摘要:

Experiments were conducted to elucidate the jet breakup, atomization, and combustion of liquid gun propellant (LGP) sprays through the use of imaging techniques. Hydroxylammonium nitrate (HAN)-based monopropellants were injected al velocities ranging from 100 to 240 m/s through circular orifices into 33 MPa. 500°C nitrogen. Spray dynamics were recorded via high speed cinematography. Breakthroughs in the imaging of combusting sprays were achieved by side illumination of The sprays with pulsed laser light sheets and by seeding the propellants with various nitrate salts to enhance flame luminosity. The sprays were observed to ignite in the far field, and flame advanced upstream, sustained by extensive turbulent gas recirculation in the closed chamber. The experiments revealed the significance of intense, random burning in vortices. The formation and combustion of large liquid droplets in the vortices were also observed. In addition to the LGP sprays, experiments were conducted with liquids whose thermodynamic c point could be exceeded. These results suggest that the LG P spray combustion was subcritical at the ambient conditions achieved in this study. The applicability of single phase incompressible turbulent jet theory and aerodynamic theory for characterizing the dynamics of these sprays is discussed.

ISSN:0010-2202    

DOI:10.1080/00102209508907764

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Ignition is an important process in many practical devices. Most studies of ignition have been performed in stagnant flows only, using asymptotic methods, numerical techniques or experiments. Although the importance of the minimum energy and ignition delay times was shown by these studies, the effects of turbulence on these processes remain unknown. In this paper Direct Numerical Simulation (DNS) is used to provide precise data on one of the mechanisms controlling ignition, i.e., the effect of a large flow velocity at the point where the spark is produced. Spark ignition in a constant speed flow is simulated using a thermal model for the spark. Results show that a minimal power is necessary for successful ignition in nonzero mean flows. This minimal power depends nearly linearly on the flow speed (except for very low speeds). For very high heating powers the ignition delay times become independent from the flow speed. A characteristic flame radius may be used to describe the ignition limit. For zero mean flows this radius is close to the one resulting from asymptotic analysis. It increases with increasing flow speed but seems to be independent from spark duration. Finally a model based on a simplified one-dimensional configuration is developed to predict ignition in a nonzero mean flow. It is shown that this model may be used over a large range of parameters to replace DNS results.

ISSN:0010-2202    

DOI:10.1080/00102209508907765

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The combustion of condensed materials is known to admit diffusional/thermal instabilities that can lead to various oscillatory modes of burning. In the present work, asymptotic analyses are developed for nonsteady multiphase deflagration of porous energetic solids, such as degraded nitramine propellants, that experience significant gas flow in the solid preheat region and are characterized by the presence of exothermic reactions in a bubbling melt layer at their surfaces. Relative motion between the gas and condensed phases is taken into account in both regions, and the derived asymptotic model is analyzed to obtain an explicit solution for steady, planar deflagration and a dispersion relation describing its linear stability. The latter determines a pulsating neutral stability boundary in the nondimensional activation energy-disturbance wavenumber plane beyond which nonsteady. nonplanar solutions are anticipated. Focusing on the realistic limit of small ratios of gas-phase to condensed-phase density and thermal conductivity, it is shown that the effect of a nonzero porosity a, of the unburned solid material is generally destabilizing, by an.amount proportional to αs(l − αs)−1. relative to the nonporous case. This effect arises both from the lower combustion temperature of the porous energetic material and from the gas-phase diffusion of heat from the reaction zone towards the porous preheat region. These results therefore suggest that degraded propellants. which exhibit greater porosity than their undamaged counterparts, may be especially prone to nonsteady deflagration.

ISSN:0010-2202    

DOI:10.1080/00102209508907766

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Temperature and species mole fraction profiles were determined in atmospheric-pressure, premixed, laminar, flat flames of 1,1-C2H4Cl2and CH4under fuel-rich conditions at different Cl/H ratios. Samples were withdrawn from within the flame using a heated microprobe followed by gas analysis by on-line capillary gas chromatography-mass spectrometry (GC/MS). The mole fraction profiles were determined for all the reactants and for CO.CO2.HCl, H2O, H2.C2H6, C2H4, C2H2, CH3Cl, C2H3Cl, CH2CCl2, CH3CHClCH3, CH2CClCHCH2, C4H4, C4H2C6H6(benzene), and C10H8(napthalene). The role of these species, in view of the current ideas of the mechanism of combustion of 1,1-DCE is discussed and compared to earlier measurements made in l,2-C2H4C12flames.

