A. A. Borisov

Numerical simulation of momentum transfer from a shock wave to a bubbly medium

Based on the system of equations of two-phase compressible viscous flow, we performed a two-dimensional numerical simulation of momentum transfer by a shock wave propagating from a gas to a continuous aqueous medium or an aqueous medium with air bubbles. When a shock wave impinges on a continuous aqueous medium, the incompressible liquid is set in motion by gas overpressure after the reflection of the shock wave from the gas-liquid interface; however, when a shock wave impinges on a bubbly aqueous medium, the compressible liquid is set in motion due to the penetration of a shock wave into it. Parametric calculations have shown that momentum transfer from a shock wave to a bubbly fluid can be accompanied by dynamic effects, which ensure that the momentum transferred to the bubbly liquid for some time by far exceeds the momentum transferred to a continuous liquid, all other things being equal. These dynamic effects can be used to develop energy-efficient hydrojet propulsion units.

Cyclic deflagration-to-detonation transition in the flow-type combustion chamber of a pulse-detonation burner

The possibility of realization of a rapid cyclic deflagration-to-detonation transition (DDT) with a frequency of up to 2 Hz under conditions of high-velocity flow (∼10 m/s) and separate supply of the combustible mixture components (methane and air) in a tube, 5.5 m in length and 150 mm in diameter, with an open end at a low ignition energy (∼1 J) is for the first time demonstrated. It is shown that such a tube with turbulizing obstacles of special shape and placement can ensure reliable DDT at a distance of 3–4 m from the ignition source within ΔτDDT ≤ 20 ms after ignition. The results will be used in the development of a new type of industrial burner—a pulse-detonation burner for high-rate heating and fragmentation, combining thermal and shock-wave (mechanical) impacts on the treated object.

Gas-phase spontaneous ignition of hydrocarbons

The ignition of hydrocarbons at low temperatures is experimentally studied in a rapid-mixture-injection static reactor. The ignition process was monitored using a high-speed color video camera. It was found that, at low temperatures, ignition starts in kernels, a feature also characteristic of methods for measuring the ignition delay time at high and medium temperatures (shock tube, rapid compression machine). Kernel-mode ignition is associated with gas-dynamic phenomena inherent in different techniques of heating the gas to the desired temperature. Ignition in the kernel is of chain-thermal nature. The emergence of a visible kernel can be considered the beginning of hot flame propagation. It is shown that, in the self-ignition mode, the propagation of the flame front from the initial kernel occurs by the induction mechanism, proposed by Ya.B. Zel’dovich, rather than by the diffusion-heat-conduction mechanism. Introduction of a platinum wire into the reactor produces a catalytic effect in the negative temperature coefficient region, while virtually unaffecting the ignition delay at lower temperatures.

Burning velocity of methane-hydrogen mixtures at elevated pressures and temperatures

The effect of the initial pressure, temperature, and equivalence ratio on a number of combustion characteristics of methane-air mixtures with hydrogen additives in a closed vessel is experimentally studied. Experiments are conducted at 1, 5, and 10 atm and temperatures from 22 to 300°C. The hydrogen content in the fuel is 0, 10, and 20 vol %. The fuel equivalence ratio varies from 0.6 to 1.0. The limitations imposed by buoyancy on measurements of the laminar burning velocity by the constant-volume bomb method with recording of pressure-time histories are analyzed. It is shown that the laminar burning velocity can be appreciably increased by adding no less than 20 vol % of hydrogen to the fuel.

Influence of inert and active additives on the initiation and propagation of laminar spherical flames in stoichiometric mixtures of methane, pentane, and hydrogen with air

The propagation of a laminar spherical flame in stoichiometric mixtures of methane, and pentane with air in the presence of argon and carbon dioxide and in hydrogen-air-propylene mixtures at atmospheric pressure in a constant-volume bomb is investigated using high-speed color cinematography. It is shown that, under the experimental conditions employed (at T0 = 298 K and a spark energy of E0 = 0.91 J), dilution of the combustible mixtures with these additives can cause a more than 10-fold increase in the time of formation of a steady flame front, with the inhibiting effect of carbon dioxide being stronger than that of argon. Small additives of propylene, a chemically active inhibitor, are demonstrated to substantially increase the time it takes to form a steady flame front and reduce the flame propagation velocity.

