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形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2021-09-01, 資料番号: PA2220046000

The present work reports thermo-acoustic instability in a mesoscale tube of diameter 8 mm (> quenching diameter) and length 702 mm. Mixtures with Lewis number, Le~ 0.8 (rich C2H4/O2/CO2), 1.05 (lean C2H4/O2/CO2) and 1.34 (lean C2H4/O2/N2) are used. Several new flame responses are observed. For lower burning velocity mixtures, flame extinction is observed due to heat loss when primary instability transforms to secondary instability for all Le mixtures. However, parametric cellular structures which are characteristic of parametric instability are observed only for Le of 0.8. It is proposed based on calculations and experiments that parametric structures will be observed only when diameter of tube is two times larger than the characteristic wavelength of parametric instability. If the tube diameter doesn't allow formation of parametric structure whirling and counter-rotating flames are observed instead of parametric structures. Suppression of acoustic parametric instability is observed for higher burning velocity mixtures with Le>1 and its mechanism is discussed. For a range of SL, a beating instability is observed for CO2 diluted mixtures of Le>1, where pressure oscillation and flame motion show beating oscillations of frequency around 15 Hz. This beating instability is believed to be caused by non-linear interaction of acoustic instability with pulsating instability of flame front which is caused due to combined radiative and convective heat loss. Due to CO2 dilution, the radiative heat losses could play a significant role in inducing pulsating instability.

International audience; The oxidation of a stoichiometric diethyl ether-oxygen-nitrogen mixture containing 5000 ppm of fuel was studied in a jet-stirred reactor at 10 atm and a residence times of 1 s. Experimental temperatures varied stepwise for 440 to 740 K and products were quantified by gas chromatography with TCD and FID, gas chromatography-mass spectrometry, and FTIR. Other experiments in the temperature range 480 to 570 K were performed for characterizing elusive cool flame products. To this end, gas samples were trapped in acetonitrile for flow injection analyses and liquid chromatography-mass spectrometry (Orbitrap Q-Exactive®). For ionization, we used positive and negative atmospheric pressure chemical ionization (APCI). Among fuel-specific products, hydroperoxides and diols (C4H10O3), carbonyl hydroperoxides (C4H8O4), acetic acid, di-keto ethers (C4H6O3), cyclic ethers (C4H8O2) and highly oxygenated molecules, i.e., keto-dihydroperoxides (C4H8O6), keto-trihydroperoxides (C4H8O8), di-keto-hydroperoxides (C4H6O5), and diketo-dihydroperoxides (C4H6O7), were detected. To confirm the presence of –OH or –OOH groups in oxidation products, H/D exchange with D2O was used. DNPH derivatization was used to identify carbonyls present in samples, especially those with a molecular weight below 50 amu which cannot be detected directly by the mass spectrometer. Chemical kinetic modeling using a mechanism taken from the literature was performed. Although reasonable agreement between the data and the simulations was observed for several species, some discrepancies between experimental and computed mole fractions were observed.

Metastable intermolecular composites (MICs), also called nanothermites, are attracting wide attention mainly due to the increased interfacial contact area of the fuels and oxidants therein. In this study, an alcohol-thermal technique is adopted to synthesize approximately core-shell structured Al@CuO MICs, in which process copper acetate is used as the precursor of CuO while Al powder with a wide size distribution (50 nm−5 µm in diameter) is used as cores. The energy-release characteristics of the materials are studied by performing thermal analysis, constant-volume combustion cell tests, and high-speed camera imaging of the combustion process. The results show that Al nanoparticles are surrounded by CuO nanoparticles that are with an average diameter of about 10 nm, while micron-sized Al particles are coated by a continuous layer of nanoplatelets that are assembled from CuO nanoparticles. Benefiting from the enhanced interfacial contact compared with that of ultrasonically mixed counterpart, Al@CuO shows a lower apparent activation energy of solid-state interfacial reaction, a higher light intensity, a shorter burning time, a larger pressure output, and a higher pressurization rate. It is also found that equivalent ratio has a great effect on the combustion process, and a slightly fuel-rich formulation is preferred in this study. The synthesis method developed in this study may also be adapted to the preparation of other core-shell structured MICs.

This work investigates the combustion velocity, thermophysical properties, and reaction activation energies of Al-Zr-C nanolayered composite microparticles undergoing gasless high-temperature propagation after preparation via additive manufacturing. High-speed videography and pyrometry of the reaction event were used to analyze two Al-Zr-C samples with varied stoichiometry. Combustion velocity of the Al-Zr-C composites was ~0.3–0.5 cm/s and varied inversely with the Al content in the system. The Al-Zr-C composites also exhibited auto-oscillations during the propagation event which were characterized to have temperature fluctuations of ~50–100 K with a periodicity of ~1 Hz. Temperature data collected via color ratio pyrometry was used to measure the thermal profile in-operando. Temperature maps were used to estimate the thermal diffusivity of the samples to be ~2 × 10−6 m2/s on the leading edge of the reaction front with a >30x increase in thermal diffusivity on the trailing edge. The activation energy for the Al-Zr-C composites was estimated to be ~30–35 kJ/mol under reacting conditions. This work ultimately demonstrates an accessible measurement methodology that could be used to estimate thermophysical changes in materials for generalized modeling purposes and confirms the functionality of the materials to create conductive pathways after reaction.

This work demonstrates single-shot, two-dimensional temperature measurements in a premixed linear detonation channel using two-color OH planar laser-induced fluorescence thermometry. Detonation environments result in extreme thermodynamic conditions which create challenges in the resulting spectroscopic behavior. However, thermometry based on the ratio of the P1(9)/Q1(14) transitions within the A2Σ+←X2Π(1,0) band of OH was determined to be well-suited for detonation environments based on the spectral characteristics over a wide range of conditions. The technique is demonstrated in the post-induction region, focusing on the nominal equilibrium state of the Chapman-Jouguet conditions, on mixtures of stoichiometric H2 and O2 diluted with either N2 or Ar. High-speed chemiluminescence imaging at 2 MHz was used to assess the qualitative dynamics of the detonation structure. The measured temperature fields for the Ar dilution case are relatively spatially uniform, with the distributions mean agreeing well with the theoretical calculation of the Chapman-Jouguet temperature, consistent with preliminary work [Grib et al., AIAA SciTech Forum, (2021). 2021-0421]. Conversely, the temperature fields for a 50% N2 dilution case are highly irregular, showing a larger dynamic range in temperature as well as pockets of unburned reactants, which emphasized the significance of this measurement by highlighting various reaction scenarios dictated by detonation waves. The irregularity of the N2 dilution cases is explained in terms of the scale similarity between the channel size and the detonation cell sizes for the N2 dilution mixtures, leading to weak or failing detonation modes. Overall, the demonstrated thermometry technique produced a precision (1-σ) of approximately 4% in atmospheric conditions and approximately 7.7% in strong detonation environments. In addition, the accuracy, defined as the percent difference between the mean and CJ temperature, was approximately 1% in the Ar diluted case. The ability to distinguish and quantify detonation burning behavior is promising for employing this approach for application in pressure-gain combustion facilities such as rotating detonation combustors, however care needs to be taken when interpreting the resulting temperature field, particularly near the wave front, due to the precision and validation range of the present line pair.

