The formation of nitrogen oxides (NOx) during combustion is a topic of substantial fundamental and practical interest, given the complex nature of its formation kinetics and the fact that, as a highly regulated pollutant emission, it is a major constraint in engineering design. To date, there are four known mechanisms by which the strong N–N bond can be broken to facilitate NOx formation from N2 present in air. Here we posit and explore the possibility of a new NOx formation route mediated by an HNNO intermediate whose reactions with common combustion species break the N–N bond. Altogether, we present results from master equation (ME) calculations for HNNO formation from H + N2O (+M), ab initio electronic structure and RRKM/ME calculations for HNNO + O2, and simulations of NO profiles in freely propagating flames using a newly constructed HNNO kinetic sub-model. Our ME results for the H + N2O reaction indicate that HNNO is the favored product channel at lower temperatures and higher pressures – e.g. favored over all other products up to ∼1100 K and over NH + NO up to ∼1500 K above 10 atm. Our ab initio electronic structure calculations for trans-HNNO + O2 show a barrier for abstraction to HO2 + N2O of 18.2 kcal/mol and a barrier for addition of 27.0 kcal/mol to form an HN(OO)NO which can decompose to NO + HNO2 over a barrier of 32.3 kcal/mol (cis-HNNO + O2 shows similar reactivity). Altogether, our rate constant calculations and kinetic modeling, which also includes estimated rate constants for HNNO + radical reactions, suggest that HNNO + O2 mainly recycles HNNO back to N2O but is sufficiently slow that the primary fate of HNNO in many combustion situations likely involves reactions with radical species, which appear likely to occur quickly and with high NOx yields.

We experimentally examine the attenuation behavior in combustion oscillations from the perspective of symbolic dynamics, dynamical systems theory, complex networks, and machine learning. The periodicity of combustion oscillations gradually decreases with increasing amount of air steadily injected into flame base, maintaining a nearly in-phase synchronized state between pressure and OH* chemiluminescence emission intensity fluctuations. Both the dynamic behaviors of pressure and OH* chemiluminescence emission intensity fluctuations in the attenuation regime close to well-suppressed combustion oscillations exhibit pseudo-periodic oscillations with deterministically nonperiodic intercycle dynamics. They lead to noisy chaotic oscillations and exhibit a desynchronized state during well-suppressed combustion oscillations.

This work reports on a theoretical and experimental study on the role of fire retardant treatments on the discontinuous ignition of wildland fuels. The effect of the concentration of fire retardant in the solution applied to the vegetation is as expected to increase the ignition delay time. We found that the fire retardant modifies the fuel bed effective thermophysical properties, delaying the thermal response of the specimen when subjected to an incident heat flux. Nevertheless, the critical heat flux remains unaltered within the experimental error. We followed a proven approach based on the thermal ignition theory and testing which however has not been previously employed to study fire retardants on wildland fuels. To carry this out, we performed experiments on the I-FIT apparatus, which yields repeatable results and controlled boundary conditions. The theoretical model shows a good agreement with the experimental results, delivering simple expressions for pencil-and-paper calculations of the ignition delay time and analytical tools to evaluate effective fuel properties. These results will help CONAF and other forest services around the world to gain insight on the optimal concentrations and delivery methods for these types of products during wildfire response.

Originally developed to predict the chemical kinetics of hydrocarbon combustion via automated generation of detailed reaction mechanisms, Reaction Mechanism Generator (RMG) contains extensive thermokinetic data for C,H,O chemisty, and has more recently been expanded to nitrogen and sulfur. In this work, we present the addition of halogen (fluorine, chlorine, and bromine) chemisty to RMG to enable automated generation of detailed kinetic models for halocarbon combustion. RMG’s existing reaction templates are updated to include halogens, and 11 new reactions families are created specific to halogen chemistry. Notably, kinetics for more than 1000 elementary reactions are calculated via ab inito methods and transition state theory, and these kinetic data are combined with kinetics from literature sources to train rate rule decision tree estimators. Additionally, halogen groups are added to RMG’s statistical mechanics database, enabling model generation with RMG’s pressure dependence module and automated computation of microcanonical rate constants for unimolecular networks. Halogen groups are also incorporated in RMG’s transport database to provide estimated parameters for the Lennard-Jones potential, important for transport-dependent simulations including laminar flame speeds. To demonstrate RMG’s capability for predicting halocarbon combustion, RMG is used to build a flame suppression model for 2-BTP (CH2=CBrCF3) in methane flames. The laminar flame speeds of RMG’s 2-BTP model show good agreement with a published model under a variety of reaction conditions. Automating the generation of detailed kinetic models for halocarbon combustion will facilitate the exploration of previously unexplored reaction pathways, thereby accelerating the development of greener refrigerants and suppressants, as well as advancing the field of automated mechanism generation.

Lightning is one of the essential causes of wildfires. However, the ignition criteria for lightning-caused wildfires have significant limitations due to a lack of understanding of the ignition mechanism. This work aims to reveal the mechanism of lightning-induced smoldering ignition of wildland fuels and explore a more reasonable ignition criterion by experimental means. The solidified peat cake samples were ignited by the electric arcs with different discharge currents (from 100 to 220 A with a step of 30 A) and durations (from 40 to 480 ms with an interval of 40 ms). The observations show that the ignition process follows a unique flaming-to-smoldering mode consisting of three stages: discharge heating (Stage I), thermal feedback (Stage II), and sustaining smoldering (Stage III) stages. The flame column that appears in Stage I is attributed to intense gasification and burnup of the peat, and seems to have little contribution to the ignition outcome. The carbonized region formed in Stage I, not the sample gasified in the discharge channel, is the main energy deposition to drive the successful smoldering ignition. It is found that the discharge duration required for smoldering ignition decreases with increasing discharge current (Iarc) when Iarc<160 A, but weakly depends on the discharge current when Iarc≥160 A. Critical discharge energy is also required when Iarc<160 A. A segmented ignition criterion is proposed based on the critical discharge duration and energy. For the peat cake sample, a minimum size, initial temperature, and absorbed energy of the carbonized zone are required for a sustaining smoldering ignition.

The sooting propensity of steady laminar coflow ethylene/air flames is modified doping the fuel stream with vapors of alcohol and evaluated as a function of both the position of the alcohol function and the ramification level (linear/branched) of the molecule. To identify general trends, a range of 3 alcohol sizes (C3, C4, and C5) are compared: the 2 propanol isomers (primary and secondary), the 4 butanol isomers (2 primaries with 1 branched, 1 secondary, 1 tertiary), and 7 pentanol isomers (3 primaries with 2 branched, 3 secondary with 1 branched and 1 tertiary). Every alcohol has been injected as a doping vapor in the central tube of a Santoro’s burner. To evaluate the sooting tendencies, a Line-of-Sight Attenuation (LOSA) setup allows the extinction of a collimated laser beam operating at 645 nm to be measured yielding the field of local soot volume fraction fv. Two indicators are used and contrasted, i.e. one using integrated data (raw data from the camera capturing the laser extinction) and one using the local field (deconvoluted data). From these results, several conclusions can be drawn. The two indicators being in good agreement, it seems unnecessary to obtain local data to compare sooting tendencies at different conditions (for a given experimental setup). The noise from the deconvolution procedure can be drastically reduced by a classical image processing described precisely in the paper, i.e. an average filter. These results reveal a good correlation between alkene produced in rich premixed C2H4 flame. The sooting tendencies are consistent with the literature: linear molecule < branched molecule and primary alcohol < secondary alcohol < tertiary alcohol. Moreover, the number of carbon NC, here varied from 3 to 5, leads to a modification of the sooting tendency that is significantly narrower than that associated with the structure of the alcohol at a given NC.

The highly dynamic response of a continuously injected liquid fuel jet to rotating detonation waves is a critical parameter in the design, performance optimization, and modeling of a rotating detonation engine (RDE). In this work, this response is spatio-temporally resolved from the fuel injection point to the detonation channel using planar laser-induced fluorescence (PLIF) at imaging rates up to 1 MHz. A rotating detonation combustor (RDC) is operated on hydrogen and air to sustain stable detonation waves that interact in a one-way coupled manner with a single liquid fuel jet that propagates into the combustion chamber with cycle periods of ∼250 µs. Diesel is utilized as a realistic fuel surrogate with higher aromatic compounds to enable fluorescence excitation using the 355 nm third-harmonic output of a burst-mode Nd:YAG laser. By optimizing the technique to accommodate orders of magnitude variations in the fuel density throughout the injection process, the PLIF data enable measurements of (i) the overall refill dynamics after the arrival of the detonation wave, (ii) changes in the liquid spray trajectory with microsecond temporal resolution, and (iii) the time required to reestablish the quasi-steady axial refilling process as a function of peak chamber pressure relative to the air- and liquid-injector pressure drops. As the passage of the detonation wave imparts significant changes in the momentum flux ratio, the qualitative liquid break-up process and spatial distribution of the spray also vary significantly in time. Only as the injection system recovers late in the cycle does the fuel spray eventually return to a quasi-steady position and allow comparisons with theoretical jet trajectories. These data, enabled by ultra-high-speed PLIF imaging, represent some of the first detailed measurements for quantifying the dynamic response and recovery of liquid jets exposed to periodic detonations in an operating RDC.

