Spinning waves of infiltration-mediated combustion

Infiltration-mediated combustion of powder compact with gas reagent was mathematically modeled in 3D formulation. The parameters of spinning waves were obtained as a function of ambient gas reactant pressure. At low gas pressures, combustion was found to propagate over the sample surface in the form of steady waves. At higher gas pressures, the onset of spinning waves was observed. With increasing gas pressure, parameters of these waves were found to change non-monotonically, a hot spot with a maximal temperature being inside the sample body. The results of modeling were found to qualitatively agree with the relevant experimental data.      

Effect of macroscopic heterogeneity of the medium on the solid-state combustion wave characteristics in thermally and chemically heterogeneous media

A macroscopically heterogeneous medium is modeled by a set of cylinders (rods) pressed from a mixture of solid reagents, which have coaxial cylindrical cores made from an inert material. A three-dimensional mathematical model describing propagation of combustion waves over one cylinder is studied by numerical methods. The influence of the distance between the centerlines of the neighboring cylinders and transverse sizes of the cylinders on characteristics of combustion waves propagating over a heterogeneous medium is considered. It is demonstrated that there exists an optimal distance between the inert rods, which ensures a much higher velocity of the combustion wave along the specimen than the theoretically predicted velocity of the classical combustion wave propagating over a solid specimen. New types of spinning waves are described, whose motion makes the high-temperature spot move inside the charge mixture. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 39–49, May–June, 2008.     

Modeling of solid-state combustion in thermally and chemically heterogeneous media

A thermally and chemically heterogeneous medium is modeled by a set of cylinders pressed from a mixture of solid reagents with coaxially aligned cylindrical rods made from an inert material. The change in velocity of combustion waves propagating over one cylinder is studied by numerical methods under the assumption that there is no heat release from the cylinder surface. The mean velocity of the combustion front in the specimen is shown first to decrease and then to increase with increasing thermal conductivity of the inert rod. Spinning waves are obtained in the range of low velocities of the combustion front. The laws of variations of the maximum temperature in the combustion front are determined. It is shown that the inert rod may serve as a heat sink from the hot charge mixture, may be manifested as dilution by an inert component, enhance heat recuperation, and increase the combustion-wave velocity. Introduction of the inert rod may either destabilize combustion-wave propagation in the region of a stable plane front or stabilize the combustion wave in the range of parameters where the plane front is unstable. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 6, pp. 21–30, November–December, 2007.     

Multi-spot modes of unsteady gasless combustion of a thick-walled cylinder under adiabatic conditions

Propagation of multi-spot spinning waves in a cylindrical specimen with a coaxial inner channel is studied by numerical methods under the assumption of the absence of heat removal from the outer surface of the specimen and from the channel surface. Variations of the spinning wave characteristics (longitudinal and tangential velocities, “pitch,” and maximum temperature and period) are demonstrated by an example of two-spot spinning waves for specimens of different sizes with a fixed channel radius, for specimens of a particular size with different channel radii, and for specimens with a constant wall thickness and varied inner and outer radii. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 3–14, January–February, 2007.     

Combustion-Front Non-One-Dimensionality in Single- and Double-Base Propellants

Parameters of transverse waves propagating over the surface of specimens pressed from colloxylin and double-base propellants A and N are studied. By means of microvideofilming and thermocouple measurements, it is shown that the burning sites on the specimen surface are formed by a set of transverse waves. Under atmospheric pressure, the transverse-wave front has the form of a step 0.5–1.1 mm high and decreases with increasing pressure or initial temperature of the specimen. The front propagates with variable velocities in the horizontal and vertical directions. The mean velocity of the transverse wave is three to eight times higher than the normal burning rate of the specimen as a whole (with a wide spread of local values) and increases with increasing pressure. Behind the front, combustion can be interrupted till the arrival of the next transverse wave. As in the SHS process, the reason for the emergence of the transverse waves is combustion-wave spatial instability.     

Chemical furnace for preheating in dynamic consolidation experiments

A new method is developed for specimen preheating with subsequent loading by shock waves and conservation of the products of shock consolidation; the method is based on the use of the chemical furnace energy produced by a thermal explosion or combustion of SHS systems. The use of the thermal explosion expands the range of pre-shock heating temperatures up to 3000 °C and the range of mean pressures in the cylindrical ampoule up to 25 GPa. Two variants of preheating are considered: an inert specimen is heated by heat release in a reacting SHS system or the specimen under study itself is an SHS system.Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 1, pp. 129–135, January–February, 2005.     

Unsteady Modes of Filtration Combustion

Unsteady modes of filtration combustion are studied with the use of a three-dimensional mathematical model. The model describes exothermic chemical interaction of condensed systems with active gases and formation of solid products. Unsteadiness of the filtration combustion modes is caused by instability of the plane combustion front. Formation of the periodic structure of the front of combustion of porous cylindrical samples with different cross-sectional shapes is studied. It is demonstrated that different periodic regimes of combustion-front propagation can occur under conditions of instability of the plane combustion front and deficit of the active gas mass inside the pores, depending on the cross-sectional shape of cylindrical samples and various methods of heat and mass transfer between the sample and the ambient gas medium. Periodic regimes of oscillatory (volume and surface) combustion for samples with a circular cross section, spinning mode for samples with a ring-shaped cross section, and complicated periodic regimes for samples with a rectangular cross section are obtained.     

Gasless combustion in Ti-xSi powder mixtures

Gasless combustion in the Ti-Si system was investigated by high-speed video recording (250 frames per second) followed by frame-by-frame treatment of the records. Steady-state combustion was observed when the combustion temperature, Tc, was above the melting points of Ti and Si. Two different modes of unsteady combustion were found when Tc was higher than the melting point of Si but lower than the melting point of Ti: (1) combined quasi-spinning combustion atTi excess and (2) meso-scintillating combustion at Si excess. The combustion faded when the adiabatic combustion temperature, Tad, became close to a lowest melting point in the system. All of the observed combustion modes can be rationalized in terms of a micro-heterogeneous model. Proposed are the methods for experimental diagnostics of 3D combustion waves inside an opaque sample and distinguishing between truly spinning, quasi-spinning, and other combustions modes.     

The Effect of Ambient Conditions on the Burning Rate of Gel Fuel Droplets

An experimental study was conducted to determine the dependence of the burning rate coefficient of gel fuel droplets on the pressure at different ambient oxygen nitrogen mixtures. Experiments were conducted using a pressure chamber, in which the droplet was suspended and the combustion process was video‐photographed by a high‐speed digital video camera. The tests were conducted at pressures between 0.1–4 MPa at different ambient oxygen nitrogen compositions (air, 40 % O₂ – 60 % N₂, and 60 % O₂ – 40 % N₂). The fuel was a compound of 95 % kerosene and 5 % gellant. At sub‐critical pressure conditions, the burning rate coefficient was found to increase with increasing ambient oxygen mass fraction. At supercritical conditions, no dependence of the burning rate coefficient on the ambient mixture was found. The results indicate that the burning rate coefficient depends on the oxygen partial pressure, at least at low pressures.     

Effect of Melting of Inert Components and Melt Flow on Nonstationary Combustion of Gasless Systems

The effect of the thermocapillary flow of melt of inert components of gasless mixtures on the spinning combustion regimes of cylindrical samples has been studied numerically. The change in the structure of the spinning combustion wave due to a change in the sample radius is discussed, and new spinning combustion regimes are found. Increasing the melt flow velocity leads to stabilization of the combustion, i.e., to the transition from spinning regimes to the stationary propagation of the combustion wave.     

