Tехнические науки/ 3. Отраслевое машиностроение

Nickolay Zosimovych, Banshidhar Choudhary

Sharda University (Greater Noida, India, UP)

THE STRUCTURE OF HETEROGENEOUS CONDENSED MIXTURES

Summary. The paper analyzes contemporary approach to the development of heterogeneous condensed systems capable of self-sustaining combustion and consisting of powder components in the continuous polymer matrix. The obtained result allows modeling the structure and combustion processes in such systems.

Key words: Heterogeneous condensed mixtures (HCM), combustion, composite, powdered metal, dispersed component, solid propellant, ballistic, solid rocket engine (SRE).

Introduction. Heterogeneous condensed mixtures (HCM) are condensed substances capable of self-sustaining combustion and consisting of the powdered ingredients, pressed, and distributed in continuous polymer matrix. The characteristic feature of heterogeneous condensed mixtures is great volume content of dispersed components that can be reaching 90% or more. Thus, the heterogeneous condensed mixtures are highly filled composites. The most known classes of HCM are: 1) mixed solid rocket fuels; 2) mixed explosives; 3) pyrotechnical compositions; 4) termites compositions (spontaneously combustible system - SCS).

All of these HCM, despite the great differences in the properties of both individual components and systems as a whole have a number of common features. First of all, it is manifested in the fact that their self-sustaining combustion is impossible without interacting (chemical, thermal, mechanical, etc.) components, which occurs in heterogeneous mode as well as in homogenous phase in the interactions of gaseous or liquid products of decomposition.

So many patterns, as well as methods and models turn out valid for a wide class of HCM, regardless of their physical properties and chemical nature.

The present study focuses to the first and most widespread aspect, from the standpoint of practical applications, of the HCM class, namely, the mixed solid propellant. In spite of many results and methods available, modeling can be extended to other HCM classes.

Mixed solid propellants are complex composite materials which contain the dispersed components, distributed in a continuous polymer matrix. Dispersed components of mixed solid rocket propellants are oxidizer (ammonium perchlorate, ammonium nitrate, etc.), energy supplements, which include powdered explosives such as nitramines (HMX) and RDX, and powdered metals (aluminum, boron, magnesium, etc.) or their hydrides. The particle size of dispersed components can vary from a fraction of a micron to several hundred microns, while the overall mass fraction of dispersed components in mixed solid rocket fuels can reach 90%.

Physical and mechanical, thermal, ballistic, electrical and other properties of mixed solid rocket fuels, as highly filled composites, are defined not only by properties of the components and their content and dispersion, but also the distribution of dispersed components in the volume of material. For example, the steady burning of composite solid rocket fuels is due to a random distribution of components in the volume, which results in that the burning surface are always present in sufficient quantities, the components providing continuity of the process.

The main area of composite solid propellant rocket propulsion has different purposes, ranging from Space Shuttle "solid booster, each of which contains approximately 500 tons of HCM, solid booster raves "Ariane 5", containing about 250 tons of HCM, sustainer rocket engines of ballistic missiles containing 1,000 kg to 60 tons of HCM, and ending with special rocket engines containing uncompensated kilograms HCM.

The ballistic efficiency of solid propellant missiles of different classes determined first of all, a ballistic efficiency of its propulsion rocket engines, so, progress in rocket technology for various purposes is largely related to improving the fuel for solid rocket engine (SRE).

Improving SRE is going in two main directions: improvement of structural materials and the improvement of solid rocket propellants. Power and fuel efficiency of ballistics, as a generalized concept, are made up of many components, basic of which are the specific impulse, the fuel density and the law of combustion. Improving of the first two parameters promotes the growth of both energy and ballistic performance characteristics of the SRE-s and rockets in general. The law of combustion is characterized by a level of burning rate at some pressure, and an exponent in the dependence of the speed of combustion pressure. The level of combustion is determined by the tasks facing the SRE and a rocket as a whole, and in each case individually, under the influence of different requirements for the trajectory of missiles, the permissible size of accelerations, etc. However, the decline right up to zero, in the exponent in the expression of the burning rate and the pressure for the main engines, and increasing this indicator up to as much as unity for controlled multi-mode propulsion system almost always leads to an increase in SRE ballistic performance.

