Home – Home – Jornals – Combustion, Explosion and Shock Waves 2023 number 5

This paper presents the results of an experimental study of flames propagating with a sign-variable speed, in methane-air and coal-methane-air mixtures in a vertically located closed pipe at coal dust concentrations of 0.10 ÷ 0.42 kg/m³. The results of the study can be useful for developing combustion models and assessing dynamic and thermal effects during combustion of methane-air suspended coal in coal mines.

Experimental studies of combustion stability to a local excess of a component in a dispersed mixture of titanium and carbon powder are presented. The influence of the spreading of a melt of a fusible or liquid reaction product on the stability of the combustion transition through a solid and perforated carbon powder barrier is considered. The direction of melt movement with respect to the direction of front propagation has been studied. It is shown that the stability of the combustion transition through the barrier is determined by the thermal interaction between the combustion wave and the barrier and is not directly related to the convective heat transfer by the melt.

The results of an experimental study of the possibility of burning separated and discharged to the Earth's surface are presented on the example of the design of elements of the head fairing of a launch vehicle. A new scheme of a honeycomb-free three-layer construction of combusted elements of the head fairing using a charge of high-energy material is proposed. Based on the formulated requirements for the characteristics of the charge and analysis of the results of thermodynamic calculations, the basic compositions of high-energy materials (HEMs) were selected. The energy and strength characteristics of the selected HEM compositions are experimentally determined, and the patterns of their combustion at subatmospheric pressure are determined. The results of laboratory tests of the combustion of elements of the structures under consideration with a VEM filler charge showed the possibility of partial utilization of spent parts of the launch vehicle.

The possibility of using the method of flameless combustion of RDX in ballasted systems to produce composite materials containing copper and zinc particles. The initial energetic material was RDX, the binder was hexamethylene diisocyanate, and precursors were copper hydroxocarbonate and copper oxalate. Highly porous composite materials containing nanosized particles of copper, zinc oxide or mixtures thereof were obtained by optimization of the flameless combustion conditions.

The synthesis process was realized in activated mixtures 5Ti + 3Si + xAl (x= 0 ÷ 40%) and in the initial mixture 5Ti + 3Si + 10%Al. The effect of mechanical activation and aluminum content on the rate and maximum combustion temperature, morphology, elongation, integrity, and phase composition of combustion products have been studied. Mechanical activation expanded the limit of Al content to 40%, at which it is possible to realize the combustion of samples without preheating. Based on the Ti-Si-Al system, intermetallic alloys - solid solutions based on titanium silicide Ti(Si_0.75Al_0.25)₂ and based on aluminide titanium Ti(Al_0.9Si_0.1)₃.

A new method for determining the parameters of the equation of state JWL of explosive products from experimental data is presented. The experimentally obtained values of pressure and mass velocity on the adiabat of expansion - deceleration of the explosion products are used as reference points. The values of the JWL equation of state parameters are determined using an iterative algorithm.

One of the shortcomings of the classical electromagnetic method of Zavoisky is sensitivity to the non-one-dimensionality of the flow behind the front of the investigated wave. In this paper, it is proposed to use a four-contact gauge to correct the measurements. Two signals are detected from frames, one of which is located in a plane tangent to the front, and the other in a plane parallel to the direction of wave propagation. Next, the true velocity signal insensitive to the curvature of the front is constructed from the two signals. The second difficulty that arises in electromagnetic measurements is the large size of the gauges. Typically, the length of the working arm L is about 1 cm. An analysis of the potential distribution in the gauge showed that the proposed combined gauge is equivalent to two frames of zero width, and the effective length L is the distance between the midlines of the supply conductors. It is shown that the value of L can be reduced to 1.5 ÷ 2 mm with a lead width of about 0.5 mm. This makes it possible to perform local measurements at spots of millimeter size and handle small-sized charges. These improvements bring electromagnetic measurements closer to the level of modern optical techniques, at a much more modest cost of hardware.

The light emission from samples representing a transparent matrix with inclusions of hot spots was studied. The matrix material was water and epoxy resin. Hot spots were generated by shock-wave compression of MS-B hollow glass microballoons. In the pressure range 0.7-29 GPa, the time of brightness decay was 280 to 70 ns. The brightness decay time increased by more than an order of magnitude when replacing the optical window made of solid epoxy resins by LiF. However, even this increase in the brightness decay time is much shorter than the estimates of the temperature relaxation of hot spots due to heat conduction mechanisms in the calculation with stationary parameters (t_a = 10^-2 s) and due to light emission (τ = 2.4 · 10^-3 s). It is concluded that the dominant mechanism of temperature relaxation is the turbulent mixing of the medium behind the shock-wave front. The experimental results show that in numerical simulations of the temperature field during the passage of a pore by a shock wave, it is necessary to take into account the viscosity and strength of the matrix substance.

Temperature distribution characteristics are important for evaluating the combustion status, safety monitoring, and disaster diagnosis of combustible gases. Traditional colorimetric thermometry is difficult to measure the temperature of combustible gases for the lack of the grey-body in the burning processes. In the present study, a visible burning facility for combustible gases is designed, and the temperature characteristics are measured using an improved colorimetric pyrometer with auxiliary solid powders as a grey-body. In order to improve the temperature measurement accuracy of the system, the type, particle size, and concentration of the powders as well as the ignition delay time are studied. After many debugging experiments, it is found that the best measurement results are obtained for the 30/70 H₂/air mixture with the tungsten powder with the mean particle size of 7.9 μm, particle concentration of 21 g/m³, and ignition delay time of 80 ms. The results are corroborated with the previous studies.

