Flame Conditions Research Articles

Flame kinetics at scramjet-engine-relevant conditions: Role of prompt dissociation of weakly-bound radicals

Combustion in high-speed ram-based propulsion engines occurs under distinct thermodynamic conditions of high reactant temperatures (greater than 1000 K) and relatively low pressures (<5 atm). There is a lack of fundamental flame measurements at such conditions that result in adiabatic flame temperatures (Tad) exceeding 2500 K. In this work, we have measured laminar flame speeds of oxygen-enriched CH4/oxidizer mixtures at sub-atmospheric conditions to probe kinetics at high Tad using the isobaric spherically expanding flame approach. Simulations with recent kinetic models revealed increasing differences between data and model predictions with increasing Tad, reaching up to 25 %. Kinetic analyses reveal that at the thermodynamic conditions in these O2-enriched flames, i.e., lower pressures and higher Tad, the effects of HCO prompt dissociation are accentuated. In addition to HCO, the prompt dissociations of CH2OH and C2H5 are also considered. The prompt dissociations of all three radicals were evaluated and their effects considered in flame speed simulations. Reaction path analysis for the present flames revealed that approximately half of the reaction flux for HCO formation undergoes prompt dissociation to H + CO. Furthermore, these analyses also revealed that the pathways and sensitive reactions are similar between oxygen-enriched fuel/oxidizer mixtures and preheated fuel/air mixtures, if both have similar Tad. Thus, flames of oxygen-enriched mixtures could be a surrogate to probe the flame chemistry of highly preheated mixtures at relatively low pressures that are often encountered in ram-based propulsion engine combustors.

A detailed kinetic submechanism for OH* chemiluminescence in hydrocarbon combustion

Here, we propose a new physically consistent modeling scheme, JIHT-OHex(CxHy), that accurately predicts the formation and consumption of electronically excited chemiluminescent OH∗ molecules in hydrocarbon flames over a wide range of temperatures, pressures, and mixture compositions. It incorporates (unchanged) our recent well-founded JIHT-OHex(H2) reaction submodel (Sharipov et al., 2024, Combust. Flame, 263, 113417), aimed at describing the OH∗ evolution in hydrogen oxidation, and contains a necessary set of elementary processes involving OH∗ and carbon-containing species with the rate constants that are based either on a critical review of known, sometimes conflicting literature data on the elementary reaction kinetics of OH∗ or, where necessary and appropriate, on semiempirical estimates. To improve the JIHT-OHex(CxHy) performance against a representative data set for the observed OH(A2Σ+→X2Π) chemiluminescent emission (near 309 nm) accompanying high-temperature oxidation of various (from C1 to C10) hydrocarbon-based mixtures that we aggregated at the preparatory stage of the work, the rate coefficients of reaction and quenching processes that the overall OH∗ kinetics is most sensitive to (or for which there is a particular scatter in the available kinetic data, if any) were jointly optimized within their theoretical expectations and experimental uncertainties. It is shown that our universal detailed OH∗ submechanism, which includes a much larger pool of elementary processes (32 reactions and 36 quenching partners) than previous essentially global models (consisting of only a few processes and tailored to specific mixtures and combustion conditions), clearly outperforms the competitors in terms of integral accuracy (when tested against the multitude of the collected OH∗ emission measurements). Accordingly, there is reason to believe, as exemplified for the conditions of the laminar premixed methane-air flame, that our detailed OH∗ submodel with physically realistic (to the extent possible) rate constants will perform adequately over a wider range of burning conditions and flow parameters than that for which it was validated and tuned.Novelty and significance statementAlthough it is widely accepted that UV chemiluminescent OH(A2Σ+−X2Π) emission as an optical signature of the combustion process as a whole and of the underlying chemistry offers unique diagnostic capabilities, in recent years, definitely insufficient attention has been paid to the development of the detailed reaction mechanisms for quantitative interpretation of the OH∗ chemiluminescence measurements in various burning environments. This is especially true for the oxidation of hydrocarbons. Indeed, the pertinent OH∗ models known to date are essentially global, meaning that they all contain a surprisingly small number of processes (in most cases, only a couple of OH∗-forming reactions are involved alongside the quenching processes) and their rate constants are adjusted to specific hydrocarbon-based mixtures and burning conditions. Accordingly, a revision of the current understanding of the OH∗ kinetics in the presence of hydrocarbons is highly desired; therefore, our detailed physically-based reaction mechanism, which comprises dozens of plausible pathways of OH∗ formation and depletion with realistic rate coefficients and thus is capable of simulating the OH∗ emission in hydrocarbon flames more accurately than previous models, is in itself a fundamentally significant and novel contribution to the practice of chemiluminescence modeling. No less importantly, the rate constant fits, recommended here, are also of independent interest in the context of excited-state chemistry.