The present work introduces an annular combustion chamber operated at intermediate pressures. The combustor is operated with CH-H blends leading to a variety of azimuthal combustion instabilities. The influence of the hydrogen content, the air mass flow rate and the equivalence ratio on the instabilities is investigated over a wide range of operating conditions with mean chamber pressures from 1.5 to 3.3 bar. This leads to a range of exit boundary conditions, from partially to fully reflecting. It is found that pure methane and methane-hydrogen mixtures with low hydrogen contents result in stable combustion. However, when the hydrogen content reaches 25% by volume high-amplitude instabilities are excited, which exhibit higher order harmonics with significant pressure amplitude contributions. Such harmonic response was not previously observed in atmospheric annular combustors. The amplitudes decrease slightly when the H content is increased further. The harmonic response is found to be amplitude dependent with fewer significant harmonic contributions occurring at low-amplitudes and a cut-on amplitude of the fundamental mode at which higher harmonics become significant. The interaction between the harmonic components of the pressure amplitudes is shown to follow a quadratic relationship. The modal response was analyzed and it was found that all high-amplitude instabilities feature clockwise spinning modes whereas lower-amplitude instabilities feature counter clockwise spinning modes. Finally, a low- and high-amplitude case were investigated in detail and phase-averaged images are discussed. The low-amplitude instabilities result in flame dynamics similar to those observed in atmospheric combustors previously whereas the high-amplitude instabilities exhibit large oscillations in the flame height and intensity. A characterization of the boundary conditions is also provided for numerical simulations which includes temperature measurements, acoustic characterization and cold flow velocity profiles. This is an open access article distributed under the terms of the Creative Commons CC-BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The controllable preparation and rational utilization of carbon species produced in flame process is a meaningful research direction. Herein, a CQDs/TiO2 C heterostructure coupling with multi-carbon species was developed in flame process. In the millisecond-level reaction process, part of the residual carbon formed by the incomplete combustion of ethanol penetrates the lattice of TiO2, forming interstitial C (Ci) and substituent C (Cs), and the remaining amorphous carbon quantum dots (CQDs) are adsorbed on the surface of TiO2. In-situ temperature-programmed X-ray photoelectron spectroscopy (In-situ TPXPS) combined with various characterizations proved that the CQDs/TiO2 C heterostructure was successfully constructed in the flame process (the atomic ratio of the two C species was close to 1:1). The carbon species lead to the reduction of the band gap of the targeted material from 2.8 eV to 2.73 eV, meanwhile, the reduction ability is increased by 21.3%. The synergistic effect enhances the separation efficiency of photogenerated electrons by 7 times, achieving a conversion efficiency of 46.21 µmol g−1 h−1 and nearly 100% CO2-to-CO selectivity. Furthermore, the systematic theoretical calculation results visualize this kind of novel electron transfer channel from C to O to CQDs (C O-CQDs).

International audience; This work studies the influence of phoretic motion (thermophoresis and diffusiophoresis) of fine alumina particles ("smoke") produced during the combustion of aluminum. Direct numerical simulations on a single aluminum droplet burning in a quiescent environment suggest that thermophoresis is the main mechanism driving smoke back to the aluminum surface, hence a major contributor to the oxide lobe development. The presence of this lobe is found to distort the flowfield, which favors hot and smoke-rich regions closer to the lobe, thereby enhancing thermophoresis. This combination of aerodynamic and thermophoretic effects leads to a mass rate of deposited smoke which is consistent with experimental data. A simplified model, deduced from simulation results, is able to predict the size of the final oxide residue in good agreement with measurements. This study supports that aluminum oxide present on the burning aluminum particle is largely due to material formed in the flame, subsequently desposited by thermophoresis.

In the present work, premixed combustion in a turbulent boundary layer under auto-ignitive conditions is investigated using direct numerical simulation (DNS). The turbulent inflow of the reactive DNS is obtained by temporal sampling of a corresponding inert DNS of a turbulent boundary layer at a location with R e τ = 360, where R e τ is the friction Reynolds number. The reactants of the DNS are determined by mixing the products of lean natural gas combustion and a H 2 /N 2 fuel jet, resulting in a lean mixture of high temperature with a short ignition delay time. In the free stream the reaction front is stabilized at a streamwise location which can be predicted using the free stream velocity U ∞ and the ignition delay time τ i g . Inside the boundary layer, combustion modifies the near-wall coherent turbulent structures considerably and turbulence results in reaction front wrinkling. The combustion modes in various regions were examined based on the results of displacement velocity, species budget and chemical explosive mode analysis (CEMA). It was indicated that flame propagation prevails in the near-wall region and auto-ignition becomes increasingly important as the wall-normal distance increases. The interactions of turbulence and combustion were studied through statistics of reaction front normal vector and strain rate tensor. It was found that the reaction front normal preferentially aligns with the most compressive strain rate in regions where the effects of heat release on the strain rate are minor and with the most extensive strain rate where its effects are significant. Negative correlations between the wall heat flux and flame quenching distance were observed. A new quenching mode, back-on quenching, was identified. It was found that the heat release rate at the wall is the highest when head-on quenching occurs and lowest when back-on quenching occurs.

International audience; This article describes an experimental investigation into the heat fluxes generated by the combustion of AP/HTPB solid propellant samples inside 1-MPa pressurized combustion chambers. Fluxmeters developed at ONERA and recently improved were used to measure the local heat flux densities. Two kind of fluxmeters were used: total fluxmeters for which the convective and the radiative components are measured and radiative fluxmeters for which the convective component is suppressed by the use of a sapphire window mounted on top of the fluxmeter. Additionally, the flame was visualized by a narrow-band filtered IR camera which provided quantitative and spatially resolved radiance field. From the IR camera recordings, a radiative heat flux equivalent to that measured by the radiative fluxmeters was constructed by means of the integration over the fluxmeters' collection volumes of the spectrally equivalent radiance. A satisfactory agreement is thus found between the two radiative heat flux measurement methods.; Cet article décrit une étude expérimentale sur les flux de chaleur générés par la combustion d'échantillons de propergol solide AP/HTPB à l'intérieur de chambres de combustion pressurisées de 1-MPa. Des fluxmètres développés à l'ONERA et récemment améliorés ont été utilisés pour mesurer les densités locales de flux de chaleur. Deux types de fluxmètres ont été utilisés : les fluxmètres totaux pour lesquels les composantes convectives et radiatives sont mesurées et les fluxmètres radiatifs pour lesquels la composante convective est supprimée par l'utilisation d'une fenêtre en saphir montée sur le dessus du fluxmètre. En outre, la flamme a été visualisée par une caméra IR à bande étroite filtrée qui a fourni un champ de radiance quantitatif et spatialement résolu. À partir des enregistrements de la caméra IR, un flux de chaleur radiative équivalent à celui mesuré par les fluxmètres radiatifs a été construit par l'intégration de la radiance spectralement équivalente sur les volumes de collecte des fluxmètres. Un accord satisfaisant est donc trouvé entre les deux méthodes de mesure du flux de chaleur radiative.

This study investigated the combustion behaviors of pure iron and mixed particles, particularly iron–aluminum and iron–coal mixtures, doped into methane (CH4)–air premixed flames. The mechanically mixed particles were prepared with a weight ratio of 1:1. Thermogravimetric analysis revealed that the Fe particles and the Fe–coal mixture underwent oxidation in similar regions of relatively low temperatures; the Fe‒Al mixture underwent a multistage oxidation process. A conical CH4–air premixed flame—with the CH4–air equivalence ratio maintained at the stoichiometric value—was doped with micron-sized solid fuels at various feed rates. Increasing the particle feed rate appeared to alter the flame front characteristics. The interdependency between solid fuels and the CH4–air premixed flame was investigated with respect to flame temperatures, gas emissions, and metal oxide products. Particle microexplosions occurred in the Fe–coal combustion. Regarding the mechanism underlying the microexplosions, we hypothesized that the bubbles inside the Fe particles may have contained dissolved O2, N2, and CO; the dissolved CO may have generated iron carbonyl (Fe(CO)5). Coalescence, repeated bubbling, and bubble expansion processes led to the expansion of iron oxides with hollow shells. The rapid increase in inner pressure and explosive internal combustion caused by the ripening and flammability of the (Fe(CO)5)/O2 bubbles engendered the microexplosions. CO was added to the Fe flame to validate this hypothesis.

Methanol is an attractive oxygenate increasingly used as primary fuel for dual-fuel combustion technology yielding beneficial thermal-efficiency and emissions in modern engines. As such, it is significant to fundamentally understand the autoignition behavior of methanol/diesel mixtures. This study measured the ignition delay times (IDTs) of methanol/diesel blends with different mixing ratios (30%, 50%, 70% methanol by mol.) on a heated shock tube and a heated rapid compression machine at temperatures of 650−1450 K, pressures of 6−20 bar, and equivalence ratios of 0.5, 1.0 and 2.0. The typical two-stage ignition characteristics with the negative temperature coefficient response were observed for dual-fuel mixtures. In general, both the total and first-stage IDTs decrease with the increment of pressure and equivalence ratio as well as diesel proportion in mixtures. The simulation results performed with a published detailed mechanism in conjunction with a tri-component diesel surrogate demonstrate generally good agreement with the experimental data at all test conditions. Moreover, a crossover of IDTs occurs at a higher temperature (~1500 K) for varying equivalence ratios, and experiment and simulation both exhibit a non-linear mixing effect of methanol addition on diesel ignition. Interestingly, simulation results clearly suggest a crossover (~940 K) for mixtures with varying methanol content at an intermediate-temperature, where the IDT of the mixture with a lower methanol ratio becomes longer as the temperature is higher than that of the crossover. Furthermore, brute-force sensitivity analyses assisted with reaction path analysis were conducted to gain deeper insights into the autoignition chemistry of dual-fuel mixtures, especially for the chemical interaction between both fuels during the low-temperature oxidation process. It is found that methanol hardly generates •OH, whereas •OH is mainly produced by the low-temperature reaction pathways of diesel. Thus, •OH is the bridge for both fuels during the ignition process. At the high methanol ratio, the H-abstraction of diesel is mainly via •HO2 while CH3OH consumes a large percentage of •OH. Consequently, the competition between methanol and diesel for •OH radicals inhibits the overall reactivity of reaction network. In addition, the original data reported here lay a foundation for the development and validation of more accurate and robust dual-fuel kinetic schemes.

