Experimental investigation of planar strained methane-air and ethylene-air flames

The extinction of planar strained methane-air flames in the stagnation-point flow is studied. A thermal analysis has been conducted in order to build a new copper stagnation plate which can be heated up to 1000K, and allows investigation of downstream heat loss as extinction driving mechanism. Since premixed stagnation flames are mostly sensitive to the composition of the mixture, axial velocity and CH radical profiles are simultaneously measured for different equivalence ratios, using respectively Particle Streak Velocimetry (PSV) and Planar Laser Induced Fluorescence (PLIF). These are compared to simulations using CANTERA stagnation flow code with a multicomponent molecular transport model, with the following chemical kinetics mechanisms: GRI-MECH 3.0, the C3-Davis, San-Diego 200308 and San-Diego 200503 mechanisms. In methane-air flames, simulations accurately predict the velocity and CH profiles from Phi=0.8 to Phi=1.2, but the flame speed turns out to be overpredicted at Phi=0.7 by all mechanisms except the C3-Davis mechanism (see Bergthorson et al. 2005a). The experiment at Phi=1.3 would need to be reconducted. Also, measured relative concentrations of CH are compared to numerical predictions using each of the four mechanisms cited above. Composition variations impact on ethylene-air flames was also investigated due to a peculiar jump in the overprediction of flame velocities from Phi=1.6 to Phi=1.8 (Bergthorson 2005). This peculiar feature was found to be repeatable, but the cause remains unclear. Methane-air laminar flame speeds Su0 were computed using CANTERA freely propagating flame code for the following chemical kinetics mechanisms: GRI-MECH 3.0, the C3-Davis mechanism, the San Diego 200308, 200503, and 200506 mechanisms, for variable pressures (1,2,5,10,20 atm) and equivalence ratios (0.6-1.4). Even for methane, whose chemistry is one of the best understood, the scatter between the different mechanisms is significant. Both composition and pressure were found to affect Su0 substantially, although composition variations seem to excite the differences in the predictions among the different mechanisms the most

Characterization of Fluidization and Mixing of Binary Mixtures Containing Biomass at Low Velocities Through Analyzing Local Pressure Fluctuations

To judge qualitatively the effect of biomass properties on its fluidizability and mixing tendency with sand particles at low fluidization velocities, local pressure gradients in the top and bottom of the bed were compared for all investigated systems. It was found that changing the mass fraction and the size of biomass impacted the onset of bubbling and the size of bubbles across the bed, which, in turn, affected the mixing/segregation of the bed content

Particle streak velocimetry and CH laser-induced fluorescence diagnostics in strained, premixed, methane–air flames

We present the use of simultaneous particle streak velocimetry (PSV) and CH planar laser-induced fluorescence (PLIF) diagnostics in the study of planar, strained, premixed, methane–air flames, stabilized in a jet-wall stagnation flow. Both PSV and PLIF data are imaged at high spatial resolution and sufficiently high framing rates to permit an assessment of flame planarity and stability. Concurrent measurements of mixture composition, (Bernoulli) static-pressure drop, and stagnation-plate temperature provide accurate boundary conditions for numerical simulations. The new PSV implementation is characterized by very low particle loading, high accuracy, and permits short recording times. This PSV implementation and analysis methodology is validated through comparisons with previous laminar flame-speed data and detailed numerical simulations. The reported diagnostic suite facilitates the investigation of strained hydrocarbon–air flames, as a function of nozzle-plate separation to jet-diameter ratio, L/d, and equivalence ratio, ɸ. Methane–air flames are simulated using a one-dimensional streamfunction approximation, with full chemistry (GRI-Mech 3.0), and multi-component transport. In general, we find good agreement between experiments and simulations if boundary conditions are specified from measured velocity fields. Methane–air flame strength appears to be slightly overpredicted, with the largest disagreements for lean flames

The ignition of fine iron particles in the Knudsen transition regime

A theoretical model is considered to predict the minimum ambient gas temperature at which fine iron particles can undergo thermal runaway--the ignition temperature. The model accounts for Knudsen transition transport effects, which become significant when the particle size is comparable to, or smaller than, the molecular mean free path of the surrounding gas. Two kinetic models for the high-temperature solid-phase oxidation of iron are analyzed. The first model (parabolic kinetics) considers the inhibiting effect of the iron oxide layers at the particle surface on the rate of oxidation, and a kinetic rate independent of the gaseous oxidizer concentration. The ignition temperature is solved as a function of particle size and initial oxide layer thickness with an unsteady analysis considering the growth of the oxide layers. In the small-particle limit, the thermal insulating effect of transition heat transport can lead to a decrease of ignition temperature with decreasing particle size. However, the presence of the oxide layer slows the reaction kinetics and its increasing proportion in the small-particle limit can lead to an increase of ignition temperature with decreasing particle size. This effect is observed for sufficiently large initial oxide layer thicknesses. The continuum transport model is shown to predict the ignition temperature of iron particles exceeding an initial diameter of 30 $\mu$m to a difference of 3% (30 K) or less when compared to the transition transport model. The second kinetic model (first-order kinetics) considers a porous, non-hindering oxide layer, and a linear dependence of the kinetic rate of oxidation on the gaseous oxidizer concentration. The ignition temperature is resolved as a function of particle size with the transition and continuum transport models, and the differences between the ignition characteristics predicted by the two models are discussed

Combustion behavior of single iron particles-part I:An experimental study in a drop-tube furnace under high heating rates and high temperatures

Micrometric spherical particles of iron in two narrow size ranges of (38–45) µm and (45–53) µm were injected in a bench scale, transparent drop-tube furnace (DTF), electrically heated to 1400 K. Upon experiencing high heating rates (104–105 K/s) the iron particles ignited and burned. Their combustion behavior was monitored pyrometrically and cinematographically at three different oxygen mole fractions (21%, 50% and 100%) in nitrogen. The results revealed that iron particles ignited readily and exhibited a bright stage of combustion followed by a dimmer stage. There was evidence of formation of envelope micro-flames around iron particles (nanometric particle mantles) during the bright stage of combustion. As the burning iron particles fell by gravity in the DTF, contrails of these fine particles formed in their wakes. Peak temperatures of the envelope flames were in the range of 2500 K in air, climbing to 2800 K in either 50% or 100% O2. Total luminous combustion durations of particles, in the aforesaid size ranges, were in the range of 40–65 ms. Combustion products were bimodal in size distribution, consisting of micrometric black magnetite particles (Fe3O4), of sizes similar to the iron particle precursors, and reddish nanometric iron oxide particles consisting mostly of hematite (Fe2O3).</p

Particle streak velocimetry and CH laser-induced fluorescence diagnostics in strained, premixed, methane–air flames

We present the use of simultaneous particle streak velocimetry (PSV) and CH planar laser-induced fluorescence (PLIF) diagnostics in the study of planar, strained, premixed, methane–air flames, stabilized in a jet-wall stagnation flow. Both PSV and PLIF data are imaged at high spatial resolution and sufficiently high framing rates to permit an assessment of flame planarity and stability. Concurrent measurements of mixture composition, (Bernoulli) static-pressure drop, and stagnation-plate temperature provide accurate boundary conditions for numerical simulations. The new PSV implementation is characterized by very low particle loading, high accuracy, and permits short recording times. This PSV implementation and analysis methodology is validated through comparisons with previous laminar flame-speed data and detailed numerical simulations. The reported diagnostic suite facilitates the investigation of strained hydrocarbon–air flames, as a function of nozzle-plate separation to jet-diameter ratio, L/d, and equivalence ratio, ɸ. Methane–air flames are simulated using a one-dimensional streamfunction approximation, with full chemistry (GRI-Mech 3.0), and multi-component transport. In general, we find good agreement between experiments and simulations if boundary conditions are specified from measured velocity fields. Methane–air flame strength appears to be slightly overpredicted, with the largest disagreements for lean flames

TRENDS AND SIMILARITIES IN C1-C4 PRIMARY ALCOHOL HIGH-TEMPERATURE IGNITION

Abstract A review of shock tube ignition of primary alcohols from methanol to n-butanol is presented. Most of these literature data were obtained in studies of individual alcohols. Recent work published by the current authors focused on the comparative study of these alcohols, showing similarity in their ignition behavior. This similarity is also observed when ignition delays obtained by other groups under the same conditions are compared. Further comparison is carried out using ignition correlations obtained in the previous study by the current authors. A similar approach in the study of flame propagation of methanol, ethanol and n-butanol was reported by Veloo et al. [Veloo et al., Combust. Flame, 157 (2010) 1989-2004, concluding that under lean conditions the flames of these alcohols propagate with the same velocity. The same trend was observed under rich conditions, apart from methanol flames which were found to propagate faster. However, the comparable behavior is not fully captured by the various literature mechanisms of these fuels. The observed similarity, which is not obvious from the analysis of individual alcohols, can be employed in the development, validation and optimization of their detailed and reduced chemical kinetic models. Furthermore, this similarity can be useful in the practical design and modification of engine systems for flexible alcohol-based fuels