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Direct numerical simulations (DNS) with detailed chemical kinetics and chemical explosive mode analysis (CEMA) are performed to investigate the effect of operating pressure and hydrogen–methane blending on laminar wrinkled flames at reheat combustion conditions. Building upon previous three-dimensional DNS datasets of reheat hydrogen flames, the present work considers two-dimensional configurations that render computationally-feasible simulations of high-pressure combustion and significantly more complex hydrocarbon chemistry. A geometrically-simplified representation of the reheat combustor is adopted and the thermodynamic states considered in the simulations are carefully chosen to mimic gas turbine operation conditions, which are constrained to achieve a target flame position and flame temperature. The two-dimensional DNS results are first assessed against the available three-dimensional data and then analyzed to extract the main trends exhibited by the reheat combustion process with respect to the fraction of the fuel consumed by spontaneous ignition and flame propagation, for increasing pressures and hydrogen fractions. Due to significant differences in the characteristic features of the velocity and thermal boundary layers between the three-dimensional and the two-dimensional configurations, a different global flame shape is observed (V-shaped flame vs W-shape flame) together with a ∼ 10 % bias in the fraction of fuel consumed by spontaneous ignition. Crucially, results from the scaling studies reveal very clear trends with a significant decrease in the fuel consumption by spontaneous ignition for increasing pressures and hydrogen fractions, at the conditions investigated. Finally, this study highlights the important role that first-principle direct numerical simulations, even when conducted in computationally affordable two-dimensional simplified geometrical configurations, can play in obtaining detailed insights and quantitative trends useful for the characterization of complex reactive flows. Novelty and significance The reheat burners of the two-stage sequential gas turbine combustors are designed to stabilize flames primarily due to spontaneous ignition. However, prior numerical simulations revealed the presence of an additional assisted-ignition mode aided by recirculation zones. In this study, we used practically feasible two-dimensional direct numerical simulations of premixed hydrogen–air mixtures under lean conditions to quantify the roles of the two flame stabilization modes at different operating pressures. Achieving fundamental understanding of the effect of pressure and hydrogen fraction on flame stabilization is crucial to the development of two-stage sequential combustion and the present two-dimensional calculations provide important new insights. We show that at high pressures, both modes consume fuel to a similar extent. We also investigated the effect of methane blending on flame stabilization. This study also discusses how two-dimensional direct numerical simulations can be leveraged in the design optimization of reheat burners at low technology readiness levels. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This article provides a computational analysis of the reaction of fully-dense layered aluminum and copper oxide systems. After the detailed presentation of the 2D nonstationary model implementing both oxygen and aluminum diffusion, the propagation of the reaction front in an Al/CuO thin film was studied. The model qualitatively reproduces the dependency of the reaction front progression rate spatially as a function of the fuel concentration. Calculations also evidence the inverse evolution of flame front width with respect to the reaction front velocity. A procedure to estimate the heat loss generated by the fact that the reactants and products may vaporize prior to reaction completion was proposed by imposing a flame temperature limit close to Cu vaporization point. This work also shows that microscopic fluctuations in the instantaneous reaction front velocity can be observed for reactant diffusion activation energy (E a) of 125 kJ/mol, before quenching for greater E a. Finally, we demonstrate the potential of this new 2D nonstationary model to investigate the thermal effect of additives such as metallic impurities in the Al/CuO thin film that can lead to the flame front corrugation at the microscale. The simulations show that a metallic particle acts first to boost the reaction velocity as its high thermal conductivity helps the upfront heating. Then, the particle being also a heat sink, a local slowing down of the front velocity is observed. [ABSTRACT FROM AUTHOR]

The study aims to identify the characteristics of ignition and flame propagation in an n-heptane/methane–air dual-fuel mixture based on a specified mixing layer scheme using a group of numerical simulations by incorporating a skeletal chemistry mechanism. The effects of different temperature stratifications and uniform temperature distribution on autoignition kernel development, flame propagation, and flame structure in the dual-fuel combustion have been comprehensively investigated. The results indicate that with the increase in the oxidizer stream temperature, the high-temperature ignition (HTI) location in the mixing layer changes non-monotonously, as determined by the most reactive mixture fraction in the mixture fraction–temperature (ξ − T) space obtained from zero-dimensional simulation. The HTI points located in the negative temperature coefficient (NTC) region shift to a richer mixture with temperature increase, whereas the points located in the high temperature region shift to the smaller mixture fraction region. Additionally, the results demonstrate that for the ignition points in the NTC region, the shift to the richer mixture weakens the combustion rate, resulting in a prolonged time interval for steady flame propagation although the oxidizer stream temperature is increased. Furthermore, the low-temperature ignition (LTI) front propagation is driven by the gradient of the ignition delay time, and the HTI propagates as a spontaneous ignition front at first and then transforms to a diffusion-driven flame. The results also demonstrate that the maximum temperature of the LTI propagates to the richer mixture with temperature stratification, in contrast to that with a uniform temperature. Lastly, the comparative analyses of chemical explosion mode (CEM) and flame structure are conducted for two representative cases. A wide non-CEM region induced by low-temperature chemistry (LTC) occurs in the oxidizer stream when its temperature is relatively lower. And double peaks of heat release rate (HRR), that represent the HTI of n-heptane and post-combustion of methane respectively, are observed within the ignition front at the oxidizer side. [ABSTRACT FROM AUTHOR]

To reduce the size inconvenience of airbag systems employed in human-body protection devices while maintaining comparable gas generation performance, a new family of gas-generating energetic composites are proposed mixing Al/CuO nanothermite with copper complex (Cu(NH 3) 4 (NO 3) 2) known for its propensity to generate gases (0.03 mol/g) such as N 2 , O 2 , N 2 O through exothermic chemical decomposition (300 J/g). The aluminum (Al)/Copper oxide (CuO) couple, known as the most widely studied nanomaterial for thermite reactions, releasing a high energy, mostly heat, through chemical reaction, is employed as a source of heat to trigger and sustain the decomposition of Cu(NH 3) 4 (NO 3) 2 complex. This work permits developing a new family of gas-generating energetic composites that takes advantage of specific chemical and thermal properties of both materials. We demonstrate its capability to tune the pressurization rate, burn rate and pressure peak by varying the Al/CuO over Cu(NH 3)(NO 2) 2 mass ratio. The peak pressure of Cu(NH 3) 4 (NO 3) 2 /Al/CuO energetic composites reaches 12 MPa/g.cm3 in a close volume, which is 3.3 higher than that of traditional Al/CuO nanothermite. They achieve much longer high-pressure duration (∼30 ms). They also exhibit very intense burning with velocity reaching hundreds of m/s in opening burning experiments. These new materials appear very promising for green gas generation. [ABSTRACT FROM AUTHOR]

Unsteady flame propagation within a narrow channel, namely a Hele-Shaw burner, exhibits complicated phenomena. Recently, a new narrow-gap-disk-burner (NGDB) was developed, of which the disk-gap could be varied continuously and precisely. Although various complicated flame structures have been observed successfully, their dependency on the initial ignition has not been clarified. In this study, the volume of the ignition part was varied to introduce disturbance at the ignition stage, and the propagation characteristics of premixed methane and propane flames were investigated. Conclusively, quenching distance was not significantly affected by the ignition volume, especially in propane-rich conditions. In contrast, the flame structure and propagation velocity were sensitive to the ignition volume if it was larger than a critical volume, and when the disk-gap was approximately 1.5 times the quenching distance. A strong initial disturbance could generate complicated cellular structures coupled not only with shear stress but also with heat transfer. These cellular structures could increase the flame propagation velocity when the Lewis number was sufficiently smaller than unity. In contrast, the flame shape became smoother when the disk-gap was sufficiently larger than the quenching distance. Thus, the flame propagation velocity was comparable to the laminar burning velocity when it was less affected by the initial disturbance. [ABSTRACT FROM AUTHOR]