Iodine pentoxide (I2O5) based nanothermites are one of the most promising candidates for biocidal energetic materials due to superior reactivity and high iodine content. However, the tunability of nanothermites, which is important for biocidal performance, has not been fully exploited for I2O5 based nanothermites. In this work, I2O5 with various fuels (Al, Ti, Si) and their mixtures (i.e., a ternary system) have been investigated. The reactivity and flame temperature were evaluated by a pressure cell coupled with a spectrometer. Temperature-Jump time-of-flight mass spectrometry (T-Jump TOFMS) was used to probe the reaction mechanism and iodine release behavior, along with a high-speed camera to capture the ignition event. I2O5 showed distinct reactivity with different fuels. As a result, by varying the fuel composition of ternary systems, the combustion properties can be tuned. Rapid heating experiments revealed that the reaction initiation was shifted from gas phase dominated to condensed phase dominated mechanism after introducing Ti or Si into Al/I2O5 system. Further analysis of the ternary systems found that the energy release rate correlates with burn time instead of flame temperature. This study shows an approach to tune the reactivity and energy release rate of I2O5 based nanothermites without compromising the energy density.

A degradation in performance parameters related to thermal decomposition and combustion behaviour of tungsten based pyrotechnic delay composition can occur when subjected to accelerated aging conditions in a controlled environment. The present study utilizes various methods to quantify the extent of degradation in thermo-kinetic and burning characteristics as well as to postulate a fundamental aging mechanism for traditional tungsten (W) pyrotechnic delay material, which has not been attempted in the past. The delay composition based on metallic fuel (W) and perchlorate oxidizer is subjected to aging at constant temperature of 71 °C and 95% relative humidity, for 2 and 12 weeks, respectively. Experiments including thermal analysis, combustion temperature profile and burning rate measurements are conducted together with numerical simulation of an actual pyrotechnic delay device. Moreover, the reaction mechanism, chemical kinetics, and combustion behaviour between pristine and aged cases are examined. Results illustrate that there exist two aging induced processes, which increases the presence of large agglomerated particles, high metal oxide content by thickening the outer oxide layer of metallic fuel, and more unreacted oxygen to instigate incomplete combustion in aged samples. This alters the reaction pathway of combustion process, lowers average thermal conductivity, and reduces diffusion of reactants in aged samples, thus causing significant decrement in the heat of reaction (31%), combustion zone temperature (10%), reactivity (12%), and burning rates (10%), such that the overall pyrotechnic delay device experiences misfiring during operation or a failure in accomplishing its actual intended task.

Burning experiments are analyzed for 59 successfully ignited tests using a gas burner (BRE) to emulate the steady burning of 25 mm diameter flat surface materials. More than half of the tests resulted in long burns of at least 2 and as long as 5 min. Mixtures of ethylene and nitrogen allow for 23.6 and 47.2 kJ/g heats of combustion (LHV). Nominal ambient conditions range from 40% to 21% oxygen and pressures of 1 to about 0.56 bar. NASA human habitat atmospheres have been used. Generally, it has been found that nominal oxygen mole fractions of 26% and above can allow steady flames. The flames are thin blue in color and grow slowly over time for minutes. Some self-extinguish, some had periodic oscillations, and others appear to have become steady and were shut down. Theoretical analyses show the shutdown flames were steady in growth, and all of the test endpoints were more than 94% of steady state in flame heat flux. Flame heat flux for set mass flux allowed the estimation of heats of gasification to relate to steady solid materials that might burn in microgravity. Radiometers allowed the computation of flame radiative fraction that appeared to correlate well with the measured flame height. Results are shown for heat flux, flame height and base radius, radiative fraction as a function of fuel mixture mass flux, and various oxygen and pressure atmospheres.

We studied the relationship between pre-chamber jet and main chamber ignition in the pre-chamber combustion (PCC) of an optical engine, fueled with methane and equipped with an active pre-chamber with two rows of orifices. Acetone planar laser-induced fluorescence (PLIF) and OH* chemiluminescence imaging techniques were simultaneously applied to visualize the pre-chamber jet and the reaction zone in the main chamber, respectively. The pre-chamber fueling was constant and the main chamber fueling was increased to form an ultra-lean case and a lean case with global excess air ratios (λ) of 2.3 and 1.8, respectively. Results indicate that a higher pressure difference between pre-chamber and main chamber ( ▵ P) produces larger pre-chamber jet penetration speed; the maximum pre-chamber jet penetration speed appears at timing around the peak ▵ P. Over enrichment of the pre-chamber charge reduces the peak ▵ P and thus does not favor a faster pre-chamber jet discharge. In addition to the main pre-chamber jet, a weaker post jet discharge process is visualized; the former is due to the pre-chamber combustion while the latter due to the ▵ P fluctuation and the cylinder volume expansion. The post pre-chamber jet is accompanied by a post reaction zone in the ultra-lean case (λ=2.3) and there are two unburned regions in the main chamber: one is around the pre-chamber nozzle and the other between the adjacent reaction zones. These two unburned regions are consumed by flame propagation in the lean case (λ=1.8). The weak pre-chamber jet from the upper-row orifice does not produce any distinct reaction zone, indicating that the pre-chamber orifice location and arrangement on the nozzle also matters in the pre-chamber design. The pre-chamber jet penetration length is longer than that of the reaction zone during pre-chamber discharge; the penetration length difference between the pre-chamber jet and reaction zone decreases with increasing main chamber fueling.

A method is proposed to assess the thermal safety of solid propellant charges by measuring the temperature and pressure of propellant specimens during cook-off tests and determining the reaction kinetics. In this work, two propellants—ammonium perchlorate/hydroxy-terminated polybutadiene/aluminum (AP/HTPB/Al), and ammonium perchlorate/hydroxy-terminated polyether/aluminum (AP/HTPE/Al)—are taken as examples to demonstrate this method in detail. Specifically, by implementing two kinds of cook-off tests—viz., multipoint temperature monitoring, and combustion pressure measurement—the thermal reaction temperature before ignition and the combustion pressure after ignition of the two propellants were respectively determined. Then, the kinetic parameters of the thermal decomposition reaction model and the parameters of the combustion reaction model were numerically simulated and calibrated to achieve mathematical descriptions of the entire process of cook off of two solid propellant charges. On this basis, the technology of grid-node separation calculation was employed to simulate and predict the rupture of a solid rocket motor case, thereby quantitatively describing the severity of the cook-off reaction. The results demonstrate that, at a heating rate of 1 K min−1, the ignition positions of the two propellant charges in the solid rocket motor are located in the annular area where the side wall and the front of the casing are joined. Compared with those of the AP/HTPE/Al charge, the ignition time of the AP/HTPB/Al charge is longer (12,714 s for AP/HTPB/Al vs. 9701 s for AP/HTPE/Al), the temperature of the casing before ignition is higher (501.5 K for AP/HTPB/Al vs. 466.2 K for AP/HTPE/Al), the reaction after ignition is more intense, and the deformation of the casing is more serious.

Alkylbenzenes constitute a significant portion of middle-distillate transportation fuels, and they are judged as the key contributors to the extent of soot production and consequent exhaust particulate emissions in combustion engines. Because of the uncertainties involved in engine experiments due to boundary condition variation among test platforms and the difficulties involved in controlling engine variables independently, benchmark information on pressure dependence of sooting propensities of alkylbenzenes is essential to understand their influence on particulate emissions and provide a more consistent interpretation of engine data. One of the approaches to acquire such benchmark data is to investigate laminar diffusion flames doped with alkylbenzenes under tractable conditions at elevated pressures. Here, we study the influence of doping a methane laminar diffusion flame with alkylbenzenes having eight carbon atoms, namely ethylbenzene, p-xylene, and o-xylene and compare their sooting characteristics to that of n-octane at pressures up to 10 bar. Liquid hydrocarbons added to methane replaced 3% of the carbon in the base fuel keeping the carbon mass flow in the fuel stream fixed, at all pressures considered, to have tractable measurements. The flames of base fuel methane and those doped with liquid C 8 hydrocarbons were stabilized on a laminar co-flow diffusion burner positioned inside a combustion chamber capable of elevated pressures and has suitable optical access. Radially resolved temperatures and soot volume fractions were evaluated from spectral emission of soot radiation from the flames. Soot yields, inferred from the radial distributions of soot volume fractions, of ethylbenzene, p-xylene, and o-xylene doped flames were found to be a factor of about 2.6–3.2 higher than that of n-octane at 2 bar; however, this difference gradually disappeared with increasing pressure and soot yields were almost the same above 6 bar. It appears that pressure dependence of sooting propensities of ethylbenzene, p-xylene, and o-xylene is weaker than that of n-octane within the pressure range of 2 to 10 bar.

