Ignition and combustion of aluminum particles in shocked H2O/O-2/Ar and CO2/O-2/Ar mixtures

Small aluminum particles (5-10 mum) are ignited in atmospheres consisting o f Ar and varying amounts of H2O, CO2, and O-2 at the endwall of a shock tub e to study the burning characteristics in various combinations of these oxi dizers. A relected shock is used to obtain pressures of similar to8.5 atm a nd temperatures of similar to 2600 K. Visible wavelength emission spectra a re recorded by using a spectrometer coupled to a streak camera, and two pho todetectors record intensity vs. time at AlO emission wavelengths of 486 an d 514 nm. The streak camera allows recording of multiple time-resolved spec tra at rates of 100 mus per spectrum. Aluminum particles ignited in mixture s of CO2/O-2/Ar exhibited a burn time of about one half that of an atmosphe re 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 no nlinear relationship, as a function of the gas composition, was observed fo r ignition delay time and burn time. Within mixtures of H2O/O-2/Ar, two dis tinct burning stages are visible for the combusting aluminum particles. As an increasing amount of H2O was substituted for O-2 in separate tests, a se cond distinct burning stage developed. Spectroscopic data recorded during s uch experiments portray AlO emission during both combustion stages. Aluminu m particles combusted in a mixture of H2O/Ar show burn times 2 to 5 times g reater than those from CO2/O-2/Ar mixtures, and ignition delay times 3 to 6 times greater. (C) 2001 by The Combustion Institute.