gas-air flames. This is done in order to numerically evaluate the measurement of equivalence ratio using OH{sup *} and CH{sup *} chemiluminescence, an experimental practise that is used in the literature. The calculations reproduced the experimental observation that there is no effect of strain rate on the chemiluminescent intensity ratio of OH{sup *} to CH{sup *}, and that the ratio is a monotonic function of equivalence ratio. In contrast, the strain rate was found to have an effect on both the OH{sup *} and CH{sup *} intensities, in agreement with experiment. The calculated OH{sup *}/CH{supmore » *} values showed that only five out of the eight mechanisms studied were within the same order of magnitude with the experimental data. A new mechanism, proposed in this work, gave results that agreed with experiment within 30%. It was found that the location of maximum emitted intensity from the excited species OH{sup *} and CH{sup *} was displaced by less than 65 and 115 {mu}m, respectively, away from the maximum of the heat release rate, in agreement with experiments, which is small relative to the spatial resolution of experimental methods applied to combustion applications, and, therefore, it is expected that intensity from the OH{sup *} and CH{sup *} excited radicals can be used to identify the location of the reaction zone. Calculations of the OH{sup *}/CH{sup *} intensity ratio for strained non-premixed counterflow methane-air flames showed that the intensity ratio takes different values from those for premixed flames, and therefore has the potential to be used as a criterion to distinguish between premixed and non-premixed reaction in turbulent flames. (author)« less

Hybrid Method of Moments for modeling soot formation and growth

Abstract: In this work, a new statistical model for soot formation and growth is developed and presented. The Hybrid Method of Moments (HMOM) seeks to combine the advantages of two moment methods, the Method of Moments with Interpolative Closure (MOMIC) and the Direct Quadrature Method of Moments (DQMOM), in an accurate and consistent formulation. MOMIC is numerically simple and easy to implement but is unable to account for bimodal soot Number Density Functions (NDF). DQMOM is accurate but is numerically ill-posed and difficult to implement. HMOM combines the best of both two methods to capture bimodal NDF while retaining ease of implementation and numerical robustness. The new hybrid method is shown to predict mean quantities nearly as accurately as DQMOM and high-fidelity Monte Carlo simulations. In addition, a model for combining particle coalescence with particle aggregation is presented and shown to accurately reproduce experimental measurements in a variety of sooting flames.

Prediction of the burning rates of non-charring polymers

Abstract: A quantitative understanding of the processes that take place in the condensed phase of a burning material is critical for prediction of ignition and growth of fires. In the current study, a model of burning of two widely-used charring and intumescing polymers, bisphenol A polycarbonate and poly(vinyl chloride), was developed and validated. The modeling was performed using a flexible computational framework called ThermaKin, which had been developed in our laboratory. ThermaKin solves time-resolved energy and mass conservation equations describing a one-dimensional material object subjected to external heat. Most of the model parameters were obtained from direct property measurements. The model was validated against the results of cone calorimetry experiments performed under a broad range of conditions. Potential sources of uncertainties in the model parameterization were analyzed.

Experiments on chemical looping combustion of coal with a NiO based oxygen carrier

Abstract: A chemical looping combustion process for coal using interconnected fluidized beds with inherent separation of CO 2 is proposed in this paper. The configuration comprises a high velocity fluidized bed as an air reactor, a cyclone, and a spout-fluid bed as a fuel reactor. The high velocity fluidized bed is directly connected to the spout-fluid bed through the cyclone. Gas composition of both fuel reactor and air reactor, carbon content of fly ash in the fuel reactor, carbon conversion efficiency and CO 2 capture efficiency were investigated experimentally. The results showed that coal gasification was the main factor which controlled the contents of CO and CH 4 concentrations in the flue gas of the fuel reactor, carbon conversion efficiency in the process of chemical looping combustion of coal with NiO-based oxygen carrier in the interconnected fluidized beds. Carbon conversion efficiency reached only 92.8% even when the fuel reactor temperature was high up to 970 °C. There was an inherent carbon loss in the process of chemical looping combustion of coal in the interconnected fluidized beds. The inherent carbon loss was due to an easy elutriation of fine char particles from the freeboard of the spout-fluid bed, which was inevitable in this kind of fluidized bed reactor. Further improvement of carbon conversion efficiency could be achieved by means of a circulation of fine particles elutriation into the spout-fluid bed or the high velocity fluidized bed. CO 2 capture efficiency reached to its equilibrium of 80% at the fuel reactor temperature of 960 °C. The inherent loss of CO 2 capture efficiency was due to bypassing of gases from the fuel reactor to the air reactor, and the product of residual char burnt with air in the air reactor. Further experiments should be performed for a relatively long-time period to investigate the effects of ash and sulfur in coal on the reactivity of nickel-based oxygen carrier in the continuous CLC reactor.

