Volume 55, Nº 3 (2019)

Flame propagation in a closed vessel containing a stoichiometric propane–air mixture and partially filled with a porous medium has been experimentally studied. It has been shown that the flammability limit in the porous medium is determined by the current pressure at the time the flame approaches it and by the transition processes occurring as the wave enters the porous medium. The dynamic limit phenomenon—the flammability limit in the porous medium associated with a continuous pressure drop—has been investigated. It has been found that the range of initial pressures in which the dynamic limit is observed is determined not only by the change in the number of moles in the gas phase, as during flame propagation in vessels completely filled with a porous medium, but also by gas cooling in the free space. The dynamic limit can occur in a significantly wider range of initial pressures than that due to a change in the number of moles. This range is mainly determined by heat exchange between the gas and the walls of the vessel in the region free from the porous medium.

Kerosene concentration distributions measured in the combustion chamber of a hotshot wind tunnel in the attached pipeline regime with the input Mach number of 2.89 are reported. Kerosene is injected through 12 jet injectors at an angle to the flow from the wall upstream of a cavity. The measurements are performed in three cross sections. Data on the kerosene concentration distribution in the initial region of the combustion chamber and its dependence on the ratio of the jet and main flow momenta are obtained. It is demonstrated that the absence of intense combustion in a model combustor under these conditions is caused by the fact that the local equivalence ratios are insufficient for ignition.

X-ray diffraction with the help of synchrotron radiation is used to analyze a sequence of phase formation in the oxidation of original and modified Al powders in the case of heating in oxidizing gaseous media with rates of 10 and 100 K/min. It is established that an increase in the heating rate of the modified ASD-4 powder leads to an active growth of metastable phase (θ- and δ′-Al₂O₃) of aluminum oxide except for the γ-Al₂O₃ phase. Assumptions about the forms of existence of vanadium in the interaction products are given. It is shown that diffusion limitations in the core—shell system (Al-Al₂O₃) can be removed under the action of an oxidizing aluminum particle on physicochemical processes at interphase boundaries.

A method of preliminary mechanical activation is used to implement the combustion of a Ti + Ni mixture, which does not burn at room temperature. The dependences of the burning rate, maximal temperature of combustion, and elongation of samples on the mechanical activation time of the Ti + Ni powder mixture are described for the first time. Moreover, the microstructure and phase composition of activated mixtures of their combustion products are studied. The mechanical activation time (9 min) during which the burning rate of the mixture and the content of the main phase (TiNi intermetallide) are maximal in the combustion products is experimentally determined. In these conditions, the combustion propagates within a narrow zone—the maximal temperature corresponds to that in the combustion front.

The standard enthalpies of formation of the compounds 1-(2,2-bis(methoxy-NNO-azoxy)ethyl)pyrazole, 1-(2,2-bis(methoxy-NNO-azoxy)ethyl)-3-nitropyrazole, and 1-(2,2-bis(methoxy-NNO-azoxy)ethyl)-4-nitropyrazole were measured experimentally to be 273.6 ± 6.7, 231.0 ± 3.3, and 213.8 ± 7.9 kJ/mol, respectively. These enthalpy values were used to determine the contribution of the replacement of the H atoms at the N atoms in the heterocycles by CH₂CH(N₂O₂Me)₂ groups (151.9 kJ/mol). Calculations have shown that 1-(2,2-bis(methoxy-NNO-azoxy)ethyl derivatives of pyrazole, 3- and 4-nitropyrazole, 3,4-dinitropyrazole, and 3,4,5-trinitropyrazole and the bis-derivative of bis-furazano[3,4-b;3′,4′-e]piperazine are inferior to HMX as gasifying components of solid composite propellants in metal-free compositions with an active binder. Only the derivative of 3,4-dinitropyrazole, which added in a small amount together with ammonium Perchlorate to aluminum-free propellant compositions provides a specific impulse of 249 s.

The combustion of 81/19 Al/B agglomerates with a diameter of 320–780 μm in free fall in air was first studied using model monodisperse agglomerates. The dependence of the burning time on size was determined. Burning residue particles have been studied by morphological, chemical, mass, particle size, and elemental (EDS) analyses. It has been found that the essential distinctive features of the combustion mechanism of Al/B agglomerates compared to aluminum are a long combustion duration; a specific core-shell structure of the particles, with boron present in the core and absent in the shell; slight changes in particle mass and diameter during combustion.

By considering the parametric variation of an individual boron particle in a boron agglomerate, the heat transfer, and the mass transfer between the boron particle agglomerate and the surroundings, an ignition and combustion model of a boron agglomerate is proposed. An experiment of a ramjet combustor using a boron-based slurry fuel is designed and operated for the purpose of validating the ramjet configuration and verifying the combustion of boron particles. Then a mathematical model for simulating a multiphase reacting flow within the combustor of a boron-based slurry fuel ramjet is established. Kerosene droplets and boron particles are injected discretely to the burner flowfield, and their trajectories are traced using the discrete phase model. The influence of the agglomerate size, bypass air mass flow rate, initial boron particle diameter, and boron particle content on the combustion efficiency of the slurry fuels is analyzed in detail. The results show that the combustion efficiency decreases with an increase in the agglomerate radius, initial boron particle diameter, and boron particle content. The combustion efficiency increases with an increase in the mass flow rate of bypass air. If the agglomerate diameter is greater than 100 μm or the bypass air mass flow rate is smaller than 50 g/s, the boron particles cannot be fully burned.