Tехнические науки/ Механика

Banshidhar Choudhary¹, Nickolay Zosimovych²

¹Sharda University (Greater Noida, India)

²Instituto Tecnólogico de Monterrey (Campus Gualajara, Mexico)

EXPERIMENTAL INVESTIGATIONS OF COMBUSTION PROCESSES OF HCM

In the report has been analyzing experimental researches of processes the combustion of aluminum agglomeration heterogeneous condensed mixtures. Heating of agglomerates on the burning surface and agglomerates, detached from the surface of the combustion happens in an oxidizing atmosphere, and always ends with their inflammation and combustion. Heterogeneous mode of burning of agglomerates connected with diffusion of gaseous oxidizer to the surface of the particle and the chemical reaction of oxidation on the surface. This results to the formation of the surface layer of aluminum oxide agglomerate.

Keywords: Heterogeneous condensed mixtures (HCM), composite, solid rocket fuels, combustion, burning surface, agglomerates, aluminum oxide, modes density function (MDF), mass fraction, concentration of metal, phase.

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 in them dispersed components that can be reaching 90% or more. Thus, the heterogeneous condensed mixtures are highly filled composites.

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% [1].

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.

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.

Problem setting. Experimental researches of processes the combustion of aluminum agglomeration HCM many papers, which address the impact of various factors on both the distribution of the agglomerates in size, and the structure and chemical composition of the agglomerates.

Results and Discussion. Experimentally founded [2-4] that the k-phase particles in combustion HCM have two modal and even polymodal size distribution. It is related with two major processes that produce. The original process is the formation of agglomerates of metal which form coarse fraction (> 10 mkm) k-phase particles in combustion products.

Heating of agglomerates on the burning surface and agglomerates, detached from the surface of the combustion happens in an oxidizing atmosphere, and always ends with their inflammation and combustion.

Burning of large aluminum particles (agglomerates) occurs in two modes: the gas-phase and heterogeneous [5, 6]. Burning in the gas-phase mode connected to evaporation of the metal and its vapor in the combustion mixture with gaseous oxidizer. The result of this process is the formation of gaseous the higher oxides of aluminum, which as a result of chemical condensation constitute fine particles of aluminum oxide [6]. These particles have a size less than 5-10 mkm and forming first (highly dispersed) mode condensed combustion products of HCM.

Heterogeneous mode of burning of agglomerates connected with diffusion of gaseous oxidizer to the surface of the particle and the chemical reaction of oxidation on the surface. This results to the formation of the surface layer of aluminum oxide agglomerate. In addition, the large agglomerates slip around fine particles of aluminum oxide generated in the gas phase by chemical condensation, which also leads to an increase in the mass of aluminum oxide which is part of agglomerate [6]. Thus, the agglomerate is a binary drop of metal oxide and is composed mainly of unburned aluminum and aluminum oxide which in case of combustion power plants are liquid. Should be noted that the structure of agglomerates can also be part products of incomplete decomposition of the organic binder, primarily carbon [7]. Due to surface tension of aluminum oxide is usually collected in compact formation (blotches) [6, 8], takes some of the surface of agglomerate.

Because normally of agglomerate temperature significantly lower than the temperature of vaporization of aluminum oxide during evolution the agglomerate is irreversible accumulation of aluminum oxide and reducing the share of pure aluminum in the agglomerate. In other words, the mass of aluminum oxide in the agglomerate increases monotonically as long as the will burn out all the free aluminum or does not stop the burning process of agglomerate. There are theoretical considerations [6] that in a chemical reaction (in k-phase sinter), the formation of gaseous oxides of higher and that should result to a certain decrease in the weight of in agglomerate. However seems, the process is slower than the accumulation in the agglomerate by heterogeneous oxidation of aluminum, and cannot change the qualitative picture of formation coarse particle fraction.

Thus the main modes density function (MDF) on of particle sizes of aluminum oxide in the HCM combustion products have a simple explanation: MDF first mode connected with highly dispersed aluminum oxide, formed by the condensation from the gas phase, while the other modes MDF describe major functions of the particles of aluminum oxide, caused by agglomeration of metal by burning HCM and accumulating them in the heterogeneous reactions with the gaseous oxidizer.