Operational characteristics of a parallel jet MILD combustion burner system

Abstract: This study describes the performance and stability characteristics of a parallel jet MILD (Moderate or Intense Low-oxygen Dilution) combustion burner system in a laboratory-scale furnace, in which the reactants and exhaust ports are all mounted on the same wall. Thermal field measurements are presented for cases with and without combustion air preheat, in addition to global temperature and emission measurements for a range of equivalence ratio, heat extraction, air preheat and fuel dilution levels. The present furnace/burner configuration proved to operate without the need for external air preheating, and achieved a high degree of temperature uniformity. Based on an analysis of the temperature distribution and emissions, PSR model predictions, and equilibrium calculations, the CO formation was found to be related to the mixing patterns and furnace temperature rather than reaction quenching by the heat exchanger. The critical equivalence ratio, or excess air level, which maintains low CO emissions is reported for different heat exchanger positions, and an optimum operating condition is identified. Results of CO and NOx emissions, together with visual observations and a simplified two-dimensional analysis of the furnace aerodynamics, demonstrate that fuel jet momentum controls the stability of this multiple jet system. A stability diagram showing the threshold for stable operation is reported, which is not explained by previous stability criteria.

Explosion bomb measurements of ethanol-air laminar gaseous flame characteristics at pressures up to 1.4 MPa

Abstract: The principal burning characteristics of a laminar flame comprise the fuel vapour pressure, the laminar burning velocity, ignition delay times, Markstein numbers for strain rate and curvature, the stretch rates for the onset of flame instabilities and of flame extinction for different mixtures. With the exception of ignition delay times, measurements of these are reported and discussed for ethanol–air mixtures. The measurements were in a spherical explosion bomb, with central ignition, in the regime of a developed stable, flame between that of an under or over-driven ignition and that of an unstable flame. Pressures ranged from 0.1 to 1.4 MPa, temperatures from 300 to 393 K, and equivalence ratios were between 0.7 and 1.5. It was important to ensure the relatively large volume of ethanol in rich mixtures at high pressures was fully evaporated. The maximum pressure for the measurements was the highest compatible with the maximum safe working pressure of the bomb. Many of the flames soon became unstable, due to Darrieus–Landau and thermo-diffusive instabilities. This effect increased with pressure and the flame wrinkling arising from the instabilities enhanced the flame speed. Both the critical Peclet number and the, more rational, associated critical Karlovitz stretch factor were evaluated at the onset of the instability. With increasing pressure, the onset of flame instability occurred earlier. The measured values of burning velocity are expressed in terms of their variations with temperature and pressure, and these are compared with those obtained by other researchers. Some comparisons are made with the corresponding properties for iso-octane–air mixtures.