1,2,4-trimethylbenzene is an important representative aromatic component of gasoline/diesel/jet fuels and thus it is necessary to understand its low-temperature chemistry. In this paper, ignition delay times (IDTs) of both 1,2,4-trimethylbenzene (124TMB) and its blends with n-heptane were measured at engine-like conditions using both a high-pressure shock tube and a rapid compression machine for fuel in ‘air’ mixtures at pressures of 10 and 30 atm and at temperatures in the range 600 – 1100 K. The experiments in this study show for the first time that 124TMB presents a two-stage ignition behavior at engine relevant conditions. Blending n-heptane with 124TMB can significantly increase mixture reactivity at temperatures below 1000 K. A new detailed mechanism has been developed to simulate the experimentally measured IDT data. The mechanism can capture well the two-stage ignition behavior as well as the ignition delays at different pressures, equivalence ratios over a wide temperature range, for both pure fuels and their blended mixtures. Flux analyses show that the benzylic radicals (formed via H-atom abstraction from the methyl groups ortho-sites on 124TMB) can add to O2 forming RȮ2 radicals, which can isomerize to Q˙OOH by intramolecular H-atom transfer from the ortho- methyl group and these Q˙OOH radicals undergo a second addition to O2. This is analogous to the chain branching reaction pathways of alkanes. The chain branching reaction pathways are responsible for the first-stage heat release of 124TMB. The competitions between chain branching and both chain propagating and chain termination reaction pathways lead to a less pronounced negative temperature coefficient (NTC) behavior for 124TMB oxidation, compared to two-stage ignition behavior observed for alkanes and other fuels.

The influence of low temperature chemistry (LTC) on the locus of steady solutions predicted by a ZND model with curvature losses and detailed kinetics was assessed using undiluted / CO2-diluted stoichiometric DME-O2 mixtures. Results show (i) the existence of an additional critical point at large velocity deficits when the LTC submechanism is included in the reaction model, and (ii) a shift in the criticality from small to large velocity deficits as CO2-dilution is increased. Detailed thermo-chemical analyses revealed the importance of LTC in enabling an increased resistance to losses at large velocity deficits. LTC results in a temperature increase of ∼200 K at the beginning of the reaction zone that activates the intermediate and high temperature reactions, thereafter leading to the main heat release stage. Without a process that replenishes the OH radical pool at postshock temperatures below 1000 K the critical point at large velocity deficits ceases to exist.

Producing high-frequency detonations is an important topic for pulse detonations which has received considerable attentions. The valveless scheme has been verified to be able to obtain high-frequency detonations more than 100 Hz. This work has been conducted to investigate the possibility to achieve a higher detonation frequency and clarify the limits of stable operations preliminarily for the valveless scheme with different purge methods. Oxygen, ethylene, and nitrogen or liquid water are utilized as oxidizer, fuel, and purge medium in the experiments while two injection configurations are employed. The maximum detonation frequencies of 180 Hz and 330 Hz have been achieved in stable operations for two different injection configurations when nitrogen is used as the purge gas. The ceiling frequency for stable detonations is 300 Hz if nitrogen is replaced by liquid water, which indicates that water vapor is capable to create an efficient buffer zone to ensure stable operations. The results imply that the injection configuration also has a great impact on the ceiling stable detonation frequency. Three operating modes have been observed in this study, i.e., a stable detonation mode, an unstable detonation mode, and a deflagration mode. In the unstable mode, failure of detonation initiation occurs frequently and one interesting phenomenon is that the detonation frequency is reduced by half exactly when insufficient filling happens. The supply pressure ratios of oxidizer to fuel and purge to fuel are obtained for different operating modes when the purge method is changed. Furthermore, the equivalence ratios have been also studied for different operating modes which reveals that the range will change when different purge methods and injection configurations are employed. According to the equivalence ratio and the mass flow rates, an equivalent volume fraction of oxygen is defined and its range for the stable detonation mode is clarified.

