Combustion Characteristics of Suspended Hydrocarbon Fuel Droplets with Various Nanoenergetic Additives

ABSTRACT

The burning process of multiple liquid fuels with various nanoenergetic additives is investigated experimentally in the suspended droplet configuration. Specifically, the burning rate constant $K$ and ignition delay ${\tau }_{\mathrm{i}\mathrm{g}\mathrm{n}}$ are measured for two base fuels, rocket grade kerosene (RP-2) and ethanol, with seven energetic additives; the investigated additives include aluminum (Al), magnesium oxide (MgO), graphene, graphene nano-platelet (GNP), as well as numerous soluble additives including ammonia borane (AB), ammonium nitrate (NaAN) and sodium borohydride ($\mathrm{N}\mathrm{a}\mathrm{B}{\mathrm{H}}_4$), all of which do not exceed 6 wt.% loading concentration. Results from these experiments show that in general, there is not a significant modification to the steady burning rate for these nanofuel combinations (averaging $<$10%) for the loading concentrations considered. Instead, the additives exhibit a majority of particle burning during the end of the droplet lifetime, which generally are short-lived events that do not affect the overall $K$. This is illustrated by calculating the instantaneous burning rate $\tilde{K}$ for these nanofuels, where $\tilde{K}$ is 2% to 18% higher than pure fuels during the last half of the droplet burn time depending on the additive type. Similarly, nanofuel ignition delays generally only exhibit modest changes for a majority of the investigated combinations, causing only a $\pm$5% change on average. However, two nanofuels consisting of ethanol with 5 wt.% soluble ammonia borane and RP-2 with 1 wt.$\mathrm{\%}$ suspended graphene do exhibit increases in ${\tau }_{\mathrm{i}\mathrm{g}\mathrm{n}}$ of approximately 20 and 30$\mathrm{\%}$, respectively. These nanofuel compounds also exhibit substantial qualitative combustion behavioral changes including flame color changes and micro-droplet shedding events for ethanol with ammonia borane, as well as a notable reduction in the average burning rate of 20$\mathrm{\%}$ for RP-2 with graphene. Therefore, these two nanofuel combinations may be candidates for further study as a means to garner control of combustion dynamics in a reacting liquid rocket spray environment.

KEYWORDS:

• Nanofuel droplets
• droplet combustion
• energetic additives
• burning rate
• ignition delay

Acknowledgments

The authors would like to acknowledge Dr. Hyung Sub Sim for performing the TEM of the various nanoparticles, as well as Professor Ann Karagozian and her research group for their insight into certain aspects of this work regarding burning nanofuel droplets.

Additional information

Funding

This work has been supported by the Air Force Office of Scientific Research (AFOSR) with Dr. Mitat Birkan as program manager.