Combustion characteristics of boron nanoparticles

Abstract: An experimental investigation of the combustion characteristics of boron nanoparticles in the post flame region of a flat flame burner has been conducted. Boron is attractive as a fuel or a fuel supplement in propellants and explosives due to its high heats of combustion on both a gravimetric and volumetric basis. A relatively large database exists for combustion characteristics of large (greater than 1 μm) boron particles, but very little exists for nano-sized boron. Ignition and combustion characteristics have been studied in the post flame region of a fuel lean CH4/Air/O2 flame, with burner temperatures ranging from about 1600 K to 1900 K, and oxygen mole fractions ranging between 0.1 and 0.3. As in earlier investigations on boron combustion, a two-stage combustion phenomenon was observed. Ensemble-averaged burning times of boron nanoparticles were obtained, while the ignition time measurements for boron nanoparticles were extended into a lower temperature range previously unavailable in the literature. The measured burning times were between 1.5 ms and 3.0 ms depending on both the temperature and oxygen mole fraction. The ignition times were relatively insensitive to oxygen concentration in the range studied, and were affected only by temperature. The measured ignition times were inversely related to the temperature, ranging from 1.5 ms at 1810 K to 6.0 ms at 1580 K. The burning time results were compared to both diffusion and kinetic limited theories of particle combustion. It was found that the size dependence on particle burning times did not follow either theory.

Influence of autoignition delay time characteristics of different fuels on pressure waves and knock in reciprocating engines

Abstract: The functional relationship of autoignition delay time with temperature and pressure is employed to derive the propagation velocities of autoignitive reaction fronts for particular reactivity gradients, once autoignition has been initiated. In the present study of a variety of premixtures, with different functional relationships, such gradients comprise fixed initial temperature gradients. The smaller is the ratio of the acoustic speed through the mixture to the localised velocity of the autoignitive front, the greater are the amplitude and frequency of the induced pressure wave. This might lead to damaging engine knock. At higher values of the ratio, the autoignition can be benign with only small over-pressures. This approach to the effects of autoignition is confirmed by its application to a variety of experimental studies involving: (i) Imposed temperature gradients in a rapid compression and expansion machine. (ii) Onset of knock in an engine with advancing spark timing. (iii) Development of autoignition at a single hot spot in an engine. (iv) Autoignition fronts initiated by several hot spots. There is much diversity in the effects that can be produced by different fuels in different ranges of temperature and pressure. Higher values of autoignitive propagation speeds lead to increasingly severe engine knock. Such effects cannot always be predicted from the Research and Motor octane numbers.

Measurement of laminar burning speeds and Markstein lengths using a novel methodology

Abstract: Three different methodologies used for the extraction of laminar information are compared and discussed. Starting from an asymptotic analysis assuming a linear relation between the propagation speed and the stretch acting on the flame front, temporal radius evolutions of spherically expanding laminar flames are postprocessed to obtain laminar burning velocities and Markstein lengths. The first methodology fits the temporal radius evolution with a polynomial function, while the new methodology proposed uses the exact solution of the linear relation linking the flame speed and the stretch as a fit. The last methodology consists in an analytical resolution of the problem. To test the different methodologies, experiments were carried out in a stainless steel combustion chamber with methane/air mixtures at atmospheric pressure and ambient temperature. The equivalence ratio was varied from 0.55 to 1.3. The classical shadowgraph technique was used to detect the reaction zone. The new methodology has proven to be the most robust and provides the most accurate results, while the polynomial methodology induces some errors due to the differentiation process. As original radii are used in the analytical methodology, it is more affected by the experimental radius determination. Finally, laminar burning velocity and Markstein length values determined with the new methodology are compared with results reported in the literature.

Visualization of the unburned gas flow field ahead of an accelerating flame in an obstructed square channel

Abstract: The effect of blockage ratio on the early phase of the flame acceleration process was investigated in an obstructed square cross-section channel. Flame acceleration was promoted by an array of top-and bottom-surface mounted obstacles that were distributed along the entire channel length at an equal spacing corresponding to one channel height. It was determined that flame acceleration is more pronounced for higher blockage obstacles during the initial stage of flame acceleration up to a flame velocity below the speed of sound of the reactants. The progression of the flame shape and flame area was determined by constructing a series of three-dimensional flame surface models using synchronized orthogonal schlieren images. A novel schlieren based photographic technique was used to visualize the unburned gas flow field ahead of the flame front. A small amount of helium gas is injected into the channel before ignition, and the evolution of the helium diluted unburned gas pocket is tracked simultaneously with the flame front. Using this technique the formation of a vortex downstream of each obstacle was observed. The size of the vortex increases with time until it reaches the channel wall and completely spans the distance between adjacent obstacles. A shear layer develops separating the core flow from the recirculation zone between the obstacles. The evolution of oscillations in centerline flame velocity is discussed in the context of the development of these flow structures in the unburned gas.

