摘要:

The nonpremixed burning of a condensed fuel with an oxidizer flow, driven by the buoyancy force, adjacent to the wall in porous media is analyzed in the reaction-sheet limit. Because of the distinct flow characteristics in porous media, similarity exists when the problem is scaled by x1/2, where x is the spatial coordinate along the wall, instead of x1/4as for the conventional gaseous natural convective flow. Results show that the profiles of the temperature and reactants are qualitatively similar to those of the gaseous combustion situation. Quantitatively, the flame temperature is reduced as a consequence of the high effective thermal conductivity, which results in high Lewis numbers. The standoff distance of the reaction sheet is relatively insensitive to the preferential diffusion of the oxidizer but is decresed with increasing Lewis number of the fuel vapor. Interestingly, the vertical induced flow velocity may have one maximum, which is located at the reaction sheet, or two maxima, depending upon the temperature dependence of the flow viscosity. The fuel consumption rate, as a result of the combined effect of the flame temperature and standofT distance, can be increased or decreased when varying the Lewis numbers.

ISSN:0010-2202    

DOI:10.1080/00102209408935434

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Instabilities occurring in downwards propagating flames (in plane geometries) are studied in two different models: the complete equations of isobaric flames with one global exothermic reaction and a model equation: a modified version of Michelson-Sivashinsky equation (including gravity effects). Results obtained in both models are compared, showing a good qualitative agreement for a gravity not too small.

ISSN:0010-2202    

DOI:10.1080/00102209408935435

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

An extensive set offlat laminar premixedNH3/H2/O2,NH3,/NO/H2/O2and NH3O2flames have been investigated by detailed chemical kinetic modelling to facilitate the construction of a reaction mechanism capable of satisfactory predictions for a wide range of flames. Available information for the rate coefficients of all the reactions in the detailed mechanism has been reviewed. An extensive sensitivity analysis has been performed to distinguish the reactions of greatest importance to the formation and destruction of nitric oxide. The relative significance of the different NO formation channels is found to depend entirely on the flame conditions: (i) for all flames the reaction of NH2with the O radical is found to be significant; (ii) in pure ammonia flames the reaction of the NH radical with OH becomes important; (iii) in hydrogen flames with ammonia and ammonia with nitric oxide dopants the Zel'dovich mechanism becomes increasingly significant with increasing fuel concentrations. The conversion of NO to N2is dominated by reactions involving the NH2and N radicals with NH providing a secondary path. In pure ammonia and doped lean hydrogen flames the reaction of NO with NH2 becomes the major NO conversion path. In doped stoichiometric and rich hydrogen flames the reaction of NO with N is dominant.

ISSN:0010-2202    

DOI:10.1080/00102209408935436

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Reduced reaction mechanisms are essential for the inclusion of direct kinetic effects into turbulent flame computations for practical problems. In the present paper reduced seven step, five step and four step reaction mechanisms for ammonia oxidation have been developed by the study ofpremixed laminar flames. A systematic reduction technique using steady state assumptions for intermediaries and radicals has been used to obtain the reduced schemes from a comprehensive detailed kinetic mechanism. Special emphasis has been given to the formation and destruction of NO and N2O

ISSN:0010-2202    

DOI:10.1080/00102209408935437

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

In relation to the criterion for ignition of a combustible gas flowing at high speed over a wedge at a high surface temperature, the method of large activation energy asymptotics was employed to investigate the characteristics of ignition and to derive an explicit expression for an ignition criterion including the effects of general Prandtl (Pr) number and wedge angle. Based on this criterion, the regions of ignition and lack of ignition were identified over the surface of the wedge for various wedge angles.

ISSN:0010-2202    

DOI:10.1080/00102209408935438

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We formulate and analyze a model describing the combustion of porous condensed materials in which a reactant melts and spreads through the pores of the sample. Thus there is liquid motion relative to the porous solid matrix. Our model describes the cases when the melt either fills all the pores or when some gas remains in the pores. In each case the melt occupies a prescribed volume fraction of the mixture. We employ both analytical and numerical methods to find uniformly propagating combustion waves, to analyze their stability and to determine behavior in the instability region.

ISSN:0010-2202    

DOI:10.1080/00102209408935439

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A numerical model is developed to examine steady, laminar flame spread and extinction over a thin solid fuel in low-speed concurrent flow. The model incorporates an elliptic treatment of the upstream flame stabilization zone near the fuel burnout point, and a parabolic treatment of the downstream flame, which has a higher flow Reynolds number. This provides a more precise fluid-mechanical description of the flame than using parabolic equations throughout, and is the first time such an approach has been used in concurrent flame spread modeling. The parabolic and elliptic regions are coupled smoothly by matching boundary conditions. The solid phase consists of an energy equation with surface radiative loss and a surface pyrolysis relation. Calculations (with the flame spread rate being an eigenvalue) are performed for forced flow without gravitational influences in a range of velocities which are lower than those induced in a normal gravity buoyant environment. Steady spread with constant flame and pyrolysis lengths is found possible for thin fuels and this facilitates the adoption of a moving coordinate system attached to the flame.

ISSN:0010-2202    

DOI:10.1080/00102209408935440

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A zero net flux boundary condition is applied to the classical Burke-Schumann diffusion flame. The numerical results indicate that the conventional constant boundary condition is valid only at a high Reynolds number (Re ⩾ 100) and strong back diffusion appears at low Reynolds numbers. The importance of applying this boundary condition in modelling of composite propellant combustion is also discussed.

ISSN:0010-2202    

DOI:10.1080/00102209408935441

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Ignition and extinction points are calculated for homogeneous-heterogeneous combustion of a premixed propane-air stream in stagnation point flow over a platinum surface as functions of inlet gas velocity and composition and power input to the catalyst surface using global rate expressions for the homogeneous and surface reactions. Curves of surface temperature and surface concentration of fuel and oxygen at the ignition and extinction points are computed at several compositions in the fuel-lean regime. [gnition and extinction of the surface reaction and autothermal behavior are found over a very broad range of velocities. Homogeneous ignition occurs only at relatively low velocities. As the velocity increases from 0.5 cm/s to 50 cm/s, autothermal behavior is supported by leaner mixtures, and the autothermal surface temperature increases substantially. Inlet compositions leaner than 1.4% propane cannot support autothermal operation at any velocity. Homogeneous ignition takes place at higher surface temperatures as the velocity is increased, while the range of compositions that exhibit homogeneous ignition shrinks markedly as velocity increases.

ISSN:0010-2202    

DOI:10.1080/00102209408935442

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor