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Powdered iron is being investigated for its potential use as a carbon-free fuel due to its ability to burn heterogeneously and produce oxide particles, which can be collected, reduced back to iron and burned again. However, high temperature oxidation of iron particles can induce partial vaporization/decomposition and evolution of nanometric iron oxide particles. To investigate the formation process of nanoparticles in iron combustion, iron powders (consisting of spheroidal 45–53 μm particles) were injected in an electrically-heated drop tube furnace, operated at a maximum gas temperature of 1375 K, where they experienced high heating rates (104 K/s). The particles reacted with oxygen at concentrations of 15, 21, 35, 50 and 100 % by volume in nitrogen diluent gas. Particles ignited and burned brightly, with peak temperatures reaching 2344–2884 K, depending on the oxygen concentration. The observed distribution of the combustion products of iron was bimodal in size and composition, containing (a) dark gray spherical micrometric particles bigger than their iron particle precursors composed of both magnetite and hematite, and (b) highly agglomerated orange-reddish nanometric particles composed of hematite. The mass fraction of nanometric particles accounted for up to 1.7–7.4 % of the collected products, increasing with the oxygen partial pressure. The nanometric particles were spherules, 30–100 nm in diameter. However, they were highly agglomerated with aggregate aerodynamic diameters peaking at 180–560 nm. The yield of nanoparticles increased with increasing oxygen concentration in the furnace. A heuristic model was used to investigate the impact and sensitivity of various strategies for modeling evaporation, aiming to identify key mechanisms that limit the evaporation rate. This study highlights that understanding the type of liquid at the particle surface is crucial, as evaporation can increase significantly with a homogeneous liquid Fe-O particle compared to a core–shell morphology. Additionally, the analysis suggests that evaporation likely occurs in an intermediate regime where gaseous Fe-containing species oxidize in the boundary layer. Understanding these boundary layer processes is essential for accurately modeling the evaporation rate while maintaining computational efficiency. 1. 

This research focused on the size and overall porosity (pore volume) of carbonaceous chars, originating from high-heating rates and high-temperature pyrolysis and/or combustion of biomass. Emphasis was given to torrefied biomass chars. First, the porosity of char residues of single biomass particles of known mass was determined, based on an assumed value of skeletal density and by comparing experimentally observed temperature-time histories with numerical predictions of their burnout times. The average char porosities (effective porosities) of several raw and torrefied biomass particles were calculated to be in the range of 92–95%. Thereafter, these deduced porosity values were input again to the model to calculate the size of chars of other biomass particle precursors, whose initial size and mass were not known. Such biomass particles were sieve-classified to different nominal size ranges. This time, besides the porosity, representative time-temperature profiles of biomass particles in the aforementioned size ranges were also input to the model. Biomass particles are highly irregular with large aspect ratios and, in many cases, they melt and spherodize under high heating rates and elevated temperatures. Knowledge of the initial size of the chars, upon extinction of the volatile flames, is needed for modeling their heterogeneous combustion phase. For this purpose, numerical predictions were in general agreement with measurements of char size obtained from both scanning electron microscopy of captured chars and real-time high-speed, high- magnification cinematographic observations of their combustion. Results showed that the generated chars of the examined biomass types were highly porous with large cavities. The average initial dimension of the chars, upon rapid pyrolysis, was in the range of 50–60% the mid-value of the mesh size of the sieves used to size-classify their highly irregular parent biomass particles. 

Liquid transportation fuels are composed of a wide range of molecular structures and weights, therefore exhibiting a relatively large distillation temperature range. When fuel chemical properties change along with the distillation temperature curve, preferential vaporization effects could play a role in near-limit combustion behaviors. The objective of this study is to experimentally evaluate the role of preferential vaporization on flame flashback behaviors. A unique spray burner is developed to control the extent of fuel spray vaporization by adjusting flow rates and/or the spray injection location from the burner exit. Spray characteristics are comprehensively determined using Phase Doppler Particle Analyzer. Two binary component mixtures are formulated (n-octane/iso-cetane and iso-octane/n-hexadecane) to exhibit common combustion behaviors in the fully vaporized condition but have considerably different preferential vaporization characteristics. Identical flashback behaviors of two mixtures are observed for fully pre-vaporized conditions by setting the burner temperature at 700 K, including both propagation- and ignition-driven flashback behaviors. Partially vaporized conditions are investigated at two global equivalence ratios (1.0 and 1.4) by setting the burner temperature at 450 K. The flashback behaviors for both global equivalence ratio conditions are found to be affected by the preferential vaporization characteristics represented by laminar flame speeds of the vaporized fuel mixture composition. The relative significance of local flow perturbation induced by instantaneous fuel droplet evaporation near the flame surface has been also investigated by analyzing planar laser-induced fluorescence images, as well as considering the changes of Markstein length with the extent of fuel vaporization. Finally, the relative contributions of local laminar flame speed representing local fuel vapor deposit, local flow perturbation, and preferential vaporization are evaluated through feature sensitivity analyses.