The main objective of this work is to identify the end-gas combustion mode transition under different initial thermodynamic conditions and to focus on the role of pressure waves in autoignition formation and detonation development in the confined space by a group of two-dimensional (2D) numerical simulations with detailed chemistry of H 2 /air mixture. Pressure waves with different strengths are obtained by the flame acceleration in closed chambers with and without obstacles under three different initial temperatures. The results indicate that with the increase of initial temperature, there exist three different end-gas autoignition transition modes: autoignition promoted by strong flame acceleration, autoignition suppressed by weak flame acceleration and autoignition independent of flame acceleration. It is also shown that there are three types of end-gas autoignition-induced detonation initiation: (1) detonation initiated directly by the pressure wave generated from the flame propagation; (2) detonation initiated directly by the pressure wave generated from other hot-spot autoignition; (3) autoignition to detonation transition based on the reactivity gradient theory. Meanwhile, to further identify the autoignition transition mode under continuous variation of pressure wave intensity and initial temperature, an idealized physical model, with the Mach number of the pressure wave as the link between the flame propagation duration and autoignition time of the end-gas, is proposed. It is shown that the autoignition can be suppressed by the elevated flame speed when the pressure wave is weak, while it cannot be prevented intrinsically when the temperature of the end-gas is high or the pressure wave is strong enough. Moreover, different autoignition propagation modes are identified from Bradley's diagram, including deflagration, developing detonation and thermal explosion, and the combined effects of reactivity and pressure wave strength are discussed as well. [ABSTRACT FROM AUTHOR]

With the development of compression ignition (CI) engines, a detailed investigation of the in-cylinder combustion process is needed under strengthened regulation on the emission. The development of a tomographic algorithm makes it possible for further investigation of the 3D structure of the in-cylinder flame. In this study, the Full-field Cross-Interfaces Computed Tomography algorithm (FCICT) is utilized to reconstruct the 3D chemiluminescence distribution of the in-cylinder flame in an optical engine. The reconstructed flame provides 3D information on exhibiting flame equivalence radius variation, as well as clarifying the effect of injection pressure on flame development. The study reveals the 3D structure of the in-cylinder flame and makes it possible for further investigation of the 3D information of the in-cylinder combustion process in the future. [ABSTRACT FROM AUTHOR]

Experiments of the interaction process between shock waves and zirconium particles have been carried out using a self-designed shock tube. The interaction process was recorded using a high-speed shadowgraph imaging system, pressure testing system, and infrared thermal imager. The wave-systems structures resulting from the interaction between shock waves and accumulated zirconium powders have been characterized. Generally, the axial distance of the powder cloud reduces with the increase in accumulation thickness, which in turn increases with the addition of shock Mach number. The incident shock wave with an intensity of 1.53 Ma was enough to ignite zirconium powders under these experimental conditions. The ignition delay time for zirconium powders decreases with the addition of shock Mach number, while it decreases for thinner accumulations. Besides, the flame propagation characteristics and the ignition mechanism of zirconium powders are discussed in detail. The results can serve as a supplementary guide for the utilization and safe operation of zirconium powders. [ABSTRACT FROM AUTHOR]

To observe the agglomeration, an important parasitic process in metal composite combustion, sandwiched structures with laminates of aluminum (Al) and ammonia perchlorate (AP) layers are prepared and characterized by high-speed macro and microscopic imaging systems with pyrometry. The bilayer structure prepared by electrospraying was confirmed by Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) and the composition was confirmed by thermogravimetric analysis/differential thermal calorimetry (TGA/DSC). We found that the burn rate of the Al/AP sandwich films increases with the decrease of bilayer thickness of Al and AP, owing to the shorter diffusion distance between fuel and oxidizer. More importantly, the complete process of the agglomeration/sintering of Al nanoparticles (NPs) is observed in-situ, which demonstrates the different stages of agglomeration process of Al NPs. Temperature measurement from pyrometry reveals the temperature of agglomerates during combustion, consistent with the agglomerate size and burn rate observation. Simple calculations based on "pocket-theory" support the observed changes in agglomeration size with bilayer thickness. [ABSTRACT FROM AUTHOR]

Previous work on spray flames has shown that different propagation mechanisms may occur depending on the size and number density of droplets. In this work, the structure and propagation of flames in uniformly dispersed sprays of low-volatility fuels is experimentally examined. The effect of the Sauter mean diameter (SMD) of the spray (16–33 µm) on the propagation modes, flame speed, and flame curvature is assessed in weakly turbulent sprays, with the ratio of axial velocity rms to the gaseous laminar burning velocity u z ′ / S L , g ranging from 0.5–2.5, and overall equivalence ratio ϕ of 0.8, 1, and 1.4. The growth of the flame is evaluated from OH*-chemiluminescence and schlieren visualisation, which combined with OH/fuel planar laser-induced fluorescence visualisation reveal details of the propagation mechanisms. The aviation fuels investigated – Jet A and a renewable alternative, ATJ-8 – exhibited similar flame speed behaviour due to changes in SMD in each of the propagation modes identified: the droplet, inter-droplet, and gaseous-like modes. Concentrated reactions around large droplets found in lean conditions (ϕ = 0.8) allowed for a slowly propagating flame front which, in turn, ignited new droplets. Stoichiometric to rich conditions (ϕ = 1, 1.4) were marked by stronger evaporation ahead of the flame and, therefore, higher and more uniform heat release across the flame. Still, droplets penetrated the flame, locally inducing regions of negative flame curvature and continuing to evaporate in the burnt products. The droplet-induced effects disappeared at low SMD (16 µm, ϕ = 1.4), giving rise to a fully gaseous layer ahead of the flame and the highest flame speeds. At rich conditions and high SMD, Jet A had a lower flame speed than ATJ-8. [ABSTRACT FROM AUTHOR]

This numerical study investigates the combustion modes in the second stage of a sequential combustor at atmospheric and high pressure. The sequential burner (SB) features a mixing section with fuel injection into a hot vitiated crossflow. Depending on the dominant combustion mode, a recirculation zone assists flame anchoring in the combustion chamber. The flame is located sufficiently downstream of the injector resulting in partially-premixed conditions. First, combustion regime maps are obtained from 0-D and 1-D simulations showing the co-existence of three combustion modes: autoignition, flame propagation and flame propagation assisted by autoignition. These regime maps can be used to understand the combustion modes at play in turbulent sequential combustors, as shown with 3-D large eddy simulations (LES) with semi-detailed chemistry. In addition to the simulation of steady-state combustion at three different operating conditions, transient simulations are performed: (i) ignition of the combustor with autoignition as the dominant mode, (ii) ignition that is initiated by autoignition and that is followed by a transition to a propagation stabilized flame, and (iii) a transient change of the inlet temperature (decrease by 150 K) resulting into a change of the combustion regime. These results show the importance of the recirculation zone for the ignition and the anchoring of a propagating type flame. On the contrary, the autoignition flame stabilizes due to continuous self-ignition of the mixture and the recirculation zone does not play an important role for the flame anchoring. These findings are important for the design and operation of practical sequential combustion systems. [ABSTRACT FROM AUTHOR]

Abstract The structure of mechanically activated (MA) energetic composites are complex, having at the same time intra-particle reactive length scales as small as sub-micrometer and inter-particle length scales as large as 1 mm. This work investigates the combustion propagation of MA aluminum and polytetrafluoroethylene (PTFE) while varying as-milled particle size, stoichiometry, particle bed density, and confinement to better understand combustion propagation. Propagation rates of MA Al/PTFE are four orders of magnitude slower than similar physically mixed nano-Al/PTFE or micron-Al/PTFE compositions reported elsewhere. Combustion of MA composite beds is found to be macroscopically stable within quartz tubes of >3 mm diameter, though microscopic relay-race combustion is observed in which slow combustion transfers between particles preclude rapid intra-particle combustion. Confined burning rate is increased with reduction of packing density below 55% TMD and burning rate is insensitive to tube end confinement. A maximum burning rate is observed at Al/PTFE ratios of 50/50 wt.%, corresponding to conditions predicted to maximize gas production. Packed beds containing larger (> 75 µm) particles produced 50% faster burning rates than smaller (< 25 µm) particles, and absence of confinement increases burning rate by a factor of ten. Taken together, these results show that the propagation of MA Al/PTFE composites contradicts combustion trends of either purely micrometer-Al/PTFE or nanometer-Al/PTFE packed beds. Findings of this work are supportive of separate results in which laser ignition of single MA Al/PTFE particles results in MA particle microexplosion. It is hypothesized that MA Al/PTFE packed bed combustion is controlled by the rate enhancing effects of microexplosion-induced forward propagation of burning ejecta within packed beds, leading to maximum burning rates in conditions which promote ejecta propagation. The findings of this work provide insight into the combustion physics of MA Al/PTFE and help inform use of MA composites in energetic formulations. [ABSTRACT FROM AUTHOR]

