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In advanced propulsion systems such as scramjet engines, endothermic decomposition of onboard large hydrocarbon fuels can be used effectively for cooling and active thermal protection. During the cooling process, large hydrocarbon fuels absorb heat and decompose into small fragments. Since fuel decomposition changes the chemical and transport properties of the reactants, it is expected to affect the combustion afterwards. In this study, forced ignition in quiescent n-decane/air mixtures with fuel decomposition is investigated via a simplified model and transient numerical simulations considering detailed chemistry and transport. The emphasis is placed on assessing the effects of fuel decomposition on ignition kernel development and minimum ignition energy (MIE) in both homogenous and fuel-stratified mixtures. Fuel decomposition is modelled by a homogeneous ignition process in n-decane/air mixture at constant atmospheric pressure and with an initial temperature of 1300 K. Small fragments appear during the pyrolysis process. The partially-reacted mixture is frozen and cooled to a lower temperature and used as the initial mixture in forced ignition. For homogeneous mixtures, fuel decomposition can greatly promote ignition for fuel-lean decane/air mixtures while it has little effect for the stoichiometric case. Fuel decomposition also affects the duration of unsteady ignition kernel transition. Besides, fuel decomposition and fuel stratification are combined to further promote forced ignition. New flame regimes are observed and an optimum stratification radius is identified. In order to promote the forced ignition, only the fuel within the optimum stratification radius needs to be decomposed. Furthermore, laser and spark ignition experiments are conducted to measure the MIE of n-decane/ethylene/air mixtures with different equivalence ratios and ethylene blending ratios. The MIE measured in experiments cannot be directly compared with simulation results since the simulation model for forced-ignition is simplified. Nevertheless, the experimental results are consistent with simulation results and thus validate some conclusions mentioned above. The present results provide useful guidance to the fundamental understanding of forced ignition in a mixture with fuel decomposition. [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]

The decomposition characteristics of five candidate polysulfide (LP-33) based ramjet solid fuel formulations are evaluated using thermogravimetric analysis and differential scanning calorimetry. The formulations are variants on the polymer crosslink mechanism and the doping mass fraction of elemental sulfur. Four fuel candidates are oxidatively cured and one formulation is cured with isocyanates. Post-cure, the Shore-A hardness of each sample is measured followed by TGA and DSC analysis. The objective of this research is to develop a solid fuel ramjet formulation with thermal decomposition onset temperatures lower than conventional ramjet fuels while maintaining a Shore-A hardness of at least 30. The results show that polysulfide based formulations decompose at temperatures approximately 120 °C lower than hydroxyl-terminated polybutadiene based fuels. Additionally, the results demonstrate that the decomposition onset temperature, and the heat of decomposition can be further reduced with the addition of elemental sulfur. [ABSTRACT FROM AUTHOR]

Mastication of shrubs and small trees to reduce fire hazard has become a widespread management practice, yet many aspects of the fire behaviour of these unique woody fuelbeds remain poorly understood. To examine the effects of fuelbed aging on fire behaviour, we conducted laboratory burns with masticated Arctostaphylos spp. and Ceanothus spp. woody debris that ranged from 2 to 16 years since treatment. Masticated fuels that were 10 years or older burned with 18 to 29% shorter flame heights and 19% lower fireline intensities compared with the younger fuelbeds across three different fuel loads (25, 50 and 75 Mg ha-1). Older fuelbeds smouldered for almost 50% longer than the younger masticated fuelbeds. Fuel consumption was 96% in the two higher fuel load categories regardless of fuelbed age, whereas consumption was 77% in the lighter fuel load. Fire intensity in masticated fuels may decrease over time owing to particle degradation, but in dry environments where decomposition is slow, combustion of the remaining fuels may still pose risks for tree mortality and smoke production associated with protracted smouldering. [ABSTRACT FROM AUTHOR]

Abstract: Two critical steps towards soot production in combustors are the decomposition of the fuel and the subsequent formation of aromatic hydrocarbons with one to three benzenoid rings. Traditionally, flame studies of these processes have used small hydrocarbons such as methane, ethylene, and acetylene as the fuel. However, recent research, which is reviewed in this article, has begun to close the ‘gap’ between these small hydrocarbons and the larger, more complex hydrocarbons that constitute all liquid combustion fuels. [Copyright &y& Elsevier]