Home – Home – Jornals – Combustion, Explosion and Shock Waves 2021 number 5

Numerical simulation has shown that replacing a part of methane by carbon monoxide in a rich mixture while maintaining the equivalence ratio leads to a decrease in the superadiabatic temperature effect due to the competition of chemical reactions. This has been explained by a decrease in the H content in the combustible mixture and a decrease in the superequilibrium concentration of water in combustion products. Using the tracer numerical simulation method and a comparative analysis of the rates of consumption of CH₄ and CO, it has been established that the consumption of both fuels in the CH₄/CO/air flame is a competitive reaction path and CO monoxide does not act as an inert component in the low-temperature region of the front (contrary to the statements of a number of authors). Furthermore, the rate of consumption of CH₄ is much higher than that of CO due to the larger number of paths of consumption of CH₄ and their higher rates. The main contribution to the increase in the concentration of H atoms in the flame when replacing a methane-air mixture with a CH₄/CO/air mixture is due to the same reactions that increase the heat release rate: O + CH₃ = H + CH₂O and CO + OH = CO₂ + H. The results obtained and their comparison with literature data lead to the conclusion that the increase in the heat release rate and, hence, flame propagation rate should be greater in rich mixtures, since there the thermophysical effect is higher.

In large-scale turbulent flame simulations, the exploitation of detailed chemistry and transport models often necessitates expensive memory and CPU requirements. To maintain the practicality and flexibility of such simulations, the combustion chemistry is commonly represented by reduced reaction mechanisms. The present paper describes the development of such a reduced short kinetic scheme for high-temperature oxidation of n -heptane suitable for application in complex turbulent flame simulations. Through a combination of the directed relation graph and quasi-steady state approximation methodologies, a skeletal 65-species kinetic model is formally reduced down to a 25-species derivative suitable for atmospheric lean to stoichiometric conditions. Further removal of appropriate reactions and species is facilitated by using the reaction path flux analysis, yielding a short chemical scheme of 25 species and 69 reactions. Particular attention is given to avoid addition of lumped reactions (for all isomer compounds) and artificial kinetic rates expressed as nonlinear algebraic combinations of excluded elementary steps. In addition, most of the original radical reaction pathways are duly preserved, and an adequate number of intermediate lighter-chain hydrocarbon species is represented in the reduced scheme to ensure a proper breakdown and oxidation of the main hydrocarbon. A series of 0D and 1D propagating and counterflowing premixed flames and axisymmetric coflowing laminar jet flames are computed throughout an iterative validation procedure. Complementary computations with the 65-species base scheme, as well as available experimental data are exploited for the assessment of the optimization effort. The comparisons demonstrate that the derived scheme ensures satisfactory agreement with these data over the investigated parameter space.

A stationary mathematical model is used to estimate the characteristics of gasification of pulverized fuel in a filtration combustion reactor with a heat carrier counterflow are estimated. The high efficiency of the gasification method under study is established. The movement of the heat carrier toward the flow of gaseous products makes it possible to recuperate a significant part of the thermal energy released, which has a significant effect on the temperature and composition of combustion products. It is shown that the gasification characteristics can be purposefully changed by varying three main parameters: air flow rate, steam flow rate, and heat carrier flow. As an example, the dependences of the main gasification characteristics (combustion temperature, product composition, and process efficiency) on control parameters are calculated. Calculations make it possible to estimate possible modes and select an optimal set of control parameters for gasification.

The effect of polymer binder on the thermal behavior, combustion, and composition of condensed gasification products of model boron-containing compositions was studied to determine the optimal fuel composition for a ducted rocket gas generator. The physicochemical transformations of boron particles during the passage of a combustion wave in a solid propellant composition and the interaction with the decomposition products of the oxidizer and binder were investigated. The binder components were oligodienurethane and polyetherurethane rubbers, plasticized with various substances: dimethyl ester of phthalic acid, transformer oil, and dioctyl sebacate. Samples of a consistently complicated composition were studied while maintaining the ratio between the rubber and plasticizer. After thermal analysis of plasticizers and rubbers, plasticized binders and compositions of these binders with boron were investigated and then the combustion parameters of model compositions obtained by adding a third component, ammonium perchlorate, were determined. It was found that the thermal destruction of the more heat-resistant binder and the melt formation of boron oxide largely overlapped and proceeded simultaneously. Features of the combustion of compositions with various binders were identified by high-speed video recording. The composition and microstructure of condensed gasification products, both taken from the gas phase directly from the burning surface and those remaining after combustion in the form of a porous skeleton, were studied in detail by electron microscopy and thermogravimetry. Based on a comparative analysis of experimental data, it is concluded that it is preferable to use plasticizers and rubbers with reduced thermal stability in inert binders for compositions with amorphous boron designed for ducted rocket gas generators. The possibility of the formation of boron suboxide (B₆O) crystals during combustion of boron-containing compositions was shown for the first time.

This paper describes the mathematical simulation of an electric thermal explosion (ETE) in a gasless system placed in an annular layer of a conductive product. The influence of changes in the thermoelectric characteristics of a sample for the implementation of an ETE, thermal conditions, and ETE heat patterns is considered. Based on the results obtained, the previously established experimental result on the anomalous effect of the power of electric heating on the ETE parameters of a mixture of titanium and carbon black powders is explained.