Abstract For the first time, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth. Differences between flame spread in purely buoyant and purely forced flows are presented. Prior to these experiments, only samples of small size in small confined volumes had been tested in space. Therefore the first and third flights in the experimental series, called "Saffire," studied large-scale flame spread over a 94 cm long by 40.6 cm wide cotton-fiberglass fabric. The second flight examined an array of nine smaller samples of various materials each measuring 29 cm long by 5 cm wide. Among them were two of the same cotton-fiberglass fabric used in the large-scale tests and a thick, flat PMMA sample (1-cm thick). The forced airflow was 20–25 cm/s, which is typical of air circulation speeds in a spacecraft. The experiments took place aboard the Cygnus vehicle, a large unmanned resupply spacecraft to the International Space Station (ISS). The experiments were carried out in orbit before the Cygnus vehicle, reloaded with ISS trash, re-entered the Earth's atmosphere and perished. The downloaded test data show that a concurrent (downstream) spreading flame over thin fabrics in microgravity reaches a steady spread rate and a limiting length. The flame over the thick PMMA sample approaches a non-growing, steady state in the 15 min burning duration and has a limiting pyrolysis length. In contrast, upward (concurrent) flame spread at normal gravity on Earth is usually found to be accelerating so that the flame size grows with time. The existence of a flame size limit has important considerations for spacecraft fire safety as it can be used to establish the heat release rate in the vehicle. The findings and the scientific explanations of this series of innovative, novel and unique experiments are presented, analyzed and discussed. [ABSTRACT FROM AUTHOR]

Measurements of flame propagation velocities and extinction states in counterflow provide a valuable source of flame data that contain information about fundamental combustion physics. The approach to properly account for stretch effects in counterflow flame measurements through non-intrusive laser-based local velocity characterization was advanced in the mid-80s by Law and coworkers at atmospheric conditions with simple fuels. Subsequently, several research groups have extended the measurements to elevated pressures and complex fuels. However, counterflow flame data at subatmospheric pressures are limited. In the present study, a method is introduced for measuring laminar flame speeds and extinction strain rates in subatmospheric counterflow flames. A numerical study was performed to assess the dynamics of tracer particles used to facilitate measurements. It was found that the particle phase dynamics used in particle velocimetry measurements are not always representative of the underlying gas phase motion due to thermophoresis and insufficient drag, especially at low pressures. A numerical scheme was implemented whereby the computed particle phases were used for proper comparison with measurements and, based on the computed results, to infer the corresponding values of the gas phase. The method was applied to premixed methane/air and non-premixed methane–nitrogen/oxygen flames at pressures as low as 0.1 atm. Complimentary flame structure simulations were carried out which show that the kinetics of formyl radical prompt dissociation strongly impact the computed subatmospheric flames and may influence the validation of unimolecular and bimolecular reactions rate constants when tested against laminar flame data. [ABSTRACT FROM AUTHOR]

The promotional effect of non-thermal plasma (NTP) on lean flame propagation in turbulent fuel-air mixtures in a reciprocating engine-like high-temperature and -pressure environment was investigated. A turbulent flow was created by installing a perforated plate with oblique holes in the combustion chamber of a rapid compression and expansion machine (RCEM). Ignition was conducted by conventional spark plug. The NTP was generated by a dielectric-barrier discharge (DBD) device installed in the combustion chamber near the spark plug. A portion of the lean fuel-air mixture (ϕ = 0.5) in the chamber passed through the NTP and diffused throughout the chamber before spark ignition. To elucidate whether the effect persisted even when the plasma-affected volume was diffused by the flow, two types of experiments with temporal delay were conducted. The fuels evaluated were n-heptane as a representative fuel with a strong low-temperature oxidation reaction, i-octane as a representative fuel with a weak low-temperature reaction, and a primary reference fuel consisting of a mixture of these two fuels. Temporal growth of the flame was observed using a high-speed camera with an image intensifier. The evolution of in-cylinder pressure was also monitored and the characteristic time of the mass fraction burned was evaluated accordingly. It was found that flame propagation was promoted by DBD for n-heptane-containing mixtures at a certain initial temperature while the i-octane-air mixture did not exhibit such enhancement. The results obtained suggest that long-lived intermediate chemical species formed by the plasma diffuse into the cylinder, affecting flame propagation through promotion of a low-temperature oxidation reaction. In addition to these findings, the effect of NTP on burning periods were evaluated. [ABSTRACT FROM AUTHOR]

Abstracts 2-methyltetrahydrofuran (MTHF) has been considered as a potential biofuel candidate for the renewable feedstock and attractive properties. In this study, the spherically propagating flames of the MTHF/isooctane-air mixtures in different blending ratios were investigated at elevated temperatures and pressures over equivalence ratios of 0.8–1.5 in a constant volume chamber using high-speed photography technique. Laminar burning velocities were calculated through nonlinear method and correlated by mixing rules as a function of initial temperature, pressure, equivalence ratio, and blending ratio. To investigate the influence of the potential interaction between MTHF and isooctane, a detailed model was established by merging the models of the two fuels and validated against laminar burning velocity, ignition delay times and flame structure. The laminar flame velocities of MTHF were observed to increase with initial temperature while the flame velocities decrease with increase in pressure. Kinetic analysis indicates that the major consumption pathways for MTHF are through H abstraction reactions at 2 and 5 sites. The H abstraction reactions at methyl group are less competitive. Unsaturated hydrocarbons and aldehydes are produced in high concentration and are the main stable intermediates of MTHF combustion. The laminar burning velocity is sensitive to the reactions of small radicals and some intermediates such as propylene and ethylene. The laminar burning velocities of MTHF/isooctane blends increase with MTHF blending ratio. The influence of blending ratio on laminar burning velocity should attribute to the chemical factors, rather than thermal or diffusion factors. MTHF oxidation generates less proportion of propylene and higher fractions of H, OH than isooctane, reflecting positive chemical effects on laminar burning velocity as MTHF blended. According to the instability analysis, the Markstein length and critical radius shows similar flame instability among different blending ratios. [ABSTRACT FROM AUTHOR]

The effect of fuel decomposition on fundamental flame properties was investigated computationally for atmospheric-pressure n- dodecane/air mixtures. The fuel decomposition was modeled under isobaric and adiabatic conditions for initial temperatures of 1100, 1200 and 1300 K, and equivalence ratios of 0.7, 1.0, and 1.4. For various extents of n- dodecane oxidative thermal decomposition, the combustion characteristic of mixtures of the resulting products with air were investigated by keeping the total enthalpy constant and equal to that of the n- dodecane/air mixture. The endothermic n- dodecane decomposition was found, to a large extent, to be decoupled from the subsequent oxidation of the attendant products that include largely hydrogen, ethylene, methane, and other small alkenes. The mass burning rates in freely propagating flames were found to increase with an increase in the extent of n -dodecane decomposition, but the change is limited to 15%, which occurs in the highest extent of decomposition. On the other hand, the extinction strain rate of decomposed, lean to stoichiometric mixtures increases notably compared to the corresponding un-decomposed fuel–air mixtures. Sensitivity analyses of mass burning rates and extinction strain rates to kinetics and binary diffusion coefficients reveal that the laminar flame speed is primarily sensitive to key heat release and radical branching reactions, and as such fuel decomposition has a small effect on the mass burning rate. On the other hand, the extinction strain rate of the fuel-lean mixtures is sensitive to the diffusivity of the fuel, and for this reason, fuel decomposition removes the difficulties associated with the transport of the large fuel molecules into the flame zone. [ABSTRACT FROM AUTHOR]