Blast waves generated in an unconfined space by a nonideal detonation of high-density aluminum-enriched formulations

Experiments on the detonation of high-density (1.8 g/cm3) aluminum-ammonium perchlorate-paraffin-RDX formulations in an unconfined space demonstrated their high efficiency at pressure amplitudes within 0.3–7.0 atm. The relative pressure amplitude and impulse of the blast waves with respect to the analogous characteristics of TNT charges of the same mass were found to be 1.3–2.4. The TNT equivalents in pressure and impulse vary with the distance nonmonotonically, ranging within 1.4–2.8. The blast wave produced by an infield explosion of a 1.42-kg composite charge demonstrated its high performance characteristics. Measurements at blast wave amplitudes of 1 to 20 atm gave a TNT equivalent in pressure of up to 3 and a TNT equivalent in impulse of 1.3 to 1.8. The high parameters of blast waves in an unconfined space originate from both the high-energy characteristics of the systems themselves and the afterburning of excess metal fuel in air. To estimate the extent of participation of the reaction of excess metal fuel with air in supporting the blast wave, numerical simulations of the generation of blast waves for various rates of mixing of detonation products with air at the contact surface were conducted. The main elements of the mechanisms of the processes that determine the efficiency of explosive systems with a heat release spread in space and time were considered. It was concluded that an optimal regime of blast wave generation, capable of ensuring a prolonged attenuation of the wave with the distance, could be realized for low-velocity detonation.

Correlation between Drop Vaporization and Self-Ignition

A parametric analysis of numerical solutions to problems of vaporization and self-ignition of liquid hydrocarbon drops was performed, and a new criterion determining the conditions of drop self-ignition was suggested. According to this criterion, self-ignition at a given reduced distance from the drop begins when the required reduced gas temperature and equivalence ratio are reached. A new model of heating and vaporization of drops in dense gas suspensions was suggested. The model was verified in multidimensional calculations of self-ignition and combustion of drop clouds. Calculations showed that the model correctly described the phenomenology of local formation and anisotropic propagation of self-ignition waves in suspensions of drops in gases.

Low-temperature autoignition of binary mixtures of methane with C3–C5 alkanes

The influence of C3–C5 alkanes on the ignition of their binary mixtures with methane in air at a temperature of 523–1000 K and a pressure of 1 atm is studied. It is shown that the presence of only 1% C3–C5 alkanes considerably reduces the ignition delay of methane. At a concentration of 10–20%, the ignition delay practically corresponds to the autoignition delay of the added alkane. The effect of additives of heavy alkanes becomes less noticeable with increasing initial temperature. These results can be used to estimate the permissible content of C5+ heavy species in gas turbine engine fuel at which their influence on the fuel knock resistance is sufficiently low. It is only 0.5%.

Momentum transfer from a shock wave to a bubbly liquid

The transfer of momentum from shock waves of various intensities (from 0.05 to 0.5 MPa) to a water column containing air bubbles of a mean diameter of 2.5 mm is studied both experimentally and by numerical simulation. The experiments are performed in a vertical hydrodynamic shock tube with a rectangular cross section of 50 × 100 mm and a length of 1980 mm. The tube consists of a 495-mm-long high-pressure section, 495-mm-long low-pressure section, and 990-mm-long test section filled with water and equipped with a bubble generator. Experiments have demonstrated that, as the gas content in the water increases from 0 to 30 vol %, the momentum transferred from the shock wave to the bubbly water increases smoothly, leveling off at a volumetric gas content of 20–25%. The experimental and 2D-simulation dependences of the shock wave velocity and the velocity of the bubbly liquid behind the shock wave front on the volumetric gas content are in close agreement.

Explosive characteristics of tetrafluoroethylene

The minimum energies required to initiate the combustion of gaseous tetrafluoroerthy1ene (TFE) and mixtures thereof with nitrogen, argon, and helium at various initial pressures and temperatures are determined. Flame transition from the gas to the liquid phase of TFE is investigated. Liquid TFE is demonstrated to be virtually nondetonable. The fuel-lean flammability and detonability limits of TFE-air mixtures under normal conditions are demonstrated to be identical, 12.2 vol %.