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.     

On the threshold nature of erosive burning

A physical explanation for the existence of the threshold of erosive burning is proposed. It is shown that this type of combustion occurs when the thickness of the laminar sublayer in the turbulent boundary layer becomes smaller than the thickness of the laminar combustion zone. In this case, turbulent flame in the gas phase is formed. Relations are obtained linking the critical (threshold) velocity of the blowing flow and the critical Vilyunov number to the properties of the propellant and the gas resulting from propellant decomposition. Simple exponential dependences on the blowing velocity are found for the burning rate. The simplest representation of the erosive burning rate is obtained using the Bulgakov-Lipanov number, whose threshold value is equal to unity. A new mechanism for the occurrence of negative erosion is proposed, according to which the burning rate decreases during blowing because the boundary layer is displaced, resulting in a decrease in the heat flux from the flame zone to the solid-phase decomposition surface. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 61–71, May–June, 2008.     

Real-time control of solid-propellant combustion by a catalytic method

Principles of contact catalysis of combustion are described. The essence of this process implies that a block catalyst contacting the surface of the burning propellant specimen increases the burning rate and allows controlling combustion at a temperature of 400 to 600°C with formation of a large volume of versatile gaseous products, i.e., is the basis for creating low-temperatures gas generators. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 86–91, January–February, 2007.     

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.     

Regimes of gas combustion in a porous body of a cylindrical heat generator

A stationary model of filtration combustion of a gas with allowance for conditions at the entrance to the porous body and conditions of heat exchange with the gas phase surrounding the burner and with the heat exchanger is proposed and numerically analyzed. Ranges of parameters where the regime of gas combustion with a narrow reaction zone near the outer surface of the porous body are determined. Mechanical stresses arising in the porous body owing to high temperature gradients and gas pressure in the pores are estimated. The rate of gas combustion and the radiative heat flux from the burner surface are plotted as functions of process-dependent parameters. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 18–29, January–February, 2009.     

Effect of convective motion on the flame structure in combustion waves propagating in heterogeneous systems under natural and artificial gravity conditions

Convective motion in combustion waves arises from natural or artificial gravity (centrifugal effect) for a wide range of heterogeneous systems, such as ammonium perchlorate containing solid additives, metal and nonmetal powders with a gas reagent, thermite mixtures, and hybrid layered structures. The paper summarizes the results obtained by the author and coworkers in studies of the effect of convective motion in combustion of heterogeneous systems for the period from the early 1970s to the present. It is shown that convective processes in combustion waves of heterogeneous systems may determine the structure and propagation mechanism of the combustion waves and the concomitant heat and mass transfer. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 86–92, July–August, 2009.     

Modeling the process of methane-air plume extinguishing by a solid-propellant pulse aerosol generator

Operation of a pulse aerosol system of fire fighting designed for effective extinguishing of fires in gas wells is modeled by an example of quenching a methane-air subsonic plume escaping from a nozzle. The system consists of two separate parts: a charge of a unitary solid propellant (gas generator) and a container with fine-grain powder of a flame retardant. The combustion of the mixture is described by a one-step global reaction; the effect of the concentration of flame-retardant vapors on the combustion process is taken into account through reduction of the pre-exponent in the Arrhenius law and is described by an empirical dependence. A computational experiment shows that the use of the pulse aerosol system of fire fighting ensures effective transport of fine aerosol particles of the flame retardant and its vapors to the combustion zone in amounts sufficient to suppress the ignition spot. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 26–34, May–June, 2009.     

Operation of a gas generator controlled by supplying a gaseous oxidizer

The possibility of arranging a gas generator with the heat-release process being controlled by supplying a gaseous oxidizer is experimentally checked. Gaseous hydrogen, liquid gasoline, and solid hexamethylene tetramine (solidified alcohol) is used as a fuel. The gas generator with a proposed configuration is demonstrated to ensure stable operation during combustion of various fuels; the pressure in the gas-generator combustion chamber does not exceed the pressure of oxidizer supply and clearly correlates with variations of the oxidizer mass flow. Quasi-steady calculations allow determining all parameters of the process, including those that are not measured in the experiment. In particular, the temperature of combustion products is found to be 600–1900 K, and the gas generator forms a high-temperature mixture containing a non-reacted fuel (the air-to-fuel ratio is α = 0.55–2.30). __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 18–25, November–December, 2008.     

Stabilized gas combustion wave in an inert porous medium

A stabilized gas combustion wave in an inert porous medium with intense internal interphase heat exchange (at low velocities) is considered to elucidate the mechanism underlying the increase in the burning velocity of the homogeneous gas mixture due to the porous medium. It is shown that the major factor of the increase in the burning velocity is conductive heat recuperation from the postflame zone to the region of the fresh mixture through the solid skeleton of the porous medium. Analytical dependences of the degree of increase in the burning velocity of the mixture in the stabilized wave on the determining parameters are obtained. The possibilities and restrictions of the use of the results to analyze the operation of porous burners are discussed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 8–15, September–October, 2008.     