Flame propagation is investigated in a designed experimental apparatus equipped with a perforated plate in a constant volume chamber. The effect of the perforated plate is to generate a rapidly accelerating flame based on Bychkov work (Bychkov et al . 2008), in which the flame across the obstacle will becomes a strong jet flame. The experiment was conducted with a hydrogen–air mixture at different conditions. In this work, six different turbulent flame propagation and combustion modes were clearly observed at various conditions in our designed experiment. In the presence of perforated plate, the turbulent flame formed through the perforated plate may perform six types of turbulent propagations at the end gas regime. These types form through the interaction between the flame and the shock or acoustic wave and because of the limited effect of the wall in confined space. The six forms are as follows: (1) a normal flame propagation with a low flame front tip velocity and combustion rate; (2) a weak pulsation propagation with weak fluctuation due to the acoustic wave; (3) a pulsation propagation only with a visible reflected shock wave; (4) a strong pulsation propagation with a forward shock wave and shock reflection; (5) a continuously accelerating flame propagation due to auto-ignition of the unburned mixture between flame front and shock wave, which also leads to strong pressure oscillation; and (6) a violent pulsation propagation with a multi-shock wave leading to end gas auto-ignition with large pressure oscillation. [ABSTRACT FROM AUTHOR]

Laminar flame speeds of methyl formate/air mixtures were measured at sub-atmospheric pressures for which limited data exist. The experiments were carried out in the counterflow configuration at an unburned mixture temperature of 333 K. The flow velocities were measured using particle image velocimetry. Particle phase slip correction was applied to low-pressure data sets for which the density disparity between the flow tracers and the gaseous phase is notable. The data were modeled using two recently developed kinetic models of methyl formate oxidation, and significant disagreements were realized at all pressures especially under fuel-rich conditions. Additionally, the computed species profiles of CO and CO 2 in the burner-stabilized flame configuration using the two models were found to differ significantly. Reaction path analysis revealed that the kinetics of CH 2 OCHO that is produced directly from the fuel affects the overall reactivity, and the attendant rate constants differ between the two models. The variation of laminar flame speed with pressure revealed also a different behavior between experiments and simulations. Further insight into the sources causing the observed discrepancies were investigated and it was determined that reactions involving formyl radical, methanol, and formaldehyde could also be responsible for the reduction in reactivity specifically under fuel-rich conditions. [ABSTRACT FROM AUTHOR]

The laminar flame speed s l is an important reference quantity for characterising and modelling combustion. Experimental measurements of laminar flame speed require the residence time of the fuel/air mixture ( τ f ) to be shorter than the autoignition delay time ( τ ). This presents a considerable challenge for conditions where autoignition occurs rapidly, such as in compression ignition engines. As a result, experimental measurements in typical compression ignition engine conditions do not exist. Simulations of freely propagating premixed flames, where the burning velocity is found as an eigenvalue of the solution, are also not well posed in such conditions, since the mixture ahead of the flame can autoignite, leading to the so called “cold boundary problem”. Here, a numerical method for estimating a reference flame speed, s R , is proposed that is valid for laminar flame propagation at autoignitive conditions. Two isomer fuels are considered to test this method: ethanol, which in the considered conditions is a single-stage ignition fuel; and dimethyl ether, which has a temperature-dependent single- or two-stage ignition and a negative temperature coefficient regime for τ . Calculations are performed for the flame position in a one-dimensional computational domain with inflow-outflow boundary conditions, as a function of the inlet velocity U I and for stoichiometric fuel–air premixtures. The response of the flame position, L F , to U I shows distinct stabilisation regimes. For single-stage ignition fuels, at low U I the flame speed exceeds U I and the flame becomes attached to the inlet. Above a critical U I value, the flame detaches from the inlet and L f becomes extremely sensitive to U I until, for sufficiently high U I , the sensitivity decreases and L f corresponds to the location expected from a purely autoignition stabilised flame. The transition from the attached to the autoignition regimes has a corresponding peak dL f / dU I value which is proposed to be a unique reference flame speed s R for single-stage ignition fuels. For two-stage ignition fuels, there is an additional stable regime where a high-temperature flame propagates into a pool of combustion intermediates generated by the first stage of autoignition. This results in two peaks in dL f / dU I and therefore two reference flame speed values. The lower value corresponds to the definition of s R for single-stage ignition fuels, while the higher value exists only for two-stage ignition fuels and corresponds to a high temperature flame propagating into the first stage of autoignition and is denoted s R ′ . A transport budget analysis for low- and high-temperature radical species is also performed, which confirms that the flame structures at U I = s R and U I = s R ′ do indeed correspond to premixed flames (deflagrations), as opposed to spontaneous ignition fronts which do not have a unique propagation speed. [ABSTRACT FROM AUTHOR]

Transient mixing and ignition mechanisms in a simplified pre-chamber/main-chamber system are investigated using direct numerical simulation (DNS) with detailed chemical kinetics. Full ignition and flame propagation processes in the premixed methane/air mixtures are simulated. Ignition, the progress and topology of flame evolution, and the mean burning velocity in the main chamber are analyzed in detail. Four important phases in the ignition and flame propagation processes are identified based on the flame structure development in the main chamber, the pressure and velocity evolution at typical points in both the pre-chamber and main chamber. Results show that the intermediate species OH, CH 2 O, and HO 2 are critical for flame stabilization and propagation in the main chamber due to their high reactivity. This is sorted as the chemical effect that the pre-chamber jet acts on the main chamber. The high temperature jet also brings heat and unburned fuel into the main chamber, which are sorted as thermal and enrichment effect, respectively. The heat release rate is found to be approximately proportional to the product of CH 2 O and OH mass fractions, which could be regarded as a reliable and effective marker for the heat release rate of methane/air mixtures. It is found that the mean burning velocity in the main chamber can be elevated up to 30 times under the condition investigated. [ABSTRACT FROM AUTHOR]

Aluminum (Al) is the most widely used metal additive in energetic materials, but its application is limited due to its oxidizable nature. However, aluminum hydride (AlH 3) could disrupt the oxide film structure and improve applications of the Al additive in energetic materials by releasing hydrogen. Therefore, this paper explored the explosion characteristics of the Al-AlH 3 composite dust by conducting composite dust explosion venting experiments. The influences of composite dust concentrations and mass ratios on the explosion pressure and flame were evaluated. The consequences indicated that when the composite dust concentration was constant, the explosion pressure and the reduction rate of explosion pressure gradually decreased as increasing the mass ratio, which confirmed that the explosion intensity of AlH 3 was greater than that of Al. When the composite dust mass ratio was the same, the increase in dust concentration caused the explosion pressure, the pressure rise rate, the flame front propagation distance, and velocity to increase first and then attenuate. 750 g/m3 was the best dust explosion concentration. On this basis, the safety design applicability of composite dust was conducted based on the NFPA 68 explosion venting design standard, which was verified to be suitable for Al dust. Notably, the safety margin should be taken into account when carrying out a safety design for composite dust using NFPA 68. In the paper, the composite dust explosion characteristics were obtained through experiments and mechanism analysis, which provided data support for the research of Al-AlH 3 composite dust for energetic material additives. [ABSTRACT FROM AUTHOR]

The influence of a periodic multiscale transient flow on isothermal-flame propagation is examined computationally. It is found that the increase in flame surface area, and hence the burning speed, due to wrinkling by the flow depends quadratically (linearly) on the intensity of the flow-field velocity fluctuations at low (high) intensities, while the dependence is consistent with a 4/3 power law for an intermediate range of intensities, consistent with results from earlier studies of flame propagation in weakly exothermic turbulent flows. Particular contributions of this work are elucidation of (a) the quantitative extent of the burning-speed enhancement for low and moderate intensities, (b) the influence of the multiscale character of the excitation flow and (c) the stabilization of flame surface-area growth after the initial period of unbounded growth revealed in earlier work. The effect of multiple excitation-flow scales on surface-area enhancement is found to be minor at low excitation-flow intensities but substantial at large intensities. [ABSTRACT FROM AUTHOR]

The ignition behaviour of a multiple-burner annular combustion chamber consisting of 12 or 18 bluff-body premixed methane-air swirl burners was investigated experimentally. The study focusses on the mechanism of lightround, namely the burner-to-burner flame propagation. Side visualization of the spreading flame by 5 kHz OH* chemiluminescence showed that propagation from burner to burner did not follow a purely azimuthal direction, but a sawtooth pattern with downstream and sideways motion from one burner to the following, bringing flame to the downstream part of the recirculation zone of the adjacent burner before being convected upstream leading to full burner ignition. This pattern was more pronounced when the burners where fitted with swirlers. Top visualization and image processing were used to quantify the speed of lightround as a function of inter-burner spacing, bulk velocity, equivalence ratio, and swirling feature. It was found that flame spread from burner to burner following two balanced modes of propagation. These are turbulent flame propagation combined with volumetric expansion in the inter-burner region and convection within the next un-ignited burner. The results presented in this paper bring new insights into the ignition of realistic gas turbines. [ABSTRACT FROM AUTHOR]