Experimentally obtained [3, 4, 6] that the size of the agglomerates MDF is also a multimodal. This is due to several processes involved in the agglomeration. For example, there is a finite probability that part of the source of metal particles will leave from the burning surface of MDF, does not combined with other particles. Namely solitary initial metal particles taken off from the burning surface, form the first mode of MDF agglomerates [6, 9]. Emergence of other modes MDF has not received satisfactory explanation to the present.

Structure of Agglomerates. Structural study of agglomerates plays an important role in understanding of the process of metal agglomeration and further evolution of particles. As we mentioned above, these agglomerates typically include unburned aluminum, aluminum oxide and products of incomplete decomposition of an organic binder. In [10, 11] you can find a description of two qualitatively different types of agglomerates which are formed during combustion of HCM composed of different particles. In [10] the authors suggests to discriminate between two classes of HCM (А and В). HCM belonging to class A are characterized by burning of metal in the lower part of the frame layer. Initially, particles burn in the heterogeneous mode and then they burn in the upper portion of the frame layer and in the vapor phase on its surface [12]. The sizes of aluminum particles which burn in heterogeneous mode are close to the sizes of primary metal particles in HCM. These mixtures are characterized by high connectivity of metal particles to carbon elements of the frame layer. As a result, pores in the carbonic frame shall be filled with liquid aluminum oxide. Large contact surface between melted aluminum oxide and the carbonic frame results in low mobility of the agglomerates and caters for strong adhesive force which keeps them on the burning surface of HCM.

When HCM class A burns, agglomerates of two types shall be formed. One type of agglomerates called “matrix agglomerates” by authors [10] is represented by spherical particles with some integrated particles of aluminum. As a rule, there is a few large and many small particles among the aluminum particles, which are generally located on the free surface of liquid oxides, however, some small particles may be fully encapsulated into aluminum oxide. The metal of "matrix" agglomerates, which has a free surface, burns in the vapor phase forming a plume with layered structure. The mass fraction of aluminum oxide in the "matrix" agglomerates may reach 60-70%. The agglomerates belonging to the second type consist of metal particles with splats of aluminum oxide on their surfaces. The proportion of the agglomerates of the first and second type changes as combustion conditions of HCM belonging to class A change. There is a threshold pressure above which the first type agglomerates are practically absent. The threshold pressure depends on the properties of HCM. Generally, the agglomerates of the first type have bigger sizes, and the sizes increase with increasing pressure.

When HCM class B burns, ignition of metal particles occurs on the burning surface (after they leave the frame layer). The upper portion of the frame layer is composed of primary metal particles covered with a solid oxide film. After ignition, the particles will be interconnected in cracks of the oxide film, and then burn in the heterogeneous combustion mode and in the vapor-phase mode [12]. During combustion of HCM class B only agglomerates of the second type will be formed, however, splats of aluminum oxide may be absent on aluminum particles in certain circumstances. The content of in the agglomerates shall be specified mainly by heterogeneous combustion on the surface of the frame layer.

The mass fraction of aluminum oxide is different in agglomerates of different types, moreover, it depends on the pressure at which HCM combustion occurs. The publication [10] specifies that the mass fraction of in the agglomerates formed during combustion of HCM class A decreases with increasing pressure, and when pressure is MPa, it levels off to approximately 0.28. At the same time, the content of in the agglomerates formed during combustion of class B compounds, is practically independent from pressure, and is approximately 0.16. It must be noted, that in general, data provided by different authors indicating mass fraction of in agglomerates differ. The reason for it is different sampling procedures used for chemical analysis. The further sampling is made from the burning surface, the lower the content of active aluminum in agglomerates [14].

The complex structure of the agglomerates containing products of incomplete decomposition of the organic binder and, in some cases, gas inclusions, results in dependence of the density of agglomerates upon their sizes, HCM composition and HCM burning conditions, but primarily upon pressure [10, 13]. In [10] you can read that in HCM class A the average density of the agglomerates increases with increasing pressure, and when MPa, it levels off corresponding to weight ratio between and in the agglomerate.

In some cases, we can observe bloating of agglomerates owing to extensional emission of gas due to chemical reactions and [10, 14]. This results in changed average densities of the agglomerates and also the difference between the mass median and volume median diameters of agglomerates.

