On the structure of the combustion wave of nitroglycerine-based propellants

The parameters of the temperature distribution in the combustion wave of nitroglycerin-based propellants N and NB are analyzed and compared. The aim of the study is to explain the known experimental fact that the size of hotspots and the critical quenching diameter for propellant NB (more rapidly burning) are larger than those for propellant N. It is demonstrated that, at a given burning rate, the burning surface temperature, heat conduction zone thickness, temperature gradient near the burning surface, and the dark zone temperature for propellants N and NB are the same, but the fizz zone thickness for NB is approximately twice as wide as that for N. The dependence of the ratio of the hotspot size to the fizz zone thickness is described by a single power law for both propellants. It is also shown that the hotspot size can be defined as the distance between two consecutive transverse waves, which, in turn, is determined by the delay in the initiation of each following wave.     

Cellular hotspot-type structure of the combustion wave of perchlorate ammonium

The non-one-dimensionality of the combustion wave of ammonium perchlorate was studied. An analysis of the surface of samples quenched by pressure drop and by coolant spray injection onto the burning surface, as well as extinction of samples with a diameter close to the quenching diameter, showed that, at pressures providing steady burning (2.5–7.0 MPa), the burning surface is nonplanar and nonuniform, but features a cellular hotspot-type pattern. Hotspots emerge and disappear at the burning surface, with their diameter and depth being pressure-dependent. The maximum diameter of hotspots depends on the pressure. It exceeds the characteristic thickness of the heat conduction zone by about two orders of magnitude but is smaller than the critical quenching diameter approximately twofold. Measurements of the electric conductivity of the surface layer and of the local luminosity of the flame revealed the existence of pulsations of approximately the same frequency. The period of these pulsations is close to the lifetime of hotspots. It was demonstrated that extinction curves for ammonium perchlorate obtained using pressure differ from the analogous curves for nitroglycerine-based propellants.     

Influence of water mist on propagation and suppression of laminar premixed flame

The combustion of premixed gas mixtures containing micro droplets of water was studied using one-dimensional approximation. The dependencies of the burning velocity and flammability limits on the initial conditions and on the properties of liquid droplets were analyzed. Effects of droplet size and concentration of added liquid were studied. It was demonstrated that the droplets with smaller diameters are more effective in reducing the flame velocity. For droplets vaporizing in the reaction zone, the burning velocity is independent of droplet size, and it depends only on the concentration of added liquid. With further increase of the droplet diameter the droplets are passing through the reaction zone with completion of vaporization in the combustion products. It was demonstrated that for droplets above a certain size there are two stable stationary modes of flame propagation with transition of hysteresis type. The critical conditions of the transition are due to the appearance of the temperature maximum at the flame front and the temperature gradient with heat losses from the reaction zone to the products, as a result of droplet vaporization passing through the reaction zone. The critical conditions are similar to the critical conditions of the classical flammability limits of flame with the thermal mechanism of flame propagation. The maximum decrease in the burning velocity and decrease in the combustion temperature at the critical turning point corresponds to predictions of the classical theories of flammability limits of Zel'dovich and Spalding. The stability analysis of stationary modes of flame propagation in the presence of water mist showed the lack of oscillatory processes in the frames of the assumed model.     

Critical condition relevant to the extinction of a carbon particle in the course of combustion

In order to examine extinction of carbon particle in the course of combustion, an attempt has been made to obtain its critical condition. Main concern has been put on the particle extinction in the lower limit that can occur at the end of the particle combustion although not only the lower limit but also the upper limit of the critical conditions has been obtained. By conducting asymptotics, with focusing on the temporal variation of the particle temperature, an analytical expression has been obtained for the limit of the particle diameter, as functions of the pressure ratio, oxygen mass-fraction, ambient temperature, and/or radiative heat flux. An approximate expression is also obtained from the analytical solution. It is found that the approximate expression can fairly represent the limits. In addition, use has been made of the Arrhenius plot of a comprehensive parameter, consisting of the particle diameter and pressure ratio, in order to identify regions for the particle combustion sustained, which are next to those for the particle extinction. Comparisons have also been conducted by use of experimental data in the literature, with presenting a fair degree of agreement, as far as the trend and approximate magnitude are concerned. It has been confirmed that the formulation has captured the essential feature, that the reduction in particle size does not necessarily favor the particle combustion, and that the particle extinction can occur when the particle diameter is reduced to the critical value.     

Thermal stability of the burning of the base subsystem of components of a composite propellant

The problem of the thermal stability of the base subsystem of components of a composite solid propellant with respect to the introduction of heat-absorbing agents, inhibitors, and burning rate catalysts was considered. It was demonstrated that the response of the base subsystem to the introduction in it of additives is equivalent to a change in the initial temperature of the propellant, i.e., is determined by its burning rate temperature sensitivity. The competition of the fuel components for oxidative species and the role of this phenomenon in the formation of the structure of the combustion wave were examined. Extensive experimental data on the effect of heterogeneous fillers of various natures on the burning rate of the composite system were obtained.     

Model of the heterogeneous catalyst of the combustion of condensed systems

A model of the heterogeneous catalysis of the combustion of condensed systems based on an analysis of local changes in the burning velocity of the condensed phase of the propellant was developed. The model expresses the catalyst efficiency through the concentration and particle size of the catalyst and the uncatalyzed propellant burning velocity. Two experimentally determined parameters were introduced to characterize the effect of the chemical properties of the propellant system. The catalysis efficiency was demonstrated to tend to finite limits at d → 0 and U ₀ → 0. This model for the first time made it possible to correctly describe a wide variety of experimentally observed regularities.     

Combustion of a propellant and its extinction upon rapid depressurization: A comparison of theory and experiment

The combustion of a nitroglycerin-based propellant and its extinction as a result of exponential pressure decay was studied on a bomb-receiver setup. At moderate depths and rates of pressure decay, the transient process ends with adoption of a new steady-state combustion mode. There are critical values of the depth and rate of pressure decay above which combustion ceases. Numerical simulations are used to determine a relationship between the critical values of these quantities (extinction curves). The calculations are performed within the framework of the phenomenological theory of unsteady combustion of energetic condensed systems using known steady-state laws of propellant combustion. A comparison of experimental and theoretical results demonstrates their satisfactory agreement.     

Physicochemical characteristics of the components of energetic condensed systems

The physicochemical characteristics of nanosized energetic materials (aluminum, ammonium nitrate, RDX) are studied and the burning rate of energetic condensed systems containing nanosized components is determined. The morphology, chemical purity, and thermal properties of nanosized powders produced by vacuum and plasma recondensation were examined. The materials obtained are identical to the precursors and are characterized by an increased reactivity compared to their microsized counterparts. At a pressure of 100 atm, a monopropellant prepared from nanosized RDX particles is found to have twice as high the burning velocity as a microsized RDX propellant. For nanoaluminum-ammonium perchlorate compositions, the burning velocity is demonstrated to be an order of magnitude higher than that for similar compositions with microsized aluminum. An important feature of nanoaluminum-based formulations is a decrease in the degree of agglomeration of metal fuel products and, hence, in two-phase losses in solid-propellant rocket engines.     

Quench collection of nano-aluminium agglomerates from combustion of sandwiches and propellants

An experimental investigation has been carried out to measure the size of nano-aluminium agglomerates emerging from the combustion of nano-aluminized sandwiches and composite solid propellants. Nano-aluminium of median size of 50 nm produced in-house by the electrical wire explosion method is used in these samples. Propellants with different sizes of coarse and fine ammonium perchlorate are considered. Surface features of sandwiches and a propellant whose burning was interrupted by rapid depressurization are examined in a scanning electron microscope. The combustion products of the sandwiches and propellants are quenched close to the burning surface and collected in a quench collection set-up. The surface features of rapid-depressurization quenched sandwiches exhibit relatively large nano-aluminium clusters—of the order a few micrometres—particularly in the binder lamina. Quench-collected nano-aluminium exhibits significant agglomeration, but only a small fraction of the agglomerates are in the 1–3 μm range, except for both the coarse and fine AP particles used in the formulation being large, but even there they do not exceed ∼5 μm in size. This is expected to be benign for reduced smoke propellant applications from exhaust signature point of view, and to decrease the specific impulse losses without sacrificing the energetics of the propellant.     

Concept and combustion characteristics of ultra-micro combustors with premixed flame

Under micro-scale combustion influenced by quenching distance, high heat loss, shortened diffusion characteristic time, and flow laminarization, we clarified the most important issues for the combustor of ultra-micro gas turbines (UMGT), such as high space heating rate, low pressure loss, and premixed combustion. The stability behavior of single flames stabilized on top of micro tubes was examined using premixtures of air with hydrogen, methane, and propane to understand the basic combustion behavior of micro premixed flames. When micro tube inner diameters were smaller than 0.4 mm, all of the fuels exhibited critical equivalence ratios in fuel-rich regions, below which no flame formed, and above which the two stability limits of blow-off and extinction appeared at a certain equivalence ratio. The extinction limit for very fuel-rich premixtures was due to heat loss to the surrounding air and the tube. The extinction limit for more diluted fuel-rich premixtures was due to leakage of unburned fuel under the flame base. This clarification and the results of micro flame analysis led to a flat-flame burning method. For hydrogen, a prototype of a flat-flame ultra-micro combustor with a volume of 0.067 cm³ was made and tested. The flame stability region satisfied the optimum operation region of the UMGT with a 16 W output. The temperatures in the combustion chamber were sufficiently high, and the combustion efficiency achieved was more than 99.2%. For methane, the effects on flame stability of an upper wall in the combustion chamber were examined. The results can be explained by the heat loss and flame stretch.     