In this work, a three-layers Mallard-Le Chatelier inspired theoretical model is developed to fully characterise the steps occurring during the flame propagation of combustible dusts/air. The model is based on the hypothesis that the dust flame propagation follows a homogeneous path: the dust-air mixture is pre-heated up to the volatile point (VP), at which production of volatiles occurs, thanks to the back-diffusion of heat from the combustion zone of the flame to the colder zones. The volatiles produced are then heated up to the ignition temperature and enter in the combustion zone. The flame burning velocity is the results of the coupling between heating rate, pyrolysis and/or evaporation/sublimation rate and volatiles combustion rate. The rate of formation of volatiles was measured by means of TG/DSC analysis. The laminar burning velocity of gases was computed by simulating the gas flame propagation in a tube starting from the measured gas compositions (by literature data or FTIR analysis). [ABSTRACT FROM AUTHOR]

Implementation of spray-guided stratified-charge direct-injection spark-ignited (DISI) engines is inhibited by the occurrence of misfire and partial burns. Engine-performance tests demonstrate that increasing engine speed induces combustion instability, but this deterioration can be prevented by generating swirling flow during the intake stroke. In-cylinder pressure-based heat-release analysis reveals that the appearance of poor-burn cycles is not solely dependent on the variability of early flame-kernel growth. Cycles can experience burning-rate regression during later combustion stages and may or may not recover before the end of the cycle. Thermodynamic analysis and optical diagnostics are used here to clarify why swirl improves the combustion repeatability from cycle to cycle. The fluid dynamics of swirl/spray interaction was previously demonstrated using high-speed PIV measurements of in-cylinder motored flow. It was found that the sprays of the multi-hole injector redistribute the intake-generated swirl flow momentum, thereby creating a better-centered higher angular-momentum vortex with reduced variability. The engine operation with high swirl was found to have significant improvement in cycle-to-cycle variations of both flow pattern and flow momentum. This paper is an extension of the previous work. Here, PIV measurements and flame imaging are applied to fired operation for studying how the swirl flow affects variability of ignition and subsequent combustion phases. PIV results for fired operation are consistent with the measurements made of motored flow. They demonstrate that the spark-plasma motion is highly correlated with the direction of the gas flow in the vicinity of the spark-plug gap. Without swirl, the plasma is randomly stretched towards either side of the spark plug, causing variability in the ignition of the two spray plumes that are straddling the spark plug. In contrast, swirl flow always convects the spark plasma towards one spray plume, causing a more repeatable ignition. The swirl decreases local RMS velocity, consistent with an observed reduction of early-burn variability. Broadband flame imaging demonstrates that with swirl, the flame consistently propagates in multiple directions to consume fuel–air mixtures within the piston bowl. In contrast, operation without swirl displays higher variability of flame-spread patterns, occasionally causing the appearance of partial-burn cycles. [ABSTRACT FROM AUTHOR]

Using one-dimensional numerical simulation, the interactions of flame propagation, auto-ignition and pressure wave during various knocking combustion scenarios are systematically investigated, for stoichiometric H 2 –air mixture fueled in a closed constant volume reactor. Different types of auto-ignition and pressure mutation are identified with various initial temperatures. It is found that as initial temperature increases, there is a transition for the auto-ignition (AI) position from the near-wall region to the region ahead of SI flame front, resulting in distinct pressure mutation and knocking intensity. Further analysis on sequential knocking combustion demonstrates that knocking intensity not only corresponds to the initial auto-ignition events induced by thermal inhomogeneity, but also to the subsequent interactions of flame front propagation, as well as AI spots initiation and pressure wave generation. Consequently, more intense pressure mutation could result from the developing detonations. Finally, the mechanism of AI occurrence and AI development is identified, which demonstrates the essential role of pressure wave disturbance in the formation of thermal inhomogeneity and detonations. [ABSTRACT FROM AUTHOR]

Copper (Cu) based chemical-looping combustion (CLC) is a promising process that utilizes solid carbon fuel such as coal and biomass. Understanding the reaction kinetics in this process can facilitate the industrial design of CLC units. In this work, molecular dynamics (MD) simulations were performed to investigate the reaction kinetics of n-butane and two simplified solid carbon fuels (lignite and anthracite) with/without copper oxide (CuO) nanoparticle. In addition, experiments were conducted on the thermal characteristics, flammability, and flame speeds of CuO and solid carbon mixtures. For n-butane, oxidation on the CuO surface is the primary reaction since the activation energy of the surface reaction is much lower than that of oxygen (O 2) combustion (9.2 vs. 53.3 kcal/mol). On the other hand, for lignite, there is a smaller difference in the activation energy of O 2 combustion and surface reaction (23.04 vs. 6.34 kcal/mol). We hypothesize that some solid carbon fuels can have different reaction kinetics dependent upon temperature. This is proven by the case of a simplified anthracite coal: an Arrhenius plot shows that this solid carbon fuel has two different reaction kinetic regimes and the critical temperature for the change in kinetics is related to the oxygen uncoupling of the CuO nanoparticle. Like the modeling simulations, a change in activation energy is observed in the experimental results, where desorption of molecular oxygen from CuO becomes important. [ABSTRACT FROM AUTHOR]

The effect of ozone (O 3 ) addition on laminar flame speeds ( S L ) across a wide pressure range was investigated experimentally and numerically using three fuels, CH 4 , C 2 H 4 and C 3 H 8 . Enhancement of S L due to O 3 addition was consistently observed for CH 4 and C 3 H 8 mixtures over a range of lean to rich equivalence ratios, based on comparisons of S L measured with and without O 3 addition. For both fuels, simulation results agree with experimental results, with the best predictions at near stoichiometric conditions and the largest discrepancies for fuel-rich cases. A significant increase in the S L enhancement was observed at elevated pressures: the enhancement in the measured S L for a stoichiometric CH 4 /air mixture with 6334 parts per million (ppm) O 3 addition increased from 7.7% at atmospheric pressure to 11% at 2.5 atm. Elevated pressure both promotes O 3 decomposition, which provides O atoms, and suppresses diffusion of H, which reduces the influence of the O 3 +H=OH+O 2 reaction. Together, these lead to the increased S L enhancement with pressure. In contrast to the results for the two saturated hydrocarbons, both detrimental and beneficial effects due to O 3 addition were observed for the unsaturated hydrocarbon fuel, C 2 H 4 in this study. With O 3 addition, C 2 H 4 /air S L decreased at room temperature and pressure, owing to the heat loss induced by the exothermic ozonolysis reaction between O 3 and C 2 H 4 in the mixing process, but increased as the ozonolysis reactions were minimized when reactants were cooled to 200 K or pressure was decreased below 0.66 atm. These experimental results were successfully explained by a numerical model that includes a new ozonolysis sub-mechanism. [ABSTRACT FROM AUTHOR]

This work addresses the experimental investigation and analytical interpretation of a flame subject to acoustic–parametric instability exited by self-generated pressure pulses. The research presented herein was carried out with lean hydrogen–air mixtures during flame propagation in a smooth channel with an open end. It was found that very lean mixtures with hydrogen concentrations in air of less than 14% vol. H 2 generate acoustic oscillations due to flame instabilities, which, in turn, significantly influence the propagation of the flame. Above a 14% vol. H 2 concentration in the air, the flame becomes relatively stable with respect to self-generated acoustic perturbations. It was also found that an external polychromatic sound with a dominant frequency of 1000 Hz inhibits the instabilities and results in a reduced flame propagation velocity. Numerical solutions of the Searby and Rochwerger analytical formulation for the acoustic–parametric instability were utilized in order to analyze the experiments and study the influence of different parameters on the existence of a spontaneous transition from the acoustic to the parametric instability. [ABSTRACT FROM AUTHOR]