In this study, explosion venting of front, centrally, and rear ignited 9% methane-air mixtures has been conducted in a 1-m³ rectangular vessel with and without cylinders placed parallel to the venting direction. Three pressure peaks P₁, P₂, and P_ext caused by vent failure, flame-acoustic interaction, and external explosion, respectively, can be distinguished. The pressure peak P₁ appears in all the tests and is insensitive to the ignition position, but the existence of obstacles increases its value. The pressure peak P₂ only appears in the centrally and front ignited explosions without obstacles. The pressure peak P_ext can be observed in the rear ignition tests and is strengthened by the cylinders. The duration of the Helmholtz oscillations is longer in front ignition tests, whereas addition of cylinders had a minor effect on their frequency. This study also validates the ability of FLACS in predicting a vented methane-air explosion by comparing the simulated pressure--time histories and flame propagations with experimental results. FLACS can basically predict the shape of overpressure curves. If cylinders exist, the simulation results ensure better agreement with the experimental data because FLACS cannot simulate the flame-acoustic-interaction-induced pressure peak P₂. The performance of FLACS is satisfactory in rear ignition tests because it calculates P_ext and obstacles' effect on P_ext exactly. The flame behavior simulated by FLACS is similar to that in experiments, but the effect of the Taylor instability on the flame is not sufficiently considered.

A unified approach to the calculation of equilibrium states of combustion products of hydrocarbons with a lack of oxygen is used to numerically construct a self-similar solution that allows simulating the structure of the detonation wave in a fuel-rich acetylene-oxygen mixture. The influence of the presence of condensed carbon particles in detonation products on this structure is analyzed.

Regimes of continuous multifront detonation of two-phase mixtures of aviation kerosene and hot air are obtained for the first time and studied in a flow-type annular combustor 503 mm in diameter and 600 mm long. Air with a flow rate of 7.8 ÷ 24 kg/s is preheated up to 600 ÷ 1200 K by a firing method in the settling chamber by means of burning a stoichiometric H₂-O₂ mixture. Liquid kerosene is bubbled with air in the fuel injection system. The equivalence ratio of the fuel is 0.66 ÷ 1.28. The influence of the air temperature on the region of continuous detonation, pressure in the combustor, and specific impulse is studied. Experiments with the air temperature in the interval 600 ÷ 1200 K reveal regimes of continuous multifront detonation with one pair (frequency 1.2±0.1 kHz) or two pairs (frequency 2.4±0.2 kHz) colliding transverse detonation waves. Based on the stagnation pressure measured at the combustor exit, the thrust force and specific impulse are determined. It is shown that an increase in the air temperature assists in detonation burning of the two-phase kerosene-air mixture, but the degree of dissociation of combustion products increases, while the specific impulse of the thrust force decreases. The specific impulse increases if the amount of the fuel in the mixture is sufficiently small, and its maximum value with allowance for the energy of compressed air in receivers is approximately 2200 for the air temperature in the settling chamber equal to 600 K.

The purpose of the present study is to investigate the detonation in air containing an n-heptane droplet cloud and the effect of the droplet size. A finite volume solver is developed to simulate the two-phase reacting compressible flow using a single-step reaction mechanism. The focus is on the impact of the droplet size on the detonation wave pressure and velocity. For the physical situation considered, the upper limit of the droplet size is determined to ensure self-sustained detonation, and it is shown that medium-size droplets initiate a stronger detonation wave than the gas fuel detonation or than large-size droplets. The distribution of the flow properties behind the wave is analyzed to demonstrate the observed behavior of the droplet size.

The explosion process of the flake aluminum powder-air two-phase flow is experimentally studied in a large-scale long straight horizontal tube with a length of 32.4 m and an inner diameter of 0.199 m. The deflagration-to-detonation transition (DDT) of the aluminum powder-air mixture is analyzed after being ignited by a 40-J electric spark, and the DDT of the mixture at different mass concentrations is compared. The results show that self-sustained detonation can be achieved in the range of 286 ÷ 532 g/m³ of the flake aluminum powder concentration, and the DDT process of the aluminum powder-air mixture at the concentration of aluminum particles 409 g/m³ (optimal concentration) is analyzed in detail. The detonation velocity and detonation pressure at the optimal concentration are 1690 m/s and 58 bar, respectively. During the self-sustained detonation stage, the detonation overpressure of the multiphase fuel-air mixture exhibits a typical constant oscillation characteristic, while the detonation velocity remains stable. In addition, a double-headed mode helical detonation phenomenon is observed in the detonation wave front of the aluminum powder-air mixture. The structure of the detonation wave, the flow field parameters, and the interaction between the shock wave and the three-wave point trajectory are analyzed. The detonation cell size at the optimal concentration is approximately 486 mm.

Experimental and numerical studies of premixed methane-air flame dynamics in an obstructed chamber are carried out. In the experiment, high-speed video photography and pressure transducer measurements are used to study the combustion dynamics. In the numerical simulation, three subgrid-scale viscosity models and three subgrid-scale combustion models are selected to evaluate their individual predictions compared to the experimental data. The high-speed photographs show that the flame propagation process can be divided into four typical stages. When the flame front passes through the obstacle, two distinct vortex structures are formed. The volute flame is the result of the flame-vortex interaction. In addition, the combustion regime experiences a transition from “wrinkled flamelets” to “corrugated flamelets” and finally arrives at a “thin reaction zone regime.”