Simulations of two cases in a novel multi-regime burner configuration are undertaken using a presumed joint probability density function (PDF) approach with tabulated chemistry. The flame conditions are varied by changing the central jet equivalence ratio, which produces different multi-regime combustion modes in the non-premixed inner flame. An outer premixed flame and recirculation zone behind a bluff body are present to supply heat and combustion products to stabilise the inner flame. A two-progress variable approach is tested to improve predictions of carbon monoxide (CO) in the post-flame regions, where CO oxidation occurs. The large eddy simulation set-up and sub-grid combustion model are assessed through comparisons with time-averaged measurements for radial profiles at different streamwise locations. The jet break-up length, the shear layers and the mixture fraction distribution are well captured in both cases. The temperature distribution is well captured for the inner flame in each case but the temperature and mixture fraction are over predicted in the downstream regions of the outer premixed flame, which is due to increased dilatation that suppresses air entrainment. Improved predictions of the CO mass fraction are obtained for the outer premixed flames with the two-progress variable approach. Over predictions are seen in the upstream regions of the inner flame when the CO mass fraction is obtained from a look-up table, suggesting that the CO mass fraction should be transported to include the convection/diffusion balance in regions where there is no flame. Furthermore, transporting the CO mass fraction with a one-progress variable approach produces over predictions in the burnt regions, suggesting a two-progress variable model is needed to capture the consumption region of CO. The multi-regime combustion characteristics are observed to be stronger in flame MRB26b, where non-premixed and rich premixed combustion is present. For flame MRB18b, the non-premixed contribution is smaller and weak stratified combustion is observed.

Secondary fuel injection affects the combustion organization and flame dynamic in axial fuel staging gas turbines. Here, we experimentally investigate a front-fuel rear-air twin-nozzle configuration in which the fuel and air can be independently injected to stabilize a lifted flame front, control the jet trajectory and tune the combustion mode in a very flexible way. A high repetition two-line formaldehyde planar laser induced fluorescence thermometry technique is used to measure the lift-off preheat zone temperature field of the flame with different amount of secondary air injection. The combustion flow field and the flame front dynamics are investigated using particle image velocimetry and CH* chemiluminescence. When the air to fuel ratio increases from 0 to 3, the preheat zone temperature, which is affected by the local equivalence ratio and injection air cooling, increases from 1300 K to 1600 K and then slightly declines. The lift-off region temperature and flow field dynamics are related to the variation of lift-off height, heat release rate and stability of the reacting jet in hot crossflow.

Cool flame oscillation of a fuel droplet array in high-temperature air was numerically simulated by using a droplet vaporization/spontaneous ignition numerical model. The time series data of temperature and chemical species spatial distributions were obtained. The data were used to train a Variational Auto-Encoder (VAE) that reduces the dimension of the data. The cool flame oscillation phenomenon was mapped as the trajectory onto a phase plane spanned by two latent variables obtained by the VAE. The oscillation phenomenon was investigated by using the distribution patterns derived by the VAE that distinguishes the temperature and the species states. Proper orthogonal decomposition was carried out on the decoder output of the VAE. The oscillation mechanism was investigated by the spatial eigenfunctions (mode maps). The temporal eigenfunctions of the three dominant modes were shown onto the trajectory of the plane. The correlation among physical variable distributions was evaluated to investigate the cool flame dynamics. From the above investigation, the plane was confirmed to distinguish the physical states, for the trajectory did not intersect during the oscillation. The plane was treated as a state space. The physical phenomena associated with each mode were identified from the mode maps and the temporal eigenfunctions. The phase in which the associated phenomenon arises was identified by checking the temporal eigenfunctions along with the trajectory. The mechanism of the oscillation was discussed with the correlation diagrams.

The paper presents a lattice Boltzmann (LB) method for premixed and nonpremixed combustion simulations with nonreflective boundary conditions, in contrast to Navier–Stokes solvers or hybrid schemes. The current approach employs different sets of distribution functions for flow, temperature and species fields, which are fully coupled. The discrete equilibrium density distributions are obtained from the Hermite expansions thus thermal compressibility is included. The coupling among the momentum, energy and species transport enables the model to be applicable for reactive flows with chemical heat release. The characteristic boundary conditions are incorporated into the LB scheme to avoid numerical reflections. The multi-relaxation-time collision schemes are applied to all the LB solution procedures to improve numerical stability. With detailed thermodynamics and chemical mechanisms for hydrogen-air, the LB modelling framework is validated against both premixed flame propagation and nonpremixed counterflow diffusion flame benchmarks. Simulations of circular expanding premixed flames further demonstrate the capability of the new reactive LB method. The developed LB methodology retains the advantages of classic LB methods and extends the LB capability to low Mach number combustion with potential applications in mesoscale and microscale combustors, catalysis, fuel cells, batteries and so on.