A nanosecond repetitively pulsed (NRP) discharge in the spark regime has been investigated as to its effectiveness in reducing ignition time, both in a flow tube and a pulsed detonation engine (PDE). The flame-development time for methane–air mixtures in the flow tube is found to be a function of the total ignition energy and the pulse repetition frequency. Schlieren imaging revealed that at low pulse-repetition frequency (0–5 kHz), ignition kernels formed by the discharge are each transported away from the discharge gap before the following pulse arrives. At higher pulse-repetition frequencies (⩾10 kHz), multiple pulses are all coupled into a single ignition kernel, thus the resulting ignition kernel size and the total energy deposition into the kernel are increased, resulting in a faster transition into a self-propagating flame. Imaging of the NRP discharge in air revealed that at high pulse frequencies (>10 kHz) and peak pulse amplitude (>9 kV), the plasma emission is not quenched in-between pulses, resulting in a building up of heat and radicals in the center of the ignition kernel. Optical emission spectra revealed the presence of electronically excited N 2 , O, and N, as well as O + and N + , during and between the discharge pulses. Numerical modeling of the plasma indicated that reactions of excited species mainly lead to the production of O atoms and the increase of gas temperature, which shortens induction chemistry timescales, and thus reduces the flame-development time through both kinetic and thermal mechanisms. Ignition of aviation gasoline–air mixtures by NRP discharge in a PDE also demonstrated a noticeable reduction in ignition time as compared to an automotive aftermarket multiple capacitive-discharge ignition system. [ABSTRACT FROM AUTHOR]

Recent modifications to the University of Michigan rapid compression facility (UM RCF) were made to allow direct imaging of flame/autoignition interactions using compression to initiate autoignition chemistry and a spark plug to initiate simultaneous flame development. The experimental data in this study quantify the effects of spark-initiated flame propagation on autoignition of iso -octane/O 2 /inert gas mixtures at well-defined initial conditions. The work leveraged the controlled environment of the UM RCF, in which temperature, pressure, and composition are nominally uniform and well-known at the end of compression. Flame initiation by the spark plasma, flame propagation, and autoignition were monitored using high-speed optical imaging of chemiluminescence and in situ pressure time histories. End-of-compression temperatures from T EOC = 942–1012 K were considered, while the end-of-compression pressures were nominally constant within the range of P EOC = 7.8–9.5 atm. The fuel-to-O 2 molar equivalence ratio was varied from ϕ = 0.20–0.99 and dilution, defined as the molar ratio of inert gases to O 2 in the reactant mixture, was varied from inert:O 2 = 3.76–7.47 to determine the effects on flame/autoignition interactions as well as to identify the lean flammability limit of the mixtures as a function of dilution. Flame propagation is generally expected to decrease autoignition delay times by compression heating the unburned portion of the mixture. The effect of flame propagation was maximized in these experiments by igniting the mixtures early during the autoignition process. Later spark timings had small to negligible effect on the autoignition delay time. Dilution had significant effect on the lean flammability limits, increasing from a lean limit of ϕ = 0.35 at air levels of dilution to ϕ = 0.65 at inert:O 2 dilution of 7.5. The flammability limit was well correlated with the theoretical adiabatic flame temperature of each experiment. The propagation rates of flames successfully initiated by the spark plasma were determined from the imaging data and were ∼1 to 12 m/s. The magnitude of the propagation rates and the effect on the time integrated temperature scaled with the energy content of the mixtures as indicated by the theoretical adiabatic flame temperature. [ABSTRACT FROM AUTHOR]

The hydrogen oxidation chemistry constitutes the foundation of the kinetics of all carbon- and hydrogen-containing fuels. The validation of rate constants of hydrogen-related reactions can be complicated by uncertainties associated with experimental data caused by the high reactivity and diffusivity of hydrogen. In the present investigation accurate experimental data on flame propagation and extinction were determined for premixed and non-premixed hydrogen flames at pressures between p = 1 and 7 atm. The experiments were designed to sensitize the three-body H + O 2 + M → HO 2 + M reaction, whose rate is subject to notable uncertainty. This was achieved by increasing the pressure and by adding to the reactants H 2 O and CO 2 whose collision efficiencies are high compared to other species. In the present study, directly measured flame properties were compared against computed ones, in order to eliminate uncertainties associated with extrapolations, as is the case for laminar flame speeds. The measured extinction strain rates exhibit both a positive and negative dependence on pressure with and without weighting with the density, and this non-monotonic behavior is caused by the competition between the H + O 2 → O + OH and H + O 2 + M → HO 2 + M reactions as well as HO 2 kinetic pathways as pressure increases. The various kinetic models considered in this investigation did not reproduce equally well the non-premixed flame extinction data with added H 2 O. On the other hand, the predicted extinction strain rates were consistent between the various models in the case of added CO 2 . Finally, it was shown that the formulation of binary diffusion coefficient pairs including H–N 2 and H 2 –N 2 has a first order effect on the prediction of extinction strain rates of non-premixed H 2 flames. [ABSTRACT FROM AUTHOR]

The mechanism of modulation of turbulent structures in the vicinity of a combustion engine wall is not yet clarified. Thus, we conducted direct numerical simulations of wall turbulence based on premixed hydrogen–air combustion using a detailed chemical reaction mechanism to clarify the mechanism of modulation of turbulence structures such as quasi-streamwise vortices by a flame propagating toward a wall. In particular, the turbulence modulation can alter the momentum and heat fluxes through the wall surface. In the simulations, we solved the fundamental equations for the compressible flow of multiple chemical species under combustion. The flame propagation speed was affected by the turbulence and was 2.04 times larger than that of the laminar flame. Moreover, upon examining the variation in the flow field, we determined that the turbulence kinetic energy in the flow field increased temporarily owing to the newly produced vortices along the flame surface including the existing turbulence vortices. In contrast, the turbulence vortices along the wall were suppressed as the flame approached the wall, and the turbulent flow tended to become laminar. In terms of vorticity transport equation, the suppression of the turbulence vortices was caused by the thermal expansion of the combustion rather than the increased viscosity. Furthermore, we determined that the vortex structure slightly influenced the chemical reaction process, but it strongly influenced the heat release rate distribution and concentration distribution of the chemical species. The current findings on the stated mechanism will aid in improving the accuracy of practical prediction and control methods for wall turbulence with combustion. [ABSTRACT FROM AUTHOR]

The paper explains the mechanism of tulip flame formation in horizontal combustion chambers closed at the ignition end. The explanations are based essentially on the PIV images and the direct visualization of the process. The obtained results demonstrate that the tulip flame is a purely hydrodynamic phenomenon which results from the competition between the backward movement of deflected burned gases expanding from the lateral flame skirts and the forward movement of unburned gases accelerated in the phase of finger-shaped flame. In some configurations a supplementary global movement imposed by the confinement (for example: acoustic waves) is superposed on the two above mentioned, and modifies the parameters of the process. The results also prove that the intrinsic instabilities of the flame front (Rayleigh–Taylor, Richtmyer–Meshkov or Darrieus–Landau) are not involved in this process. The convex shape of the flame front has no influence on the phenomenon. [ABSTRACT FROM AUTHOR]

Direct numerical simulation (DNS) results of turbulent MILD premixed and conventional (undiluted) premixed combustion have been investigated to shed light on the physical aspects of reaction zones and their morphology in MILD combustion. Results of a premixed case are used for comparative analyses. The analyses show that the regions with strong chemical activity in MILD combustion are distributed over a substantial portion of the computational domain unlike in the premixed case where these regions are confined to a small portion of the domain. Also, interactions of reaction zones are observed in MILD combustion with their spatial extent increasing with dilution level. These interactions give an appearance of distributed combustion for MILD conditions. The morphology of these reaction zones is investigated using the Minkowski functionals and shapefinders commonly employed in cosmology. Predominant sheet-like structures are observed for the premixed combustion case whereas a pancake-like structure is observed as the most probable shape for the MILD cases. Spatial and statistical analyses of various fluxes involved in a progress variable transport equation are conducted to study autoignitive or propagative characteristics of MILD reaction zones. The results suggest that there are local regions with autoignition, propagating-flames, and their coexistence for the conditions considered in this study. Typically, reaction dominated or ignition front and propagating-flame dominated regions are entangled for high dilution cases. Scalar gradient plays a strong role on whether reaction or propagating-flame dominated activities are favoured locally. [ABSTRACT FROM AUTHOR]

