Effects of Inert and Energetic Nanoparticles on Burning Liquid Ethanol Droplets

ABSTRACT

The effect of nanoscale energetic aluminum (nAl) and inert silicon dioxide (nSiO₂) particulate additives on ethanol droplet combustion was studied under atmospheric conditions. Three different types of droplet experiments were performed to study the influence of the experiment itself on combustion behavior. Simultaneous visible and intensified ultraviolet (UV) images were taken to determine the burning rate constant (K) as well as flame dynamics via OH* chemiluminescence imaging. The addition of nAl appeared to yield a systematic increase in K, by up to 13%, and increasing loading concentrations led to changes in droplet combustion dynamics. Flow instabilities, including liquid jetting and altered droplet deformation, were observed, creating unsteady combustion when the nAl-laden droplet was continuously fed via a quartz capillary. In contrast, the addition of nSiO₂ showed relatively small changes in K, possibly only as large an increase as 5%, with a lack of consistent trends for increasing nSiO₂ concentration for different fuel delivery methods, in part due to the formation of large residual shell-like structures in the later stages of combustion. A simple droplet combustion model suggests that possible enhancement mechanisms for K are related to alterations in thermal conductivity as well as flame temperature with the nAl additive. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of particulate residue revealed further differences in morphology and residue constituents after combustion.

KEYWORDS:

• Nanofluid Fuels
• Droplet Combustion
• Aluminum Nanoparticles

Acknowledgments

The authors would like to thank Dr. Alireza Badakhshan of the Air Force Research Laboratory (AFRL) for helpful advice and Daniel Ahn for his assistance with this study. The authors acknowledge the use of TEM at the Electron Imaging Center for NanoMachines (EICN) and the use of SEM facilities at the Molecular & Nano Archaeology (MNA) Laboratory at UCLA.

Supplementary material

Supplemental material for this article can be accessed on the publisher’s website.

Additional information

Funding

This work was supported by the Air Force Office of Scientific Research (AFOSR) [Award No. FA9550-15-1-0339] with Dr. Mitat Birkan as program manager. The high-speed visible camera was purchased with funds generously donated to the UCLA EPRL by Mr. Foster Stanback. The authors also acknowledge Elite Motion Systems, LLC. (Larry Vladic, President) for their donation of the 200-mm macro lens used in the visible imaging.