Thermal Decomposition and Combustion of Ammonium Dinitramide (Review)

A comprehensive review of thermal decomposition and combustion of ammonium dinitramide (ADN) has been conducted. The basic thermal properties, chemical pathways, and reaction products in both the condensed and gas phases are analyzed over a broad range of ambient conditions. Detailed combustion-wave structures and burning-rate characteristics are discussed. Prominent features of ADN combustion are identified and compared with other types of energetic materials. In particular, the influence of various condensed- and gas-phase processes in dictating the pressure and temperature sensitivities of the burning rate is examined. In the condensed phase, decomposition proceeds through the mechanisms ADN → NH₄NO₃ + N₂O and ADN → NH₃ + HNO₃ + N₂O, the former mechanism being the basic one. In the gas phase, the mechanisms ADN → NH₃ + HDN and ADN → NH₃ + HNO₃ + N₂O are prevalent. The gas-phase combustion-wave structure in the range of 5–20 atm consists of a near-surface primary flame followed by a dark-zone temperature plateau at 600–1000°C and a secondary flame followed by another dark-zone temperature plateau at 1000–1400°C. At higher pressures (60 atm and above), a final flame is observed at about 1800°C without the existence of any dark-zone temperature plateau. ADN combustion is stable in the range of 5–20 atm and the pressure sensitivity of the burning rate has the form r _b = 20.72p ^0.604 [mm/sec] (p = 0.5–2.0 MPa). The burning characteristics are controlled by exothermic decomposition in the condensed phase. Above 100 atm, the burning rate is well correlated with pressure as r _b = 8.50p ^0.608 [mm/sec] (p = 10–36 MPa). Combustion is stable, and intensive heat feedback from the gas phase dictates the burning rate. The pressure dependence of the burning rate, however, becomes irregular in the range of 20–100 atm. This phenomenon may be attributed to the competing influence of the condensed-phase and gas-phase exothermic reactions in determining the propellant surface conditions and the associated burning rate. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 6, pp. 54–79, November–December, 2005.     

Response functions of HMX and RDX burning rates with allowance for melting

Experimental values of the burning rate with variations in pressure and initial temperature are given, and identity of gasification laws in the wave for RDX and HMX is demonstrated. Differential parameters (sensitivities) of the burning rate and surface temperature are found. Based on these parameters, response functions of the burning rate to pressure oscillations are calculated. The calculations are performed with and without allowance for condensed phase melting. Allowance for melting is shown to reduce the amplitude of the real part of the response functions, while the thickness of the melted layer does not produce any significant effect on the response functions. Errors of calculating the response functions are presented. The calculated response functions are compared with experimental data. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 72–82, May–June, 2007.     

Combustion mechanism of nitro ester binders with nitramines

The combustion of binary compositions of nitramines (HMX, RDX, Bi-HMX, and CL-20) with two nitro ester binders, one of which is characterized by gas-phase combustion, and the other by the burning-rate controlling reaction in the condensed phase, was studied in the pressure range 2–15 MPa. It is shown that in compositions with the binder characterized by gas-phase combustion, HMX in concentrations up to 50% acts as an inert additive. Depending on the size and concentration of HMX particles, three types of combustion of the mixtures can be identified: combustion along the binder layers, combustion as a single system, and the model of combustion with a coolant. At higher nitramine concentrations, combustion control passes to nitramine, and the burning-rate controlling reaction occurs in the liquid phase of nitramine. For Bi-HMX and CL-20 nitramines, which are less stable and more fast burning than HMX and in a mixture with a binder characterized by gas-phase combustion, only two combustion models are observed: the model with fast burning additives or combustion as a single unit. In compositions of the nitramines RDX and HMX with a binder burning by the c-phase mechanism, the combustion model with fast burning additives is applicable only in a narrow range of conditions. The compositions mostly burn only as a single unit, and the addition of nitramine increases the burning rate of nitro ester by transferring heat from the overlying zone to the condensed phase.     