Impact of Flame Conditions on the Pd‐O Structure and Methane Oxidation Activity over Ceria Support

AbstractIn this work, we employed flame spray pyrolysis (FSP), a high‐temperature synthesis method, to control the formation of Pd structures on the CeO2 support. Multiple types of Pd structures deposited on CeO2 are observed on FSP‐made samples. Our results show that the oxidizing environment during FSP synthesis facilitates the formation of incorporated Pd2+ structures, along with highly dispersed Pd2+, Pd0 nanoparticles, and Pd° clusters formed under the reducing synthesis condition. Notably, these Pd2+ species remained stable at temperatures up to 400 °C. The catalysts containing both highly dispersed Pd2+ nanoparticles and incorporated Pd2+ species demonstrated superior methane oxidation activity, with higher turnover frequencies than those containing only one type of Pd2+ structure. However, hydrothermal pretreatment in the presence of water vapor led to partial deactivation, likely due to structural alterations in the Pd species or the interaction with the CeO2 support, which reduced the stability and effectiveness of the active sites. This study underscores the importance of both highly dispersed and incorporated Pd2+ species in enhancing catalytic performance and highlights the challenges posed by water‐induced deactivation in practical applications.

1D modeling of plasma streamers at ammonia-air flame conditions

Abstract This work is a self-consistent 1D modeling of streamers in ammonia-oxygen-nitrogen-water mixtures. A fluid model that includes species transport, electrostatic potential, and detailed chemistry was developed and verified. This model is then used to simulate the avalanche, streamer formation and propagation phases, driven by a nanosecond voltage pulse, at different thermochemical conditions derived from a 1D laminar premixed ammonia-air flame. The applicability of the Meek’s criterion in predicting the streamer inception location was successfully confirmed. Streamer formation and propagation duration were found to vary significantly with different thermochemical conditions, due to the difference in ionization rates. The thermochemical state also affected the breakdown characteristics which was tested by maintaining the background reduced electric field constant. Detailed kinetic analyses revealed the importance of O(1D) in the production of key radicals, such as O, OH, and NH2. Furthermore, the contributions of the dissociative electronic excitation of NH3 towards the production of H and NH2 radicals have also been reported. Spatial and temporal evolution of the electron energy loss fractions for various inelastic collision processes at different thermochemical states uncovered the input plasma energy spent of fuel dissociation and the large variability in the dominant processes during the avalanche and streamer propagation phases. The methodology and analyses reported in this work are key towards developing effective strategies for controlled nanosecond-pulsed non-equilibrium plasma sources used for ammonia ignition and flame stabilization.

Flame transfer functions of asymmetric conical and V-shaped flames

Transfer functions of asymmetric flames subjected to incident flow perturbations are explored using the classical kinematic model, the G-equation, parameterized by flame tip angle α, flame-base incline angle β, and eccentricity e. With perturbations of various frequencies, we analyze perturbed asymmetric flames and derive analytical formulations of Flame Transfer Functions (FTF) for various flame conditions. It is observed that the gain of the asymmetric conical flame is increased compared to the symmetric case. Meanwhile, the amplification effect of the V-shaped flame is considerably suppressed as the flame becomes asymmetric. That means with a proper asymmetric geometry setting, the V-shaped flame no longer tends to amplify the perturbation and in other words becomes less susceptible to combustion instability. These findings have significant value for the future design and optimization of propulsion systems, where improving flame stability and reducing thermoacoustic instabilities are critical. Subsequently, a numerical study is conducted to verify the FTF results and examine the response of asymmetric flames under various perturbation levels. It has been demonstrated that the asymmetric conical flame is minimally affected by the amplitude of these perturbations, and the asymmetric V-shaped flame is highly sensitive to the convective amplitudes.

