摘要:

A two-dimensional axisymmetrical numerical model for combustion of a carbon packed bed is presented. The model is set up by coupling a gas-particle two-phase flow inside the bed to a stagnation point flow in the boundary layer on the bed surface. The PISO algorithm is employed for the solution. Computations have been performed for the two limiting cases of a frozen and of an equilibrium flow as well as for finite homogeneous reactions rates. The combustion rates of the packing for various parameters and the parameter distribution profiles are obtained. The ignition and quenching behaviour of the packing is also investigated and discussed. The model is useful for the simulation of the smoldering combustion and its transition into flaming.

ISSN:0010-2202    

DOI:10.1080/00102209708935653

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

An analytical formula has been derived for the turbulent diffusivity at the leading edge of a stationary turbulent planar flame, described by the Bray-Moss-Libby (BML) model- The formula is derived from an expression for the turbulent burning velocity, formulated by Hakberget al. (1993) and validated in the case of negligible heat release. The turbulent diffusivity at the leading edge depends on the straining of the flame front, for the BML flamelet model, but is close to the cold flow diffusivity in the limit of high rates of strain.

ISSN:0010-2202    

DOI:10.1080/00102209708935654

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A new model is presented to construct a realistic form for the joint-scalar probability density function, as opposed to assuming its shape, with application to the moment equation solution of turbulent (non-premixed) combustion. The model approximates the thermo-chemical pdf in a general turbulent reacting flow by the scalar pdf that decayed from an initial binary field in homogeneous turbulence, parameterized by an appropriate set of lower moments. Full resolution, complex chemistry, scalar decay simulations are achieved with the computationally affordable Linear Eddy Model. The constructed pdf, in contrast to the assumed shape approach, incorporates the underlying physics of advection by all turbulent scales, molecular diffusion and chemical reaction. The constructed pdf has no adjustable constants or calibration factors, and its tabulation allows the expensive turbulence-chemistry interactions to be decoupled from the moment equation solution. In ana-prioriexperimental comparison using a two variable chemical mechanism, the constructed pdf shows qualitative superiority over a common assumed shape pdf. In application to the steady-state solution of a turbulent, hydrogen, round jet flame, the model demonstrates comparative agreement with the conventional assumed shape pdf, and predicts improved OH concentration fluctuation intensity.

ISSN:0010-2202    

DOI:10.1080/00102209708935655

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Acetylene constitutes one of the major intermediates in hydrocarbon flames and is important through its links to soot inception and mass growth processes. In the present study a detailed kinetic mechanism is developed and tested against experimental data for six lean (φ = 0.12) to sooting (φ = 2.50) laminar, premixed, low-pressure acetylene flames. Generally the agreement between computations and experiments is acceptable. It is suggested that OH attack competes with O attack as the major acetylene breakdown path in rich flames. It is further shown that the balancing of the ketyl radical destruction chemistry to a significant extent determines important flame features such as CO/C02ratio and H radical concentrations. The balancing of the methylene and methyne radical chemistry in both lean and rich environments is discussed in detail and the importance of molecular oxygen attack on3CH2is outlined. It is further shown that the present mechanism accurately predicts the qualitative evolution of methylene and methyne radical levels as a fucntion of stoichiometry. The present study incorporates the benzene oxidation mechanism by Lindstedt and Skevis (1994) and the benzene formation steps, involving isometri-zation reactions between linear and cyclic C6intermediates, reported by Leung and Lindstedt (1995). The results obtained show that for rich acetylene flames the primary path for benzene formation passes via propargyl radical recombination and that benzene levels are generally satisfactorily predicted. However, computations also indicate that for leaner flames paths involving acetylene addition ton-C4H3and 1,3-C3H5radicals become increasingly important. This study also identifies reactions where further experimental investigations are required.

ISSN:0010-2202    

DOI:10.1080/00102209708935656

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This paper examines the fundamental characteristics of a 2 MW swirl burner/furnace system and in particular the excitation of the Precessing Vortex Core (PVC) in diffusion controlled combustion with, natural gas being introduced axially on the axis of symmetry.

