Powdered metals as fuel for hypersonic ramjets

Abstract

The concept of a hypersonic ramjet fueled by powdered metals (B, Al, Mg, and MgB2) is considered. Thermodynamic calculations of the combustion heat release, specific impulse, and volumetric specific impulse show that metal fuels can exceed hydrocarbon fuels in volumetric energy content and, in the case of boron, exceed conventional fuels on a mass basis as well. The refractory nature of metal fuels and their combustion products also suggest they may permit ramjets utilizing subsonic combustion to extend their operation to hypersonic Mach numbers (greater than Mach 5). The feasibility of stabilizing combustion using a pure powdered-metal fuel without the use of hydrocarbons is investigated experimentally. The ability to effectively inject powdered metal into a combustor is demonstrated using a laboratory-sca le dispersion apparatus. This apparatus is then used to measure fundamental burning characteristics of aluminum powder suspensions in air. The burning rate of micron-size aluminum air suspensions is seen to be similar to gaseous hydrocarbons in air, but the dependence of burning rate on fuel equivalence ratio is very different from gas flames. A nearly constant plateau in burning velocity obtained with fiiel-rich mixtures suggests that the combustor must operate rich for stable operation. Thermodynamic calculations show that maximum specific impulse is obtained for lean metal-air mixtures. Thus, a conceptual design of the dust combustor is proposed that uses a preburner to stabilize a fiiel-rich metal flame followed by gradual mixing of the burning suspension with a secondary airflow to obtain a high air/fuel excess ratio.