Ignition and combustion of aluminum particles in shocked H 2O/O 2/Ar and CO 2/O 2/Ar mixtures

Abstract

Small aluminum particles (5–10 μm) are ignited in atmospheres consisting of Ar and varying amounts of H 2O, CO 2, and O 2 at the endwall of a shock tube to study the burning characteristics in various combinations of these oxidizers. A reflected shock is used to obtain pressures of ∼8.5 atm and temperatures of ∼2600 K. Visible wavelength emission spectra are recorded by using a spectrometer coupled to a streak camera, and two photodetectors record intensity vs. time at AlO emission wavelengths of 486 and 514 nm. The streak camera allows recording of multiple time-resolved spectra at rates of 100 μs per spectrum. Aluminum particles ignited in mixtures of CO 2/O 2/Ar exhibited a burn time of about one half that of an atmosphere containing only a mixture of O 2/Ar, holding the argon constant at 40%. In addition, as CO 2 was substituted for O 2 in successive experiments, a nonlinear relationship, as a function of the gas composition, was observed for ignition delay time and burn time. Within mixtures of H 2O/O 2/Ar, two distinct burning stages are visible for the combusting aluminum particles. As an increasing amount of H 2O was substituted for O 2 in separate tests, a second distinct burning stage developed. Spectroscopic data recorded during such experiments portray AlO emission during both combustion stages. Aluminum particles combusted in a mixture of H 2O/Ar show burn times 2 to 5 times greater than those from CO 2/O 2/Ar mixtures, and ignition delay times 3 to 6 times greater.