微尺度扩散火焰特性的数值解析

本文以均匀空气流中圆管形成的甲烷射流扩散火焰为对象,用数值解析的方法研究了微尺度扩散火焰的火焰结构和燃烧特性。燃烧反应采用甲烷/空气一步总包反应,喷管壁面采用绝热条件。在Re一定情况下,改变喷口尺寸和喷口流速考察了微扩散火焰的结构和火焰熄灭的尺度效应。数值结果表明,随着喷口直径的增大,微火焰的上方出现回流; Re=12条件下,在喷口直径=0.07 mm时存在熄灭极限;稳定燃烧区的最小发热率约为0.5 W;微尺度条件下,Da数对火焰结构和火焰的熄灭有一定的影响。     

Burning of magnesium particles under zero-gravity and convective blow conditions

The combustion of Mg particles 400 and 600 μm in diameter in O₂-N₂, O₂-Ar, and CO₂ media at 0.5–4.0 MPa under zero-gravity and free-fall conditions were experimental studied. The burning time, light emission flux from the luminous zone, temperature and size of the zone, modes of combustion, and dispersity and character of the condensed products were determined. The burning time laws were obtained, and the main physical regularities of the combustion of magnesium particle were established.     

Formation of dynamic spatial flame structures for gas burning in microchannels with temperature gradients on walls

The thermal-diffusive model was applied to the problem of flame propagation in a microchannel with controlled temperature distribution in the walls; this demonstrated the possibility of formation of oscillating or rotating spatial flame structures, which were described previously in experimental works on microcombustion. Two cases were considered: combustion in a rectangular channel and in the clearance between two disks with radial feeding of premixture. In both cases, the typical across size of the channel was lower than the critical diameter determined with respect to the ambient temperature. The gas flow was assigned and described by the Poiseuille-flow velocity profile. Formation of oscillating flame in a rectangular channel and rotating patterns in a radial channel was observed for a certain range of gas flow rate. At low flow rates beyond this range, repetitive ignition/extinction of flame took place; at high flow rate we observed a steady flame mode. Formation of these special flame structures is related to heat transfer between gas and hot walls of the channel, as well as to velocity maldistribution in the microchannel.     

The role of radiative losses in self-extinguishing and self-wrinkling flames

We have developed a general theory of non-adiabatic premixed flames that is valid for flames of arbitrary shape that fully accounts for the hydrodynamic and diffusive-thermal processes, and incorporates the effects of volumetric heat losses. The model is used to describe aspects of experimentally observed phenomena of self-extinguishing (SEFs) and self-wrinkling flames (SWFs), in which radiative heat losses play an important role. SEFs are spherical flames that propagate considerable distances in sub-limit conditions before suddenly extinguishing. Our results capture many aspects of this phenomenon including an explicit determination of flame size and propagation speed at quenching. SWFs are hydrodynamically unstable flames in which cells spontaneously appear on the flame surface once the flame reaches a critical size. Our results yield expressions of the critical flame size at the onset of wrinkling and expected cell size beyond the stability threshold. The various possible burning regimes are mapped out in parameter space.     

Theoretical and experimental studies on mesoscale flame propagation and extinction

Mesoscale flame propagation and extinction of premixed flames in channels are investigated theoretically and experimentally. Emphasis is placed on the effect of wall heat loss and the wall–flame interaction via heat recirculation. At first, an analytical solution of flame speed in mesoscale channels is obtained. The results showed that channel width, flow velocity, and wall thermal properties have dramatic effects on the flame propagation and lead to multiple flame regimes and extinction limits. With the decrease in channel width, there exist two distinct flame regimes, a fast burning regime and a slow burning regime. The existence of the new flame regime and its extended flammability limit render the classical quenching diameter inapplicable. Furthermore, the results showed that at optimum conditions of flow velocity and wall thermal properties, mesoscale flames can propagate faster than the adiabatic flame. Second, numerical simulation with detailed chemistry demonstrated the existence of multiple flame regimes. The results also showed that there is a non-linear dependence of the flame speed on equivalence ratio. Moreover, it is shown that the Nusselt number has a significant impact on this non-linear dependence. Finally, the non-linear dependence of flame speed on equivalence ratio for both flame regimes is measured using a C₃H₈–air mixture. The results are in good agreement with the theory and numerical simulation.     

