摘要:

Dynamics of laminar flames in closed tubes is studied by means of two-dimensional numerical simulations taking into account thermal conduction, fuel diffusion, viscosity and chemical kinetics. Development of the hydrodynamic instability of a flame front is investigated for flames with chemical reactions of the first and the third order. We found that for a flame with the first order reaction the hydrodynamic instbility is strongly reduced or even suppressed in sufficiently short tubes. Unlike this, in the case of a flame with the third order reaction the instability is enhanced due to significant increase of the normal velocity of the planar flame under confinement. The instability development for flames of both first and third order reactions is strongly affected by acoustic waves generated by the flame in a closed chamber. Particularly, a weak shock colliding with the flame front may lead to temporary stabilization of the flame instability. On the contrary,when flame comes to the end of the tube the acoustic waves may cause significant increase of the flame instability. We studied a possibility of the detonation ignition ahead of the flame front as well. We found that the detonation can be ignited at the far end of the tube by the weak shocks and sound waves generated by the flame in a closed tube. Triggering of the detonation ahead of the flame propagating in a closed tube is related to the knock problem in spark-ignition engines.

ISSN:0010-2202    

DOI:10.1080/00102209808924172

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The paper presents some theoretical and experimental data on self–propagating high-temperature synthesis (SHS) of high–porosity materials. A classification of SHS systems by their sensitivity to deformation has been proposed. The classification takes into account the formation and lifetime of the volume fraction of melt in the combustion wave. To produce high-porosity cermet foams, a system should meet the following requirements: it has (i) to contain an optimal amount of liquid and solid phases in the product, (ii) to evolve impurity-born gases during formation of the liquid phase without breakdown of steady combustion, (iii) to burn at a high velocity. Systems with the melt content about 80 vol.%, were found to form closed spherical macropores. High–porosity ceramics (TiC + TiB2) and cermet foams(NiAl–TiCo.6) were obtained by SHS. The porous ceramics contained largely open pores while the cermets closed pores. The effect of gravity on the formation of porous SHS was investigated. SHS was found to yield on inhomogeneous product in a vertically positioned sample and a homogeneous product in horizontally positioned sample.

ISSN:0010-2202    

DOI:10.1080/00102209808924173

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The complete vaporization of a three-dimensional submicron liquid oxygen (LOX) droplet into quiescent environments comprised of either hydrogen or helium has been simulated using molecular dynamics. The environment pressures and temperatures ranged from 2 to 20 MPa and 200 to 300 K, respectively. Droplet vaporization rates and thermodynamic property histories were obtained. Results show that at low to moderate pressures the droplet remains spherical and retains a distinct temperature profile throughout the entire vaporization process. In contrast, at very high pressures, the droplet vaporizes in a could-like manner with vanishing surface tension. Droplet temperature histories show that the quasi-steady approximation is valid even when the environment pressure is above the critical pressure of either chemical species. In addition, the environment pressures required to cause the observed transition in vaporization behavior were well above the pure species' critical pressures, which confirms that many of the assumptions used in subcritical droplet evaporation analyses may be valid at pressures much higher than the critical pressure of oxygen. It was also found that significant concentration of the gas phase species was present in the droplet even at moderate pressures.

ISSN:0010-2202    

DOI:10.1080/00102209808924174

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The joint scalar-velocity probability density function (pdf) modelling approach is here applied to the problem of mixing and chemical reaction in a scalar mixing layer. The cases considered are of direct relevance to the modelling of turbulent combustion applications and the main objectives of the work are: (i) To assess the applicability of joint scalar-velocity pdf solution methods in the context of realistic low Damkohler number turbulent reacting flows. (ii) To investigate a multi-scalar extension of a modified binomial Langevin scalar mixing model of Valifio and Dopazo. The latter is here used in conjunction with the generalised Langevin model for velocity statistics of Haworth and Pope. The experimental data sets by Bilger et al., obtained in chemically reacting flows with Darnköhler numbers of the order unity have been used to validate the approach. The excellent agreement obtained for both conserved and reacting scalars indicates that the reaction -diffusion coupling is accurately represented by the binomial Langevin mixing model for this flow. Comparisons with linear-eddy model results or Kerstein are also presented.

ISSN:0010-2202    

DOI:10.1080/00102209808924175

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Three different air-assist nozzles were investigated with a Phase-Doppler-Analyzer and high speed photography in a cold environment and then used in a pilot scale furnace at a thermal input of 200 kW. Waste water was simulated by a water-glycol mixture. Measurements showed temperatures above 1100°C and homogeneous oxygen concentrations except close to the burner. With such conditions evaporation and, hence, atomization quality set the limits on burnout, as oxidizing reactions are fast. The Sauter Mean Diameter (SMD) did not correlate closely with burnout since SMD indicates a mean droplet size while performance is limited by the number oflarge droplets present. Betteragreement was achieved for D90%. which is a parameter that represents the upper end of the size distribution. High speed photography is recommended to detect and measure the largest droplets in the spray. Their size correlated closely with the onset of incomplete burnout and is, therefore, an appropriate parameter to characterize atomization quality with respect to burnout.

ISSN:0010-2202    

DOI:10.1080/00102209808924176

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The behavior of an energetic solid subjected to shear is modelled. Data from torsional splitHopkinson bar (TSHB) experiments are first reduced to calibrate a constitutive model for stress, accounting for the effects of strain and strain rate hardening, in an inert simulant, Mock 900–20, of the high explosive LX–14. This stress model is employed in a transient one dimensional model which includes effects of thermal softening, thermal diffusion, plastic heating, and exothermic reaction. For an event with an average strain rate of 2.8×103S-1, reactive shear localization with local strain rates and temperatures in excess of 5×104s-1and 5000K, respectively, is predicted. It is predicted that within a domain of length comparable to that of the largest heterogeneities, around 250 µm, reaction evolves over sub-nanosecond times scales. Localization and ignition are found to be sensitive to changes in mechanical properties, specific heat. and activation energy, and insensitive to changes in kinetic rate constant, heat release, and thermal conductivity.

ISSN:0010-2202    

DOI:10.1080/00102209808924177

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Supercritical pyrolysis mechanisms of potential endothermic fuels and hydrogen donor additives decahydronaphthalene (decalin) and tetrahydronaphthalene (tetralin) were examined in a specially constructed silica–lined flow reactor in which pressure and temperature were varied independently. Pressure and temperature were varied over ranges of 700—810 K and 0.2 to 10 MPa. Under these conditions, major products of supercritical decalin pyrolysis included: light alkanes and alkenes, methylhexahydroindane, indene, methylcyclohexenes, and indane. Major products of supercritical tetralin pyrolysis included: naphthalene, methylindane, ethane, methane, ethene and phenyl butane. The major products found in these experiments are contrasted with those found in gas phase pyrolysis studies. Quantification of the major products indicated that C6to C5ring contraction was found to occur preferentially with increasing pressure. The formation of these more compact products is qualitatively consistent with a reaction model that incorporates cage effects.The addition of hydrogen donors decalin and tetralin to decane resulted in a slight increase in the thermal stability of decane. A 30% conversion of decane in the decane/H–donor mixtures was observed to occur at approximately 790 K versus 770 K in the neat decane for a fixed residence time and pressure. However, the real potential of these H–donor additives with regard to fuel thermal stability may be in increasing the overall volatility of the products of pyrolysis by promoting the formation of saturated products. Finally, global kinetic parameters are presented for each neat fuel and mixture

ISSN:0010-2202    

DOI:10.1080/00102209808924178

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor