The inception of soot remains one of the least understood steps in hydrocarbon combustion. Different clustering mechanisms have been proposed in the literature, with each mechanism emphasizing a particular class of chemical species. While gas-phase analyses often suggest the predominance of closed-shell polycyclic aromatic hydrocarbons (PAHs), the chemical analysis of the particle phase indicates a more complex interplay involving radical populations. The relative importance of π radicals and closed-shell PAHs in particle inception, therefore, remains controversial. In this study, incipient and primary particles collected from an ethylene/air premixed flame were analyzed using ultra-high-resolution mass spectrometry to track the evolving molecular composition during the early stages of soot formation. This approach enables the simultaneous tracking of closed-shell PAHs, radicals, and oxygenated species within the same particles, providing a detailed view of their concurrent evolution and insights into the underlying dominant growth pathway. The results show that the earliest incipient particles initially contain a substantial presence of oxygenated radicals, which declines rapidly with increasing flame height. Concurrently, hydrocarbon radicals increase in abundance and extend to higher C numbers before giving way to thermodynamically more stable closed-shell PAHs at higher flame heights. The radicals exhibit systematically higher hydrogen-to-carbon (H/C) ratios and distinct evolution trends compared to closed-shell PAHs. The broad, continuous radical distributions further suggest the continuous formation of large species with radical character during inception and early particle growth. As incipient particles evolve into primary particles, radical intensities approach a quasi-steady state while closed-shell PAHs increase significantly. The results demonstrate that the early stages of soot growth progressively shift from kinetically driven radical chemistry to thermodynamically driven stabilomers, offering new molecular-level insight into the transition from gas-phase precursors to solid particles. Novelty and significance statement The mechanisms governing soot inception can be better understood by tracking the evolution of different chemical classes within the early particles. However, progress on this aspect has been limited by the difficulty of collecting incipient soot particles and accurately resolving their complex molecular composition. High mass resolution is necessary because insufficient resolution can merge contributions from different species families, leading to ambiguous identification and distorted evolution trends. In this study, ultra-high-resolution mass spectrometry was used to obtain the chemical composition of incipient soot with high mass accuracy. This approach enabled the identification and tracking of four distinct classes of species: closed-shell PAHs, hydrocarbon radicals, closed-shell oxygenated PAHs (OPAHs), and oxygenated radicals. A dynamic interplay among these species classes occurs as the incipient particles evolve. By providing molecular-level insights into the coexistence and evolution of these species, the work offers new experimental constraints to improve mechanistic soot models and better understand soot inception.
Maintaining high safety standards in the nuclear industry, and in the rapidly growing hydrogen industry, is a continuing challenge for the scientific research. Lower Flammability Limits (LFL) are often employed to assess the explosion risk or design protective procedures and equipment. However, large discrepancies can be observed in the published LFL, and few studies on its pressure and temperature dependence are available. In this work, the lower flammability limits (LFL) of H2/Air/H2Ovap mixtures are first determined from ignition tests in a spherical vessel for a large range of initial temperatures (298 – 473 K) and pressures (0.3 – 2.0 bar) by using a parametric logistic distribution function and the pressure signals. It is found that the LFL decreases linearly with the temperature. The effect of the pressure is however more contrasted: the LFL determined from the visual criterion is shifted down as the pressure increases from 0.3 to 1.0 bar but then increases with the pressure. However, the pressure-dependence of the LFL derived from the pressure signals is positive on the whole pressure range. The dilution of the H2/air mixtures by 40% steam results in an increase of the LFL, and a total inerting of the mixture is observed for steam dilution between 61 and 65%, depending on the thermodynamic conditions. Ignition tests are then extended above the LFL, and the evolution of combustion parameters such as the relative pressure increase and the deflagrative index are measured. Upon these parameters, a new and more relevant limit for risk assessment, called the Lower Severity Explosivity Limit (LSEL) is proposed that accounts not only for the flammability of the mixture, but also the destructive capacity of the pressure wave resulting from the mixture ignition. To assess the appropriateness of the LSEL to turbulent conditions, experiments with mixtures, the composition of which is within the LFL and the LSEL, are carried out in a flame acceleration tube. Investigated flames are then discriminated into slow and fast flames, based on their maximum propagation velocity and also on the “Sigma” criterion. Great consistency is observed between two the classification methodologies as well as with the newly proposed LSEL, validating this friendlier-more accessible metric as relevant for hazard and risk assessment.Novelty and significance statementThis study introduces a novel approach to safety related to hydrogen (and other fuels) combustion hazards. A new limit, beyond the Lower Flammability Limit (LFL) and called the Lower Severity Explosivity Limit (LSEL), is introduced based on measurements in spherical vessels. Unlike the Lower Flammability Limit, the LSEL considers the potential mechanical consequences of the propagating pressure wave. The pertinence of this LSEL, obtained from laminar flow measurements, is confronted to results obtained in a flame acceleration tube, i.e. under turbulent conditions, and is found to be consistent with the definition of slow and fast flames based on the “Sigma” criterion.
