Combustion Measurements of Fuel‐Rich Aluminum and Molybdenum Oxide Nano‐Composite Mixtures

Abstract

AbstractFuel rich nano‐composite powders of aluminum and molybdenum oxide were tested for ignition and combustion behind the incident and reflected shock waves in a shock tube. The powders consisted of approximately 10 μm particles, each of which contained Al and MoO3 mixed by mechanical alloying on the nano‐scale. These powders were aluminum rich with composition ratios of 4 : 1, 8 : 1, and 16 : 1 Al : MoO3 by mass. Ignition tests were performed behind incident shocks for temperatures in the range of 900 to 1500 K. From these tests, ignition delay times were obtained, and some information on combustion duration was also derived. Samples were tested in air at 0.2 MPa, and compared against nano‐Al, 2.7 μm Al, and 10 μm Al baselines. Ignition results for the baseline Al cases were as expected: 10 μm Al not igniting until 2000 K, 2 μm Al igniting down to ∼1400 K, and n‐Al igniting as low as 1150 K. The thermite samples showed considerable improvement in ignition characteristics. At the lowest temperature tested (900 K), both the 8 : 1 and 4 : 1 samples ignited within 250 μs. The 16 : 1 sample (94% Al) ignited down to 1050 K – which represents an improvement of roughly 1000 K over baseline Al with only a small energetic penalty. In all cases, the ignition delay increased as the amount of MoO3 in the composite was reduced. The 4 : 1 nano‐composite material ignited as fast or faster than the n‐Al samples. Ignition delay increased with decreasing temperature, as expected. Emission spectra and temperature data were also taken for all samples using high‐speed pyrometry and time‐integrated spectroscopy. In these cases, measurements were made behind the reflected shock using end‐wall loading, though the conditions (temperature, pressure, and gas composition) were identical to the incident shock tests. Spectroscopy showed strong AlO features in all the samples, and the spectra fit well to an equilibrium temperature. Broadband, low resolution spectra were also fit to continuum, gray body temperatures. In general, the observed temperatures were reasonably close to 3500 K, which is similar to the combustion temperatures of pure aluminum under these conditions.