Metallic Nano-particles as Fuel Additives in Air-breathing Combustion

Title: METALLIC NANO-PARTICLES AS FUEL ADDITIVES IN AIR-BREATHING COMBUSTION Gregory Young, Doctor of Philosophy, 2007 Directed By: Associate Professor, Dr. Kenneth H. Yu, Department of Aerospace Engineering For volume limited propulsion systems, the use of metals as fuel additives offers to increase performance by increasing the volumetric energy stored in the fuel. Highspeed airbreathing combustors generally have very short residence times which do not allow for complete combustion of traditional metal particles such as aluminum and boron, which are typically larger than 1 m. The development of nano-scale metals has opened up the possibility of utilizing metals in high-speed propulsion systems. This research effort was conducted in order to determine the viability of utilizing metallic nanoparticles as fuels or fuel supplements for high-speed airbreathing applications. The study was broken into two main parts; the first was a fundamental investigation into the combustion behavior of boron nanoparticles in a controlled setting, while the second was a demonstration of the performance potential of the nanoparticles in a realistic airbreathing combustor configuration. Initially, the combustion behavior of boron nanoparticles was studied in a controlled flame environment to extract basic combustion parameters, such as ignition criterion and burning time as functions of surrounding temperature and oxygen concentration. Ensemble average burning times of boron nanoparticles were obtained for the first time, while the range of ignition time data was extended into a lower temperature range. The burning time results were compared to both diffusion and kinetic limited theories of particle combustion. It was found that the size dependence on particle burning times did not follow either theory. A kinetic limited burning time correlation has been proposed based upon Arrhenius parameters extracted in this study. Finally, the fundamental combustion behavior of both boron and aluminum nanoparticles was studied extensively in an airbreathing combustor simulating ramjet conditions in an effort to determine the viability of using metallic nanoparticles as fuels or fuel supplements in high-speed airbreathing propulsion system. Experiments were conducted with hydrocarbon only (ethylene), mixtures of ethylene and boron, and mixtures of ethylene and aluminum. The oxidation of the metals was studied through the emission of BO2 and AlO at wavelengths of 546 nm and 488 nm respectively. Temperature measurements inside the combustor using thermocouples were made in order to determine boundaries in which the addition of boron or aluminum provided a positive thermal output for a variety of equivalence ratios ranging from 0.52 to 0.7, metal loadings ranging from 9.7 to 15.2% by weight, and combustor residence times ranging from 6 to 10.5 milliseconds (combustor inlet velocities of 40 – 70 m/s). Both BO2 emission data and temperature measurements indicated that a critical temperature exists for sustained combustion of the boron particles. Tests with measured peak temperatures below 1700 K indicated no benefit of boron addition, while experiments with measured peak temperatures above 1770 K showed a positive thermal contribution from boron addition. All tests using nanoaluminum displayed a noteable positive addition to the thermal output of the system. These results suggest that even when employing nanosized boron, only a small envelope for complete energetic extraction exists in combustors with short residence times. On the other hand, great hope exists for the use of nanoaluminum in high speed airbreathing combustors. METALLIC NANOPARTICLES AS FUEL ADDITIVES IN AIRBREATHING COMBUSTION