Home – Home – Jornals – Combustion, Explosion and Shock Waves 2022 number 5

This paper presents the results of experimental panoramic measurements of the temperature field by a method based on the registration of laser-induced fluorescence of the hydroxyl radical (OH) upon excitation of two different transitions (two-line PLIF) in a laminar conical flame of a gas-droplet mixture of ethyl alcohol and air. Laminar flow of the uniformly mixed ethyl alcohol/air mixture with fine droplets is produced by an ultrasonic atomizer inside the liquid ethanol tank. The properties of the two-phase flow at the nozzle outlet without combustion was controlled by a time-of-flight optical sensor. The temperature field was estimated based on the excitation of the Q₁(5) and Q₁(14) lines of the (1-0) transition of the A²∑⁺ - X²π electronic system. The spatial unevenness of the energy distribution in the laser knife illuminating the central plane of the flame cone and the change in the pulse energy from frame to frame were compensated by the use of an additional camera recording the laser radiation intensity distribution in the calibration cuvette.

Structurally stressed hydrocarbons (SSHs) have improved energy performance compared to conventional liquid hydrocarbons. The ignition induction period of mixtures of SSH vapors with air has been studied as a function of temperature and excess air ratio. The results are compared with similar results for mixtures of vapor of n -alkanes with air. The ignition induction periods of mixtures of vapors of two SSH with air are determined. Data on the ignition delay of air-fuel mixtures are required to design engine combustors and to develop and evaluate the possibilities of using new fuels.

This paper presents the results of an experimental study of the characteristics of a swirling flow with the formation of vortex structures in a radial burner under isothermal and reacting conditions for various flow swirling parameters. For isothermal conditions, the distributions of average and fluctuating velocity fields, including those tied to the phase of a precessing vortex, are obtained, the pressure fluctuations induced by the precessing vortex are analyzed, and the contribution of the precessing vortex structure to the overall level of turbulence is determined. The studies are carried out using modern contactless experimental methods of flow diagnostics, such as optical imaging and particle image velocimetry (PIV). An acoustic field generated by the vortex core precession (VCP) is recorded using four measuring microphones with pressure taps. The contribution of the VCP to the overall level of turbulence is revealed using the method of proper orthogonal decomposition (POD), also applied to analyze the velocity distributions obtained by the PIV method. The experiments show that, in the isothermal case, the VCP that occurs after the twist parameter overcomes a value S = 0.6 is a single-helix vortex structure, whose contribution to the overall level of kinetic turbulence energy is up to 27 %. For reacting conditions, the flame is visualized at different flow swirls and the frequency characteristics of the VCP occurring in the flow at S ≥ 0.6 are measured. It is shown that the dependence of the dimensionless frequency of the VCP as a function of the flow swrl S has the same nonmonotonic character both in the case of combustion and in the isothermal case.

Results of studying kerosene combustion in constant-section channels for the Mach number at the channel entrance M = 1.7 are reported. The experiments are performed in channels with variations of the duct shape. The existence of flow regimes with wave structures that do not lead to the development of the pseudoshock mode of combustion is demonstrated. Critical conditions that have to be satisfied for initiation and realization of pre-detonation quasi-stationary combustion are determined.

Results of experimental investigations of the flow in a channel with sudden expansion without and with heat supply into a supersonic air flow are presented. Based on processing experimental data on the time evolution of static pressure on the channel walls, the spectral power of oscillations are determined. The analysis reveals an increase in the spectral power of pressure oscillations in the frequency range of 250 ÷ 400 Hz. The greatest increase in the spectral power is observed in the initial period of the process during ignition and at the end of flame stabilization. In the period corresponding to developed combustion, the peak value of the power spectrum of oscillations decreases, while the range of frequencies is extended to 400 ÷ 600 Hz.

Results of numerical simulations of turbulent reacting flows in a channel with sudden expansion with due allowance for injection of hydrogen jets into a supersonic (M = 4) air flow are reported. The simulations are performed in a three-dimensional unsteady formulation with the use of the ANSYS Fluent software under the test conditions of experiments performed in the IT-302M high-enthalpy wind tunnel. The computations predict a self-oscillatory regime with intense oscillations of pressure and integral heat release. The period-averaged pressure distribution is in reasonable agreement with the experimental measurements, and the frequency of pressure oscillations is within the range obtained in the experiments. Based on a detailed analysis of the flow characteristics within the full cycle of oscillations, the feedback mechanism responsible for the emergence of self-supported oscillations is refined.

Experimental data on the velocity of propagation of a combustion wave in a coal-methane-air mixture with respect to the walls of a closed channel for various concentrations of coal dust are presented. A physico-mathematical model of combustion of this mixture on the basis of equations of gas dynamics and mechanics of disperse media in the one-velocity one-temperature approximation is developed. In the proposed model, the velocity of propagation of the combustion wave with respect to the gas suspension and the burning rate of the coal dust particle are parameters of the model and are determined by providing the consistency between the computed and experimental results. A comparison of the calculated flame velocity with respect to the channel walls in a wide range of mass fractions of coal dust reveals reasonable agreement with the experiments. The proposed approach can be used for estimating the influence of coal dust combustion on the intensity of shock waves formed in coal mines in the case of accidental explosions of methane.

