Freely-propagating flames in aluminum dust clouds

Abstract

The free propagation of isobaric flames through aluminum dust clouds is investigated in an extensive series of experiments using two facilities with different scales. In small-scale laboratory experiments, spherical flame propagation occurs in aluminum dust clouds contained within 30-cm-diameter latex balloons, whereas in large-scale tests, flames propagate vertically through unconfined aluminum dust clouds with a vertical scale of about 4 m. The balloon experiments are performed with suspensions of aluminum powder in oxygen mixed with nitrogen, argon, or helium with various concentrations of oxygen and aluminum. It is found that stable flame propagation is only observed for aluminum concentrations near stoichiometric to rich conditions. Pulsating and spiral-like flames are discovered in fuel-lean mixtures, and flames with cellular patterns occur in very-fuel-rich suspensions. The burning velocities in the stable propagation regime derived from the balloon experiments correlate well with data previously obtained with stabilized Bunsen-type flames. The flame speed of stable flames is found to be a strong function of the heat conductivity of the gas mixture. In addition, the oxygen concentration has a strong influence on the flame speed for fuel-rich mixtures but dependence is reduced for fuel-lean mixtures. In the large-scale experiments, the burning velocity is estimated to be about two times larger than for the small-scale experiments. The increase in burning velocity is attributed to preheating of the unburned mixture by radiation from the condensed-phase combustion products. The degree of preheating, determined with an array of fine thermocouples, is found to be in the range of 150-200 K. The propagation of stable flames is discussed in light of existing qualitative dust flame models, whereas the pulsating flame propagation regime observed is interpreted in terms of the thermo-diffusive instability theory developed for high Lewis number flames in gases and solid reactive powder mixtures.

Direct combustion of recyclable metal fuels for zero-carbon heat and power

“…While the burning velocity is a well-established concept that scales the turbulent flame speeds of gaseous and low-boilingpoint liquid fuels [116,126,127], it is not completely clear yet if this concept can be applied in the same way to metal-fuel flames of all scales and turbulence intensities. Our recent experiments have demonstrated that the flame speeds of both laboratory-scale metal flames, including stabilized and propagating flames, and largescale metal flames studied during field trials are indeed scalable using the burning velocity concept [128]. These promising results suggest that burning velocities of metal flames derived from laboratory experiments can be used as a first approximation in designing turbulent metal-fuel combustors required for transportation and industrial applications.…”

“…Little data on the effect of turbulence on the flame speed of metal fuels is available [122], but our recent measurements show a large increase in burning rate for large-scale flames in field tests that contain some residual turbulence created during the required metal-fuel dispersal process [75,128]. Turbulent combustion of traditional fuels remains a challenging area of active research [126,127] and, therefore, much remains to be learned about turbulent metal flames.…”

“…Methane data from our laboratory [75] compared to simulations using Cantera and GRI-Mech [131,132]. Aluminum curve obtained by fits to recent data obtained in our laboratory [128] and previously-published data is plotted for comparison [113]. nitrogen) to control the percentage of oxygen in the oxidizing gas mixture.…”

Front roughening of flames in discrete media

“…This situation is distinct from homogenous reactive media, as exemplified by gaseous mixtures, where fluctuations in concentration only occur on scales much smaller (e.g., mean molecular spacing) than the flame thickness, permitting a clear separation of scales by several orders of magnitude. In heterogeneous media (e.g., fuel particulates suspended in gaseous oxidizer [1,2] or mixtures of solid reactants as encountered in Self-propagating High-temperature Synthesis, SHS [3,4]), the scale of the heterogeneity can be comparable to or greater than the flame thickness, raising the question of how the statistical properties of the media influence the flame dynamics.…”

Analysis of thermodiffusive cellular instabilities in continuum combustion fronts

“…Even though this choice of kinetics is a qualitatively good description for such systems, Brailovsky et al [24] have shown more recently that ignition reaction kinetics and Lewis number. Thermo-diffusive instabilities for these systems in two forms of "Cells" for low Lewis number values (Le < 1), and "Oscillations" for high Lewis number values (Le > 1), have been observed in previous studies [20][21][22][23]. This paper studies the regime of rich metal particles in gas.…”