Laminar flame speeds of nano-aluminum/methane hybrid mixtures
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2016 The Combustion Institute. An existing flame speed bomb was utilized to study the fundamental phenomena of flame propagation through a uniformly dispersed aerosol with a relatively small mass of metal nanoparticles. The aerosol was dispersed with a burst of gas and then allowed to settle for at least 45s, to ensure that the conditions inside the test chamber were quiescent and uniform. Extinction of the light from a HeNe laser was used so that the mass of suspended nano-particles with a fundamental size of 100nm could be determined as a function of time prior to combustion, and the suspended mass of aluminum (up to 90mg) was measured in situ during an experiment. A particle size distribution was measured as well, resulting in an average pre-combustion agglomeration size of 446.1nm. Two series of experiments were performed with CH4 fuel, both at stoichiometric conditions: one with the mixture in air and the second with the mixture in a 70/30 N2/O2 mix. The results herein show a maximum decrease in flame speed, 5-7% for the baseline mixture, when nano-aluminum was introduced at the suspended masses utilized in the present study. For the 70/30 N2/O2 mixture, the aluminum resulted in a maximum decrease of 5cm/s from the baseline value of 80.5cm/s; and in the air mixture, a 2cm/s maximum decrease from 35.3cm/s was observed. Simple modeling of the process extremes indicates that the observed decrease in flame speed is somewhere between the limits of simple particle heating with no particle reaction and the kinetics limit where the Al reacts in the gas phase. Interestingly, the trends qualitatively matched those of the kinetics model applied herein. It was also found that the addition of nanoparticles caused the flame to become unstable much sooner when compared to the baseline mixture of CH4/air.