Construction of kinetic models to predict real-fuel combustion properties requires significant human and computational resources. In the first of this two-part study, a functional group correlation approach called FGMech was proposed for predicting the stoichiometric parameters in lumped pyrolysis reactions. The stoichiometric parameters were implemented in a recent real-fuel kinetic model, HyChem (Xu et al., 2018), and the validity of this approach was demonstrated for simulating real-fuel combustion. The present work extends the FGMech approach for developing surrogate and real-fuel kinetic models. Our approach is fundamentally different from the HyChem development approach in that no parameters are tuned to match actual real-fuel pyrolysis/oxidation data, and all model parameters are derived only from functional group data. Along with the stoichiometric parameters obtained in the first part of this study, the thermodynamic data, lumped reaction rate parameters and transport data were predicted in this work based on the functional group characterization of real fuels. The Benson group additivity method was adopted to estimate the thermodynamic data of real fuels, while rate rules developed for pure fuels were used to estimate the rate constants of lumped reactions in real-fuel models. For transport data, normal boiling point, critical temperature and pressure (estimated using the Joback group contribution method) were used to obtain Lennard-Jones parameters. The format of lumped reactions in FGMech followed the HyChem approach, and the base mechanism was adopted from the AramcoMech 2.0 and USC Mech II, respectively, to compare the model performance with different base mechanisms. Fourteen surrogate and twelve real-fuel models were developed based on this approach; they were validated against the experimental data in the literature. FGMech's performance was also compared with detailed and reduced models available in the literature. FGMech reasonably captures the experimental data in the literature, indicating that the present modeling approach is promising for modeling the combustion behavior of fuel, including surrogate mixtures and real fuels.

The HITEMP 2010 databases was employed in line by line (LBL) method to calculate an accurate total emissivity benchmark for oxy-fuel combustion atmosphere. This paper focuses on the effect of CO in the oxy-fuel gas mixture on the total emissivity. CO is significant in the primary combustion zone of boiler, generating from incomplete combustion and gasification reactions in oxy-fuel combustion mode. Based on the total emissivity benchmark from LBL, the new correlation coefficients for weighted sum of gray gases model (WSGGM) are fitted for mole fraction of CO from 0% to 45%, molar ratio of H2O to CO2 between 0.01 and 4, temperature range of 400 K–3000 K and path length varying from 0.01 m to 60 m. Radiative heat flux and radiative source term results from solving the radiation transfer equation (RTE) in one-dimensional slab system are used to evaluate the accuracy and effectiveness of new correlation coefficients for WSGGM. By comparing with the results from LBL and models in other literature, new correlation coefficients for WSGGM are validated no matter in non-isothermal and non-homogenous conditions considering CO in gas mixture. At last, the new correlation coefficients are implemented in the numerical simulation of oxy-natural gas furnace to show the effect of gas radiation model on the wall surface radiative heat flux.

Ignition delay times (IDTs) and laminar flame speeds (SL) of C9H20 (nonane) isomers are systematically investigated. IDTs of normal nonane (n-C9), 2-methyloctane (2mC8), 2,4-dimethylheptane (24mC7), and 2,2,4,4-tetramethylpentane (2244mC5) are experimetally obtained by a shock tube facility and numerically simulated by a chemkin 0-D reactor model. Further, laminar flame speeds (SL) of n-C9 and 2244mC5 are measured by spherical expanding flames in a constant-temperature, constant-pressure dual-chamber cruciform burner over a wide range of the equivalence ratio (Φ = 0.7–1.4), which are used to compare with numerically simulated results obtained by chemkin 1-D flame speed model. Detailed reaction mechanisms of KUCRS, LLNL and JetSurF ver.02 are used for numerical simulations. It is found that experimental IDTs increase with the number of methyl branches, especially in low-temperature and negative temperature coefficient (NTC) regions, where the increase of IDT with the number of methyl branches are well predicted by KUCRS. We also find that the measured values of SL of highly branched 2244mC5 are smaller than those of n-C9 at all values of Φ studied, of which measured SL data are successfully reproduced by the 1-D flame speed model with KUCRS. These results are important to our understanding of reaction characteristics for highly branched nonane isomers and for the designing of optimal alternative fuels in internal combustion engines.

The pyrolysis of 1-heptene was studied in a flow reactor using synchrotron vacuum ultraviolet photoionization mass spectrometry at 0.04 and 1 atm and in a jet-stirred reactor using gas chromatography at 1 atm. Flow reactor pyrolysis products, including the allyl radical, cycloalkenes and aromatics, were identified and quantified. Alkenes are found to be the dominant product family, among which ethylene is the most abundant product. A detailed intermediate-to-high temperature model of 1-heptene was developed and validated against the new pyrolysis data in this work, as well as previous data of 1-heptene combustion in literature over a wide range of pressures, temperatures and equivalence ratios. Rate of production analysis and sensitivity analysis were performed to reveal the key pathways in fuel decomposition and product formation. The allylic C C bond dissociation reaction is concluded as the most important pathway in 1-heptene decomposition. Reactions of allyl, propargyl and cyclopentadienyl radicals play important roles in the formation of cycloalkenes and aromatics. Furthermore, comparative pyrolysis experiments of 1-hexene and n-heptane were also performed in the jet-stirred reactor at 1 atm using gas chromatography to explore fuel molecular structure effects on pyrolysis reactivity and product distributions among 1-alkene and n-alkane fuels. The comparison between 1-heptene and 1-hexene pyrolysis demonstrates that similar fuel molecular structure results in the similarities in primary fuel decomposition pathways and pyrolysis reactivity. Ethylene is the most abundant product in both 1-alkene pyrolysis, and the feature in 1-heptene molecular structure leads to enhanced formation of ethylene in its pyrolysis. The abundant formation of ethyl and methyl radicals leads to higher production of 1-pentene and 1-butene in 1-heptene and 1-hexene pyrolysis, respectively. The comparison between 1-heptene and n-heptane pyrolysis reveals that the existence of C C double bond enhances the pyrolysis reactivity of 1-heptene. Different from 1-heptene consumption, n-heptane consumption is dominantly controlled by H abstraction reactions instead of unimolecular decomposition reactions. Propene and 1-butene are prone to be produced in 1-heptene pyrolysis, while 1-hexene has higher mole fractions in n-heptane pyrolysis.

Smouldering combustion is emerging as a valuable tool for energy conversion purposes. However, the effects of radial/lateral heat losses, while critical to its viability, are not well understood. It is known that heat losses weaken the smouldering reaction near the walls. It is less known that these losses generate non-uniform air flux across the system cross-section, potentially changing conversion rates and quenching limits. This study integrated: (i) highly instrumented smouldering experiments across numerous scales, (ii) a novel method of estimating non-uniform air flux in the experiments, (iii) analytical modelling to predict non-uniform cooling, and (iv) energy balance calculations to quantify the non-uniform heat of smouldering. Altogether, this work demonstrates that heat loss-induced non-uniform air flux is significant, affecting key smouldering propagation and cooling characteristics. The uniform air flux injected at the base became redistributed with a ~50% decrease at the centreline and a ~50% increase at the wall. This was shown to cause a concave (in the direction of air flow) smouldering front and a concave cooling front. The former was shown to cause radial heat transfer inwards, leading to super-adiabatic heating towards the centre of the reactor. The latter was shown to inhibit cooling along the centreline, which progressed ~40% slower than expected during propagation. Altogether, the multiple and integrated analyses used reveal the magnitude and significance of heat losses in smouldering systems. This insight is valuable to better harness smouldering for engineering applications.