The influence of a relatively small additive of unsaturated gaseous hydrocarbon (propylene) on the detonation wave dynamics in a stoichiometric hydrogen-air mixture was studied by numerical simulation. The influence of the propylene concentration on the development of detonation in the mixture caused by direct initiation with an energy input from the outside in a small volume in a short time was studied at different initial temperatures of the mixture. A detailed mechanism of hydrogen combustion and key propylene hydrogenation reaction were used. Propylene is readily hydrogenated with the removal of atomic hydrogen from the reaction chain. This leads to an increase in the self-ignition delay of the mixture behind the leading shock wave, and at a sufficient concentration of the inhibitory additive, to the decomposition of the cellular detonation wave structure and detonation degeneration.

Interaction of a shock wave in a gas with a combustible gas bubble of an elevated density is numerically simulated based on the Euler equations. Three qualitatively different regimes of detonation initiation are described: direct initiation of detonation in the frontal part of the bubble at sufficiently high Mach numbers of the incident wave and detonation initiation in the rear part of the bubble due to wave refraction and focusing of secondary shock waves at lower Mach numbers. It is shown that the detonation initiation regime depends to a large extent both on the shock wave intensity and on the density of the mixture in the bubble. Based on a series of computations, a diagram of initiation regimes is composed. It is demonstrated that the effect of shock wave focusing ensures successful initiation of detonation with a much lower intensity of the incident wave as compared to direct initiation.

The flow in a combustion chamber in the form of an annular gap between plates with multi-headed rotating detonation has been studied numerically. It is assumed that a homogeneous propane-air mixture with given stagnation parameters enters the combustion chamber through elementary nozzles evenly filling the outer bounding ring. The gas-dynamic parameters of the mixture are determined as functions of the stagnation parameters and static pressure in the gap. The conditions for the formation of a given number of waves in the multi-headed detonation wave related to the dimensions of the combustion chamber and parameters of the initiators are obtained. The maximum number of waves for given dimensions of the combustion chamber are established. The existence of the maximum critical number of waves in multi-headed detonation is associated with blocking of the supply of the combustible mixture. Under the considered geometrical parameters of the flow region, one to eight rotating detonation waves are formed. It is found that in the case of an uneven arrangement of initiators, there is gradual alignment of the mutual angles between the waves making up the multi-headed detonation. Calculations were performed on the Lomonosov supercomputer at the Moscow State University using an original software package implementing a modified Godunov method and one-step reaction kinetics.

A new microwave technique for measuring the gasification rate in a non-stationary mode is described. A feature of the technique is that to determine the mass loss during gasification, the time-varying resonant frequency of the microwave resonator with the sample under study is measured by sequential registration of the resonant characteristics of the sensor. This provides independence of the measurement results from the change in the quality factor of the resonator during fuel sample gasification. A sensor prototype which is a coaxial resonator in which the sample under study is placed in the region of maximum electric field was tested. Experiments have shown that the sensitivity (the ratio of the change in resonant frequency to the change in the inner diameter of the sample) of the new sensor design is two to four times higher than that of the previous sensor model.

This paper presents results of an experimental study of the processes of thermal decomposition and ignition of high-energy materials (HEMs) containing an oxidizing agent, a combustible agent, and dispersed additives of aluminum, aluminum borides (AlB₂ and AlB₁₂), and amorphous boron. A Netzsch STA 449 F3 Jupiter thermal analyzer and an experimental testbench, which includes a continuous-wave CO₂ laser, are used to investigate the response and ignition characteristics of two basic HEM compositions based on AP/SKDM/Me and PCA/AN/MPVT/Me at different heating rates. It has been established that ammonium nitrate at low heat flux densities (q < 130 W/cm²) decomposes and melts, forming a liquid layer on the reaction surface and increasing the delay time of the emergence of a HEM flame containing Al, AlB₂, and AlB₁₂. With an increase in the heat flux density, the effect of the liquid layer on the reaction surface of the sample decreases due to the rising surface temperature, the outflow rate of gaseous decomposition products, and the layer evaporation.

A conjugated physical and mathematical model for the combustion of a mixed solid fuel with the addition of polydisperse boron powder is presented. The gas-dynamic processes in a two-phase, multi-velocity, and multi-temperature heat-conducting medium, as well as heat transfer and reaction processes in solid fuel are taken into account in this model above the solid fuel surface. Boundary conditions for the equality of heat and mass flows of fuel components are set on the fuel surface. From the numerical solution of the system of equations, the dependence of the burning rate of a mixed solid fuel containing boron particles on the pressure above the fuel surface is determined.

This paper presents the results of a study of the ignition of nine brands of coal from the Kuznetsk Basin by laser pulses (120 μs and 1064 nm). Depending on the laser pulse energy density, three types of processes can be distinguished in the general process of coal ignition, and they differ in threshold, temporal, and spectral-kinetic characteristics. The first type is associated with flame ignition on the surface of coal particles, and the second and third types are related to the initiation of various thermochemical reactions within particles. The laser pulse energy density defines the intensity and duration of the flame glow, as well as the flame column height.

The detonation characteristics of an aluminized EMX with a density of ≈0.5 g/cm³ and with a mass content of aluminum up to 20 % were determined. The addition of aluminum leads to an increase in detonation velocity and detonation pressure in comparison with the parameters of the pure EMX. Replacing aluminum with ground glass leads to a decrease in both detonation velocity and pressure. The increased detonation characteristics of the aluminized EMX can be due to the partial reaction of aluminum before the Chapman-Jouguet plane. The brightness temperature of detonation products of the aluminized EMX was measured. It is shown that it decreases with the addition of aluminum to the EMX. This result demonstrates the complex effect of aluminum addition on the detonation characteristics of the EMX.