Tabulated chemistry methods often make a quasi steady-state assumption, which leads them to give poor predictions of slow-reacting species. For example, the NO mass fraction keeps increasing post-flame while progress variable has effectively reached equilibrium. In this work, we explore the Aging Progress Variable (APV), which tracks post-flame processes by adding an’aging’ source-term once a threshold of chemical progress is reached. Two different manifold approaches are used in LES simulations of Sandia flames D and F with a focus on predicting NO mass fraction. For both manifolds, both the NO mass fraction and its source-term are tabulated and the results of solving a transport equation for NO are compared to direct look-up of the mass fraction. Results show good agreement for both manifolds using direct look-up when compared to the experimental measurements, highlighting the benefit of tracking post-flame processes with the APV.Novelty and significance statement: Tabulated chemistry methods are popular in industry for their reduced computational cost, but struggle to capture post-flame processes for slow-reacting pollutant species such as NO. Current approaches — such as MFM and UCS — rely on tabulating a source-term and solving an additional transport equation for NO, a method not guaranteed to generalize to species like soot, whose formation and destruction are strongly dependent on particle age. We propose using the aging progress variable (APV) as a novel conditioning variable, enabling direct tabulation of the NO mass fraction without an auxiliary transport equation. To our knowledge, this is the first application of an APV in LES of a turbulent flame. The approach yields NO predictions in good agreement with experimental measurements of a well-studied flame, motivating future extension to soot modelling.
This study examines heat release rates (HRR) and smoke emissions from battery failures resulting from direct fire, mechanical damage, and overcharging across various states of charge. Measurements were taken to characterize the critical components influencing HRR and smoke emission calculations in each experiment. The HRR was characterized using oxygen consumption calorimetry, carbon dioxide generation calorimetry, and radiative heat exposure (RHE) measurements with a heat flux gauge. The HRR measured with all three methods showed both quantitative and qualitative agreement, with the median and maximum measurements differing by no more than 10% in most cases. A larger difference in PHRR was observed when fully charged batteries were exposed to direct fire, with the RHE-derived measurement showing more variable local extrema than the calorimetry-derived methods. As the maxima were higher and the minima were lower, this difference was attributed to the sensors’ response time; however, further research is needed to verify these findings. Speciated gas and particulate measurements were collected from plumes generated during battery failure. The mass-specific extinction coefficient of particulates, derived from an optical laser and gravimetric measurements of a filter sample, is not a universal constant for battery fires because the resulting smoke is test-specific and time-dependent, evolving throughout the thermal runaway process. Hydrocarbons and gaseous carbonates released during thermal runaway are inversely related to total heat release, meaning more are released when the batteries are not fully ignited. The results presented here provide a more accurate understanding of the limitations of existing methodologies and have implications for improving standard test methods.Novelty and significance statement: The study characterized rechargeable lithium-cell batteries across various charge states during thermal runaway when subjected to direct exposure to fire, overheating, puncturing, or overcharging triggering mechanisms. It compared three heat release rate (HRR) methods (oxygen consumption calorimetry, carbon dioxide generation, and radiative heat exposure) and found agreement among them (<10% difference in most cases). HRR accuracy improved by characterizing unburned fuel to better determine the gas expansion factor. The mass-specific extinction coefficient for battery smoke was shown not to be a universal constant, as it varies across tests and evolves dynamically. This research provides detailed thermal data and smoke profiles, improving the understanding of combustion mechanisms and limitations in current battery fire testing. The findings have far-reaching implications for the design and testing of lithium-cell batteries, enabling safer storage designs, developing a safer approach to address battery fires, and establishing protocols to manage residual smoke.