Energy transfer in a pulsed-microwave enhanced flame is investigated using hybrid fs/ps coherent anti-Stokes Raman scattering (CARS) to monitor both vibrational and rotational temperatures of nitrogen in an atmospheric pressure laminar premixed natural gas/air stagnation flame. Temperatures were measured throughout the laminar flame structure following a 30-kW peak power, 2 μ s duration, 3 GHz microwave pulse in a resonant waveguide cavity. CARS measurements show a delayed increase in vibrational temperature, indicating energy loading via electron impact and subsequent energy cascade. Vibrational energy thermalization was observed over timescales faster than transport through the flame zone, but slower than predicted by known vibrational-translational rates, suggesting a long-lived pathway for increased vibrational temperature. Peak vibrational temperature increases of 100 K were observed and thermalize over 100 ′ s of microseconds, resulting in a measurable increase in the rotational temperature over the same time interval. The magnitude of vibrational excitation and rate of thermalization in such plasma-assisted combustion environments is critical for applications including combustion ignition and control, and hybrid fs/ps CARS measurements provide the necessary detail on vibrational-translational relaxation processes of ground state nitrogen.

The Cambridge/Sandia turbulent stratified flame (SwB5) is simulated with the LES and Flamelet-Generated Manifolds (FGM) combustion model. Three 3D FGM manifolds are adopted. With the purpose to examine the influence of transport properties, unity and non-unity Lewis numbers ( L e ) are included in the first two manifolds, respectively. The combined effects of non-unity L e and stretch are investigated in the third manifold. Heat loss to the wall is also modeled. Good agreement is found between the simulation and experiment. The equivalence ratio, temperature and mass fractions of CO and H 2 are all well reproduced in contrast with previous simulations. It is found that using non-unity L e can even deteriorate the near-wall temperature modeling. Non-unity L e is proposed to be crucial for the CO prediction as well, besides H 2 . The equivalence ratio modeling is observed to be very important, which accounts for several non-unity L e effects. Flame stretch shows almost no impact on the velocity fields, whereas its effects on the species, equivalence ratio and temperature are identified, although to a limited extent for the Cambridge/Sandia flame.

In this work, pyrolysis experiments of ethyl valerate were performed in a flow reactor over 705–1051 K at low and atmospheric pressures and in a jet-stirred reactor over 633–1013 K at near-atmospheric pressure. Products were measured with synchrotron vacuum ultraviolet photoionization mass spectrometry in the flow reactor pyrolysis and gas chromatography in the jet-stirred reactor pyrolysis. Valeric acid and ethylene were observed as the most abundant pyrolysis products in both experiments. Laminar burning velocities of ethyl valerate/air mixtures were also measured in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 443 K and initial pressures of 1–10 atm. A kinetic model of ethyl valerate combustion incorporated with recent theoretical progress was developed to predict the new experimental data in this work, as well as the speciation data under flame conditions and laminar burning velocities at different initial temperatures and pressures in literature. Experimental observations and modeling analyses both confirm the significant role of the intramolecular elimination reaction of ethyl valerate producing valeric acid and ethylene. In particular, this reaction has exclusive significance in decomposition of ethyl valerate under pyrolysis conditions, indicating pyrolysis experiments can provide crucial constraints for its rate constant. Subsequent decomposition reactions of valeric acid at higher temperatures enrich the intermediate pool, especially radicals, and can continue producing ethylene to make its mole fraction keep growing under the investigated temperature ranges in the jet-stirred reactor pyrolysis. Under the flame propagation conditions, C0 C1 reactions have the highest sensitivity coefficients to the flame propagation, while ethylene- and vinyl-involved reactions also play important roles due to the abundant production of ethylene.

In spite of widely use of ethanol, mostly as a fuel extender in ground transportation engines, its sooting propensity with pressure and with ethanol content in the base fuel has not been clarified. Although, information on ethanol’s sooting and other combustion characteristics at atmospheric conditions is extensive, scaling this information to elevated pressures is problematic. Information that could be gathered from tractable laminar diffusion flames on the soot formation of ethanol’s response to pressure would be beneficial in assessing the soot emissions from engines fuelled with hydrocarbons supplemented with ethanol. To address this lack of information at pressures above atmospheric, high pressure soot formation in laminar co-flow diffusion flames fuelled by ethanol was investigated using nitrogen-diluted ethylene as the base fuel on a burner of 3 mm diameter fuel nozzle. Base fuel nitrogen to ethylene mass ratio was kept fixed at 6 for all experimental cases. In terms of total carbon mass flow, which was kept constant at 0.41 mg/s, ethanol’s contribution was varied as 0%, 10%, 30% and 40%, and the experiments were conducted at 3, 4, and 5 bar pressures. The main aim of the investigation was to evaluate the soot production at elevated pressures with increasing ethanol fraction in the fuel stream treating nitrogen-diluted ethylene as the other fuel component. Flame heights defined by the luminous visible tips did not display any significant changes as the ethanol fraction or pressure was changed. Spectrally-resolved line-of-sight soot radiation collected along the radial distance from the flame centreline at various heights above the burner exit with 1 mm increments was converted to radial soot temperature and volume fraction distributions through an Abel-type inversion algorithm. Within the bounds of pressure considered, soot production increased with the amount of ethanol added to the fuel stream. Largest incremental increase in soot production over ethylene diluted with nitrogen was observed at the condition with 10% carbon from ethanol in the fuel blend; further increases in ethanol fractions kept increasing the soot but at relatively lower rates. This non-linear effect with ethanol addition was discussed and it was argued that one of the reasons for this behavior is the influence of methyl radical produced by radical-induced decomposition of ethanol enhancing soot production for 10% ethanol case. With further increases in ethanol amount, this effect seems to slow down because of reduction in ethylene fraction and the increase in hydrogen to carbon ratio in the fuel stream at fixed carbon flow rate.

Although C5H4 isomers are detected in flames, they are not thoroughly incorporated into detailed chemical kinetics models (DCKMs). Here we use RRKM/ME modelling to simulate C5H4 + H reactions on a C5H5 potential energy surface. Kinetic studies indicate that C3H3 + C2H2 is the main fate but fall-off from the initial adduct isomer back to C5H4 + H cannot be ignored at relevant combustion temperatures of 900 to 2000 K. Calculated rate coefficient expressions were incorporated into a DCKM for a toluene flame, along with updates to other relevant reactions from the recent literature, particularly the open-chain 1-vinylpropargyl radical, l-C5H5. Obtained species mole fractions were found to be in good agreement with published experimental data for a low-pressure toluene flame, with a significant improvement in predicted concentration of the cyclopentadienyl radical. The presented DCKM will allow for further reactions of C5Hx species such as 1-vinylpropargyl to be included in combustion simulations.

Oxy-combustion is an effective carbon capture technology. Many oxy-combustion technologies utilize recycled flue gas (RFG) for dilution to control temperature and heat flux. As the concentration of dilution gas on the fuel and oxidant side changes, stoichiometric mixture fraction (Zst) and flame temperature will change and significantly impact the flame structure and soot formation characteristics. In this work, the effects of stoichiometric mixture fraction and flame temperature on soot formation characteristics in laminar diffusion flames are studied by diluting the fuel and changing the oxygen concentration. CO2 is used as a dilution gas to simulate the RFG in oxy-combustion. The numerical calculation combines gas reaction kinetics with a soot formation model. Soot nucleation, surface growth, oxidation processes are considered, as well as the distributions of temperature, soot concentration, and key substances. The experimental flame appearances and spectral radiation data are imaged by a hyperspectral imager, and the temperature and soot concentration are reconstructed. The results of experimental measurement and numerical calculation are compared to evaluate the applicability of the soot formation mechanism to oxy-combustion with elevated Zst and using CO2 as diluent. As the Zst increases, the flame changes into a blue flame, soot concentration and temperature in flames decrease, because nucleation and surface growth are both inhibited and oxidation is enhanced. As the flame temperature increases, the flame becomes brighter, numerical results indicate that soot formation and oxidation are both enhanced, while the promoting effect of temperature increase on surface growth is stronger than that of oxidation, resulting in an increase in soot concentration. This study provides a fundamental understanding of the effects of RFG utilization (fuel dilution and oxygen enhancement) on flame structure, temperature distribution, soot concentration and formation characteristics in non-premixed oxy-combustion systems.

Real fuels are complex mixtures of hundreds of molecules, which makes it challenging to unravel their combustion chemistry. Several approaches in the literature have helped to clarify fuel combustion, including multi-component surrogates, lumped fuel chemistry modeling, and functional-group based methods. This work presents an innovative advancement to the lumped fuel chemistry modeling approach, using functional groups for mechanism development (FGMech). Stoichiometric parameters of lumped fuel decomposition reactions dictate the population of the key pyrolysis products, previously obtained by fitting experimental data of real-fuel pyrolysis. In this work, a functional group-based approach is proposed, which can account for real-fuel variability and predict stoichiometric parameters without experimentation. A database of the stoichiometric parameters and/or yields of key pyrolysis products was first constructed for approximately 50 neat fuels, based on previous pyrolysis data and a lumped kinetic model we developed. The effects of functional groups on the stoichiometric parameters and/or yields of key pyrolysis products were then identified and quantified. A quantitative structure-stoichiometry relationship was developed by multiple linear regression (MLR) model, which was used to predict the stoichiometric parameters and/or yields of key pyrolysis products based on ten input features (eight functional groups, molecular weight, and branching index). Products from the pyrolysis of surrogate mixtures and real-fuels were predicted using the MLR model and validated against experimental data in the literature. Comparison with the stoichiometric parameters from the HyChem experiment-based approach (Xu et al., 2018) showed that the predicted values in this work were in reasonable agreement (generally within a factor of two). When the stoichiometric parameters in the jet fuel (POSF 10325) HyChem kinetic model were replaced with this functional-group based prediction, only minor discrepancies were observed in the predictions of key pyrolysis products and global combustion parameters (such as ignition delay times and laminar flame speeds). Sensitivity analysis on stoichiometric parameters revealed their different roles in predicting speciation and global parameters. The functional group approach for predicting stoichiometric parameters in this work was the first step towards developing FGMech for modeling real-fuel combustion chemistry. Further development of the FGMech model's thermodynamic, kinetic, and transport data will be presented in a following study.

One of the major concerns in high pressure combustion is its high soot yield. An exact and comprehensive mechanism behind this phenomenon, from a chemical kinetics perspective, is still elusive. In this study, a series of pressurized (1–16 atm) co-flow ethylene diffusion sooting flames are simulated with detailed finite-rate chemistry and molecular transport. The experimental maximum soot volume fraction and its scaling law with pressure are well reproduced by the simulations. To extract kinetic information from the complex sooting reacting system, a Soot-based Global Pathway Analysis (SGPA) method is developed to identify the dominant Global Pathways (GPs) from fuel to soot by considering carbon element flux from gaseous species to soot. Using SGPA, the dominance and sensitivity of soot chemical pathways at elevated pressures are revealed. It is found that increasing pressure shifts the first ring Polycyclic Aromatic Hydrocarbon (PAH) formation from C 3 H 3 recombination to reactions involving C 2 H 2 . At 1 atm, the production of C 2 H 2 for surface growth is purely controlled by the H-abstraction of C 2 H 4 and C 2 H 3 . In contrast, at elevated pressures, the production of C 2 H 2 for surface growth is also influenced by many other reactions including some third body reactions. The SGPA method reveals that the mismatch of predicted PAH with the experimental data at 12 atm is majorly caused by the rate coefficient uncertainty of the reaction C 2 H 2 + A1CH 2 = C 9 H 8 + H. Based on the analysis by SGPA, the mechanism reduction based on Directed Relation Graph with Error Propagation (DRGEP) with A2 and C 2 H 2 as the target species deleted significant species such as C 9 H 8 , C 9 H 7 , incurring inaccurate soot field prediction. It is also found that the combined dominance of GPs with heavier PAH species (A4-A7) is even greater than the most dominant GP at the flame wing regions, indicating that heavier PAH species play critical roles for soot nucleation and condensation, especially at the flame wing regions.

Carbon free fuels such as hydrogen/ammonia blends show a promising potential to become sustainable and renewable fuels for gas turbines and other combustion systems. One interesting aspect about these blends is the possibility to adjust different combustion properties like the laminar burning velocity or ignition delay time by changing the ratio between H 2 and NH 3 . Such fuel blends can be produced via partial catalytic decomposition of NH 3 . However, such mixtures can lead to flame instabilities such as flashback, especially if the hydrogen content is high. In the present study, the boundary layer flashback of premixed hydrogen/ammonia/air mixtures is investigated experimentally for non-swirling flows at normal temperature (293 K) and normal pressure (101 kPa). A new experimental setup for boundary layer flashback investigation with a fully automated measurement procedure is introduced. With preliminary studies, the influence of various measurement procedures on the flashback limits is firstly investigated. For a broad flashback study, the data of 351 flashback experiments are collected. The ammonia content in H 2 / NH 3 fuel mixtures is varied from 0 vol% to 50 vol% in 10 vol% steps. The fuel–air equivalence ratio is ranging from 0.38 to 1.17. As the ammonia content is increasing, the mean flow velocities at flashback are exponentially decreasing. Additionally, theoretical modeling is performed. A model is derived based on the concept of the critical velocity gradient which is able to predict the measured data with high accuracy. For two exemplary cases with H 2 /air and 80% H 2 /20% NH 3 /air mixtures, the process of boundary layer flashback is investigated in detail with low and high speed direct imaging and image post-processing. During the flashback onset of H 2 / NH 3 /air flames a separate reaction of H 2 followed by the reaction of NH 3 can be observed. Also, a flame-oscillation between fused silica tube and burner head with approximately 10 Hz was observed. Furthermore, indications for an adverse pressure gradient based on the flame propagation speed is seen. Details about the flame structure during the flashback process of H 2 / NH 3 /air flames are shown. During the upstream flame propagation of H 2 / NH 3 /air flames, high frequency oscillations with about 830 Hz of the leading flame tip are observed, which are assumed to be related to thermoacoustic instabilities.

Thermochemical interaction – represented by CO mole fraction and gas phase temperature measurements – between flame and cooling air is investigated in a close-to-reality effusion-cooled single sector model gas turbine combustor. To investigate the influence of effusion cooling air mass flow on the thermochemical state, a parametric study is conducted. Temperature measurements are performed using ro-vibrational N 2 coherent anti-Stokes Raman spectroscopy (CARS). CO mole fraction is measured by means of quantitative CO two-photon laser-induced fluorescence (CO-LIF) using a temperature dependent calibration acquired in an adiabatic pressurized laminar flame. Significantly different thermochemical states are observed in the inner and outer shear layer of the swirl stabilized flame. Within the primary zone, increasing cooling air mass flow leads to decreased CO concentrations. Close to the effusion cooled liner, the interaction varies with axial coordinate. In the outer recirculation zone, increased CO mole fractions were measured with increasing cooling air mass flow, indicating occurrence of chemical quenching in the late oxidation branch in the CO-T diagram. Further downstream, processes are dominated by mixing and CO concentrations decrease with the amount of supplied effusion cooling air. To our best knowledge, this is the first time that these effects has been shown experimentally.

Here we present analysis of a novel reactive material system that employs dinitrogen tetroxide (N2O4) as a liquid oxidizer with metal powder fuels. The oxidizer was added to micron scale aluminum and zirconium powders by a remote injection system. When ignited with a high voltage spark, the mixtures were observed to possess reactivity comparable to nanocomposite reactive materials, with open-tube flame expansion velocities from 500 to 1400 m/s depending on fuel/oxidizer ratio and tube diameter. Temperatures were observed to range from 3000 to 3500 K as measured with gray body fits to 16-channel time-resolved pyrometer and time-integrated spectrometer data. These values were significantly below calculated adiabatic flame temperatures, which we attribute to local deviations from stoichiometry and kinetic/energetic limitations similar to those observed in studies of particles burning in high pressures gaseous oxidizers. Al/N2O4 reactivity was found to be most likely limited by the vaporization of the metal from the particle surface and Zr/N2O4 was limited by slower burning and complex interactions involving the solubility of nitrogen and oxygen in the molten Zr. We also discuss the potential for these materials to be used to create an “on/off” reactive material, since N2O4 can be added remotely and driven to evaporate via vacuum or purge of inert gases to return it to a safe condition.

The pyrolysis of three-component surrogate fuel for jet fuel has been studied experimentally in flow reactor using synchrotron photoionization and molecular beam mass spectrometry techniques with temperature range of 850–1150 K. Alkenes are the most abundant products in the decomposition process. Other important intermediates such as alkanes, alkynes, polycyclic aromatic hydrocarbons were also identified and quantified. Detailed kinetic reaction model involving 462 species and 3170 reactions have been developed by validating against the measured results as well as oxidation data reported previously with reasonable predictions. Detailed rate of production and sensitivity analysis indicated T135MCH mainly decay through demethylation and H-abstraction reactions. Moreover, n-propylbenzene consumption is more sensitive to CH 3 than H, while n-dodecane and T135MCH prefer H radical. The reactions between fuel and fuel-derived radicals show limited effect on surrogate fuel consumption. Reactivity changes of NPB and NC12H26 were investigated through the comparison of fuel conversion ratio. The coupling effect was displayed via the common and important sensitive reactions which are related to the H and CH3 radicals. Present experimental data and reaction model will contribute to a better understanding of combustion behavior of 3C surrogate fuel. Thus, the results of present work could contribute to comprehensive investigation of jet fuel combustion properties.

This work examined combustion modes in a fundamental, axisymmetric, supersonic combustor at flight equivalent Mach numbers ranging from 7.5 to 9.0. Ethylene was injected at a variety of mass flow rates to examine both scram-mode, jet wake stabilized and dual-mode combustion. At higher flight Mach number conditions, the fuelling rate required to transition from scram-mode to dual-mode combustion increased. Distributions of the OH radical were observed using planar laser-induced fluorescence (PLIF) in a cross-sectional plane immediately downstream of the combustor exit, and analyzed according to their variation in the radial and circumferential dimensions. It was observed that radial centroids of the ring-like OH PLIF signal for the scram-mode cases approached the centerline linearly with increasing equivalence ratio, but the dual-mode cases appeared to randomly fluctuate in the observed exhaust cross-section. The distributions of scram- and dual-mode cases were clustered in specific regions on plots of circumferential variation as a function of radial centroid, with a jet wake stabilized case appearing between the two clusters. These clusters may be used to help identify scram- and dual-mode combustion in future experiments.

Combustion instabilities constitute a well pronounced problem in all large rocket motors due to the inherent acoustic pressure oscillations established based on the port geometries. Suppression of the combustion instabilities in the solid rocket motor is required for controlling the mean burning rate variation which arises due to the interaction of acoustic pressure wave with propellant combustion. An experimental study has been carried out to investigate the effects of acoustic pressure oscillations on mean burning rates of non-aluminized and aluminized propellants which exhibit low pressure exponent index (n) in the burning rate trends. Steady and unsteady mean burning rates are determined from combustion photography method using a window bomb test facility over the pressure range of 17 MPa. A rotary valve is coupled with the window bomb setup to generate acoustic pressure oscillations inside the test chamber (cylindrical pressure vessel), which imposes the required frequencies of 140, 180 and 240 Hz respectively. The acoustic pressure amplitudes are varied from 0.04% to 1.4% of the mean chamber pressure. Both non-aluminized propellants and aluminized propellants have shown significant enhancement in the mean burning rate due to the fluctuations imposed by acoustic pressure amplitudes and frequencies on the propellant combustion. The enhancement in the mean burning rate also depends upon dynamic response of the flame to the excited frequencies and acoustic pressure amplitudes. The plateau burning behaviour of the non-aluminized propellant is completely distorted whereas it is retained in aluminized compositions. Conversely, it also shifts the mean pressure range of plateau burning rate trend. The maximum burning rate augmentation factors resulted from imposed acoustic pressure wave on non-aluminized and aluminized propellants are observed as 1.27 and 1.47 respectively.

In this study, an on-the-fly artificial neural network (ANN) framework has been developed for the tabulation of chemical reaction terms in direct numerical simulations (DNS) of premixed and igniting flames. The procedure does not require any preliminary knowledge to generate samples for ANN training; the whole training process is based on the detailed simulation results and takes place on-the-fly, so that the obtained ANN model is perfectly adapted to the specific problem considered. The framework combines direct integration (DI) and ANN model in an efficient way to overcome the extrapolation issue of the monolithic ANN model. Auto-ignition processes as well as the characteristics of established flames can be very well predicted using the ANN model. In the final simulations, involving a case with 3D turbulent hot-spot ignition, and a flame propagating in a turbulent flow, the developed procedure reduces the computational times by a factor of almost 5, while keeping the error for all species below 1 % compared to the standard, monolithic DI solution.

A model of combustion of aluminum nanopowder in water vapor has been analyzed with allowance for the formation of a condensed phase of aluminum oxide on aluminum particles. Various processes affecting the growth rate of the oxide coating have been considered including adsorption and surface diffusion of Al2O3 molecules on aluminum particles, and condensation of the molecules on the oxide coating. Model calculations have yielded the time profiles of system temperature, phase and fractional composition, as well as the shape of oxide coated aluminum particles. The heterogeneous processes forming the condensed phase of aluminum oxide on Al particles have shown to increase heat generation in the system. On the other hand, shielding aluminum particles by oxide coating retards aluminum evaporation. The competition of these processes during the formation of oxide coating is one of the factors affecting the combustion of aluminum nanopowder in water vapor.

A recent analysis (Lipatnikov et al., 2020) of complex-chemistry direct numerical simulation (DNS) data obtained from lean hydrogen-air flames associated with corrugated-flame (case A), thin-reaction-zone (case B), and broken-reaction-zone (case C) regimes of turbulent burning has shown that the flamelet concept (i) can predict mean concentrations of various species in those flames if the probability density function (PDF) for the fuel-based combustion progress variable c is extracted from the DNS data, but (ii) poorly performs for the mean rate W¯c of product creation. These results suggest applying the concept to evaluation of mean species concentration (but not the mean rate) in combination with another closure relation for W¯c whose predictive capabilities are better. This proposal is developed in the present paper whose focus is placed on studying a new flamelet-based presumed PDF P(c) for predictions of mean concentration of radicals in engineering computational fluid dynamics (CFD) applications. Analysis of the DNS data shows that (i) the flamelet PDF performs well at intermediate values of c in cases A and B, but should be truncated at small and large c, (ii) modeling P(c) in the radical recombination zone (i.e., at large c) is of importance for predicting mean concentrations of H,O, and OH. Accordingly, the flamelet PDF is truncated and combined with a uniform P(c) at large c. Moreover, the mean rate W¯c extracted from the DNS data is used to calibrate the PDF (the rate is considered to be given by another model). Assessment of the approach against the DNS data shows that it well predicts mean density, temperature, and concentrations of reactants, product, and the aforementioned radicals in cases A and B. In case C, the approach performs worse for OandOH at large c¯ and moderately underestimates the mean concentration of H in the entire flame brush.

Electrostatic discharge (ESD) or spark produced by breakdown of a gap while discharging a capacitor is a common ignition stimulus for powders of reactive and energetic materials. It is a safety hazard, but is also used to initiate pyrotechnic formulations and as a characterization method of reactivity of energetic materials. In previous experimental studies, the powder served as one of the ESD electrodes. Ignition was found to be caused by Joule heating, with the main heat release by current passing through points of contact between particles. However, ignition may occur when the ESD does not pass directly through the powder. ESD generates a shock wave, which can lift the powder from a substrate even if the substrate is not an electrode. ESD also generates a plasma kernel heated to ca. 10,000 K, which exists for hundreds of µs, often much longer than the ESD current. Here, the effect of the ESD-induced shock and plasma on a reactive powder was studied experimentally for the first time. A thin layer of Zr powder was placed on a substrate located under a spark gap between two pin electrodes. Photo-sensors and high-speed video were used to document ignition and combustion events. The ESD energy and distance from the spark gap and powder were varied. At close distances, particles ignited consistently. Distance limits were determined, beyond which ignition was no longer observed. Higher ESD energies as well as longer ESD pulses led to ignition at greater distances. Ignition involved two distinct processes. First, submicron particles were lifted from the substrate on a time scale of single microseconds, interacted directly with the plasma kernel, and combusted. Optical emissions from this combustion lasted 100 – 600 µs, as expected for the burn time of submicron Zr particles in air. The second ignition event occurred several milliseconds later. It involved a flame propagating through Zr powder aerosol or combustion of individual lifted particles. This event was caused by coarser Zr particles radiatively heated during ESD. Importantly, the powder was aerosolized by convective flows caused by the rising plasma kernel making it possible for the self-heating of particles oxidizing in air to eventually lead to their ignition and combustion. Observed times for these delayed ignition and combustion events were in the order of tens of milliseconds.

The exergy loss characteristics of combustion processes under homogeneous-charge compression ignition (HCCI) and stratified-charge compression ignition (SCCI) conditions are numerically investigated by analyzing two-dimensional (2-D) direct numerical simulation (DNS) data. Two fuels, dimethyl ether and ethanol, together with the initial conditions of different mean temperatures, and levels of temperature and concentration fluctuations relevant to HCCI/SCCI conditions were investigated. It is found that the prevalent deflagration mode significantly decreases the maximum exergy loss rates and spreads out the exergy loss rate for all the cases regardless of fuel types, temperature regimes, and temperature and/or concentration fluctuations. The primary irreversible sources of exergy loss are also identified. The chemical reaction is found to be the primary contributor to the total exergy loss, followed by heat conduction and mass diffusion, regardless of the fluctuation levels. It is also found that the relative change of exergy loss due to chemical reactions, EL chemrel , correlates strongly with the heat release fraction by deflagration. The maximum EL chemrel is found to be less than 10%. Chemical pathway analysis reveals that the exergy loss induced by low-temperature reactions, represented by the decomposition of hydroperoxy–alkylperoxy and the H-abstraction reactions of the fuel molecule, is much lower under the SCCI conditions than that under the HCCI conditions. Generally, the dominant reactions contributing to the exergy loss in the high-temperature regime are nearly identical for the HCCI and SCCI combustion. Key reactions, including the H 2 O 2 loop reactions, the reactions of the H 2 –O 2 mechanism, and the conversion reaction of CO to CO 2 , CO + OH = CO 2 + H , are found to contribute more than 50% of the total exergy loss. Due to locally higher reactivities by temperature and concentration fluctuations inducing deflagration dominance, these reactions occur at a relatively higher temperature (1600 K–1900 K) compared with the homogeneous zero-dimensional cases ( ∼ 1400 K), resulting in a net reduction in exergy loss.