2-Phenylethanol (2-PE) is an aromatic alcohol with high research octane number, high octane sensitivity, and a potential to be produced using biomass. Considering that 2-PE can be used as a fuel additive for boosting the anti-knocking quality of gasoline in spark-ignition engines and as the low reactivity fuel or fuel component in dual-fuel reactivity controlled compression ignition (RCCI) engines, it is of fundamental and practical interest to understand the autoignition chemistry of 2-PE, especially at low-to-intermediate temperatures (<1000 K). Based upon the experimental ignition delay time (IDT) results of neat 2-PE obtained from our previous rapid compression machine (RCM) investigation and the literature shock tube study, a detailed chemical kinetic model of 2-PE is developed herein, covering low-to-high temperature regimes. Besides, RCM experiments using binary fuel blends of 2-PE and n-heptane (nC7) are conducted in this work to investigate the nC7/2-PE blending effects, as they represent a dual-fuel system for RCCI operations. Furthermore, the newly developed 2-PE model is merged with a well-validated nC7 kinetic model to generate the current nC7/2-PE binary blend model. Overall, the consolidated model reasonably predicts the experimental IDT data of neat 2-PE and nC7/2-PE blends, as well as captures the experimental effects of pressure, equivalence ratio, and blending ratio on autoignition. Finally, model-based chemical kinetic analyses are carried out to understand and identify the controlling chemistry accounting for the observed blending effects in RCM experiments. The analyses reveal that nC7 enhances 2-PE autoignition via providing extra ȮH radicals to the shared radical pool, while the diminished nC7 promoting effect on 2-PE autoignition with increasing temperature is due to the negative temperature coefficient characteristics of nC7.

The physical aggregation of polycyclic aromatic compounds (PACs) is a key step in soot inception. In this work, we set out to elucidate which molecular properties of PACs influence the physical growth process and develop a machine learning framework to quantitatively relate these features to the propensity of PACs to physically dimerize. To this end, we identify a pool of compounds with a diverse range of properties and create a dataset of PAC monomers along with their calculated free energies of dimerization, obtained via molecular dynamics simulations enhanced by well-tempered Metadynamics. We then demonstrate that a machine learning model based on the least absolute shrinkage and selection operator (Lasso) is able to quantitatively learn how molecular features contribute to physical aggregation and predict the free energy of dimerization for new pairs of molecules. Results show that our model is able to accurately determine the stability of dimers obtained from both homo- and hetero-molecular dimerization cases. Our approach provides also a data driven method to determine the molecular features most important to predicting the dimer stability. Indeed, we identified size, shape, oxygenation, and presence of rotatable bonds as the most influential characteristics of PACs that contribute to physical dimerization. This work highlights the molecular complexity of the PAC monomers that must be accounted for in order to accurately represent physical aggregation. We anticipate that our approach is key to modeling soot inception as it allows for the efficient prediction of dimerization propensity from easily calculable molecular features.

Oxymethylene ethers (OMEn) are an important family of e-fuels that can be produced sustainably from carbon dioxide and hydrogen via renewable electricity. In this work, laminar flame propagation of dimethyl ether (DME, which can be deemed as OME0), dimethoxymethane (OME1) and methoxy(methoxymethoxy)methane (OME2) was investigated in a constant-volume cylindrical combustion vessel. Laminar burning velocities (LBVs) of the three fuels were derived at 423 K, 1–10 atm and equivalence ratios of 0.7–1.5. A kinetic model for the high-temperature oxidation of the three fuels was developed with the isomerization and decomposition reactions of OME2 radicals theoretically calculated. Reasonable predictions can be achieved by the present model during the validation against the new data in this work and previous data in literature. Based on the modeling analysis, fuel-specific flame chemistry of the three fuels was analyzed, especially for the key formation pathways of major intermediates including formaldehyde, methyl formate and CH3. Special attentions were paid on the role of CH2O moiety, which is demonstrated by the variation of LBV and flame chemistry with the ratio (α) of CH2O moiety to the rest moiety in the fuel molecule (α = 1, 2 and 3 for DME, OME1 and OME2). It is observed from the experimental and simulated results that as α increases, the LBV profile has close peak values and peaks towards rich conditions, which results in the crossings of profiles and ascending LBV values under the richest conditions. Reactions involving fuel-specific radicals HCO and CH3 result in the peak shift of H profile and different LBV values, especially under the richest conditions. Furthermore, extended α values at 0 and ∞ by using methane and formaldehyde respectively were also explored with kinetic modeling to provide more insight into the effects of fuel molecular structures.