Improved thermal stability and flame retardancy of soybean oil-based polyol rigid polyurethane foams modified with magnesium borate hydroxide and ammonium polyphosphate

The concept of sustainable development has led to a growing research interest in bio-based flame-retardant polyurethane foams. These foams offer environmentally friendly, sustainable, and flame-retardant raw materials for the construction, automotive, and furniture industries. The 15 wt% soybean oil-based polyol rigid polyurethane foams (RPUF-S3A20B system) were modified with 20 wt% ammonium polyphosphate (APP) and homemade magnesium borate hydroxide (MgBO2(OH)). Thermogravimetric analysis, pyrolysis kinetic analysis, limiting oxygen index (LOI) test, cone calorimetry (CONE), morphological analysis, and smoke density (Ds) were employed to investigate the impact of MgBO2(OH) on the thermal stability and flame-retardant properties of the RPUF-S3A20B system. The results indicated that RPUF-S3A20B12.5 with 12.5 wt% MgBO2(OH) had better thermal stability and higher activation energy. In addition, its LOI was increased by 3.1% compared to RPUF-S3A20 without MgBO2(OH). The peak heat release rate (PHRR) and total heat release rate (THR) of RPUF-S3A20B12.5 at a radiant flux of 25 kW/m2 were reduced by 45.8% and 35.0% compared with RPUF-S3A20. RPUF-S3A20B12.5 demonstrated the lowest smoke density (17.4 and 17.5) and highest light transmission (73.9% and 73.7%) in both flameless and flame conditions, indicating superior flame-retardant and smoke-suppression properties. These findings offered valuable insights for further research on synergistic flame-retardant systems in bio-based polyurethane foams.

Fabrication of nickel phytate modified bio-based polyol rigid polyurethane foam with enhanced compression-resistant and improved flame-retardant

A bio-based flame retardant nickel phytate (PA-Ni) was synthesized and combined with soybean oil-based polyol (SO) to create a green rigid polyurethane foam (RPUF) with enhanced compressive strength, good thermal stability and flame retardant. The results showed that the RPUF-SO2/Ni3 with 3 wt% PA-Ni had the highest initial and termination temperature, maximum thermal decomposition rate temperature and carbon residue, and better thermal stability. Its limiting oxygen index was increased by 2.6% compared with RPUF-SO2 without PA-Ni added, and the peak heat release rate (PHRR) and total heat release rate (THR) were reduced by 14.92% and 19.92%, respectively. In addition, RPUF-SO2/Ni3 had the lowest Ds under the conditions of flame (18.90) and flameless (22.41), and had the best smoke suppression effect. And the compressive strength of RPUF-SO/Ni3 was significantly enhanced by the addition of PA-Ni. The results show that the improvement of flame retardancy of RPUF is mainly the result of the combined effect of gas-phase and condensed-phase flame retardancy, among which the flame retardancy of RPUF-SO/Ni3 was the best. The current findings offer a practical way to produce green and low-carbon RPUF as well as promising prospects for the material's safe application.

Mechanistic study of OH radical-promoted decomposition of polycyclic aromatic hydrocarbon oxyradicals

Polycyclic aromatic hydrocarbons (PAHs) are widely recognized for their detrimental impact on human health and the atmospheric environment. One of the primary challenges in mitigating these effects is the efficient and rapid decomposition of PAH oxyradicals, a byproduct of PAH oxidation. This study aims to elucidate the promoting mechanism of hydroxyl (OH) radicals on the decomposition of PAH oxyradicals. Employing density functional theory and transition state theory, we systematically investigate the decomposition mechanism of PAH oxyradicals under the influence of OH radicals. Our findings indicate that the interaction between OH radicals and PAH oxyradicals constitutes a pivotal oxidation process within flames. Notably, PAH oxyradicals are more inclined to undergo pyrolysis subsequent to their reaction with OH radicals within a flame, as opposed to direct pyrolysis. This observation underscores the role of OH radicals in facilitating the decomposition of PAH oxyradicals in flame conditions. Further analysis reveals that OH radicals can effectively react with various forms of PAH oxyradicals—zigzag, free, and armchair—leading to the formation of OH-OPAH compounds characterized by HO-C-O functional groups. Importantly, this addition reaction between OH radicals and PAH oxyradicals is found to be independent of the edge types of the PAH oxyradicals. The pyrolysis of OH-OPAH predominantly yields products such as COOH, CO2, and PAHs with five-membered rings, with COOH emerging as the primary oxidation product. This pyrolysis predominantly proceeds via a ring-opening reaction, rather than through cyclization. This study also reveals that the likelihood of PAH oxyradical pyrolysis increases with the number of free edges surrounding the oxygen-containing functional groups. Furthermore, the oxidation activity of the HO-C-O functional group is demonstrated to be more potent than that of the C-O functional group, significantly enhancing the reactivity of PAH oxyradical degradation in the presence of OH radicals. These insights not only contribute to refining the combustion model of PAH oxidation but also advance our understanding of PAH oxyradical decomposition. Such knowledge is crucial for the development and optimization of control technologies targeting PAH pollutants emanating from combustion devices.