Thermochemistry serves as the foundation for accurate chemical kinetic models, playing a role as crucial as that of meticulously selected reaction rate constants in ensuring the accuracy of predictions. The determination of thermochemical parameters via theoretical means has increased relative to experiments and has become established as common practice. In order to enhance the comparability and provide a better assessment of the uncertainties associated with theoretically derived thermochemistry, a systematic comparison is performed. The data are generated using a newly developed automated framework for conformational energy minimization in quantum chemical calculations. A set of 42 closed-shell molecules with available experimental data for the enthalpy of formation, the isobaric heat capacity, and the molar entropy at standard conditions is taken as reference from a NIST database. A test matrix comprising eleven different methods in combination with six basis sets is benchmarked against the experimental reference. This yields a comprehensive compilation of over 3000 sets of thermochemical properties. Standard enthalpies of formation were calculated at the G4 level of theory and corrected using the atomization scheme and connectivity-based hierarchy. The statistical evaluation of the results, combined with the resource usage for the calculations, facilitates the assessment of the accuracy and costs associated with different levels of theory. The results indicate that the accuracy of calculations is more sensitive to the chosen method than to the basis set choice. B3LYP-D4/def2-TZVP shows the lowest mean absolute relative deviation (MARD) for the calculation of Cp,298°, while RI-MP2/6-31G(d,p) shows the lowest MARD for S298°. Furthermore, the MARDs of B97M-V in combination with several basis sets are competitively low. The idealized entropy at 1000 K is evaluated as combined performance indicator and by considering the necessary calculation wall time, for which ωB97X-D3(BJ)/6-31G(d,p), B97M-V/cc-pVTZ, and B97M-V/def2-QZVP performed best on average. The present work demonstrates the capability to accurately calculate thermochemical data and identify deviations, thereby facilitating a more precise quantification of uncertainties in kinetic models. Furthermore, the work enables the selection of precise and efficient levels of theory.Novelty and significance statement: The present study reports the first comprehensive uncertainty assessment of over 3000 theoretically derived thermochemical datasets, obtained from eleven methods combined with six basis sets, for 42 molecules with experimental references. A newly developed automated framework named Thermodynamics & Conformational Optimizer (TACO) integrates conformer searches, rigid rotor and harmonic oscillator/hindered-rotor treatments, and enthalpy corrections within a unified workflow, thereby enabling parallel evaluation of accuracy and computational cost across levels of theory. This modular benchmarking approach provides an unprecedented quantitative map for the performance of different levels of theory versus resource demand for thermochemistry calculations and the potential to expand the resulting database in the future.Reliable combustion mechanisms require thermochemical parameters of known accuracy. The present benchmark identifies uncertainties associated with method/basis set combinations for enthalpies, heat capacities, and entropies. Consequently, it can serve as guidance for choosing a level of theory as well as provide support for kinetic modelers in the assessment of theoretically derived thermochemical data. The accompanying error statistics and openly available TACO code provide a route to expanding databases, propagating quantified uncertainties, and rationally balancing fidelity against computational expense in future combustion studies.