This paper is part of a broader program aimed at investigating the effects of co-firing clean fuels such as ammonia or hydrogen with hydrocarbons. The focus is on soot formation as well as flame stability in turbulent mixed-mode combustion, which is highly relevant in practical combustors. Ammonia substitution for nitrogen results in reduced flame stability, and this is correlated to differences in flame speed and extinction strain rate. While it is known that the addition of ammonia suppresses soot, visual inspection of compositionally inhomogeneous flames of ethylene-ammonia indicates a reduction in ammonia's ability to suppress soot formation. Measurements of soot volume fraction and laser-induced fluorescence in selected UV and visible bands are made along the centreline in selected flames to test this hypothesis. Experimental results are then compared to simulations in laminar diffusion flames, stratified counterflow flames, and partially premixed flames. All results confirm the soot-inhibiting ability of ammonia. Increasing inhomogeneity, leading to higher centreline mixture fractions, enhances soot formation, and the level of enhancement is greater for flames with ammonia than without. Moreover, it is found that partial premixing is ultimately responsible for determining the amount of soot formed as opposed to stratification of fuel mixtures near the pilot.

The extinction stretch/strain rate is an important global parameter for validating kinetics and modeling laminar and turbulent flame extinctions. Here, we propose a generalized approach to extrapolate the extinction stretch rate from spherically expanding flames, based on the nonlinear relation between the flame speed and the stretch rate applicable to general Lewis numbers (Le’s). Results show that the extinction events for Le1, rich hydrogen/air flames, for which the extinction mechanism is that of preferential diffusion. The effects of equivalence ratio and pressure have also been examined and explained, which highlight the essential role of proper definitions in explaining the extinction stretch rate responses. The present generalized description is particularly useful for the investigations of flame extinction and chemistry at elevated pressures within internal combustion engines.

To investigate the effects of fuel composition on the production and growth of polycyclic aromatic hydrocarbons (PAH) at conditions relevant to the pre-combustion environment of fuels in future high-speed aircraft, we have conducted supercritical pyrolysis experiments in an isothermal, silica-lined stainless-steel flow reactor at 568 °C, 94.6 atm, and 133 s, with the model fuels n-decane and ethylcyclohexane—as well as n-decane/ethylcyclohexane blends in which the fraction of fuel carbon coming from ethylcyclohexane is 0.25, 0.50, and 0.75. High-pressure liquid-chromatographic analyses of the reaction products have led to the isomer-specific identification and quantification of 169 three- to nine-ring PAH—159 of which have never before been reported as products of a cyclic-alkane fuel. Quantification of the aliphatic products by gas chromatographic techniques reveals that n-decane produces mostly 1-alkenes and n-alkanes, whose radicals generate more radicals; whereas ethylcyclohexane produces mostly C5-ring and C6-ring alkanes and alkenes, whose radicals show a propensity for ring dehydrogenation. These contrasts in the aliphatic-product distributions have major impacts on PAH growth, which, in this reaction environment, occurs chiefly through the reactions of resonance-stabilized arylmethyl radicals with the C2-C4 1-alkenes. For fuel compositions rich in n-decane, all the ingredients necessary for these PAH-growth reactions to thrive are in place: methyl-substituted PAH, as sources of arylmethyl radicals; a radical-rich environment that fosters H abstraction and arylmethyl-radical formation; and C2-C4 1-alkenes, as growth species. However, an increase in the level of ethylcyclohexane in the fuel brings about substantial reductions in the levels of H-abstracting radicals and C2-C4 1-alkenes as well as an increase in the supply of “donated” hydrogen for stabilizing radicals. These factors combine to bring about marked reductions in PAH growth at fuel compositions rich in ethylcyclohexane, resulting in much lower production of the high-ring-number PAH that are the precursors to solids in the supercritical fuel-pyrolysis environment.