The influence of the ratio of the initial components on the combustion parameters and modes of mixtures of aluminum with copper oxide has been studied. It has been shown that under normal conditions, such mixtures can burn steadily when they contain not less than 30 % copper oxide. Moreover, as the content of copper oxide increases to the stoichiometric ratio, the combustion mode changes in the following sequence: self-oscillatory combustion, spin combustion, a combination of convective and multi-hotspot combustion, flame, and fireball. In addition, the effect of the ratio of the components on the combustion of ternary mixtures of copper oxide-aluminum-titanium was studied, and the concentration regions were determined for four main modes of their combustion: self-oscillatory, hotspot, flame, and fireball. It has been shown that the hotspot combustion mode of such mixtures can exist in five different forms: spin, multi-hotspot, a combination of convective and multi-hotspot modes, a multi-hotspot mode with the formation of an opposed front, and a multi-hotspot mode with a periodic separation of combustion products. Depending on the ratio of the initial components, the condensed combustion products of mixtures of copper oxide with aluminum and titanium were found to contain copper, intermetallic compounds Al₃Ti, Ti₃Al, and CuAl₅Ti₂, and oxides Al₂O₃, TiO₂, TiO, Cu₂O, Al₂TiO₅, and Cu₃Ti₃O.

A model of thermodynamics of a reacting mixture of rarefied gases and a suspension of condensed species is developed by using statistical physics methods. An NVT ensemble is considered for determining the detailed equilibrium chemical composition, and the minimum of the free energy of the mixture of possible species is found numerically. Tabular data for the species are used for determining the enthalpy and free energy of chemical compounds. An algorithm that allows the Chapman-Jouguet detonation parameters to be determined for a wide range of combustible mixtures is developed. The model is tested through comparisons of the predicted and experimental detonation velocities. Good agreement for mixtures with oxygen excess is demonstrated. For compositions with the formation of a significant amount of condensed carabon, the predicted and experimental detonation velocities agree reasonably well.

This paper describes an experimental study and a quasi-one-dimensional calculation of acceleration of dispersed particles by gas detonation products in an expanding channel. The calculations show the possibility of a significant increase in the velocity of powder particles due to the conical expansion of a detonation channel. For particles with sizes of 30 ÷ 40 mm at cone angles of 2 ÷ 4 ^ºC, the maximum velocity increase reaches 35 ÷ 60 %. A method is developed for fixing the luminescence of a packet of dispersed particles accelerated in a detonation channel by a photosensor, which makes it possible to measure the particle velocity with an accuracy of ±5 %. The calculation results are in good agreement with the experimental data.

Explosion venting experiments of corn starch are carried out in a small-scale container. With the help of a high-speed camera and a pressure sensor, an interesting multiple venting behavior and its physical mechanism are comprehensively analyzed. The results suggest that the failure of the venting structure could cause three times intermittent explosion venting at most, which is negatively correlated with the opening pressure. Only the first venting significantly changes the internal explosion pressure. The flame behavior changes substantially from one venting to the other, even under the same operating conditions. The first venting flame occurs in an underexpanded jet form (with point sparks at the front), the second one is bright and spherical (without sparks), and the third one is dim and exhibits a striped pattern. In addition, the earlier the venting, the larger the average velocity of the external flame. With the opening pressure increasing, a decrease in the maximum propagation distance and time of the first venting flame is observed; simultaneously, the average flame velocity and the maximum instantaneous flame velocity increase. There is no such a phenomenon in secondary venting, although the flame brightness gradually decreases. Overall, the results may provide a theoretical basis for the safety design of explosion venting and further exploration of the explosion mechanism.

Insensitive munition assessments are required to carry out sympathetic detonation experiments, in which detonation of an explosive charge triggers detonation of another, and a chain reaction subsequency starts. In this paper, a numerical simulation method is developed to predict the sympathetic detonation behavior of fuze explosive trains, which includes the ignition and growth model, the Jones-Wilkins-Lee equations of state, and the constitutive relationship. The sympathetic detonation behaviors of a fuze is studied for a single donor and a single acceptor. The causes of sympathetic detonation of the fuze explosive trains are analyzed for different fragment and shock wave loading conditions. A sympathetic detonation criterion in different modes of loading is established, which provides a theoretical model for predicting the relationship between the detonation sequence and the placement distance of the fuze. The conclusions obtained in this paper can provide a reference for studying sympathetic detonation of fuze conditions.

This paper presents a comparative computational analysis of the penetration of shaped charges with copper liners in the form of a cone of progressive thickness with a cone angle of 60º, an equal-thickness hemisphere, a hemisphere and semi-ellipsoid of degressive (decreasing from top to bottom) thickness. The polar semiaxis of the semi-ellipsoid liner slightly exceeded the equatorial semiaxis. The parameters of the generated shaped jets were determined by numerical simulation within the framework of a two-dimensional axisymmetric problem of continuum mechanics, and the depth of their penetration into a steel target was determined using an engineering technique that takes into account the influence of the manufacturing accuracy of charges. The parameters of the liners of degressive thickness were selected so that the speed of the head of the jets formed from them was close to the speed provided by conical liners. In this case, the penetration of the shaped charge with a hemispherical liner of degressive thickness was significantly lower than that of the charge with a conical liner, and the latter, in turn, was slightly inferior in the maximum depth of penetration to the charge with a semi-ellipsoidal liner. The physical reasons for the results are discussed.