Theory of nonstationary combustion of homogeneous propellants

Conclusions A comparison of the results of the theory outlined above and the experimental data shows that theory and experiment are in qualitative agreement. Thus, taking into account the variability of the surface temperature eliminates the principal difficulty of the theory with constant temperature—instability of the steady—state regimes of propellant combustion under ordinary practical conditions (k>1)—and, hence, leads to the possibility of investigating nonstationary phenomena associated with the combustion of actual systems. It is also possible to explain experimental facts such as combustion stability in a chamber and the existence of a natural frequency of the burning rate oscillations. The order of the frequency and its pressure dependence are in agreement with experiment. The theory is also in qualitative agreement with experimental data on quenching and on the nonstationary burning rate at variable pressure. A number of problems remain to be investigated from the standpoint of the theory described; for example, the dynamic erosive burning regime, or a propellant burning in the presence of a variable-velocity gas flow at its surface. Further development of the theory and its quantitative comparison with experiment must be based on the steady-state dependences of burning rate and surface temperature on initial temperature and pressure. To obtain these relations is the primary task of experimenters and theorists in the area of steady-state combustion. In conclusion, the directions to be followed are now plain: it is necessary to account for the inertia of the reaction layer in the condensed phase and to improve the theory by accounting for the variability in the thermal diffusivity of the propellant, which increases with temperature. As calculations show [46, 47], this effect increases the regions of stable combustion at constant pressure and possible oscillatory combustion modes. Fizika Goreniya i Vzryva, Vol. 4, No. 4, pp. 482–493, 1968 Contribution to a group discussion on the unstable and nonstationary combustion of condensed systems. 19680715     

Low-temperature combustion of energetic materials in filled polymer systems

In systems filled by inert additives, combustion of cellulose nitrate (CN) proceeds in a flameless low-temperature regime with a low linear burning rate. At a standard temperature, the exponent in the low of CN combustion in ballasted mixtures with inert additives in the pressure range of 0.1 to 10 MPa is several times lower than that of pure cellulose nitrate and amounts to 0.23. The qualitative and quantitative composition of gaseous products of flameless CN conversion is found. It is noted that this composition approximately corresponds to data available in the literature for the products of thermal decomposition of cellulose nitrate at comparatively low temperatures. Based on this fact and on a weak dependence of the CN burning rate on pressure in ballasted systems, the process under these conditions is assumed to be controlled by conversion of the energetic component predominantly in the condensed phase. In the case of a composite consisting of cellulose nitrate, silicon carbide, and polymer binder, for samples 10–25 mm in diameter, armoring exerts practically no effect on combustion parameters. Combustion of the same mixture with smaller diameters of non-armored samples is unstable. The presence of a liner establishes a clearly expressed critical combustion diameter in the examined systems. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 98–102, May–June, 2007.     

Erosive burning. Modeling problems

Existing models for the erosive burning of homogeneous energetic materials ignore the fact that the burning regime can be changed radically by exposure to a hot gas blowing over the burning surface. In other words, transition can occur from the gasification regime at very high blowing rates (where the burning rate is determined primarily by the heat transfer from the flow core) to the self-heating regime of the condensed phase at low or zero blowing if the heat release in the condensed phase is sufficient to heat it to the surface temperature. A possible method for solving this problem is proposed. The approach proposed provides a plausible explanation for the experimentally observed singular (kinked) dependence of the magnitude of the negative erosion effect on the initial temperature. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 47–58, May–June, 2007.     

Kinetics of heterogeneous reactions of carbon and oxygen during combustion of porous carbon particles in oxygen

A model of combustion of a high-porosity carbon particle in oxygen is considered, which takes into account heterogeneous and homogeneous chemical reactions inside the particles and radiative heat transfer. The boundaries of the domain where the burning rate depends on the particle temperature are determined. The possibility of two combustion regimes is demonstrated: regime with a high burning rate, where the carbon-oxygen reaction proceeds in a layer adjacent to the particle surface, and regime with a low burning rate, where the reaction proceeds in the entire particle volume. In the regime with a high burning rate, the main product of the reaction between carbon and oxygen is carbon monoxide, whereas both carbon monoxide and carbon dioxide can be formed in the regime with a low burning rate. The kinetic equations of heterogeneous reactions C + O₂ = CO₂ and 2C + O₂ = 2CO are determined, which reveal the retarding effect of carbon monoxide and dioxide on the rates of these reactions. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 11–22, May–June, 2006.     