ISSN:0010-2202    

DOI:10.1080/00102209708935657

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The thermal nitric oxide formation in a one-dimensional tubular flow reactor has been investigated. A mixture of oxygen and nitrogen was continuously supplied into an electrically heated reactor of wall temperature within the range 1653-I798K. Five different reactor diameters made of two materials were used. The measured rate of nitric oxide formation was much higher than the calculated by the Zeldovich thermal mechanism with the available rate constants. During the experiments no fuel was supplied into the reactor, thus the concept of the Fenimore prompt NO could not be used to clear up these discrepances. Also the nitrous oxide mechanism with the recommended rate constants could not explain the results.

ISSN:0010-2202    

DOI:10.1080/00102209708935658

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Computational results are reported for structures of laminar counterflow diffusion flames between carbon monoxide and air, initially at room temperature and pressure from 1 to 100 atm, with total hydrogen-atom mole fractions in the system ranging from zero to about 0.02. All strain rates considered are within a factor of ten of the critical extinction strain rate. This critical strain rate is calculated as a function of pressure and of hydrogen content and is shown to lie below measured values under most conditions. For hydrogen-free flames, activation-energy asymptotics is employed and supports the computational results. It is reasoned that trace hydrogen amounts in air and preferential hydrogen diffusion through nonplanar diffusive-thermal instability contribute to enhanced flame robustness in the experiments, while increasing buoyant convective heat loss with increasing pressure promotes extinction at the higher pressures.

ISSN:0010-2202    

DOI:10.1080/00102209708935659

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Analytical and numerical calculations of the structure and burning rate of a deflagrating porous energetic material are presented for the limiting case of merged condensed and gas-phase reaction zones. The reaction scheme is modeled by a global two-step mechanism, applicable to certain types of degraded nitramine propellants and consisting of sequential condensed and gaseous steps. Taking into account important effects due to multiphase flow and exploiting the limit of large activation energies, a theoretical analysis may be developed based on activation-energy asymptotics. For steady, planar deflagration, this leads to an eigenvalue problem for the inner reaction-zone, the solution of which determines the burning rate. Numerical solutions give a reasonably complete description of the dependence of the structure and burning rate on the various parameters in the problem, and show excellent agreement with analytical results that are obtained in a more limited parameter regime in which most of the heat release is produced by the condensed-phase reaction and the porosity of the solid is small. These calculations indicate the significant influences of two-phase flow and the multiphase, multi-step chemistry on the deflagration structure and the burning rate, and thus serve to define an important parameter regime that supports the intrusion of the primary gas flame into the two-phase condensed decomposition region at the propellant surface.

ISSN:0010-2202    

DOI:10.1080/00102209708935660

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Modelled transport equations for the conditional mean and r.m.s. temperature increments θ before autoignition in a turbulent non-premixed flow have been developed. The model is based on the recent finding that a well-defined mixture fraction, fMR, first ignites and the concept that the conditional scalar dissipation rate, χ|fMR, controls the heat losses and hence the ignition time. By introducing the fluctuations of χ|fMR, and by modelling the conditional correlation coefficients between χ and θ from results from Direct Numerical Simulations, the model is closed and calculates autoignition times in good agreement with DNS results. Extensions to complex chemistry and the relationship with the more rigorous Conditional Moment Closure are discussed. Finally, the model is incorporated in a Favre-averaged k – ϵcode(KIVA-II)to predict autoignition of a transient fuel jet in hot air with promising results.

ISSN:0010-2202    

DOI:10.1080/00102209708935661

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A theoretical study of combustion in porous media driven by a gravity induced gas flux is conducted. Filtration of the oxidizer carrying gas arises in response to heating of the gas due to exothermic conversion of the solid fuel. Specifically, we consider a reaction front propagating through a porous matrix consisting of reactive (fuel) and inert components. Gas, consisting of both oxidizer and inert components, filters through the matrix and reacts with the solid fuel. The hot gases in the medium rise due to buoyancy, thus drawing in fresh gas from below. We employ approximate analytical methods and numerical simulations to analyze all the basic combustion phenomena, including self ignition, external ignition, both upward and downward as well as adiabatic and nonadiabatic propagating combustion waves. Our simulations also describe the dynamics of buoyancy driven combustion waves.

ISSN:0010-2202    

DOI:10.1080/00102209708935662

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor