Dynamics of nanoparticle combustion

Aluminum;Nanoparticle;Energetics;Combustion

Allen, David

关键词: Aluminum;    Nanoparticle;    Energetics;    Combustion;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/78299/Allen_David.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

A heterogeneous shock tube was used to ignite and measure the combustion behavior of the nano-aluminum suspension behind reflected shock waves. The burning time and particle temperatures were measured using optical diagnostics. In order to use pyrometry measurements for nano-aluminum particles, the emissivity of nano-alumina particles was also measured using the shock tube to heat the particles to known temperatures. The burning time and peak particle temperature results suggested that heat transfer models currently used for burning nanoparticles may significantly overestimate heat losses during combustion. By applying conventional non-continuum heat transfer correlations to burning nano-aluminum particles, the observed peak temperatures, which greatly exceed the ambient temperature, should only be observable if the burning time were very short, of the order of 1 $\mu$s, whereas the observed burning time is two orders of magnitude larger. These observations can be reconciled if the energy accommodation coefficient for these conditions is of the order of 0.005, which is the value suggested by Altman, instead of approximately unity, which is the common assumption. A simple model was developed for nano-aluminum particle combustion focusing on a surface controlled reaction as evidenced by experimental data and heat transfer to the surroundings. The simple model supports a low energy accommodation coefficient as suggested by Altman.This result has significant implications on the heat transfer and performance of the nanoparticles in combustion environments. Direct measurement is needed in order to decouple the accommodation coefficient from the assumed combustion mechanism in the simple model. Time-resolved laser induced incandescence measurements were performed to measure the accommodation coefficient of nano-alumina particles in various gaseous environments. The accommodation coefficient was found to be 0.03, 0.07, and 0.15 in helium, nitrogen, and argon respectively at 300 K and 2 atm is each environment. These values represent upper limits for the accommodation coefficient as scaling suggests that the accommodation coefficient will decrease with increasing particle and ambient temperature to values similar to those observed during shock tube measurements. The accommodation coefficient values measured using LII are similar to what has been seen for other metallic nanoparticles and significantly smaller than values used in soot measurements. The results will allow for additional modeling of the accommodation coefficient to be extended to other environments and support previous measurements of high combustion temperatures during nano-aluminum combustion. Further constant volume combustion measurements were used to determine the macroscopic effect of a low energy accommodation coefficient on the heat release to the ambient surrounding in a aersolized aluminum combusting medium.