Rotational and vibrational temperatures of transient atmospheric glow plasma

Plasma ignition can significantly improve the efficiency and performance of combustion devices through the enhancement of combustibility limits. Investigating plasma development for fundamental experimental flame conditions (i.e. spherical flame experiments) can provide insight into how plasma thermalizes the combustible mixture and, therefore a better understanding of flame development in future experimental studies. This study observed an ignition system designed to produce spherical flames in quiescent gas inside a constant-volume combustion chamber. Rotational and vibrational temperature measurements of dry atmospheric air glow plasma are reported. Measurements were taken for a transient discharge with currents less than 0.5 A. The electrode wire geometry and discharge variation resulted in an ellipsoid-shaped kernel and plasma region with an abnormal glow discharge. The measured temperatures were compared to the conductive thermal kernel boundary observed with Schlieren imaging. Maximum rotational and vibrational temperatures of 3000 K and 10 000 K, respectively, were observed near the anode electrode for a 0.5 A current. The temperature decreased with the axial distance from the anode, while a constant temperature was observed in the radial direction. Lower currents resulted in a smaller temperature, with minimum measured rotational and vibrational temperatures of 1500 K and 5000 K, respectively. The results were compared with available experimental literature and the variation observed was a result of the transient nature, which resulted in hysteresis in temperature vs discharge current measurements.

Investigation of differential diffusion in lean, premixed, hydrogen-enriched swirl flames

Hydrogen combustion is a promising alternative to fossil fuel combustion in an effort to reduce our carbon footprint. However, hydrogen combustion is prone to thermodiffusive instabilities largely dependent on differential diffusion, a phenomenon that can lead to higher probabilities of flashback in industrial burners, given hydrogen’s high reactivity and diffusivity. This paper evaluates low-swirl flames of methane and air enriched with hydrogen to highlight the onset of differential diffusion. Testing was conducted in a fully controllable swirl burner, where bulk velocity Uav = 13 m/s and swirl number S = 0.6 were kept constant for each hydrogen–methane blend to isolate increases in flame surface area from increases in turbulence intensity. Furthermore, each fuel blend of hydrogen and methane is evaluated at the same laminar flame speed of SLo = 0.267 m/s to isolate flame stretch effects on the turbulent burning rate. Combined hydroxyl (OH) PLIF and stereoscopic PIV at the National Research Council of Canada were used to analyze the OH fluorescence in a 2D-3C velocity field for each flame condition. High-speed PIV at McGill University was used to resolve local flame phenomena, such as local flame displacement velocity and flame stretch rate. Using these techniques, it can be observed that the flame displaces axially in response to turbulent flame speed while exhibiting increases in flamefront wrinkling. This increased corrugation due to flame stretch is highlighted in the PDFs of local curvature and κSf and is further evidenced by a shift towards positive curvatures (κ> 0) for increasing H2 volume fraction. This trend suggests that there is a strong correlation with increases in turbulent burning rate and positive curvature as a result of differential diffusion, but it is not necessarily a control mechanism of the most forward propagating points proposed by the theory of leading points.

