摘要:

This paper presents a direct simulation of low frequency combustion instabilities in premixed turbulent combustors. The simulation is based on the thin flame sheet (TFS) model proposed by Marble and Candel in their investigation of ramjet and afterburner instabilities. This model is solved numerically to allow the prediction of the flame response to any excitation. The numerical calculations are performed with an original implicit extension of a two phase Row TRAC method. The numerical properties of the scheme (dispersion, dissipation) are obtained by comparison with exact solutions in the case of acoustic wave propagations in isothermal ducts. Results obtained for reactive Rows show that stability is mainly determined by the steady flow configuration and by the interaction parameter introduced to represent the effects of pressure on the local normal flame speed. Quarter-wave excitations induce the strongest instabilities when convective perturbations travelling on the flame front couple with the acoustic oscillation. A criterion for quarter-wave instability is derived and compared with experimental results.

ISSN:0010-2202    

DOI:10.1080/00102208808915760

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The combustion of an uncoated boron particle in an oxidizing environment is theoretically studied. Chemical reactions considered include the surfaceB-O2andB-B2O3reactions and the gas-phaseBO-O2reaction. Generalized coupling functions are identified and three limiting solutions respectively describing frozen, detached flame-sheet. and attached flame-sheet combustion for the gas-phase reaction are derived. Effects of reactivities, surface temperature, pressure. particle size, and oxide condensation are studied, while the importance of finite rale surface reactions are also demonstrated. Approximate expressions are derived for the particle burning time; the predictions compare well with the experimental data of Macek.

ISSN:0010-2202    

DOI:10.1080/00102208808915761

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

When "containerized" liquid wastes, bound on sorbents. are introduced into a rotary kiln in a batch mode, transient phenomena in-volving heat transfer into, and waste mass transfer out of, the sorbent can oromote the raoid release of waste vaoor into the kiln environment. This raoid vaoor release can cause depletion and displacement of the excess oxygen from the primary flame, and formation ofa "puff," which can result in a temporary Failure orthe incinerator system. Parametric studies on a specially designed rotary kiln incinerator simulator showed that puffs are easily generated even with very small quantities of surrogate wastes and at excess air values exceeding 100 percent. Furthermore, their magnitudes and intensities increase with increasing kiln temperature and kiln rotation speed. A theoretical model describing simultaneous heat and mass transfer through a sorbent aggregate, coupled with vapor pressure driven waste vaporization within the sorbent aggregate, was combined with a fragmcntauon model and was able qualitatively to predict experimentally observed effects relating to puff duration, kiln rotation speed, kiln temperature. and stoichiometric oxygen requirement of the surrogate waste. Extrapolation of the model to conditions beyond the experimental test matrix indicated very strong influences of waste boiling point (and consequently latent heat), and of sorbent parameters such as overall void fraction in the container and the sorption characteristics of the individual sorbent particles. The theoretical results support the experimental data from the rotary kiln incinerator simulator and suggest that the experimentally observed trends have general practical validity. The model constitutes a first step in being able to rank wastes and sorbents with respect to their propensity to produce puffs.

ISSN:0010-2202    

DOI:10.1080/00102208808915762

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The detailed structure of laminar opposed-flow diffusion flames with a fuel composition of 40% CO, 30% H2, and 30% N2and an oxidizer composition of 79% N2and 21% O2has been calculated over a wide range of stretch conditions (α = 0.1-5000s-1) using 32 elcmenlal chemical reactions and realistic transport properties. Previously, when compared to experimental measurements at χ = 70, 180, and 330s-', this model was found to predict peak flame temperatures within 150K. widths of temperature profiles to 20%, and positions of temperature maxima to 0.1 cm. The present paper presents calculations over a wider range of stretch (χ = 0.1-5000s-1) and quantifies the effect of stretch on fundamental flame processes such as preferential diffusion. overlap of fuel and air. superequilibrium radical formation. deviations from partial equilibrium, and extinction. Preferential diffusion of H2leads to superadiabatic temperatures in the α = 0.1 s-1flame. Pronounced nonequilibrium effects occur even in the α = 0.1 s-1flame and increase markedly in flames with higher stretch. Water gas equilibria occur only when the temperatures are larger than 1800 K, with deviations greater than a factor of 20 in low temperitlure. lean flame zones. Results are compared to previous calculations on stretched flames with H2and CH4fucls. The complexity of the finite rate chemical processes shown here in the laminar opposed-flow CO/H2/N2diffusion flames suggests that the assumptions of full or partial equilibrium used in previous models of turbulent CO/H2/N2jet diffusion flames are inadequate. The present results provide the basis for turbulent combustion models using the laminar flamclet approach which may give a more realistic description of turbulence-chemistry interactions.

ISSN:0010-2202    

DOI:10.1080/00102208808915763

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor