Laminar Flame Speeds of Nano-Aluminum/Methane Hybrid Mixtures

An existing flame speed bomb, which uses optical techniques to measure laminar flame speed, was employed to study the fundamental phenomena of flame propagation through a uniformly dispersed aerosol. In a previous thesis by Andrew Vissotski, the groundwork was laid to begin studies of hybrid flames. Beginning from these preliminary findings, the facility was upgraded to disperse dust into the test chamber through a strong burst of gas. This aerosol was then allowed to settle for a minimum of 45 seconds to ensure that the conditions inside the test chamber were quiescent and that the dust was uniformly distributed. Extinction of laser light through the resulting aerosol was measured through the large optical access with a 632.8-nm, 5-mW HeNe laser so that the mass of suspended nano-particles could be determined as a function of time up until combustion has occurred.

The particles used in these experiments were aluminum nano-particles with a manufacturer-stated average fundamental particle size of 100 nm. To properly quantify the particle distribution inside of the vessel, a scanning mobility particle sizer was employed to characterize the aluminum, resulting in an average particle size of 446.1 nm. With a calibrated extinction measurement, experimental suspended mass of aluminum was measured up to 90 mg. A hybrid mixture of Al/CH4 was chosen to serve as the combustion medium and to provide a well-characterized parent fuel to measure the contribution of nano-aluminum on combustion.

Two series of experiments were performed, 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% from the neat mixture, when nano-aluminum is introduced. In the 70/30 N2/O2 mixture, the addition of aluminum results in a maximum decrease of 5 cm/s from the neat flame speed of 80.5 cm/s and in the air mixture, a 2 cm/s maximum decrease from 35.3 cm/s. A preliminary spectroscopic analysis was performed but was inconclusive. It was also found that the addition of nanoparticles cause the flame to become unstable faster when compared to the neat mixture of CH4/air.