Formation Mechanism of AlN–SiC Solid Solution by Combustion Nitridation in Si3N4–Si-Al-C System

The synthesis of solid solutions of AlN–SiC was investigated through the combustion reaction between Si₃N₄, aluminum, and carbon powders and nitrogen gas at pressures ranging from 0.1 to 6.0 MPa. The combustion reaction was initiated locally and then the wave front propagated spontaneously, passing through the cylindrical bed containing the loose powder. In the presence of Si₃N₄ as a reactant, it was feasible to synthesize solid solutions at an ambient pressure (0.1 MPa). The relationship between nitrogen pressure and full-width at half-maximum of the (110) peak of the product showed that lower pressures produced more-homogeneous solid solutions. Some aspects of formation of the AlN–SiC solid solutions were discussed with special emphasis on the influence of nitrogen pressure and reactant stoichiometry.     

Propagation of a burning front over a condensed material from a focus

Measured propagation speeds are given for the combustion front in a blind gap in a condensed substance, which are related to gap height and mean pressure. The front may propagate monotonically, which is characteristic of relatively high pressures and wide gaps, or as a result of additional burning foci arising at a certain distance from the main front. At low pressures and particularly with small gap heights, the front propagates in an unstable fashion over the surface.Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 28, No. 4, pp. 49–53, July–August, 1992.     

Influence of ambient flow conditions on the droplet atomization characteristics of dimethyl ether (DME)

This paper describes the effects of ambient flow conditions on the droplet atomization characteristics of dimethyl ether (DME) both experimentally and numerically.In this investigation, the droplet atomization of DME fuel affected by ambient flow conditions was studied in terms of droplet mean size and detected droplet percentage under elevated ambient pressures and temperatures. In order to predict the DME spray atomization, the hybrid breakup model combined with KH-RT (Kelvin-Helmholtz and Rayleigh-Taylor) and KH-DDB (Kelvin-Helmholtz and Drop Deformation Breakup) models was applied in this study.It was revealed that the spray arrival time of DME fuel under a high ambient pressure increased in accordance with the increase in ambient pressure in the spray chamber. It can be seen that more small droplets are distributed at high ambient flow pressure conditions than at atmospheric conditions. This is a consequence of enhanced atomization of DME fuel. On the other hand, when the ambient pressure increases to 2 MPa, the Sauter mean diameter (SMD) increases only slightly compared with that at 1 MPa of pressure. The SMD value of droplets is increased as ambient temperature is increased. Under the high temperature condition in the chamber, the small droplets of DME fuel evaporate quickly and mix with the ambient air. As a result, it promotes the air-fuel mixing in a combustion chamber.     

Effects of NTO Oxidizer Temperature and Pressure on Hypergolic Ignition Delay and Life Time of UDMH Organic Gel Droplet

Organic gel propellants are promising candidates for a variety of rocket motor and scramjet applications, since they are intrinsically safe and provide high performance. It is well known that organic gel fuel droplets exhibit distinct combustion characteristics compared with conventional liquid fuel droplets, and furthermore an understanding of the ignition delay and lifetime of these droplets is critical to the improvement of combustor design. In this work, investigations of the combustion of unsymmetrical dimethylhydrazine (UDMH) organic gel droplets in different nitrogen tetroxide (NTO) oxidizing atmospheres were conducted using two sets of experimental apparatus. The combustion characteristics under different conditions of temperature and pressure were compared and analyzed based on the flame shapes observed during experimentation. From these trials, an unsteady combustion model was developed and used for the numerical simulation of spray‐sized UDMH organic gel droplet combustion in an NTO atmosphere. The hypergolic ignition and burning characteristics of the organic gel droplets under conditions simulating either engine startup or steady state combustion were compared, and changes in ignition delay and droplet lifetime with ambient temperature and pressure were analyzed. The experimental and numerical results show that the UDMH organic gel droplets exhibit periodic swell‐burst behavior following the formation of an elastic film at the droplet surface. Each droplet burst results in fuel vapor ejection and flame distortion, the intensity of which declines with increasing ambient pressure. However, the swell‐burst period is extended with increasing ambient pressure, which results in potential flameout. Under conditions of low temperature and pressure similar to those at engine startup, the ignition delay and lifetime of spray‐sized gel droplets decrease with increasing temperature or pressure, although there is a sharp increase in droplet lifetime when the ambient pressure reaches a critical value associated with flameout. The ignition delay was found to be a rate‐limited phenomenon linked to the droplet heating rate. The proportion of ignition delay and droplet lifetime due to droplet heating‐up decreased with increasing temperature or decreasing pressure. Conversely, at high temperatures and pressures simulating the engine’s steady state operating conditions, the droplets were observed to flameout after several swell‐burst periods and both ignition delay and lifetime decreased monotonically with increasing temperature or pressure. The ignition delay time was determined to be rate‐limited by gas phase chemical reactions and contributed very little to the overall droplet lifetime compared with the engine startup condition.     

On the combustion mechanism of HMX

The papers on the combustion mechanism of HMX published in Russian journals in 2008–2010 are analyzed. The causes of different interpretations of experimental and theoretical results are discussed. Based on the analysis, it is concluded that the most reasonable mechanism for HMX combustion is the mechanism which assumes the leading role of the decomposition reaction in the melt at the surface temperature over a wide range of pressures.     

Combustion of energetic materials controlled by condensed-phase reactions

A combustion model controlled by condensed-phase reactions (c-phase combustion model) is considered. Based on an analysis of numerous thermocouple studies of combustion of energetic materials, it is shown that the condensed phase is heated in the combustion wave to the maximum possible temperature—the boiling point at this pressure. It is established that the combustion of representatives of the class of onium salts such as ammonium perchlorate, ammonium nitrate, ammonium dinitramide, and hydrazinium nitroformate over a wide range of pressures and initial temperatures obeys the the c-phase-model. The kinetic parameters of the burning-rate controlling reaction of these salts are given.     

Measurement of the impurity gas pressure during combustion of a “gasless” system in a long cylindrical shell

A method for estimating the maximum impurity gas pressure averaged over the specimen cross section in the combustion wave for different cross sections of a long specimen is proposed. The impurity gas pressure in a given specimen cross section is understood as the gas pressure that induces cross-sectional fracture of the specimen enclosed into a shell and certain displacement along the shell of the pressed matter portion located ahead of the combustion wave front at the instant of fracture initiation. Special composite specimens are used to ensure constant conditions of impurity gas removal from the volume and to model the SHS process and associated impurity degassing in the case of long (L/d γ 1) cylindrical specimens under the condition L = const. The model mixtures are chosen to be typical compositions for SHS: Ti+C and Ti + 2B.     

Droplet size distribution and evaporation characteristics of fuel spray by a swirl type atomizer

Spray atomization and evaporation play extremely important roles in mixture formation and combustion processes of direct injection (DI) gasoline engines. In this study, the fundamental characteristics of a swirl spray injected into a constant volume vessel are investigated by means of several laser diagnostic techniques including the laser diffraction-based method for droplet size distribution, the laser induced fluorescence-particle image velocimetry for velocity distributions of droplets and spray-induced ambient air flow, and the two-wavelength laser absorption-scattering technique for concentration distributions of liquid and vapor phases in the spray. The results show that the droplets at outer zone of the spray exhibit larger diameter than those at inner zone under both ambient pressures 0.1 and 0.4 MPa. While this can be partially attributed to the effect of spray-induced ambient air flow, the strength of ambient air flow become small when increasing the ambient pressure from 0.1 to 0.4 MPa, indicating the strong influence of spray dynamics on the droplet size distribution. In the evaporating spray, there are higher vapor concentrations near the spray axis than at peripheral zones. At 4.0 ms after start of injection, spray droplets almost completely evaporate under ambient temperature 500 K and pressure 1.0 MPa, but there are significantly amount of fuels with equivalence ratio below 0.5 in the spray. Reduction in ambient pressure promotes the air entrainment and droplet evaporation, but lowered ambient pressure results in more fuel vapor of equivalence ratio above 1.3 along the spray axis.