Condensed combustion products of aluminized propellants. IV. Effect of the nature of nitramines on aluminum agglomeration and combustion efficiency

The condensed combustion products of two model propellants consisting of ammonium perchlorate, aluminum, nitramine, and an energetic binder were studied by a sampling method. One of the propellants contained HMX with a particle size D ₁₀ ≈ 490 μm, and the other RDX with a particle size _{D 10} ≈ 380 μm. The particle-size distribution and the content of metallic aluminum in particles of condensed combustion products with a particle size of 1.2 μm to the maximum particle size in the pressure range of 0.1–6.5 MPa were determined with variation in the particle quenching distance from the burning surface to 100 mm. For agglomerates, dependences of the incompleteness of aluminum combustion on the residence time in the propellant flame were obtained. The RDX-based propellant is characterized by more severe agglomeration than the HMX-based propellant — the agglomerate size and mass are larger and the aluminum burnout proceeds more slowly. The ratio of the mass of the oxide accumulated on the agglomerates to the total mass of the oxide formed is determined. The agglomerate size is shown to be the main physical factor that governs the accumulation of the oxide on the burning agglomerate. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 78–92, July–August, 2006.     

Burning of hydroxylammonium nitrate

Burning of solid crystalline hydroxylammonium nitrate (HAN) and its water solutions is studied in a constant-pressure bomb within the pressure range from 0.1 to 36 MPa. Abnormally high pressure exponents are found to be typical of combustion of the crystalline substance, its ≈9-mole/liter water solution, and a solution containing ethanolamine nitrate as a fuel: for pressures below ≈10 MPa, the burning rate is proportional to the 1.5–2.6 power of pressure. Crystalline HAN retains this tendency up to ≈20 MPa, whereas the solutions burning in the turbulent regime show a sharp decrease in the pressure exponent if the pressure exceeds 10 MPa. In some instances, as was found in previous works, the burning rate even decreases with increasing pressure. Special features of turbulent burning are explained. A model is proposed to describe the steady-state burning of solid HAN. Kinetic parameters of the leading reaction are derived. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 1, pp. 149–160, January–February, 2000. This work was partly supported by the Russian Foundation for Fundamental Research (Grant Nos. 98-32164 and 98-32167).     

Temperature profile in the combustion wave of low calorific value propellants

Combustion wave temperature profiles are determined for two low calorific value propellants (Q _c = 2189 and 2518 kJ/kg). It is shown that the structure and parameters of the combustion wave differ significantly from those for previously studied propellants of medium (propellant N) and high (propellant NB) calorific values. At a relatively short distance from the burning surface, the temperature is significantly (180–270 K) higher than the calculated value due to fact the combustion products contain carbon black from the decomposition of heat-resistant dibutyl dinitrotoluene and dibutyl phthalate. Then, part of carbon black reacts endothermically with CO₂ and H₂O, leading to a decrease in temperature, which for the first sample is nevertheless 100–140 K higher than the thermodynamic value. For the investigated propellants, the activation energy of the leading reaction is the same as for the previously studied propellants, suggesting a common decomposition kinetics of the condensed phase regardless of the propellant composition. However, a uniform dependence of the burning rate on surface temperature is not observed. For low calorific value propellants, the surface temperatures are close to those for propellant N although their burning rate is significantly (2–2.2 times) lower. The causes of this fact are considered.     

Mechanism and laws of combustion of composite solid propellants with a coolant

The model of combustion of a composition consisting of a quasi-homogeneous composite propellant (matrix) and coolant particles is considered. The model is based on the leading role of exothermal decomposition of the matrix and on the cooling effect of the second component by virtue of transverse heat transfer between the components in the condensed and gas phases. Formulas for combustion characteristics (temperature, burning rate, its sensitivity to pressure, and initial temperature) are derived and analyzed. The calculated dependences of these characteristics on pressure, particle size, concentration, and thermal effects of decomposition of the components show that transitional regimes with a stronger dependence of the burning rate on pressure than that of the initial propellant are reached in a certain range of parameters. An algorithm is proposed, and a parametric identification of the model on the basis of experimental data is performed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 6, pp. 51–60, November–December, 2007.     

Effect of the addition of ultrafine aluminum powders on the rheological properties and burning rate of energetic condensed systems

The effect of small additives (1.25–5.00%) of ultrafine aluminum powders (UFAP) on the rheology and combustion of model four-component energetic condensed systems is studied. It is found that the addition of UFAP decreases the temperature of HMX decomposition. Small additives of UFAP increase the burning rate of model energetic condensed systems and decrease the exponent ν in the burning rate law without deteriorating the rheological characteristics of the model propellants. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 54–59, January–February, 2007.     

