Home – Home – Jornals – Combustion, Explosion and Shock Waves 2022 number 3

Results of numerical simulation of two-dimensional flows of cellular detonation in plane channels in gas suspensions of submicron aluminum particles (0.6 and 0.3   m) in oxygen are presented. Mixtures with homogeneous and heterogeneous concentrations are considered. A previously developed model of reduced kinetics verified on the basis of the detonation velocity dependence on the particle concentration and extended to heterogeneous mixtures is used. The size and character of detonation cells are found as functions of the particle size and concentration. Problems of detonation propagation in channels with transverse concentration gradients and intermittent distributions of concentration are considered.

Results of numerical simulations of mixing, ignition, and combustion of a cold supersonic (М _jet = 1.46) hydrogen jet injected coaxially into an annular supersonic (M _air = 1.86) jet of hot vitiated air expanding into a still space are reported. The simulations are performed within the framework of the ANSYS Fluent 2020 R1 software in a transient two-dimensional axisymmetric approach based on the Reynolds-averaged Navier-Stokes equations supplemented with the k-w SST turbulence model and a detailed mechanism of hydrogen combustion in air. The geometry and simulation parameters are chosen to be those of the experiment of Cohen and Guile (1969), whose data were used for verification of the numerical algorithm. The structure of the reacting jet is studied, and the hydrogen combustion efficiency is evaluated for various values of the jet pressure ratio. The instantaneous, mean, and RMS components of the main gas-dynamic quantities and species mass fractions in the reacting mixture are obtained.

This paper presents the results of a study carried out using numerical simulation of flame front dynamics in a gaseous reacting mixture, including those in the presence of a suspended phase of liquid microdroplets. It is shown that the local effect on the flame front is one of the leading factors determining the development of combustion. Thus, the local dynamic effect of relatively large droplets on the flame front contributes to its curvature, which, in turn, determines the corresponding local acceleration of individual sections of the front. Further unstable growth of such perturbations leads to an integral acceleration of the flame. At the same time, local stretching by the flow in depleted compositions can lead to combustion extinction.

A numerical model for heterogeneous combustion of axisymmetric porous objects has been proposed that allows one to simulate processes under both forced filtration natural convection. The influence of the location of the ignition zone on combustion in a cylindrical porous reactor has been investigated. It has been shown that under forced filtering, the process similar to plane case: the combustion wave moves upward and sideways from the ignition source, completely burning out the solid fuel, while the gas tends to bypass hot zones and flow through colder regions. Under natural convection conditions, as in the plane case, the oxidizer flow into the reaction zone is significantly affected by vortex gas flows that arise in the vicinity of the combustion center at the initial time. In this case, the direction propagation of combustion waves in the axisymmetric case can significantly differ from thatin the plane case.

A physical and mathematical model of spark ignition and combustion of boron powder suspension in a propane-air mixture is presented. Dependences of the critical energy of spark ignition on the radius and mass concentration of particles and propane content in the boron gas suspension are obtained. Dependences between the steady flame propagation velocity in a boron powder suspension in a propane-air mixture on particle radius and particle mass concentration are obtained, and the propane content in a boron gas suspension is determined. Quantitative correspondence of the computational and theoretical values of the flame propagation velocity in a boron powder suspension in a propane-air mixture with known experimental data has been obtained.

An improved mathematical model of gasification is proposed solid porous fuel when hot gases are filtered through it. On the example of polymethyl methacrylate, the gasification regimes were studied both at constant pressure drop at the inlet and outlet of the gasifier, and at constant velocity of the gas at its inlet. In the event of a constant drop pressure gasification of combustible material takes longer and the gas temperature at the outlet increases more slowly than in the case of a constant gas velocity at input under comparable conditions.

Qualitative transformation of a low-velocity laminar flow to a turbulent state (owing to natural or artificial instability) and formation of compression waves passing ahead have been studied in much detail. A disputable issue is the nature of the emergence of a reaction site in the region between the bow compression wave and the flame front moving at a certain distance behind this wave, as well as the dynamics of interaction of this site with the main structural elements. It is the type of this site (slow or explosive combustion) that defines its subsequent interaction with the compression wave front: shockless or shock-induced expansion capable of forming a detonation wave. As a method of transforming the reaction site to an explosion site, its amplification owing to the resonance of streamwise acoustic oscillations of hot reaction products with the initial combustible mixture induced by flame propagation is discussed. It is the resonance with its multiple enhancement of the amplitude of gas-dynamic parameters that can effectively initiate the deflagration-to-detonation transition. Various stages of this transition are discussed; the corresponding estimates are made and are found to be consistent with experiments.

Interaction of homogeneous and heterogeneous detonation waves in mixtures of aluminum in oxygen and hydrogen in oxygen with a cloud of water droplets is studied by methods of mechanics of multiphase media. The main interaction mechanisms are determined: propagation of an attenuated detonation wave with a velocity smaller than the Chapman-Jouguet velocity and detonation failure. Critical conditions of detonation propagation in water sheets are determined. These critical conditions are compared with the results of modeling detonation suppression with the use of clouds of inert particles.