The combustion-wave propagation of nickel-coated aluminum particles is studied theoretically for packing densities in the range of 10–100% of the theoretical maximum density. Emphasis is placed on the effect of packing density on the burning properties. The energy conservation equation is solved numerically and the burning rate is determined by tracking the position of the flame front. Atomic diffusion coefficients and reaction rate of isolated nickel-coated aluminum particles are input parameters to the model. The burning behaviors and combustion wave structures are dictated by the heat transfer from the flame zone to the unburned region. Five different models for the effective thermal conductivity of the mixture are employed. The impact of radiation heat transfer is also assessed. As a specific example, the case with a particle size of 79 μm is considered in detail. The burning rate remains nearly constant (<1 cm/s) up to a packing density of 60%, and then increases sharply toward the maximum value of 11.55 cm/s at a density of 100%. The Maxwell–Eucken–Bruggeman model of thermal conductivity offers the most accurate predictions of the burning rate for all loading densities. [ABSTRACT FROM AUTHOR]

The combustion of aluminum particle, liquid water, and hydrogen peroxide (H2O2) mixtures is studied theoretically for a pressure range of 1-20 MPa and particle sizes between 3 and 70 μm. The oxidizer-to-fuel (O/F) weight ratio is varied in the range of 1.00-1.67, and four different H2O2 concentrations of 0%, 30%, 60%, and 90% are considered. A multi-zone flame model is developed to determine the burning behaviors and combustion-wave structures by solving the energy equation in each zone and enforcing the temperature and heat-flux continuities at the interfacial boundaries. The entrainment of particles is taken into account. Key parameters that dictate the burning properties of mixtures are found to be the thermal diffusivity, flame temperature, particle burning time, ignition temperature, and entrainment index of particles. When the pressure increases from 1 to 20 MPa, the flame thickness decreases by a factor of two. The ensuing enhancement of conductive heat flux to the unburned mixture thus increases the burning rate, which exhibits a pressure dependence of the form rb = apm . The exponent, m , depends on reaction kinetics and convective motion of particles. Transition from diffusion to kinetically-controlled conditions causes the pressure exponent to increase from 0.35 at 70 μm to 1.04 at 3 μm. The addition of hydrogen peroxide has a positive effect on the burning properties. The burning rate is nearly doubled when the concentration of hydrogen peroxide increases from 0 to 90%. For the conditions encountered in this study, the following correlation for the burning rate is developed: rb[cm/s]=4.97(p[MPa])0.37(dp[μm])-0.85(O/F)-0.54exp(0.0066CH2O2). [ABSTRACT FROM AUTHOR]

Driven by pollutant emissions stringent regulations, engines manufacturers tend to reduce the number of injectors and rely on lean combustion which impacts the light-around phase of ignition. To improve knowledge of the ignition process occurring in real engines, current research combines fundamental and increasingly complex experiments with high fidelity numerical simulations. This work investigates the flame propagation, using a multi-injector experiment located at CORIA (France) in combination with Large Eddy Simulation (LES) obtained by CERFACS (France). The comparison of numerical fully transient ignition sequences with experimental data shows that LES recovers features found in the experiment. Global events such as the propagation of the flame front to neighboring swirlers are well captured by LES, with the correct propagation mode (spanwise or axial) and the correct overall ignition time delay. The detailed analysis of LES data allows to identify the driving mechanisms leading to each propagation mode. [ABSTRACT FROM AUTHOR]

This study investigates flame propagation in small thermally-participating tubes where the wall acts as a heat-recirculating medium. This fundamental configuration allows heat in the combustion products to be recirculated into the reactants, resulting in excess enthalpy and enhanced burning rates. Preheating of the reactants by heat recirculation has traditionally been considered to be the dominant mechanism leading to large burning rates observed in such systems. This is mainly supported by results from physical models based on a one-dimensional (1-D) representation of the system, where the radial diffusion of heat from wall surface to channel centerline is not accurately captured. In this study, a 2-D formulation with conjugate heat transfer, which accurately resolves the transport of heat inside the gas-wall system, is used to model the excess-enthalpy phenomenon. Steadily-propagating stoichiometric methane-air flames are simulated inside an adiabatic tube of finite wall-thickness, over a wide range of inlet flow velocities and small tube diameters. Burning-rate enhancement is found to be caused not only by preheating, associated with heat recirculation, but also by an increase in flame-front area. Flame elongation is more pronounced with increasing tube diameter and inlet velocity, up to a point where the change in flame-front area becomes dominant in enhancing burning rate. In that case, heat recirculation is a necessary condition for flames to couple to the thermal wave in the wall and elongate, but does not provide a significant increase in enthalpy or temperature that would otherwise be needed for high burning rates to be observed. As the diameter is reduced, the effect of preheating becomes increasingly important for burning-rate enhancement compared to flame area increase. At very small diameters, smaller than the flame thickness, the increase in burning rate is seen to be predominantly attributable to preheating. However, preheating is seen to become limited as inflow velocity is increased, due to 2-D effects inside the fluid that interfere with heat recirculation. These findings demonstrate that 2-D effects inside the fluid can have a prohibitive influence on the burning-rate enhancement attributed to preheating, but that they also give rise to an additional mechanism, associated with the change in flame surface area, responsible for burning-rate enhancement in heat-recirculating burners. [ABSTRACT FROM AUTHOR]

The uncertainties associated with the extraction of laminar flame speeds through extrapolations from directly measured experimental data were assessed using one-dimensional direct numerical simulations with focus on the effects of molecular transport and thermal radiation loss. The simulations were carried out for counterflow and spherically expanding flames given that both configurations are used extensively for the determination of laminar flame speeds. The spherically expanding flames were modeled by performing high fidelity time integration of the mass, species, and energy conservation equations. The simulation results were treated as "data" for stretch rate ranges that are encountered in experiments and were used to perform extrapolations using formulas that have been derived based on asymptotic analyses. The extrapolation results were compared then against the known answers of the direct numerical simulations. The fuel diffusivity was varied in order to evaluate the flame response to stretch and to address reactant differential diffusion effects that cannot be captured based on Lewis number considerations. It was found that for large molecular weight hydrocarbons at fuel-rich conditions, the flame behavior is controlled by differential diffusion and that the extrapolation formulas can result in notable errors. Analysis of the computed flame structures revealed that differential diffusion modifies the fluxes of fuel and oxygen inside the flame and thus affect the reactivity as stretch increases. Radiation loss was found to affect notably the extracted laminar flame speed from spherically expanding flame experiments especially for slower flames, in agreement with recent similar studies. The effect of radiation could be eliminated however, by determining the displacement speed relative to the unburned gas. This can be achieved in experiments using high-speed particle image velocimetry to determine the flow velocity field within the few milliseconds duration of the experiment. In general, extrapolations were found to be unreliable under certain conditions, and it is proposed that the raw experimental data in either flame configurations are compared against results of direct numerical simulations in order to avoid potential falsifications of rate constants upon validation. [ABSTRACT FROM AUTHOR]

Abstract: Flame propagation studies for Al nanoparticles (80nm) and micron particles (3–4.5μm) mixed with MoO3 in both an open and confined burn setup were examined. A scanning electron microscopy (SEM) analysis of the reactants and products reveals quantitative size data that contributes toward an understanding of the governing reaction mechanisms. For the confined burn tube experiments, nanoscaled reactants exhibited a flame speed of 960m/s, the same as has been reported in previous experiments. Micron scale particles exhibited a flame speed of 402m/s, much higher than the 244m/s obtained previously for 1–3μm particles. These flame speeds are in quantitative agreement with predictions based on the recently developed melt-dispersion mechanism (MDM) describing the reaction of Al particles. It also demonstrates that some micron particles can reach flame speeds just 58% lower than the fastest nanoparticles, while micron scale particles are less expensive and do not have the pre-combustion safety and environmental issues typical of nanoparticles. The SEM analysis reveals a significant (at least by factor of 3.7 for nanoparticles) reduction in Al particle size post combustion, which is in agreement with the MDM and in contrast to the predictions based on diffusion mechanisms. Open burn experiments with nanoscale reactants have flame speeds of 12m/s and product particle sizes almost as small as those in the burn tube experiments. However, the presence of some large particles, which may grow based on the diffusion mechanism, exclude evaporation and the homogenous nucleation mechanism. For open burn experiments with micron reactants, with flames speeds of 9m/s, SEM analysis shows a molten-resolidified product with no distinguishable particles and cavities containing numerous nanoparticles with a measured diameter of 36nm. [Copyright &y& Elsevier]