Influence of Various Factors on the Process of Agglomeration. The process of agglomeration is influenced by a large number of factors which can be divided into two groups. The first group includes factors characterizing HCM itself (composition, structure, dispersion of components, burning rate, etc.), the second group includes factors characterizing external conditions under which HCM combustion occurs (pressure, blowing of burning surface, overloads etc.).

Concentration and Sizes of Metal Particles in HCM. It was discovered that there is a certain threshold concentration of metal in HCM, starting from which agglomeration becomes noticeable [15]. The threshold concentration of aluminum is directly dependent on the particle sizes: the smaller the particles, the less their concentration at which agglomeration occurs. Table 1 shows the data from [15], which demonstrate how the threshold concentration of aluminum changes in HCM depending on the mass median diameter of primary aluminum particles.

Table 1

Dependence of threshold concentration of aluminum under which agglomeration starts upon the mass median diameters of primary particles of aluminum in HCM

Mass median diameter of Al particles, mkm		10	50-70	160
Threshold concentration, %	1	2-3	5-7	10

The agglomeration process may be characterized by the degree of agglomeration equal to the ratio of the average diameter of agglomerates to the average diameter of primary metal particles in HCM.

The transition from finely divided to large particles of aluminum will be accompanied by increase in the mass median diameter of agglomerates and simultaneous decrease in the degree of agglomeration [4, 16].

With growing concentration of aluminum in HCM the mass median sizes of agglomerates increases exponentially [4].

The influence of dispersion and concentration of the metal fuel in HCM upon the sizes of agglomerates can be simply explained [4]: the smaller the particles of the primary metal and the larger its mass concentration in HCM, the greater concentration of metal particles in HCM, the higher the number of contact particles and, consequently, it is more likely that particles will form agglomerates and a heat wave will pass through phase of HCM [17].

Concentration and Sizes of AP and Other Disperse Components of HCM. With the increase of mass median diameter of particles of the oxidizer (AP) the degree of agglomeration and mass median sizes of agglomerates increase as well [4, 11, 18, 19], and the larger the sizes of AP particles, the larger the agglomerates.

However, direct relationship between the average sizes of agglomerate particles and AP particles shall be observed only for relatively coarse AP powders. The publication [6] presents data demonstrating the increase in the sizes of agglomerates with decrease of average diameter of AP particles for mixtures containing fine AP powder (less than 30 mkm). This effect can’t be explained by a "pocket" model.

Burning Rate. Almost all researchers pointed to a positive role of HCM burning rate upon agglomeration process: any changes increasing the burning rate of HCM help to reduce agglomeration of the metal during combustion of metallized HCM [4, 18, 19]. In fact, it is very difficult to separate the influence of HCM burning rate from the influence of other factors (pressure, oxidizer dispersity, etc.), which in their turn, influence the burning rate.

For example, it has been established [20] that aluminum agglomeration will be practically unobserved in mixtures with fine components containing 15-20 % aluminum and having the combustion rate of 40-50 mm/s. It is hard to say which fact plays the major role in this case: either increased dispersion of AP, and hence, reduction in the sizes of "pockets", or increased burning rate caused by it. In [21] the burning rate of HCM changed due to changes in the gravitational density of a compressed sample. By changing in this way the burning rate of HCM from 5.8 to 7.6 mm/s, the volume median diameter of agglomerates was reduced by one third as well. However, in this case the mass flow rate of gas from the burning surface (mass burning rate of HCM) varied within a narrow range between 0.98 and . Thus, decrease in the degree of agglomeration was caused rather by changes in the structure of HCM, than by the burning rate itself (rate of generation of combustion gases).

The answer to the question could be obtained in experiments in which the burning rate would vary widely due to introduction of catalysts and flame retardants while chemical and fractional composition of HCM remained unchanged. Unfortunately, such quantitative data are not currently available.

Role of Fuel and Oxidant. All available data point to the importance of the kind of the binder and its concentration in HCM. It is common knowledge that replacement of the binder in HCM under otherwise equal conditions directly influences the sizes of the agglomerates formed. This is primarily due to the behavior of the binder when it is heated during combustion of HCM. At the moment, we have achieved only qualitative understanding of the role of binder in the agglomeration process: the more carbonaceous residue is formed and less issuance of the binder volatile under thermal decomposition, the larger (on the average) sizes of agglomerates [9, 21-24].

