Ignition and combustion of aluminum particles in shocked H2O/O2/Ar and CO2/O2/Ar mixtures

Small aluminum particles (5-10 mum) are ignited in atmospheres consisting of Ar and varying amounts of H2O, CO2, and O-2 at the endwall of a shock tube to study the burning characteristics in various combinations of these oxidizers. A relected shock is used to obtain pressures of similar to8.5 atm and temperatures of similar to 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 mus per spectrum. Aluminum particles ignited in mixtures of CO2/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 CO2 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 H2O/O-2/Ar, two distinct burning stages are visible for the combusting aluminum particles. As an increasing amount of H2O 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 H2O/Ar show burn times 2 to 5 times greater than those from CO2/O-2/Ar mixtures, and ignition delay times 3 to 6 times greater. (C) 2001 by The Combustion Institute.