Considerable toxic gases and high temperature smoke will be generated during the combustion of polystyrene (PS), which restricts its application. Here, a cobalt-based MOF-71-NH2 (hereinafter referred to as MOF-NH2) has been synthesized and further modified with phosphonitrilic chloride trimer (PCT) by a post-synthesis modification (PSM) strategy, named as PCT@MOF-NH2, which was used to enhance the flame retardancy of PS. Desirable results were obtained as expected: the fire safety and tensile strength of PS were prominently improved after adding PCT@MOF-NH2. Compared with pure PS, there were more than 40% and 31% decreases in the value of pHRR and THR with 3.0 wt% content of PCT@MOF-NH2. From the analysis of gaseous and condensed products after combustion, the possible flame retardancy mechanism of PS nanocomposites can be attributed to the barrier effect of PCT@MOF-NH2, which provides a promising application field of MOFs to improve flame retardation of polymer materials.

Numerical simulations of one-dimensional pulsating detonation in off-stoichiometric n-heptane/air mixtures are conducted by solving the reactive Navier–Stokes equations with a skeletal chemical mechanism. The effects of mixture equivalence ratio, initial pressure and temperature on pulsating detonations are studied. The results show that the pulsating instabilities in n-heptane/air mixtures are strongly affected by equivalence ratio. It is seen that pulsating instability only occurs in the fuel-lean or fuel-rich cases, whereas stable detonation is obtained for near-stoichiometric mixtures. Low-frequency pulsating detonations with single mode are observed, and decoupling / coupling of the reaction front and leading shock front occur periodically during the pulsating detonation propagation. The heat release and flame structure at the reaction front of the fuel-lean case differ from those in the fuel-rich case, and thus affects the DDT process of the reaction front. The pulsating detonation frequency is considerably influenced by equivalence ratio, initial pressure and temperature. The results of chemical explosive mode analysis and budget analysis of energy equation reveal that the mixture between the reaction front and shock front is highly explosive and thermal diffusion would promote the periodic dynamics of the reaction front and shock front. It is also found that the chemical explosion mode in the induction zone consists of two parts, i.e. the autoignition dominated reaction immediately behind the leading shock front and a following propagating reaction front.

High-resolution synchrotron x-ray radiography with computed tomography is used to observe the evolution of porosity created by thermal exposure in two HMX-based polymer-bonded explosive compositions; LX-04 and BX-63. The measurements were made in situ, over an extended period of time, during which the samples were heated on a slow-rate thermal trajectory. The tests ended with thermal-runaway to ignition after which the samples were consumed by combustion. The primary means of damage appears to be from mechanical debonding of the HMX-binder interface with secondary contribution from chemical decomposition. Confinement and binder properties affect the amount of porosity and permeability that develops. Additionally, observations were made describing the emergence and structure of an internal ignition volume, the formation and transport of a pre-ignition melt layer, and how the early stages of combustion were affected by material morphology, mechanical confinement and melt. The contact angle between molten HMX and the fluoropolymer, Viton A, is also presented. For the first time we have time-resolved x-ray images of ignition in sufficient detail to verify the mechanism of cookoff in polymer-bonded explosive compositions.

The incorporation of aluminum (Al) into explosives represents a facility and efficient way to enhance the energy output through the secondary reaction between Al and the detonation products of explosive. The Al/O ratio and microstructure of HMX/Al play critical roles in combustion performance and energy output. Herein, a gradient structured HMX/Al composite has been designed and fabricated via 3D printing technology using well-dispersed HMX/Al-based ink. For HMX/Al lines, the second reaction heat increases from 196.5 J/g to 1644.8 J/g and the burning rate would decrease from 14.6 to 12.0 mm/s with Al content increased from 10 to 30 wt%. For gradient structured HMX/Al, the burning rate could be controlled by varying the component ratio and the burning rate of each component in the gradient structure was higher than that of the normal HMX/Al counterpart. Furthermore, a gradient evolution of pressure output was monitored for gradient structured HMX/Al cylinder ignited by the laser. These results indicate that the gradient structure as an effective approach can tune combustion performance and energy output of HMX/Al.

A prospective method to assess thermo-acoustic instabilities based on two reflection coefficients measured from the upstream side of the burner is presented and experimentally validated. In order to compose a model which allows predicting the onset of thermo-acoustic instability of combustion in a practical appliance, one has to characterize the thermo-acoustic properties of the burner including the flame as an acoustically active element and acoustic properties of all other (usually passive) components of the combustion appliance both upstream as well as downstream of the burner. This kind of modeling strategy usually faces serious practical problems related to the need of measurements/modeling at the hot downstream part of the system. In the present work, we propose a measurement and a system modeling approach which relies on two acoustic measurements, namely reflection coefficients, only at the cold (burner upstream) part of the combustion appliance. Both reflection coefficients, termed upstream and input, can be readily measured using standard acoustic techniques. The need to measure the input reflection coefficient of an acoustically active subsystem may impose difficulties related to the acoustic instability of the measurement setup itself. The approach and technical solution to handle this problem via a special modification of the excitation source (loudspeaker box) is proposed. The dispersion relation to search for system eigen frequencies is represented in a form that couples the reflection coefficients of the upstream part of the appliance and input reflection coefficient from the downstream part as observed through the burner with flame. This form of the dispersion relation is commonly used in the theory of radio-frequency circuits and recently introduced for thermo-acoustic problems. The proposed method is applied to burners with premixed burner-stabilized Bunsen-type flames. The observed instability conditions and oscillation frequencies are compared with predictions of the proposed modeling approach and reveal good correspondence.

Ozone (O3) has been shown to accelerate low and high temperature chemistry and is a promising strategy to effectively enhance and control combustion and ignition processes, such as for engines. There has, however, been a lack of experimental investigation of O3 enhancement effects on low temperature chemistry and premixed cool flames. This study undertook a detailed experimental and numerical assessment of O3 enhancement effects on ignition behavior, propagation speeds, flame temperatures, and CH2O production of n-heptane cool flames as functions of O3 addition. Zero-dimensional simulations were used to gain insights into the effects of O3 on two-stage ignition behavior and gradual suppression of NTC phenomena. Propagation speeds of O3-enhanced cool flames were experimentally determined using a flame lift-off technique, and one-dimensional simulations utilizing several full and reduced mechanisms were performed for comparison. Enhancement by O3 versus enhancement by preheating were experimentally compared and analyzed to evaluate relative effectiveness of these enhancement strategies. Finally, downstream temperature measurements and formaldehyde planar laser-induced fluorescence imaging (CH2O PLIF) were performed at different O3 enhancement levels to further elucidate O3 impacts, and numerical simulations were performed to support experimental insights. Results of this study reveal O3 impacts on low temperature kinetics and resulting impacts on low temperature ignition and cool flames.

Using traditional metallic combustion catalysts in composite solid propellant (CSP) can lead to sharp energy losses of CSP due to the non-energetic properties of the catalysts. In the current study, the energetic ligands based 3D [Cu(atrz)3(NO3)2]n metal-organic frameworks (MOF(Cu)) were first used as the combustion catalysts to study their effects on the performance of CSP. It was found that the variation of MOF(Cu) content in the range of 0–5% had no significant effects on the theoretical specific impulse (Isp) and characteristic velocity (C*) of CSP. MOF(Cu) reduced the high decomposition temperature of AP to 344 °C and the complete decomposition temperature of CSP to 358 °C. It also decreased the thermal decomposition activation energy of CSP from 74.1 kJ mol−1 to 63.3 kJ mol−1. The CSP containing MOF(Cu) exhibits higher burning rates, particular flame structure and lower pressure index, as compared with that with CuO as the catalyst. The mechanical sensitivity test results reveal that MOF(Cu) can reduce the friction sensitivity and impact sensitivity of CSP from 40% and 39.4 cm to 32% and 63.5 cm, respectively. The current study is promising to develop a new kind of energetic combustion catalysts for CSP.