This paper presents the results of mathematical modeling of the propagation of air blast waves during methane explosion in mine workings taking into account their interaction with prefabricated parachute stoppings. Parachute stoppings are able to reduce the shock-wave intensity when the intensity of the incoming shock wave does not exceed the critical failure pressure of the stopping. The gas-dynamic method of calculating explosion-proof distances allows one to take into account parachute stoppings installed in various places of workings and to calculate the parameters of shock waves that have passed beyond the stopping.

The present paper describes the effects of hydrogen addition and carbon dioxide dilution in the natural gas on the velocity profiles and on the turbulent quantities (integral scale and Kolmogorov scale) in a cylindrical burner. The hydrogen content in the fuel is varied from 0 to 20 % in volume, and the volume of carbon dioxide is varied between 0 and 50 %. The velocity fields and the root mean square value of velocity are determined by the particle image velocimetry technique in the reacting flow. The concentrations of CO and NO _x are found using the corresponding analyzers. The turbulent quantities are determined by a numerical method. The results show that the absence of hydrogen and the carbon dioxide content greater than 20 % lead to flame blow-out. Therefore, the flame is hooked to the burner if hydrogen is added. In this study, with hydrogen addition, the difference in the maximum velocity ( U _max/ U ₀) along the bio-hythane jet is less important far from the burner due to the low density and high molecular diffusivity of hydrogen. The studies of the root mean square values of two velocity components ( U'_x and U'_z ) indicate that turbulence is more important for the U'_z component.

Even a slight change in the content of impurity gases during a self-propagating high-temperature synthesis can lead to a change in the combustion regime and the characteristics of the target products. In this work, the dependence of the burning rate of Ti + C granular mixtures on a titanium particle size is determined for the first time, and the effect of impurity gas evolution when using various allotropic modifications of carbon (graphite/soot) is studied. Experimental results are analyzed using the convective-conductive combustion model, which explains the strong influence of impurity gas release on the front velocity. Interaction rate of the components becomes a key factor for granular mixtures in which the influence of impurity gases is leveled. Experiments show that the burning rates of granular mixtures of titanium with soot are noticeably higher than the burning rates of a mixture of titanium with graphite. The curves approximating the dependence of the burning rate of a granular mixture of titanium and graphite on the size of titanium particles correspond to the linear law of interaction of the initial components. The interaction in a mixture of titanium and soot occurs according to the parabolic law.

This paper presents a combustion model for a solid fuel consisting of a matrix capable of self-sustained combustion and particles of a polydisperse coolant distributed in it. Heat transfer between the exothermically decomposing matrix and coolant particles in the condensed phase and the gas phase products of their gasification. The leading reaction in the region of the matrix and the evaporation surface of the coolant are assumed to be located on the interface. The heat consumption for the evaporation of the coolant is determined by the depth of its gasification during passage through the interface, which depends on the dispersion of the coolant and the burning rate of the fuel. Parametric identification of the model was carried out using data of sieve analysis of the granulometric composition of the coolant and an experimental dependence of the burning rate on pressure. It is shown that the model can be used to predict and providing the required ballistic characteristics of the fuel at the stages of its testing and series production.

Low-temperature (1 023 K) vacuum reduction of a mixture of zirconium dioxide with calcium is used to obtain a powder with an active zirconium content of 98.4 % (wt.). Physicochemical properties that determine the thermal and ignition characteristics of the powder and pyrotechnic compositions based on it are studied.

The explosive properties of cyclodextrin nitrates with various degrees of substitution of nitrate groups for hydroxyl groups in cyclodextrins. It is shown that charges of b-cyclodextrin nitrate with 100 % substitution at a density of 1.576 g/cm³ detonate with a relative explosion momentum equal to 96.4 % of its value for the 50/50 TNT/RDX composition with a density of 1.66 g/cm³, whose momentum is taken as 100 %. In this case, the detonation velocity is 7.15 km/s. It is concluded that the substance belongs to powerful blasting explosives. The sensitivity of cyclodextrin nitrates to mechanical influences was studied as a function of the degree of substitution. The obtained values of the explosive properties and sensitivity of cyclodextrin nitrates to impact and friction are compared with the properties of nitrocellulose.

The shock-wave initiation of an emulsion explosive was studied by flash radiography. Initiation was performed by impact of a thin Duralumin plate at a small angle to a flat surface of the explosive. The parameters of the initiating shock wave in the investigated explosive were estimated, and the depth of detonation initiation was measured.

Detonation overcompression during detonation convergence in a hemispherical charge of plasticized triaminobtrinitrobenzene with outer and inner radii of 75 and 20 mm after its initiation along the outer surface is studied. The experiment is numerically simulated with account for the transformation kinetics of an explosive into explosion products. The overcompressed detonation parameters in the explosive under study at a diagnosable charge radius of 20 mm are obtained via experiments and calculations: in the profile maximum, the pressure is 70 GPa, the front velocity is 9.45 km/s, and the mass velocity behind the front is 3.88 km/s. The overcompression achieved in the experiment under consideration is 2.3. The adiabatintersection point of the “nonreacting” explosive and its explosion products is revealed, which is implemented at a radius of 31 mm and a pressure of 52 GPa. The corresponding front velocity and the mass velocity behind the front at this point are 8.55 and 3.18 km/s. The resulting parameters at the adiabat intersection point are compared with the available literature data for triaminotrinitrobenzene and compositions based on it. A fairly large scatter of data is revealed. Suggestions are made about the causes of the scatter.