Phosphorus features halogen –calcium hypophosphite replaces antimony trioxide, reduces smoke, and improves flame retardancy

Replacing antimony trioxide (ATO) in flame retardant formulations is an urgent task due to its toxicity. There are indications that calcium hypophosphite (CaP) may be a promising replacement. This study investigates the decomposition, fire behavior, and smoke release of brominated flame-retarded acrylonitrile butadiene styrene (ABS) under various fire scenarios like ignition, developing fire and smoldering, while replacing ATO with CaP and CaP/talc. Adding 4 wt.-% of talc to CaP formulations showed beneficial effects on flammability due to changes in the viscosity and barrier properties. Synergism between 8 wt.-% talc and CaP improved the protective layer in the developing fire scenario, resulting in a ∼60 % decrease in the peak of heat release rate and reduction of ∼21 % in total smoke production (ref. ABS+Br+ATO). With a conventional index of toxicity (CIT) of below 0.75, ABS+Br+CaP passes the highest requirements according to EN 45545-2. Overall, the CaP/talc materials improve flame retardancy, show less smoke emission under forced flaming conditions, and prevent chronic intoxication and environmental pollution through smoke particles contaminated with antimony.

Characterizing carbonaceous aerosols in residential coal combustion: Insights from thermal/spectral carbon analyzer coupled with photoionization mass spectrometry analysis

This study aims to identify unique signatures from residential coal combustion in China across various combustion conditions and coal types. Using a Thermal/Spectral Carbon Analyzer with a Photoionization Time-of-Flight Mass Spectrometer (TSCA-PI-TOF-MS), we focus on the optical properties and organic mass spectra of the emissions. Bituminous coal emerged as the primary emitter of total carbon, releasing 729 μg C/mg PM2.5 under smoldering and 894 μg C/mg PM2.5 under flaming. Carbon fractions mainly comprised OC1 and OC2, except for anthracite's dominance of EC1 under smoldering. Pyrolysis carbon absorption shifted from 405, 445 and 532 nm during smoldering to near-infrared bands (635–980 nm) during flaming for both bituminous and anthracite coal. Conversely, clean coal exhibited an inverse trend, attributed to additives enhancing oxygen-containing organic compounds and long-chain hydrocarbons released in charring process. Sample of bituminous coal began charring at OC3 step, while anthracite began earlier at OC2 step, particularly pronounced under flaming. Clean coal displayed unconventional charring at OC1 step under smoldering condition, producing signature compounds like butenal, methylfuran, furanylalcohol, and naphthol. The mass spectra of bituminous coal featured characteristic peaks, including m/z 192 (methylphenanthrene), 206, 220 (alkylated phenanthrenes), and 234 (retene). Anthracite coal showed a potential tracer at m/z 223, shifting from OC1 in smoldering to OC2 in flaming. Clean coal under flaming condition exhibited elevated levels of aromatic compounds, indicating potential toxicity, with peaks at m/z 178 (phenanthrene), 228 (chrysene/benz[a]anthracene), 234 (retene), 242 (methylchrysene), and 252 (benzo[a]pyrene, benzo[k]fluoranthene). Results also showed that the broader mass spectra range in the OC3 and OC4 steps across all coal types suggests that high-temperature pyrolysis promotes diversity. These findings contribute to refined source apportionment of carbon emissions from residential coal combustion and provide the scientific basis for the formulation of air pollution prevention strategies, crucial for coal-dependent regions.

Effect of flame characteristics on an isolated ethanol droplet evaporating through stagnation methane/air flames: An experimental and numerical study

A single ethanol droplet evaporation through laminar methane/air stagnation flame is investigated at lean, stoichiometric and rich conditions experimentally and numerically. For the droplets having initial diameters of 20-70μm, the particle Reynolds number is measured via PIV/PTV, resulting in the slip velocity between the droplet and gas flow being small and the surrounding gas being able to carry without any significant convective effects. Evaporation rates, computed from Abramzon–Sirignano for a moving droplet towards a stagnation flame field, satisfy the ones computed from empirically determined evaporation rates from ILIDS measurements as a function of flame temperature, flame speed and flame thickness. Nevertheless, Abramzon–Sirignano model slightly overestimates the vaporization constant for lean cases. The distance traveled by the droplet after leaving the flame zone determines the average critical diameter. Accordingly, droplets larger than 18μm can cross the flame, leading to local modifications in the flame region due to heat loss and vapor diffusion from the droplet. Novelty and Significance Energy conversion in combustion should be carried out by optimizing efficiency, minimizing pollution, and preventing further climate change. The multiphase nature of spray combustion particularly adds further complexity due to the strong coupling of atomization, evaporation, mixing, turbulence, and chemical reactions. Although spray combustion involves a cloud of polydispersed droplets, it is of fundamental importance to understand the dynamics of an isolated droplet and its interactions with the flame front, which can provide indispensable information for spray combustion models. In this study, an experimental and computational framework has been developed to investigate droplet-flame interactions. The dynamics and evaporation characteristics of an isolated droplets interacting with stagnation flames have been determined by coupling several laser diagnostics, while further enhanced with Eulerian–Lagrangian simulations under flame conditions. Thereupon, the outcomes help the design of injectors of spray combustion systems and enhancement of evaporation models.