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.     

Critical combustion diameter

The heat loss in combustion of gaseous and condensed materials is determined in plane and axisymmetric formulations. Generalized expressions are derived for the combustion quenching conditions versus pressure and the stability boundaries for steady-state combustion of condensed materials are obtained. It is shown that theoretical dependences of the critical diameter on pressure agree with experimental data on combustion of gas mixtures and condensed liquids and solids. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 29–39, January–February, 2007.     

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.     

Modeling of ignition of a solid-propellant plate by a low-temperature plasma jet

A problem of ignition of a semi-infinite solid-propellant plate by a low-temperature plasma jet formed in an igniter under a high-power electric discharge is considered. Particular attention is paid to formation and evolution of zones of forced gasification of the fuel under the action of a heat flux from the plasma to the propellant. The locations of these zones are determined by satisfying two conditions: the propellant surface reaches the gasification temperature and the heat flux exceeds a certain “threshold” value. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 23–28, January–February, 2007.     

Combustion limits of liquid monofuels

A mathematical model for steady-state nonadiabatic waves of filtration combustion of liquid monofuels in narrow tubes is proposed. Using this model, it is shown that combustion in this system can proceed in two regimes with different dominant mechanisms of heat transfer from combustion products to the preflame zone. The nature and parametric dependences of the limits of both regimes are analyzed. Calculations using the model are in good agreement with experimental data on the combustion of liquid hydrazine in narrow tubes. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 2, pp. 29–39, March–April, 2008.     

Relay-Race Combustion of Heterogeneous Systems

The structure of a combustion front in a multilayer system is analyzed taking into account radiative heat transfer in the gas interlayer. The effect of radiative heat loss on the structure and characteristics of the relay-race transformation regime is studied. The characteristics of the front and the transformation dynamics of the elements of the discrete system are determined. For the case of heat loss to the ambient medium, it is shown that the steady-state regimes are nonunique — high-temperature and low-temperature relay-race combustion fronts are detected. The absolute instability of the low-temperature regime and the dynamics of establishment of the high-temperature regime are studied in computational experiments under unsteady conditions. It is established that there is a critical heat loss above which there is flame extinction. The change in the thermokinetic characteristics during the process is analyzed, and it is shown that during combustion superadiabatic temperatures are reached. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 5, pp. 70–77, September–October, 2005.     

The evolution of 100-µm aluminum agglomerates and initially continuous aluminum particles in the flame of a model solid propellant. II. Results

A quenching/sampling method was used to study the evolution of monodisperse 100-μm aluminum agglomerates and continuous particles in the flame of combustion products — composite propellant at a pressure of 0.7–8 MPa. The particle residence time in the flame was varied in the range 6–170 msec, whereas the calculated time of their combustion was ≈25 msec. The evolution of a burning particle is related to the consumption of metallic aluminum and the deposition of oxide in the form of a cap, which, after complete combustion of aluminum, is transformed to the final spherical oxide particle. The density of the final oxide particles was measured. The ratio of the diameters and masses of the initial metal particle and the final oxide particles was determined. Data on particle fragmentation during combustion were obtained by comparing the numbers of the initial metal particles and the final oxide particles. Considerable differences in the combustion behavior of the agglomerates and continuous particles of size 100 μm were not found. It was established that the smaller the size of the burning particle the less oxide is deposited on the particle and the more oxide is carried away in the form of oxide smoke. For 100-μm particles, the fraction of deposited oxide was found to be ≈0.1 of the total mass of oxide formed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 61–71, November–December, 2008.     

Hybrid autowaves in filtration combustion of gases in a catalytic fixed bed

A mathematical model of autowave processes in a heterogeneous medium with chemical reactions in the gas phase and on the catalyst is constructed. The coefficients of heat and mass transfer between the phases and the thermal conductivity of the solid phase are determined from the current values of system parameters. The model describes three types of autowaves: waves due to filtration combustion of gases in the low-velocity regime, waves due to a catalytic reaction, and hybrid waves. The behavior of phase trajectories of the dynamic system is studied by methods of qualitative and numerical analysis, and an effective technique is developed for searching for a physically grounded autowave solution of the problem. The laws of propagation of autowaves due to filtration combustion of gases and hybrid autowaves are numerically examined, and the effect of system parameters on the basic technological characteristics of autowave processes is studied. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 2, pp. 3–13, March–April, 2008.     

Cross-flow effect on combustion of a solid propellant with harmonically varying pressure

The response function of the burning rate of a solid propellant to periodically varying pressure is found with allowance for the cross-flow (erosion) effect in the linear approximation of the phenomenological theory of nonsteady combustion. Numerical results are obtained for the simplest propellant model with the minimum number of parameters. An elementary acoustic perturbation (planar monochromatic traveling acoustic wave) is considered. The role of steady-state and nonsteady cross-flow components at low and high values of the erosion ratio is elucidated. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 4, pp. 66–72, July–August, 2007.