Edge flames stabilized in a non-premixed microcombustor

The dynamics of an edge flame confined in a non-premixed microcombustor model is studied numerically within the context of a diffusive-thermal model. Fuel and oxidizer, separated upstream by a thin plate, flow through a channel with a prescribed velocity. At the tip of the plate, the fuel and oxidizer mix and, when ignited, an edge flame is sustained at some distance from the plate. The objective in this work is to consider the effects of confinement, differential diffusion, and heat loss on the dynamics of an edge flame in a narrow channel. We consider a wide range of channel widths and allow for changing Lewis numbers, and both adiabatic conditions and heat losses along the channel walls. The results illustrate how the flame shape and standoff distance are affected by the channel width, by mixture composition through variations in Lewis numbers and by heat losses. Conditions for flame stabilization, flame oscillations and flame extinction or blowoff are predicted.     

Plasma losses in a toroidal theta-pinch with superposed hexapole

A toroidal theta-pinch discharge with superposed non-helical hexapole field is investigated. The characteristic data of the discharge are: major diameter 52 cm, minor inner diameter of the vacuum vessel 6 cm, maximum magnetic field between 10 and 21 kG, rise time (quarter-cycle) 3.0 μs, maximum temperature between 40 and 100 eV, maximum density between 1 and 3×10¹⁶ cm^?3, beta-values between 0.3 and 1. The plasma confinement times are determined by measuring particle density, temperature and plasma radius. The confinement times are compared with those of models which account for cusp losses, resistive losses, and Bohm diffusion. Measured confinement times are consistent with those expected from cusp losses with a cusp slit-width of one ion gyro-radius. Above electron temperatures of 20 eV, resistive losses are negligible. Bohm diffusion is not consistent with measurements, but is of the same order of magnitude.     

Exploring a critical diameter for thermo-acoustic instability of downward propagating flames in tubes

Thermo-acoustic oscillations are observed when a flame ignited at open end of a tube propagates towards the closed end due to interaction between unsteady heat release rate fluctuations from flame and acoustic fluctuations. In our past work, it was found that thermo-acoustic instability increases with decreasing diameter from 7.0 cm to 3.0 cm. A recent study in flame propagation in Hele–Shaw cells showed that thermo-acoustic instability is not observed for plate separation less than or equal to 0.4 cm. Thermoacoustic instabilities cannot be observed in very narrow tubes due to excessive damping from the wall. This opens up the possibility of a critical diameter where thermo-acoustic instability would be maximum. In this work we perform flame propagation experiments with diameter of combustion tube in the range 0.5 cm to 3 cm for a fixed length of 70.2 cm. It was found that thermo-acoustic parametric instability begins at lowest laminar burning velocity when the diameter is around 1.0 cm. This diameter is termed as critical diameter. Critical diameter is found to be independent of Lewis number of mixtures. Existence of a critical diameter is thus proved experimentally. Growth rates of primary instability increase with decreasing diameter and show a maximum around the critical diameter and decrease with further decrease in tube diameter. But, growth rates of secondary instability as well as maximum pressure fluctuation amplitude decreases continuously with decreasing diameter. Mechanisms responsible for these observations and existence of a critical diameter are clarified.     

Combustion of volatile condensed systems during pressure decay

The combustion and extinction of volatile condensed systems during pressure decay are studied. In contrast to the existing theories of this phenomenon, where only the thermal inertia of the condensed phase (t _c -approximation) is considered, an analysis of the time-dependent behavior of the gas phase is also included. Extinction curves, i.e., dependence between the pressure decay depth and the pressure decay rate at which the burning of the propellant ceases are calculated. The analysis is performed within the framework of the Belyaev model. A comparison of the results with calculations based on the t _c -approximation shows that, at high pressures, the thermal inertia of the gas phase is of considerable importance.     

Nonane droplet combustion with and without buoyant convection: Flame structure, burning rate and extinction in air and helium

The burning and extinction characteristics of isolated small nonane droplets are examined in a buoyant convective environment and in an environment with no external axial convection (as created by doing experiments at low gravity) to promote spherical droplet flames. The ambience is air and a mixture of 30%O₂/70%He to assess the influence of soot formation. The initial droplet diameter (D_o) ranges from 0.4 to 0.95 mm. Measurements are reported of the extinction diameter and time to extinction, and of the evolution of droplet diameter, flame diameter, soot shell diameter, burning rate, and broadband radiative emissions.In a buoyancy-free environment for air larger droplets burn slower than smaller droplets for the range of D_o examined, which is attributed to the influence of soot. In the presence of a buoyant flow in air, no influence of D_o is observed on the burning rate while the buoyant flames are still heavily sooting. The effect of D_o is believed to be due to a combination of dominance of the nonluminous, nonsooting, portion of the buoyant flame around the forward half of the droplet on heat transport and the secondary role of the luminous wake portion of the flame. In a non-sooting helium inert at low gravity, no effect of D_o is found on the evolution of droplet diameter.Flame extinction is observed only in the 30%O₂/70%He ambience. For all of the observations, extinction appears to occur before the disappearance of the droplet which is then followed by a period of evaporation. The extinction diameter and time to extinction increases with D_o and an empirical correlation is presented for these two variables.