Burn Pit Smoke Condensate-Mediated Toxicity in Human Airway Epithelial Cells.

Burn pits are a method of open-air waste management that was common during military operations in Iraq, Afghanistan, and other regions in Southwest Asia. Veterans returning from deployment have reported respiratory symptoms, potentially from exposure to burn pit smoke, yet comprehensive assessment of such exposure on pulmonary health is lacking. We have previously shown that exposure to condensates from burn pit smoke emissions causes inflammation and cytotoxicity in mice. In this study, we explored the effects of burn pit smoke condensates on human airway epithelial cells (HAECs) to understand their impact on cellular targets in the human lung. HAECs were cultured at the air-liquid interface (ALI) and exposed to burn pit waste smoke condensates (plywood, cardboard, plastic, mixed, and mixed with diesel) generated under smoldering and flaming conditions. Cytotoxicity was evaluated by measuring transepithelial electrical resistance (TEER) and lactate dehydrogenase (LDH) release; toxicity scores (TSs) were quantified for each exposure. Pro-inflammatory cytokine release and modulation of gene expression were examined for cardboard and plastic condensate exposures. Burn pit smoke condensates generated under flaming conditions affected cell viability, with flaming mixed waste and plywood exhibiting the highest toxicity scores. Cardboard and plastic smoke condensates modulated cytokine secretion, with GM-CSF and IL-1β altered in more than one exposure group. Gene expression of detoxifying enzymes (ALDH1A3, ALDH3A1, CYP1A1, CYP1B1, NQO1, etc.), mucins (MUC5AC, MUC5B), and cytokines was affected by several smoke condensates. Particularly, expression of IL6 was elevated following exposure to all burn pit smoke condensates, and polycyclic aromatic hydrocarbon acenaphthene was positively associated with the IL-6 level in the basolateral media of HAECs. These observations demonstrate that exposure to smoke condensates of materials present in burn pits adversely affects HAECs and that aberrant cytokine secretion and altered gene expression profiles following burn pit material smoke exposure could contribute to the development of airway disease.

Combining performances of E(m)-corrected LII and absorption for in situ measurements of the volume fraction of 2–4 nm soot particles.

Determining the soot volume fraction (fv) in combustion environments requires detailed knowledge of the optical properties of the soot particles, and in particular of their absorption function E(m). This study addresses a fundamental lack of information on the optical properties of 2–4 nm soot particles. Recent works based on the modeling of the photoelectron emission yields and UV-vis-NIR-absorption measurements found a sharp decrease of E(m) with the particle size in the vis-NIR spectral region, which is inconsistent with the in situ detection of 2–4 nm particles in the near-infrared region by laser-induced incandescence (LII) or sensitive absorption methods like cavity ring-down extinction (CRDE). The objective of this study is twofold: first, an original method for the determination of E(m) of soot particles, including 2–4 nm particles is proposed. Then, the dynamic of two widespread in situ diagnostics, LII and CRDE, are compared over three orders of magnitude of fv in atmospheric premixed ethylene/air flames with different flow rates and C/O. The determination of the absolute value of E(m) and of its variation in the flames is derived from an original analysis, which does not require complex LII modeling. This analysis is based on the comparison between the experimental and calculated LII/LIImax signals in the low fluence regime, LIImax being the plateau value of the fluence curve, which is reached at fluence larger than 1 J/cm2 for the smallest C/O. E(m) is found to vary between 0.15 at low C/O up to 0.36 for the richest flames. Concerning the comparison of the dynamics of LII and CRDE, an excellent agreement is found above a threshold (C/O)limit, while LII exhibits a stronger decrease with C/O below (C/O)limit. This discrepancy is attributed to the spectral dependence of E(m) which is negligible above (C/O)limit, but increases when C/O decreases below (C/O)limit. The particle size distribution function (PSD), measured by scanning mobility particle sizing, reveals monomodal or bimodal PSDs with soot having mobility diameter in the range 2.3–7.5 nm depending on the flame conditions. It is suggested that the particles contained in the first PSD mode, which is dominant in the low C/O range, could be affected by a significant spectral dependence of E(m) in comparison with the second PSD mode.