Providing high performance fuels as well as continued progress in creating new high-performance propellants is impossible without a detailed understanding of the complexity of physical and chemical processes that occur during combustion of HCM and without creating a theories and models to predict the impact of various factors, including the structure and composition of HCM and laws of their burning. This is particularly true at present time when there has been a significant progress in solid rocket fuels, associated with using new components such as ADN, CL-20 etc., as well as with the use of ultra component. For example, HMX with particle sizes less than 10 microns, and aluminum with particle sizes less than 1 micron and even nanoaluminum with particle sizes of 10-100 nm [1].

Characteristic feature of HCM is synergism, in which combustion of individual components is significantly different from burning a mixture of components in the HCM. This leads to the fact that burning HCM cannot be considered simply as the sum of the burning of the individual components, as great importance is in the interaction of components during combustion (combustion of each component depends on the surroundings in which it takes place). For example, at normal temperature, steady burning of HCM, as an individual substance is only possible at pressures higher than 2 MPa [2], while in the mixture with the polymer binder, burning it is sustainable, even at pressures below 0.1 MPa [2-6]. Similarly, inert binders in normal conditions are not able of self-sustaining combustion, but the composite solid rocket propellants are stable in a wide range of external conditions [2,7-9]. This is explained by the fact that determining the combustion of HCM is the interaction of components: chemical, thermal, mechanical, etc. The interaction of components depends not only on their properties, but also on their relative position within the dispersed components HCM and on the burning surface.

How this interaction occurs, inside the structure, depends on the reaction zone, the burning rate of HCM as a macroscopic system, as well as chemical composition and structure of the combustion products (primarily condensed). This is evident most strongly in the combustion of metalized HCM, which is followed by fusion of the metal particles to agglomerates, and the metal fuel may actively engage in a chemical reaction with oxygen-products of the decomposition, other components being in solid, and liquid and gas phases. Dispersed components are distributed randomly inside the HCM and thus the conditions of their interaction will vary in different parts of HCM. In other words, the interaction of components inside the HCM, on the surface of combustion in the gas phase will be random, and that randomness will be determined by random structure of the described HCM. This means that talking about processes occurring during combustion of HCM, is possible only in a statistical sense. At the same time, from the macroscopic point of view, the combustion process of HCM is determined. This is evident in that, for example, the dependence of the burning rate of HCM, of pressure and initial temperature for a macroscopic sample, of the dimension which is considerably larger than internal structural irregularities of HCM, is deterministic, like the energy characteristics of the HCM, such as a specific impulse, or an adiabatic combustion temperature. The macroscopic deterministic behavior in combustion of HCM related to the fact that random processes taking place in different parts of the burning surface, averaged out for macroscopic samples and casual local fluctuations, cancel each other. Thus, we can conclude that noncontradictory description of the processes occurring in combustion of HCM, is only possible in the statistical approach, using the apparatus of probability theory, stochastic processes and statistical physics.

Statistical approach to the description of combustion processes is currently widely used in the simulation of turbulent combustion of gases [10], but practically not used to describe the combustion of heterogeneous condensed mixtures with initially random structure. This is one of the main internal contradictions of the modern theory of combustion of HCM. Typically the description of combustion processes in HCM, with a random structure, or used homogeneous models in which HCM is replaced by a microheterogeneous averaged homogeneous system, or considered some averaged characteristic of cell of HCM, which is preserved with microheterogeneity, but loses the statistical nature of the process.

Conclusion. The above analysis allows using statistical modeling of the structure and combustion processes HCM, based on methods used in statistical physics and stochastic processes, allowing a single statistical position to investigate the processes that occur during the combustion of HCM.

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