For example in [22, 23], the transition from binders containing 30-40% of the carbon to binders containing 80-85% of carbon leads to heavy increase by 2-5 times in the average sizes of agglomerates, as well as to qualitative changes in the dependence of mass median sizes of agglomerates from pressure.

This is because of the fact that agglomerates can be bound to the burning surface of HCM through carbon frame or carbon fibers - products of incomplete decomposition of the binder. The larger and stronger the carbon residue, the stronger agglomerates bound to the burning surface. Consequently, the longer they stay on the surface, the more likely their enlargement.

An important role in the process of agglomeration plays behavior of the binder and oxidizer when they are heated. For example, HCM with potassium perchlorate melting during combustion produce agglomerates smaller than HCM ( other conditions being equal) [6, 25]. The same applies to the binder: HCM with the binder melting during combustion produces agglomerates which are smaller compared to agglomerates produced with the binder decomposing without the liquid phase formation. This behavior can be explained, presumably, by the fact that the connection with the agglomerates with a liquid layer on the surface of the HCM is weaker on the average than with condensed porous frame layer, which contributes to an earlier separation of the agglomerates that have no opportunity to enlarge on the burning surface.

Accelerations. Experiments show, that normal accelerations on the burning surface, speed up the agglomeration process [9, 18, 22, 26]. The smaller primary aluminum particles in HCM, the higher the degree of agglomeration under normal accelerations directed towards the burning surface. This is due to the increase in time during which agglomerates remain on the burning surface under the influence of the pressing acceleration which contributes to enlargement of agglomerates.

Under accelerations directed towards the burning surface, agglomerate is pressed against the surface of the binder, thereby accelerating pyrolysis of the binder and, ultimately, increasing the burning rate of HCM [27-29].

Merging of agglomerate and aluminum particles protruding to the surface from k-phase, leads to a rapid enlargement of the agglomerates and contributes to fusion of agglomerates on the burning surface. As a result, the burning surface shall be coated with molten aluminum and its oxides.

Increase of the burning rate of HCM by reduction of AP dispersion of or by introduction of a catalyst enables to reduce the degree of agglomeration, but, anyway, there are large agglomerates on the burning surface which are unable to leave it due to acceleration. The sizes of such agglomerates increase over time, and may reach several mm.

HCM Mixing Methods. The end sizes of the agglomerates are influenced not only by dispersion, chemical and fractional composition of HCM, but also by the method of preparing a mixture that defines the internal structure of HCM and uniform distribution of components in it. For example, in [21], the same mixture (10 % PMMA) was made in two ways: by a conventional mechanical mixing of components in a rotating drum, when uneven distribution of components is natural, and by "gelation" when aluminum and oxidant were injected into the previously prepared dichloroethane solution of propellant.

After thorough mixing and evaporation of the solvent a relatively homogeneous mass was obtained in which propellant enveloped aluminum and AP particles with a thin layer. After combustion of gelled samples containing 7% Al, the average sizes of the agglomerates were 1.5 times less than in the samples made by the usual method. This fact also points to the important role of the structure of HCM in the agglomeration process. However, it should be noted that the burning rate was slightly higher in the gelled mixture.

Combustion of Aluminum Particles and Agglomerates. Ignition and combustion of aluminum particles and agglomerates on the burning surface of HCM is a very important factor affecting the sizes of agglomerates. There is a large number of publications dedicated to a detailed study of ignition and combustion of aluminum particles, both single particles and in conditions relevant to HCM combustion [5, 6, 8-10, 12, 16, 19, 20, 24, 29-33].

Conclusions

In this paper, the authors considers the basic problems in experimental investigations of combustion processes in HCM and in their practical application, such as:

1. Investigation of the internal structure of heterogeneous condensed mixtures containing disperse components with different distribution of particles according their sizes by methods of statistical physics, definition of statistical characteristics describing the internal structure of HCM.

2. Analysis of stationary combustion of HCM with consideration of the statistical structure of its burning surface, development of a statistical model of stationary-state combustion of HCM.

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