Study on flame retardant properties and thermal stability of synergistically modified polyurethane foam with ammonium polyphosphate and barium phytate

Abstract Barium phytate (Pa–Ba) was prepared by phytic acid and barium carbonate, and then the flame-retardant modified polyurethane foam (PUF) was synergized with Pa–Ba and ammonium polyphosphate (APP). The flame retardant properties and thermal stability of the modified PUFs were investigated by thermogravimetric analysis, cone calorimetry (CONE) and smoke density (Ds). The results showed the modified PUF with the addition of 5 % Pa–Ba and 10 % APP (PUF-A10-PB5) had the highest integral programmed decomposition temperature and the activation energy, indicating that its thermal stability was better compared with other samples. In addition, PUF-A10-PB5 had the lowest total heat release under different radiation intensities, and it had the smallest Ds and the highest light transmittance under the flame and flameless condition. The current results indicated that PUF-A10-PB5 had better flame-retardant properties and thermal stability, which can provide a useful reference for future experimental studies on the flame retardant properties of phytate-modified PUF.

Variations in gaseous and particulate emissions from flaming and smoldering combustion of Douglas fir and lodgepole pine under different fuel moisture conditions

The effects of fuel moisture content (FMC) on emissions under both flaming and smoldering combustion conditions were studied for two different vegetative fuels, Douglas fir and lodgepole pine. A custom linear tube-heater apparatus was developed to produce steady-state emissions for two extreme FMC, recently live (FMC ∼ 50%) and fully dead and dry (FMC ∼ 3%). The concentrations of particulate matter and gaseous species measured using a TSI DustTrak and Fourier transform infrared spectroscopy (FTIR), respectively, showed a significant relationship with FMC for both flaming and smoldering combustion conditions. As expected, smoldering combustion showed larger production of CO, particulate matter, and unburned hydrocarbons when compared to flaming combustion. Unburnt hydrocarbons with higher emission factors produced during smoldering are CH4, C2H2, C2H6, C4H10, C4H8, C2H4, C3H6, CH2O2, C3H4O, CH2O, and CH3OH. This work also identified butane (C4H10) and hydrogen bromide (HBr) as gaseous species, which have not been commonly reported in literature. Acrolein (C3H4O) was also identified during combustion and it has previously reported only by ion detection method. These emissions were further investigated by performing a carbon balance of the carbon in the fuel vs. emitted carbon, and these were compared to literature values. Flaming-mode emission factors significantly correlated with FMC, while FMC had little influence on emission factors for smoldering combustion, when considering the dry mass of the burned fuel. These variations were not the same for each fuel species, suggesting that the type of fuel plays an important role in burning behavior and emissions, possibly due to the chemical makeup of moist and recently live fuels. The results were discussed and compared with previous measurements, where good agreement was observed. This study provides an important emissions dataset for modeling emissions from common wildland fuels under varying FMC and combustion conditions.Novelty and significanceThis research investigates the impact of fuel moisture content (FMC) on emissions during flaming and smoldering combustion of Douglas fir and lodgepole pine. A unique linear tube-heater apparatus was developed to maintain steady-state emissions for extreme FMC levels of recently live (FMC 50%) and fully dead/dry (FMC 3%). The study reveals a noteworthy correlation between FMC and concentrations of particulate matter and gaseous species. This finding provides valuable insights into combustion behavior. It also identifies butane (C4H10), acrolein (C3H4O), and hydrogen bromide (HBr), not commonly reported in literature. Additionally, the study highlights the varying influence of FMC on emission factors for flaming and smoldering combustion, indicating the pivotal role of fuel type in emissions. The dataset obtained serves as a valuable resource for modeling emissions from wildland fuels under diverse FMC and combustion conditions, advancing our understanding of fire behavior and environmental impacts.

Temperature and Thermal Diffusivity Diagnostics in Laminar Methane Flames Using Infrared Four-Wave Mixing Techniques.