Critical radius for sustained propagation of spark-ignited spherical flames

Abstract: An experimental study was performed to determine the requirements for sustained propagation of spark–ignited hydrogen–air and butane–air flames at atmospheric and elevated pressures. Results show that sustained propagation is always possible for mixtures whose Lewis number is less than unity, as long as a flame can be initially established. However, for mixtures whose Lewis number is greater than unity, sustained propagation depends on whether the initially ignited flame can attain a minimum radius. This minimum radius was determined for mixtures of different equivalence ratios and pressures, and was found to agree moderately well with the theoretically predicted critical radius beyond which there is no solution for the adiabatic, quasi-steady propagation of the spherical flame. The essential roles of pressure, detailed chemistry, and the need to use local values in the quantitative evaluation of the flame response parameters are emphasized.

Quantitative measurement of soot particle size distribution in premixed flames – The burner-stabilized stagnation flame approach

Abstract: A burner-stabilized, stagnation flame technique is introduced. In this technique, a previously developed sampling probe is combined with a water-cooled circular plate such that the combination simultaneously acts as a flow stagnation surface and soot sample probe for mobility particle sizing. The technique allows for a rigorous definition of the boundary conditions of the flame with probe intrusion and enables less ambiguous comparison between experiment and model. Tests on a 16.3% ethylene–23.7% oxygen–argon flame at atmospheric pressure show that with the boundary temperatures of the burner and stagnation surfaces accurately determined, the entire temperature field may be reproduced by pseudo one-dimensional stagnation reacting flow simulation using these temperature values as the input boundary conditions. Soot particle size distribution functions were determined for the burner-stabilized, stagnation flame at several burner-to-stagnation surface separations. It was found that the tubular probe developed earlier perturbs the flow and flame temperature in a way which is better described by a one-dimensional stagnation reacting flow than by a burner-stabilized flame free of probe intrusion.

Effect of nano-aluminium in plateau-burning and catalyzed composite solid propellant combustion

Abstract: Nano-aluminium particles of ∼50 nm size, produced at this laboratory, are added to composite solid propellants based on ammonium perchlorate and hydroxyl-terminated poly-butadiene binder that exhibit plateau burning rate trends and those including burning rate catalysts. The nano-aluminized propellant burning rates are compared with corresponding micro-aluminized and non-aluminized ones in the 1–12 MPa pressure range. The mid-pressure extinction of the matrixes containing the fine-sized ammonium perchlorate particles in the propellant along with the binder is investigated in all the cases to understand the mechanism of plateau-burning. Further, the variations in aluminium content, the aluminium size (within nano- and micro-ranges), bimodal combination of nano- and micro-aluminium are considered. Ferric oxide and titanium dioxide are the burning rate catalysts considered. Large scale accumulation of aluminium is observed not only in micro-aluminized matrixes, but also in nano-aluminized ones as clusters of 1–5 μm size. Since aluminium is added at the expense of the coarse ammonium perchlorate particles to preserve the total-solids loading in the present formulations, addition of micro-aluminium decreases the burning rate; whereas, nano-aluminized propellants exhibit ∼80–100% increase in the burning rate under most conditions. The near-complete combustion of nano-aluminium close to the burning surface of the propellant provides heat feedback that controls the burning rate. Mid-pressure extinctions of matrixes and plateau burning rates of propellants are washed out when nano-aluminium is progressively added beyond 50% in bimodal aluminium blends, but low pressure-exponents are observed in the nano-aluminized propellant burning rates at elevated pressures. Adjusting the plasticizer content in the binder alters the pressure range of plateau burning rates in non-aluminized propellants. Catalysts increase the burning rate by ∼50–100% in non-aluminized and micro-aluminized propellants, but in nano-aluminized propellants, the μm-sized catalyst does not affect the burning rate significantly; whereas, the nanometre size catalysts increases the burning rate merely by ∼5–15%.

