Home – Home – Jornals – Combustion, Explosion and Shock Waves 2013 number 5

A comparison of the characteristics of the boundary layer with combustion with flame stabilization by a rib and a backward-facing step is performed. The data on the thermal boundary layer, the flame blow-off velocity, and the rate of ethanol evaporation into the air flow at a turbulence of up to 26% are obtained. It is shown that the temperature of the outer region of the boundary layer and the flame blow-off velocity behind the rib is higher than behind the backward-facing step. With both methods of flame intensity, the stabilization of evaporation corresponds to the transitional regime of mass transfer.

Results of an experimental study of the operation process in a model of an air-breathing combustor operating on heated kerosene with a supersonic air flow at the combustor entrance (Mach numbers 3 and 3.5) are presented. The tests are performed in a hotshot wind tunnel in the attached pipeline configuration with total temperatures of 2350–3250 K and pressures of 3.2–12.0 MPa. Data on ignition conditions and kerosene combustion efficiency are obtained. A principal possibility of using Ozawa's criterion and its modification for the description of the range of stable combustion of the kerosene–air mixture in a combustor duct with flame holders shaped as cavities and steps in an essentially three-dimensional flow is demonstrated.

A kinetic model is developed for calculating the emission characteristics of homogeneous combustors using methane and synthesis gas (syngas) as a fuel. The model is validated over a large set of experimental data on concentrations of NO, CO, and OH in laminar flames and in the Bunsen burner and on concentrations of OH, NO, and CO in a homogeneous combustor operating on a mixture of syngas with air. At an identical temperature of combustion products, i.e., identical thermodynamic efficiency, the combustor operating on syngas is demonstrated to emit a greater amount of NO, CO, and CO₂, as compared with the combustor operating on methane. Though the use of syngas allows one to organize stable combustion of ultralean mixtures and to obtain extremely low concentrations of NO and CO at the combustor exit ( ≈ 1–3 ppm), the amount of CO₂ in the exhaust of even extremely lean mixtures (α ≈ 3) is appreciably greater than that in the case of using methane.

For the diffusion-kinetic model of combustion of a porous carbon particle in air, the structure of the reaction zone of carbon with reactive gases inside the porous particle is studied. It is shown that, for the given kinetics of reaction of carbon with oxygen, the dependence of combustion rate of the porous particle on its internal surface area is ambiguous, which is related to the strong dependence of the reaction rate of carbon with oxygen on temperature. To obtain an unambiguous dependence, it is necessary to use the kinetic equation for the reaction rate of carbon with oxygen with an activation energy lower than the experimentally determined value.

A high-pressure combustor and a metal/steam reactor are used to simulate the two-stage combustion of hydro-reactive propellants used for a water ramjet. Raw metal powders added to the propellants are the aluminum power, magnesium powder, 50/50 aluminum–magnesium alloy (AM), and ball-milled 50/50 aluminum–magnesium alloy (b-AM), which are characterized by using scanning electron microscopy (SEM), x-ray diffraction (XRD), and simultaneous thermogravimetric analysis (TGA). The efficiencies of the Al reaction in the raw metal in heated steam and in the propellants during the two-stage combustion are calculated. The results indicate that both Mg and Al in the alloys, whether b-AM or AM, can reactcompletely in air when heated up to 950 ^oC. The XRD patterns for the combustion products of the AM and b-AM alloys in heated steam contain magnesium oxide MgO, spinel Al₂MgO₄, and Al diffraction peaks. The Al reaction efficiencies of the AM and b-AM alloy powders in heated steam are much higher than that of the Al powders. The hydroxyl-terminated polybutadiene (HTPB)-ammonium perchlorate (AP)-(b-AM)-Mg and HTPB-AP-AM-Mg propellant systems exhibit good performance in terms of the Al reaction efficiency, which are better than that of the HTPB-AP-Al-Mg and HTPB-AP-Al systems.

Methods of reconstruction of the gas flow velocity from the measured velocity of disperse phase particles are considered. It is demonstrated that available approaches are limited by the low velocity nonequilibrium of the phases. A method of correction of particle image velocimetry (PIV) data on the basis of measuring the parameters of velocity relaxation of particles is proposed for measuring the gas velocity in high-gradient flows. The method is experimentally tested in the flow behind a shock wave with submicron tracers. A possibility of using this method in flows with gas-dynamic discontinuities is demonstrated.

Combustion of a palletized mixture of titanium and carbon black placed in a quartz tube and exposed to a flow of argon or nitrogen is studied. The gas flow (cocurrent filtration) is provided by a fixed pressure gradient at the inlet and outlet of the tube, which did not exceed 1 atm. The possible modes of combustion of pelletized mixtures related to the presence of a more complex hierarchy of scales (micro, macro, and meso) compared to that of powder mixtures (micro, macro) are analyzed. A comparison is made of the burning rates of powder and palletized mixtures. An increase in the burning rate when using palletized mixtures was found experimentally. It is shown that the gas coflow through the pelletized mixture of Ti + 0.5C leads to an increase in the burning rate. It is established that the propagation of the flame front of the pelletized mixture of Ti + 0.5C in flows of nitrogen and argon is controlled by different reactions. In contrast to combustion of powder mixtures of Ti + 0.5C, in combustion of pelletized mixtures of Ti + 0.5C in a nitrogen flow, only one front is observed. It is proved that radiation plays a significant role in the propagation of the combustion front in the pelletized mixture of Ti + 0.5C.

A heterogeneous model of gasless combustion of binary disperse systems is proposed. The model combines the description of microscale processes of interphase interaction in the cell of the mixture with the macroscale description, which allows physicochemical transformations to be described on the basis of the continuum approach of mechanics of continuous media and the phase composition of the final products to be calculated. Schemes of metallochemical reactions of formation and decomposition of intermetallic phases are proposed, based on an analysis of the state diagram of the Ni–Al system. A problem of SHS wave propagation and evolution of the spatial distribution of concentrations of intermetallic phases during combustion of a mixture of Ni and Al powders is solved numerically. A two-wave structure of the combustion wave is obtained, and comparisons with experimental data are performed.

A physicomathematical formulation of the coupled gas-dynamic and geometric problem of modeling intrachamber processes and calculating the internal ballistics of nozzleless solid rocket motors is given, and a method and algorithm for solving the problem are developed. The parameters in the forward section of the motor are calculated using averaged unsteady equations of internal ballistics, and the parameters of the grain channel and the exit cone are determined using one-dimensional gas-dynamic equations in a quasi-steady formulation. The software package is verified by calculating the internal ballistics of a motor which is utilized without nozzle cluster and simulates intrachamber processes in a nozzleless solid rocket motor during the full-duration firing. The design features, motor operating parameters, and the composition characteristics influencing the energetics of propellants in nozzleless solid rocket motors are calculated. It is shown that, depending on comparison conditions (identical expansion ratios and identical profiles of the nozzle and exit cone), the specific impulse of nozzleless solid rocket motors (the main energy parameter) is slightly smaller than or nearly comparable to that of conventional solid rocket motors.

In this study, characteristics of the burning rate anomaly in composite propellant grains are investigated. The burning rate anomaly has been known as the “midweb anomaly” or the “hump effect.” This paper describes some results of an experimental study on effects of propellant formulations, casting processes, and viscosity of the propellant slurry on the phenomena. According to some past studies, it has been suggested that the geometry of the “isochrone surface” of the propellant slurry affects the local burning rate. To investigate the effect of the isochrone surface, visualization of the isochrone surface in composite propellant grains is carried out. A relation between the geometry and the local burning rate measured in motor firing tests and strand tests is proposed. As a result, besides the common static characteristics of the burning rate anomaly, i.e., the pressure hump effect and the nonisotropic characteristic of the local burning rate, a peculiar burning rate distribution in connection with the isochrone surface is obtained.

Nonlinear response functions of the burning rate to pressure oscillations are calculated for RDX and HMX in a quadratic approximation. Values of the burning surface temperature of HMX and RDX and the sensitivities of the burning rate and surface temperature to the initial temperature and pressure are given. A statistical analysis of the measurement and calculation errors is performed. The characteristics of the nonlinear response functions are considered. Some issues of the physics of combustion of nitramines are briefly discussed.

Pyrolysis of wood in vibro-fluidized beds of disperse packings of deep oxidation catalysts and inert materials is investigated. It is shown that the properties of the disperse packing material of vibro-fluidized beds do not have a significant effect on the rate and degree of conversion of wood to volatiles. The presence of catalysts in the vibro-fluidized bed leads to an increase in the amount of CO₂, CO, H₂, and CH₄ in the gas phase compared to the pyrolysis of wood in vibro-fluidized beds of inert materials. The greatest activity in the conversion of volatiles to CO₂, CO, H₂, CH₄ was found for the catalyst IK-12-73 (Mg_0.5Cu_0.5Cr₂O₄/Al₂O₃). The accumulation of carbon in IK-12-73 catalyst has little effect on the conversion of volatile substances to gaseous products. The degree of burnout of wood particles under conditions of a vibro-fluidized bed of IK-12-73 catalyst is 99.7%, which is consistent with data on the catalytic combustion of wood in the fluidized bed.

Preliminarily results on the reaction mechanism of detonation of composite emulsion explosives sensitized by MgH₂, which simultaneously plays the role of an energetic material, are presented. Compared to emulsion explosives sensitized by glass microspheres, emulsion explosives sensitized by magnesium hydride have a different reaction mechanism of detonation. The shock wave overpressure, specific impulse, shock wave energy, and bubble energy are all greatly increased with the use of MgH₂, and it is noticeable that the shock wave overpressure and shock wave energy increase by 17% and 24%, respectively. In addition, emulsion explosives sensitized by MgH₂ improve significantly in terms of detonation velocity and brisance. These emulsion explosives also meet safety requirements.

This paper presents the results of a study of the formation of localized shear in M1 copper of two types: as-received and after preloading by a quasi-entropic compression wave. The experiments were performed with hat-shaped samples using the split Hopkinson bar method. For both types of copper, dynamic compression diagrams were obtained at strain rates of 2100–2500 s^–1. The copper structure was subjected to metallographic analysis, and the effect of preliminary shock deformation on the dynamic mechanical properties of the material was estimated. It is shown that preloaded higher-strength metals with a smaller degree of strain hardening are more prone to the formation of adiabatic shear bands.