Combustion studies of metallized fuels for solid-fuel ramjets

Abstract

Combustion phenomena of highly metallized, boron containing, solid fuels in solid-fuel ramjets (SFRJ) were studied by means of high-speed photography using a windowed two-dimensional SFRJ combustor. The ex- periments indicated the existence of a gas-phase diffusion of the volatile fuel ingredients within the bound- ary layer above the fuel surface. It was also revealed that material is often emitted from the surface in the form of large pieces arid segments. Flow impingement on the surface may cause surface heating and glowing by chemical reactions, which promote the high-speed ejection of hot particles and the emittance and disintegration of large glowing segments and pieces from the fuel surface layer into the gas stream. HERMOCHEMICAL evaluation of fuel candidates for advanced solid-fuel ramjet (SFRJ) systems reveals the high energy and energy density of some metals, especially boron and boron compounds.1 Compared with the commonly used hydrocarbon (HC) fuels, boron exhibits remarkable theoretical heat of combustion per unit mass (about 30% higher than HCs) and per unit volume (almost three times that of HC). As an ingredient of solid fuels for ramjets, metals, in- cluding elemental boron or boron compounds (e.g., boron carbide), are usually introduced as fine powders into a matrix of polymeric material. In spite of the promising potential, the practical use of solid-fuel formulations highly loaded with metal particles can present severe problems associated with complex burning phenomena, which have major effects on the energy genera- tion process within the combustor and may lead to poor combustion efficiencies and low motor performance. Experimental investigations and modeling of the combus- tion of nonmetallized fuels in SFRJ combustors have pro- vided a good description of the main combustion and flow characteristics.2'3 See Fig. la. The foremost region of the combustor is characterized by a separated recirculation flow zone generated by the inlet step used for stabilization. Downstream of the reattachmeht zone, often along most of the fuel grain length, a gas-phase diffusion flame, typically a narrow flame sheet, is established within the turbulent boundary layer that develops over the condensed fuel sur- face.4 Fuel vapors diffuse from the decomposing fuel beneath the flame, while oxygen is transported to the from the core stream along the combustor centerline. Heat feedback from the to the condensed fuel determines the fuel regression rate and completes the combustion cycle. The situations encountered when using highly metallized fuels are somewhat different (Fig. Ib). Although the flow characteristics are similar to those existing in the combustor employing nonmetallized fuels, the combustion phenomena are not the same. Metal particles tend to accumulate and coalesce at the condensed fuel surface prior to their ejection into the gas stream. The result may be the formation of