A general flamelet transformation useful for distinguishing between premixed and non-premixed modes of combustion

Abstract: The flame index was originally proposed by Yamashita et al. as a method of locally distinguishing between premixed and non-premixed combustion. Although this index has been applied both passively in the analysis of direct numerical simulation data, and actively using single step combustion models, certain limitations restrict its use in more detailed combustion models. In this work a general flamelet transformation that holds in the limits of both premixed and non-premixed combustion is developed. This transformation makes use of two statistically independent variables: a mixture fraction and a reaction progress parameter. The transformation is used to produce a model for distinguishing between premixed and non-premixed combustion regimes. The new model locally examines the term budget of the general flamelet transformation. The magnitudes of each of the terms in the budget are calculated and compared to the chemical source term. Determining whether a flame burns in a premixed or a non-premixed regime then amounts to determining which sets of these terms most significantly contribute to balancing the source term. The model is tested in a numerical simulation of a laminar triple flame, and is compared to a recent manifestation of the flame index approach. Additionally, the model is applied in a presumed probability density function (PDF) large eddy simulation (LES) of a lean premixed swirl burner. The model is used to locally select whether tabulated premixed or tabulated non-premixed chemistry should be referenced in the LES. Results from the LES are compared to experiments.

Reduction of very large reaction mechanisms using methods based on simulation error minimization

Abstract: A new species reduction method called the Simulation Error Minimization Connectivity Method (SEM-CM) was developed. According to the SEM-CM algorithm, a mechanism building procedure is started from the important species. Strongly connected sets of species, identified on the basis of the normalized Jacobian, are added and several consistent mechanisms are produced. The combustion model is simulated with each of these mechanisms and the mechanism causing the smallest error (i.e. deviation from the model that uses the full mechanism), considering the important species only, is selected. Then, in several steps other strongly connected sets of species are added, the size of the mechanism is gradually increased and the procedure is terminated when the error becomes smaller than the required threshold. A new method for the elimination of redundant reactions is also presented, which is called the Principal Component Analysis of Matrix F with Simulation Error Minimization (SEM-PCAF). According to this method, several reduced mechanisms are produced by using various PCAF thresholds. The reduced mechanism having the least CPU time requirement among the ones having almost the smallest error is selected. Application of SEM-CM and SEM-PCAF together provides a very efficient way to eliminate redundant species and reactions from large mechanisms. The suggested approach was tested on a mechanism containing 6874 irreversible reactions of 345 species that describes methane partial oxidation to high conversion. The aim is to accurately reproduce the concentration–time profiles of 12 major species with less than 5% error at the conditions of an industrial application. The reduced mechanism consists of 246 reactions of 47 species and its simulation is 116 times faster than using the full mechanism. The SEM-CM was found to be more effective than the classic Connectivity Method, and also than the DRG, two-stage DRG, DRGASA, basic DRGEP and extended DRGEP methods.

Mathematical modeling of MILD combustion of pulverized coal

Abstract: MILD (flameless) combustion is a new rapidly developing technology. The IFRF trials have demonstrated high potential of this technology also for N-containing fuels. In this work the IFRF experiments are analyzed using the CFD-based mathematical model. Both the Chemical Percolation Devolatilization (CPD) model and the char combustion intrinsic reactivity model have been adapted to Guasare coal combusted. The flow-field as well as the temperature and the oxygen fields have been accurately predicted by the CFD-based model. The predicted temperature and gas composition fields have been uniform demonstrating that slow combustion occurs in the entire furnace volume. The CFD-based predictions have highlighted the NO x reduction potential of MILD combustion through the following mechanism. Before the coal devolatilization proceeds, the coal jet entrains a substantial amount of flue gas so that its oxygen content is typically not higher than 3–5%. The volatiles are given off in a highly sub-stoichiometric environment and their N-containing species are preferentially converted to molecular nitrogen rather than to NO. Furthermore, there exists a strong NO-reburning mechanism within the fuel jet and in the air jet downstream of the position where these two jets merge. In other words, less NO is formed from combustion of volatiles and stronger NO-reburning mechanisms exist in the MILD combustion if compared to conventional coal combustion technology.