ISSN:0010-2202    

DOI:10.1080/00102209508907767

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Ignition diagram of methane has been simulated, in three different reaction vessels, by means of a model based on detailed kinetic scheme involving 21 reaction intermediates and 88 reactions. Comparison with literature experimental diagrams appears satisfactory, all the phenomena being well or fairly predicted.

ISSN:0010-2202    

DOI:10.1080/00102209508907768

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A two-dimensional, unsteady, variable property model of the smoldering combustion of an insulating celiulosic bed, in still air, is presented. Chemical processes account for pyrolytic (endothermic)and oxidative (exothermic) degradation of cellulose to volatiles and char, followed by (exothermic) oxidation of char. Physical processes are described by energy, momentum and mass balances for the highly porous solid. The smoldering wave presents a multi-dimensional structure, affected by oxygen transport to the reaction zone (km) and heat losses from the top (hc) and bottom (hw) surfaces of the bed. In particular, as the mass transport coefficient is increased, not only reaction extent but also reaction selectivity varies, with solid pyrolysis becoming more important than solid oxidation. The dependence of the smoldering process on the chemical pathways has also been studied through variations in the pre-exponential factors of the reaction rates.

ISSN:0010-2202    

DOI:10.1080/00102209508907769

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We use recent advances in material flame spread physics to emphasize that empirical flammability indices might be replaced by fundamentally derived parameters. Flame spread rates in upward wall fire situations can be described both for charring and non-charring materials by using a characteristics length scale,lm, and a characteristic time,tp. The length scale,lm, is related to the distribution and magnitude of the heat flux from the flames to the wall surface as well as to burning material properties and mixing processes in the turbulent wall flow. The same characteristic time,tpdescribes both the spread rate, by the determination of ignition time of yet unpyrolyzed material, and the transient pyrolysis of the solid for situations where such pyrolysis can be approximated by a thermal pyrolysis model.

ISSN:0010-2202    

DOI:10.1080/00102209508907770

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Crack appearance in the combustion of a porous medium is studied both theoretically and experimentally. A simplified model that treats the phenomenon as a result of the pressure rise caused by heating is formulated and used to analyze the dynamics of crack appearance. An experimental study is carried out for the combustion of porous samples of ferrum-zirconium thermites.

ISSN:0010-2202    

DOI:10.1080/00102209508907771

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The effects S02and NO concentrations on the emissions of N20 and NO from a single particle of coal burning in a fluidized bed combustor is reported. The experiments were carried out in a laboratory scale quartz glass fluidized bed reactor in the temperature range 1023-1123 K. Since the combustion processes for the volatiles and char are consecutive, the effects of added NO and SO2occurring during the volatiles combustion can be distinguished from those occurring during the subsequent char combustion. During the combustion of the volatiles, addition of NO (250 ppm) results in an increased formation of N20, whereas the net NO formation decreases. This effect is enhanced when both NO (250 ppm) and SO2(900 ppm) are included in the fluidizing gas mixture. The effect of NO can be explained by the generally accepted gas phase mechanism of NO and N2O formation and destruction. The role of SO2is less clear, but is likely to involve its impact on the radical concentrations. During the char combustion stage, similar effects are seen, i.e., the N2O formation increases in the presence of NO, and is further increased in the presence of both NO and SO20ver the temperature range investigated, the addition of NO, resulted in an increased emission of N2O: whereas the effect of SO2decreased with increasing temperature.

ISSN:0010-2202    

DOI:10.1080/00102209508907772

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The detonation properties of several gaseous systems at elevated initial pressures were examined theoretically. The systems investigated include; hydrogen (H2), methane (CH4), ethylene (C2H4, ethane (C2H6), and propane (C3H8). Chemkin Real Gas, a computer program capable of calculating real-gas thermodynamic properties and chemical kinetic reaction rates, was expanded to utilize the ideal gas, van der Waals, Redlich-Kwong, Soave, and Peng-Robinson equations of state to describe the P-V-T behavior of the gaseous mixtures used in this investigation. The Chapman-Jouguet (CJ) theory of detonation was used to examine the variation of the detonation properties as a function of the initial pressure and temperature. The CJ numerical calculations are shown to be relatively insensitive to equation of state dependent constants (critical properties) estimated for the radical species present in the detonation products. The real-gas equations of state are shown to accurately predict the variation of the experimentally measured detonation velocities with initial pressure.

ISSN:0010-2202    

DOI:10.1080/00102209508907773

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor