Home – Home – Jornals – Combustion, Explosion and Shock Waves 2022 number 6

The mechanisms of excitation of low-frequency unstable combustion of a methane-air mixture in full-scale low-emission combustion chambers are experimentally studied. Experimental investigations of the flow characteristics without combustion in low-emission combustion chambers shows that the central zone of reverse flows can serve as a source of regular hydrodynamic pressure oscillations in a wide range of flow regimes. A model of low-frequency unstable combustion is proposed. The model is based on hydrodynamic instability of the flow in the central zone of reverse flows, which can excite low-frequency regimes of unstable combustion. Methods for suppressing thermohydrodynamic instability of combustion are developed. Based on the proposed model and with the use of methods that ensure suppression of combustion instability, a low-emission combustion chamber with a stable process of combustion in the entire range of its operation conditions is created and tested, which confirms the feasibility of the proposed approach.

An increase in the rate of combustion (gasification) of solid propellants exposed to a sufficiently intense blowing gas flow was discovered experimentally in the 1940s and called erosive burning. Later it was found that for some propellants at relatively low blowing speeds, there may be a decrease in the burning rate, called negative erosion. Attempts to theoretically study negative erosion have been made since 1971 and focused on analyzing changes in the intensity of heat transfer on a burning surface with the leading role of gas-phase reactions, taking into account the effect of large-scale fluctuations in gas velocity and temperature, the effect of the differences in diffusion coefficients and thermal diffusivity of the gas, and the effect of flame stretching. This paper presents an analysis of propellant combustion under blowing conditions in the presence of intense subsurface heat release due to exothermic reactions in the condensed phase. In the case where the surface temperature is limited by the boiling point, local temperature maxima can form in the subsurface layer, leading to an uneven response on the surface and to the occurrence of “intrinsic” turbulence in the adjacent gas layer. This turbulence leads to a change in the heat input to the propellant and allows one to qualitatively explain the effect of negative erosion.

The propagation of a high-temperature synthesis wave through a perforated metal plate mounted inside a cylindrical sample of a powder mixture of Ni + Al was studied experimentally and theoretically. Copper and steel plates of different thickness were used. The passage of the exothermic reaction front through a hole in the barrier was investigated for different thermophysical characteristics of the plate and different geometric dimensions of the hole. Depending on the parameters of the plate, the minimum critical diameter of the hole required for the propagation of the combustion wave in the sample was determined as a function of plate parameters.

The influence of mechanical activation (MA) and Mn content on the rate and maximum combustion temperature, sample elongation during combustion, size of composite particles, mixture yield after MA, phase composition, and morphology of synthesis products in the Ni-Al-Mn system was studied. The mechanical activation of the Ni + Al + Mn mixture expanded the manganese content limit, at which it is possible to realize the combustion of samples without preheating, from 14 to 49 % (wt.). Preliminary MA made it possible to synthesize a compound containing all three initial metals - the (Ni, Mn)Al phase, which is an ordered solid solution of variable composition based on NiAl. In addition, after MA, the burning rate, elongation of product samples and their porosity increased, and the maximum combustion temperature decreased. An increase in the proportion of manganese in the Ni + Al + Mn mixture led to a decrease in the size of composite particles, elongation of product samples, the maximum combustion temperature, and an increase in the yield of the mixture after MA. The dependence of the combustion rate on the proportion of manganese in the activated mixture Ni + Al + Mn has a maximum.

Methods for the selection and analysis of condensed combustion products (CCPs) of large monolithic titanium particles with a diameter of 350 ÷ 460 μm in air at atmospheric pressure are described. Detailed data on the granulometric, morphological, and phase composition of CCPs and the number of particles produced by a single burning mother particle are presented. The following morphological types of CCP particles were identified: compact spheres (combustion residues of mother particles and their fragments) and airgel round and elongated comet-shaped objects (sparse fine particles consisting of chains of nanosized spherules). According to the ratio of O/Ti atoms, all types of CCP particles are oxide particles. The mass fraction of airgel objects in CCPs is 0.52 ÷ 0.98, and their physical density is about 0.8 g/cm³. The characteristic dimensions of compact spheres are 2 ÷ 410 μm, those of airgel round objects are 11 ÷ 470 μm, and the length of airgel comet-shaped objects can reach 13 mm. Typical sizes of spherules are 25 ÷ 100 nm. Large compact spheres 200 ÷ 400 μm in size typically have a gaseous bubble and a density of about 0.9 g/cm³.

Synthesis products in mechanically activated powder mixtures of titanium and nickel of three compositions corresponding to double intermetallic compounds have been studied. The mechanical activation of the mixtures was carried out in a planetary mill at an intensity of 40 g and a processing time of 20 min. The synthesis was carried out in the thermal explosion mode by heating mechanically activated mixtures in a sealed reactor in an argon atmosphere at an average heating rate of 70 ˚C/min. The phase composition of the powder products after synthesis and additional annealing was studied by X-ray diffraction analysis, and the results were discussed using literature data on the temperature dependences of the Gibbs energy of intermetallic compounds. It has been found that, regardless of the elemental composition of the mixtures, the intermetallic compound TiNi₃, which has the highest negative Gibbs energy, is predominantly formed during synthesis. Therefore, a single-phase target product was obtained only from a mixture of composition corresponding to TiNi₃. Thermal explosion products in mixtures of the other two compositions are multiphase. After annealing, the phase composition does not change qualitatively, and the quantitative changes in the phase content are insignificant.