Four-wave mixing techniques, such as coherent anti-Stokes Raman spectroscopy (CARS), laser-induced grating spectroscopy (LIGS), and degenerate four-wave mixing (DFWM), have been widely used in combustion diagnostics due to their advantages of high signal-to-noise ratio (S/N), coherent signal, and spatial resolution. In this work, a nano-second pulsed laser is utilized to generate mid-infrared (near 3 µm) pump beams, exciting the rovibrational transitions of nascent water in flames. Combined LIGS and DFWM measurements are demonstrated in premixed laminar CH4/O2/N2 flames with equivalence ratios from 0.6 to 1.5, to achieve precise thermometry in a wide range of flame conditions. The flame temperatures were also measured by thermocouple as a reference, and the results from LIGS and DFWM align well with the trends shown in the thermocouple measurements. In fuel-lean flames, where the mass-to-specific-heat ratio variation is minimal, LIGS provides temperature data with a precision better than 16 K (0.8%). In fuel-rich flames, where the increased H2 concentration in the flame introduces uncertainty in gas constants thus affecting the accuracy of LIGS thermometry, DFWM is instead employed for temperature measurement since it is less sensitive to the gas composition within the measured volume. The high-precision LIGS temperatures in lean flames serve as temperature reference during the DFWM calibration of the degree of saturation, and a precision better than 90 K (4.5%) is achieved for DFWM thermometry. In addition to temperature, a theoretical model is employed to fit LIGS signal time waveforms, extracting the local speed of sound and thermal diffusivity with precisions better than 0.5% and 1.3%, respectively. These high-precision measurements contribute additional data for flame research and simulation calculations. This way, the combined use of DFWM and LIGS proves the potential for accurate thermometry and diagnostics of other thermodynamic parameters across a wide range of flame conditions.

Unraveling the surface heat flux heterogeneity in cone calorimetry of cylindrical charring material: Insights from experiments and 2D modeling

Although originally designed for planar samples, cone calorimeter is frequently used for the flammability assessment and pyrolysis modeling of non-planar objects, like electrical cables. Prescribing the boundary conditions for the numerical models of such objects requires knowledge about the radiative and convective heat fluxes along the sample circumference. In this work, we performed gasification and flaming experiments and 2D numerical simulations on birch cylinders. During the gasification of a single-rod, the char front propagated faster vertically than horizontally, while similar rates were observed in the flaming condition. With five rods, ∪- and ∩-shaped char fronts were observed in gasification and flaming conditions, respectively. From the simulations, we extracted detailed heat flux distributions, with main deviation from flat samples being the steep reduction in incident radiation between the top and sides of the cylinders. Moreover, in five-rod configurations, radiative flux to the sides of the central rod increases up to 10 (gasification) and 20 kW m−2 (flaming). On the outermost rod, the descending flame induces downward-moving waves of heat fluxes with amplitudes 20 kW m−2 for convection and 15 kW m−2 for radiation. Simplified expressions for the heat transfer boundary conditions were tabulated for practical engineering applications.

Comparison between Different Optical Configurations of Active-FRAME Setup in Multispectral Imaging of Flames

Snapshot multispectral imaging of chemical species in the flame is essential for improved understanding of the combustion process. In this article, we investigate the different configurations of a structured laser sheet-based multispectral imaging approach called the Frequency Recognition Algorithm for Multiple Exposures (FRAME). Using FRAME, a snapshot of Laser-Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAH) excited by 283.5 nm laser and Laser-Induced Incandescence (LII) of soot particles excited by 532 nm laser are acquired simultaneously on a single FRAME image. A laminar diffusion flame of acetylene produced by a Gülder burner is used for the experiments. The standard FRAME approach is based on creating two spatially modulated laser sheets and arranging them in a cross-patterned configuration (X). However, the effect of using different configurations (angles) of the two laser sheets on the multispectral planar imaging of the flame has not yet been studied. Therefore, we have compared the FRAME approach in four different configurations while keeping the same flame conditions. First, we have compared the relation between laser fluence and LII signals with and without spatial modulation of the 532 nm laser sheet and found that both detections follow the same curve. When comparing the maps of flame species reconstructed from the standard FRAME configuration and other configurations, there are some dissimilarities. These differences are attributed to minor changes in the imaging plane, optical alignment, laser path length, different modulation frequencies of the laser sheet, laser extinction, laser fluence, etc.