Researchers led by Professor Michael R. Zachariah at the University of Maryland have utilized “mesoparticle” composites of nanoaluminum and nitrocellulose by electrospray particle assembly to promote the burning rates of liquid droplets upon incorporation of the particles. These “nanofuel” suspensions of mesoparticles exhibit higher maximum particle loadings than suspensions of unassembled nanoaluminum and nitrocellulose.
Metal nanoparticles have been previously identified to boost ignition and higher burning rates in combustion systems as compared to their micron-sized counterparts. This is based on their increasing surface to volume ratio as particle size drops.
The effects of addition of nanoparticle additives in liquid fuels have been investigated featuring increased energy densities, higher heats of combustion, shortened ignition delay times, reduced emissions, and promotion of evaporation and combustion rates. The high enthalpy of combustion of metals can therefore be exploited to enhance the volumetric energy density of a fuel.
Several nanoparticle additives were identified to decrease hydrocarbon, carbon-monoxide emissions and NOx when added to diesel fuel for engines. These additives include aluminum oxide, iron, and carbon nanotubes. However, these energetic metal nanoparticles are susceptible to aggregation occurring at an increased rate near the surface of a burning liquid droplet where concentrations increase. This may lead to the formation of a transport-inhibiting shell, which may decrease the droplet-burning rate.
Researchers led by Professor Michael R. Zachariah at the University of Maryland investigated the effects of chemically stabilized Nano-aluminum-based additives to kerosene fuel with and without a gas generating polymeric co-additive, nitrocellulose in a drop-tower arrangement designed to approximate combustion rates in the presence of disruptive burning. Their research work is now published in Combustion and Flame.
The researchers needed a surfactant in order chemically stabilize the additive particles in the Nano fuels. They proposed trioctylphosphine oxide composed of two carbon chains for the study. Trioctylphosphine oxide was then added to all nanofuels.
The research team assembled the Nano-fuels by adding selected solid loadings including nanoaluminum and nitrocellulose to premixed trioctylphosphine oxide in kerosene solutions. They also used the same solutions as control fuels without additives for every loading. To enhance the suspension, they agitated nanofuel mixtures. Mesoparticle nanofuels that exhibited higher suspension stability than nanoaluminum or nitrocellulose particles required less agitation.
The research team observed that nitrocellulose was a suitable gas generator and was capable of increasing the burning rates of hydrocarbon droplets laden with nanoaluminum particles. Without a gas-generating co-additive, this would have decreased the burning rate of the fuel. A physical mixture of nanoaluminum and nitrocellulose in the kerosene fuel were limited by poor stability with increasing particle loading. This was the case even with the use a hydrocarbon surfactant, trioctylphosphine oxide.
Their study realized that composite nanoaluminum and nitrocellulose would be applied to make stable nanofuels with twice the maximum particle loadings of physically mixed nanofuels without clogging the droplet generation capillary. Mesoparticles were also observed to enhance high burning rates at increased loadings where detrimental agglomeration effects are pronounced for physically mixed additives relative to mesoparticle additives. Moreover, cyclical droplet inflations as well as deflations was observed to be a necessary mechanism whereby increased gasification rates, promoted the overall burning rate of the fuel.
Philip M. Guerieri, Jeffery B. DeLisio, Michael R. Zachariah. Nanoaluminum/Nitrocellulose microparticle additive for burn enhancement of liquid fuels. Combustion and Flame, volume 176 (2017), pages 220–228.Go To Combustion and Flame