RDX-CMDB推进剂燃速温度敏感系数的实验研究

为了揭示RDX-CMDB推进剂中各常见组分对其燃速温度敏感系数的影响规律,制备了一系列含RDX、铝粉及燃烧催化剂的CMDB推进剂样品。采用氮气靶线法测得其在2~14MPa下的燃速温度敏感系数(σp)。讨论了RDX含量、铝粉、燃烧催化剂对RDX-CMDB推进剂燃速温度敏感系数的影响。结果表明,提高工作压强、增加RDX含量、添加燃烧催化剂均有助于降低RDX-CMDB推进剂在一定初始条件下的燃速温度敏感系数。配方中引入铝粉后可降低中低压下RDX-CMDB推进剂的燃速温度敏感系数,且燃速温度敏感系数几乎不随压强变化而变化。选用含邻苯二甲酸铅和没食子酸铋锆作燃烧催化剂,均可在2~10MPa下降低RDX-CMDB推进剂的燃速压强指数,同时降低燃速温度敏感系数。     

Frontal regime of exothermal conversion of refractory condensed compounds

We consider a mathematical model of propagation of the combustion front in heterogeneous condensed compounds. For these compounds, the adiabatic combustion temperature is lower than the melting point of the initial reagents and condensed products; it is also lower than the minimum temperature of their eutectics. The chemical interaction in the combustion front is accomplished by a gas transport mechanism due to the gasification of additive oxides. Within the framework of the investigated model, values are obtained for the front velocity, which are of the same order as those observed experimentally. Periodic unstable combustion regimes (fluctuating and spinning) arise when the macrokinetics of the chemical transformation is strongly activated. It is shown that the activation can depend on the gasification energy and, consequently, the evaporation of additive oxides influences the stability of a steady-state combustion regime. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 3, pp. 77–83, May–June 1998.     

Instability of a combustion model with surface vaporization and overheat in the condensed phase

Using numerical modeling, we have revealed the instability of a steady-state combustion regime which was previously obtained using analytical methods in the substance-combustion model with surface vaporization and with exothermic reactions in the condensed phase which are intense enough to form a maximum of temperature underneath the surface. The instability has been studied analytically using the method of small perturbations to eliminate the version of its nonphysical (numerical) character. Steady-state combustion regimes with maxima on the condensed-phase temperature profile are shown to be actually unsteady. It is suggested that convection in a liquid-subsurface layer owing to bubble motion caused by the Marangoni effect should be taken into account to describe correctly the experimentally observed steady-state regimes with a leading role of the condensed phase. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 1, pp. 43–50, January–February, 1997.     

Combustion of Energetic Materials in an Acoustic Field (Review)

The review summarizes the long-term experience in theoretical research of combustion of gasifying condensed systems with periodically varied pressure. Most results are obtained within the framework of the Zel'dovich-Novozhilov theory. The main properties of the linear function of the burning rate response to harmonically varied pressure are discussed. The concept of nonlinear response functions is introduced, which is illustrated by the explicit form of a number of second-order response functions. A new phenomenon is described: bifurcations of response functions with a varied amplitude or frequency of pressure oscillations. For the simplest gunpowder model containing three parameters only, the sequence of bifurcations of doubling of the burning rate oscillation period is studied, which finally leads to a random combustion regime. An analytical relation between the linear response functions to harmonically varied pressure and to an oscillating radiant heat flux is noted. An example of calculating the response function with allowance for thermal inertia of the gas phase is presented. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 6, pp. 116–136, November–December, 2005.     

Filtration combustion of liquid monofuels

A one-dimensional two-temperature model for filtration combustion of liquid monofuels is proposed. The model is used to analyze the filtration combustion of liquid hydrazine in narrow tubes. Two steady-state regimes are found. In regime I, the dominant mechanism of heat transfer from the combustion products in the preflame zone is heat conduction in the gas, and in regime II, this is interfacial convective heat transfer and heat conduction in the solid phase. Parameter ranges for the existence of the regimes are established. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 21–30, July–August, 2008.     

Some aspects of the effect of the porosity of the condensed phase on the combustion of nitrocellulose propellants

This paper considers the mechanisms involved in the effect of the unconnected porosity of the condensed phase of nitrocellulose propellants on the linear burning rate with a pressure rise. A model based on experimental data is proposed for the variation in the linear burning rate taking into account the compaction of the fuel cells resulting in a nonlinear dependence of the burning rate on pressure. The temperature coefficient of the linear burning rate is found to exhibit a nontrivial behavior under pressure rise conditions in various ranges of initial temperatures. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 58–63, September–October, 2008.