METALLIC NANOPARTICLES AS FUEL ADDITIVES IN AIRBREATHING COMBUSTION

Abstract

Fundamental and applied studies on combustion of nano-scale metal particles were conducted to explore the possibility of using metals as fuel additives in high-speed air-breathing propulsion. Recent development of nano-scale metals made it interesting to consider the metallic fuels in high-speed air-breathing applications with restrictive conditions, such as short flow residence time and limited storage volumes. In particular, SB99 boron nanoparticles and 50-nm ALEX nanoaluminum were selected for flame and combustor experiments because of their high volumetric energy contents as well as their availability. The investigations consisted of 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 fundamental combustion behavior of boron nanoparticles was studied in a controlled flame environment to understand the ignition criterion and characterize the complete burning times, which were characterized as functions of surrounding temperature, ranging between 1580K and 1870K, and oxygen concentration, ranging between 0.1 and 0.3. Ensemble-averaged burning times of boron nanoparticles were obtained for the first time, while the ignition delay measurements for boron nanoparticles were extended into a lower temperature range previously unavailable in the literature. The measured burning times were between 1.5 msec and 3.0 msec depending on both the temperature and oxygen mole fraction. On the other hand, the ignition times were relatively insensitive to oxygen concentration in the range studied, and were affected only by temperature. The measured ignition times were inversely related to the temperature, ranging from 1.5 msec at 1810K to 6.0 msec at 1580K. 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 is proposed based upon Arrhenius parameters extracted in this study. Finally, the characteristic combustion behavior of both boron and aluminum nanoparticles was studied parametrically in a confined combustor setting, simulating air-breathing ramjet conditions. Systematic experiments were conducted (i) with mixtures of ethylene and nanoaluminum, (ii) with mixtures of ethylene and boron nanoparticles, and (iii) with ethylene only to provide the baseline comparison. The oxidation of the metals was studied through the chemiluminescence emission of BO2 and AlO at wavelengths of 546 nm and 488 nm respectively. Temperature measurements inside the combustor using thermocouples were made over a variety of equivalence ratios ranging from 0.52 to 0.70 in order to determine boundaries in which the addition of boron nanoparticles or nanoaluminum provided a positive thermal output. The metal loadings in the experiments ranged from 9.7 to 15.2% of total fuel by weight, while the combustor residence times ranged from 6.0 to 10.5 milliseconds, corresponding to the combustor inlet velocity variation between 40 and 70 m/s. Both BO2 emission data and temperature measurements indicated that a critical temperature exists for sustained combustion of the boron nanoparticles. The boron addition in the experiments with measured peak temperatures below 1700 K yielded no benefit, while a positive thermal contribution was obtained in experiments with measured peak temperatures above 1770 K. All tests using nanoaluminum displayed a substantial increase in the thermal output of the system. These results suggest that, for using boron in combustors with short residence times, only a small envelope for complete energetic extraction exists even when the particles are nano-sized. On the other hand, the use of nanoaluminum in the combustor experiments consistently increased volumetric heat release over a wide range of operating conditions.