The classical models of steady propagation of combustion and detonation waves in a combustible mixture describe the increase in the system entropy to a maximum value in the case of deflagration (subsonic) combustion of the mixture driven by slow processes of heat conduction and diffusion. In the detonation (supersonic) regime, however, where one of the leading roles belongs to the bow shock wave, the models predict that the combustible system after completion of the chemical reaction “chooses” the minimum increase in entropy. These predictions are inconsistent with the formulation of chemical thermodynamics that the entropy of the system reaches its maximum value after the spontaneous irreversible chemical reaction is finalized and the equilibrium state is established. It is shown in the present study that the predictions of the classical models on the minimum increase in entropy in the case of detonation are eliminated if detonation is considered as a process of combustion of a mixture preliminary subjected to an irreversible process of compression and heating of the initial mixture in the bow shock wave (chemical spike) with a corresponding increase in entropy of the initial mixture and subsequent energy release from the mixture in an irreversible process of mixture conversion to chemical reaction products.

On a pulsed gas detonation apparatus (PGDA) at initial atmospheric pressure, a study was made of the process of initiation and detonation in acetylene-oxygen mixtures C₂H₂ + k O₂, including those with a low content of oxygen near the upper concentration limit of detonation. The cell sizes, detonation velocities, and pressures in the detonation products were measured in the range k from zero to unity, and the composition of the detonation products was calculated. The upper limits of detonation in IGDA barrels with diameters of 14, 26, 46, and 104 mm and the volume of booster charges required to initiate detonation in the limiting modes are found. With regard to hydrogen energy, the technological chain methane ® acetylene ® hydrogen + nanosized detonation carbon is considered and an assessment is made of the characteristics of PGDA as a hydrogen generator.

The previously developed wide-range multiphase equation of state of Fe was used to calculate the shock pressure causing its evaporation during isentropic unloading up to 10^-4 GPa (1 atm). Calculations were made for three initial states of matter: pressure 1 atm and temperature 298 K (“cold” initial state), 1 GPa and 1 500K (“warm” state), and 40 GPa and 4 000 K (“hot” state). The shock pressure is 359, 261, and 132 GPa, respectively. These values are generally lower than the estimates of other authors. Arguments are given in their justification.

The aim of the article is to point out the dangers arising from the properties of plastic dust and what influence its properties have on the origin and course of the explosion. The present study deals with a sample of polyethylene dust, by-product of granulate production and storage. The explosion tests are performed on containers of a similar shape to those found in plants. The volumes of the closed vessels are 1.35 m³ (N1) and 5.45 m³ (N2). The tests are conducted in vessel N1 with a venting area DN 250 on the top of the vessel, in vessel N2 with a venting area DN 585 or DN 775 installed on the upper flanges of the vessels, and in vessels interconnected by pipes with a diameter DN 150 and length of 3, 6, and 10 m. The experiments show that the pressure of the explosion in technological equipment may reach higher values than those obtained in laboratory tests. During the explosion propagation in the connected vessels, the effect of overpressure appears due to precompression; as a result, the measured pressure and the rate of pressure rise are many times higher than the values measured only in the vessel itself with a venting area. As the explosion propagates from a larger volume vessel to a smaller volume vessel, the effect of precompression of the mixture increases the explosion parameters in the smaller vessel despite the opening areas of both vessels. Other elements can be also used to ensure sufficient explosion protection. The results also describe the effect of the length of the pipeline route by which the vessels are connected.

The explosive formability of common age-hardenable Al alloys (2024-T4, 2024-T6, 6061-T4, 6061-T6, 7075-T4, and 7075-T6) is investigated experimentally. Manufacturers utilize explosive forming to produce parts having large and complex geometries in a single operation. To investigate the explosive formability of the most common Al alloys used in the aerospace/aircraft industry, an experimental die design is constructed. Tensile tests are carried out to determine the mechanical behavior of the alloys at low strain rates. Charpy V-notch tests are used as input values to determine the fracture toughness and fracture energy. The strain rate considering the expansion angle is calculated through the analytical formula of the Gurney metal velocity. The strain rates obtained by explosive forming conducted in air are calculated roughly as 1.1·10⁵ s^-1, which is an extreme rate. The experimental results reveal that the 6061-T4 Al alloy could be explosively formed to the demanded geometry without any hollows, cracks, tearing, and fractured regions. It is clear that the T6 temper condition significantly improves the strength of the tested alloys, accompanied by reduction of ductility and crack initiation potential. Except for the 6061-T4 Al alloy, all of the tested alloys are fractured during explosive forming tests. Stereo microscopy and scanning electron microscopy investigations reveal transgranular fracture and cleavage facets resulting from brittle fracture initiated by high strain rates, which occurs in the peak-strength temper condition-T6 of the tested Al alloys.