Abstract: The combustion characteristics of promising alternative fuels have been studied extensively in the recent years. Nevertheless, the pyrolysis and oxidation kinetics for many oxygenated fuels are not well characterized compared to those of hydrocarbons. In the present investigation, the first chemical kinetic study of a long-chain linear symmetric ether, di-n-butyl ether (DBE), is presented and a detailed reaction model is developed. DBE has been identified recently as a candidate biofuel produced from lignocellulosic biomass. The model includes both high temperature and low temperature reaction pathways with reaction rates generated using appropriate rate rules. In addition, experimental studies on fundamental combustion characteristics, such as ignition delay times and laminar flame speeds have been performed. A laminar flow reactor was used to determine the ignition delay times of lean and stoichiometric DBE/air mixtures. The laminar flame speeds of DBE/air mixtures were measured in the stagnation flame configuration for a wide rage of equivalence ratios at atmospheric pressure and an unburned reactant temperature of 373K. All experimental data were modeled using the present kinetic model. The agreement between measured and computed results is satisfactory, and the model was used to elucidate the oxidation pathways of DBE. The dissociation of keto-hydroperoxides, leading to radical chain branching was found to dominate the ignition of DBE in the low temperature regime. The results of the present numerical and experimental study of the oxidation of di-n-butyl ether provide a good basis for further investigation of long chain linear and branched ethers. [Copyright &y& Elsevier]

Abstract: The oxidation characteristics of several small methyl and ethyl esters with carbon number less than six were investigated in laminar flames. The kinetics of such fuels are subsets of those of larger alkyl esters that are constituents of practical biodiesel fuels. A total of seven fuels, namely methyl formate, methyl acetate, methyl propionate, methyl butanoate, ethyl formate, ethyl acetate, and ethyl propionate were considered. Experiments were conducted at atmospheric pressure, elevated reactant temperatures, and over a wide range of equivalence ratios. Laminar flame speeds were determined in the counterflow configuration in which flow velocities were measured using particle image velocimetry. Several detailed kinetic models were tested against the experimental data, and insight was provided into the high-temperature combustion kinetics of the aforementioned fuels. Based on comparisons between experimental and computed results it became apparent that the chemistry of alkyl-ester combustion chemistry is evolving and much needs to be done in order to derive improved rate constants for a wide range of elementary steps. [Copyright &y& Elsevier]

Abstract: Laminar flame speeds of iso-cetane/air and decalin/air mixtures were measured in the counterflow configuration at atmospheric pressure and an elevated unburned mixture temperature of 443K. Axial flow velocities were measured along the stagnation streamline using the digital particle image velocimetry. The laminar flame speeds were determined by determining the variation of a reference flame speed as a function of strain rate and computationally assisted non-linear extrapolations. The data are the first to be reported in the literature, and they were modeled using a recently developed kinetic model that includes 187 species and 6086 elementary reactions. In general, the computed results were found to be in close agreement with the data. In order to get insight into kinetic effects on flame propagation, detailed sensitivity and reaction path analyses were performed using the computed flame structures. The results revealed that at the same equivalence ratio, laminar flame speeds of iso-cetane/air mixtures are lower than those of n-hexadecane/air mixtures. Additionally, it was found that the laminar flame speeds of iso-cetane/air and decalin/air mixtures are sensitive largely to C0–C4 kinetic subset, and that the lower reactivity of iso-cetane compared to n-hexadecane could be attributed to the higher production of relatively stable intermediates. [Copyright &y& Elsevier]

Abstract: We investigate the effect of thermal expansion and gravity on the propagation of a triple flame in a horizontal channel with porous walls, where the fuel and oxidiser concentrations are prescribed. The triple flame therefore propagates in a direction perpendicular to the direction of gravity, a configuration that does not seem to have received any dedicated investigation in the literature. In particular, we examine the effect of the non-dimensional flame-front thickness ∊ on the propagation speed of the triple flame for different values of the thermal expansion coefficient α and the Rayleigh number Ra. When gravity is not accounted for (Ra =0), and for small values of ∊, the numerically calculated propagation speed is found to agree with predictions made in previous studies based on scaling laws [1]. We show that the well known monotonic relationship between U and ∊, which exists in the constant density case when the Lewis numbers are of order unity or larger, persists for triple flames undergoing thermal expansion. Under strong enough gravitational effects (Ra ≫1), however, the relationship is no longer found to be monotonic. For a fixed value of ∊, the relationship between the Rayleigh number and the propagation speed is shown to vary qualitatively depending on the value of ∊ chosen, exhibiting hysteresis if ∊ is small enough and displaying local maxima, local minima or monotonic behaviour for other values of ∊. All of the steady solutions presented in the paper have been found to be stable, except for those on the middle branches of the hysteresis curves. [Copyright &y& Elsevier]

Abstract: Understanding the kinetics of large molecular weight aldehydes is essential in the context of both conventional and alternative fuels. For example, they are key intermediates formed during the low-temperature oxidation of hydrocarbons as well as during the high-temperature oxidation of oxygenated fuels such as alcohols. In this study, an experimental and kinetic modeling investigation of n-butanal (n-butyraldehyde) and iso-butanal (iso-butyraldehyde or 2-methylpropanal) oxidation kinetics was performed. Experiments were performed in a jet stirred reactor and in counterflow flames over a wide range of equivalence ratios, temperatures, and pressures. The jet stirred reactor was utilized to observe the evolution of stable intermediates and products for the oxidation of n- and iso-butanal at elevated pressures and low to intermediate temperatures. The counterflow configuration was utilized for the determination of laminar flame speeds. A detailed chemical kinetic interpretative model was developed and validated consisting of 244 species and 1198 reactions derived from a previous study of the oxidation of propanal (propionaldehyde). Extensive reaction pathway and sensitivity analysis was performed to provide detailed insight into the mechanisms governing low-, intermediate-, and high-temperature reactivity. The simulation results using the present model are in good agreement with the experimental laminar flame speeds and well within a factor of two of the speciation data obtained in the jet stirred reactor. [Copyright &y& Elsevier]

Abstract: An extension of the turbulent flame speed closure model rendering the model applicable to multiphase flow and ignition is presented. As formerly no coupling between reaction progress variable and enthalpy was existent, except through the temperature dependency of the laminar flame speed, an adaptation is proposed which offers an interface to initiate the combustion process. The modification to incorporate multiphase conditions is achieved by substituting the mixture fraction variable as representation of the composition in the original implementation of the turbulent flame speed closure model with independent species. Source terms to correlate the species progress to the reaction progress variable are derived in this work. The additional transport equations serve a higher generality of the model and enable the proper treatment of vaporizing fuel droplets. It is demonstrated that limitations which arise in the standard formulation of the model, stemming from differences in the transport equation for the reaction progress variable and the mixture fraction, are addressed and resolved by the new approach. Regarding the initiation of the flame, an additional source term for the reaction progress variable is introduced, which relates the reaction progress to the auto-ignition time. This allows the development of the flame without imposing artificial boundary conditions. The correct model behavior is established by means of a series of widely used test cases. The results of these simulations show that the model’s potential to predict flame growth and more generally the flame evolution as a function of time and space is preserved. At the same time more sophisticated test case boundary conditions involving multiphase conditions and variable inflows in terms of composition can be incorporated. As a thorough assessment of the extended model capabilities, a multiphase lab scale set-up, which provides a comprehensive data set, is presented. The good agreement of the obtained results underline the range of applicability of the extended model and its accuracy, albeit its simplicity, for multiphase conditions. [Copyright &y& Elsevier]

Abstract: This study proposes a numerical and experimental investigation of the propagation characteristics of CO-added CH4 flames in a confined quartz tube. The transient propagation of a CH4/CO–air flames were modeled using GRI-Mech 3.0, and the propagation characteristics are discussed based on the calculated results. This study characterizes the formation of the reaction zone, the transformation of the flame base structure, the ignition of fuel, and the propagation phenomena of the leading point of the flame base. The leading point of methane flame with a large amount of added CO is found to be difficult to define unambiguously. During flame propagation, a complex combination of chemical reactions coupled with the fluid dynamics between the stoichiometric line and the preferred equivalence ratio line occurs. The results suggest that the leading point of the propagating flame is still dominated by the redirection effect, while the effect of the intrinsic chemical properties of the fuel mixture on a propagating flame has finite thickness cannot be neglected. [Copyright &y& Elsevier]